WO2009084859A2

WO2009084859A2 - Time-difference estimation method for location based services in a cellular network

Info

Publication number: WO2009084859A2
Application number: PCT/KR2008/007662
Authority: WO
Inventors: Zongchuang Liang
Original assignee: Samsung Electronics Co., Ltd.
Priority date: 2007-12-28
Filing date: 2008-12-24
Publication date: 2009-07-09
Also published as: CN101472330A; WO2009084859A3

Abstract

A time-difference estimation method for location based services in a cellular network comprises: estimating, by a terminal, the received power of a serving base station BS and the received power of other adjacent non-serving BSs, dividing the serving sector of the serving BS into a plurality of areas according to the estimation values; calculating, by the terminal, a compared parameter T and sending the compared parameter T to a location server via the serving BS; judging, by the location server, whether the compared parameter T is greater than a threshold or not, and if yes, then estimating, by the terminal, the power levels of the adjacent sectors of the serving BS and reporting these parameters to the system; determining, by the system according to said parameters, which specific area of the serving BS the terminal is located nearby; calculating user location information by the system or the terminal. With the present invention, defects of T-DOA and E-OTD based approaches can be compensated to improve precision for positioning a user who is located nearby the serving BS.

Description

TIME-DIFFERENCE ESTIMATION METHOD FOR LOCATION BASED SERVICES IN A CELLULAR NETWORK

Technical Field

[1] The present invention relates to mobile communication, in particular to an improved time difference estimation method for location based services in a cellular network. Background Art

[2] In recent years, with the rapid development of cellular mobile communication technology, more attention is paid to cellular wireless cellular positioning technology.

[3] The main reason includes compulsory requirements from governments and market- driven demands. FCC issued the wireless E-911 emergency call service function in October, 1996. The key requirement of the function is that all mobile communication networks shall offer longitude and latitude location information on a user who calls in emergency on a phase-wise basis.

[4] In 1996, U.S. Federal Communications Commission (FCC) published E-911

(Emergency call'9H') positioning requirements, requiring that, before October 1, 2001, all kinds of wireless cellular networks shall be able to offer location based service with a precision of 125m or less (the probability that this precision is met is no less than 67%) to any mobile station which sends out an E-911 call, and after 2001, they shall offer higher location precision and three-dimensional location information. Relevant requirements are also proposed in Europe and Japan, indicating that E-911 location based service will be a necessary and fundamental function of any future cellular network.

[5] Major companies all over the world commence on their own positioning implementation solutions for GSM, IS-95 CDMA and the 3^rd generation mobile communication system according to the E-911 positioning requirements. Especially, more detailed requirements on positioning are specified for 3GPP and 3GPP2. This is a positive indication that the wireless cellular positioning technique bears great market potential. In addition, the demand for mobile location based service from mobile communication subscribers is getting stronger and stronger. With the technology of wireless positioning in a cellular network, information on geographic position of a mobile station can be obtained when the mobile station is in an idle or operational state. With the information on the mobile station's location, value-added services like environment information inquiry, emergency rescue, intelligent transport, advertisement publication can be offered by operators for subscribers. Meanwhile, the information can be auxiliary data for mobile communication network operation, maintenance and management.

[6] Figure 1 illustrates an example of architecture of a wireless positioning system. A location based service provider provides subscribers with location information and location sensing service. Once a subscriber requests to obtain location information of some mobile station (MS, hereafter, MS is also referred to as terminal), the location based service provider first communicates with a location control center and queries for the MS' s location coordinates. The subscriber may be a business user who hopes to keep track of some mobile device, or may be a PSAP (Public Safety Answering Point) which attempts to answer an E-911 call. Then the location control center collects necessary information to calculate the MS's location. The necessary information may be parameters such as the received signal's intensity, BS ID, signal's TOA, etc. According to the MS's history information, a series of BSs can be utilized to paging the MS and thus directly or indirectly obtain positioning parameters. Once the necessary information has been collected, the location control center can determine the MS's location with certain precision and feed back this information to the location based service provider. Then the service provider displays the MS's location visually using the information.

[7] Choice of positioning system

[8] The process of positioning an MS in a wireless positioning system is estimating the target MS's geometrical location by detecting the characteristic parameters of the signals transmitted between the MS and several stationary transceivers. According to location and positioning data for positioning estimation, solutions of positioning an MS can be classified into two categories, one being Terminal based, and the other being network based. Correspondingly, there are two types of positioning systems.

[9] (1) Terminal based positioning system. This type of positioning system is also called

MS self-positioning system. It is also called forward link positioning system in a cellular network. The positioning process is implemented in such a manner that the MS determines the geometrical location relationship between itself and respective transmitters of known locations according to the characteristic information about the MS's location carried on the signals received from these transmitters, then the MS estimates its own location with some relevant algorithm, the estimated location information is under the control of the MS's subscriber. The well-known Global Positioning System (GPS) belongs to this category of positioning system.

[10] (2) Network based positioning system. This category of positioning system is also called inverse link positioning system. The positioning process is implemented in such a manner that several location-fixed receivers simultaneously detect a signal transmitted from an MS and transfer the characteristic information on MS location carried in the received signal to an information processing center for estimation of the MS' s location. The auto- vehicle location (AVL) system belongs to this category of positioning system.

[11] From the basic characteristics of the above positioning systems, it can be seen that proper modifications on the known MS, such as integrating a GPS receiver or any processing component capable of simultaneously receiving signals from several BSs for self-positioning, should be made, if the Terminal based forward link positioning solution is applied in a cellular network. Also, the location information should be fed back to the cellular network in a proper method. And for the network-based inverse link positioning solution, it is not necessary to make any modification to the existing MS. Only proper expansion and modification to the cellular network component is needed. With this positioning solution, huge resources of the existing cellular systems can be fully used to protect subscriber's investments. It is relatively easier to implement. So it is preferred in terms of E-911 positioning requirements. [12] There are mainly four techniques for using a mobile cellular network to location an

MS: (1) positioning technique based on electric wave field intensity; (2) positioning technique based on angle of arrival (AOA) of electric wave; (3) positioning technique based on time of arrival (TOA) or time difference of arrival (TDOA) of electric wave; (4) hybrid positioning technique.

[13] 1. Positioning technique based on electric wave field intensity

[14] With this technique, distance between a receiver and a transmitter can be estimated by measuring the field intensity of the received signal, the known model of channel fading and the field intensity of the transmission signal, according to the fact that the intensity of the signal received by an MS is inversely proportional to the distance between the MS and a BS. And by several distance measurements (at least three), the MS' s location can be estimated. The key of this technique is how to model the radio wave propagation process to accurately reflect the service propagation range. This is very difficult to implement in practice. Besides, positioning precision may be affected by such factors as the cell BS' s sector characteristic, possible antenna tilting, constant adjustment of wireless system, geographic environment, vehicles and so on. This technique is limited in positioning precision because that the propagation of electric wave in a mobile communication environment is very complicated. But it is simple to implement. In the case of less precision, it can be adopted. To improve its performance, research is launched that ray tracking method in electric wave propagation is used to further improve the positioning precision.

[15] 2. Positioning technique based on angle of arrival (AOA) of electric wave

[16] This technique is implemented in such a manner that two or more BSs estimate an

MS' s location by measuring the AOA (angle of arrival) of received signals, as illustrated in figure 2. With the AOA positioning method, a unique two-dimensional location point can be determined.

[17] The MS transmits a signal, and BSl receives it. In this way, a line between BS and

MS can be obtained. Similarly, BS2 receives the signal from the MS. And another line between BS2 and MS can be obtained. The two lines intersect to generate a positioning angle. The coordinate positions of BSl and BS2 are known. With the North direction being the reference direction, +0-+360 degrees clockwise and-0— 360 degrees counter- clockwise, a determined triangular relation can be obtained with the three vertices MS, BSl and BS2.

[18] With this method, higher accuracy can be obtained in an area with a small number of obstacles. But in an area with more obstacles, error is increased due to the multipath effect of wireless propagation.

[19] In addition, the AOA technique must be implemented on the basis of it smart antenna

(SA). So, if the AOA technique is adopted for positioning, it is necessary to replace antennas in BTS in a current GSM system with smart antennas to implement AOA positioning technique.

[20] 3. Positioning technique based on time of arrival (T0A)/time difference of arrival

(TDOA) of electric wave

[21] This technique is most widely applied in the cellular network-based wireless positioning system. In the TOA positioning technique, the time taken by a signal in its propagation directly from the target MS to the BS is measured first, then according to the velocity the electro-magnetic wave propagates in the air, we can calculate the distance between the target MS and the BS. That is, the MS locates on a circle with the BS being the center and the distance between the MS and the BS being the radius. With the measurements and calculations by several BSs, the MS's two-dimensional location can be determined by the cross points of three circles. In the TOA positioning technique, it is necessary for BSs receiving signals to learn about the time moment when the MS transmits the signal. And the requirement on clock precision is very strict for the BSs. To overcome these disadvantages, the TDOA technique is put forward. It is implemented according to the difference between the times taken by a signal in its propagation from MS to two BSs, instead of the absolute time. In this way, requirement on time synchronization is greatly reduced. As shown in figure 3, it is obvious that the MS is certainly located on a hyperbola with the two BSs as its focuses. Therefore, by establishing more than two hyperbola equations, we can obtain the MS's two-dimensional location by finding the cross point of the hyperbola.

[22] In the two time-based positioning techniques, it is not necessary to make large-scale modifications to existing networks equipments. These techniques only require that BSs are able to accurately extract the estimation value of delay from received RF signals. Under this condition, high precision can be reached in terminal based positioning. So, research in these two techniques is on focus for cellular networks. Generally speaking, such network-based positioning technique is called E-OTD method, and such terminal based positioning technique is called T-DOA method.

[23] 4. Hybrid positioning technique

[24] This technique simultaneously applies different types of signal characteristic measurements like TOA, AOA, etc. in positioning estimation. And in a cellular network, the MS' s location can be determined according to the TOA and AOA data measured by the serving BS. To realize accurate positioning process with the above fundamental positioning techniques, line-of-sight (LOS) propagation of electro-magnetic wave must be well performed between a receiver and a transmitter. In a cellular network, such factors as non-line-of-sight propagation, multipath effect and noise interferences often cause great effects on positioning precision. Among these positioning techniques, the one based on electric wave field intensity is the most simple but of less positioning precision, AOA positioning technique has higher precision but with complicated receiving equipment, TOA positioning technique has higher precision but has strict requirement on time synchronization, TDOA positioning technique can remove the dependency on time baseline so that costs can be reduced on the premise of keeping higher location precision, and the hybrid positioning technique has higher location precision but large-scale modifications on network equipments in existing cellular systems. By far, the positioning technique under more attention and deeper research is the TDA-based /TDOA-based inverse link positioning technique.

[25] In a cellular system, known methods, such as phase ranging method, pulse ranging method and spectrum spread ranging method can be adopted to obtain estimation of TOA or TDOA. For instance, the spectrum spread ranging method is often used in a CDMA cellular system. In general, two kinds of spectrum spread ranging methods are adopted in a CDMA system, one of which is called coarse time detection method using a sliding correlator or a matching filter. The coarse detecting process is realized by a sliding correlator, a matching filter or a continuous detection circuit. The delay estimation value is confined within one chip interval. The other method is called fine detection method using delay phase locked loop (DLL). During the process of fine detection, the local PN sequence is kept consistent with the input PN sequence by DLL. This scheme is successfully applied in a GPS system.

[26] In a CDMA system, multi-access interference will severely affect coarse time capturing in a time-based positioning system, and it will greatly affect time measurement by DLL. With the use of power control, it is difficult for multiple non- serving BSs to simultaneously measure TOA or TDOA correctly. At present, some methods have been proposed attempting to settle this problem. One example is that transmitting power of an MS is instantly increased to maximum when an E-911 call happens. Other examples includes improved soft switching, utilization of a anti- far/near-effect delay estimator and multi-user detector. The time-based positioning technique requires that the transmitter at a BS can accurately extract TOA or TDOA estimation value from received RF signals.

[27] In the E-OTD (downlink) and the T-DOA (uplink) positioning methods, the predominant technical constraint is the limit on the number of BSs that participate in the positioning process. This positioning technique requires more than three BSs to participate in the positioning process. However, in an environment where the distribution of BSs is sparse, it is difficult to find more than three BSs to participate in the positioning process at the same time. Even in an urban environment populated with BSs, reception of LOS signals may be failed because of blocking problem from many buildings. More seriously, when an MS approaches a serving BS, other non-serving BSs have difficulty in receiving the positioning measurement signal from the MS, or the MS has difficulty in receiving signals from the other non-serving BSs. This is because adaptive power control is used to overcome the far-and-near effect (for CDMA based BS) and the cell boundary interference (for OFDM based BS). For the purpose of illustration, Table 1 lists the statistic data of the number of BSs participating in an MS positioning process in all kinds of conditions. The statistic data indicates that the possibility of three or more BSs simultaneously keeping in touch with an MS in countryside or suburb is small even with the measurement carried along the diameter of the range.

[28] Table 1 [Table 1]

Table 1 Statistic data of different value of X in all kinds of conditions

[29] Meanwhile, it can not be ignored that, even three BSs can participate in the implementation of positioning, the signals received by the non-serving BSs drastically degrades when a user is close to the serving BS. This leads to a degradation of performance of E-OTD or T-DOA, etc., as shown in Figure 4. Disclosure of Invention Technical Solution

[30] The object of the present invention is to provide an improved Time-Difference estimation method for Location based services in a cellular-based network.

[31] To reach the object mentioned above, a Time-Difference estimation method for

Location based services in a cellular-based network comprises:

[32] a) estimating, by a terminal, the received power of a serving base station BS and the received power of other adjacent non-serving BSs, dividing the serving sector of the serving BS into a plurality of areas according to the estimation values;

[33] b) calculating, by the terminal, a compared parameter T and sending the compared parameter T to a location server via the serving BS;

[34] c) judging, by the location server, whether the compared parameter T is greater than a threshold or not, and if yes, then

[35] d) estimating, by the terminal, the power levels of the adjacent sectors of the serving

BS and reporting these parameters to the system;

[36] e) determining, by the system according to said parameters, which specific area of the serving BS the terminal is located nearby;

[37] f) calculating user location information by the system or the terminal.

[38] With the present invention, defects of T-DOA and E-OTD based approaches can be compensated to improve precision for positioning a user who is located nearby the serving BS. Brief Description of the Drawings

[39] Figure 1 shows the architecture of a wireless positioning system;

[40] Figure 2 illustrates the principle of AOA positioning method;

[41] Figure 3 illustrates the operation principle of T-DOA and E-OTD methods;

[42] Figure 4 illustrates the relationship between the magnitude of a received signal and parameter estimation precision;

[43] Figure 5 illustrates the principle of the solution proposed in the present invention;

[44] Figure 6 shows the composition of the solution proposed in the present invention;

[45] Figure 7 illustrates a flow chart of terminal (MS) -based E-OTD method;

[46] Figure 8 illustrates a flow chart of terminal (MS) -based -based sector information method;

[47] Figure 9 illustrates a flow chart of network-based T-DOA method;

[48] Figure 10 illustrates a flow chart of network-based sector information method.

Best Mode for Carrying Out the Invention

[49] Different positioning methods are adopted by the system according to different locations of a user in a network. The primary composition of the present invention is illustrated in figure 6. [50] 701: The terminal first estimates the received power of a serving base station (BS) and the received power of other adjacent non-serving BSs. Then according to the estimation, the terminal divides the serving sector of the serving BS into area 1, area 2, area 3 and area 4, as shown in figure 5.

[51] Specifically, the terminal first calculates the power levels of specified sectors of three adjacent BSs:

[53] where, i=0,l,2; when i=0, P, denotes the received power of the serving sector of the serving BS; when i=l,2, P _t denotes the received power of the adjacent sectors of the serving BS;

denotes sub-carrier j of BS /; M≤N, N denotes the maximum number of sub-carriers available for signal receiving. Then the parameter below is calculated: [54] p

T = °

P + P

[55] 702: The terminal sends this information to a location server via the serving BS.

[56] Here, it is necessary for the system to learn about the power levels of the serving BS

(the serving sector) and the adjacent non-serving BSs (adjacent sectors). The terminal user implements such measurement by certain signal manner and reports the measurements to the terminal's serving BS. The information to be reported includes the power levels of relevant BSs (for the user to fulfill functions like switching, etc.) and the parameter T (for the user system to make further determination).

[57] 703: The location server judges whether the parameter T is greater than a threshold or not (black bold solid line in figure 4 indicates the threshold):

^[58^] T < γ

[59] If T is less than the threshold, the process goes to 704, as terminal 1 and terminal 2 shown in figure 5. Otherwise, the process goes to 705, as terminal 3 in figure 5.

[60] 704: The terminal estimates the user's location with the conventional E-OTD

(downlink) method, or the system estimates the user's location with T-DOA (uplink) method.

[61] 705: The terminal estimates the power levels of adjacent sectors of the serving BS in the following manner

[63] where, i=0,l,2; when i=0, P, denotes the received power of the serving sector of the serving BS; when i=l,2, P _t denotes the received power of the adjacent sectors of the serving BS; e(0

denotes sub-carrier j of BS /; M≤N, N denotes the maximum number of sub-carriers available for signal receiving. Then the two parameters below are calculated: [64] p

T — —0-

T _

2 = 0

P>

[65] The terminal reports these parameters to the system. And the system determines which of the areas 2, 3 and 4 the user is located in. [66] 706: The system determines the specific area in which the terminal user is located according to the reported parameters. [67] Details on the method are if

[68] I 1"₁ ^{^} ^> p R and _i ¹ T₂ — ^*> P R ,

[69] where β denotes the threshold, then the system determines that the terminal user is in area 2. Thus, the average location in area 2 is used by the system as the terminal user's location information; else, if

¹⁷⁰¹ τ_x ≤ β _anά τ₂ ≥ β ,

[71] then the system determines that the terminal user is in area 3. Thus, the average location in area 3 is used by the system as the terminal user's location information; else the system determines that the terminal user is in area 4. Thus, the average location in area 4 is used by the system as the terminal user's location information.

[72] Here, it is necessary for the system to learn about the power levels of the serving BS

(the serving sector) and adjacent non-serving BSs (adjacent sectors). The terminal user implements such measurement with certain signal manner and reports the measurements to the terminal's serving BS. The information to be reported includes the power levels of relevant BSs (for the user to perform functions like switching, etc.) and parameters Tl and T2 (for next judgment made by the user).

[73] 707: Finally, the system or the terminal user obtains the final user location information.

[74] Embodiments

[75] For network-based and terminal (MS)-based positioning information and different situations, the following two embodiments are described.

[76] 1. Terminal (MS)-based positioning method, the signaling flows are illustrated in figure 7 and figure 8.

[77] Signaling 1: the terminal (MS) sends out a positioning request and reports necessary information to the serving sector of the serving BS according to steps 701 and 702.

[78] Signaling 2: the serving BS forwards the request and relevant information to the location server at the network side.

[79] Now, it is necessary for the system to determine whether to adopt the E-OTD based or the sector information based method, according to the reported information. Detailed determination process is illustrated in steps 701-704 in figure 6.

[80] If the system decides to adopt the E-OTD based method, the operation flow is illustrated in figure 7.

[81] Signaling 3: the system informs the BSs relevant to the terminal user to prepare to transmit relevant measurement signals like Preamble or Pilot information in an OFDM system or PN sequence in a CDMA system.

[82] Signaling 4: the serving BS sends service acknowledgement (ACK) information to the terminal user.

[83] Signaling 5: the relevant BSs transmit corresponding measurement signals for reception by the terminal user.

[84] Then, the terminal calculates its own location information.

[85] If the system decides to adopt the sector information based method, the operation flow is illustrated in figure 8.

[86] Signaling 3: the location server in the system feeds the information back to the serving BS.

[87] Signaling 4: the serving BS forwards the information to the terminal user.

[88] The terminal user intercepts information on adjacent sectors of the serving BS to determine more detailed location information of the terminal user. Detailed determination process is illustrated in steps 705-706 in figure 6.

[89] 2. Network-based positioning method. The signaling flows are illustrated in figure 9 and figure 10. [90] Signaling 1: the terminal (MS) sends out a positioning request and reports necessary information to the serving sector of the serving BS according to steps 701 and 702. [91] Signaling 2: the serving BS forwards the request and relevant information to the location server at the network side. [92] Now, it is necessary for the system to determine whether to adopt the E-OTD based method or the sector information based method, according to the reported information.

Detailed determination process is illustrated in steps 701-704 in figure 6. [93] If the system decides to adopt the T-DOA based method, the operation flow is illustrated in figure 9. [94] Signaling 3: the system informs the BSs relevant to the terminal user to prepare to transmit measurement signals like Preamble or Pilot information in an OFDM system, or PN sequence in a CDMA system.

[95] Signaling 4: the serving BS sends service ACK information to the terminal user.

[96] Signaling 5: the terminal user transmits reference measurement signal to the relevant

BSs. [97] Signaling 6: the relevant BSs perform time measurement after they receive the relevant measurement signal, and feed the measurements back to the system's location server.

[98] The system calculates the location information of the terminal user.

[99] If the system decides to adopt the sector information based method, the operation flow is illustrated in figure 10. [100] Signaling 3: the location server in the system feeds the information back to the serving BS.

[101] Signaling 4: the serving BS forwards the information to the terminal user. [102] The terminal user intercepts information on adjacent sectors of the serving BS to determine more detailed location information of the terminal user. Detailed determination process is illustrated in steps 705-706 in figure 6.

[103] Signaling 5: the terminal user reports the information to the serving BS. [104] Signaling 6: the serving BS forwards the information to the location server in the system. [105] The location server determines the final location information of the terminal user according to the information.

Claims

[1] A time-difference estimation method for location based services in a cellular network comprising: a) estimating, by a terminal, the received power of a serving base station BS and the received power of other adjacent non-serving BSs, dividing the serving sector of the serving BS into a plurality of areas according to the estimation values; b) calculating, by the terminal, a compared parameter T and sending the compared parameter T to a location server via the serving BS; c) judging, by the location server, whether the compared parameter T is greater than a threshold or not, and if yes, then d) estimating, by the terminal, the power levels of the adjacent sectors of the serving BS and reporting these parameters to the system; e) determining, by the system according to said parameters, which specific area of the serving BS the terminal is located nearby; f) calculating user location information by the system or the terminal.

[2] The method of Claim 1, wherein said step a) comprises: estimating the power of the serving BS and the power of the other adjacent non- serving BSs with the formula

where, i=0,l,2; when i=0, P, denotes the received power of the serving sector of the serving BS; when i=l,2, P, denotes the received power of the adjacent sectors of the serving BS;

denotes sub-carrier y of BS /; M≤N, N denotes the maximum number of sub- carriers available for signal receiving.

[3] The method of Claim 2, wherein the compared parameter T is calculated with the formula

T = P_n 0

P + P

[4] The method of Claim 1, wherein the number of said plurality of sectors is four.

[5] The method of Claim 1, wherein said step e) comprises:

If T₁ ≥ β and T₂ ≥ determining that the terminal user is in area 2; If

T₁ ≤ β and T₂ ≥ β , determining that the terminal user is in area 3; else, determining that the terminal user is in area 4; β denotes the threshold.

[6] The method of Claim 1, wherein said step d) comprises: the power levels of adjacent sectors of the serving BS are estimated with the formula

where i=0,l,2; when i=0, P _; denotes the received power of the serving sector of the serving BS; when i=l,2, /^denotes the received power of the adjacent sectors of the serving BS; e(0

denotes sub-carriery^' of BS /; M≤N, N denotes the maximum number of sub- carriers available for signal receiving.

[7] The method of Claim 1, wherein the compared parameters Tl and T2 are calculated with the formulas

T - O 1I

P_n

-*^■ ? —

P,