WO2000011900A1 - Wireless communication system and method of assigning communication resources - Google Patents

Wireless communication system and method of assigning communication resources Download PDF

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Publication number
WO2000011900A1
WO2000011900A1 PCT/US1999/012452 US9912452W WO0011900A1 WO 2000011900 A1 WO2000011900 A1 WO 2000011900A1 US 9912452 W US9912452 W US 9912452W WO 0011900 A1 WO0011900 A1 WO 0011900A1
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WO
WIPO (PCT)
Prior art keywords
traffic
cell
coverage
mobile
cells
Prior art date
Application number
PCT/US1999/012452
Other languages
French (fr)
Inventor
Dean M. Picha
Dennis W. Gilliland
David J. Bielat
Original Assignee
Motorola Inc.
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Motorola Inc. filed Critical Motorola Inc.
Publication of WO2000011900A1 publication Critical patent/WO2000011900A1/en

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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W72/00Local resource management
    • H04W72/50Allocation or scheduling criteria for wireless resources
    • H04W72/54Allocation or scheduling criteria for wireless resources based on quality criteria
    • H04W72/542Allocation or scheduling criteria for wireless resources based on quality criteria using measured or perceived quality
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W16/00Network planning, e.g. coverage or traffic planning tools; Network deployment, e.g. resource partitioning or cells structures
    • H04W16/24Cell structures
    • H04W16/32Hierarchical cell structures
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W24/00Supervisory, monitoring or testing arrangements
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04WWIRELESS COMMUNICATION NETWORKS
    • H04W28/00Network traffic management; Network resource management
    • H04W28/16Central resource management; Negotiation of resources or communication parameters, e.g. negotiating bandwidth or QoS [Quality of Service]
    • H04W28/26Resource reservation

Definitions

  • the present invention relates generally to wireless communication systems, and more particularly, to a wireless communication system having a layered configuration and a method of allocating communication resources in a layered wireless communication system.
  • Wireless communication systems for example cellular radiotelephone systems, are well known. These systems provide communication services utilizing radio frequency transmission techniques between land based radio transceiver stations, or base stations, and mobile radio transceivers, or mobiles.
  • Each of the base stations and the mobiles thus include a radio transceiver for communicating voice, data, or other information utilizing a communication resource, i.e., a radio link, and a standard communication protocol.
  • a communication resource i.e., a radio link
  • a standard communication protocol i.e., several such protocols for cellular radiotelephony are the Narrowband Advanced Mobile Phone System (NAMPS), the Advanced Mobile Phone System (AMPS), the Global System for Mobile Communications (GSM), the Personal Digital Communications (PDC), the United States Digital Cellular (USDC), or the Code Division Multiple Access (CDMA) protocols.
  • NAMPS Narrowband Advanced Mobile Phone System
  • AMPS Advanced Mobile Phone System
  • GSM Global System for Mobile Communications
  • PDC Personal Digital Communications
  • USDC United States Digital Cellular
  • CDMA Code Division Multiple Access
  • a common feature of these wireless communication systems is that a large geographic area is divided into smaller areas ("cells"), which are serviced by a base station.
  • the use of cells to subdivide the large geographic area permits enhanced geographic coverage with efficient use of radio frequency resources.
  • radio frequency resources assigned to one cell may be reused in cells located a sufficient distance from this cell without the resources interfering.
  • careful planning is undertaken to assign communication resources to cells to ensure sufficient capacity, i.e., to assign a sufficient number of resources to handle an anticipated number of calls at a given time within the cell, and to ensure that communication resources assigned to one cell do not cause unacceptable levels of interference in adjacent or neighboring cells.
  • An advantage of the cellular system in providing greater capacity is that cell sizes may be reduced, by limiting the transmission power of the base stations and mobiles, permitting increased reuse of communication resources. Greater reuse of communication resources in a given geographic area equates to increased capacity.
  • Picocellular systems find particularly advantageous use as in-building cellular radiotelephone systems.
  • the picocellular systems require a high degree of optimization to successfully interact with the surrounding "macrocellular" environment, as well as to provide the highest quality of service to the in-building users.
  • Perhaps the largest optimization effort is due to the installation of the many required base stations, one each for each picocell.
  • Each picocell requires assignment of a broadcast carrier including a broadcast control channel for supplying "coverage" in the picocell, and the assignment of traffic carriers for handling the call traffic within the cells.
  • the broadcast control channel is utilized by the base station and the mobiles to request and establish service, i.e., to make and complete calls. Once service is established, the mobile is assigned a traffic channel resource on which the call is conducted.
  • the broadcast control channel is referred to as a dedicated channel. That is, its signal characteristics, e.g., carrier, transmission power, timing, etc., are necessarily fixed. However, fixing certain characteristics of the broadcast control channel, i.e., its carrier and power, limit the potential for reusing this carrier within the cellular communication system. For example, it is known to use frequency hopping techniques in order to provide statistically enhanced carrier reuse. However, with the broadcast carriers fixed for each cell, these carriers are typically unavailable for reuse in the frequency hopping plan. In addition, the transmit power of the broadcast control channel requires substantial geographic spacing of cells reusing the associated broadcast carrier thus exacerbating the optimization problem particularly in the picocellular environment. The amount a cell size may be reduced is further limited by the time it requires the mobile to acquire the broadcast control carrier. If the cell sizes are too small, the mobile may not be able to adequately acquire a cell's broadcast control carrier before moving into another cell.
  • a system in which certain of the cells in a layered picocell system and/or a layered microcell / macrocell system are designated as "traffic zones" or traffic cells.
  • the traffic cells are not allocated a broadcast control channel.
  • Coverage cells are provided and include a broadcast control channel.
  • the coverage cells are utilized primarily to provide coverage and not to provide capacity. Service requests are received within one of the coverage cells, and a communication resource is preferentially allocated from a traffic cell, if possible, otherwise a communication resource is allocated from the coverage cell.
  • a communication resource may not be allocated from a traffic cell is that the mobile's location within the traffic cell may not be sufficiently determined to allow a resource allocation.
  • the system of the above- referenced patent is robust in that should a resource allocation fail, an alternative resource allocation strategy is provided. Still, more efficient utilization of communication resources may be achieved if the traffic cell communication resources are more fully utilized.
  • One prior proposal for identifying cells from which communication resources may be allocated provides for the cell base station receiver retuning to the specified frequency on the specified time slot that the mobile is assigned.
  • the cell base station then reports the uplink signal strength to a control entity. If any of the cells report a signal level which qualifies to provide service, the control entity will assign a traffic channel resource from this cell to service the mobile. For example, in one approach measurements may be made while the mobile is communicating on a dedicated control channel. This case requires at least one carrier at the cell to be configured with the same dedicated control channel configuration to listen to the mobile on this control channel, either with a dedicated configuration or retuned to the configuration.
  • a second approach is to monitor the random access channel during a channel request.
  • FIG. 1 is a diagram illustrating a layered wireless communication system in accordance with a preferred embodiment of the present invention.
  • FIG. 2 is a chart illustrating traffic cell resources and coverage cell resources and illustrating an allocation of traffic cell resources in accordance with a preferred embodiment of the present invention.
  • FIG. 3 is a chart similar to FIG. 2, and further illustrating traffic cell resources and coverage cell resources with a number of active calls.
  • FIG. 4 is a flow chart illustrating a method of allocating resources in accordance with a preferred embodiment of the present invention.
  • FIG. 5 is a flow chart further illustrating the method of allocating resources in accordance with a preferred embodiment of the present invention.
  • wireless communication services are provided in a geographic area 10, utilizing a plurality of cells.
  • the geographic area is preferably an interior and immediate surrounding portions of a building, an area of a city or town, an airport, or any geographic area over which wireless communication services are desired.
  • Wireless communication system 100 includes a coverage cell 102.
  • the primary purpose of coverage cell 102 is to provide coverage and not to provide the required system capacity. It will be further appreciated that additional coverage cells such as shown and described in the above-referenced commonly assigned United States patent application may be provided without departing from the fair scope of the present invention.
  • Wireless communication system 100 further includes "traffic zones" defined by a plurality of traffic cells, individually cell 104, cell 106, cell 108, cell 110 cell 112, cell 114 and cell 116, which are distributed through area 10. Traffic cells 104 - 116 are arranged to provide capacity.
  • cells 104 - 116 also allow reuse of communication resources, i.e., reuse of the radio frequencies/carriers available for providing wireless communication services.
  • communication resource and communication channel are used to refer to a physical link between the base station and the mobile.
  • TDM time division multiplexed
  • CDM code division multiplexed
  • a communication channel is a spreading code on a particular carrier.
  • the broadcast control channel is a fixed time slot on a fixed carrier within a cell.
  • a traffic channel is one of a plurality of time slots available on one or more traffic channel carriers assigned to a cell and the otherwise available time slots on, for example, the broadcast carrier.
  • cell 102 includes a broadcast carrier for providing a broadcast communication channel.
  • Each of cells 104 - 116 preferably include only a single traffic channel carrier and do not include a broadcast channel carrier. While presently not preferred due to a size and cost restrictions, it will be appreciated that additional traffic channel carriers may be provided in cells 104 - 116. Additionally, cells 104 - 116 are preferably not provided retuning capability. As is well known, cells 102 - 116 include a suitable base station or base station system at least including a radio frequency transmitter and receiver and an antenna structure for communicating on its assigned traffic carrier.
  • the respective base stations may be coupled to a base station controller or similar device but are at least coupled to a mobile switching center, which controls the operation of the respective base stations for assigning communication resources.
  • Cell 102 is also arranged to operate at substantially higher power than cells 104 - 116, and accordingly, the relative size of cell 102 is larger than the size of cells 104 - 116.
  • Cells 104 - 116 are thus contained within the coverage area of cell 102, and as used herein, cell 102 defines an upper layer while cells 104 - 116 define a lower layer of wireless communication system 100. Additionally, it will be appreciated that one or more mobile radiotelephones
  • mobile operates within area 10, and particularly, within the coverage area of one or more of cells 102 - 116.
  • a mobile within cell 104 is likewise located within cell 102.
  • five traffic cells e.g., traffic cells 104 - 110, are respectively allocated carriers, i.e. frequencies, F(l) 120, F(2) 122, F(3) 124 and
  • Traffic cell 112 is also allocated carrier F(l), designated 128, in a reuse configuration. Each traffic cell 104 - 112 is assigned a single carrier and, as noted, is not provided with carrier/frequency retuning capability.
  • Coverage cell 102 is allocated two carriers, respectively designated carrier 130 and carrier 132, and further includes retuning capability. One of these carriers may be reserved for dedicated use as a dedicated control channel carrier, e.g., the broadcast carrier. It will also be appreciated that coverage cell 102 may be assigned additional carriers without departing from the fair scope of the present invention.
  • Wireless communication system 100 is configured as a TDM system, and therefore, each of carriers 120 - 132 have associated therewith a plurality of time slots designated 0 -
  • carriers 130 and 132 are arranged to frequency hop among carriers F(l) - F(4), in each of time slots 0 - 7.
  • the frequency hopping sequence within each time slot 0 - 7, and respectively for each of carrier 130 and carrier 132, is illustrated by the legend fj, fj, f k , f m .
  • legend 134 When a timeslot in any of carriers 120 - 132 is allocated to a mobile such designation is indicated using legend 134.
  • traffic cells 104 - 112 are reporting uplink signal strength for a respective time slot such reporting is indicated using legend 136.
  • a single call is active. As will be described, it originated in time slot 0 of carrier 132 of coverage cell 102 and was assigned to time slot 7 of carrier 120.
  • FIG. 3 similarly illustrates several call assignments to each of traffic cells 104 - 112 as well as to coverage cell 102.
  • Method 200 begins at step 202 with a channel request from a mobile.
  • the channel request is received within coverage cell 102 and a coverage cell communication resource is assigned.
  • a coverage cell communication resource is assigned.
  • the lowest level time slot on a coverage carrier is assigned.
  • time slot 0 being available on either carrier 130 or carrier 132
  • time slot 0 of carrier 132 is assigned to the mobile.
  • Coverage cell 102, and/or a control entity informs each of traffic cells 104 - 112 of the frequency hopping sequence and that a call is assigned in a particular time slot, namely time slot 0 of carrier 132.
  • each of traffic cells 104 - 112 monitors at the appropriate time, i.e., during the appropriate time slot and when carrier 132 is tuned to the traffic cell carrier in accordance with the hopping sequence, and measures signal strength of an uplink signal from the mobile, step 206.
  • signal strength may also or additionally be measured without departing from the fair scope of the present invention.
  • the highest available time slot on the traffic cell carrier is assigned to the mobile, step 210.
  • the assignment is graphically illustrated by arrow 138 in FIG. 2 with the mobile being assigned time slot 7 of carrier 120. If a suitable traffic cell is not identified, each of the traffic cells continue to monitor during the appropriate time periods and to report the mobile signal strength to coverage cell 102 or the control entity.
  • the communication resource assigned in coverage cell 102 is released, step 212. Also, because the mobile is now assigned to time slot 7 of carrier 120, the hopping sequence is modified, step 214. To avoid creating an interference condition due to the coverage cell frequency hopping, when a mobile is assigned a communication resource in a traffic cell the traffic cell carrier is removed from the hopping sequence for that time slot. This is illustrated in FIG 2 by the extended arrow 140, and the modification of the associated hopping sequence for time slot 7 to omit carrier 120, i.e., f ⁇ .
  • a plurality of mobiles are assigned communication resources on carriers 120, 122 and 128. Additionally, mobiles are assigned to time slots 0 of coverage cell carriers 130 and 132 and to time slot 1 of coverage carrier 132. Accordingly, each of the traffic cells monitor in time slot 0 and time slot 1 at the appropriate time and report accordingly signal strength. Additionally, the hopping sequences for time slots 4 - 7 are modified accordingly given the assignment of mobiles therein.
  • method 200 further includes a background processing portion 216. Background processing portion
  • step 216 runs continuously to "clean up" communication resource assignments as mobiles complete calls within the traffic cells and begins at step 218.
  • a check is made to determine if assignments to higher time slots should be performed, step 222. If not, the clean up process returns to step 218. Otherwise, communication resources are reassigned to move mobiles into the higher level time slots within the traffic cells, step 224.
  • carriers are preferably reused within the traffic cells. This is possible because the traffic cells operate at reduced power as compared to the coverage cell carriers and interference between traffic cells is avoided. As noted above, to further reduce the potential for interference, the coverage cell hopping sequence is accordingly modified when a traffic cell channel assignment is made. Additionally, the coverage cell may be segmented and the traffic cells oriented to reduce potential interference between traffic cells, and likewise, between other cells in the remainder of the communication system, which is contemplated to contain additional cells, coverage cells and the like.
  • the coverage cell must be capable of synthesizing each of the traffic cell carrier frequencies and the coverage cell carrier frequency. If the coverage cell is not capable of retuning fast enough, it is preferred to reserve lower time slots for measurement purposes until the traffic load reaches a point that the lower time slots are required. The above described clean up process 216 assists in ensuring the lower time slots remain available for measurement. Then, if the retune period is for example one time slot, time slots 0 - 2 may be reserved for measurement. Time slot 0 is used to retune from a first frequency to a second frequency, time slot 1 is used to measure the signal strength of the second frequency, and time slot 2 is used to retune back to the first frequency or another frequency to monitor and so on.
  • An alternate preferred embodiment of the present invention is particularly well suited for use where in fact the traffic cells include receivers capable of fast retuning.
  • Such traffic cells may include multiple receivers wherein a first is tuned to the traffic channel carrier frequency and a second is tuned and/or retuned, as necessary, to monitor broadcast control channel carrier frequencies. Switching between the two receivers provides a fast retune capability.
  • a single receiver may be adapted for retuning quickly enough to monitor a first frequency in a first time slot and a second frequency in a second time slot.
  • the traffic cell may "listen" on a broadcast control channel frequency at appropriate times, as informed by the coverage cell or other control entity, and on the traffic channel frequency during time slots on which active calls are assigned. Implementations of such receivers, either multiple receivers and/or fast retuning single receivers, are known in the art.
  • all time slots within the traffic cells are configured as traffic channels, and consistent with the preferred embodiments of the present invention, the traffic cells do not include a broadcast control channel. All calls will originate within coverage cell 102, and all calls are now first assigned a traffic channel resource within coverage area 102. After the call is assigned in coverage area 102, a request is made to each of the traffic cells 104 - 116, providing carrier frequency, time slot and sub-channel information of the assigned call. Each of the traffic cells begin "listening" for the call on the frequency and the time slot. A check may be made to first determine if the time slot is idle within the particular traffic cell.
  • Listening occurs by retuning the traffic cell receiver, either actual retuning of the traffic cell receiver and/or switching between multiple receivers, to the particular frequency during the particular time slot, measuring signal strength of the mobile signal, and reporting the measured signal strength to the coverage cell and/or control entity. Of course, other signal characteristics may be measured and reported. If a suitable traffic cell is identified, i.e., a traffic cell reports sufficiently strong signal strength from the mobile, a reassignment of the call to a traffic channel from the traffic cell is made. It is contemplated that reassignments of calls within traffic cells to other time slots may be made in order to free time slots allowing traffic cell monitoring.
  • traffic cell to traffic cell reassignments may be made using a similar technique wherein traffic cells with idle time slots monitor calls assigned to traffic channels in other traffic cells to determine an alternate suitable traffic cell. If an alternate suitable traffic cell is identified, an alternate traffic cell traffic channel is assigned from the alternate suitable traffic cell.

Abstract

A layered wireless communication system (100) includes a coverage cell (102) and a plurality of traffic cells (104-116). The traffic cells (104-116) are assigned a single traffic channel carrier and do not include a broadcast channel carrier. The coverage cell frequency hops on the traffic channel carriers, and the traffic cells listen at the appropriate times to an uplink signal from a mobile. Mobiles are assigned communication resources in traffic cells returning reports indicating that the mobile is sufficiently well received in the traffic cell.

Description

WIRELESS COMMUNICATION SYSTEM AND METHOD OF ASSIGNING COMMUNICATION RESOURCES
Cross-reference to Related Applications The present application is related to commonly assigned United States
Patent Application, attorney docket no. CE03772R, entitled "Layered Wireless Communication System and Method" filed of even date herewith, the disclosure of which is hereby expressly incorporated herein by reference.
Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to a wireless communication system having a layered configuration and a method of allocating communication resources in a layered wireless communication system.
Background of the Invention Wireless communication systems, for example cellular radiotelephone systems, are well known. These systems provide communication services utilizing radio frequency transmission techniques between land based radio transceiver stations, or base stations, and mobile radio transceivers, or mobiles.
Each of the base stations and the mobiles thus include a radio transceiver for communicating voice, data, or other information utilizing a communication resource, i.e., a radio link, and a standard communication protocol. For example, several such protocols for cellular radiotelephony are the Narrowband Advanced Mobile Phone System (NAMPS), the Advanced Mobile Phone System (AMPS), the Global System for Mobile Communications (GSM), the Personal Digital Communications (PDC), the United States Digital Cellular (USDC), or the Code Division Multiple Access (CDMA) protocols.
A common feature of these wireless communication systems is that a large geographic area is divided into smaller areas ("cells"), which are serviced by a base station. The use of cells to subdivide the large geographic area permits enhanced geographic coverage with efficient use of radio frequency resources. For example, radio frequency resources assigned to one cell may be reused in cells located a sufficient distance from this cell without the resources interfering. During the design and implementation of a cellular communication system, careful planning is undertaken to assign communication resources to cells to ensure sufficient capacity, i.e., to assign a sufficient number of resources to handle an anticipated number of calls at a given time within the cell, and to ensure that communication resources assigned to one cell do not cause unacceptable levels of interference in adjacent or neighboring cells. The ever increasing popularity of cellular systems with their users has created a need to increase system capacity. An advantage of the cellular system in providing greater capacity is that cell sizes may be reduced, by limiting the transmission power of the base stations and mobiles, permitting increased reuse of communication resources. Greater reuse of communication resources in a given geographic area equates to increased capacity.
One type of cellular system utilizing particularly small cell sizes is referred to as a "picocellular" system including very small geographic sized "picocells." Picocellular systems find particularly advantageous use as in-building cellular radiotelephone systems. Implementing wireless communication systems within a building poses numerous problems because of the building topology. The picocellular systems require a high degree of optimization to successfully interact with the surrounding "macrocellular" environment, as well as to provide the highest quality of service to the in-building users. Perhaps the largest optimization effort is due to the installation of the many required base stations, one each for each picocell. Each picocell requires assignment of a broadcast carrier including a broadcast control channel for supplying "coverage" in the picocell, and the assignment of traffic carriers for handling the call traffic within the cells. The broadcast control channel is utilized by the base station and the mobiles to request and establish service, i.e., to make and complete calls. Once service is established, the mobile is assigned a traffic channel resource on which the call is conducted.
The broadcast control channel is referred to as a dedicated channel. That is, its signal characteristics, e.g., carrier, transmission power, timing, etc., are necessarily fixed. However, fixing certain characteristics of the broadcast control channel, i.e., its carrier and power, limit the potential for reusing this carrier within the cellular communication system. For example, it is known to use frequency hopping techniques in order to provide statistically enhanced carrier reuse. However, with the broadcast carriers fixed for each cell, these carriers are typically unavailable for reuse in the frequency hopping plan. In addition, the transmit power of the broadcast control channel requires substantial geographic spacing of cells reusing the associated broadcast carrier thus exacerbating the optimization problem particularly in the picocellular environment. The amount a cell size may be reduced is further limited by the time it requires the mobile to acquire the broadcast control carrier. If the cell sizes are too small, the mobile may not be able to adequately acquire a cell's broadcast control carrier before moving into another cell.
In the above-referenced United States Patent Application, it is proposed to provide a system in which certain of the cells in a layered picocell system and/or a layered microcell / macrocell system are designated as "traffic zones" or traffic cells. The traffic cells are not allocated a broadcast control channel. Coverage cells are provided and include a broadcast control channel. The coverage cells are utilized primarily to provide coverage and not to provide capacity. Service requests are received within one of the coverage cells, and a communication resource is preferentially allocated from a traffic cell, if possible, otherwise a communication resource is allocated from the coverage cell.
One reason why a communication resource may not be allocated from a traffic cell is that the mobile's location within the traffic cell may not be sufficiently determined to allow a resource allocation. The system of the above- referenced patent is robust in that should a resource allocation fail, an alternative resource allocation strategy is provided. Still, more efficient utilization of communication resources may be achieved if the traffic cell communication resources are more fully utilized.
One prior proposal for identifying cells from which communication resources may be allocated provides for the cell base station receiver retuning to the specified frequency on the specified time slot that the mobile is assigned. The cell base station then reports the uplink signal strength to a control entity. If any of the cells report a signal level which qualifies to provide service, the control entity will assign a traffic channel resource from this cell to service the mobile. For example, in one approach measurements may be made while the mobile is communicating on a dedicated control channel. This case requires at least one carrier at the cell to be configured with the same dedicated control channel configuration to listen to the mobile on this control channel, either with a dedicated configuration or retuned to the configuration. A second approach is to monitor the random access channel during a channel request. This case requires a timeslot be configured the same as the random access channel. Either of these proposals will typically require at least two receiver elements and/or fast retuning receivers. Providing additional receiver capability adds not only to the expense of the base station but also to its physical size. For picocellular applications in particular, it is very desirable to keep the size of the base station equipment as small as possible at least for placement and aesthetic reasons, and because of the number of required base stations the cost as low as possible. In picocellular applications specifically, and other applications more generally, redundant or retunable receivers present cost and space penalties.
Thus there is a need for a wireless communication system and a method for assigning communication resource in the wireless communication system which takes full advantage of the statistical capacity enhancement of frequency hopping, provides mobile location determination and does so with low cost, small footprint base stations.
Brief Description of the Drawings
FIG. 1 is a diagram illustrating a layered wireless communication system in accordance with a preferred embodiment of the present invention.
FIG. 2 is a chart illustrating traffic cell resources and coverage cell resources and illustrating an allocation of traffic cell resources in accordance with a preferred embodiment of the present invention.
FIG. 3 is a chart similar to FIG. 2, and further illustrating traffic cell resources and coverage cell resources with a number of active calls.
FIG. 4 is a flow chart illustrating a method of allocating resources in accordance with a preferred embodiment of the present invention. FIG. 5 is a flow chart further illustrating the method of allocating resources in accordance with a preferred embodiment of the present invention.
Detailed Description of the Preferred Embodiments The present invention is described in terms of several preferred embodiments directed to layered cellular communication systems. Such systems include in-building picocellular environments as well as a microcell /macrocell environments. It will be appreciated that the present invention will have application apart from the preferred embodiments described herein, and the description is provided merely to illustrate and describe the present invention and it should in no way be taken as limiting of the present invention. Referring to FIG. 1, wireless communication services are provided in a geographic area 10, utilizing a plurality of cells. The geographic area is preferably an interior and immediate surrounding portions of a building, an area of a city or town, an airport, or any geographic area over which wireless communication services are desired. Coverage of area 10 is provided by a wireless communication system 100 including a coverage cell 102. The primary purpose of coverage cell 102 is to provide coverage and not to provide the required system capacity. It will be further appreciated that additional coverage cells such as shown and described in the above-referenced commonly assigned United States patent application may be provided without departing from the fair scope of the present invention. Wireless communication system 100 further includes "traffic zones" defined by a plurality of traffic cells, individually cell 104, cell 106, cell 108, cell 110 cell 112, cell 114 and cell 116, which are distributed through area 10. Traffic cells 104 - 116 are arranged to provide capacity. In accordance with the present invention, cells 104 - 116 also allow reuse of communication resources, i.e., reuse of the radio frequencies/carriers available for providing wireless communication services. It should also be noted from the outset that communication resource and communication channel are used to refer to a physical link between the base station and the mobile. For example, in a time division multiplexed (TDM) system such as GSM or USDC, a communication channel is one of a number of time slots on a particular carrier. In a code division multiplexed (CDM) system such as PCS or CDMA, a communication channel is a spreading code on a particular carrier. For the TDM system, therefore, the broadcast control channel is a fixed time slot on a fixed carrier within a cell. A traffic channel is one of a plurality of time slots available on one or more traffic channel carriers assigned to a cell and the otherwise available time slots on, for example, the broadcast carrier.
As will be described more fully below, cell 102 includes a broadcast carrier for providing a broadcast communication channel. Each of cells 104 - 116 preferably include only a single traffic channel carrier and do not include a broadcast channel carrier. While presently not preferred due to a size and cost restrictions, it will be appreciated that additional traffic channel carriers may be provided in cells 104 - 116. Additionally, cells 104 - 116 are preferably not provided retuning capability. As is well known, cells 102 - 116 include a suitable base station or base station system at least including a radio frequency transmitter and receiver and an antenna structure for communicating on its assigned traffic carrier. Furthermore, the respective base stations may be coupled to a base station controller or similar device but are at least coupled to a mobile switching center, which controls the operation of the respective base stations for assigning communication resources. Cell 102 is also arranged to operate at substantially higher power than cells 104 - 116, and accordingly, the relative size of cell 102 is larger than the size of cells 104 - 116. Cells 104 - 116 are thus contained within the coverage area of cell 102, and as used herein, cell 102 defines an upper layer while cells 104 - 116 define a lower layer of wireless communication system 100. Additionally, it will be appreciated that one or more mobile radiotelephones
("mobile") operates within area 10, and particularly, within the coverage area of one or more of cells 102 - 116. For example, a mobile within cell 104 is likewise located within cell 102.
With reference to FIG. 2, five traffic cells, e.g., traffic cells 104 - 110, are respectively allocated carriers, i.e. frequencies, F(l) 120, F(2) 122, F(3) 124 and
F(4) 126. Traffic cell 112 is also allocated carrier F(l), designated 128, in a reuse configuration. Each traffic cell 104 - 112 is assigned a single carrier and, as noted, is not provided with carrier/frequency retuning capability. Coverage cell 102 is allocated two carriers, respectively designated carrier 130 and carrier 132, and further includes retuning capability. One of these carriers may be reserved for dedicated use as a dedicated control channel carrier, e.g., the broadcast carrier. It will also be appreciated that coverage cell 102 may be assigned additional carriers without departing from the fair scope of the present invention. Wireless communication system 100 is configured as a TDM system, and therefore, each of carriers 120 - 132 have associated therewith a plurality of time slots designated 0 -
7. In accordance with the preferred embodiments of the present invention, carriers 130 and 132 are arranged to frequency hop among carriers F(l) - F(4), in each of time slots 0 - 7. The frequency hopping sequence within each time slot 0 - 7, and respectively for each of carrier 130 and carrier 132, is illustrated by the legend fj, fj, fk, fm. When a timeslot in any of carriers 120 - 132 is allocated to a mobile such designation is indicated using legend 134. When traffic cells 104 - 112 are reporting uplink signal strength for a respective time slot such reporting is indicated using legend 136. As shown in FIG. 2, a single call is active. As will be described, it originated in time slot 0 of carrier 132 of coverage cell 102 and was assigned to time slot 7 of carrier 120. FIG. 3 similarly illustrates several call assignments to each of traffic cells 104 - 112 as well as to coverage cell 102.
With reference then to FIG. 4, and continued reference to FIG. 2 and FIG. 3, a method 200 of allocating communication resources in accordance with a preferred embodiment of the present invention is shown. Method 200 begins at step 202 with a channel request from a mobile. The channel request is received within coverage cell 102 and a coverage cell communication resource is assigned. Preferably, the lowest level time slot on a coverage carrier is assigned. As shown in FIG. 2, with time slot 0 being available on either carrier 130 or carrier 132, time slot 0 of carrier 132 is assigned to the mobile. Coverage cell 102, and/or a control entity, informs each of traffic cells 104 - 112 of the frequency hopping sequence and that a call is assigned in a particular time slot, namely time slot 0 of carrier 132. As shown in FIG. 2, with a call assigned in time slot 0 of coverage carrier 132 each of traffic cells 104 - 112 monitors at the appropriate time, i.e., during the appropriate time slot and when carrier 132 is tuned to the traffic cell carrier in accordance with the hopping sequence, and measures signal strength of an uplink signal from the mobile, step 206. Of course some other signal characteristic may also or additionally be measured without departing from the fair scope of the present invention.
If a traffic cell reports a sufficiently high signal strength, thus indicating that the mobile is located within the traffic cell, step 208, the highest available time slot on the traffic cell carrier is assigned to the mobile, step 210. The assignment is graphically illustrated by arrow 138 in FIG. 2 with the mobile being assigned time slot 7 of carrier 120. If a suitable traffic cell is not identified, each of the traffic cells continue to monitor during the appropriate time periods and to report the mobile signal strength to coverage cell 102 or the control entity.
With the assignment made, the communication resource assigned in coverage cell 102 is released, step 212. Also, because the mobile is now assigned to time slot 7 of carrier 120, the hopping sequence is modified, step 214. To avoid creating an interference condition due to the coverage cell frequency hopping, when a mobile is assigned a communication resource in a traffic cell the traffic cell carrier is removed from the hopping sequence for that time slot. This is illustrated in FIG 2 by the extended arrow 140, and the modification of the associated hopping sequence for time slot 7 to omit carrier 120, i.e., f\.
Referring to FIG. 3, a plurality of mobiles are assigned communication resources on carriers 120, 122 and 128. Additionally, mobiles are assigned to time slots 0 of coverage cell carriers 130 and 132 and to time slot 1 of coverage carrier 132. Accordingly, each of the traffic cells monitor in time slot 0 and time slot 1 at the appropriate time and report accordingly signal strength. Additionally, the hopping sequences for time slots 4 - 7 are modified accordingly given the assignment of mobiles therein.
When a mobile assigned a traffic cell communication resource completes its call, that communication resource becomes available for use. It is desirable from a traffic channel packing standpoint to keep assigned resources in the traffic cells in the highest level time slots and assigned resources in the coverage cell in the lowest level time slots. This provides several advantages, including it ensures that the lower time slots are available for the required monitoring thus maximizing the number of monitoring traffic cells. It also takes full advantage of frequency hopping in the coverage cell by providing the longest possible hopping sequence in the lowest time slots. With reference to FIG. 5, method 200 further includes a background processing portion 216. Background processing portion
216 runs continuously to "clean up" communication resource assignments as mobiles complete calls within the traffic cells and begins at step 218. Upon detecting the release of a traffic cell communication resource, step 220, a check is made to determine if assignments to higher time slots should be performed, step 222. If not, the clean up process returns to step 218. Otherwise, communication resources are reassigned to move mobiles into the higher level time slots within the traffic cells, step 224.
As discussed and shown in FIG. 2 and FIG. 3, carriers are preferably reused within the traffic cells. This is possible because the traffic cells operate at reduced power as compared to the coverage cell carriers and interference between traffic cells is avoided. As noted above, to further reduce the potential for interference, the coverage cell hopping sequence is accordingly modified when a traffic cell channel assignment is made. Additionally, the coverage cell may be segmented and the traffic cells oriented to reduce potential interference between traffic cells, and likewise, between other cells in the remainder of the communication system, which is contemplated to contain additional cells, coverage cells and the like.
Of course, in the foregoing discussion, the coverage cell must be capable of synthesizing each of the traffic cell carrier frequencies and the coverage cell carrier frequency. If the coverage cell is not capable of retuning fast enough, it is preferred to reserve lower time slots for measurement purposes until the traffic load reaches a point that the lower time slots are required. The above described clean up process 216 assists in ensuring the lower time slots remain available for measurement. Then, if the retune period is for example one time slot, time slots 0 - 2 may be reserved for measurement. Time slot 0 is used to retune from a first frequency to a second frequency, time slot 1 is used to measure the signal strength of the second frequency, and time slot 2 is used to retune back to the first frequency or another frequency to monitor and so on.
An alternate preferred embodiment of the present invention is particularly well suited for use where in fact the traffic cells include receivers capable of fast retuning. Such traffic cells may include multiple receivers wherein a first is tuned to the traffic channel carrier frequency and a second is tuned and/or retuned, as necessary, to monitor broadcast control channel carrier frequencies. Switching between the two receivers provides a fast retune capability. Still, in other embodiments, a single receiver may be adapted for retuning quickly enough to monitor a first frequency in a first time slot and a second frequency in a second time slot. In either embodiment, the traffic cell may "listen" on a broadcast control channel frequency at appropriate times, as informed by the coverage cell or other control entity, and on the traffic channel frequency during time slots on which active calls are assigned. Implementations of such receivers, either multiple receivers and/or fast retuning single receivers, are known in the art.
Given such a configuration for the traffic cells, all time slots within the traffic cells are configured as traffic channels, and consistent with the preferred embodiments of the present invention, the traffic cells do not include a broadcast control channel. All calls will originate within coverage cell 102, and all calls are now first assigned a traffic channel resource within coverage area 102. After the call is assigned in coverage area 102, a request is made to each of the traffic cells 104 - 116, providing carrier frequency, time slot and sub-channel information of the assigned call. Each of the traffic cells begin "listening" for the call on the frequency and the time slot. A check may be made to first determine if the time slot is idle within the particular traffic cell. Listening occurs by retuning the traffic cell receiver, either actual retuning of the traffic cell receiver and/or switching between multiple receivers, to the particular frequency during the particular time slot, measuring signal strength of the mobile signal, and reporting the measured signal strength to the coverage cell and/or control entity. Of course, other signal characteristics may be measured and reported. If a suitable traffic cell is identified, i.e., a traffic cell reports sufficiently strong signal strength from the mobile, a reassignment of the call to a traffic channel from the traffic cell is made. It is contemplated that reassignments of calls within traffic cells to other time slots may be made in order to free time slots allowing traffic cell monitoring.
It is further contemplated that traffic cell to traffic cell reassignments may be made using a similar technique wherein traffic cells with idle time slots monitor calls assigned to traffic channels in other traffic cells to determine an alternate suitable traffic cell. If an alternate suitable traffic cell is identified, an alternate traffic cell traffic channel is assigned from the alternate suitable traffic cell.
As with the other embodiments of the present invention, it is again preferred to maintain call assignments in the coverage cell in the lowest time slots while biasing calls in the traffic cells to higher time slots to ensure the time slots will be idle in the traffic cells. This results in maximizing the number of traffic cells monitoring the coverage cell carrier. Also, in this embodiment of the invention since the coverage cell carrier does not hop through the traffic cell carriers interference introduced to the external network is minimized.
As is appreciated from the foregoing discussion the requirement of providing a broadcast channel in each cell is eliminated as is the requirement of retuning traffic cells in order to perform resource allocation. Still further, the capacity enhancing capability of frequency hopping is advantageously employed. Many additional changes and modifications could be made to the invention without departing from the fair scope and spirit thereof. The scope of some changes is discussed above. The scope of others will become apparent from the appended claims.

Claims

We claim:
1. A wireless communication system comprising: a coverage cell arranged to receive an uplink signal from a mobile on a plurality of traffic channels in accordance with a hopping sequence; a plurality of traffic cells, each traffic cell assigned a portion of the traffic channels and arranged to monitor the uplink mobile signal in its portion of the traffic channels in accordance with the hopping sequence; and whereby, a traffic channel is assigned to the mobile from one of the plurality of traffic cells reporting a sufficient characteristic of the uplink signal.
2. The wireless communication system of claim 1, wherein the traffic channel comprises one of a time slot and a spreading code.
3. The wireless communication system of claim 1, wherein the traffic channel comprises a highest time slot of a plurality time slots.
4. The wireless communication system of claim 1 comprising one of a time division multiplexed system and a code division multiplexed system.
5. A method for assigning communication resource in a wireless communication system comprising: receiving within a coverage cell a channel request from a mobile located in the coverage cell; assigning a coverage communication resource to the mobile within the coverage cell, the coverage communication resource subject to a hopping sequence; monitoring in each of a plurality of traffic cells, and in accordance with the hopping sequence, an uplink signal from the mobile on the coverage communication resource; identifying a traffic cell from the plurality of traffic cells; and assigning a traffic communication resource from the traffic cell to the mobile.
6. The method of claim 5, the step of monitoring comprising measuring a signal strength of the uplink signal.
7. The method of claim 5, each traffic cell being assigned one of a plurality of traffic carriers, the hopping sequence comprising a frequency hopping sequence between each of the plurality of traffic carriers, and the step of monitoring comprising: monitoring within a traffic cell the uplink signal during a hopping sequence period associated with the one of a plurality of traffic carriers assigned to the traffic cell.
8. A method for assigning communication resources in a wireless communication system comprising the steps of: receiving within a coverage cell a channel request from a mobile located in the coverage cell; assigning a coverage cell traffic channel to the mobile within the coverage cell; providing carrier and time slot information associated with the coverage cell traffic channel to each of a plurality of traffic cells located substantially within the coverage cell; for each traffic cell, listening on the coverage cell traffic channel and measuring a signal characteristic; reporting the signal characteristic measured at each traffic cell to a control entity; determining a suitable traffic cell; and reassigning the mobile to a traffic cell traffic channel from the suitable traffic cell.
9. The method of claim 8, wherein the step of for each traffic cell, listening on the coverage area traffic channel and measuring a signal characteristic further comprises determining a time slot availability within the traffic cell.
10. The method of claim 8, further comprising the step of reassigning an active traffic channel assignment to provide a time slot availability.
PCT/US1999/012452 1998-08-19 1999-06-04 Wireless communication system and method of assigning communication resources WO2000011900A1 (en)

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