Abstract:
A mobile communication station in a wireless communication network is used to measure the respective times of arrival of radio signals respectively transmitted by a plurality of radio transmitters in the network. The mobile communication station is provided with real time difference information indicative of differences between a time base used by a radio transmitter serving the mobile communication station and respective time bases used by the other radio transmitters. The mobile communication station determines, in response to the real time difference information and relative to the time base used by the radio transmitter serving the mobile communication station, a plurality of points in time at which the respective radio signals are expected to arrive the mobile communication station. For each radio signal, the mobile communication station monitors for arrival of the radio signal during a period of time after the point in time at which the radio signal is expected to arrive.

Description:
BACKGROUND OF THE INVENTION 
     The ability to locate the position of a mobile communication unit operating in a wireless communication system (for example, a cellular communication system) provides many well known advantages. Exemplary uses of such position location capability include security applications, emergency response applications, and travel guidance applications. Several known techniques for providing position location involve the measurement of certain characteristics of communication signals, such as the time of arrival (TOA), the round trip delay, or the angle of arrival of a communication signal. Some of these techniques can be further divided into uplink or downlink approaches In the uplink category, a base transceiver station (BTS) or other receiver performs the measurements on communication signals originating at a mobile communication unit (or mobile station). In downlink approaches, the mobile station performs the measurements on signals originating at base transceiver stations or other transmitters. 
     One example of a downlink technique for locating the position of a mobile station is the observed time difference (OTD) technique. This technique will now be described with respect to the Global System for Mobile Communication (GSM), which is exemplary of a cellular communication system in which downlink observed time difference techniques are applicable. The OTD technique is implemented, for example, by having the mobile station measure the time difference between arrival times of selected radio signals transmitted from different base transceiver stations. Assuming the geometry shown in FIG. 1, and further assuming that two signals are transmitted simultaneously from the base transceiver stations BTS 1  and BTS 2 , and letting T 1  and T 2  denote the times of arrival of the respective signals at the mobile station, then the observed time difference OTD is given by the following equation: 
     
       
         T 1 −T 2 =(d 1 −d 2 )/c,  (Eq. 1) 
       
     
     where d 1  and d 2  are the respective distances from BTS 1  and BTS 2  to the mobile station. The locations of BTS 1  and BTS 2  are known, and the possible locations of the mobile station are described by the hyperbola  15  shown in FIG.  1 . By combining measurements from at least three base transceiver stations, a position estimate for the mobile station can be obtained. 
     Most conventional cellular communication systems (including GSM systems) are asynchronous, that is, each base transceiver station uses its own internal clock reference to generate its frame and time slot structure. Therefore, the frame structures of the different base transceiver stations will tend to drift in time relative to one another, because clocks are not perfectly stable. As a consequence, an OTD measurement is not really meaningful for locating the position of a mobile station unless the differences in timing between the base transceiver stations being used is known. This difference, often referred to as the real time difference or RTD, represents the actual difference in absolute time between the transmission of respective signals (e.g., respective synchronization bursts in GSM) from respective base transceiver stations, which signals would be transmitted simultaneously if the frame structures of the base transceiver stations were perfectly synchronized. 
     Among several possible approaches to determine the real time difference RTD between base transceiver stations, two conventional examples are: absolute time stamping in the respective base transceiver stations; and use of stationary reference mobiles located in known positions. In the latter example, the reference mobile measures downlink signals sent from various base transceiver stations. Because the respective distances between the various base transceiver stations and the stationary reference mobile station are known, the expected time difference in arrival times of the respective signals from the base transceiver stations can be easily calculated. The real time difference RTD between base transceiver stations is simply the difference between the expected time difference of arrival and the observed time difference of arrival actually observed at the reference mobile station. The reference mobile station can periodically make the downlink time of arrival measurements and report them to a mobile location node in the network so that the network can maintain an updated record of the RTDs. 
     The techniques underlying known OTD methods are very similar to procedures used conventionally by mobile stations to synchronize to a serving base transceiver station and make measurements on a number of neighboring base transceiver stations as instructed by the serving cell (as in mobile assisted hand-off operations). The mobile station needs to know which base transceiver stations are to be monitored for OTD measurements. This information can typically be provided in conventional system information messages broadcasted in the cell, for example on a GSM cell&#39;s BCCH (broadcast control channels) frequency. This system information typically includes a list of frequencies of neighboring cells which are to be measured. The mobile station scans the designated frequencies to detect a frequency correction burst, which is an easily identifiable burst that appears approximately every 50 ms in GSM. 
     After successful detection of a frequency correction burst, the mobile station knows that in GSM the next frame will contain a synchronization burst SB. The synchronization burst SB contains the Base Station Identity Code (BSIC) and information indicative of the frame number of the current frame in which the burst SB is occurring. The mobile station measures the time of arrival of the synchronization burst SB at the mobile station relative to the timing of mobile station&#39;s own serving cell. Since now the mobile station knows the frame structure of the neighboring base transceiver station relative to its own serving base transceiver station timing, it is possible to repeat the time of arrival measurements to improve accuracy. This procedure is repeated until all frequencies (i.e., all BTSs) on the list have been measured. The observed time difference values recorded by the mobile station are then transferred to a mobile station location node in the cellular system, which node performs the position determination based on the observed time difference values, the real time difference values and the geographic locations of the base transceiver stations. 
     Because the mobile station does not know when the frequency correction burst (and thus the following synchronization burst SB) will appear, the brute force method described above, namely monitoring for the frequency correction burst, must be used. 
     The time required to capture a synchronization burst will depend on the measurement mode. OTD measurements can be made, for example, when call setup is being performed on a GSM SDCCH (Stand-alone Dedicated Control Channel), or during idle frames when the mobile station is in call mode, or during speech interrupt. For example, if the mobile station makes the measurements in call mode, then the mobile station can only make measurements during idle frames, which conventionally occur in GSM systems every 120 ms. The probability that a particular synchronization burst will appear within the idle frame is approximately 1 in 10, because the synchronization burst conventionally occurs once every ten frames in GSM. Accordingly, on average, 5 idle frames will be needed, meaning 0.6 seconds per base transceiver station. Thus, if it is desired to measure at least 6 neighboring base transceiver stations, an average measurement time of 3 or 4 seconds will be required, which may be prohibitively long in many applications. 
     The mobile station is guaranteed to have measured the synchronization burst SB if the mobile station captures and stores all signals (for example, all signals on the BTS&#39;s BCCH frequency in GSM) for 10 consecutive frames. However, providing the mobile station with the memory and computational capacity to capture (and thereafter process) all signal information in 10 consecutive frames is disadvantageously complex. 
     Moreover, in areas such as urban areas characterized by high interference levels, and in rural areas with large distances between base transceiver stations, the probability of detecting the synchronization burst SB may be unacceptably low, because the signals will typically be characterized by low signal-to-noise ratios. 
     Due also to the low signal-to-noise ratio, it is typically very difficult to decode the BSIC in the synchronization burst SB. The probability of taking ghost spikes instead of a synchronization burst SB is therefore disadvantageously increased in instances of low signal-to-noise ratio. 
     It is therefore desirable to improve the mobile station&#39;s ability to detect downlink signals used in conventional downlink observed time difference approaches. 
     The present invention attempts to overcome the aforementioned disadvantages of conventional downlink observed time difference approaches by providing for improved sensitivity in detecting the downlink communication signals used for making observed time difference measurements at mobile stations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 diagrammatically illustrates how the location of a mobile station can be determined using downlink observed time difference measurements. 
     FIG. 2 is a block diagram of an exemplary wireless communications system including downlink observed time difference measurement capability according to the present invention. 
     FIG. 3 illustrates one example of relative timing difference between base transceiver stations such as shown in FIG.  2 . 
     FIG. 4 illustrates an exemplary time slot structure of the frames of FIG.  3 . 
     FIG. 5 illustrates an exemplary quarter bit structure of the time slot of FIG.  4 . 
     FIG. 6 illustrates pertinent portions of a mobile station having the downlink observed time difference measurement capability according to the present invention. 
     FIG. 7 illustrates how an example downlink monitoring window is determined according to the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 illustrates one example of a pertinent portion of a wireless communication system including the downlink observed time difference measurement capability according to the present invention. The invention is implemented in a GSM network in the example of FIG.  2 . As shown in FIG. 2, a GSM mobile switching center MSC is coupled for communication with a plurality of GSM base station controllers BSCs, which are in turn coupled to communicate with one or more GSM base transceiver stations BTSs. The base transceiver stations are capable of radio communication with a plurality of mobile stations MSs via the air interface. Communication from the MSC to the MSs via the BSCs and the BTSs is well known in the art. 
     FIG. 2 also includes a mobile location center MLC coupled to communicate bidirectionally with the mobile switching center MSC using conventional GSM signaling protocol. In FIG. 2, the MLC can receive a request to locate the position of a mobile station MS 1 . Such a request is typically received from a location application  21  coupled to communicate with the MLC. The location application  21  can be a node within the network itself, or an external location application. In response to the request to locate the position of mobile station MS 1 , the MLC interrogates the network to thereby determine the serving BTS  23  (i.e., the serving GSM cell), and decides which BTSs should be selected for the downlink observed time difference measurements. 
     The MLC can then generate a positioning request message for mobile station MS 1 , indicating the frequencies and BSICs (the BSICs are conventionally available in networks such as the GSM network) of the base transceiver stations selected to be monitored, and the real time differences RTDs between the serving BTS and each of the selected BTSs. The positioning request message can be communicated from the MLC to MS 1  via MSC, BSC  21 , BTS  23 , and the air interface between BTS  23  and MS 1 . Because MS 1  knows when synchronization bursts will arrive from its own serving BTS, MS 1  can use the RTD information to calculate approximately when synchronization bursts will arrive from the selected neighboring BTSs. This will be described in more detail hereinafter. 
     The aforementioned information can also be sent to MS 1  as a dedicated message during, for example call setup. Moreover, the aforementioned information can also be sent to MS 1  periodically on a broadcast control channel as a system information message. The RTDs can be calculated by the MLC using OTD information received from a reference mobile station, as described hereinabove, or the RTDs can be provided to the MLC using other conventional techniques. 
     FIGS. 3-5 illustrate the concept of real time differences among base transceiver stations in GSM networks such as the example GSM network portion of FIG.  2 . 
     FIG. 3 illustrates the real time difference between the frame structure timing of a pair of base transceiver stations designated in FIG. 3 as BTS 2  and BTS 1 . In GSM, the TDMA frames used by the base transceiver stations are numbered in a repetitive cyclic pattern, each cycle (also called a hyperframe) including 2,715,648 frames numbered as frame  0  through frame 2,715,647. In the example of FIG. 3, frame  0  of BTS 1  timewise overlaps with frame  828  of BTS 2 . 
     Referring now to FIG. 4, each TDMA frame in GSM is divided into eight time slots TS, numbered time slot  0  through time slot  7 . As shown in FIG. 5, each GSM time slot is further divided into 625 quarter bits QB, so that during each time slot a total of 625/4=156.25 bits are transmitted. The real time difference RTD between BTS 2  and BT 1  is thus conventionally expressed as the triplet (FND, TND, QND), wherein FND is the difference (FN 2 −FN 1 ) between the TDMA frame numbers of BTS 2  and BTS 1 , TND is the difference (TN 2 −TN 1 ) between the time slot numbers of BTS 2  and BTS 1 , and QND is the difference (QN 2 −QN 1 ) between the quarter bit numbers of BTS 2  and BTS 1 . For example, with reference to FIGS. 3-5, if quarter bit  0  of time slot  0  of frame  0  of BTS 1  is aligned in time with quarter bit  37  of time slot  6  of frame  828  of BTS 2 , then the real time difference RTD between BTS 2  and BTS 1  is given by the triplet (FN 2 −FN 1 , TN 2 −TN 1 , QN 2 −QN 1 ), where FN 2 , TN 2  and QN 2  are the frame number, time slot number and quarter bit number of BTS 2 , and FN 1 , TN 1  and QN 1  are the same parameters of BTS 1 . Thus, the triplet is (828−0, 6−0, 37−0), or simply (828, 6, 37). 
     When the mobile station MS 1  receives from MLC the real time difference RTD between its own serving base transceiver station, for example BTS 1  of FIG. 3, and another base transceiver station on which it is to make downlink time of arrival measurements, for example BTS 2  of FIG. 3, the mobile station MS 1  can use the RTD triplet (FND, TND, QND) along with the known frame structure timing (FN 1 , TN 1 , QN 1 ) of the serving base transceiver station BTS 1  to determine the frame structure timing of BTS 2  relative to that of BTS 1 . The following calculations can be made by the mobile station MS 1  to determine the current frame number FN 2  of BTS 2  at any given point (FN 1 , TN 1 , QN 1 ) in the time base of BTS 1 . 
     
       
         QN 2 ′=QN 1 +QND  (Eq. 2) 
       
     
     
       
         TN 2 ′=TN 1 +TND+(QN 2 ′div625)  (Eq. 3) 
       
     
     
       
         FN 2 ′=FN 1 +FND+(TN 2 ′div8)  (Eq. 4) 
       
     
     
       
         FN 2 =FN 2 ′mod2,715,648  (Eq. 5) 
       
     
     In the foregoing equations, “div” represents integer division, and “mod” is modulo n division, wherein “x mod n”=“the remainder when x is divided by n”. 
     The synchronization burst SB in GSM contains 78 encoded information bits and a predetermined 64 bit training sequence, as is well known in the art. The 78 encoded information bits contain BSIC and the so-called reduced frame number, conventionally expressed in three parts, T 1 , T 2  and T 3 ′. The conventional relationship between the frame number (FN) of the synchronization burst SB and the parameters T 1 , T 2  and T 3 ′ is as follows: 
     
       
         T 1 =FNdiv(26x51)  (Eq. 6) 
       
     
     
       
         T 2 =FNmod26  (Eq. 7) 
       
     
     
       
         T 3 =FNmod51  (Eq. 8) 
       
     
     
       
         T 3 ′=(T 3 −1)div10  (Eq. 9) 
       
     
     Thus, once the current frame number FN 2  of BTS 2  has been calculated as shown above with respect to equations 2-5, then the parameter T 3  can be determined by plugging FN 2  into equation 8 above. 
     In conventional GSM networks, the synchronization burst SB occurs in time slot  0  of frames  1 ,  11 ,  21 ,  31  and  41  of a 51-frame repeating sequence of TDMA frames transmitted on the BTS&#39;s BCCH (broadcast control channels) carrier. Thus, T 3  above indicates where the current frame FN 2  is located within the 51-frame repeating sequence. Because, as mentioned above, the synchronization burst SB occurs in time slot  0  of frames  1 ,  11 ,  21 ,  31  and  41  of this 51-frame repeating sequence, the next T 3  (call it T 3 n) that satisfies the relationship, (T 3 −1) mod 10=0, will designate the frame of BTS 2  in which the next synchronization burst SB will occur. The corresponding frame number (call it FN 2 n) is then determined by: 
     
       
         FN 2 n=(FN 2 +DT 3 )mod2,715,648,  (Eq.10) 
       
     
     where DT 3 =(T 3 n−T 3 ) mod 51. 
     Now, the parameters T 1 , T 2  and T 3 ′ can be determined by plugging FN 2 n into equations  6  and  7  and plugging T 3 n into equation 9. According to the GSM standard, the parameters T 1 , T 2  and T 3 ′, along with the BSIC, can be expressed using  25  bits. The BSIC bits can be determined from the BSIC information received at MS 1 , and the bits representing T 1 , T 2  and T 3 ′ can be determined from equations 6, 7 and 9. The mobile station MS 1  can then apply to the aforementioned 25 bits, a well known coding algorithm described in the GSM standard (ETSI GSM Specification 05.03), in order to generate from those 25 bits the 78 encoded bits in the synchronization burst. 
     In this manner, the mobile station MS 1  now knows, with respect to the frame structure timing of its own serving BTS 1 , the frame number FN 2 n of BTS 2  in which the synchronization burst will occur. As mentioned above, the synchronization burst always occurs in time slot  0 , so the mobile station MS 1  now knows exactly when the synchronization burst will be transmitted by BTS 2 . Moreover, the mobile station MS 1  now also knows all 78 encoded bits along with all 64 training bits of the synchronization burst. With knowledge of 142 bits rather than just 64 bits, the mobile station can achieve better accuracy in making time of arrival measurements than in the conventional situation wherein only 64 bits are known. Moreover, with 142 known bits, it is possible for the mobile station MS 1  to achieve, in a far noisier environment, the same accuracy as could be achieved using 64 bits in a less noisy environment. 
     Because the position of the mobile station MS 1  relative to a given neighboring BTS (e.g., BTS  28  of FIG. 2) is not known, the synchronization burst SB from that BTS will not arrive at the mobile station MS 1  at precisely the time that was calculated by the mobile station. FIG. 7 illustrates one example of how a search window can be defined to encompass the time at which the synchronization burst can be expected to arrive at the mobile station MS 1 . Let FN denote the frame number of the next SB (SB 2 ) that is expected to arrive from neighboring (non-serving) BTS 2 . How this frame number is calculated can be found in Eq. 10. MS 1  knows when the corresponding SB (SB 1 ) with the same frame number will arrive, or would have arrived, from the serving BTS 1 . Let this time instant be denoted by T 0 , relative to the mobile station&#39;s timebase. 
     MS 1  is within the circle  71 . The radius r of this circle can e.g., be determined by the cell radius or derived from the timing advance value. Consider the two extreme cases. One extreme case is when MS 1  is at 74. Then SB 2  arrives at time T 0 +RTD+d 12 /c since SB 2  travels d 12  further than SB 1  does. The other extreme case is when MS 1  is at 75. Then SB 2  arrives at T 0 +RTD+(d 12 − 2 r)/c. Thus, when the mobile is between 75 and 74, SB 2  arrives in the window [T 0 +RTD+(d 12 − 2 r)/c−k, T 0 +RTD+d 12 /c+k], where k accounts for inaccuracies in the provided RTD and d 12  values. 
     Since RTD is known, MS 1  can predict with a certain uncertainty when the SB 2  from BTS 2  (non serving) will arrive. 
     The ability to calculate a search window permits the synchronization burst to be detected with higher reliability compared to when the arrival time is completely unknown, and the complexity of the mobile station is reduced compared to prior art mobile stations. For example, data from the whole search window can be received in real time and stored for later processing, which is not realistically feasible if the search window is required to be 10 TDMA frames long, as is necessary to guarantee capturing the synchronization burst using conventional techniques. In addition, the search window permits the total measurement time to be reduced. 
     Use of the RTD knowledge to calculate the starting time and search window for the synchronization burst SB can significantly reduce the measurement time in making downlink OTD measurements. Without receiving the RTD information, the mobile station is conventionally required to search continuously until the frequency correction burst is detected, so that the mobile station knows the synchronization burst will occur in the next frame. With RTD information corresponding to all base transceiver stations to be measured, the mobile station can schedule the various measurements and limit the monitoring times to the search window periods, which is not possible using the prior art scanning techniques. 
     FIG. 6 illustrates an example implementation of a pertinent portion of the mobile station MS 1  of FIG. 2 for making downlink observed time difference measurements according to the present invention. The mobile station includes a synchronization burst determiner  61  which receives as input (for example from MLC of FIG. 2 via MSC, BSC  21  and BTS  23 ) the frequency, the BSIC, and the RTD relative to the serving base transceiver station, of each base transceiver station selected for OTD measurements. The synchronization burst determiner also receives information about the distances between its serving base transceiver station and all neighboring base transceiver stations, along with cell radius information for all neighboring base transceiver stations. This information can be periodically updated by MLC (as MS 1  roams), and stored into a memory as shown at 63 in FIG.  6 , or the information can be included in the positioning request message provided to the synchronization burst determiner by MLC. 
     The synchronization burst determiner  61  determines for each selected BTS the approximate expected arrival time of the synchronization burst relative to the frame structure time base  60  of the serving cell (serving base transceiver station), and outputs this information at 64 to a time of arrival monitor  65 . Also at 64, the synchronization burst determiner outputs to the time of arrival monitor the 78 encrypted bits and the 64 training bits of the synchronization burst of each selected BTS. The synchronization burst determiner also calculates search windows for each selected base transceiver station, and outputs this search window information at 62 to the time of arrival monitor. 
     The time of arrival monitor makes time of arrival measurements on the signals received from the BTSs at 68. The time of arrival monitor can use the calculated arrival time information, the window information and the 142 bit sequence information to make time of arrival measurements for each selected base transceiver station. With this information, the time of arrival monitor can efficiently schedule the various measurements and, as necessary, can capture and store the signals received during the various search windows, and then process those signals at a later time. The processing of the received signals for determination of time of arrival can be done in any desired conventional manner, or in the manners described in detail in copending U.S. Ser. No. 08/978,960 filed on Nov. 26, 1997, which is hereby incorporated herein by reference. 
     After the desired time of arrival measurements have been made, the time of arrival monitor can output at  66  either the time of arrival information or the observed time difference information to the MLC (via BTS  23 , BSC  21  and MSC). The MLC then uses this information in conventional fashion to determine the location of the mobile station MS 1 , which location is then provided in a suitable message to the requesting application  21  in FIG.  2 . Alternatively, if MS 1  knows the geographic locations of the measured BTSs, then MS 1  can calculate its own position. 
     It will be evident to workers in the art that the exemplary mobile station portions of FIG. 6 can be readily implemented by suitably modifying hardware, software, or both, in a data processing portion of a conventional mobile station. 
     Although OTD measurements on the GSM synchronization burst are described in detail above, it should be clear that the techniques of the invention are applicable to various other types of bursts as well. 
     In view of the foregoing description, it should be clear that the downlink observed time difference techniques of the present invention improve the sensitivity of downlink observed time difference measurements by providing the mobile station with more known bits from the synchronization burst SB, enhances the accuracy of the time of arrival and observed time difference measurements, reduces the risk of measurement errors, reduces the time required to make the necessary measurements, and requires less data processing capability in the mobile station. 
     Although exemplary embodiments of the present invention have been described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.