CA2169646C - Method and apparatus for balancing the forward link handoff boundary to the reverse link handoff boundary in a cellular communication system - Google Patents
Method and apparatus for balancing the forward link handoff boundary to the reverse link handoff boundary in a cellular communication systemInfo
- Publication number
- CA2169646C CA2169646C CA002169646A CA2169646A CA2169646C CA 2169646 C CA2169646 C CA 2169646C CA 002169646 A CA002169646 A CA 002169646A CA 2169646 A CA2169646 A CA 2169646A CA 2169646 C CA2169646 C CA 2169646C
- Authority
- CA
- Canada
- Prior art keywords
- base station
- reverse link
- power level
- coverage area
- forward link
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/06—TPC algorithms
- H04W52/14—Separate analysis of uplink or downlink
- H04W52/143—Downlink power control
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/18—TPC being performed according to specific parameters
- H04W52/24—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters
- H04W52/245—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters taking into account received signal strength
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/18—TPC being performed according to specific parameters
- H04W52/24—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters
- H04W52/246—TPC being performed according to specific parameters using SIR [Signal to Interference Ratio] or other wireless path parameters where the output power of a terminal is based on a path parameter calculated in said terminal
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
- H04W52/04—TPC
- H04W52/38—TPC being performed in particular situations
- H04W52/40—TPC being performed in particular situations during macro-diversity or soft handoff
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W36/00—Hand-off or reselection arrangements
- H04W36/06—Reselecting a communication resource in the serving access point
Abstract
A method and apparatus for matching the location of the forward link handoff boundary to the reverse link handoff boundary. A system constant is chosen which defines the product of the received power and the transmitted pilot power at every base station. The reverse link power level is measured at the base station and the forward link power level is compensated for the reverse link loading to maintain the constant product. Thereby the forward link handoff boundary to the reverse link handoff boundary are aligned to the same location.
Description
2 1 6 9 6 ~ 6 PCT/US95/09212 METHOD AND APPARATUS FOR BALANCING THE
FC)RWARD LINK HANDOFF BOUNDARY TO THE REVERSE
LINK HANDOl~F BOUNDARY IN A CELLULAR
COMMUNICATION SYSTEM
BACKGROUND OF THE INVENTION
I. Field of ~e Invention The present invention relates to communication systems, particularly to a method and apparatus for performing handoff between two sectors of a common base station.
II. lDescription of ~e R~l~te~
In a code division multiple access (CDMA) cellular telephone ~y~Lel--or personal communications ~y~te~l~, a common frequency band is used for communication with all base stations in a ~ysleln. The common frequency band allows simultaneously communication between a mobile unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving terminAl (either within the mobile unit or base station) through the spread spectrum CDMA waveform properties based on the use of high speed pseudonoise (PN) codes and orthogonal Walsh codes. The high speed PN codes and orthogonal Walsh codes are used to modulate signals tran~mitte-l from the base stations and the mobile units. Transmitting tPrminAl~ (either within a mobile unit or within a base station) using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving terminAl.
In an exemplary CDMA ~y~leln, each base station tr~n~mit~ a pilot signal having a rolnmon PN spreading code that is offset in code phase frorn the pilot signal of other base stations. During ~y~Lelll operation, the mobile unit is provided with a list of code phase offsets col.es~onding to neighhoring base stations surrolln~ling the base station through which communication is established. The mobile unit is equipped with a searching element that allows the mobile unit to track the signal strength of the pilot signal from a group of base stations in~ ling the neighboring base stations.
A method and ~ysLem for providing communication with the mobile unit through more than one base station during the handoff process are disclosed in U.S. Patent No. 5,267,261 issued November 30, 1993, entitlell "MOBILE STATION AS~ L~ SOFT HANDOFF IN A CDMA CELLULAR
W096/03845 ,~ ~ ~9 ~ .. PCTIUS95/09212 ~t COMMUNICATION SYSTEM," assigned to the assignee of the present invention. Using this system, communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This ty-pe of handoff may be considered 5 as a "soft" handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the mobile unit is in communication with two base stations, a single signal for the end user is created from the signals from each base station by a c~ lAr or personal communication system controller.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. The Active Set is the set of base stations through which active comm1mi~ ~tion is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having 15 a pilot signal strength of sllffi~ient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, a mobile unit communicates through a first base station and the Active Set contains only 20 the first base station. The mobile unit monitors the pilot signal strength ofthe base stations of the Active Set, the Candidate Set, and the Neighbor Set.
When a pilot signal of a base station in the Neighbor Set exceeds a predet~rmine~l threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile 25 unit communicates a mPssAge to the first base station identifying the new base station. A c~ lAr or personal communication ~y~Lell~ controller decides whether to establish commlmi~ Ation between the new base station and the mobile unit. Should the cell~ r or personal communication sy~lell~ controller decide to do so, the c~ llAr or personal communication 30 ~y~lelll controller sends a message to the new base station with identifying information about the mobile unit and a commAnd to establish communications therewith. A message is also tran~mitte-l to the mobile unit through the first base station. The mess~ge i~l~ntifi~s a new Active Set that includes the first and the new base stations. The mobile unit searches 35 for the new base station's transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with A(l~litional base stations.
WO 96/03845 ~ ~ B ~ ~ 4 6 PCT/US95/09212 When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a 5 predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The cellular or personal communication sysl~m controller receives this m~s~ge through at least one of the base stations with which the mobile unit is commlmi~ Ating.
The celllll~r or personal communication ~ysle~l~ controller may decide to 10 terminate communications through the base station having a weak pilot signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set 15 does not contain the base station through which communication is to be prrnin~ted. The base stations through which communication is established send a m~s~ge to the mobile unit. The c~lhll~r or personal co~mlmirAtion ~yslell~ controller also communicates information to the base station to terminate communications with the mobile unit. The mobile unit 20 communications are thus routed only through base stations ifl~ntifie~l in the new Active Set.
Because the mobile unit is commllnicating with the end user though at least one base station at all times throughout the soft handoff processes, no interruption in communications occurs between the mobile unit and the 25 end user. A soft handoff provides si~nifi~ ~nt benefits in its inherent "makebefore break" communication over conventional "break before make"
terhniques employed in other rell~ r commllni~ ~tion ~yslems.
In a cellular or personal communication telephone system, maximi7.ing the capacity of the system in terms of the number of 30 simultaneous telephone calls that can be h~n~lle~ is extremely important.
System capacity in a spread spectrum ~yst~ can be maximized if the transmitter power of each mobile unit is controlled such that each tr~n~mitte-l signal arrives at the base station receiver at the same level. In an actual ~ysleln, each mobile unit may transmit the minimum signal level 35 that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal tr~n~mitte~l by a mobile unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to inle~ferel,ce from the other mobile units.
WO 96/03845 ~ PCT/US95/09212 ~
,; ., . :
On the other hand, if the mobile unit trAn~mitted signal is at a power level that is too high when received at the base station, communication with this particular mobile unit is acceptable but this high power signal acts as inle.ference to other mobile units. This interference may adversely affect 5 communications with other mobile units.
Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each mobile unit in communication with a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the 10 ideal case, the total signal power received at the base station is equal to the nominal power received from each mobile unit multiplied by the number of mobile units transmitting within the coverage area of the base station plus the power received at the base station from mobile units in the coverage area of neighboring base stations.
The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile unit, operates on a different frequency than the reverse link, from the mobile unit to the base station. However because the forward link and reverse link freqllPnriP~ are within the same frequency 20 band, a signifirAnt correlation between the average path loss of the two links exists. On the other hand, fading is an indepen(iPnt phenomenon for the forward link and reverse link and varies as a function of time. However, the characteristics of the fading on the rhAnnel are the same for both the forward and reverse link because the freqll~nries are within the same band.
25 Therefore the average of fading over time of the rhAnnPl for both links is typically the same.
In an exemplary CDMA ~y~ , each mobile unit estimAtP~ the path loss of the forward link based on the total power at the input to the mobile unit. The total power is the sum of the power from all base stations 30 operating on the same frequency ~ignmPnt as perceived by the mobile unit. From the estimate of the average forward link path loss, the mobile unit sets the transmit level of the reverse link signal.
Mobile unit transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is in commlmi~ ~tion 35 measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile unit at that base station. A power adjustment commAn~l is generated by each base station and sent to the mobile unit on the forward f WO 96/03845 - PCT/US95109212 2~6~646 link. In response to the base station power adjustment commAn~l~, the mobile unit increases or decreases the mobile unit transmit power by a predet~rminerl amount.
When a mobile unit is in communication with~ more than one base station, power adjustment commands are provided from each base station.
The mobile unit acts upon these multiple base station power adjustment co~mAncl~ to avoid transmit power levels that may adversely interfere with other mobile unit communications and yet provide sllffit ient power to support communication from the mobile unit to at least one of the base stations. This power control mechanism is accomplished by having the mobile unit increase its transmit signal level only if every base station with which the mobile unit is in communication requests an increase in power level. The mobile unit decreases its transmit signal level if any base station with which the mobile unit is in commllnication requests that the power be decreased. A system for base station and mobile unit power control is disclosed in U.S. Patent No. 5,056,109 entitled "METHOD AND
APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A
CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued October 8, 1991, ~signe~l to the A~signee of the present invPntion Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit commlmi- Ates with each base station through which rommlmirAtion is possible. In doing so, the mobile unit avoids inadvertently illL~lfering with communications through a base station receiving the mobile unit's signal at an excessive level but unable to communicate a power adjustment commAntl to the mobile unit because rommlmi~ Ation is not established therewith.
Each base station coverage area has two handoff boundaries. A
handoff boundary is riefine~l as the physical location between two base stations where the link would perform the same regardless of which of the base stations the mobile unit was in commlmi~ Ation with. Each base station has a forward link handoff boundary and a reverse link handoff boundary.
The forward link handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is i~fine-l as the location of the mobile unit where two base station receivers would perform the same with respect to that mobile unit.
WO 96/03845 ~ 9 6 ~ 6 PCT/US95/092~2 ~
"
Ideally these boundaries should be the balanced meaning that they have the same physical location. If they are not, network capacity may be reduced as the power control process is disturbed or the handoff region unreasonably expands. Note that handoff boundary balance is a function of 5 time in that the reversing link power increases as the number of mobile units increases. An increase reverse power decreases the effective size of the coverage area of the base station and causes the reverse link handoff boundary to move inward toward the base station. Unless a comp~n~Atir,n mechanism for the forward link is incorporated in the base station, even a 10 ~y~lelll that is initially perfectly balanced will be unbalanced periodically dependent on the loading.
The present invention is an apparatus and method for compensation of a base station to achieve a balanced handoff boundary condition under varying loading conditions. The balancing of a base station increases and 15 decrease the coverage area of the base station automatically as needed to match the forward link handoff boundary to the reverse link handoff boundary. This process is called base station breathing.
It is therefore the object of the present invention to provide method and apparatus for matching the forward link handoff boundary to the 20 reverse link handoff boundary.
It is another object of the present invention to provide a method and apparatus for continuously monitoring and reacting to the reverse link IOA~1;n~ to maxirnize ~y~e~ll capacity.
SUMMARY OF THE INVENTION
The present invention defines a method and apparatus for mAtrhin~
the forward link handoff boundary to the reverse link handoff boundary.
The nnethod and apparatus is based on measurement the reverse link power 30 level at the base station and adjustment of the forward link power level to compensate for the reverse link loading.
Each base station in the ~yslell- is initially calibrated such that the sum of the unloaded receiver path noise and the desired pilot power is equal to some constant. The calibration constant is consistent throughout 35 the system of base stations. As the system becomes loaded (i.e. mobile units begin to communicate with the base stations), a comp~n~Ation network rnAint~inS the constant relationship between the reverse link power received at the base station and the pilot power trar-~itte-l from the base station. The loading of a base station effectively moves the reverse link ~ WO 96/03845 PCT/US9S/09212 s 2~69~
handoff boundary closer in toward the base station. Thererore to imitate the same effect on the forward link, the pilot power is decreased as loading is mcreased.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the present invention will become more apparent from the letAiie~ description set forth below when taken in conjunction with the drawings in which like reference characters 10 identify colles~ondingly throughout and wherein:
Figures lA - lC show three unbAlAnce~l handoff conditions;
Figures 2A- 2C illustrate the effect of loading on the handoff boLln~lAri~ and the effect of the breathing mechanism compensation; and Figure 3 is a highly simplified block diagram of the breathing 15 met~hAni~m in a base station.
DET~TT.l~n DESCRIPTION OF THE PREFERRED
EMBODIMENTS
Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit communicates with each base station through which communication is possible. In doing so, the mobile unit avoids inadve~lel,tly inlelfefiQg with commtlni~Ations through a base station receiving the mobile unit's signal at an excessive level but unable to communicate a power adjustment commAn~ to the mobile unit because commllnicAtion is not established therewith.
A typical cellular, wireless local loop, or personal commlmication system contains some base stations having multiple sectors. A multi-sectored base station comprises multiple independent tr~n~mit and receive antennas as well as independent processing circuitry. The present il~ve~llion applies equally to each sector of a sectorized base station and to single sectored independent base stations. The term base station can be assumed to refer to either a sector of a base station or a single sectored base station.
Each base station has a physical coverage area in which communication with the base station is possible. Each base station coverage area has two handoff boundaries. A handoff boundary is defined as the physical location between two base stations where the link would perform in the same mAnn~r regardless of which of the base station a mobile unit at W0 96/03845 ~ 6 PCT/US95/09212 ~
that location was in communication with. Each base station has a forward link handoff boundary and a reverse link handoff boundary. The forward link handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was 5 receiving. The reverse link handoff boundary is defined as the location of the mobile unit where two base station receivel~ would perform the same with respect to that mobile unit.
The present invention is described herein based on a system having soft handoff capability. However the invention is equally applicable hard 10 handoff operation.
A handoff boundary is always defined between at least two base stations. For example in Figure lA forward link handoff boundary 60 is a function of the power tr~n~mittetl from base station 10 and from base station 40 as well as inlelrere~lce from other surrounding base stations (not 15 shown) and other inband sources. Reverse link handoff boundary 50 is a function of the power level received at base station 10 and base station 40 from a mobile unit at that location and the power level received at base station 10 and base station 40 from the other mobile units and other inband sources. Note that the power level received at base station 10 and the power 20 level received at base station 40 are somewhat independent in that if base station 10 has a large number of mobile units located within its coverage area and base station 40 has only one mobile unit, the ..-~e.~rellce for base station 40 will be much less.
Ideally the forward link handoff boundary and the reverse link 25 h~n~loff boundary are co-located so that the optional system ~ ArA~ ity may be achieved. If they are not co-located then three situations that are detrimental to capacity can occur. Figure lA shows the first of these situations. A soft handoff region is the physical region between two base stations where a mobile unit located within the region is likely to establish 30 commlmic~Ation with both base stations. In Figure lA the shaded portion represents soft handoff region 20.
In mobile unit assisted soft handoff, the handoff region is defined by the forward link characteristics. For example, in Figure lA soft handoff region 20 represents the region where both the signal quality from base 35 station 10 and the signal quality from base station 40 are sllffi~iPnt to support communications. When mobile unit 30 enters soft handoff region 20, it will notify which ever base station it is in communication with that the second base station is available for communications. The system controller ~ 69~46 (not shown) establishes communication between with the second base station and mobile unit 30 as described in above mentioned U.S. Patent No. 5,267,261. When mobile unit 30 is in soft handoff between base station 10 and base station 40, both base stations control the trAn~mit power 5 from mobile unit 30. Mobile unit 30 decreases its transmit power if either base station requests a decrease and increases its transmit power only if each base station asks for an increase as disclosed in the above m~ntioned U.S.
Patent No. 5,056,109.
Figure lA shows the first situation which is detrimental to ~y~
10 capacity. In Figure lA forward link handoff boundary 60 and reverse link handoff boundary 50 are significantly unbalanced (i.e. spaced apart). Mobile unit 30 is located in a position where communication is established only with base station 40. In the region where mobile unit 30 is located, the forward link performance is best with base station 40 but the reverse link 15 performAnce would be better if mobile unit 30 were commlmir~ting with base station 10. In this situation mobile unit 30 is transmitting more power than it would be tr~n~mitting if it were in communication with base station 10. The increased trAn~mit power adds lmnecess~rily to the total inlelrer~"ce in the ~y~lem thereby adversely effecting capacity. It also 20 increases the overall power consumption of mobile unit 30 thereby decreasing its battery life. And it entl~n~ers the communication link if mobile unit 30 reaches its maximum transmit power and is unable to respond to commAn~l~ for increased power.
Figure lB show an alternative but also detrimental result of an 25 unbalanced handoff con(lition In Figure lB, soft handoff region 70 is positioned about reverse link handoff boundary 50. This handoff position could be the result of an altemative handoff s~Pme where handoff is based on the reverse link performance instead of the forward link performAnce.
In one such case, each base station would ~lle.-~l,t to measure the power 30 received from each mobile unit. When the measured power level exceeds a threshold or exceeds the level received at other base stations, communication with a second base station is established. In Figure lB, mobile unit 30 is located in a region where commlmil~ticn is established only with base station 10. As in Figure lA in the region where mobile 35 unit 30 is located, the forward link perform~nce is best with base station 40but the reverse link performance is best with base station 10. Unlike the reverse link, the forward link does not have a large dynamic range of transmit power and as mobile unit 30 moves toward base station 40, 2 ~
i~lel~erellce from base station 40 increases as the received power level from base station 10 decreases. If the power level from base station 10 falls below a sl7ffi~i~nt signal to inlelrele"ce level or below a certain absolute level, the communication link is in danger being of lost. The power level tran~mi~te~i from base station 10 is slowly increased within a limite~l dy-namic range as mobile unit 30 moves away from base station 10. This increase in power adversely interferes with other users in base station 10 and base station 40 thus unnec~ss~rily decreasing capacity.
Yet another altemative is a combined handoff srheme based on both the forward link performance and the reverse link performAnce Figure lC
shows one such scenario. In Figure 1C, handoff region 80 is large and encompasses both reverse link handoff boundary 50 and forward link handoff boundary 60. But u~nec~s~ry soft handoff directly decreases the capacity- of the ~yslelll. The purpose of soft handoff is to provide a make before break handoff between base stations and to provide an efflcient power control merh~ni~m However if the soft handoff region is too large, the negative effects become significant. For example, in Figure 1C, both base station 10 and base station 40 must tr~n~mit to mobile unit 30 while mobile unit 30 is in soft handoff region 80. Thus the total ~y~le~ er~ert:l~ce is increased while mobile unit 30 is in soft handoff region 80. In addition, resources at both base station 10 and base station 40 must be ~ie~lic~te-l to the signal received from mobile unit 30. Therefor~ increasing the size of th soft handoff region is not an efficient use of the syslell, capacity and resources.
The solution to these adverse effects is to b~l~n~ e (i.e. physically align) the reverse link handoff boundary to the forward link handoff boundary or vice versa. Even if this was done at each base station in a static con~lition, the balance would be lost as the system was used. For example, the signal to il,ter~erence level of the reverse link signal received at a base station is a function of the number, location, and trAn~ sion power level of the mobile un*s within its coverage area. As the loading on one base station increases, inlerference increases and the reverse link handoff boundary shrinks toward the base station. The forward link boundary is not effected in the same mAnner thus a ~y~telll that is initially b~l~nce~l may become unb~l~ncell over time.
To m~int~in balance, the present invention defines a method "breathingn the size of the base station coverage area. The breathing met h~ni~m effectively moves the forward link handoff boundary to the same location as the reverse link handoff boundary. Both of the boundaries are dependent on the performance of at least two base stations. For breathing to be effective, the reverse link handoff boundary and the forward link handoff boundary must be initially aligned. The boundaries can remain aligned if the performance of each base station is controlled as described below.
The forward link performance can be controlled by the base station.
In an exemplary CDMA ~y~lem, each base station tr~n~mit~ a pilot signal.
The mobile units per~lln handoff based on the perceived pilot signal strength as described above. By c~nging the power level of the pilot signal transmitted from the base station, the forward link handoff boundary location may be maniptll~te~i.
The reverse link performance can also be controlled by the base station. The noise performance of the base station receiver sets the minimum receive power level which can be detected. The noise performance of the receiver is typically defined in terms of an overall ~y~lelll noise figure. By controlling the noise figure of the receiver, such as by injecting noise or adding attPnll~tif n, the reverse link pefrollllance, and hence the reverse link handoff boundary, may be adjusted.
To balance the handoff boundaries, the performAnce of each base station must be controlled to be the same as the perform~nce of other base stations in the system. Therefore, we define a ~y~Lell~ wide perform~nce constant to be used by each base station in the ~y~l~l.. A dynamic constant that is equal for every base station but allowed to change over time could 25 also be define~l- In the inleresl of simplicity of ~i~fiigll and impl~mentAtion, a fixed constant is ple~elred in this embo~liment The constant is defined in terms of the sum of the receiver path noise in dlecibels (dB) and the maximum desired pilot signal power in dB as ~)l'UVell below. The best choice constant takes advantage of the perform~nce 30 available from the ~ysle~.. Therefore to define the constant, KleVel~ the following equation is used:
MAX
Klevel = alli [NRx:i+PMax:i] Eq. 1 where:
35 NRX i is the receiver path noise of base station i in dB;
PMaX:i is the m~cimllm desired pilot signal power of base station i in dB; and MAX
all i [ ] finds the largest such sum of all base stations in a system.
wo sG/03s4s 2 ~ PCT/US9S/09212 Note that once KleVel is chosen, artificial means can be used to increase the path noise of the unloaded system of each base station to meet the constant.
To prove that setting the sum of the received power and the 5 transmitted power to a KleVel indeed balances the system, several assumptions are made. The first assumption is that in any base station using multiple redundant receive and transmit antennas, the antennas have been balanced to have the same performance. Also the analyses assumes that the identical decoding perform~nce is available at each base 10 station. It assumes a constant ratio between total forward link power and pilot signal power. And it assumes reciprocity in the forward link path loss and the reverse link path loss.
To find the forward link handoff boundary between two arbitrary base stations, base station A and base station B, start by noting that the 15 forward handoff boundary occurs where the ratio of the pilot signal power of the two base stations to the total power is equal. Assume that mobile unit C is located at the boundary, mathematically in units of linear power (such as Watts):
Pilot Power of A Rx'd at C Pilot Power of B Rx'd at C
20 Total Power Received at C = Total Power Received at C Eq. 2 Noting that the power received at the mobile unit is equal to the power trAn~mitte~ times the path loss, Equation 2 becc)mes:
25 Pilot Power Tx'd from A X Pa~ loss from A to C Pilot Power Tx'd from B X Pa~ loss from B to C
Total Power Received at C Total Power Received at C Eq. 3 Re-arranging Equation 3 and Plimin~ting the common denomin~tor, yields:
Pilot Power Tx'd from A Path loss from B to C
Pilot Power Tx'd from B = Path loss from A to C Eq. 4 Following the same procedure for the reverse link and noting that the reverse link handoff boundary occurs where each base station pe.ceives the same signal to inLelfere~lce ratio for that mobile unit:
Power of C Rx'd at A Power of C Rx'd at B
35 Total Power Received at A = Total Power Received at B Eq. 5 Noting that the power received at the base station is equal to the power tr~n~mitte~ from the mobile unit times the path loss, Equation 5 becomes:
Power Tx'd from C X Path loss from C to A Power Tx'd from C X Path loss from C to B
Total Power Received at A Total Power Received at B Eq. 6 wo 96/03845 ~ 1 ~ g ~ 4 6 PCT/US95/09212 Re-arranging Equation 6 and eliminating the common numerator, yields:
Total Power Received at A Path loss from C to A
Total Power Received at B Path loss from C to B Eq. 7 Due to the assumed reciprocity in the forward and reverse link path loss at any location, Equations 4 and 7 may be combined to yield:
Total Power Received at A Pilot Power Tx'd from B
Total Power Received at B Pilot Power Tx'd from A Eq. 8 ('h~nging the units of Equation 8 from linear power to dB yields:
Total Power Received at A (dB) - Total Power Received at B (dB) =
Pilot Power Tx'd from B (dB) - Pilot Power Tx'd from A (dB) Eq. 8' Equation 8' is equivalent to premise set forth in that:
if Total Power Received at A (dB) ~ Pilot Power Tx'd from A (dB) = Kleve and Total Power Received at B (dB) + Pilot Power Tx'd from B (dB) = Kleve then equation 8 will be satisfied.
And the forward link handoff boundary and the reverse link handoff boundary are co-located.
Three mechanisms are needed to perform the breathing function: a means of initially setting performance to KleVel~ a means of monitoring the fll1ctll~tions in the reverse link, and a means of changing the performance of the forward link in response to the reverse link flllchlAtio~.
One method of initially setting the perform~nce to KleVel is to set the maximum desired pilot signal strength taking into account the variations over temperature and time and adding att~ml~ticn in line with the receiver in a no input signal condition until the KleVel perfo~ nce is achieved.
Adding attenuation desensitize the receiver and effectively increases the noise figure thereof. This also requires that each mobile unit transmit proportionately more power. The added attenuation should be kept to the minimum dictated by Klevel Once initial balance is achieved, the power coming into the base station can be measured to monitor the reverse link performance. Several methods can be used. Measurement can be done by monitoring an AGC
(automatic gain control) voltage or by directly measuring the incoming level. This method has the advantage that if an inleLrerer is present (such as am FM signal) this energy will be measured and the handoff boundaries will be drawn closer to the base station. By drawing the handoff boundary WO 96/0384S ~ i 9 6 PCT/US95/09212 closer to the base station, the interferer may be eliminated from the coverage area of the base station and its effect minimized. Measurement could be made by simply counting the number of users communicating through the base station and estimating the total power based on the fact 5 that each mobile unit's signal nominally arrives at the base station at the same signal level.
As the reverse link power increases, the forward link power should be decreased. This can be easily achieved by using an existing AGC circuit within the transmit circuitry or by providing a controllable ~ nll~tor in the 10 transmit path.
In the exemplary handoff scheme described above, handoff boundaries are based on the measurement of the pilot signal strength at the mobile unit. An alternative to controlling the total transmit power would be to control only the pilot signal level. To the coverage area d~igner, this 15 scheme might have a certain sense of appeal but controlling the total transmit power including the traffic (e.g. active calls) and pilot signals together has some advantages. First, the ratio of the pilot signal strength to the traffic channel signal strength remains fixed. The mobile unit may be expecting a fixed ratio and may allocate its resources based on the ratio. If 20 the mobile unit were to receive two equally powerful pilot ~ign~l~ each corresponding to a traffic channel having a different power level, a suboptimal decision on the allocation of mobile unit resources could result.
Adjusting the total power is also advantageous because it reduces the inlelrerellce to other base station coverage areas. If the pilot signal is not 25 strong enough to warrant a handoff in the coverage area of a neighboring base station, the high powered traffic ~hAnnel signal adds unusable and unnec~s~ry inlel~rel.ce to that area. Of course, in some applications, it may be advantageous to combine the methods by controlling the power of the pilot signal in some cases and the total transmit power in other cases. In 30 still another application, it may be advantageous to change the ratio of the pilot power to the traffic channel power.
In an ideal configuration, the bre~thing mechanism would measure the receive power and change the transmit power proportionately.
However, some ~ysl~llls may not use the proportional method and may 35 instead change the transmit level only a fraction of the perceived changed in receive power. For example, if a ~yslelll was designed in which the estimation of the received power was difficult and inaccurate, the ~y~lell-designers may wish to decrease the sensitivity to the inaccuracy. A change ~6~6~.6 in transmit level which is only a fraction of the change in receive power achieves the desensitization while preventing gross unbalance of the handoff boundaries.
Another alternative changes the transmit level only when the 5 receiver level exceeds a predetermine threshold. This method could be used to primarily deal with inlerfeLers. Of course this method may be combined with a system which changes the trAn~mit level only a fraction of the perceived change in receive power.
The breathing mechanism must have a carefully considered time 10 constant. The breathing mechanism may cause mobile unit handoffs. To perform a handoff, the mobile unit must detect the change in power and send a message to the base station. The system controller must make a decision and notify base stAtion~ A m~sAge must be sent back to the mobile unit. This process takes time and the breathing process should be slow 15 enough to allow this process to happen smoothly.
The process of breathing will naturally limit itself to ~vel-t the total convergence of the coverage area of the base station due to excess users on the ~yslelll. The CDMA ~yslem has a large and soft limitell capacity. The terrn sof~ limited capacfty refers to the fact that one more user can always be 20 addLed but at some number of users each additional user effects the communication quality of all the other users. At some greater number of users, each user's commllnicAtion quality becomes unusable and the entire link is lost to every mobile unit. To prevent the loss of the link, each base station limits the number of mobile units with which it will establish 25 communication. Once that limit has been reached, the ~ysle", will refuse attempts to establish additional calls, i.e. new call originations are blocked.
The limit is a design parameter and is typically set at about 75% of theoretical capacity. This gives some margin to the ~y~lelll and allows the ~ysle"~ to accept an emergency call even while in the limite~l condition.
30 This limit of the total number of mobile units communicating within the coverage area of a single base station naturally limits the maximum received power and therefore limits the breathing process range of operation.
Figures 2A - 2C illustrate the base station breathing me~ hAnism. In 35 Figure 2A, base station 100 has circular coverage area 130 in an unloaded condition. The coverage area of base station 100 has been balanced in an unloaded condition and the forward and reverse links coverage areas are aligned with circular coverage area 130. Base station 110 has circular W096/03845 ~ 1 ~ 9 ~ 4 S PCT/US95/09212 coverage area 140 in an unloaded condition. The coverage area of base station 110 has also been balanced in an unloaded con~ition and the forward and reverse links coverage areas are aligned with circular coverage area 140.
The operation of base stations 100 and 110 have been balanced to Klevel in 5 an unloaded condition and line 120 represents the location at which operation with each base station is same and hence both handoff boundaries.
In Figure 2B, base station 110 has be come heavily loaded and base station 100 is lightly loaded. The coverage area of the reverse link has 10 shrunk to the location of circular coverage area 145 while the forward link coverage area remains at circular coverage area 140. The light loading of base station 100 has not effected the coverage area of base station 100 which is still at circular coverage area 130. Note that the reverse link handoff boundary between base station 100 and base station 110 has moved to 15 line 125 while the forward link handoff boundary remains at line 120. Thus the undesirable unbalanced handoff boundary condition has been created.
In Figure 2C, base station 110 has implemented the base station breathing mech~ni~m The effect has been to move the forward link handoff boundary to circular coverage area 145. Line 125 now represents 20 both the forward and reverse link handoff boundaries.
In Figures 2B and 2C, the X's represent ~yslell- users. In particular user X 150 is located at the handoff boundary in Figure 2B. Due to his location, user X is in soft handoff between base station 100 and base station 110. Note that in Figure 2C, user X 150 is now deep into the coverage 25 area of base station 100 and not in the soft handoff region between base station 100 and base station 110. Therefore, the heavily loaded base station 110 has effectively transferred some of its load to the lightly load base station 100.
Figure 3 is a block diagram showing an exemplary base station 30 breathing configuration. Antenna 270 receives sign~l~ at base station 300.
The receive signals are then passed to variable attenuator 200 which has been used to initially set Klevel operation. The receive signals are passed to power ~ietertor 210. Power detector 210 generates a level in~lic~ing the total power in the received signal. Low pass filer 220 averages the power 35 indication and slows the breathing response time. Scale and threshold 230 sets the desired ratio and offset of the relation between increases in the reverse link power and decreases in the forward link power. Scale and threshold 230 outputs a control signal for variable gain device 240. Variable wo 96/03845 ~ :L 6 ~ ~ ~ 6 PCT/US9S/09212 gain device 240 accepts the transmit signal and provides a gain controlled output signal to high power amplifier (HPA) 250. HPA 250 amplifies the transit signal and passes to antenna 260 for transmission over the wireless link.
Many variations to the configuration of Figure 3 exist. For example, antennas 260 and 270 may each comprise two antennas. Or conversely antennas 260 and 270 may be the same antenna. The power detection in Figure 3 is based on all incoming signal power within the band of interest.
As discll~se-l above, power detection can be based solely on the number of mobile units which have established communication with the base station.
Also low pass filer 220 may be a linear filter or nonlinear filter (such as a slew rate limitin~ filter).
There are many obvious variations to the present invention as presented inclllrlin~ simple architectural changes. The previous description of the ~refelred embo-lim~nt is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embo~lim~nt~ will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other emboriimPnt~
without the use of the inventive faculty. Thus, the present illvenlion is not intended to be limited to the embo(limPnt~ shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
WE CLAIM:
FC)RWARD LINK HANDOFF BOUNDARY TO THE REVERSE
LINK HANDOl~F BOUNDARY IN A CELLULAR
COMMUNICATION SYSTEM
BACKGROUND OF THE INVENTION
I. Field of ~e Invention The present invention relates to communication systems, particularly to a method and apparatus for performing handoff between two sectors of a common base station.
II. lDescription of ~e R~l~te~
In a code division multiple access (CDMA) cellular telephone ~y~Lel--or personal communications ~y~te~l~, a common frequency band is used for communication with all base stations in a ~ysleln. The common frequency band allows simultaneously communication between a mobile unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving terminAl (either within the mobile unit or base station) through the spread spectrum CDMA waveform properties based on the use of high speed pseudonoise (PN) codes and orthogonal Walsh codes. The high speed PN codes and orthogonal Walsh codes are used to modulate signals tran~mitte-l from the base stations and the mobile units. Transmitting tPrminAl~ (either within a mobile unit or within a base station) using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving terminAl.
In an exemplary CDMA ~y~leln, each base station tr~n~mit~ a pilot signal having a rolnmon PN spreading code that is offset in code phase frorn the pilot signal of other base stations. During ~y~Lelll operation, the mobile unit is provided with a list of code phase offsets col.es~onding to neighhoring base stations surrolln~ling the base station through which communication is established. The mobile unit is equipped with a searching element that allows the mobile unit to track the signal strength of the pilot signal from a group of base stations in~ ling the neighboring base stations.
A method and ~ysLem for providing communication with the mobile unit through more than one base station during the handoff process are disclosed in U.S. Patent No. 5,267,261 issued November 30, 1993, entitlell "MOBILE STATION AS~ L~ SOFT HANDOFF IN A CDMA CELLULAR
W096/03845 ,~ ~ ~9 ~ .. PCTIUS95/09212 ~t COMMUNICATION SYSTEM," assigned to the assignee of the present invention. Using this system, communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This ty-pe of handoff may be considered 5 as a "soft" handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the mobile unit is in communication with two base stations, a single signal for the end user is created from the signals from each base station by a c~ lAr or personal communication system controller.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. The Active Set is the set of base stations through which active comm1mi~ ~tion is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having 15 a pilot signal strength of sllffi~ient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, a mobile unit communicates through a first base station and the Active Set contains only 20 the first base station. The mobile unit monitors the pilot signal strength ofthe base stations of the Active Set, the Candidate Set, and the Neighbor Set.
When a pilot signal of a base station in the Neighbor Set exceeds a predet~rmine~l threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile 25 unit communicates a mPssAge to the first base station identifying the new base station. A c~ lAr or personal communication ~y~Lell~ controller decides whether to establish commlmi~ Ation between the new base station and the mobile unit. Should the cell~ r or personal communication sy~lell~ controller decide to do so, the c~ llAr or personal communication 30 ~y~lelll controller sends a message to the new base station with identifying information about the mobile unit and a commAnd to establish communications therewith. A message is also tran~mitte-l to the mobile unit through the first base station. The mess~ge i~l~ntifi~s a new Active Set that includes the first and the new base stations. The mobile unit searches 35 for the new base station's transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with A(l~litional base stations.
WO 96/03845 ~ ~ B ~ ~ 4 6 PCT/US95/09212 When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a 5 predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The cellular or personal communication sysl~m controller receives this m~s~ge through at least one of the base stations with which the mobile unit is commlmi~ Ating.
The celllll~r or personal communication ~ysle~l~ controller may decide to 10 terminate communications through the base station having a weak pilot signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set 15 does not contain the base station through which communication is to be prrnin~ted. The base stations through which communication is established send a m~s~ge to the mobile unit. The c~lhll~r or personal co~mlmirAtion ~yslell~ controller also communicates information to the base station to terminate communications with the mobile unit. The mobile unit 20 communications are thus routed only through base stations ifl~ntifie~l in the new Active Set.
Because the mobile unit is commllnicating with the end user though at least one base station at all times throughout the soft handoff processes, no interruption in communications occurs between the mobile unit and the 25 end user. A soft handoff provides si~nifi~ ~nt benefits in its inherent "makebefore break" communication over conventional "break before make"
terhniques employed in other rell~ r commllni~ ~tion ~yslems.
In a cellular or personal communication telephone system, maximi7.ing the capacity of the system in terms of the number of 30 simultaneous telephone calls that can be h~n~lle~ is extremely important.
System capacity in a spread spectrum ~yst~ can be maximized if the transmitter power of each mobile unit is controlled such that each tr~n~mitte-l signal arrives at the base station receiver at the same level. In an actual ~ysleln, each mobile unit may transmit the minimum signal level 35 that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal tr~n~mitte~l by a mobile unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to inle~ferel,ce from the other mobile units.
WO 96/03845 ~ PCT/US95/09212 ~
,; ., . :
On the other hand, if the mobile unit trAn~mitted signal is at a power level that is too high when received at the base station, communication with this particular mobile unit is acceptable but this high power signal acts as inle.ference to other mobile units. This interference may adversely affect 5 communications with other mobile units.
Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each mobile unit in communication with a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the 10 ideal case, the total signal power received at the base station is equal to the nominal power received from each mobile unit multiplied by the number of mobile units transmitting within the coverage area of the base station plus the power received at the base station from mobile units in the coverage area of neighboring base stations.
The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile unit, operates on a different frequency than the reverse link, from the mobile unit to the base station. However because the forward link and reverse link freqllPnriP~ are within the same frequency 20 band, a signifirAnt correlation between the average path loss of the two links exists. On the other hand, fading is an indepen(iPnt phenomenon for the forward link and reverse link and varies as a function of time. However, the characteristics of the fading on the rhAnnel are the same for both the forward and reverse link because the freqll~nries are within the same band.
25 Therefore the average of fading over time of the rhAnnPl for both links is typically the same.
In an exemplary CDMA ~y~ , each mobile unit estimAtP~ the path loss of the forward link based on the total power at the input to the mobile unit. The total power is the sum of the power from all base stations 30 operating on the same frequency ~ignmPnt as perceived by the mobile unit. From the estimate of the average forward link path loss, the mobile unit sets the transmit level of the reverse link signal.
Mobile unit transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is in commlmi~ ~tion 35 measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile unit at that base station. A power adjustment commAn~l is generated by each base station and sent to the mobile unit on the forward f WO 96/03845 - PCT/US95109212 2~6~646 link. In response to the base station power adjustment commAn~l~, the mobile unit increases or decreases the mobile unit transmit power by a predet~rminerl amount.
When a mobile unit is in communication with~ more than one base station, power adjustment commands are provided from each base station.
The mobile unit acts upon these multiple base station power adjustment co~mAncl~ to avoid transmit power levels that may adversely interfere with other mobile unit communications and yet provide sllffit ient power to support communication from the mobile unit to at least one of the base stations. This power control mechanism is accomplished by having the mobile unit increase its transmit signal level only if every base station with which the mobile unit is in communication requests an increase in power level. The mobile unit decreases its transmit signal level if any base station with which the mobile unit is in commllnication requests that the power be decreased. A system for base station and mobile unit power control is disclosed in U.S. Patent No. 5,056,109 entitled "METHOD AND
APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A
CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued October 8, 1991, ~signe~l to the A~signee of the present invPntion Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit commlmi- Ates with each base station through which rommlmirAtion is possible. In doing so, the mobile unit avoids inadvertently illL~lfering with communications through a base station receiving the mobile unit's signal at an excessive level but unable to communicate a power adjustment commAntl to the mobile unit because rommlmi~ Ation is not established therewith.
Each base station coverage area has two handoff boundaries. A
handoff boundary is riefine~l as the physical location between two base stations where the link would perform the same regardless of which of the base stations the mobile unit was in commlmi~ Ation with. Each base station has a forward link handoff boundary and a reverse link handoff boundary.
The forward link handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is i~fine-l as the location of the mobile unit where two base station receivers would perform the same with respect to that mobile unit.
WO 96/03845 ~ 9 6 ~ 6 PCT/US95/092~2 ~
"
Ideally these boundaries should be the balanced meaning that they have the same physical location. If they are not, network capacity may be reduced as the power control process is disturbed or the handoff region unreasonably expands. Note that handoff boundary balance is a function of 5 time in that the reversing link power increases as the number of mobile units increases. An increase reverse power decreases the effective size of the coverage area of the base station and causes the reverse link handoff boundary to move inward toward the base station. Unless a comp~n~Atir,n mechanism for the forward link is incorporated in the base station, even a 10 ~y~lelll that is initially perfectly balanced will be unbalanced periodically dependent on the loading.
The present invention is an apparatus and method for compensation of a base station to achieve a balanced handoff boundary condition under varying loading conditions. The balancing of a base station increases and 15 decrease the coverage area of the base station automatically as needed to match the forward link handoff boundary to the reverse link handoff boundary. This process is called base station breathing.
It is therefore the object of the present invention to provide method and apparatus for matching the forward link handoff boundary to the 20 reverse link handoff boundary.
It is another object of the present invention to provide a method and apparatus for continuously monitoring and reacting to the reverse link IOA~1;n~ to maxirnize ~y~e~ll capacity.
SUMMARY OF THE INVENTION
The present invention defines a method and apparatus for mAtrhin~
the forward link handoff boundary to the reverse link handoff boundary.
The nnethod and apparatus is based on measurement the reverse link power 30 level at the base station and adjustment of the forward link power level to compensate for the reverse link loading.
Each base station in the ~yslell- is initially calibrated such that the sum of the unloaded receiver path noise and the desired pilot power is equal to some constant. The calibration constant is consistent throughout 35 the system of base stations. As the system becomes loaded (i.e. mobile units begin to communicate with the base stations), a comp~n~Ation network rnAint~inS the constant relationship between the reverse link power received at the base station and the pilot power trar-~itte-l from the base station. The loading of a base station effectively moves the reverse link ~ WO 96/03845 PCT/US9S/09212 s 2~69~
handoff boundary closer in toward the base station. Thererore to imitate the same effect on the forward link, the pilot power is decreased as loading is mcreased.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, objects, and advantages of the present invention will become more apparent from the letAiie~ description set forth below when taken in conjunction with the drawings in which like reference characters 10 identify colles~ondingly throughout and wherein:
Figures lA - lC show three unbAlAnce~l handoff conditions;
Figures 2A- 2C illustrate the effect of loading on the handoff boLln~lAri~ and the effect of the breathing mechanism compensation; and Figure 3 is a highly simplified block diagram of the breathing 15 met~hAni~m in a base station.
DET~TT.l~n DESCRIPTION OF THE PREFERRED
EMBODIMENTS
Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit communicates with each base station through which communication is possible. In doing so, the mobile unit avoids inadve~lel,tly inlelfefiQg with commtlni~Ations through a base station receiving the mobile unit's signal at an excessive level but unable to communicate a power adjustment commAn~ to the mobile unit because commllnicAtion is not established therewith.
A typical cellular, wireless local loop, or personal commlmication system contains some base stations having multiple sectors. A multi-sectored base station comprises multiple independent tr~n~mit and receive antennas as well as independent processing circuitry. The present il~ve~llion applies equally to each sector of a sectorized base station and to single sectored independent base stations. The term base station can be assumed to refer to either a sector of a base station or a single sectored base station.
Each base station has a physical coverage area in which communication with the base station is possible. Each base station coverage area has two handoff boundaries. A handoff boundary is defined as the physical location between two base stations where the link would perform in the same mAnn~r regardless of which of the base station a mobile unit at W0 96/03845 ~ 6 PCT/US95/09212 ~
that location was in communication with. Each base station has a forward link handoff boundary and a reverse link handoff boundary. The forward link handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was 5 receiving. The reverse link handoff boundary is defined as the location of the mobile unit where two base station receivel~ would perform the same with respect to that mobile unit.
The present invention is described herein based on a system having soft handoff capability. However the invention is equally applicable hard 10 handoff operation.
A handoff boundary is always defined between at least two base stations. For example in Figure lA forward link handoff boundary 60 is a function of the power tr~n~mittetl from base station 10 and from base station 40 as well as inlelrere~lce from other surrounding base stations (not 15 shown) and other inband sources. Reverse link handoff boundary 50 is a function of the power level received at base station 10 and base station 40 from a mobile unit at that location and the power level received at base station 10 and base station 40 from the other mobile units and other inband sources. Note that the power level received at base station 10 and the power 20 level received at base station 40 are somewhat independent in that if base station 10 has a large number of mobile units located within its coverage area and base station 40 has only one mobile unit, the ..-~e.~rellce for base station 40 will be much less.
Ideally the forward link handoff boundary and the reverse link 25 h~n~loff boundary are co-located so that the optional system ~ ArA~ ity may be achieved. If they are not co-located then three situations that are detrimental to capacity can occur. Figure lA shows the first of these situations. A soft handoff region is the physical region between two base stations where a mobile unit located within the region is likely to establish 30 commlmic~Ation with both base stations. In Figure lA the shaded portion represents soft handoff region 20.
In mobile unit assisted soft handoff, the handoff region is defined by the forward link characteristics. For example, in Figure lA soft handoff region 20 represents the region where both the signal quality from base 35 station 10 and the signal quality from base station 40 are sllffi~iPnt to support communications. When mobile unit 30 enters soft handoff region 20, it will notify which ever base station it is in communication with that the second base station is available for communications. The system controller ~ 69~46 (not shown) establishes communication between with the second base station and mobile unit 30 as described in above mentioned U.S. Patent No. 5,267,261. When mobile unit 30 is in soft handoff between base station 10 and base station 40, both base stations control the trAn~mit power 5 from mobile unit 30. Mobile unit 30 decreases its transmit power if either base station requests a decrease and increases its transmit power only if each base station asks for an increase as disclosed in the above m~ntioned U.S.
Patent No. 5,056,109.
Figure lA shows the first situation which is detrimental to ~y~
10 capacity. In Figure lA forward link handoff boundary 60 and reverse link handoff boundary 50 are significantly unbalanced (i.e. spaced apart). Mobile unit 30 is located in a position where communication is established only with base station 40. In the region where mobile unit 30 is located, the forward link performance is best with base station 40 but the reverse link 15 performAnce would be better if mobile unit 30 were commlmir~ting with base station 10. In this situation mobile unit 30 is transmitting more power than it would be tr~n~mitting if it were in communication with base station 10. The increased trAn~mit power adds lmnecess~rily to the total inlelrer~"ce in the ~y~lem thereby adversely effecting capacity. It also 20 increases the overall power consumption of mobile unit 30 thereby decreasing its battery life. And it entl~n~ers the communication link if mobile unit 30 reaches its maximum transmit power and is unable to respond to commAn~l~ for increased power.
Figure lB show an alternative but also detrimental result of an 25 unbalanced handoff con(lition In Figure lB, soft handoff region 70 is positioned about reverse link handoff boundary 50. This handoff position could be the result of an altemative handoff s~Pme where handoff is based on the reverse link performance instead of the forward link performAnce.
In one such case, each base station would ~lle.-~l,t to measure the power 30 received from each mobile unit. When the measured power level exceeds a threshold or exceeds the level received at other base stations, communication with a second base station is established. In Figure lB, mobile unit 30 is located in a region where commlmil~ticn is established only with base station 10. As in Figure lA in the region where mobile 35 unit 30 is located, the forward link perform~nce is best with base station 40but the reverse link performance is best with base station 10. Unlike the reverse link, the forward link does not have a large dynamic range of transmit power and as mobile unit 30 moves toward base station 40, 2 ~
i~lel~erellce from base station 40 increases as the received power level from base station 10 decreases. If the power level from base station 10 falls below a sl7ffi~i~nt signal to inlelrele"ce level or below a certain absolute level, the communication link is in danger being of lost. The power level tran~mi~te~i from base station 10 is slowly increased within a limite~l dy-namic range as mobile unit 30 moves away from base station 10. This increase in power adversely interferes with other users in base station 10 and base station 40 thus unnec~ss~rily decreasing capacity.
Yet another altemative is a combined handoff srheme based on both the forward link performance and the reverse link performAnce Figure lC
shows one such scenario. In Figure 1C, handoff region 80 is large and encompasses both reverse link handoff boundary 50 and forward link handoff boundary 60. But u~nec~s~ry soft handoff directly decreases the capacity- of the ~yslelll. The purpose of soft handoff is to provide a make before break handoff between base stations and to provide an efflcient power control merh~ni~m However if the soft handoff region is too large, the negative effects become significant. For example, in Figure 1C, both base station 10 and base station 40 must tr~n~mit to mobile unit 30 while mobile unit 30 is in soft handoff region 80. Thus the total ~y~le~ er~ert:l~ce is increased while mobile unit 30 is in soft handoff region 80. In addition, resources at both base station 10 and base station 40 must be ~ie~lic~te-l to the signal received from mobile unit 30. Therefor~ increasing the size of th soft handoff region is not an efficient use of the syslell, capacity and resources.
The solution to these adverse effects is to b~l~n~ e (i.e. physically align) the reverse link handoff boundary to the forward link handoff boundary or vice versa. Even if this was done at each base station in a static con~lition, the balance would be lost as the system was used. For example, the signal to il,ter~erence level of the reverse link signal received at a base station is a function of the number, location, and trAn~ sion power level of the mobile un*s within its coverage area. As the loading on one base station increases, inlerference increases and the reverse link handoff boundary shrinks toward the base station. The forward link boundary is not effected in the same mAnner thus a ~y~telll that is initially b~l~nce~l may become unb~l~ncell over time.
To m~int~in balance, the present invention defines a method "breathingn the size of the base station coverage area. The breathing met h~ni~m effectively moves the forward link handoff boundary to the same location as the reverse link handoff boundary. Both of the boundaries are dependent on the performance of at least two base stations. For breathing to be effective, the reverse link handoff boundary and the forward link handoff boundary must be initially aligned. The boundaries can remain aligned if the performance of each base station is controlled as described below.
The forward link performance can be controlled by the base station.
In an exemplary CDMA ~y~lem, each base station tr~n~mit~ a pilot signal.
The mobile units per~lln handoff based on the perceived pilot signal strength as described above. By c~nging the power level of the pilot signal transmitted from the base station, the forward link handoff boundary location may be maniptll~te~i.
The reverse link performance can also be controlled by the base station. The noise performance of the base station receiver sets the minimum receive power level which can be detected. The noise performance of the receiver is typically defined in terms of an overall ~y~lelll noise figure. By controlling the noise figure of the receiver, such as by injecting noise or adding attPnll~tif n, the reverse link pefrollllance, and hence the reverse link handoff boundary, may be adjusted.
To balance the handoff boundaries, the performAnce of each base station must be controlled to be the same as the perform~nce of other base stations in the system. Therefore, we define a ~y~Lell~ wide perform~nce constant to be used by each base station in the ~y~l~l.. A dynamic constant that is equal for every base station but allowed to change over time could 25 also be define~l- In the inleresl of simplicity of ~i~fiigll and impl~mentAtion, a fixed constant is ple~elred in this embo~liment The constant is defined in terms of the sum of the receiver path noise in dlecibels (dB) and the maximum desired pilot signal power in dB as ~)l'UVell below. The best choice constant takes advantage of the perform~nce 30 available from the ~ysle~.. Therefore to define the constant, KleVel~ the following equation is used:
MAX
Klevel = alli [NRx:i+PMax:i] Eq. 1 where:
35 NRX i is the receiver path noise of base station i in dB;
PMaX:i is the m~cimllm desired pilot signal power of base station i in dB; and MAX
all i [ ] finds the largest such sum of all base stations in a system.
wo sG/03s4s 2 ~ PCT/US9S/09212 Note that once KleVel is chosen, artificial means can be used to increase the path noise of the unloaded system of each base station to meet the constant.
To prove that setting the sum of the received power and the 5 transmitted power to a KleVel indeed balances the system, several assumptions are made. The first assumption is that in any base station using multiple redundant receive and transmit antennas, the antennas have been balanced to have the same performance. Also the analyses assumes that the identical decoding perform~nce is available at each base 10 station. It assumes a constant ratio between total forward link power and pilot signal power. And it assumes reciprocity in the forward link path loss and the reverse link path loss.
To find the forward link handoff boundary between two arbitrary base stations, base station A and base station B, start by noting that the 15 forward handoff boundary occurs where the ratio of the pilot signal power of the two base stations to the total power is equal. Assume that mobile unit C is located at the boundary, mathematically in units of linear power (such as Watts):
Pilot Power of A Rx'd at C Pilot Power of B Rx'd at C
20 Total Power Received at C = Total Power Received at C Eq. 2 Noting that the power received at the mobile unit is equal to the power trAn~mitte~ times the path loss, Equation 2 becc)mes:
25 Pilot Power Tx'd from A X Pa~ loss from A to C Pilot Power Tx'd from B X Pa~ loss from B to C
Total Power Received at C Total Power Received at C Eq. 3 Re-arranging Equation 3 and Plimin~ting the common denomin~tor, yields:
Pilot Power Tx'd from A Path loss from B to C
Pilot Power Tx'd from B = Path loss from A to C Eq. 4 Following the same procedure for the reverse link and noting that the reverse link handoff boundary occurs where each base station pe.ceives the same signal to inLelfere~lce ratio for that mobile unit:
Power of C Rx'd at A Power of C Rx'd at B
35 Total Power Received at A = Total Power Received at B Eq. 5 Noting that the power received at the base station is equal to the power tr~n~mitte~ from the mobile unit times the path loss, Equation 5 becomes:
Power Tx'd from C X Path loss from C to A Power Tx'd from C X Path loss from C to B
Total Power Received at A Total Power Received at B Eq. 6 wo 96/03845 ~ 1 ~ g ~ 4 6 PCT/US95/09212 Re-arranging Equation 6 and eliminating the common numerator, yields:
Total Power Received at A Path loss from C to A
Total Power Received at B Path loss from C to B Eq. 7 Due to the assumed reciprocity in the forward and reverse link path loss at any location, Equations 4 and 7 may be combined to yield:
Total Power Received at A Pilot Power Tx'd from B
Total Power Received at B Pilot Power Tx'd from A Eq. 8 ('h~nging the units of Equation 8 from linear power to dB yields:
Total Power Received at A (dB) - Total Power Received at B (dB) =
Pilot Power Tx'd from B (dB) - Pilot Power Tx'd from A (dB) Eq. 8' Equation 8' is equivalent to premise set forth in that:
if Total Power Received at A (dB) ~ Pilot Power Tx'd from A (dB) = Kleve and Total Power Received at B (dB) + Pilot Power Tx'd from B (dB) = Kleve then equation 8 will be satisfied.
And the forward link handoff boundary and the reverse link handoff boundary are co-located.
Three mechanisms are needed to perform the breathing function: a means of initially setting performance to KleVel~ a means of monitoring the fll1ctll~tions in the reverse link, and a means of changing the performance of the forward link in response to the reverse link flllchlAtio~.
One method of initially setting the perform~nce to KleVel is to set the maximum desired pilot signal strength taking into account the variations over temperature and time and adding att~ml~ticn in line with the receiver in a no input signal condition until the KleVel perfo~ nce is achieved.
Adding attenuation desensitize the receiver and effectively increases the noise figure thereof. This also requires that each mobile unit transmit proportionately more power. The added attenuation should be kept to the minimum dictated by Klevel Once initial balance is achieved, the power coming into the base station can be measured to monitor the reverse link performance. Several methods can be used. Measurement can be done by monitoring an AGC
(automatic gain control) voltage or by directly measuring the incoming level. This method has the advantage that if an inleLrerer is present (such as am FM signal) this energy will be measured and the handoff boundaries will be drawn closer to the base station. By drawing the handoff boundary WO 96/0384S ~ i 9 6 PCT/US95/09212 closer to the base station, the interferer may be eliminated from the coverage area of the base station and its effect minimized. Measurement could be made by simply counting the number of users communicating through the base station and estimating the total power based on the fact 5 that each mobile unit's signal nominally arrives at the base station at the same signal level.
As the reverse link power increases, the forward link power should be decreased. This can be easily achieved by using an existing AGC circuit within the transmit circuitry or by providing a controllable ~ nll~tor in the 10 transmit path.
In the exemplary handoff scheme described above, handoff boundaries are based on the measurement of the pilot signal strength at the mobile unit. An alternative to controlling the total transmit power would be to control only the pilot signal level. To the coverage area d~igner, this 15 scheme might have a certain sense of appeal but controlling the total transmit power including the traffic (e.g. active calls) and pilot signals together has some advantages. First, the ratio of the pilot signal strength to the traffic channel signal strength remains fixed. The mobile unit may be expecting a fixed ratio and may allocate its resources based on the ratio. If 20 the mobile unit were to receive two equally powerful pilot ~ign~l~ each corresponding to a traffic channel having a different power level, a suboptimal decision on the allocation of mobile unit resources could result.
Adjusting the total power is also advantageous because it reduces the inlelrerellce to other base station coverage areas. If the pilot signal is not 25 strong enough to warrant a handoff in the coverage area of a neighboring base station, the high powered traffic ~hAnnel signal adds unusable and unnec~s~ry inlel~rel.ce to that area. Of course, in some applications, it may be advantageous to combine the methods by controlling the power of the pilot signal in some cases and the total transmit power in other cases. In 30 still another application, it may be advantageous to change the ratio of the pilot power to the traffic channel power.
In an ideal configuration, the bre~thing mechanism would measure the receive power and change the transmit power proportionately.
However, some ~ysl~llls may not use the proportional method and may 35 instead change the transmit level only a fraction of the perceived changed in receive power. For example, if a ~yslelll was designed in which the estimation of the received power was difficult and inaccurate, the ~y~lell-designers may wish to decrease the sensitivity to the inaccuracy. A change ~6~6~.6 in transmit level which is only a fraction of the change in receive power achieves the desensitization while preventing gross unbalance of the handoff boundaries.
Another alternative changes the transmit level only when the 5 receiver level exceeds a predetermine threshold. This method could be used to primarily deal with inlerfeLers. Of course this method may be combined with a system which changes the trAn~mit level only a fraction of the perceived change in receive power.
The breathing mechanism must have a carefully considered time 10 constant. The breathing mechanism may cause mobile unit handoffs. To perform a handoff, the mobile unit must detect the change in power and send a message to the base station. The system controller must make a decision and notify base stAtion~ A m~sAge must be sent back to the mobile unit. This process takes time and the breathing process should be slow 15 enough to allow this process to happen smoothly.
The process of breathing will naturally limit itself to ~vel-t the total convergence of the coverage area of the base station due to excess users on the ~yslelll. The CDMA ~yslem has a large and soft limitell capacity. The terrn sof~ limited capacfty refers to the fact that one more user can always be 20 addLed but at some number of users each additional user effects the communication quality of all the other users. At some greater number of users, each user's commllnicAtion quality becomes unusable and the entire link is lost to every mobile unit. To prevent the loss of the link, each base station limits the number of mobile units with which it will establish 25 communication. Once that limit has been reached, the ~ysle", will refuse attempts to establish additional calls, i.e. new call originations are blocked.
The limit is a design parameter and is typically set at about 75% of theoretical capacity. This gives some margin to the ~y~lelll and allows the ~ysle"~ to accept an emergency call even while in the limite~l condition.
30 This limit of the total number of mobile units communicating within the coverage area of a single base station naturally limits the maximum received power and therefore limits the breathing process range of operation.
Figures 2A - 2C illustrate the base station breathing me~ hAnism. In 35 Figure 2A, base station 100 has circular coverage area 130 in an unloaded condition. The coverage area of base station 100 has been balanced in an unloaded condition and the forward and reverse links coverage areas are aligned with circular coverage area 130. Base station 110 has circular W096/03845 ~ 1 ~ 9 ~ 4 S PCT/US95/09212 coverage area 140 in an unloaded condition. The coverage area of base station 110 has also been balanced in an unloaded con~ition and the forward and reverse links coverage areas are aligned with circular coverage area 140.
The operation of base stations 100 and 110 have been balanced to Klevel in 5 an unloaded condition and line 120 represents the location at which operation with each base station is same and hence both handoff boundaries.
In Figure 2B, base station 110 has be come heavily loaded and base station 100 is lightly loaded. The coverage area of the reverse link has 10 shrunk to the location of circular coverage area 145 while the forward link coverage area remains at circular coverage area 140. The light loading of base station 100 has not effected the coverage area of base station 100 which is still at circular coverage area 130. Note that the reverse link handoff boundary between base station 100 and base station 110 has moved to 15 line 125 while the forward link handoff boundary remains at line 120. Thus the undesirable unbalanced handoff boundary condition has been created.
In Figure 2C, base station 110 has implemented the base station breathing mech~ni~m The effect has been to move the forward link handoff boundary to circular coverage area 145. Line 125 now represents 20 both the forward and reverse link handoff boundaries.
In Figures 2B and 2C, the X's represent ~yslell- users. In particular user X 150 is located at the handoff boundary in Figure 2B. Due to his location, user X is in soft handoff between base station 100 and base station 110. Note that in Figure 2C, user X 150 is now deep into the coverage 25 area of base station 100 and not in the soft handoff region between base station 100 and base station 110. Therefore, the heavily loaded base station 110 has effectively transferred some of its load to the lightly load base station 100.
Figure 3 is a block diagram showing an exemplary base station 30 breathing configuration. Antenna 270 receives sign~l~ at base station 300.
The receive signals are then passed to variable attenuator 200 which has been used to initially set Klevel operation. The receive signals are passed to power ~ietertor 210. Power detector 210 generates a level in~lic~ing the total power in the received signal. Low pass filer 220 averages the power 35 indication and slows the breathing response time. Scale and threshold 230 sets the desired ratio and offset of the relation between increases in the reverse link power and decreases in the forward link power. Scale and threshold 230 outputs a control signal for variable gain device 240. Variable wo 96/03845 ~ :L 6 ~ ~ ~ 6 PCT/US9S/09212 gain device 240 accepts the transmit signal and provides a gain controlled output signal to high power amplifier (HPA) 250. HPA 250 amplifies the transit signal and passes to antenna 260 for transmission over the wireless link.
Many variations to the configuration of Figure 3 exist. For example, antennas 260 and 270 may each comprise two antennas. Or conversely antennas 260 and 270 may be the same antenna. The power detection in Figure 3 is based on all incoming signal power within the band of interest.
As discll~se-l above, power detection can be based solely on the number of mobile units which have established communication with the base station.
Also low pass filer 220 may be a linear filter or nonlinear filter (such as a slew rate limitin~ filter).
There are many obvious variations to the present invention as presented inclllrlin~ simple architectural changes. The previous description of the ~refelred embo-lim~nt is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embo~lim~nt~ will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other emboriimPnt~
without the use of the inventive faculty. Thus, the present illvenlion is not intended to be limited to the embo(limPnt~ shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
WE CLAIM:
Claims
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a system having a plurality of base stations capable of bi-directional communication with a mobile unit, wherein information is communicated to said mobile unit from said plurality of base stations on a forward link and information is communicated to said plurality of base stations from said mobile unit on a reverse link, and wherein each base station defines a forward link coverage area and a reverse link coverage area, a method of controlling said base station coverage areas comprising the steps of:
measuring a reverse link power level received at a first base station and at a second base station;
adjusting a forward link handoff performance location defined by said forward link coverage area by changing a forward link power level at said first base station and at said second base station, said adjustment based on a reverse link power level measurement of said first and second base stations, said adjustment balancing said forward link performance in relation to said reverse link performance between said first and said second base stations; and handing off a communication from said first base station to said second base station.
2. The method recited in claim 1, wherein a product of said reverse link and said forward link power levels at said first base station are equal to a product of said reverse link and said forward link power levels at said second base 3. The method recited in claim 2, wherein:
the product of said reverse link and said forward link power levels at said first base station is equal to a constant when said reverse link power level at said first base station is greater than a threshold; and the product of said reverse link and said forward link power levels at said second base station is equal to said constant when said reverse link power level at said second base station is greater than said threshold.
4. In a system having a plurality of base stations, each of said plurality of base stations having a corresponding forward link coverage area and a corresponding reverse link coverage area wherein each of said plurality of base stations is for communicating to a mobile unit located within said corresponding forward link coverage area and each of said plurality of base stations is for receiving communication from a mobile unit located within said corresponding reverse link coverage area, a method of aligning a location of a first forward link coverage area to a location of a first reverse link coverage area corresponding to a first base station comprising the steps of:
measuring a level of loading of said reverse link coverage area indicative of said location of said reverse link coverage area; and changing said location of said first forward link coverage area based on said measured level of loading.
5. The method of aligning of claim 4 wherein said measured level of loading of said reverse link coverage area comprises energy received from a set of mobile units located within said first reverse link coverage area.
6. The method of aligning of claim 5 wherein said level of loading of said reverse link coverage area further comprises energy received from a non system user and a set of mobile units located within a reverse link coverage area corresponding to a second base station.
7. The method of aligning of claim 4 wherein said step of changing said location of said first forward link coverage area is limited to a minimum coverage area boundary.
8. The method of aligning of claim 4 wherein said step of measuring comprises the step of counting the number of mobile units in communication with said first base station.
9. A method for balancing base station boundaries in a system comprising a plurality of base stations, comprising the steps of:
transmitting a forward link signal at a selected power level from a first base station defining a first forward link coverage area;
receiving a reverse link signal at a first power level at said first base station defining a first reverse link coverage area;
transmitting a forward link signal at a selected power level from a second base station defining a second forward link coverage area, wherein said first forward link coverage area and said second forward link coverage area intersect to define a forward link equality location at which a mobile unit receives communication with the same performance level with said first base station and said second base station; and receiving a reverse link signal at a power level at said second base station defining a second reverse link coverage area, wherein said first reverse link coverage area and said second reverse link coverage area intersect to define a reverse link equality location wherein said first base station and said second base station receive communication from a mobile unit at said reverse link equality location with the same performance level;
wherein said selected power level of said forward link signal from said first base station and said selected power level of said forward link signal from said second base station are selected such that said forward link equality location and said reverse link equality location are the same.
10. The method for balancing base station boundaries of claim 9 further comprising the steps of:
receiving at said first base station said reverse link signal at a second power level higher than said first power level received at said first base station thereby defining a second smaller reverse link coverage area of said first base station and defining a new reverse link equality location; and transmitting from said first base station said forward link signal at a lower power level defining a second forward link coverage area and a new forward link equality location such that said new forward link equality location is the same as said new reverse link equality location.
11. The method for balancing base station boundaries of claim 9 wherein each of said plurality of base stations in said system transmits a pilot signal and wherein said forward link signal from said first base station is said pilot signal corresponding to said first base station.
12. The method for balancing base station boundaries of claim 9 wherein each of said plurality of base stations in said system transmits a pilot signal and message signals and wherein said forward link signal from said first base station is said pilot signal and said message signals corresponding to said first base station.
13. The method for balancing base station boundaries of claim 9 wherein the product of said selected power level of said forward link signal from said first base station and said first power level of said reverse link signal from said first base station is equal to a constant.
14. The method for balancing base station boundaries of claim 13 wherein the product of said selected power level of said forward link signal from said second base station and said power level of said reverse link signal from said second base station is equal to said constant.
15. The method for balancing base station boundaries of claim 13 wherein said constant is dynamic and varies over time.
16. The method for balancing base station boundaries of claim 9 wherein said first power level of said reverse link signal at said first base station comprises an amount of artificial power such that the product of said selected power level of said forward link signal from said first base station and said power level of said reverse link signal from said first base station is equal to a constant.
17. The method for balancing base station boundaries of claim 16 further wherein said power level of said reverse link signal at said second base station comprises an amount of artificial power to such that the product of said power level of said forward link signal from said second base station and said power level of said reverse link signal from said second base station is equal to said constant.
18. An apparatus for controlling a location of a forward link coverage area and a reverse link coverage area of a base station in a system of base stations for bi-directional communication with a set of mobile units comprising:
an antenna system for receiving an incoming signal at a receive power level and for providing a transmit signal at a transmit power level;
a power detector having an input coupled to said antenna system and having an output for providing a power level output indication proportional to said receive power level ;and a variable attenuator coupled said output of said power detector for receiving a power control signal and receiving an information signal and providing a power controlled information signal wherein the output of said variable attenuator is coupled to said antenna system thereby setting said transmit power level;
wherein the product of said receive power level of said incoming signal and said transmit power level of said transmit signal is controlled to maintain balance in said location of said forward link coverage area and said reverse link coverage area.
19. The apparatus for controlling the coverage area of a base station of claim 18 further comprising an attenuator disposed between said receive antenna and said power detector for setting said product to a constant when said power level of said incoming signal is minimum.
20. The apparatus for controlling the coverage area of a base station of claim 18 further comprising means of scaling and gating said power level output indication disposed between said power detector and said variable attenuator.
--21. In a system having a plurality of base stations capable of bi-directional communication with a mobile unit wherein information is communicated to said mobile unit from said plurality of base stations on a forward link and information is communicated to said plurality of base stations from said mobile unit on a reverse link and wherein each base station defines a forward link coverage area and a reverse link coverage area, a method of controlling said base station coverage areas comprising the steps of:
measuring a reverse link power level received at a first base station;
and adjusting a forward link power level at said first base station based on said reverse link power level measurement at said first base station to preserve a balanced between said forward link coverage area and said reverse link coverage area.
--22. The method of claim 21 wherein the forward link power level is adjusted at said first base station such that the product of said reverse link power level at said first base station and said forward link power level at said first base station remains equal to a constant. ---- 23. The method of claim 21 further including the steps of:
measuring a reverse link power level received at said second base station; and adjusting a forward link power level at said second base station based on said reverse link power level measurement at said second base station such that the product of said reverse link power level at said second base station and said forward link power level at said second base station remains equal to a constant. ---- 24. In a system having a plurality of base stations, each of said plurality of base stations having a corresponding forward link coverage area and a corresponding reverse link coverage area wherein each of said plurality of base stations is capable of communicating to a mobile unit located within said corresponding forward link coverage area and each of said plurality of base stations is capable of receiving communication from a mobile unit located within said corresponding reverse link coverage area, a method of aligning a location of a first forward link coverage area to a location of a first reverse link coverage area corresponding to a first base station comprising the steps of:
changing, at said first base station, a level of artificial loading associated with said first reverse link coverage area to alter a location of said first reverse link coverage area; and changing a level of transmit power from said first base station based upon change in said level of artificial loading in order to correspondingly alter said location of said first forward link coverage area. ---- 25. The method of aligning of claim 24 wherein said level of artificial loading of said reverse link coverage area is changed in response to a measured level of loading, said measured level of loading being determined from energy received at said first base station from a set of mobile units located within said first reverse link coverage area. ---- 26. The method of aligning of claim 25 wherein said measured level of loading is further determined on the basis of energy received from a non system user and a set of mobile units located within a reverse link coverage area corresponding to a second base station. ---- 27. In a system including a plurality of base stations, a method for defining a coverage boundary associated with a first base station comprising the steps of:
transmitting a forward link signal at a selected power level from a first base station, said selected power level defining a first forward link coverage area and being chosen such that said first forward link coverage area intersects a second forward link coverage area of a second base station and thereby defines a forward link equality location at which a mobile unit receives communication with the same performance level with said first base station and said second base station;
receiving a reverse link signal at a first power level at said first base station, said first power level corresponding to a first reverse link coverage area which intersects a second reverse link coverage area of a second base station and thereby defines a reverse link equality location at which said first base station and said second base station receive communication from a mobile unit with the same performance level, said selected power level being further chosen such that said forward link equality location and said reverse link equality location are the same. ---- 28. The method of claim 27 further comprising the steps of:
receiving at said first base station said reverse link signal at a second power level higher than said first power level received at said first base station, thereby defining a second smaller reverse link coverage area of said first base station and defining a new reverse link equality location; and transmitting from said first base station said forward link signal at a lower power level defining a second forward link coverage area and a new forward link equality location such that said new forward link equality location is the same as said new reverse link equality location.---- 29. The method of claim 27 wherein each of said plurality base stations in said system transmits a pilot signal, said forward link signal from said first base station consisting of said pilot signal corresponding to said first base station. ---- 30. The method of claim 27 wherein each of said plurality of base stations in said system transmits a pilot signal and message signals, said forward link signal from said first base station consisting of said pilot signal and said message signals corresponding to said first base station. ---- 31. The method of claim 27 wherein the product of said selected power level of said forward link signal from said first base station and said first power level of said reverse link signal from said first base station is equal to a constant. --32. The method of claim 31 wherein said constant is dynamic and varies over time.
-- 33. The method of claim 27 wherein said first power level of said reverse link signal at said first base station includes a level of artificial power chosen such that the product of said selected power level of said forward link signal from said first base station and said power level of said reverse link signal from said first base station is equal to a constant. ---- 34. An apparatus for controlling a location of a forward link coverage area and a reverse link coverage area of a base station in a system of base stations capable of bi-directional communication with a set of mobile units comprising:
a receiver unit for receiving an incoming signal at a receive power level and for generating a power level output indication proportional to said receive power level;
a transmitter for transmitting a power controlled information signal at a transmit power level, said transmit power level being adjusted so that the product of said receive power level of said incoming signal and said transmit power level of said transmit signal is controlled in a predefined manner. ---- 35. The apparatus of claim 34 further including means for setting said product to a constant when said power level of said incoming signal is minimum. --
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a system having a plurality of base stations capable of bi-directional communication with a mobile unit, wherein information is communicated to said mobile unit from said plurality of base stations on a forward link and information is communicated to said plurality of base stations from said mobile unit on a reverse link, and wherein each base station defines a forward link coverage area and a reverse link coverage area, a method of controlling said base station coverage areas comprising the steps of:
measuring a reverse link power level received at a first base station and at a second base station;
adjusting a forward link handoff performance location defined by said forward link coverage area by changing a forward link power level at said first base station and at said second base station, said adjustment based on a reverse link power level measurement of said first and second base stations, said adjustment balancing said forward link performance in relation to said reverse link performance between said first and said second base stations; and handing off a communication from said first base station to said second base station.
2. The method recited in claim 1, wherein a product of said reverse link and said forward link power levels at said first base station are equal to a product of said reverse link and said forward link power levels at said second base 3. The method recited in claim 2, wherein:
the product of said reverse link and said forward link power levels at said first base station is equal to a constant when said reverse link power level at said first base station is greater than a threshold; and the product of said reverse link and said forward link power levels at said second base station is equal to said constant when said reverse link power level at said second base station is greater than said threshold.
4. In a system having a plurality of base stations, each of said plurality of base stations having a corresponding forward link coverage area and a corresponding reverse link coverage area wherein each of said plurality of base stations is for communicating to a mobile unit located within said corresponding forward link coverage area and each of said plurality of base stations is for receiving communication from a mobile unit located within said corresponding reverse link coverage area, a method of aligning a location of a first forward link coverage area to a location of a first reverse link coverage area corresponding to a first base station comprising the steps of:
measuring a level of loading of said reverse link coverage area indicative of said location of said reverse link coverage area; and changing said location of said first forward link coverage area based on said measured level of loading.
5. The method of aligning of claim 4 wherein said measured level of loading of said reverse link coverage area comprises energy received from a set of mobile units located within said first reverse link coverage area.
6. The method of aligning of claim 5 wherein said level of loading of said reverse link coverage area further comprises energy received from a non system user and a set of mobile units located within a reverse link coverage area corresponding to a second base station.
7. The method of aligning of claim 4 wherein said step of changing said location of said first forward link coverage area is limited to a minimum coverage area boundary.
8. The method of aligning of claim 4 wherein said step of measuring comprises the step of counting the number of mobile units in communication with said first base station.
9. A method for balancing base station boundaries in a system comprising a plurality of base stations, comprising the steps of:
transmitting a forward link signal at a selected power level from a first base station defining a first forward link coverage area;
receiving a reverse link signal at a first power level at said first base station defining a first reverse link coverage area;
transmitting a forward link signal at a selected power level from a second base station defining a second forward link coverage area, wherein said first forward link coverage area and said second forward link coverage area intersect to define a forward link equality location at which a mobile unit receives communication with the same performance level with said first base station and said second base station; and receiving a reverse link signal at a power level at said second base station defining a second reverse link coverage area, wherein said first reverse link coverage area and said second reverse link coverage area intersect to define a reverse link equality location wherein said first base station and said second base station receive communication from a mobile unit at said reverse link equality location with the same performance level;
wherein said selected power level of said forward link signal from said first base station and said selected power level of said forward link signal from said second base station are selected such that said forward link equality location and said reverse link equality location are the same.
10. The method for balancing base station boundaries of claim 9 further comprising the steps of:
receiving at said first base station said reverse link signal at a second power level higher than said first power level received at said first base station thereby defining a second smaller reverse link coverage area of said first base station and defining a new reverse link equality location; and transmitting from said first base station said forward link signal at a lower power level defining a second forward link coverage area and a new forward link equality location such that said new forward link equality location is the same as said new reverse link equality location.
11. The method for balancing base station boundaries of claim 9 wherein each of said plurality of base stations in said system transmits a pilot signal and wherein said forward link signal from said first base station is said pilot signal corresponding to said first base station.
12. The method for balancing base station boundaries of claim 9 wherein each of said plurality of base stations in said system transmits a pilot signal and message signals and wherein said forward link signal from said first base station is said pilot signal and said message signals corresponding to said first base station.
13. The method for balancing base station boundaries of claim 9 wherein the product of said selected power level of said forward link signal from said first base station and said first power level of said reverse link signal from said first base station is equal to a constant.
14. The method for balancing base station boundaries of claim 13 wherein the product of said selected power level of said forward link signal from said second base station and said power level of said reverse link signal from said second base station is equal to said constant.
15. The method for balancing base station boundaries of claim 13 wherein said constant is dynamic and varies over time.
16. The method for balancing base station boundaries of claim 9 wherein said first power level of said reverse link signal at said first base station comprises an amount of artificial power such that the product of said selected power level of said forward link signal from said first base station and said power level of said reverse link signal from said first base station is equal to a constant.
17. The method for balancing base station boundaries of claim 16 further wherein said power level of said reverse link signal at said second base station comprises an amount of artificial power to such that the product of said power level of said forward link signal from said second base station and said power level of said reverse link signal from said second base station is equal to said constant.
18. An apparatus for controlling a location of a forward link coverage area and a reverse link coverage area of a base station in a system of base stations for bi-directional communication with a set of mobile units comprising:
an antenna system for receiving an incoming signal at a receive power level and for providing a transmit signal at a transmit power level;
a power detector having an input coupled to said antenna system and having an output for providing a power level output indication proportional to said receive power level ;and a variable attenuator coupled said output of said power detector for receiving a power control signal and receiving an information signal and providing a power controlled information signal wherein the output of said variable attenuator is coupled to said antenna system thereby setting said transmit power level;
wherein the product of said receive power level of said incoming signal and said transmit power level of said transmit signal is controlled to maintain balance in said location of said forward link coverage area and said reverse link coverage area.
19. The apparatus for controlling the coverage area of a base station of claim 18 further comprising an attenuator disposed between said receive antenna and said power detector for setting said product to a constant when said power level of said incoming signal is minimum.
20. The apparatus for controlling the coverage area of a base station of claim 18 further comprising means of scaling and gating said power level output indication disposed between said power detector and said variable attenuator.
--21. In a system having a plurality of base stations capable of bi-directional communication with a mobile unit wherein information is communicated to said mobile unit from said plurality of base stations on a forward link and information is communicated to said plurality of base stations from said mobile unit on a reverse link and wherein each base station defines a forward link coverage area and a reverse link coverage area, a method of controlling said base station coverage areas comprising the steps of:
measuring a reverse link power level received at a first base station;
and adjusting a forward link power level at said first base station based on said reverse link power level measurement at said first base station to preserve a balanced between said forward link coverage area and said reverse link coverage area.
--22. The method of claim 21 wherein the forward link power level is adjusted at said first base station such that the product of said reverse link power level at said first base station and said forward link power level at said first base station remains equal to a constant. ---- 23. The method of claim 21 further including the steps of:
measuring a reverse link power level received at said second base station; and adjusting a forward link power level at said second base station based on said reverse link power level measurement at said second base station such that the product of said reverse link power level at said second base station and said forward link power level at said second base station remains equal to a constant. ---- 24. In a system having a plurality of base stations, each of said plurality of base stations having a corresponding forward link coverage area and a corresponding reverse link coverage area wherein each of said plurality of base stations is capable of communicating to a mobile unit located within said corresponding forward link coverage area and each of said plurality of base stations is capable of receiving communication from a mobile unit located within said corresponding reverse link coverage area, a method of aligning a location of a first forward link coverage area to a location of a first reverse link coverage area corresponding to a first base station comprising the steps of:
changing, at said first base station, a level of artificial loading associated with said first reverse link coverage area to alter a location of said first reverse link coverage area; and changing a level of transmit power from said first base station based upon change in said level of artificial loading in order to correspondingly alter said location of said first forward link coverage area. ---- 25. The method of aligning of claim 24 wherein said level of artificial loading of said reverse link coverage area is changed in response to a measured level of loading, said measured level of loading being determined from energy received at said first base station from a set of mobile units located within said first reverse link coverage area. ---- 26. The method of aligning of claim 25 wherein said measured level of loading is further determined on the basis of energy received from a non system user and a set of mobile units located within a reverse link coverage area corresponding to a second base station. ---- 27. In a system including a plurality of base stations, a method for defining a coverage boundary associated with a first base station comprising the steps of:
transmitting a forward link signal at a selected power level from a first base station, said selected power level defining a first forward link coverage area and being chosen such that said first forward link coverage area intersects a second forward link coverage area of a second base station and thereby defines a forward link equality location at which a mobile unit receives communication with the same performance level with said first base station and said second base station;
receiving a reverse link signal at a first power level at said first base station, said first power level corresponding to a first reverse link coverage area which intersects a second reverse link coverage area of a second base station and thereby defines a reverse link equality location at which said first base station and said second base station receive communication from a mobile unit with the same performance level, said selected power level being further chosen such that said forward link equality location and said reverse link equality location are the same. ---- 28. The method of claim 27 further comprising the steps of:
receiving at said first base station said reverse link signal at a second power level higher than said first power level received at said first base station, thereby defining a second smaller reverse link coverage area of said first base station and defining a new reverse link equality location; and transmitting from said first base station said forward link signal at a lower power level defining a second forward link coverage area and a new forward link equality location such that said new forward link equality location is the same as said new reverse link equality location.---- 29. The method of claim 27 wherein each of said plurality base stations in said system transmits a pilot signal, said forward link signal from said first base station consisting of said pilot signal corresponding to said first base station. ---- 30. The method of claim 27 wherein each of said plurality of base stations in said system transmits a pilot signal and message signals, said forward link signal from said first base station consisting of said pilot signal and said message signals corresponding to said first base station. ---- 31. The method of claim 27 wherein the product of said selected power level of said forward link signal from said first base station and said first power level of said reverse link signal from said first base station is equal to a constant. --32. The method of claim 31 wherein said constant is dynamic and varies over time.
-- 33. The method of claim 27 wherein said first power level of said reverse link signal at said first base station includes a level of artificial power chosen such that the product of said selected power level of said forward link signal from said first base station and said power level of said reverse link signal from said first base station is equal to a constant. ---- 34. An apparatus for controlling a location of a forward link coverage area and a reverse link coverage area of a base station in a system of base stations capable of bi-directional communication with a set of mobile units comprising:
a receiver unit for receiving an incoming signal at a receive power level and for generating a power level output indication proportional to said receive power level;
a transmitter for transmitting a power controlled information signal at a transmit power level, said transmit power level being adjusted so that the product of said receive power level of said incoming signal and said transmit power level of said transmit signal is controlled in a predefined manner. ---- 35. The apparatus of claim 34 further including means for setting said product to a constant when said power level of said incoming signal is minimum. --
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| US278,347 | 1994-07-21 | ||
| PCT/US1995/009212 WO1996003845A1 (en) | 1994-07-21 | 1995-07-21 | Method and apparatus for balancing the forward link handoff boundary to the reverse link handoff boundary in a cellular communication system |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4435840A (en) * | 1981-06-22 | 1984-03-06 | Nippon Electric Co., Ltd. | Radio mobile communication system wherein probability of loss of calls is reduced without a surplus of base station equipment |
| US4968489A (en) * | 1988-09-13 | 1990-11-06 | Peroxidation Systems, Inc. | UV lamp enclosure sleeve |
| US5276907A (en) * | 1991-01-07 | 1994-01-04 | Motorola Inc. | Method and apparatus for dynamic distribution of a communication channel load in a cellular radio communication system |
| US5241685A (en) * | 1991-03-15 | 1993-08-31 | Telefonaktiebolaget L M Ericsson | Load sharing control for a mobile cellular radio system |
| JP2674404B2 (en) * | 1991-12-13 | 1997-11-12 | 日本電気株式会社 | Base station coverage area control method |
| US5504938A (en) * | 1994-05-02 | 1996-04-02 | Motorola, Inc. | Method and apparatus for varying apparent cell size in a cellular communication system |
| US5548812A (en) * | 1994-07-21 | 1996-08-20 | Qualcomm Incorporated | Method and apparatus for balancing the forward link handoff boundary to the reverse link handoff boundary in a cellular communication system |
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1994
- 1994-07-21 US US08/278,347 patent/US5548812A/en not_active Expired - Lifetime
-
1995
- 1995-07-12 ZA ZA955809A patent/ZA955809B/en unknown
- 1995-07-19 IL IL11466795A patent/IL114667A/en not_active IP Right Cessation
- 1995-07-21 RU RU96107751/09A patent/RU2158481C2/en active
- 1995-07-21 JP JP08505872A patent/JP3086257B2/en not_active Expired - Lifetime
- 1995-07-21 MX MX9601063A patent/MX9601063A/en unknown
- 1995-07-21 DE DE69531853T patent/DE69531853T2/en not_active Expired - Lifetime
- 1995-07-21 CA CA002169646A patent/CA2169646C/en not_active Expired - Lifetime
- 1995-07-21 AT AT95927333T patent/ATE251374T1/en not_active IP Right Cessation
- 1995-07-21 KR KR1019960701455A patent/KR100432566B1/en not_active IP Right Cessation
- 1995-07-21 BR BR9506274A patent/BR9506274A/en not_active IP Right Cessation
- 1995-07-21 KR KR10-2003-7006653A patent/KR100432565B1/en not_active IP Right Cessation
- 1995-07-21 EP EP95927333A patent/EP0720808B1/en not_active Expired - Lifetime
- 1995-07-21 CN CNB951906437A patent/CN1152592C/en not_active Expired - Lifetime
- 1995-07-21 WO PCT/US1995/009212 patent/WO1996003845A1/en active IP Right Grant
- 1995-07-21 ES ES95927333T patent/ES2208684T3/en not_active Expired - Lifetime
- 1995-07-25 TW TW084107667A patent/TW285794B/zh not_active IP Right Cessation
-
1996
- 1996-01-17 US US08/587,697 patent/US5722044A/en not_active Expired - Lifetime
- 1996-03-21 FI FI961318A patent/FI113728B/en not_active IP Right Cessation
-
1998
- 1998-12-28 HK HK98116176A patent/HK1015218A1/en not_active IP Right Cessation
-
2003
- 2003-09-26 FI FI20031395A patent/FI114533B/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| US5722044A (en) | 1998-02-24 |
| BR9506274A (en) | 1997-08-12 |
| CN1130975A (en) | 1996-09-11 |
| AU3139395A (en) | 1996-02-22 |
| JPH09506230A (en) | 1997-06-17 |
| ATE251374T1 (en) | 2003-10-15 |
| HK1015218A1 (en) | 1999-10-08 |
| IL114667A (en) | 1998-09-24 |
| FI961318A (en) | 1996-05-20 |
| MX9601063A (en) | 1997-06-28 |
| TW285794B (en) | 1996-09-11 |
| KR20040004438A (en) | 2004-01-13 |
| KR100432565B1 (en) | 2004-05-28 |
| FI113728B (en) | 2004-05-31 |
| AU701240B2 (en) | 1999-01-21 |
| JP3086257B2 (en) | 2000-09-11 |
| WO1996003845A1 (en) | 1996-02-08 |
| KR100432566B1 (en) | 2004-08-04 |
| US5548812A (en) | 1996-08-20 |
| FI114533B (en) | 2004-10-29 |
| DE69531853T2 (en) | 2004-08-19 |
| CA2169646A1 (en) | 1996-02-08 |
| FI20031395A (en) | 2003-09-26 |
| FI961318A0 (en) | 1996-03-21 |
| IL114667A0 (en) | 1995-11-27 |
| DE69531853D1 (en) | 2003-11-06 |
| EP0720808B1 (en) | 2003-10-01 |
| EP0720808A1 (en) | 1996-07-10 |
| ES2208684T3 (en) | 2004-06-16 |
| ZA955809B (en) | 1996-04-18 |
| RU2158481C2 (en) | 2000-10-27 |
| CN1152592C (en) | 2004-06-02 |
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