MXPA00007813A - Method and system for facilitating timing of base stations in an asynchronous cdma mobile communications system - Google Patents

Abstract

A method and system are disclosed for facilitating the timing (e.g., the known relative timing differences) of base stations (Bss) (BS1, BS2, and BS3) in asynchronous CDMA mobile communications systems (200). A plurality of mobile stations (MSs) (MS1, MS2, and MS3) measure the relative time differences between various pairs of BSs, and these measurements are stored by the BSs. A source BS sends (106) to an MS, in a neighbor list message, estimates of the relative time difference between the source BS and each of the BSs on the neighboring cell list. Each BS on the list can maintain a relative time difference estimate table, which can be updated continuously from the reports received from MSs. Subsequently, the BSs can send entries from this table to the MS in the neighbor list message. Using this novel technique, the BSs have known relative timing differences. Consequently, when the MS initiates a cell-search for a candidate BS, the MS already has an estimate of the timing of that BS as compared to its source BS. As such, the resulting cell-search procedure has a lower level of complexity and thus can be accomplished much quicker than with prior procedures. In addition, the relative time difference estimates can be compared with corresponding time differences that are measured by a second mobile station. Based on this comparison, the propagation delays of signals between the second MS and various BSs can be calculated to determine the position of the second MS.

Description

METHOD AND SYSTEM TO FACILITATE THE SINCROTS1 ZATION OF BASE STATIONS IN A MOBILE COMMUNICATIONS SYSTEM ASYNCHRONOUS CDMA RELATED APPLICATION This application claims the benefit of the filing date of the provisional application North American No. 60 / 074,494, filed on February 12, 1998. BACKGROUND OF THE INVENTION TECHNICAL FIELD OF THE INVENTION The present invention relates to in general, to the field of mobile communications, and, particularly, to a method and system for facilitating the synchronization of base stations in an asynchronous Code Division Multiple Access (CDMA) mobile communication system. DESCRIPTION OF THE RELATED ART Direct sequence CDMA mobile communication systems (DS-CDMA) can be either synchronous intercell systems or asynchronous intercell systems. In other words, the base stations (BSs) in a synchronous intercellular system are precisely synchronized with each other and the base stations in an asynchronous intercellular system are not. More specifically, asynchronous base stations do not share a common time reference, and their transmissions, therefore, have arbitrary non-predetermined portions in relation to each other. An example of a synchronous intercellular system is the North American IS-95 System. Examples of asynchronous intercell systems are the broadband CDMA systems (WCDMA) proposed in the technical specifications CODIT, ETSI, SMG2 alpha group, and ARIB. The main disadvantage of synchronous intercell systems is that the base stations must be synchronized very accurately (up to the μs level). This high level of accuracy is typically offered through the use of very precise time references or localized by base stations such as, for example, Global Positioning System (GPS) receivers. However, due to the nature of line of sight of satellite signal propagation, the use of such localized references is probably not feasible for base stations located below the ground in buildings or tunnels. Another related disadvantage is that the GPS system is controlled by a government agency. Accordingly, the use of GPS receivers for synchronization with base station networks may be undesirable in certain national regions. These disadvantages are the main reasons why asynchronous intercell systems are currently considered. For asynchronous intercell systems to work correctly, there are two essential functional issues that must be resolved: (1) soft transfers (SOHO); and (2) cell searches. In a SOHO state, a mobile station (MS) is in communication with more than one base station at the same time. To facilitate the SOHOs, the mobile station constantly explores to determine the presence of other base stations in the vicinity. The mobile station can therefore monitor the quality of the received signal from the various base stations and determine the time delay of the base stations. For a SOHO to occur, the mobile station transferred must be able to receive the "white" base station signal at approximately the same time as the "source" base station signal. They are intended to minimize buffer requirements (ie say, a smaller time difference between BS signals requires smaller buffer area than larger time difference). Likewise, the white base station must be able to find the mobile station signal without an excessive expenditure of processing resources. These SOHO issues are solved in the case of asynchronous systems through a synchronization technique "per call", which is disclosed in "A Design Study for a CDMA-Based Third-Generation Mobile Radio System" (A design study for a third-generation mobile radio system based on C MA), by A- Baier et al, IEEE JSAC, volume 12, pages 733-743, May 1994. Using this technique, the mobile station involved in the SOHO calculates and reports to the network the time difference between the white base station and the base station source. The network notifies the white base station through the base station controller (BSC) or radio network controller (RNC) regarding the time difference. The white base station can then adjust its reception and transmission timing for the signal provided for the mobile station involved, in order to compensate for the difference. A similar known SOHO technique is employed in which the mobile station reports the timing difference between the white base station transmission and its own transmission, rather than the difference between the white base station transmission and the station transmission of source base. However, since the mobile station transmission / reception timing ratio is always fixed, the two SOHO techniques described above are essentially equivalent. These techniques are known as a mobile assisted transfer (MAHO). In other words, the mobile station assigns the white base station in compensation for the timing difference between the white base station and the source base station. A cell search generally refers to a method by which a mobile station achieves a lacquer, segment and frame synchronization with a base station, and detects the downlink coding code of the base station. This procedure is used both during the connection (initial synchronization) and continues later during the inactive or active modes while the mobile station is searching for candidate base stations for SOHO. In a synchronous system, the cell search can be carried out efficiently (ie with a relatively low level of complexity) since the same coding code can be used for all base stations. Accordingly, the mobile station can carry out the complete search of the base stations using only a single balanced filter (or similar functionality). However, this same technique can not be easily employed in an asynchronous system due to the different coding codes employed by different base stations. Accordingly, the need arises for a fast cell search procedure, of low complexity for asynchronous CDMA systems. A rapid, multi-step cell search procedure for asynchronous CDMA systems has been proposed, where each base station transmits an unmodulated symbol. This transmitted symbol is extended by a globally known short code, without a coding code., In each segment of each frame. In a proposal of this type, this symbol is indicated as a "Perch I Long Code Masked Symbol (LCMS)" (masked symbol of long code Perch 1). In a second proposal, this symbol is indicated as a "Primary Synchronization Channel" or primary (SCH). With the proposed multistep method, a mobile station can thus find the flake and segment synchronization of a base station, employing a unique balanced filter corresponding to the primary synchronization channel. Subsequently, the mobile station has to find the downlink encoding code and frame synchronization of the base station (which encompasses a frame in the proposed multi-step procedure). The mobile station can find the base station frame synchronization by detecting a second symbol transmitted in a regular manner, indicated as a "Perch 2 LCMS" or "Secondary SCH". This second symbol is transmitted in parallel with the first symbol, but the second symbol is extended by a second short code (again without coding code). The second symbol can have a unique repetitive modulation pattern per frame, and by detecting this pattern, the mobile station can determine the frame synchronization of the base station. The extension code used for the second symbol indicates to the mobile station to which group of possible coding codes a coding code actually used belongs. The mobile station can then find the coding code used by correlating with the coding codes belonging to the indicated group, in the frame synchronization identified above (or in different possible frame synchronizations). However, a problem with the proposed multi-step procedure is that the level of complexity of the cell search is relatively important, especially in the case of a candidate search for SOHO (which the mobile station has to perform regularly) . Another problem with intercell asynchronous systems is that the synchronization difference between the base stations makes it difficult to determine the position of the mobile stations. Mobile communications systems capable of determining the position of mobile stations in the system are increasingly desirable. Nowadays, the location of a mobile phone is usually carried out through the use of external systems such as a GPS system. Preferably, however, the location of mobiles must be carried out through the cellular system itself without the need for external systems of this type. To carry out said cellular location, a method is required to accurately determine the absolute or relative distances between a mobile station and each of several different base stations. The distances can be calculated using measurements of propagation time, arrival time (TO), or difference of arrival time (TDOA) in the signals transmitted between the mobile stations and each of the several different base stations. Once these measurements are available, there are numerous algorithms for calculating the geographical location of the mobile station. For example, in accordance with the TOA method, the distance from a mobile station to each of the base stations is obtained using TOA measurements. Each of these distances can be conceptualized as the radius of a circle with the respective base station in the center. In other words, the TOA measurement can be used to determine the radial distance of the mobile station from a particular base station, but the direction _ can not be determined based on a single TOA measurement; therefore, the mobile station can be anywhere in the circle defined by the calculated radius. By determining the intersection of the circles associated with each of the several different base stations, however, the position of the mobile station can be determined. The TDOA method, on the other hand, uses the difference in TOA between two base stations to determine a TDOA between these two base stations. The position of the mobile station can then be estimated along a curve, i.e. a hyperbola, in accordance with the calculation of TDOA. By using three or more base stations, more than a curve of this type can be obtained. The intersection of these curves gives the approximate position of the mobile station. In the simplest mobile location technique, SOHO is made to several base stations. During each of these transfers, the propagation time between each base station and the mobile station can be measured. The location of the mobile station can then be determined by triangulating the position of the mobile. This method of location is the simplest to implement since it includes very little change in terms of mobile radio design. In addition, the base stations do not require an absolute time reference; that is, this method can be employed in an asynchronous cellular system. However, due to the geographical separation between the base stations, the transfer to two other base stations located geographically is possible only in a small number of cases. In other words, when the mobile station is near a base station, it is often not possible to perform a SOHO with other base stations. This is because the "ability to hear" the signals between the mobile station and several base stations is usually unsatisfactory. Another possible solution is the use of a set of antennas in the base station. When the base station has a set of antennas, the position of the mobile station can be calculated by estimating the direction from which the uplink signals propagate and by measuring the round trip delay of the signal from the mobile station. communications. In this method, the mobile station only requires to be in communication with a base station to calculate the position. However, the widespread use of antenna arrays for location purposes is a costly method. In addition, the effects of multipath propagation characteristics of uplink and downlink signals frequently make a set of antennas undesirable, especially in cities where signals are frequently reflected in buildings and other structures. As mentioned above, it is also possible that a Global Location System can be incorporated into the mobile without the use of an additional radio receiver. This method, however, results from an excessive computation and receiver complexity in the mobile station. Another solution is the measurement of the propagation time, TOA, or TDOA of signals transmitted by the base stations to the mobile station or by the mobile station to the base stations. For example, a downlink solution can be employed there, in the case of CDMA, the mobile station measures the TOA of a pilot channel data transmitted by several different base stations. Alternatively, an uplink solution may be employed where several base stations each measure the TOA of a signal transmitted by the mobile station to the base stations. However, both methods require a precise absolute or relative time reference in the base stations or synchronization of the base stations. Therefore, downlink and uplink solutions usually require additional equipment (for example, GPS receiver located at base stations to obtain synchronization of base stations) in an asynchronous network. A system and method is required to reduce the complexity of the processing resources used during cell search and mobile location processes in asynchronous networks. In particular, it would be advantageous to employ as much a priori search information as possible to help reduce the level of complexity and increase the search speed for cell searches and to allow simplified mobile location solutions. In accordance with what is described in detail below, the present invention successfully solves the problems described above. COMPENDIUM OF THE INVENTION _ A method and system for facilitating the synchronization of base stations in asynchronous CDMA mobile communication systems, where a source BSC (or RNC) is sent to a mobile station (eg, in a message) is provided. neighbor cell list) Estimates of the Relative Time Difference (RTD) between the source base station and each of the base stations in the list of neighboring cells. For SOHO purposes, several mobile stations can report to the network the estimated relative time differences with the signal quality information for the neighboring base stations. Each base station can maintain a table of RTD estimates, which can be updated continuously from RTD reports received from mobile stations. Subsequently, the base stations can send inputs from the RTD estimation table to the mobile station in the message of neighboring cell lists, together with corresponding coding codes. Using this novel technique, the base stations have known relative synchronization differences. Accordingly, when the mobile station initiates a cell search for a potential target base station, the mobile station already has an estimate of the synchronization of this base station compared to its source base station. Thus, the resulting cell search procedure employed in an asynchronous CDMA system has a lower level of complexity and therefore can be achieved more quickly than with previous methods. In another aspect of the invention, the accuracy of the estimated relative time differences can be greatly improved by taking into account the propagation delays between the mobile station and the base stations that are used to estimate the RTD. These improved RTDs can be used to further improve the synchronization estimates to carry out cell searches. The improved RTDs can also be used to calculate the position of mobile stations in the mobile communication system. Once known RTDs with high accuracy, distances can easily be determined between a mobile station and several base stations by using the propagation times TOAs or All of signals traveling between the mobile station and several base stations. An important technical advantage of the present invention is that neighboring base stations in an asynchronous CDMA-type mobile communication system have known relative synchronization differences. Another important technical advantage of the present invention is that the computer and programmatic complexity of the mobile stations in an asynchronous CDMA mobile communication system is reduced. Another important technical advantage of the present invention is that the overall level of complexity of the cell search procedure in an asynchronous CDMA mobile communication system is significantly reduced. Another important technical advantage of the present invention is that the speed of cell searches performed on asynchronous CDMA mobile communication systems increases from significantly compared to previous procedures. Another important technical advantage of the present invention is that the location of mobiles can be determined in an asynchronous mobile communication system by performing simple calculations on easily obtainable data and without the need for an external system. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention can be obtained with reference to the following detailed description when taken in conjunction with the accompanying drawings in which: Figure 1 is a flow diagram illustrating a exemplary method that can be employed to facilitate the synchronization of base stations in an asynchronous CDMA mobile communication system in accordance with a preferred embodiment of the present invention; Figure 2 is a simplified block schematic diagram of an exemplary mobile communication system that can be employed to implement the method illustrated in Figure 1, in accordance with the preferred embodiment of the present invention; Figure 3 is a simplified block schematic diagram of a mobile station that is in a SOHO or that is about to enter a SOHO and that can be used to facilitate improved synchronization calculations of base stations in a wireless system. asynchronous CDMA mobile communication, in accordance with a preferred embodiment of the present invention; Figure 4 is a diagram of the relative timing of signals involved in a SOHO scenario illustrated in Figure 3; Fig. 5 is a flow diagram illustrating an exemplary method that can be employed to determine the position of a mobile station in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS The preferred embodiment of the present invention and its advantages are better understood with reference to Figures 1-5 of the drawings, similar numbers being used for similar and corresponding parts of the various drawings.

Essentially, in the asynchronous CDMA system, a base station controller "knows" the downlink coding codes for all its base stations. Typically, a list of neighboring stations is broadcast in each cell (for mobile stations operating in the idle mode) or transmitted in a dedicated control channel (in the case of mobile stations operating in the active mode). When a mobile station receives the information from neighboring cells, they determine the coding codes of the listed neighboring cells which are candidate cells for potential SOHOs. Having said a priori knowledge of this coding code information for the cells for SOHO candidates allows the mobile station to reduce the total SOHO cell search time (or the level of complexity), since the number of coding codes possible are reduced compared to the number for initial synchronization (connection). Furthermore, if the set of coding codes to be searched by the mobile station is relatively pegueño, the mobile station does not know the synchronization of these codes. This lack of synchronization information is the main reason why current proposals for a cell search in asynchronous system require more time (and are more complex) than a cell search in an asynchronous system. The present invention solves this problem of lack of synchronization information to the extent that the source base station sends to the mobile station (together with the list of neighboring cells) an estimated RTD between the source base station and that each one of the base stations in the list of neighboring cells. In other words, instead of sending only the coding codes of the neighboring stations to the mobile station, the source base station also transmits each of its estimated RTDs. For SOHO purposes the mobile stations can report (on a regular basis, triggered by some event or at the request of the BSC) the estimated RTD network together with the signal cavity information (eg, signal strength, signal to signal ratio). interference or SIR, etc.) for the neighboring base stations. Accordingly, each BSC can maintain a table of RTD estimates that can be updated continuously from RTD reports received from MSs. In one embodiment of the present invention, the RTD estimation table is maintained in a database in the BSC. Subsequently, the base station controllers (BSC) can send inputs from this RTD estimation table to the mobile station in the neighboring cell list message, together with corresponding coding codes (with the base station controller following the estimated RTD information that has already sent the previous message to the mobile station). Using this novel technique, the base stations have known relative synchronization differences. Accordingly, in an exemplary embodiment, when a mobile station begins searching for a potential objective base station, the mobile station already has an estimate of the synchronization of this base station (i.e., from the RTD information) in comparison with your BST source. As such, the resulting cell search procedure that is employed in an asynchronous CDMA system can be accomplished much more quickly than in the case of prior art methods. When the mobile station has synchronized with the potential target base station, the mobile station has an improved estimate of the RTD, which in turn, the mobile station can report back to the source base station (preferably together with quality information for the potential target base station). The source base station (or its associated base station controller) can even update this entry in the RTD estimation table. More specifically, Fig. 1 is a flow chart illustrating an exemplary method 100 that can be employed to facilitate base station synchronization and that can increase the search speed of transfer candidate cells in an asynchronous CDMA type mobile communication system, in accordance with a preferred embodiment of the present invention. In step 104 of the exemplary method illustrated in FIG. 1 a base station controller prepares a list of neighboring cells (ie, a "set of neighboring cells" in an IS-95 system) with respective coding codes, together with several RTD estimates between a source base station and the respective transfer candidate base stations from a table of RTD estimates (preferably maintained in a database in the base station controller). In step 106, the source base station broadcasts or transmits the list of neighboring cells with RTD coding and estimation codes in a "neighbor cell list message" to the mobile station involved. In fact, the BSC follows the estimated RTDs it sends to the mobile station in order not to unnecessarily duplicate the RTD information that the mobile station may already have. At this point, the mobile station has now received a list of base stations with which it can be synchronized (and also a quality information report). The received neighbor cell list message may also include an uncertainty estimate (in accordance with what is described in more detail below). The mobile station stores the list information of neighboring cells in a local memory. In step 108, with the a priori neighboring cell RTD estimation (synchronization) information already available, along with other corresponding neighbor cell information, the mobile station can initiate a primary cell search using a conventional balanced array of filters. The use by the mobile filter stations of balanced search of primary cells produces peaks of signals corresponding to the base stations that the mobile station can receive with sufficient quality to qualify as candidate cells for transfer. In step 110, the mobile station in relation to the RTD estimates with the balanced filter signal peaks produced to determine which peaks will most likely correspond to which codes of coding in the list of neighboring cells (step 112). In step 114, based on the correlations produced in step 112, the mobile station can select the coding codes for the candidate cells for most likely transfer among the list of neighboring cells. The mobile station can then start the cell search (step 116). Theoretically, if the RTD estimates described above are fully accurate, then the mobile station could discard (in advance) all peaks of balanced filter output signals that do not correspond to the "estimate" RTD information. In this hypothetical situation, the coding code correlation procedure (e.g., step 112) could be omitted. However, in any way, according to the present invention, the use by the mobile stations of the RTD estimates to determine the most likely candidate cells for transfer from the list of neighboring cells allows the mobile station not to take into account a significant number of the types of balanced filters, and / or associate some of these peaks with corresponding coding codes, which significantly reduces the complexity of the cell search procedure and which substantially increases the speed of the search. By using the method of the present invention described above, each base station (cell) with the help of the mobile stations connected there, has a known relative synchronization difference in relation to its neighboring base stations (cells). If, for some reason, no mobile station is connected to a particular base station, the RTD estimation table corresponding to this base station is not updated. Accordingly, since the relative synchronization between the neighboring base stations can be continuously shifted, the uncertainty (or variation) of the table entries of RTD estimates for this base station will increase. In general, the uncertainty of the RTD estimate may increase over time, but this uncertainty is typically minimal immediately after an update (eg, based on an RTD report received from a mobile station). Therefore, for the communication system to be more robust during periods of inactivity of mobile stations (for example, during the night, or during holidays in private internal systems), as mentioned above, an estimate of RTD uncertainty can be issued. or transmitted from a base station together with the RTD estimate, in the neighbor cell list message.

The base station can then, for example, establish (for example, increase), its time search window correspondingly to allow the additional level of uncertainty. The mobile station can thus manage these base stations that have relatively uncertain knowledge of their RTDs, and also minimize their level of complexity when relatively safe RTD estimates were provided. An additional method to further mitigate the uncertainty problem that occurs when there is too low a number of active mobile stations for relatively long periods is to place "dummy" mobile stations in fixed locations throughout the system. These "dummy" mobile stations may have limited functionality, and base stations that have RTD estimates table entries of relatively large uncertainty may use them to provide more current RTD updates. Such "fictitious" mobile stations can therefore be advantageously located where numerous base stations (e.g., near the edges of cells) can reach them. Figure 2 is a simplified schematic block diagram of an exemplary mobile communications system 200 that can be used to implement method 100 (Figure 1) to facilitate synchronization (eg, known relative timing differences) of base stations and to increase the cell search speed, in accordance with the preferred embodiment of the present invention. The system 200 is preferably an asynchronous CDMA mobile communication system which includes, for illustrative purposes, three base stations and three mobile stations. However, it will be understood that the number of base stations and mobile stations shown is for illustrative purposes only and that a typical system may include more than three base stations and three mobile stations. In this example, MSI operates in the active mode and is connected through an air interface link 202 to BS1. In accordance with step 106 of method 100 (FIG. 1), MSI has received a neighbor cell list message that preferably includes respective RTD estimates and, optionally, associated uncertainty estimates on a dedicated control channel from the BSC 203 (through BSl once "connected" to MSI). At least two of the neighboring cells that appear in the list as entries in the table of RTD estimates are BS2 and BS3. On a periodic basis (or on request), MSI monitors and reports the quality (signal strength, SIR, signal-to-noise ratio, or SNR, bit error frequency, or BER, etc.) of these base stations to base station controller 204 (through BSl). Since MSI has received estimates of RTD from BSC 204 (through BSl), MSI can synchronize relatively quickly with BS2 and BS3, at least during the first opportunity when MSI searches for BS2 and BS3. When MSI has been synchronized with BS2 (or BS3), it can be considered that MSI has a "good" RTD estimate for this base station. Periodically, or upon request, MSI can report the quality of the estimated signal from at least one of the entries in the list of cells neighboring BSC 204 (through BSl). In addition to quality estimates, MSI can also report the current estimate of RTD to BSC 204.

The cell search situation for MS2 is similar to the cell search situation for MSI, except that in the example shown, MS2 participates in a SOHO with both BSl and BS2, and monitors only another BS (for example, BS3 a through air interface link 214). For this example, MS3 is operating in the idle mode (it has no established connection), but it can monitor the base stations in accordance with the neighbor cell list received in the base station sender channel that MS3 considered the "best" " (for example, in this case, BS3 via air interface link 218). As such, MS3 can also monitor BSl (via air interface link 208) and BS2 (through an air interface link 212). Again, the RTD estimates issued by BS3 help MS3 to synchronize more quickly with BSl and BS2, or at least the first time the synchronization procedure occurs. In this way, the complexity of cell searches is reduced, and the speed of cell searches is significantly improved. Preferably, each mobile station operating in the mobile communication system 200 will transmit its measured RTD estimate on a periodic basis, or on request, to the base station controller 204 (via BS1). The BSC 204 stores the RTD estimates received from the MS in a table of RTD estimates. Alternatively, each entry stored in the RTD estimate table (ie, representing an estimated difference between a pair of base stations) can be calculated based on estimates received from several different mobile stations. For example, the stored estimate can be an average of the previous x estimates received, or of the estimates received in the preceding minutes and minutes. The values in the table can be updated by replacing previous estimates or by re-calculating particular estimates based on newly received data. The values stored in the table are then sent to other mobile stations, in accordance with what is described above, together with the list of neighboring cells, to help synchronize these mobile stations with neighboring base stations, according to the need. In addition, experts in the field will note that the table of RTD estimates does not have to be stored in the BSC 204; the table can be stored in one or more databases located virtually anywhere in the network (for example, in a register associated with the MSC or in a completely separate database). In another aspect of the invention, the RTD estimates can be used to determine the position of the mobile station. Location calculations, however, require more accurate RTD estimates than in the case of cell searches. This is due to the fact that the concept of location of mobile is essentially based on a determination of the propagation delay between the mobile station and each of several base stations or in TOA or TDOA measurements between the various base stations. In most cases, the speed of cell search can be significantly improved without having to take into account propagation delays. Thus, it is usually sufficient to base estimates of RTD in the time difference between two BS in accordance with measured by one or more mobile without considering the effect of propagation delays of signals down link received by a mobile station from of each of the base stations. To carry out the location of the mobiles, on the other hand, a more accurate estimation of the RTD is required. The present invention solves this problem by calculating an improved RTD which takes into account the propagation delays of uplink and downlink signals. Essentially, the enhanced RTD is the difference between the time at which a first base station begins to transmit its downlink signal and the time at which a second base station begins to transmit its downlink signal. This improved RTD estimate can be calculated using: (1) receive times and local transmission of uplink signals and downlink base stations of interest, in accordance with measured in each of the respective base stations , and (2) the TOA difference in the mobile station of the downlink signals coming from the base stations, in accordance with that reported by the mobile. This enhanced RTD information can then be used by other mobiles for location purposes. In a preferred embodiment, the improved RTD estimates are stored in a database table in the BSC or in the MSC. Subsequently, a position determination is desired for a second mobile station (on a regular basis, triggered by some event, or at the request of the BSC or MS). The second mobile station measures the time difference between the base stations based on the reception time in the second mobile station of the downlink signal from each base station and reports the time differences measured to the BSC. The BSC then compares the improved RTD estimate between a particular pair of base stations with the time difference measured between the same pair of base stations in accordance with that reported by the mobile station. Based on this comparison, the propagation delays between each of the various base stations and the mobile station can be calculated, and an accurate determination of the location of the mobile stations can be made. Again, TOA or TDOA measurements can be used to determine the location. Regardless of which location method is used, however, location calculations are essentially based on the existence of propagation delays in the mobile environment. Generally, the determination of each RTD estimate by a mobile station includes only two base stations, even if a three-part SOHO may occur (ie, a SOHO in which three different base stations participate). By repeating the determination of RTD during several different SOHO procedures, an estimated estimated RTD can be determined between a substantial number of possible pairs of base stations. The improved RTD estimates are then normally employed by other mobile stations (i.e., mobile stations that did not participate in the estimated RTD calculations) to determine the position of these other mobile stations. It will be noted, however, that the position of a mobile station that participated in the estimated RTD calculations can also be determined using the improved RTD estimates. In any case, the location procedure preferably employs the largest possible number of base stations in order to improve the accuracy of the estimated location. Referring now to Figure 3, there is shown a schematic illustration of a mobile station that is in a SOHO or that is about to initiate a SOHO. A first base station BS1 transmits a frame of a downlink signal 302 (either a pilot box or a traffic data frame) at a time 1x1, as measured in the time base of the first base station BS1. . An uplink signal 304 from the mobile station MSI is received by the first base station BS1 at a time Tri, also measured in the time base of the first base station BS1. Similarly, a second base station BS2 transmits a downlink signal 306 at a time Tl2 and receives the uplink signal 304 from the mobile station MSI at a time Tr2, in accordance with the measured time base of the second base station BS2. Generally, the time base of the two base stations in an asynchronous network will have a relative time difference (RTD)? In other words, if an event (for example the transmission of a pilot frame) occurs at a time Ti at the first base station BS1, a corresponding event will occur at a time T2 = Ti +? (1) in the second base station BS2. Once known RTD?, It can be used by other mobile stations for mobile location. In addition, each downlink signal has a shift ti relative to the transmission time of the pilot channel frame. Thus, the traffic channel data from the first base station BSl is transmitted at a time where Tp? is the transmission time of the pilot channel frame from the first base station BSl. Similarly, the traffic channel data of the downlink signal from BS2 is transmitted in time Tr2 = Tp2 + t2 (3) When the SOHO is started, the mobile station MSI simply listens to the pilot and t2 = 0. Subsequently, when the mobile station MSI is in the SOHO, the second base station BS2 will transmit data and the displacement t2 of the signals will be adjusted from such that the data from the first base station BS1 and the second base station BS2 arrive at the mobile approximately at the same time. In the following commentary, a generic scenario can be considered in which it is considered that the displacements ti and t2 are known. This scenario covers both the SOHO initialization cases and in the case of an already established SOHO. Referring now to Figure 4, a relative timing diagram of the various SOHO signals that are transmitted and received in the system of Figure 3 is illustrated. All times in the figure are illustrated in a common arbitrary time base. For the purposes of making RTD calculations in accordance with the present invention, however, the time of each event is reported in the local time base of the station (i.e., the mobile station or the base station) associated with this event. At time t, according to what is measured in the time base of the first base station BS1, a pilot frame or traffic frame 402 is transmitted by the first base station BS1. The frame 402 is received in the mobile station MSI at a time Tmr ?, which is measured in the time base of the mobile station MSI. The time Tmr? it is delayed after the transmission time Tu by a propagation delay time taui, which is the time required for the signal to travel from the first base station BSl to the mobile station MSI and vice versa. The mobile station MSI transmits its uplink signal 404 in the time mi-For simplicity and without losing the capacity of general application, it can be considered that the mobile station MSI transmits its uplink signal 404 at the same time as it receives the signal from downlink 402 of the first base station BSl. Thus, Tmr = mri, and (4) the uplink signal 404 is received by the first base station BSl at time Tri = ti + 2t ?. (5) The uplink signal 404 from the mobile station MSI is received at the second base station BS2 at a time Tr2 which is delayed after the transmission time Tm? for a propagation delay time tau2. The second base station BS2 also transmits a pilot or traffic frame 406 at a time Tt2. After the propagation delay time t2, the downlink signal 404 is received by the mobile station MSI at time Tmr2. To calculate the? of RTD, the mobile station MSI reports the time difference td? ff between the reception time Tmr2 of the downlink signal 406 from the second base station BS2 and the transmission time Tmr of the uplink signal 404 coming from of the MSI mobile station. Therefore Txff = Tmr - Tmr2. (6) It will be noted that in Figure 4, the time difference tdiff is relatively large, which is a typical indication of an initial acquisition scenario. Using the above designations, we can then formulate the following expression for the reception time Tr2 of the uplink signal in the second base station BS2: Tr2 = 2t2 + tdiff + Tr2. (7) Finally, we can formulate the following expression for tdiffL tdiff = Tn - Tr2 + ti - t2 +?, (8) which is obtained by subtracting the arrival time Tmr2 from the downlink signal 406 coming from the second base station BS2 (either the pilot frame in the SOHO initialization or the traffic data during a SOHO) of the transmission time Tmi of the uplink signal 404 from the mobile station MSI, all measured in the time base of the second base station BS2. Thus, as will be understood by people with a normal knowledge of the technique, in the time base of the second base station: Tmr2 = Tr2 - t2, and (9) Tmt = Tri +? + you (10) Now there are three equations: (5), (7), and (8) and three unknowns: 1) the propagation delay time Ti between the mobile station MSI and the first base station BS1; 2) the propagation delay time x2 between the mobile station MSI and the second base station BS2; and 3) the time difference? between the first base station BSl and the second base station BS2. Is it easy to solve? to get ? =% (tdiff - Tn - Tn + Ti2 + T ^), (11) what provides a solution for? desired RTD between base stations BSl & BS2. According to a preferred embodiment of the invention, the base station MSI reports the tdiff time difference and each of the base stations BSl and BS2 reports their respective times of transmission and reception to the network. The calculation of? RTD is then performed in the BSC or in the MSC. In the alternative, the calculation can be carried out in the mobile station MSI or in a base station provided by necessary synchronization data. By calculating improved RTD estimates between several pairs of base stations in an asynchronous mobile communication system, an uplink solution or downlink solution can be employed to determine the position of mobile stations in the system without the need for an absolute time reference. For example, Figure 5 is a flowchart illustrating a possible method 500 for facilitating the synchronization of base stations and the determination of the position of a mobile station in an asynchronous CDMA mobile communication system, in accordance with a mode of the present invention. As will be appreciated by those of ordinary skill in the art, numerous other location methods such as TOA or TDOA can also be employed in relation to the improved RTD estimation to facilitate location in accordance with the invention. In step 504, a BSC calculates several improved RTD estimates between several pairs of base stations controlled by the BSC or listed in the list of neighboring cells. This calculation is made by using data provided by other mobile stations in the mobile communication system. Therefore, the effect of propagation delays is taken into account to calculate very accurate RTD estimates. Preferably, a table of these improved RTD estimates is maintained in a database in the BSC. In step 506, the selected mobile station monitors the base stations in neighboring cells. For purposes of the present location method 500, this includes monitoring, for example, of a known sequence that periodically transmitted by the base stations. This monitoring procedure may include the regular monitoring of base stations for potential candidates for transfer. It will be noted that the monitoring of a known sequence from a base station can be carried out usually even in cases where a limited "listening capacity" prevents a SOHO with this base station. In step 508, the mobile station measures the TOA of downlink signals transmitted by several different base stations, each TOA measurement can be measured in the time base of the mobile station or as a relative value for the base station of source or another base station. The TOA measurements are temporarily stored in a local memory together with information identifying the base station corresponding to each TOA measurement. These data are then sent to the BSC for further processing. The measurements of step 508 may be made in the pilot channel data or the traffic channel data. Since the base stations generally "know" the displacements ti, the time difference between the base stations (ie, the time difference between the pilot frame transmissions of the base stations) is known even when a traffic channel. In step 510, the BSC adjusts the TOA measurements to take into account the RTDs between the various base stations by adding the RTD estimates to the TOA measurements. In step 512, a propagation delay time is calculated for each downlink signal using the adjusted TOA measurements, and the location of the mobile station is estimated in step 514 using the calculated propagation delay times. The positioning information can then, for example, be transmitted to the mobile station, stored in the BSC, or sent to the Home Location Registry (HLR). In an alternative embodiment, the calculations of steps 510, 512 and 514 can also be processed at the mobile station, the mobile station controller or else at another location in the network. The method 500 illustrated in Figure 5 provides location estimates based on measurements made in the TOA mobile station of a downlink signal. In another alternative mode, the location of the mobile is determined using an uplink signal. The uplink solution is essentially the same as the downlink solution, except that, instead of measuring the TOA of downlink signals in step 508, TOA measurements are made on multiple base stations in a link signal ascending transmitted by the mobile station. These uplink signal measurements are then provided to the BSC or the MSC, and adjusted TOA measurements and propagation delay times are calculated, as in step 510 and step 512 of the downlink solution method 500. As discussed above in relation to the standard RTD estimates, the uncertainty regarding the improved RTD estimates also increases with the passage of time if the RTD estimation table for a particular base station is not updated. For location purposes, however, the accuracy of the RTD estimates is much greater than in the context of cell searches. Thus, the improved RTD estimates obtained during a SOHO should be recent enough in such a way that the clocks at the base stations do not show divergences between them. Otherwise, it will be difficult if not impossible to carry out precise determinations of mobile position. Many of the same methods described above for solving the uncertainty problem the standard RTD estimation context can be similarly used to solve the uncertainty in the context of improved RTD estimation. The method for obtaining an improved RTD estimate can also be used to further improve the cell search process described in relation to figures 1 and 2. In a preferred embodiment of the cell search method 100 (see figure 1), the difference The time reported by the mobile station in SOHO is directly used to calculate the RTD estimates; no information is required from the base station. Thus, with reference again to Figure 4, the standard RTD estimate is equal to a MS reception time Tmr? of a downlink signal from a first base station BS1, subtracted from the MS reception time Tmr2 of a downlink signal from a second base station BS2. In Figure 4, this value is illustrated by the time differential tdiff. The use of direct time difference in accordance with that reported by the mobile station offers a significant improvement compared to previous cell search procedures and, in most cases, sufficiently reduces the complexity of the cell search process for superior problems found in other potential location methods. If there is greater accuracy, however, the synchronization uncertainty in the mobile stations doing the SOHO searches can be further reduced by up to fifty percent through the use of an improved RTD estimate that takes into account delays of propagation. By using improved RTD estimates, the set of time delays that the mobile station has to search during the cell search process decreases considerably, especially compared to previous cell search methods. The cell search uncertainty interval will then depend on the size of the cell and the amount of sectorization of the cell. For example, in a non-sectorized cell system having a cell radius of approximately 30 kilometers, the uncertainty is less than 300 microseconds, considering that the position of the cell can be estimated within 3 cell radii. The normal search window for previous cell search methods, in contrast, is approximately 10 milliseconds. Thus, the use of an improved estimated RTD offers an improvement of two orders of magnitude in terms of the complexity of the search. The results are even better in cellular systems with smaller cells or with sectorized cells. It is also possible to decrease the uncertainty interval for the cell search even more by estimating the round trip delay between a target base station and the mobile that is performing the cell search, especially in the case of sectorized cells. An estimated round trip delay can be easily calculated from the available data when doing RTD calculations or if the approximate location of the mobile is known. Although several preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying drawings and described in the above detailed description, it will be understood that the invention is not limited to the disclosed embodiments. For example, measurements of the relative synchronization of base stations in accordance with the present invention can also be used for the pseudo-synchronization of base stations. Thus, the invention can present numerous modifications and substitutions without departing from the spirit of the invention presented and defined in the appended claims.

Claims (48)

1. CLAIMS A method for facilitating the synchronization of multiple base stations in mobile communication system asynchronous, gue comprising the steps of: at least one of said various base stations send at least one estimated value of relative time difference to a mobile station said at least one estimated value of relative time difference comprises an estimated synchronization difference between said at least one of said several base stations and a neighboring base station; and said mobile station receives said at least one estimated value of relative time difference.
2. The method according to claim 1, further comprising the steps of: said mobile station correlates said at least one estimated value of relative time difference with a balanced filter output signal; and start a cell search based on a result of the correlation step.
3. The method according to claim 2, further comprising the step of estimating a propagation delay between said mobile station and at least one of said base stations, said delay estimated propagation is used to reduce uncertainty in said correlating step .
4. The method according to claim 2, wherein the correlation step comprises: comparing said at least one estimated value of relative time difference with said balanced filter output signal; and determining whether said at least one estimated value of relative time difference is likely to correspond to a balanced filter output signal peak.
5. The method according to claim 4, wherein the initiation step comprises the selection of an encoding code based on a result of the determination step.
6. The method according to claim 1, wherein the mobile communication system comprises an asynchronous DS-CDMA system.
7. The method according to claim 1, wherein said sending step comprises the diffusion or transmission of said at least one estimated value of relative time difference in a neighboring list message.
8. The method according to claim 7, wherein the neighbor base station list message includes at least one encoding code associated with said neighboring base station.
9. The method according to claim 1, wherein the sending step further comprises sending an uncertainty value associated with said at least one estimated value of relative time difference.
10. The method according to claim 1, wherein said mobile station transmits said at least one estimated value of relative time difference together with a cell quality report neighboring said at least one of said several base stations.
11. The method according to claim 10, wherein a base station controller associated with said at least one of said several base stations stores said at least one estimated value of relative time difference in a database.
12. The method according to claim 1, further comprising the step of determining an approximate position of the mobile station employing said at least one value of relative time difference.
13. The method according to claim 12 wherein said step of determining an approximate position comprises the steps of: calculating a time difference in the mobile station between times of reception of a first downlink signal transmitted by said at least one of said several base stations and a second downlink signal transmitted by said neighboring base station; and comparing said at least one relative time difference value with said calculated time difference to determine at least one possible location of said mobile station with respect to said at least one of said several base stations and said neighboring base station.
14. A system for synchronizing a plurality of base stations in a mobile communication system, comprising: a first base station of said plurality of base stations, said first base station operates to broadcast or transmit at least one estimated difference value relative time, where at least one estimated value of relative time difference comprises an estimated synchronization difference between said first base station and a neighboring base station; and a mobile station for receiving said at least one estimated value of relative time difference.
15. The system according to claim 14, wherein said mobile station operates to correlate said at least one estimated value of relative time difference with a balanced filter output signal and to initiate a cell search.
16. The system according to claim 15, wherein said mobile station operates to compare said at least one estimated value of relative time difference with said balanced filter output signal, and determines whether said at least one estimated value of relative time difference probably corresponds to a balanced filter output signal peak.
17. The system according to claim 15, wherein said mobile station operates to select an encoding code based on a result of the correlation of said at least one estimated value of relative time difference with a peak of the output signal of balanced filter.
18. 18. The system according to claim 14, wherein the mobile communication system comprises an asynchronous DS-CDMA system.
19. The system according to claim 14, wherein the first base station operates to broadcast or transmit said at least one estimated value of relative time difference in a list message of neighboring base stations.
20. The system according to claim 14, wherein said first base station further operates to send an uncertainty value associated with said at least one estimated value of relative time difference.
21. The system according to claim 14, wherein said mobile station transmits said at least one estimated value of relative time difference together with a cell quality report neighboring said first base station.
22. 22. The system according to claim 14, wherein a base station controller associated with said first base station stores said at least one estimated value of relative time difference in a database.
23. 23. The system according to claim 14, further comprising a processor for determining an approximate position of the mobile station employing said at least one estimated value of relative time difference.
24. The system according to claim 23, wherein said processor compares said at least one estimated value of relative time difference with a time difference measured by said mobile station to determine at least one possible location of said mobile station in relation to said first base station and said neighboring base station.
25. 25. A method for estimating a relative synchronization of a plurality of base stations in an asynchronous mobile communication system, comprising the steps of: receiving in a first mobile station a first downlink signal transmitted by a first of said subscriber stations; base and a second downlink signal transmitted by a second of said base stations; calculating an estimated value of relative time difference between the time base of said first base station and the time base of said second base station using said first downlink signal and said second downlink signal; and store the calculated difference in a table of relative time differences.
26. 26. The method according to claim 25, wherein said first mobile station is in a transfer state
27. 27. The method according to claim 25, wherein the step of calculating an estimated value of relative time difference comprises furthermore, the step of taking into account a propagation delay of the first downlink signal and the second downlink signal
28. 28. The method according to claim 27, further comprising the step of transmitting the estimated value of the relative time difference to a second mobile station
29. 29. The method according to claim 28, further comprising the steps of: said second mobile station correlates the value being scaled for relative time difference with a balanced filter output signal; and start a cell search based on the result of the correlation step.
30. 30. The method according to claim 28, further comprising the steps of: estimating a propagation delay of signals transmitted between the second mobile station and the first base station and signals transmitted between the second mobile station and the second station base, based on an approximate location of the second mobile station; adjusting the estimated value of relative time difference by factoring in the estimated propagation delay to determine an estimated value of relativ difference "time at the mobile station delays, said second correlation mobile station correlates said estimated value of relative time difference with an output signal of the matched filter, and initiate a cell search based on a result of the correlation step 31.
31. A method of estimating a relative timing of various base stations in a system for asynchronous mobile communication, comprising the steps. of: receiving at a first mobile station a first downlink signal transmitted by a first of said base stations and a second downlink signal transmitted by a second of said base stations; transmitting an uplink signal from said first station mobile to the first base station and the second base station, and calculate an estimated value of relative time difference between the time base of said first base station and the time base of said second base station using reception times in the first base station and in the second base station base station said uplink signal, transmission times of the first downlink signal and the second downlink signal, and a time difference in the first mobile station between a time of receiving the second downlink signal and the transmission time of said uplink signal, said calculation takes into account the propagation delays between said first mobile station and said first and second base stations.
32. The method according to claim 31, wherein the times of reception of the uplink signal and the transmission times of the first downlink signal and the second downlink signal are in the time base of the radio station. base that transmits or receives the respective signal.
33. The method according to claim 32, further comprising the step of determining at least one possible location of a second mobile station by using reception times in the second mobile station of downlink signals transmitted by each of said first and second stations. and using said estimated value of relative time difference.
34. The method according to claim 32, further comprising the step of determining at least one possible location of a second mobile station using the reception times at said first base station and said second base station of a transmitted uplink signal by said second mobile station and using said estimated value of relative time difference.
35. The method according to claim 32, wherein said estimated values of relative time differences are used to synchronize said first base station and said second base station.
36. The method according to claim 31, further comprising the step of using the estimated value of relative time difference to calculate at least one possible location of a second mobile station relative to at least one of said first and second base stations. by determining a distance between the second mobile station and at least one of said first base station and said second base station.
37. 37. The method according to claim 36, which further comprises the step of determining a location of the second mobile station by using several relative time differences between a plurality of different pairs of base stations to calculate a distance between the second mobile station and each of several stations of base.
38. 38. A method for facilitating synchronization between mobile stations and base stations in an asynchronous mobile telecommunication network, comprising the steps of: receiving synchronization relative difference data from each of several mobile stations, the relative difference data of synchronization of each mobile station includes a measured difference between the time bases of at least two base stations in accordance with what is measured by said mobile stations; determining a relative difference estimate of synchronization based on the received data of relative synchronization difference, the relative difference estimate of synchronization represents an estimate of the difference between the time bases of at least two base stations; store the relative difference estimate of synchronization in a table of synchronization relative differences; transmitting the relative difference estimate of synchronization to a receiving mobile station.
39. The method according to claim 38, wherein data from a list of neighboring cells are transmitted together with the relative difference estimate of synchronization.
40. The method according to claim 38, wherein the synchronization relative difference estimate transmitted to the receiving mobile station is employed to help synchronize the receiving mobile station with a base station.
41. The method according to claim 38, wherein the relative difference estimate of synchronization comprises the measured difference received from one of several mobile stations.
42. The method according to claim 38, wherein the estimation of relative timing difference is determined by calculating a relative difference estimate value of synchronization from several measured differences received from the plurality of mobile stations.
43. The method according to claim 38, wherein said step of determining the relative difference estimate of synchronization further comprises the step of taking into account the propagation delays between a mobile station that measures and the at least two base stations of which the time base difference is measured.
44. The method according to claim 43, wherein the relative difference estimate of synchronization transmitted to the receiving mobile station is used to estimate a position of the receiving mobile station.
45. The method according to claim 38 further comprising the steps of: estimating a range of errors for the estimate of relative synchronization difference; and transmitting the range of errors to the receiving mobile station.
46. An asynchronous mobile telecommunications system, comprising: a plurality of base stations for transmitting data to a plurality of mobile stations and for receiving data from a plurality of mobile stations, the various base stations individually receive synchronization relative difference data, the synchronization relative difference data of each mobile station comprises a measured difference between the time bases of two of the plurality of base stations in accordance with that measured by the mobile station; a register storing a table of relative synchronization differences, each of several entries in said table comprises a relative difference estimate of synchronization calculated from the synchronization relative difference data; and wherein a first of the base stations transmits a relative difference estimate of synchronization to a receiving mobile station to facilitate communication synchronization between the mobile station and a second of the several base stations.
47. 47. The system according to claim 46 wherein the register further stores a list of neighboring cells.
48. 48. The system according to claim 46 wherein the register further stores error data for each estimate of relative synchronization difference. SUMMARY OF THE INVENTION A method and system for facilitating synchronization (eg, known relative synchronization differences) of base stations (BSS) (BS1, BS, &BS3) in asynchronous CDMA mobile communication systems ( 200) . Several mobile stations (MSs) (MSI, MS2, &MS3) measure the relative differences of time between several pairs of base stations, and these measurements are stored by the base stations. A source base station sends (106) to a mobile station, in a neighbor cell list message, estimates of the relative time difference between the source base station and each of the base stations in the list of neighboring cells. Each base station in the list can maintain a table of relative estimation of time differences, which can be updated continuously from the reports received from the mobile stations. Subsequently, the base stations can send entries of this table to the mobile station in the neighbor cell list message. Using this novel technique, the base stations have known relative synchronization differences. Accordingly, when the mobile station initiates a cell search for a candidate base station, the mobile station already has an estimate of the synchronization of this base station compared to its source base station. Thus, the resulting cell search procedure has a lower level of complexity and therefore can be achieved much more quickly than in the case of the prior art methods. In addition, estimates of relative time difference can be compared with corresponding time differences that are measured through a second mobile station. Based on this comparison, the propagation delays of signals between the second mobile station and several base stations can be calculated to determine the position of the second mobile station.