WO2005013636A1 - Location determination of a local transmitter using a database - Google Patents

Abstract

A method and apparatus for identifying wireless communication base stations from incomplete signal data reported by a Mobile Station (MS). In one embodiment, a database includes a plurality of key BTS data entries, each key BTS data entry including a unique global identifier that uniquely identifies a BTS in the system, and wherein the BTS database further includes associated data entries corresponding to and associated with each key BTS data entry. BTS signal data is obtained from an MS, the BTS signal data including a unique identifier of a first BTS and at least partial identifying information for at least one BTS other than the first BTS. A match is determined between the first associated data entries and the BTS signal data, and a second BTS is identified responsive to the match.

Description

LOCATION DETERMINATION OF A LOCAL TRANSMITTER

USING A DATABASE

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 60/490,820, filed July 28, 2003 and entitled "OPTIMIZATION OF BASE STATION ALMANAC LOOK-UP FOR IDENTIFICATION OF MOBILE STATION REPORTED CELLS," the contents of which are hereby incorporated in their entirety by reference.

BACKGROUND

Field 001 The disclosed method and apparatus relates to location services for mobile communication devices, and more particularly to a system and method for determining the location of transmitters transmitting signals used to locate a mobile station.

Description of Related Art 002 Location services (abbreviated as LCS, for "LoCation Services") for wireless communication devices, referred to as Mobile Stations (MSs), are an increasingly important business area for wireless communication providers. Location information can be used to provide a variety of location services to MS users. For example, public safety authorities can use location information to pinpoint the precise geographical location of an MS. Alternatively, an MS user can use location information to locate the nearest automatic teller machine (ATM), as well as the fee charged by that ATM. As another example, location information can assist a traveler in obtaining step-by-step directions to a desired destination while in route.

003 Technologies that permit a large number of system users to share a wireless communication system play an important role in meeting the ever-increasing demands of mobile computing, including the demands for location services. Such systems include Code Division Multiple Access (CDMA) and Wideband CDMA (WCDMA) technology, for example. As is well known, CDMA and WCDMA communication devices are assigned a pseudo noise (PN) code or sequence. Each device uses its PN code to spread its communication signals across a common spread-spectrum frequency band. As long as each communication device uses the correct code, each such device can successfully detect and select a desired signal from among the signals concurrently transmitted within the same frequency band.

004 Two types of positioning systems are commonly known. The first is referred to as an MS-based positioning system. In MS-based positioning systems the computations for determining the MS location are performed within the MS. The second is referred to as an MS- assisted positioning system. In MS-assisted positioning systems, the network provides assistance data to the MS to enable location measurements and/or to improve measurement performance by the MS. The MS provides the signal measurements to the network. A component of the network then computes an estimate of the location of the MS. One particular method of MS-assisted positioning employs the Global Positioning System (GPS) and is referred to as "assisted GPS" or simply AGPS. In accordance with the AGPS method, the MS acquires measurements from GPS satellites (commonly referred to as "GPS measurements") using assistance data provided by the network. In addition to GPS measurements, the MS acquires terrestrial measurements, such as forward link measurements from a ground reference station, such as a Base Transceiver Station (BTS). "Forward link" refers to communications transmitted from the BTS and received by the MS. "Reverse link" refers to the communications transmitted from the MS and received by the BTS. Terrestrial measurements can also be acquired on the reverse link, measured at the BTS. Other measurements include altitude assistance and timing information. In MS-assisted operation, regardless of the origin of such measurement information, all of the measurements made for the purpose of servicing a given location request are typically sent to a position deteπiiination entity (PDE) for geolocation calculations.

005 One method that may be used in conjunction with GPS or AGPS systems is commonly referred to as Advanced Forward Link Trilateration (AFLT). This is a geolocation technique that utilizes the measured time of arrival (TO A) of radio signals transmitted from a plurality of BTSs and received by an MS. Other methods that utilize TOA include Enhanced Observed Time Difference (E-OTD) and Observed Time Difference of Arrival (OTDOA).

006 In order to implement TOA-based geolocation techniques, the MS "reports" the receipt of signals transmitted from BTSs. The MS may provide a PDE with PN measurement data for each BTS signal that it receives. The PN measurement data is derived from a phase- coherent sequence of data. The data is commonly referred to as "chips". The sequence of chips is commonly referred to as a pilot chip sequence. The signal that carries the pilot chip sequence is commonly referred to as a pilot signal. Methods for acquiring the pilot signals by the MS are well known to persons skilled in the arts of wireless communications.

007 Within a given geographical region each BTS periodically broadcasts the same pseudo-noise (PN) code pilot signal, but with a different time offset. That is, each BTS transmits the same PN code. However, the start of transmission of the PN code from each BTS is delayed in time by a different precisely known offset with respect a common timing reference. Because different BTSs transmit PN codes with different offsets, the PN offset of a pilot signal may be used to identify the corresponding BTS. Consequently, a PDE may identify the BTSs that have transmitted signals received by an MS by referring to a database relating BTS identities to PN offset. It should be noted for the purposes of brevity, reference is made to the "PN offset of the signal" being transmitted rather than to the PN offset of the start of the PN code modulated on the signal.

008 Alternatively, other variations in the PN code may be used to distinguish signals transmitted by different BTSs. PN offset can typically be measured on the incoming signals received from a BTS. One database known to persons skilled in the art is a Base Station Almanac (BSA), which contains information about the terrestrial wireless network. In particular, the BSA may relate the location of a BTS to the PN offset of signals transmitted by that BTS.

009 Unfortunately, due to the limited number of PN offsets available, some BTSs are assigned to transmit signals with the same PN offset. However, BTSs transmitting signals with the same PN offsets are typically sufficiently far away from each other that no one MS can receive signals from two BTSs assigned the same PN offset. Nonetheless, the PN offset alone may be insufficient to uniquely identify a BTS because an MS may receive signals from a distant BTS having the same PN offset as a more proximate BTS.

010 In addition to problems in uniquely identifying the BTS from which signals were transmitted, the relatively large number of BTSs in a typical system results in a significant amount of time being spent searching through a database in order to identify the particular transmitting BTS.

011 An accurate determination of the location of the MS requires accurate information regarding the location of each BTS from which the MS receives signals. This, in turn, requires rapid and unique identification of each BTS from which a signal is received. Therefore, it can be appreciated that there is a significant need for improved methods by which transmitters may be identified using the limited data received by an MS. Accordingly, there is a need for a method and apparatus for determining the location of BTSs that can be "heard" by an MS.

SUMMARY 012 A system and method is disclosed for determining the location of transmitters of signals, such as Base Transceiver Stations (BTSs) reported by a Mobile Station (MS). In one embodiment of the disclosed method and apparatus, a database includes a plurality of key BTS data entries. The term "key" is used to indicate that the key BTS data entry is used as a "search key" to assist in identifying the particular record of interest within the database. Each key BTS data entry corresponds to a unique BTS in the system. The BTS database further includes associated data entries corresponding to and associated with each key BTS data entry. One such associated data entry is the Neighbor List for storing "Neighbor" BTS data, including pseudo-random noise (PN) offsets of signals transmitted by BTSs that are geographically proximate to the BTS corresponding to the key BTS entry.

013 Another associated data entry is the Hearable List for storing "Hearable" BTS data, including the PN offset of signals transmitted by BTSs and received by an MS, and not transmitted by BTSs in the Neighbor List. Yet another associated data entry is referred to as the Remaining List for storing the "Remaining" BTS data, including PN offsets of signals transmitted by BTSs that do not belong to the Neighbor List or Hearable List, but that may potentially be received by an MS. One additional associated data entry includes a threshold, the use of which will become clear below.

014 In accordance with one embodiment of the disclosed method and apparatus, BTS data is received from an MS. The BTS data includes a unique identifier associated with a "primary serving" BTS. In one embodiment of the disclosed method and apparatus, the unique identifier could be the SID/NID/BaselD or in another embodiment the identifier could be the Switch Number, Market ID and/or Base ID. The BTS data also includes Pseudo-random noise (PN) data associated with both the primary serving BTS and other BTSs. These other BTSs are hereafter referred to as "non-serving" BTSs. However, it will be understood that some of these non-serving BTSs may in fact be secondary serving BTSs. That is, they may be providing a communication link between the MS and a communication network. In addition, the BTS data includes an indication of the signal strength of signals transmitted by other BTSs and received by the MS. A match is sought between the received unique identifier and the key BTS data entries within the database. Once the match is made, the location of the primary serving BTS identified by the key BTS data entry is known from the primary serving BTS data associated with that key BTS data entry.

015 Next, data associated with the signals that are reported by the MS as having been received by the MS from non-serving BTSs are examined. The examination is made to determine whether the signal strength reported by the MS for each non-serving BTS signal is greater than the threshold associated with the primary serving BTS in the database. If so, then the method checks whether the PN offset of signals transmitted by the non-serving BTSs match the PN offset stored in the Neighbor List. The PN offset of the signals transmitted by the non- serving BTSs is considered to be ambiguous identification data, since the PN offsets may be associated with more than one particular BTS. However, other identification data may be considered to be ambiguous as well. For example, a "System IDentification" number (SID) can be assigned to more than one BTS, making the SID ambiguous as to the identify of the BTS associated with that SID. Accordingly, any information that identifies two or more sources is considered to be ambiguous identification data for the purposes of this disclosure.

016 In accordance with one embodiment of the disclosed method and apparatus, it can be assumed that the PN offset of a signal having signal strength that is above the threshold will match a PN offset associated with a Neighbor BTS listed in the Neighbor List. In another embodiment, if: (1) the signal strength is above the threshold; (2) the PN offset is not found in the Neighbor List, and (3) the PN offset is found in one of the other lists, then the entry associated with the PN offset is moved from the list in which the PN offset is found to the Neighbor List. In this way, the Neighbor List can be dynamically constructed rather than having to be downloaded from the BTS, or other BTS network database equipment.

017 However, if the signal strength is not greater than the threshold, then we cannot assume that the PN offset will be on the Neighbor List. Accordingly, even if the PN offset associated with a non-serving BTS matches the PN offset of a BTS listed in the Neighbor List associated with the primary serving BTS, confirmation of that PN offset is required. That is, an additional step is performed to confirm that the PN offset indicates that the BTS associated with that PN offset in the database is the same as the BTS associated with the PN offset sent by the MS. In one embodiment, this confirmation is made by determining the likelihood that the BTS associated with the PN offset in the database transmitted the signal having the associated PN offset. In an alternative embodiment of the disclosed method and apparatus, the confirming step may be omitted. However, there would then be a chance that the match could be wrong. Any wrong matches would result in the location of at least some of the BTSs from which the MS is receiving signals to be incorrectly determined.

018 If the PN offset sent by the MS does not match any of the PN offsets stored in the Neighbor List, then a match is sought from among the PN offsets stored in the Hearable List entry. In one embodiment of the disclosed method and apparatus, if such a match is found, then the match is confirmed. However, in another embodiment, the match can be assumed to be correct at some risk of that assumption being wrong.

019 If the PN offset sent by the MS does not match any of the PN offset stored in the Hearable List entry, then a match is sought from among the PN offset stored in the Remaining List. If a match is found, then in accordance with one embodiment of the disclosed method and apparatus, the match is confirmed via a further step. Alternatively, the match might be assumed to be correct without further confirmation. In response to determining that there is a match, all of the data associated with the PN offset is transferred from the Remaining List to the Hearable List.

BRIEF DESCRIPTION OF THE DRAWINGS

020 FIGURE 1 is a functional block diagram of a wireless communication system to provide wireless communications including location services.

021 FIGURE 2 is a functional block diagram of a wireless communication system to provide wireless communications including location services, showing additional components.

022 FIGURE 3 is a conceptual illustration of a database, shown in the form of a table.

023 FIGURE 4 is a flow chart illustrating a method of identifying base transceiver stations according to signal data reported by a Mobile Station (MS).

024 FIGURES 5 and 6 comprise a unitary flow chart illustrating a second method of identifying base transceiver stations according to signal data reported by a MS.

025 FIGURE 7 illustrates a system that performs a statistical analysis based on the amount of overlap in coverage areas between a primary serving Base Transceiver Station (BTS) and a BTS represented by a particular set of entries to a database.

026 FIGURE 8 illustrates the coverage area of a primary serving BTS. 027 FIGURE 9 shows the calculation of the relative phase of signals received from different BTSs.

DETAILED DESCRIPTION

028 FIGURE 1 illustrates a simplified general wireless communication system 100 that may be adapted to provide location services. As shown in FIGURE I, a Mobile Station (MS) 102 communicates with Base Transceiver Stations (BTSs) 112, 114, 116 and 142 via a plurality of wireless links 122. While four such BTSs are shown, it should be understood that the MS 102 may communicate with one or more such BTSs without limits on the number of such BTSs. The MS 102 may be a cell phone, a wireless telephone, a personal digital assistant (PDA) with wireless communication capabilities, a laptop computer having wireless communication capabilities, or any other mobile device for personal communication via wireless connection. The MS 102 receives pilot signals transmitted by the BTSs 112, 114, 116, 142. In accordance with one embodiment of the disclosed method and apparatus, the pilot signals, referred to herein as "pilots," are modulated with a "pseudo-random noise" (PN) code. In another embodiment of the disclosed method and apparatus, the pilots may be any signal that permits the identification of BTSs based on some characteristic of the signal. In one embodiment in which the pilot is modulated with a PN code, the PN code is a string of digital values. Modulation of pilots with such PN codes is commonly used to allow Code Division Multiple Access (CDMA) communication systems to discriminate between different signals sent on the same frequency. In addition, it is common in CDMA cellular telephone systems to modulate the pilot signals transmitted by many, or all, of BTSs in the system with the same PN code. To distinguish the pilot transmitted by one BTS from the pilots transmitted by other BTSs, the PN code from each BTS has a time offset that is unique to the particular transmitting BTS. The offsets are each referenced to a common clock. Therefore, by determining the particular timing offset with respect to the common clock, the MS can determine which BTS transmitted that pilot.

029 However, in most such systems, the same offset is assigned to more than one BTS in accordance with an assignment plan. In accordance with the assignment plan, BTSs that have the same offset assigned should be far enough away from each other that the signals transmitted by them cannot be received by the same MS, for ordinary communications purposes. Therefore, in theory there should not be any ambiguity when attempting to use the offset to determine which particular BTS has transmitted a pilot that an MS received at signal levels strong enough for communication. However, in practice, there remains a potential for ambiguity. This is especially true in cases where an MS can detect signals weaker than those used for ordinary communications purposes, and thus may detect signals that are coming from farther away than the communications assignment plan anticipated. In any case, such potential ambiguities need to be accounted for when attempting to identify the location of the source of a pilot for the purpose of using that pilot to locate an MS.

030 The BTSs 112, 114 and 116 are coupled to a Base Station Controller/Mobile Switching Center (BSC/MSC) 110. The BSC/MSC is coupled to a Position Determination Entity (PDE) 130. The PDE 130 may be incorporated into other components of a communication system (such as a BSC/MSC, a Communication Service Provider Network 150, or some combination thereof). The PDE 130 may provide location services to multiple devices (such as a plurality of MSs similar to the MS 102) communicating through multiple BSC/MSCs, and BTSs, such as the BSC/MSC 140 and the BTSs 112, 114, 116, and 142. In one alternative embodiment, the location of the MS 102 may be determined solely by processing that occurs within the MS itself. In one embodiment of the disclosed method and apparatus, information required to locate all BTSs may be stored in a centralized PDE, rather than in the MS 102. However, such data might be stored anywhere that would permit it to be accessed, including in the MS 102.

031 hi the present example, the BTS 142 is connected to a separate BSC/MSC 140 to illustrate that a typical cellular telephone system will include more than one BSC/MSC. The BSC/MSC 110, 140 provide an interface between the BTS and other network elements, such as the PDE 130 and a communications service provider network 150, such as a Public Switched Telephone Network (PSTN).

032 In addition to the signals received from BTSs, the MS 102 may also receive signals (such as GPS signals) from one or more other sources (such as satellites) 126 and 128 via communication links 123 and 124. Similarly, the BSC/MSC 110 may also receive signals, such as GPS signals, from one or more satellites 126 and 128 via communication links 123 and 124. Although two satellites are illustrated by way of example, no satellites, one satellite, or a plurality of satellites and/or other sources, may be employed when providing location services to an MS. Further, although the satellites are shown communicating with the BSC/MSC 110, persons skilled in the arts of wireless communications will understand that satellite data may also be received by other receivers (not shown) such as a Wide Area Reference Network (WARN). The BSC/MSCs 110 and 140 are connected to the communication service provider network 150 to receive and transmit data such as audio/video/text communication and programming data, position requests or data from the WARN.

033 FIGURE 2 provides additional details regarding the components within the MS 102 and the PDE 130. For simplicity, the GPS satellites that may be used in a positioning system, and their associated communication links, are not shown in FIGURE 2. The PDE 130 has a memory 234, and a processor 232 that controls operation of the PDE 130. The term "processor", as used throughout this description, is intended to encompass any processing device, alone or in combination with other devices (such as a memory), capable of controlling operation of a device in which it is included (such as a PDE 130, an MS 102, a BSC/MSC 110, or a portion thereof). For example, a processor, such as the processor 232, can include microprocessors, embedded controllers, application specific integrated circuits (ASICs), digital signal processors (DSPs), state machines, dedicated discrete hardware, and the like. The system, apparatus, and method described herein are not limited by the specific hardware component selected to implement the processor 232.

034 The memories 234 and 206 may include read-only memory (ROM) components, random-access memories (RAM), non-volatile RAM components or any other means by which information can be stored and later accessed. The memory 234 stores and provides instructions and data for the processor 232. The components of the PDE 130 are linked together by an internal bus system 236, and the components of the MS 102 are linked together by an internal bus system 207. As described in more detail below, the memory 234 includes a database used to locate the source of signals (i.e., BTSs) according to a PN offset provided by the MS 102.

035 As shown in FIGURE 2, the MS 102 includes a processor 204, a memory 206, and a transceiver 208. The memory 206 stores and provides instructions and data for the processor 204. The transceiver 208 allows the transmission and/or reception of data, such as audio, video, text, and programming data, between the MS 102 and a remote location, such as the BTSs 112, 114, 116 and 142, or GPS satellites (not shown). An antenna 209 is coupled to the transceiver 208. The basic operation of the MS 102 is well known in the art and need not be described herein.

036 PDE 130 may have a database stored that maps PN offset to the location of an associated BTS. As noted above, CDMA systems commonly use pilot PN offsets as a means of identifying BTSs. PN offsets are commonly known as "transmit PN sequence offsets". In another embodiment, signals transmitted by different BTSs may be distinguished based on the PN code values or any other attribute of the modulation of the signals rather than PN offset. However, for the sake of simplicity in describing the disclosed method and apparatus, reference is made to the PN offset. However, it will be understood by those skilled in the art that any other attribute of the signal, including the particular PN code, or some other distinction in the manner in which the signals transmitted by BTSs may be used in place of the PN offset.

037 In one embodiment, the database may be stored in the memory 234. However, in another embodiment, the database may be stored in the memory 206 within the MS 102. For this embodiment, the MS 102 may require information relating the location of the BTS from which a pilot having a particular PN offset is located. This information may come from a BSC/MSC, a PDE, or other location.

038 FIGURE 3 is a conceptual illustration of a database, shown in the form of a table 300. The table 300 contains a number of records 301, 303, 305. Each record 301, 303, 305 includes a key BTS entry 302 for storing key BTS data entries. The data stored in each key BTS data entry 302 relates to a particular "primary serving" BTS. The primary serving BTS is that BTS with which the MS 102 is registered. "Registration" refers to the network recording that the MS 102 is communicating with the network through a particular BTS. In the case in which the MS 102 is registered with more than one BTS, the primary serving BTS might be the BTS from which the MS 102 is receiving the strongest signal, or alternatively, the primary serving BTS might be arbitrarily designated from among all of the BTSs with which the MS 102 is registered.

039 In one embodiment, each key BTS data entry 302 includes at least two sub-fields. The first sub-field 307 is referred to herein as the PN data sub-field. The PN data sub-field 307 includes PN data, such as PN offset, used to determine with which BTS 112, 114, 116, 142 the MS 102 is registered. The phrase "PN data" can be used to generally refer to information such as: 1) the offset of the start of a PN code modulated onto a carrier signal transmitted by a BTS, 2) the particular PN code modulated onto a carrier signal transmitted by a BTS, or 3) any other information modulated onto a carrier signal transmitted by a BTS from which signals transmitted by different BTSs can be distinguished from one another, whether that information relates to a pseudo-random noise code or not. However, for the sake of clarity and simplicity, the presently disclosed method and apparatus is described using the PN offset as the particular type of PN data.

040 Accordingly, in one embodiment of the disclosed method and apparatus, the data stored in the key BTS data entry 302 includes the PN offset of the start of the PN code modulated onto the signals transmitted by the primary serving BTS. For the sake of simplicity and brevity, we refer simply to the "PN offset of the signals" rather than the longer phrase "PN offset of the start of the PN code modulated on the signals". However, those skilled in the art will understand the former phrase to mean the same as the later phrase. In addition, the key BTS data entry 302 includes the location of the primary serving BTS. While the information in the key BTS data entry 302 is typically related to the primary serving BTS, the key BTS data entry 302 may include the PN offset of signals transmitted by a BTS other than the primary serving BTS. For example, the PN offset may be associated with a secondary serving BTS.

0,41 The second sub-field 309 is referred to as the Serving BTS Location sub-field. In the example shown in FIGURE 3, the Serving BTS Location sub-field 309 provides the location of the primary serving BTS. The location may be provided in any particular format and/or form that would make the location information useful for at least the purpose of identifying from where a signal transmitted by the BTS originated. The location is provided in order to use such signals to assist in determining the location of a receiving MS 102. For the purpose of the disclosed method and apparatus, the location of the MS 102 might be determined using any of the following methods: 1) time of arrival method, 2) time difference of arrival method, 3) angle of arrival method, 4) triangulation method, 5) trilateration method or 6) any other method that might benefit from knowledge of the location of a source of a transmitted signal.

042 In addition, in some embodiments, information that identifies the primary serving BTS might also be included in one or more additional sub-fields (not shown) in the key BTS data entry 302. In one embodiment, the key BTS data entries may also include other data relevant to the BTS.

043 Associated with each key BTS data entry 302 (i.e., contained within the same record 301, 303, or 305 as the key BTS data entry) are associated data entries, la accordance with one embodiment of the disclosed method and apparatus, for each key BTS data entry 302, there is an associated Neighbor List 311. The Neighbor List 311 includes two sub-fields. The first sub- field is the Neighbor PN data sub-field 313. The second sub-field is the Neighbor location sub- field 315. In one embodiment of the disclosed method and apparatus, there is a one-to-one correspondence between the entries in the Neighbor PN data subfield 313 and the entries in the location sub-field 315. The Neighbor PN data sub-field 313 includes the PN offset of the signals transmitted by a Neighboring BTS. The corresponding Neighbor location sub-field 315 includes the location of that neighboring BTS. For example, FIGURE 3 shows a database that has n Neighbor BTSs. Each Neighbor BTS has a corresponding Neighbor PN data entry 317 and a corresponding Neighbor location entry 319 in the database 300. Ellipses are shown in FIGURE 3 between the first Neighbor PN data sub-field 317 and the nth PN data sub-field 318 to indicate that there are n minus 2 additional PN data entries not expressly shown. Similarly, ellipses are shown between Neighbor location data entry 319 and Neighbor location data entry 320. A first Neighbor Location sub-field entry 319 corresponds to, and locates, the BTS that transmitted the signal with the PN offset residing in the first Neighbor PN data sub-field entry 317. An rath Neighbor Location sub-field entry 320 corresponds to, and locates, the BTS that transmitted the signal with the PN offset residing in the rath Neighbor PN data sub-field entry 318.

044 The associated data entries also include a "Hearable List" 322 associated with a particular primary serving BTS identified in the key BTS data entry 302 within the same record 301, 303, 305. The Hearable List 322 shown in FIGURE 3 includes two sub-fields with m entries in each sub-field. The first sub-field is the Hearable PN data sub-field 321. The Hearable PN data sub-field 321 includes a first Hearable PN data sub-field entry 325 separated by ellipses from an zth Hearable PN data sub-field entry 327. The ellipses indicate the existence of m entries within the sub-field 321.

045 The second sub-field is the Hearable Location sub-field 323, including a first entry 329 to the Hearable Location sub-field 323 separated by ellipses from an mth Hearable PN data sub-field entry 331 to indicate the existence of m entries within the Hearable Location sub-field 323. The data in the Hearable List 322 allows BTSs to be located that: 1) have transmitted signals received by an MS 102 that is currently being served by the BTS corresponding to the key BTS data entry 302 within the same record 301, 303, 305; and 2) are not in the Neighbor List 311.

046 The associated data entries also include a "Remaining BTS List" 333. The Remaining BTS List 333 identifies a set of BTSs that are not identified in either the Neighbor List 311 or the Hearable BTS List 322. In addition, in accordance with one embodiment of the disclosed method and apparatus, in order to be listed in the Remaining BTS List 333, a BTS should be detectable by an MS 102 being served by the primary serving BTS responsible for transmitting the signal having the PN offset indicated in the key BTS data entry 302. In one particular embodiment, any BTS in the system is considered to be potentially detectable. In yet another embodiment, all BTSs in the system are included in the Remaining List excluding those listed on the Neighbor or Hearable Lists.

047 The Remaining BTS List 333 includes two sub-fields. The first is a Remaining BTS PN sub-field 335. The second is the Remaining BTS Location sub-field 337. The Remaining BTS PN sub-field 335 includes several Remaining BTS PN sub-field entries as depicted in FIGURE 3 by the ellipses between a first Remaining BTS PN sub-field entry 339 and an Zth Remaining BTS PN sub-field entry 341. Similarly, the Remaining BTS Location sub-field includes several Remaining BTS Location sub-field entries as depicted in FIGURE 3 by the ellipses between a first Remaining BTS Location sub-field entry 343 and an fth Remaining BTS Location sub-field 345. Each of the entries in the Remaining BTS Location sub-field 337 provides the location of the BTS transmitting the PN code indicated in the corresponding entry in the Remaining BTS PN sub-field 335.

048 While the embodiment depicted in FIGURE 3 shows storing only the PN offset and BTS location in each of the Lists, the Neighbor List 311, Hearable BTS List 322 and Remaining BTS List 333 may each include any combination of the PN offset, PN code, and/or data pertaining to other attributes, such as, for example, a unique global identifier, geographical coordinates, altitude information, antenna range, etc., that may be used for position determination purposes. Accordingly, table 300 may optionally contain additional fields, and may refer to other databases or tables, or it may not contain all of the fields described herein. For example, in one embodiment, one or more separate databases containing a Remaining List may exist. In this embodiment, the table 300 might contain a field that refers to the desired Remaining List in the external database. In this embodiment, the PDE 130 (FIGURE 2), or a processor located in another device, looks up the corresponding Remaining BTS data in another database or table, rather than storing it in the field 333. Other servers, such as those in the BSC/MSC 110 or 140, may maintain separate databases and information retrieved by the PDE 130, as needed. In yet another embodiment, the database represented by Table 300 might be incorporated in the MS 102. In this embodiment, in accordance with the methods described in more detail below, the MS 102 may identify the BTSs based upon the PN offset and report the BTS location to the PDE 130, rather than merely reporting the PN offset.

049 FIGURE 4 is a flowchart illustrating a method of identifying BTSs according to PN offset reported by an MS. The method may be employed, for example, by the system 100 shown in FIGURE 2. It should be noted that while the present example illustrates the disclosed method and apparatus using PN offset, other forms of ambiguous transmitter identifying information might be used.

050 The method is initiated at STEP 402, wherein a BTS database, such as the database 300, is created. At STEP 404, data is obtained from an MS. Typically, the MS will provide the information when requesting that its position be determined. In accordance with one embodiment of the disclosed method and apparatus, the data includes a unique Global Identifier which identifies the serving BTS. The Global Identifier is included in the information modulated on signals transmitted to the MS by the serving BTS. Accordingly, the MS will receive and demodulate the Global Identifier modulated onto the signals transmitted by the serving basestation. Furthermore, the MS sends a set of PN offsets designated by PN,- (i = 1, ra), where i is an index indicating a particular one of ra instances of PN offset. Each instance of PN offset is associated with one BTS from which the MS receives signals. The value ra is the number of BTSs from which the MS receives signals. The data is provided to a processor. The processor may be incorporated in a PDE or an MS, such as either the processor 204 or the processor 232 of FIGURE 2. The processor is coupled to the database to receive and store data.

051 The method then proceeds to STEP 406. At STEP 406 the processor finds the key BTS data entry 302 in the database 300 having the unique ID that matches the unique ID obtained from the MS. The method then proceeds to STEP 408. At STEP 408, a counter is initialized by setting the index i = 1. The method then proceeds to STEP 410.

052 At STEP 410, the strength of the signal received by the MS with a PN offset of PN,- is compared to a desired threshold level (THRESH). In accordance with one embodiment of the disclosed method and apparatus, the value used for THRESH may be the minimum signal level required to allow an assumption to be made that the transmitting source (i.e., BTS) is close enough to the serving BTS to be considered to be a communications neighbor of the serving BTS. One particular parameter that might be useful is TT_ADD. TT_ADD is a typical value for the signal strength threshold T_ADD. TT_ADD reduces variations in T_ADD, given that T_ADD may vary in different systems or locations. In another embodiment of the disclosed method and apparatus, THRESH may be the particular T_ADD for the system and location wherein the MS is operating. In this case, the particular T_ADD level to be used for THRESH may be stored in a field associated with the key BTS data entry 302 of FIGURE 3. Alternatively, THRESH may be stored in a sub-field of the key BTS data entry 302. If the signal strength for the signal having a PN offset equal to PN,- is greater than THRESH, the method proceeds to STEP 412.

053 At STEP 412, the entry corresponding to PN,- in the Neighbor List 311 is identified. The offset value, PN,-, should be present in the Neighbor List 311, because any signal having a strength greater than THRESH can be assumed to have originated within the area associated with the Neighbor BTSs. Therefore, there should not be any ambiguity regarding the mapping of the PN offset and a Neighbor BTS. However, if the PN,- offset is not identified in the Neighbor List 311, then the PN,- offset is sought from among the other lists in STEP 413. If found, then the information associated with PN,- is moved to the Neighbor List 311. In accordance with one embodiment of the disclosed method and apparatus, the information to be moved includes the PN,- offset and the location of the BTS from which that PN,- is transmitted.

054 From STEP 412, the method proceeds to STEP 428 at which the method determines whether the most recent value of PN,- is the last value to be considered (i.e., "i" = "n"). If the answer is YES, the method proceeds to STEP 432, which terminates the method. If the answer is NO, the method proceeds to STEP 430 wherein the index "i" is incremented, and then returns to STEP 410.

055 If the signal strength is equal to or less than THRESH, the method proceeds from STEP 410 to STEP 414. At STEP 414, the Neighbor List 311 is searched to determine whether there is a match with the PN,- offset. Even though the signal strength is below the threshold level, such a match may occur if the signal from a Neighbor BTS is blocked by a building or other obstacle. If there is a match, the method proceeds to STEP 416. If not, then the method proceeds to STEP 418. It should be noted that there is a relatively high likelihood that PN,- will match one of the entries 317, 318 in the Neighbor List 311, since many of the BTSs from which the MS 102 will receive signals will be on the Neighbor List 311. However, this assumes that the Neighbor List 311 has been completely compiled. In one embodiment of the disclosed method and apparatus, the Neighbor List 311 is generated by gathering reports from MSs that have received signals from a BTS that are above THRESH.

056 At STEP 416, since the signal level was below the threshold level, a coverage and phase test (CPT) is applied to the PN,- offset to determine if the PN,- corresponds to a Neighbor BTS. The CPT is performed because it is possible that two BTSs may have the identical PN,- offset. The CPT confirms whether a signal having the particular PN offset value found in the Neighbor List 311 was actually transmitted by a Neighbor BTS or was transmitted by a BTS that is farther away then would allow that BTS to qualify as a Neighbor BTS. A "YES" answer to this test signifies and confirms that the PN,- offset has been unambiguously identified by the CPT as being from a Neighbor BTS. If the answer at STEP 416 is YES, the method proceeds to STEP 428. If the answer is NO, the method proceeds to STEP 418. The CPT test is described in detail further below. Accordingly, for the sake of clarity, no further information regarding CPT is provided at this point. 057 At STEP 418, the Hearable List 322 is searched to determine whether there is a match or matches for the PN,- offset. As described in more detail below, the Hearable data is initially empty, and these entries become populated as a result of the operation of the disclosed method. If the answer at STEP 418 is YES, the method proceeds to STEP 420. If the answer is NO, the method proceeds to STEP 422.

058 At STEP 420, a CPT is applied to the PN,- offset to confirm a positive match in STEP 418. As indicated above, there may be more than one match found for the PN,- offset at the preceding STEP 418. A "YES" answer to this test signifies that the PN,- offset has been unambiguously identified by the CPT. If the answer at STEP 420 is YES, the method proceeds to STEP 428 for further processing. If the answer is NO, the method proceeds to STEP 422.

059 At STEP 422, the Remaining BTS data is searched to determine if there is a match or matches for the PN,- offset. If the answer is YES, then the method proceeds to STEP 424. If the answer is NO (no match found), the method proceeds to STEP 427.

060 At STEP 424, a CPT is applied to the PN,- offset to confirm a positive identity determination for the match(s) identified in STEP 422. As noted above, there may be more than one match found for the PN,- offset at the preceding STEP 422. In this case, the result of the CPT test will verify which one, if any, of the matches is correct. A "YES" answer to this test signifies that the PN,- offset has been unambiguously identified by the CPT. If the answer at STEP 424 is YES, the method proceeds to STEP 426. If the answer is NO, the method proceeds to STEP 427 where the PN,- offset is labeled with an error flag.

061 At STEP 426, the data for the Remaining BTS identified for the current PN,- offset is added to the Hearable List 322. Adding to the Hearable List 322 facilitates the identification of the BTS for future searches. This process of adding to the Hearable List 322 also improves the efficiency of the search process. That is because the Remaining List 333 constitutes a much larger data set than the Hearable List 322. By adding a new BTS to the Hearable List 322, the new BTS can be identified in the future without the need for searching the larger Remaining List 333. Thus, in accordance with one embodiment of the disclosed method and apparatus, adding to the database reduces the search time. The particular set of BTSs that can be heard may change with time. Therefore, in accordance with one embodiment of the disclosed method and apparatus, BTSs that have not been recently detected and reported are removed from the Hearable List 322. Removing BTSs improves search efficiency and/or reduces memory requirements. 062 Determining when to remove a BTS may be implemented, for example, by time stamping entries each time they are detected by an MS being served by the serving BTS identified in the key BTS data entry residing in the same record with the Hearable List 322. Entries may then be deleted if an MS has not detected them for a predetermined period of time. Alternatively, the location of the Hearable BTS entries in the list can be modified by placing the most recently detected BTSs at the top of the list. In this way, BTSs that are not recently detected are migrated to the bottom of the list and become eligible for deletion. A combination of these two approaches may also be employed. From STEP 426, the method proceeds to STEP 428.

063 At STEP 427, the processor notifies the operator, by means of an error flag, or some other notification message, that a BTS corresponding to the current PN,- offset was not uniquely identified by the search process. The error flag alerts the operator so that corrective action may be taken. In particular, if the PN,- offset cannot be uniquely identified, it should not be used for computation of a location estimate. From STEP 427, the method proceeds to STEP 428.

064 As noted above, at STEP 428 the data index, i is tested to determine whether all the PN,- offset have been evaluated. If the answer is NO, the method proceeds to STEP 430, where the index i is incremented. The next PN,- offset is identified by searching the database as described above. If the answer is YES, the method proceeds to STEP 432 and the process terminates.

065 FIGURES 5a and 5b comprise a unified flow chart illustrating another operation of a system, such as the system 100 of FIGURE 2, for locating BTSs based on data received from the MS. The method is initiated at STEP 502, wherein a database, such as the database 300, is created. At STEP 504, data is obtained from an MS requiring its position to be determined. The data is provided to a processor, which may be incorporated in a PDE or an MS. The data is obtained from an MS requiring its position be determined and provided to a processor. The process may be incorporated in a PDE or an MS. The processor is operatively connected to the database to receive and store data, and may be implemented in a processor such as either the processor 204 or the processor 232 of FIGURE 2. The data includes a Global Identifier (GIo) for at least one BTS signal received by the MS, and the PN,- offset (i = 1, ra) of each received signal provided by the MS for evaluation, where ra is the number of BTS signals provided by the MS for evaluation. The method then proceeds to STEP 506. At STEP 506, the processor finds the key BTS data entry in the database corresponding to the GIQ. The method proceeds to STEP 508. At STEP 508, a counter is initialized by setting an index i = 1, and the method proceeds to STEP 510.

066 At STEP 510, the signal strength of the signal having the PN offset, PN,-, (hereafter referred to as "the signal having the PN") is compared to a signal strength threshold level THRESH. If the signal strength for the signal having the PN,- is greater than THRESH, the method proceeds to STEP 512. At STEP 512, the PN,- is sorted into a "Candidate list", referred to as PN_C data, and the method proceeds to STEP 516. At STEP 516, the index "i" is tested to determine whether all of the PN,s for i equal to 1 through n have been sorted. If the answer is YES, the method proceeds to STEP 520. If the answer is NO, the method proceeds to STEP 518. At STEP 518, the index i is incremented. The method then returns to STEP 510 and the sorting process is repeated as described above.

067 If at STEP 510 the signal strength is less than or equal to THRESH, the method proceeds to STEP 514. At STEP 514, the PN,- is sorted into an "Unknown List", referred to as PN„, and the method proceeds to STEP 516. When the sorting process implemented by STEPS 510, 512, 514, 516 and 518 is completed, the entries PN,- (i = 1, ra) will have been sorted into one of the two sets PN_C (c = 1, c^ or PN„ (u = 1, u-^, where c^ + u^ = ra.

068 At STEP 520, all of the data in the PN_C data is assumed to be in the Neighbor List 311. This assumption will typically be valid, since any signal having a signal strength exceeding THRESH can be assumed to have originated within the proximate geographical areas associated with the Neighbor BTSs. Therefore, there is no ambiguity regarding the mapping of the PN offset and the BTS identities for Neighbor BTSs. The method then proceeds to STEP 524 (FIGURE 5(b)) via a flow connector 522.

069 At STEP 524 (FIGURE 5(b)), the index V is initialize by setting u = 1. The method then proceeds to STEP 526. At STEP 526, the Neighbor List 322 is searched to determine whether there is a match for the PN_W data. Such a match will occur, for example, when the signal from a Neighbor BTS is blocked by a building or other obstacle, causing the signal strength to fall below THRESH. If a match is found at STEP 526, the method proceeds to STEP 528. At STEP 528, a CPT is applied to the PN_a data. A "YES" answer to this test signifies that the PN„ has been unambiguously identified by the CPT. Accordingly, if the answer at STEP 528 is YES, then the process proceeds from STEP 540. At STEP 540, the index u is tested to determine whether all of the PN„ offset have been evaluated. As noted above in reference to STEPS 510, 512, 514, 516 and 518, the entries in the PN,- (i = 1, ra) list were sorted into the sets PN_C (c = 1, cj and PN„ (u = I, W_ma ), where c^ + u^_x = n. If the answer at STEP 540 is NO, the method proceeds to STEP 542 where the index u is incremented, and the next PN„ offset is identified by searching the database as described above. If the answer at STEP 540 is YES, the method proceeds to STEP 544 and the process terminates.

070 Returning to STEP 526, if the answer to STEP 526 is "NO", then the method proceeds to STEP 530. Similarly, if the answer at STEP 528 is NO, the method proceeds to STEP 530. At STEP 530, the Hearable BTS data is searched to determine whether there is a match or matches for the PN„ measurement. If the answer at STEP 530 is YES (match was found), the method proceeds to STEP 532. At STEP 532, a CPT is applied to the PN„ data to confirm a positive identity determination for the match(s) identified during STEP 530. A "YES" answer to this test signifies that the PN,- offset has been unambiguously identified by the CPT. If the answer at STEP 532 is YES, the method proceeds to STEP 540 for further processing as described below. If the answer is NO, the method proceeds to STEP 534. Returning to STEP 530, if the answer is NO (no match found), the method proceeds to STEP 534.

071 At STEP 534, the Remaining List 333 is searched to determine whether there is a match or matches for the PN„ measurement. If a match is found, the method proceeds to STEP 536. At STEP 536, a CPT is applied to the PN_M offset to confirm a positive identity determination for the match or matches identified in STEP 534. The CPT verifies which, if any, of the matches is correct. A "YES" answer to the CPT test signifies that the PN„ has been unambiguously identified by the CPT. If the answer at STEP 536 is YES, the method proceeds to STEP 538. At STEP 538, the offset for the Remote BTS identified for the current PN_U offset are added to the Hearable List 322. The method then proceeds to STEP 540 for further processing as described below.

072 Returning to STEP 534, if the answer is NO, the method proceeds to STEP 539. Similarly, if the answer at STEP 536 is NO, the method proceeds to STEP 539. At STEP 539, the processor notifies the operator by means of an error flag that a BTS corresponding to the current PN„ offset was not uniquely identified by the search process. The method then proceeds to STEP 540 and proceeds as noted above.

073 STEPs described above in reference to FIGURES 4, 5(a) and 5(b) may be implemented by a processor within a PDE, such as the PDE 130 (FIGURE 2), using a processor such as the processor 232, executing according to software instructions stored in a memory such as the memory 234. 074 As previously noted, in an alternative embodiment, the database may be located in an MS rather than in the PDE. In this alternative embodiment, the method steps described above may be implemented by a processor within an MS such as the MS 102, using a processor such as the processor 204, and a memory such as the memory 206. In this embodiment, the MS reports the BTS identities to the PDE rather than reporting the PN offset, so that the PDE is not required to determine the BTS identities.

075 As noted above, the CPT is used to select the correct entry from those stored in the Neighbor location sub-fields 315, Hearable location sub-fields 323, or Remaining location sub- fields 337 from within the location entries stored in the record associated with the key BTS entry 302. The particular name or other identifying information, such as SID/NID/BaselD of the BTS need not be known. What is important is that the correct entries within the database 300 be correctly selected. This is important, since the objective is to properly locate the source of the signals received by the MS. In one embodiment of the disclosed method and apparatus, the relative times of arrival of those signals are used, together with the location of the BTS from which the signals were sent, to determine the location of the MS.

076 In one example of the disclosed CPT, if the primary serving BTS is located in Seattle, Washington, then the MS 102 must be located close enough to communicate with the primary serving BTS. Accordingly, the MS 102 must be in or near Seattle. In addition, each other BTS from which the MS is receiving signals must be sufficiently close to the MS to permit the MS to receive those signals.

077 FIGURE 7 illustrates a system 700. The system 700 comprises a CPU 702, a memory 704, a transceiver 712 including a transmitter 708 and receiver 710, a signal analyzer 720, a statistical model 722 and a timer 724. The system 700 performs a statistical analysis based on the amount of overlap in coverage areas between primary serving BTS and a BTS represented by a particular set of entries to the database 300. This geographic region analysis assists in determining the likelihood that the particular BTS represented by the set of entries is the source of signals received by the MS. Once the likelihood is determined, the CPT outputs a decision as to whether the MS received signals from that particular BTS. The system 700 may also perform a phase measurement analysis using relative phase measurements. The coverage overlap and relative phase measurement processes are described in greater detail below.

078 The system 700 is provided with information regarding one or more candidate BTSs including, the PN offset, BAND_CLASS, and frequency of the signals received by the MS 302. The system 700 can limit the candidate list to BTSs that are located near the coverage area of the primary serving BTS. The coverage area may be refined based on the identify of any other BTSs that have been previously identified as having transmitted signals received by the MS 102. When the entries associated with a first BTS have been uniquely identified, that information may be used to uniquely identify entries from the database 300 associated with other BTSs from which the MS 102 is receiving signals. As more and more entries are identified, further information is provided to the system 700 to help identify additional entries, and thus locate the remaining BTSs from which the MS 102 has received signals.

079 In certain cases, the geographic region analysis described above may be sufficient to uniquely locate the BTS from which signals associated with a particular PN offset were transmitted. For example, there may be only one BTS represented by a particular PN offset that is located within the vicinity of the primary serving BTS. As previously discussed, uniquely locating one BTS from which signals were received by the MS 102 can provide further data used to locate additional BTSs from which signals have been received by the MS 102.

080 A one-dimensional probabilistic calculation is relatively simple to perform using a Gaussian distribution. The HEPE is based on an assumption that the density of MSs that hear a BTS is distributed as a two dimensional Gaussian distribution centered at the center of the BTS coverage area. The system 700 may calculate probabilities in two dimensions to accommodate variations in the location of the MS 102 in the North-South direction as well as variations in the East-West direction. To accommodate such two-dimensional probabilities, the system 700 calculates a "horizontal estimated position error" (HEPE) value based on possible errors in two directions. In one example, the HEPE value of a known coverage area is calculated as the square root of the sum of squares of error estimates in each of the two directions. If one assumes the MS 102 to be located within one-sigma (i.e., one standard deviation) from the mean in a Gaussian distribution of the location of MSs, the HEPE value may be represented by the following:

081

082 HEPE = τ]σ_N ² + σ_E ² (1)

083

084 where σ_N 2 i •ndicates a one-sigma error in the location of an MS in the North-South direction and σ_E 2 indicates a one-sigma error in the location of an MS in the East-West direction. Those skilled in the art will recognize that because the coverage areas are considered to be circles, the HEPE value represents the diagonal of a square, the sides of the square being equal in length to the radius of the circle. FIGURE 8 illustrates a coverage area 850 of a primary serving BTS. Associated with the area 850 is a HEPE value illustrated in FIGURE 8 as ti.

085 Since the MS 102 is known to be within the coverage area of the primary serving BTS, the coverage area of the primary serving BTS can be referred to as a "known area" 850. In addition, the known area 850 can include the intersection of the coverage areas of the primary serving BTS and the coverage area of other BTSs. Accordingly, the known area 850 may be smaller than the coverage area of the primary serving BTS if there is additional information available regarding other BTS from which the MS 102 is known to be receiving signals.

086 Also illustrated in FIGURE 8 are the coverage areas of three BTSs from which the MS 102 might have received signals based on the fact that each has an identical PN offset of 25 (i.e., 25 x 64 chips). The coverage areas 852 and 856 do not overlap with the area 850. In contrast, there is overlap between the coverage area 850 and a coverage area 854 corresponding to the PN 25 candidate 2. The one-sigma value for the PN 25 candidate 2 is illustrated in FIGURE 8 by the value r₂. The values r_x and r₂ indicate a metric to be used in determining the relative size of coverage area 850 with respect to the candidate coverage area 854. The distance from the center of the coverage area 850 and the center of the coverage area 854 is illustrated in FIGURE 8 by the reference D.

087 The statistical model 722 (see FIGURE 7) of the system 700 calculates a measure of coverage area separation using the relative size of coverage areas and the distance D separating the centers of coverage areas. This separation may be represented by the following:

088

D 089 Separation -» (2)

090 091 where all of the terms have been previously defined. A normal distribution statistical evaluation may be made of the term in equation (2) to generate a probabilistic measure of separation between the coverage area 850 and the coverage area 854.

092 The normal distribution is sometimes calculated using the following: 093

1 — 094 N nDυ (xx)j == e 2π (3)

095

096 where x is a number representative of the amount of separation away from a perfect overlap between the coverage area 850 and the coverage area 854. In one embodiment, the value x is chosen to be the Separation value from equation (2). This equation may be simplified as the following:

097

-x~ 098 ND(x) ^ e ² (4)

099

0100 where all of the terms have been previously defined.

0101 As an example of the application of the model 722 illustrated above, consider that the values r_ and r₂ are 2.0 and 1.0, respectively, while the distance D is 1J. Note that these distances may be measured in convenient units, such as kilometers or miles. Inserting these values into equation (2) provides a result of 0.49 for the separation. Substituting that value as x in equation (4) provides a result of 0.886. This indicates an 88.6% probability of perfect overlap between the coverage area 850 and the coverage are 854. Note that a perfect overlap gives the result of 1.0.

0102 In contrast, the one-sigma value for the coverage area 852, r₂, is equal to 1.5 while the distance D between the center of the coverage area 852 and the center of the coverage area 850 is 4.0. Applying these values to equation (2) provides a result of 1.6 for the separation. Substituting that value into equation (4) provides a result of 0.278, which indicates a 27.8% probability of perfect overlap between the coverage area 850 and the coverage area 852. Thus, it can be seen that there is a greater probability (i.e., higher likelihood) that signals received by the MS were fransmitted by the BTS at the center of coverage area 854 then by either the BTS at the center of coverage area 856 or coverage area 852.

0103 The system 700 can eliminate BTSs based solely on the geographic region analysis. However, those skilled in the art will recognize that there is some probability, however small, that signals received by the MS could have been transmitted by the BTS at the center, of coverage area 852 or coverage area 856. Therefore, in accordance with one embodiment of the disclosed method and apparatus, the system 700 will only eliminate a candidate if the probabilities calculated using equation (4) differ by a factor of 10. That is, a candidate will be eliminated based solely on coverage area overlap only if some other candidate is at least 10 times more likely to be the detected BTS. In the example illustrated above, candidate 2 is slightly more than three times more likely to be the BTS detected by the MS 102 than the candidate 1. Therefore, the system 700 will perform additional analysis to uniquely identify the candidate BTS.

0104 In one embodiment, the system 700 will analyze any candidate BTS using equation (4) if the result of equation (2) is less than 8. This first step of analysis ensures that even candidates with a very low probability of coverage overlap will be analyzed using equation (4). If the amount of the one-sigma separation in equation (2) equals 8, the probability using equation (4) is very small. As a practical matter, the system 700 will eliminate any candidate whose one-sigma overlap has such a large value. This may typically occur in a situation where great distances separate the coverage area of a candidate BTS from the coverage area of the primary serving BTS. For example, if the coverage area 850 is in Seattle, Washington and another BTS is in San Francisco, California, the distance D separating the two BTSs is so large that the probability of reception from the San Francisco BTS can be ignored.

0105 In addition to a coverage area overlap analysis described above, the system 700 uses a relative phase model to further narrow the list of candidate BTSs. The term "relative phase" is used to indicate the difference between the measurement phases between a known BTS and a reference BTS. This "relative phase" (when adjusted for known biases, including the PN offset) should be approximately equal to the difference between the distance from the known BTS and the MS 102, and a candidate BTS to the MS 102. As discussed above, each BTS transmits an identical PN sequence, but with known time delays or PN offsets. When two candidate BTSs have an identical PN offset, the signal will be detected by the MS 102 at different times (or phase offsets) based on the distance from the candidate BTS to the MS 102. hi one example, the MS 102 is known to be within the coverage region of the primary serving BTS 112. If two candidate BTSs are also within that coverage region, it may be possible to eliminate one of the candidate BTSs based on the relative phase, which is indicative of the propagation delay. For example, if one candidate BTS is within two miles of the Reference BTS while the other candidate BTS is twenty miles from the primary serving BTS, the relative phase between the two can often be used to eliminate one of the candidate BTSs.

0106 In one embodiment, the statistical model 722 (see FIGURE 7) uses a double- difference relative phase model as follows:

0107

0108 ND ([(dK-dCi)-(pK-pC)]/SC) (5)

0109

0110 where dK is the distance from the center of the combined coverage area (i.e., the combined coverage area of the candidate BTS and the primary serving BTS or another BTS, the location of which has been identified) to an already known BTS, dCi is the distance from the combined coverage area center to the tth candidate BTS, pK is the phase measurement to the known BTS, pC is the phase measurement to the candidate BTS, and SC is the size of the expected double-difference phase error based on the combined coverage area. The term "double difference" refers to a statistical calculation based on two difference measurements (i.e., the difference in distance minus the difference in phase).

0111 The combined coverage area is a probabilistic measure of the combined areas of coverage of the known BTS and the candidate BTS. Details on the measurement of the combined coverage area are provided below. The relative phase model is used to determine whether the phase delay measured by the MS 102 is consistent with the distances between the known BTS and the candidate BTS. As discussed above, the known BTS may be the primary serving BTS or any other measurement BTS that has already been uniquely identified.

0112 The example presented above is one technique that may be used to determine such relative phase differences. Those skilled in the art will recognize that other techniques may be used to determine such phase differences. The present invention is not limited by the specific analysis described above to determine the relative phase differences. 0113 The calculation of the relative phase is illustrated in FIGURE 9 where the approximate center of a combined coverage area 960 is indicated by the reference numeral 964.

The distance dK is the distance between the center 964 of the combined coverage area 960 and a known BTS 966. As discussed above, the known BTS 966 may be the primary serving BTS or any other uniquely identified BTS.

0114 A candidate BTS 968 has a coverage area 962, which in this example, is modeled as a circle. As shown in FIGURE 9, the candidate BTS 968 is not located at the center of the candidate coverage area 962. This is due to the fact that a typical BTS is not omni-directional, but is broken up into a number of sectors. The sector could be modeled by the system 700 as a pie-shaped sector. However, such modeling is often inaccurate due to back scatter from the antenna, as well as reflection off buildings, natural terrain, and other objects. Thus, the candidate coverage area 962 may be modeled as a circle. Similarly, the known BTS 966 is typically not located at the center of the known coverage area (not shown in FIGURE 9) for the reasons discussed above.

0115 The coverage areas of each BTS (or each cell sector) is determined at the time of installation and is known. The combined coverage area, indicating the coverage area of the known BTS 966 and the candidate BTS 968, can be calculated linearly by calculating an area of overlap of circular areas of coverage. Alternatively, the combined coverage area may be calculated weighting the coverage areas. The determination of the combined coverage area is described in greater detail below.

0116 The combined coverage area 960 is determined based on coverage areas mapped when a BTS is installed and calibrated. The combined coverage area 960 is a probabilistic estimation of coverage areas of the known BTS 966 and the candidate BTS 968. As discussed above, the two-dimensional positional error, referred to as HEPE value provides a measure of the statistical uncertainty in measuring the combined coverage area 960. In the system 700, a distance SC is based on HEPE value coverage and represents a one-sigma uncertainty in the relative phase.

0117 The distance between the center 964 of the combined coverage area 960 to the candidate BTS 968 is indicated by d{. Phase measurements p_κ and p_c are measured by the MS 102 and provided to the BTS using telecommunication standard IS-801.

0118 As noted above, the system 700 can calculate the expected relative phase difference and compare the expected phase difference with actual distance measurements. The system 700 may apply the normal distribution equation (4) to calculate the probability that the candidate BTS is consistent with the phase and distance measurements. If multiple candidate BTSs (with the same PN) are detected by the system 700, it may be possible to eliminate one or more the candidate BTSs based on the relative phase difference. That is, the candidate BTS must have a phase difference that is reasonable given the location of the known BTS from the center 964 of the combined coverage area 960 to the distance from the candidate BTS from the center of the combined coverage area. Candidate BTSs that are inconsistent can be eliminated as candidates to have been the source of signals received by the MS 102.

0119 The relative phase model is applied to other candidate BTSs as well. For example, FIGURE 8 illustrates three candidates that all have the identical PN 25 offset. The analysis process described above is applied to each of the candidate BTSs (e.g., the BTSs fransmitting PN 25 located in the center of the circles 850, 852, 854 in FIGURE 8) with a probability calculated for each candidate BTS. As noted above, a candidate BTS may be eliminated based solely on the coverage area overlap model if the coverage overlap of another BTS is at least 10 times more likely than the coverage area overlap of the BTS to be eliminated. Similarly, a particular candidate BTS may be eliminated based solely on the relative phase model if the phase difference probability of another BTS is at least 10 times more likely than the phase difference probability of the BTS to be eliminated. This process assures that a low probability candidate BTS will be eliminated while maintaining a low likelihood of eliminating the wrong BTS.

0120 The probabilities of the coverage area overlap model and the relative phase model may be combined to eliminate candidate BTSs. In one example, the probability of the coverage area overlap model is multiplied by the probability of the relative phase model. The combination of probabilities serves to further eliminate unlikely BTSs from the set of candidates. A candidate BTS may be eliminated based on the combined probability model if the combined probability overlap of another BTS is at least 10 times more likely than the coverage area overlap probability of the BTS to be eliminated.

0121 In addition to the analysis described above, the system 700 may also use signal strength and cell sector coverage models to uniquely identify candidate BTSs. As discussed above, a typical BTS has multiple transmitters and multiple antenna elements, each of which is directed for operation in a sector. In a typical embodiment, a BTS may have three sectors, each of which may be considered a separate BTS. The area of coverage of a typical sector may have a pie-shaped coverage area. 0122 The system 700 may calculate scale factors based on received signal strength. One measure of received signal strength is Ec/Io, which is a measure of the pilot energy accumulated over a 1 PN chip period (i.e., Ec) to the total power spectral density (i.e., Io) in the received bandwidth. Those skilled in the art will recognize that other power measurements may also be used with the system 700. The system 700 assigns a scale factor based on the strength or weakness of the received signal. If the received signal strength is relatively weak, then the MS 102 may be located within a relatively wide area with respect to the BTS. In this event, the circular coverage area may be expanded by a scale factor to produce a larger circular coverage area. In contrast, the system 700 may reduce the coverage area if the received signal strength is strong since the MS is more likely to be close to the BTS.

0123 In one embodiment of the disclosed method and apparatus, the system 700 may apply a scale factor of 0.9 for a strong signal (i.e., a signal above a threshold) and may apply a scale factor of 1J for weak signals (below the threshold). In a simple calculation, the coverage area of a single known BTS may be identified as a known area for the coverage area overlap model. Similarly, a single known BTS may be used in combination with a single candidate BTS to generate the combined coverage area used in the relative phase model. However, the system 700 can also accommodate calculations of the known area or combined coverage area that may result from mixing coverage areas from multiple cells. The cells may be combined in a linear fashion or may include weighting.

0124 Those of ordinary skill in the art shall recognize that computer readable medium that tangibly embodies the method steps of any of the embodiments described herein may be used in accordance with the present teachings. Such a medium may include, without limitation, RAM, ROM, EPROM, EEPROM, floppy disk, hard disk, CD-ROM, etc. The disclosure also includes the method steps of any of the foregoing embodiments synthesized as digital logic in an integrated circuit, such as a Field Programmable Gate Array, or Programmable Logic Array, or other integrated circuits that can be fabricated or modified to embody computer program instructions.

0125 The MS 102, in accordance with the present teachings may include, without limitation: wireless telephone, a personal digital assistant with wireless communication capabilities, a laptop having wireless communication capabilities, and any other mobile digital device for personal communication via wireless connection.

0126 A number of embodiments of disclosed method and apparatus have been described. Nevertheless, it will be understood that various modifications may be made to the disclosed method and apparatus. For example, the methods can be executed in software or hardware, or a combination of hardware and software embodiments. As another example, it should be understood that the functions described as being part of one module may, in general, be performed equivalently in another module. As yet another example, steps or acts shown or described in a particular sequence may generally be performed in a different order. In yet one more example, the disclosed method and apparatus is described with reference to the example of PN offset. However, persons skilled in the communications arts will understand that the disclosed method and apparatus may use other forms of ambiguous transmitter identifying information.

0127 Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments of the disclosed method and apparatus, but only by the scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A method for identifying the location of a transmitter of signals received by a mobile station, including: a) receiving a signal having ambiguous identification data; b) checking a neighbor list for data that matches the ambiguous identification data provided in the received signal; c) determining whether the received signal is above a threshold; and d) if above the threshold, then determining the location of the transmitter from location data associated with the matching data in the neighbor list.

2. The method of Claim 1 , wherein: a) if the received signal is not above the threshold, then verifying that the transmitter associated with the data that matches is likely to have transmitted the received signal; and b) if the transmitter associated with the data that matches is likely to have transmitted the received signal, then determining the location of the transmitter from data in the neighbor list associated with the data that matches.

3. The method of Claim 2, wherein: a) if either i) no data in the neighbor list matches or ii) no transmitter associated with data that matches is likely to be the transmitter of the received signal, then checking a hearable list of all transmitters from which signals had been previously received; b) if the received ambiguous data matches data associated with at least one transmitter in the hearable list, then determining from among transmitters on the hearable list associated with data that matches, the most likely transmitter to be the source of the received signal; and c) determining the location associated with the most likely transmitter on the hearable list from data in the hearable list.

A method for identifying the location of a transmitter of signals received by a mobile station, including: a) checking the neighbor list to determine whether the location of the transmitter might be provided by a candidate transmitter on the neighbor list and if so, then verifying that the candidate is a likely candidate; b) if no neighboring transmitter is likely to be the transmitter of the received signal, then checking a hearable list of all transmitters from which signals had been previously received;

c) if at least one transmitter in the hearable list is a candidate to be the source of the received signal, then determining how likely is that candidate to be the source of the received signal; and d) if the candidate transmitter is a likely candidate, then assuming that transmitter to be the source of the received signal and so determining the location of the transmitter from a database including the candidate transmitter.

A method for identifying the location of a base station transceiver (BTS), including:

a) receiving in a mobile station (MS) a signal from a primary serving BTS;

b) receiving in the MS an additional signal from a second BTS, the additional received signal having ambiguous data regarding the identity of the second BTS, the ambiguous data potentially being similar to data transmitted from at least a third BTS but not necessarily received by the MS; c) checking a neighbor list having less then all of the BTSs that could have transmitted the received signal, to determine whether the ambiguous data matches data in the neighbor list associated with at least one BTS, the location of the at least one BTS being disceπiable from data in the neighbor list; d) if the ambiguous data matches data associated with at least one BTS in the neighbor list, then verifying which BTS associated with the matching data is most likely to have transmitted the received signal; and

e) if either the ambiguous data does not match data associated with any of the BTSs on the neighbor list, or none of the BTSs associated with data that matched is likely to have transmitted the received signal, then checking a hearable list containing all BTSs from which signals had been previously received by MSs in communication with the primary serving BTS; f) if data associated with at least one BTS on the hearable list matches the ambiguous data of the received signal, then determining from among those BTSs associated with the matching data, the most likely BTS to have transmitted the received signal; g) determining the location of the transmitter most likely to have transmitted the ambiguous data from data on the hearable list.

6. The method of Claim 5, wherein the data on the hearable list is the location of the transmitter most likely to have transmitted the ambiguous data.

7. The method of Claim 5, wherein the data on the hearable list is a link to a memory that stores the location of the transmitter most likely to have transmitted the ambiguous data.

8. A method for identifying the location of a base station transceiver (BTS), including: a) receiving in a mobile station (MS) a signal from a primary serving BTS; b) receiving in the MS an additional signal from a second BTS, the additional received signal having first data regarding the identity of the second BTS; c) checking a neighbor list having less then all of the BTSs that could have transmitted the received signal, to determine whether the first data matches second data in the neighbor list; d) if the first data matches the second data associated with a BTS in the neighbor list, then determining the location of the BTS in the neighbor list from third data in the neighbor list; e) if the first data does not match the second data associated with any of the BTSs on the neighbor list, then checking a hearable list containing all BTSs from which signals had been previously received by MSs in communication with the primary serving BTS; f) if fourth data associated with a BTS on the hearable list matches the first data of the received signal, then determining from the BTS associated with the matching fourth data, the location of the transmitter from fifth data on the hearable list associated with the matching fourth data.

9. A method for determining the location of a Base Transceiver Station (BTS), including: a) receiving BTS data from a Mobile Station (MS) including: i) a unique identifier associated with a primary serving BTS; ii) PN data associated with non-serving BTSs from which signals were received by the MS; and iii) an indication of the signal strength of signals received from non-serving BTSs by the MS; b) seeking a match between the received unique identifier and key BTS data entries within a database; c) once the match is made, examining the received BTS data to determine whether one signal strength reported by the MS for a non-serving BTS signal is greater than a threshold associated with the primary serving BTS identified by the unique identifier in the database; d) if the signal strength of one of the signals transmitted by a non-serving BTS for which associated PN data was received by the MS is greater than the threshold, then determining the location of the non-serving BTS by matching the PN data associated with received signal, the strength of which was below the threshold, with PN data from a neighbor list associated with the matching key BTS data entry; e) if the signal is not greater than the threshold, then determining the likelihood that a non-serving BTS indicated by the PN data in an entry in the neighbor list was the source of the signal received by the MS from a non-serving BTS at a signal strength that is below the threshold; and f) if it is determined that it is likely that that non-serving BTS indicated by the PN data is the source of the signal received by the MS at a signal strength below the threshold, then determining the location of that non-serving BTS from data in the neighbor list.

10. A method for building a database used to determine the location of a Base Transceiver Station (BTS), including: a) receiving BTS data from a Mobile Station (MS) including: i) a unique identifier associated with a primary serving BTS; ii) PN data associated with signals received by the MS; and iii) an indication of the signal strength of signals received from non-serving BTSs by the MS; b) seeking a match between the received unique identifier and key BTS data entries within a database; c) once the match is made, examining the received BTS data to determine whether the strength of at least one signal strength received by the MS from a non-serving BTS is greater than a threshold associated with the primary serving BTS identified by the unique identifier in the database; d) if the strength of at least one signal is greater then the threshold, determining whether a neighbor list has been created within the database; and e) if no neighbor list has yet been created, then creating a neighbor list in the database and adding an entry for the PN data associated with each signal having a signal strength greater than the threshold.

11. A method for determining the location of a Base Transceiver Station (BTS), including: a) receiving from a mobile station (MS) data including: i) at least one Global Identifier; ii) at least one pseudorandom noise (PN) offset of signals received by the MS; iii) the signal strength of at least one of the signals that have the received PN offset; b) sorting a PN offset into a Candidate List if the strength of the signal received by the MS and having that PN offset is above a predetermined threshold; and c) determining whether each of the PN offsets of the Candidate List is on a neighbor list, the neighbor list including a location associated with each PN offset, and determining the location associated with each PN offset found in the neighbor list, the associated location being the location of the source of the signal having that PN offset.

12. The method of Claims 11 further including: a) sorting a PN offset into an Unknown List if the strength of the signal having that PN offset is not above a predetermined threshold; b) determining whether each of the PN offsets of the Unknown List is on a neighbor list, the neighbor list including a location associated with each PN offset; c) testing whether each PN offset in the Unknown List has been unambiguously associated with a PN offset in the neighbor list; and d) if at least one PN offset has been unambiguously associated, then determining the location associated with unambiguously associated PN offset found in the neighbor list, the associated location being the location of the source of the signal having that PN offset.

13. A position determination entity (PDE), including: a) a memory; and b) a central processing unit (CPU) for executing program instructions from the memory to: i) receive ambiguous identification data regarding the source of a signal received by a mobile station (MS); ii) receive data regarding the strength of the signal received by the MS; iii) check a neighbor list for data that matches the ambiguous identification data; iv) determine whether the strength of the received signal is above a threshold; and v) if the strength of the received signal is above the threshold and a match is found in the neighbor list, then determine the location of the transmitter of the signal from location data associated with the matching data in the neighbor list.

14. The PDE of Claim 13, wherein the CPU further executes instructions to: a) verifying that the transmitter associated with the data that matches is likely to have transmitted the received signal if the received signal is not above the threshold; and b) if the transmitter associated with the data that matches is likely to have transmitted the received signal, then determine the location of the transmitter from data in the neighbor list associated with the data that matches.

15. The PDE of Claim 14, wherein the CPU further executes instructions to: a) check a hearable list of all transmitters from which signals had been previously received if either: i) no data in the neighbor list matches; or ii) no transmitter associated with data that matches is likely to be the transmitter of the received signal; b) determine from among transmitters on the hearable list associated with data that matches, the most likely transmitter to be the source of the received signal if the received ambiguous data matches data associated with at least one transmitter in the hearable list; and c) determine the location associated with the most likely transmitter on the hearable list from data in the hearable list.

16. A mobile station (MS), including: a) a memory; and b) a central processing unit (CPU) for executing program instructions from the memory to: i) receive ambiguous identification data regarding the source of a signal received by the MS; ii) receive data regarding the strength of the signal received by the MS; iii) check a neighbor list for data that matches the ambiguous identification data; iv) determine whether the strength of the received signal is above a threshold; and v) if the strength of the received signal is above the threshold and a match is found in the neighbor list, then determine the location of the transmitter of the signal from location data associated with the matching data in the neighbor list.

17. A position determination entity (PDE), including: a) memory; and b) a central processing unit (CPU) coupled to the memory and capable of executing program instructions stored in the memory to: i) check a neighbor list to determine whether the location of a transmitter might be provided by a candidate transmitter on the neighbor list and if so, then verify that the candidate is a likely candidate; and ii) check a hearable list of all transmitters from which signals had been previously received if no neighboring transmitter is likely to be the transmitter of the received signal; iii) determine how likely is that candidate to be the source of the received signal if at least one transmitter in the hearable list is a candidate to be the source of the received signal; and iv) assume that candidate fransmitter to be the source of the received signal and so determining the location of the candidate transmitter from a database that includes the candidate transmitter if the candidate transmitter is a likely candidate.