Abstract:
A method and apparatus for locating, in relation to a plurality of cell sites, a mobile cellular transceiver in a conversation state. The method includes the steps of transmitting from one of the plurality of cell sites to the mobile transceiver a handoff signal for causing the mobile transceiver to continually transmit a known beacon signal while still in the conversation state, at at least some of the plurality of base stations, receiving the known beacon signal and measuring the time at which the known beacon signal was received, and calculating the location of the mobile transceiver from the respective locations of each of the at least some of the plurality of cell sites and the respective times at which the beacon signal was received at each of the at least some of the plurality of cell sites.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for locating a mobile transceiver operating in its conversation state. More particularly, the invention is directed to causing a mobile transceiver, while in conversation state, to transmit to a plurality of cell sites a predetermined beacon signal as a time difference of arrival reference datum. 
     2. Description of Related Art 
     It is desirable that wireless telecommunications service providers be able to geographically locate a mobile wireless transceiver in emergency situations. For example, a kidnap victim might be tracked via his mobile transceiver, including a cellular or PCS telephone. In fact, the United States Federal Communication Commission has mandated under Docket No. CC94-102 that wireless telecommunications service providers be able to locate a mobile transceiver within 125 m with a 67% degree of confidence as of Oct. 1, 2001. This ability is colloquially referred to as enhanced 911 (“E911”) service. 
     E911 service can be implemented from either the mobile transceiver perspective or the network perspective. From the mobile transceiver perspective, one might combine a global positioning system (“GPS”) receiver with a wireless mobile transceiver, such that the combination might periodically or on command report its geographic location to a proximate cell site. This solution has a number of disadvantages, including that the mobile transceiver becomes heavy, bulky, power hungry, and complicated compared to a conventional wireless transceiver. 
     From the network perspective, the problem is approached by realizing that a mobile transceiver in communication with one cell site is generally also detectable by other proximate cell sites. Thus, one can determine the geographic location of the mobile transceiver with reference to the geographic location of each of the cell sites detecting the mobile transceiver and the relative times at which each of these cell sites respectively receives a particular signal from the mobile transceiver. This network approach implements a time difference of arrival (TDOA) calculation. 
     In estimating at each cell site the time of arrival of a signal from the mobile transceiver, one conventionally detects both the beginning of the mobile transceiver signal and the signal&#39;s phase difference, or intra-symbol time delay. In this regard, it is advantageous that the cell sites know in advance the specific signal to be transmitted by the mobile transceiver. 
     For example, in a system compliant with interim standard IS-136, this predetermined signal can be obtained from the shortened burst that a mobile transceiver transmits when it initiates a call. This arrangement is satisfactory so long as the mobile transceiver is in a state from which it can initiate a call. For reference, the shortened burst is specified in interim standard IS-136.2. 
     However, it is also desirable that the mobile transceiver be able to summon help to its location while its user is engaged in an ongoing call, i.e. while the mobile transceiver is in a conversation state. For example, a mobile transceiver user might be speaking with a friend when he detects a need to summon emergency personnel to his location, in which case it might be unsafe or traumatic to terminate the conversation and then dial 911 to determine location. 
     Unfortunately, current solutions do not support generating a known beacon signal, for example a shortened burst, while a wireless transceiver is in the conversation state. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention are directed to locating a mobile transceiver in the conversation state. They provide for deliberately forcing the mobile transceiver into a handoff process, wherein without leaving the conversation state, the mobile transceiver transmits a predetermined beacon signal for example a shortened burst—to be received at proximate cell sites. Conventionally, this handoff process is initiated when poor signal quality indicates a need to determine which of the cell sites can best communicate with the mobile transceiver in its current location. However, according to aspects of the invention, this handoff process can also be used to generate a time difference of arrival dataset from the relative times at which the beacon signal respectively arrives at each proximate cell site. 
     More particularly, according to a preferred embodiment of the invention, the user initiates the locating process by transmitting a flash 911 (“*911”) signal from the mobile transceiver to the cell site with which it is currently in communication, requesting that the wireless communication network determine the geographic location of the mobile transceiver. 
     In response, the cell site receiving the *911 signal transmits to the mobile transceiver a handoff signal, including a synchronization signal. Thereafter, the mobile transceiver synchronizes to the synchronization signal and begins continually transmitting a predetermined beacon signal, for example a shortened burst. 
     Cell sites sufficiently proximate to the mobile transceiver receive the predetermined beacon signal and with reference to a time standard, ascertain the time of arrival of the beacon signal at that cell site. Each such cell site communicates to a mobile telephone switching office the time at which it received the predetermined beacon signal and, in the event that the mobile telephone switching has not previously stored the cell site&#39;s geographic location, the cell site&#39;s geographic location. 
     Upon receiving from such cell sites a sufficient dataset of location and time of arrival data, the mobile telephone switching office performs time difference of arrival calculations and thereby resolves the location of the mobile transceiver. 
     In another embodiment of the invention, another node on the mobile telephone system may initiate the locating process, either with or without the locating process being detectable at the mobile transceiver. In the case of a detectable locating process initiated for example by a friend or a family member, the mobile transceiver may prompt the user before transmitting the beacon signal to determine whether or not the user wants to be located. In the case of an undetectable locating process, advantageous for law enforcement applications, the mobile transceiver would transmit the beacon signal without prompting the user and without leaving the conversation state. 
     In yet another embodiment of the invention, the mobile transceiver may initiate the locating process for other than emergency purposes. Thus, the mobile telephone network may provide additional services to mobile transceiver users, such that when a user dials a predetermined code or number the mobile transceiver transmits a subscription service signal to the mobile telephone network, which responds by transmitting to the mobile transceiver location data in either visual or audible form. 
     Thus, more precisely, there is provided according to one aspect of the invention a method including the steps of: transmit ting from one of a plurality of cell sites to a mobile transceiver a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal without leaving a conversation state; at at least some of the plurality of cell sites, receiving the predetermined beacon signal and measuring the time at which the beacon signal was received; and calculating the location of the mobile transceiver from the respective locations of each of the at least some of the plurality of cell sites and the respective times at which the beacon signal was received at each of the at least some of the plurality of cell sites. 
     The handoff signal might be a signal to the mobile transceiver to handoff to a large diameter cell, might be a Fast Associated Control Channel Handoff (FACCH) signal, and might include a synchronization signal. 
     It is desirable that the method further include transmitting from one of the plurality of cell sites to the mobile transceiver a cancel signal for causing the mobile transceiver to stop transmitting the predetermined beacon signal and to resume normal conversation. 
     The cancel signal might include a physical layer control signal, which might in turn include a time alignment signal. 
     Preferably, calculating the location of the mobile transceiver includes calculating at a mobile telephone switching office the location of the mobile transceiver. In such case, the method might further include receiving at the mobile telephone switching office a query signal and transmitting the handoff signal from one of the plurality of cell sites in response to receiving at the mobile telephone switching office the query signal. 
     Receiving at the mobile telephone switching office a query signal might advantageously include receiving at the mobile telephone switching office a query signal from the mobile transceiver, including a *911 signal. 
     According to another aspect of the invention, there is provided a system, including: a mobile telephone switching office having a processor and memory storing codes for instructing the processor to calculate a location of a mobile transceiver based upon the respective locations of a plurality of receivers and the respective times that a beacon signal was received at each of the plurality of receivers; and a plurality of cell sites in communication with the mobile telephone switching office, at least some of the plurality of cell sites having: a transmitter for transmitting to the mobile transceiver a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal without leaving a conversation state; a receiver for receiving from the mobile transceiver the predetermined beacon signal continually transmitted from the mobile transceiver; a clock for measuring the time at which the predetermined beacon signal was received at the receiver; and a communication channel for communicating to the mobile telephone switching office the time at which the predetermined beacon signal was received at the receiver. 
     According to yet another aspect of the invention, there is provided a method including: receiving at a mobile transceiver from one of a plurality of cell sites a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal without leaving a conversation state; and transmitting to at least some of the plurality of cell sites, the predetermined beacon signal for time difference of arrival calculation. 
     According to still another aspect of the invention, there is provided a cellular mobile transceiver apparatus locatable in a conversation state with respect to a plurality of cell sites, having: a receiver for receiving from at least one of the plurality of cell sites a handoff signal for causing the mobile transceiver to continually transmit a predetermined beacon signal while still in the conversation state; and a transmitter for transmitting the predetermined beacon signal to at least some of the plurality of cell sites for time difference of arrival calculation in response to the handoff signal being received at the receiver. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In drawings which illustrate embodiments of the invention, 
     FIG. 1 is an overview block diagram of a mobile telephone system, including a mobile telephone switching office, a plurality of cell sites, and at least one mobile transceiver. 
     FIG. 2 is a block diagram detailing the architecture of the mobile telephone switching office (MTSO), including an MTSO microprocessor, an MTSO random access memory (MTSO RAM) and a MTSO read only memory (MTSO ROM). 
     FIG. 3 is a block diagram detailing the architecture of a cell site, including a cell microprocessor, a cell site random access memory (cell site RAM) and a cell site read only memory (cell site ROM). 
     FIG. 4 is a block diagram detailing the architecture of a mobile transceiver, including a mobile microprocessor, a mobile transceiver random access memory (mobile RAM) and a mobile transceiver read only memory (mobile ROM) 
     FIG. 5 is a table detailing the allocation of the MTSO RAM of FIG.  2 . 
     FIG. 6 is a table detailing the allocation of the MTSO ROM of FIG.  2 . 
     FIG. 7 is a table detailing the allocation of the cell RAM of FIG.  3 . 
     FIG. 8 is a table detailing the allocation of the cell ROM of FIG.  3 . 
     FIG. 9 is a table detailing the allocation of the mobile RAM of FIG.  4 . 
     FIG. 10 is a table detailing the allocation of the mobile ROM of FIG.  4 . 
     FIG. 11 is a flowchart of an MTSO Main Routine encoded in the MTSO ROM for instructing the MTSO microprocessor. 
     FIG. 12 is a flowchart of a Locate? Routine encoded in the MTSO ROM for instructing the MTSO microprocessor. 
     FIG. 13 is a flowchart of a Locate! Routine encoded in the MTSO ROM for instructing the MTSO microprocessor. 
     FIG. 14 is a flowchart of a Collect Dataset Routine encoded in the MTSO ROM for instructing the MTSO microprocessor. 
     FIG. 15 is a flowchart of a Cell Main Routine encoded in the cell ROM for instructing the cell microprocessor. 
     FIG. 16 is a Generate Data Sample Routine encoded in the cell ROM for instructing the cell microprocessor. 
     FIG. 17 is a flowchart of a Mobile Main Routine encoded in the mobile ROM for instructing the mobile microprocessor. 
     FIG. 18 is a flowchart of a Beacon? Routine encoded in the mobile ROM for instructing the mobile microprocessor. 
     FIG. 19 is a flowchart of a Beacon! Routine encoded in the mobile ROM for instructing the mobile microprocessor. 
    
    
     DETAILED DESCRIPTION 
     Description of Preferred Embodiments 
     FIG. 1 illustrates a mobile telephone system generally indicated at  10 . The mobile telephone system  10  includes a mobile telephone switching office (“MTSO”)  12  in communication with a plurality of cell sites  14  over a trunk  16 . The mobile telephone system  10  further includes at least one mobile transceiver  18  bidirectionally connectable to one or more of the plurality of cell sites  14  by a radio link  19 . 
     Each of the plurality of cell sites  14  defines a cell  20  within which the mobile transceiver  18  is likely to be in communication with that one of the plurality of cell sites  14 . Nevertheless, when the mobile transceiver  18  is outside of the cell  20  defined by any of the plurality of cell sites  14 , such cell sites  14  might still detect or even be in communication with the mobile transceiver  18 . 
     Thus, it will be seen that a mobile transceiver  18  located within a particular cell  20  and in communication with one of the plurality of cell sites  14  may transmit a signal receivable at more than one of the plurality of cell sites  14 . For clarity of explanation, this embodiment is being described in terms of a plurality of cell sites  14 . However, a broader contemplated embodiment would extend to a plurality of cell site partitions or sectors. In this sense, the term cell site  14  should be understood to mean and include cell site partition on cell site sector. 
     It will be appreciated that this network configuration supports time difference of arrival calculations to determine the location of the mobile transceiver  18  relative to proximate cell sites  14 . In particular, the configuration is well suited for supporting a method for locating the mobile transceiver  18  in a conversation state, where one of the plurality of cell sites  14  transmits to the mobile transceiver  18  a handoff signal for causing the transceiver to continually transmit a predetermined beacon signal, for example a shortened burst, without leaving the conversation state; at at least some of the plurality of cell sites  14 , receiving the predetermined beacon signal and measuring the time at which the beacon signal was received; and calculating the location of the mobile transceiver  18  from the respective location of each of the receiving plurality of cell sites  14  and the respective times at which the beacon signal was received at each of the receiving plurality of cell sites  14 . Preferably, this calculation step is carried out at the mobile telephone switching office  12 , being the common and controlling node of the mbbile telephone system  10 . 
     The embodiments that follow are described in significant detail for purposes of illustration, but it must be borne in mind that the invention itself is significantly broader as is set out in the claims. 
     FIG. 2 illustrates the architecture of the MTSO  12 . The MTSO  12 ; includes a microprocessor circuit (“MTSO microprocessor circuit”), generally illustrated at  40 . The MTSO microprocessor circuit  40  is in communication with memory devices, including random access memory (“MTSO RAM”)  42  and read only memory (“MTSO ROM”)  44 . Conventional address, data and control signal lines forming an MTSO local bus  46  are used by the MTSO microprocessor circuit  40  to read from each of the memory devices and to write to the MTSO RAM  42 . 
     In this embodiment, the MTSO microprocessor circuit  40  includes a microprocessor  48  (“MTSO microprocessor”) and various other conventional microprocessor circuit components including signal buffers, timers, and the like as will be appreciated by those skilled in the art, rendering the MTSO microprocessor  48  operable to communicate with the MTSO RAM  42  and the MTSO ROM  44 . Generally, the MTSO microprocessor circuit  40  establishes an address space with the MTSO RAM  42  and the MTSO ROM  44  mapped to respective areas of the address space. 
     The MTSO microprocessor circuit  40  is in communication with a plurality of interface components through an MTSO input/output (“I/O”) module  50 , including a communication switch  52 . 
     The communication switch  52  includes a plurality of contacts  54  which may be connected together in various combinations at various times to conduct various communication channels through the fabric of the communication switch  52 . 
     The MTSO microprocessor circuit  40  includes a plurality of interface circuits, some of which may be located on the MTSO microprocessor  48  and some of which may be remote from the MTSO microprocessor  48 , including on the MTSO I/O module  50 . These interface circuits establish a plurality of I/O ports within a designated address space through which communications between the MTSO microprocessor circuit  48  and the various interface components described above are conducted. Such communications are conducted by writing to or reading from ports associated with a given interface circuit or component described above. 
     In this embodiment, the interface circuits include a communication switch port  56 , in bidirectional communication with the communication switch  52 . 
     The communication switch  52  is in communication with the plurality of cell sites  14  through the trunk  16 , which connects to some of the plurality of contacts  54 . Similarly, the communication switch  52  is in communication with a public switch telephone network  58  via some of the plurality of contacts  54 . 
     FIG. 3 illustrates the architecture of one of the plurality of cell sites  14  in greater detail. The cell site  14  includes a microprocessor circuit (“cell microprocessor”), generally illustrated at  80 . The cell microprocessor circuit  80  is in communication with memory devices, including random access memory (“cell RAM”)  82  and read only memory (“cell ROM”)  84 . Conventional address, data and control signal lines forming a cell local bus  86  are used, by the cell microprocessor circuit  80  to read from each of the memory devices and to write to the cell RAM  82 . 
     In this embodiment, the cell microprocessor circuit  80  includes a microprocessor  88  (“cell microprocessor”) and various other conventional microprocessor circuit components, including signal buffers, timers, and the like as will be appreciated by those skilled in the art, rendering the cell microprocessor  88  operable to communicate with the cell RAM  82  and the cell ROM  84 . Generally, the cell microprocessor circuit  80  establishes an address space with the cell RAM  82  and the cell ROM  84  mapped to respective areas of the address space. 
     The cell microprocessor circuit  80  is in communication with a plurality of interface components through a cell input/output (“I/O”) module  90 , including a global positioning system (“GPS”) receiver  92 , a plurality of modems  94 , and a section of the trunk  16  connecting the cell site  14  to the MTSO  12 . 
     The GPS receiver  92  is connected by a radio link (not illustrated) to a plurality of geosynchronous satellites and, in response to signals received form the plurality of geosynchronous satellites, generates signals representing the current time and geographic coordinates at the location of the GPS receiver  92 . 
     The plurality of modems  94  are in communication with a multi-carrier radio transceiver  96 . The radio transceiver  96  receives a plurality of radio frequency signals respectively modulated onto a plurality of carriers and provides such plurality of signals to respective ones of the plurality of modems  94 . Similarly, the radio transceiver  96  receives a plurality of signals from respective ones of the plurality of modems  94  and modulates those signals into respective ones of the plurality of carriers. 
     The cell microprocessor circuit  80  includes a plurality of interface circuits, some of which may be located on the cell microprocessor  88  and some of which may be remote from the cell microprocessor  88 , including on the cell I/O module  90 . These interface circuits establish a plurality of I/O ports within a designated address space through which communications between the cell microprocessor circuit  80  and the various interface components described above are conducted. Such communications are conducted by writing to or reading from ports associated with a given interface circuit or component described above. 
     In this embodiment, the interface circuits include a GPS port  98  in communication with the GPS receiver  92 , a plurality of modem ports  100  in respective bidirectional communication with the plurality of modems  94 , and a trunk port  102  in communication with the trunk  16 . 
     FIG. 4 illustrates in greater detail the architecture of the mobile transceiver  18 . The mobile transceiver  18  includes a microprocessor circuit (“mobile microprocessor circuit”) generally illustrated at  120 . The mobile microprocessor circuit  120  is in communication with memory devices including random access memory (“mobile RAM”)  122  and read only memory (“mobile ROM”)  124 . Conventional address data and control signal lines forming a mobile local bus  126  are used by the mobile microprocessor circuit  120  to read from each of the memory devices and to write to the mobile RAM  122 . 
     In this embodiment, the mobile microprocessor circuit  120  includes a microprocessor (“mobile microprocessor”)  128  and various other conventional microprocessor circuit components, including signal buffers, timers, and the like as will be appreciated by those skilled in the art, rendering the mobile microprocessor  128  operable to communicate with the mobile RAM  122  and the mobile ROM  124 . Generally, the mobile microprocessor circuit  120  establishes an address space with the mobile RAM  122  and the mobile ROM  124  mapped to respective areas of the address space. 
     The mobile microprocessor circuit  120  is in communication with a plurality of interface components through a mobile input/output (I/O) module  130 , including an audio speaker  132 , a microphone  134 , a keypad  136 , and a wireless telephony module  138 . 
     The audio speaker  132  has a speaker input  140 . The audio speaker  132  generates audio waves in response to electrical signals received at the speaker input  140 . 
     The microphone  134  has a microphone output  142 . The microphone  134  generates electrical signals at the microphone output  142  in response to audio waves arriving at the microphone  134 . 
     The keypad  136  has a keypad output  144 . In response to predetermined keypress actions at the keypad  136 , predetermined electrical signals are generated at the keypad output  144 . 
     The wireless telephony module  138  has a circuit-side terminal  146  connected to the mobile I/O module  130  and a line-side terminal  148  connected to a radio transceiver  150 . The wireless telephony module  138  and the radio transceiver  150  are conventional. Radio frequency signals received at the radio transceiver  150  are down converted to provide a baseband analog signal to the wireless telephony module  138  at its line-side terminal  148 . The baseband analog signal is decoded at the wireless telephony module  138  and routed to the audio speaker  132  or the mobile microprocessor circuit  120  via the mobile I/O module  130 . Similarly, voice signals from the microphone  134  and command signals from the keypad  136  and the mobile microprocessor circuit  120  are provided to the wireless telephony module  138  via the mobile I/O module  130 . These voice and command signals are converted into baseband analog signals at the wireless telephony module  138  and passed to the radio transceiver  150  via the wireless telephony module line-side terminal  148  for modulation onto a radio frequency carrier. 
     The mobile microprocessor circuit  120  includes a plurality of interface circuits, some of which may be located on the mobile microprocessor  128  and some of which may be remote, from the mobile microprocessor  128 , including on the mobile I/O module  130 . These interface circuits establish a plurality of I/O ports within a designated address space through which communications between the mobile microprocessor circuit  120  and the various interface components described above are conducted. Such communications are conducted by writing to or reading from ports associated with a given interface circuit or component described above. 
     In this embodiment, the interface circuits include a speaker port  152  in communication with the audio speaker  132 , a microphone port  154  in communication with the microphone  134 , a keypad port  156  in communication with the keypad  136 , and a wireless telephony module port  158  in communication with the wireless telephony module  138 . 
     FIG. 5 illustrates in greater detail the allocation of the MTSO RAM  42 . The MTSO RAM includes a request array  200 , a fast associated control channel handoff (“FACCH”) array  202 , a timer buffer  204 , an index buffer  206 , an input queue  208 , a next_packet buffer  210  and a dataset array  212 . For reference, fast associated control channel handoff !signals are specified in interim standard IS-136, part  2 . 
     The request array  200  includes a network.request element  214 , a mobile.request element  216  and a mobileID.request element  218 . The network.request element  214  is loaded with codes representing an active or inactive digital state to indicate whether there exists a request on the network to locate a mobile transceiver  18 . The mobile.request element  216  is loaded with codes representing a digital active or inactive state to indicate whether a mobile transceiver has requested that it be located. The mobileID.request element  218  is loaded with codes representing a predetermined unique identifier respectively associated with each mobile transceiver  18 . 
     The FACCH array  202  includes a burst.FACCH element  220  and a timealign.FACCH element  222 . The burst.FACCH element  220  is loaded with codes representing a digital active or inactive state to indicate whether the MTSO is commanding a mobile transceiver  18  to transmit a shortened burst signal. The timealign.FACCH element  222  is loaded with codes representing an allocated time slot in a time division multiple access scheme. 
     The timer buffer  204  is loaded with codes representing a time. 
     The index buffer  206  is loaded with codes representing an integer number. 
     The input queue  208  is loaded with codes representing an input data stream received at the MTSO I/O module  50  from the communication switch  52 . The input queue  208  is associated with a read pointer and a write pointer (not illustrated) for respectively pointing to the location in the input queue  208  at which the next data is to be read or written. 
     The next_packet buffer  210  is loaded with codes representing a data packet read from the input queue  208  at the location indicated by the read pointer. 
     The dataset array  212  includes a place.dataset element  224  and a time.dataset element  226 . The dataset array  212  can store multiple sets of the place.dataset element  224  and the time.dataset element  226 , as referenced by an integer subscript  228 . The place.dataset element  224  is loaded with codes representing geographic coordinates in a format corresponding to that used in a global positioning satellite system. The time.dataset element  226  is loaded with codes representing a time in a format corresponding to that used in a global positioning satellite system. 
     FIG. 6 illustrates the allocation of the MTSO ROM  44  in greater detail. The MTSO ROM  44  is programmed with sets of codes readable by the MTSO microprocessor  48  for directing the MTSO microprocessor  48  to interact with the I/O ports to establish certain functionality according to conventional algorithms  251  and according to new algorithms described herein. New algorithms according to this embodiment of the invention are implemented by routines including an MTSO Main Routine  250 , a Locate? Routine  252 , a Locate! Routine  254 , a Collect Dataset Routine  256 , a Set Timealign.FACCH Routine  258  and a TDOA routine  260 . 
     Furthermore, the MTSO ROM  44  encodes a set of mnemonic constants, including an ACTIVE constant  262 , an INACTIVE constant  264 , an END_SAMPLING constant  266 , and a SAMPLE_HEADER constant  268 . 
     The mnemonic constant ACTIVE identifies a code representing a digital active state. The mnemonic constant INACTIVE  264  identifies a code representing a digital inactive state. The mnemonic constant END_SAMPLING  226  identifies a code representing a maximum sampling time. The mnemonic constant SAMPLE_HEADER  268  identifies a code representing the header of a data packet storing a sample of signal time of arrival data. 
     FIG. 7 illustrates the allocation of the cell RAM  82  in greater detail. The cell RAM  82  includes an input queue  280 , a next_packet buffer  282 , an ID buffer  284 , a time buffer  286 , a place buffer  288 , and a sample buffer  290 . 
     The input queue is loaded with codes representing a data stream received at the cell I/O module from the multi-carrier radio transceiver  96 . Associated with the input queue are a read pointer and a write pointer (not illustrated), for respectively indicating the location in the input queue  280  at which the next data is to be read or written. 
     The next_packet buffer  282  is loaded with codes representing the next data packet read from the input queue  280  at the location indicated by the read pointer. 
     The ID buffer  284  is loaded with codes representing a predetermined unique identifier associated with each mobile transceiver  18 . 
     The time buffer is loaded with codes representing a time in a format compatible with a global positioning satellite system. 
     The place buffer  288  is loaded with codes representing a geographic location in a format compatible with a global positioning satellite system. 
     The sample buffer  290  is loaded with codes representing a time of arrival data sample, and includes codes representing a header identifying the encapsulated data as a sample, an identifier identifying a mobile transceiver  18  associated with the sample in a format corresponding to the ID buffer  284 , a sample receipt time in a format corresponding to the time buffer  286 , and a sample location in a format corresponding to the place buffer  288 . 
     FIG. 8 illustrates the allocation of the cell ROM  84  in greater detail. The cell ROM  84  is programmed with sets of codes readable by the cell microprocessor  88  for directing the cell microprocessor  88  to interact with the I/O ports to establish certain functionality according to conventional algorithms  301  and according to new algorithms described herein. New algorithms according to this embodiment of the invention are implemented by routines including a Cell Main Routine  300  and a Generate Data Sample Routine  302 . 
     The cell. ROM  84  further includes codes representing mnemonic constants, including a mnemonic constant BEACON_HEADER  304  and a mnemonic constant SAMPLE_HEADER  306 . The mnemonic constant BEACON_HEADER  304  identifies codes representing the header of a packet associated with a beacon signal (a shortened burst) from a mobile transceiver  18 . The mnemonic constant SAMPLE_HEADER  306  identifies codes representing the header of a packet generated at the cell site  14  encapsulating time of arrival data. 
     FIG. 9 illustrates the allocation of the mobile RAM  122  in greater detail. The mobile RAM includes an input queue  320 , a next_packet buffer  322 , an FACCH array  324 , and a beacon buffer  326 . 
     The input queue is loaded with codes representing a data stream received at the mobile I/O module from the wireless telephony module  138 . Associated with the input queue  320  are a read pointer and a write pointer (not illustrated), for respectively indicating the location at which the next data is to be read and written. 
     The next_packet buffer  322  is loaded with codes representing the next data packet read from the input queue  320  at the location indicated by the read pointer. 
     The FACCH array  324  includes a burst.FACCH element  328 . The burst.FACCH element  328  is loaded with codes representing a digital active or digital inactive state indicating whether the mobile transceiver  18  has been commanded to transmit a shortened burst. 
     The beacon buffer  326  is loaded with codes representing a digital active or digital inactive state indicating whether the mobile transceiver  18  is set to transmit a shortened burst. 
     FIG. 10 illustrates the allocation of the mobile ROM  124  in greater detail. The mobile ROM  124  is programmed with sets of codes readable by the mobile microprocessor  128  for directing the mobile microprocessor  128  to interact with the I/O ports to establish certain functionality according to conventional algorithms  341  and according to new algorithms described herein. New algorithms according to this embodiment of the invention are implemented by routines including a Mobile Main Routine  340 , a Beacon? Routine  342 , and a Beacon! Routine  344 . 
     The mobile ROM  124  is also programmed with codes representing a mnemonic constant ACTIVE  346  and a mnemonic constant INACTIVE  348 . The mnemonic constant ACTIVE  346  identifies codes representing a digital active state. The mnemonic constant INACTIVE  348  identifies codes representing a digital inactive state. 
     Operation 
     The operation of this embodiment of the invention will now be described with reference to FIGS. 11 through 19. 
     FIG. 11 illustrates the MTSO Main Routine  250 . The MTSO Main Routine begins with a block of conventional code  251  which direct the MTSO microprocessor  48  to perform the functionality conventional to a mobile telephone switching office. Thereafter, block  400  directs the MTSO microprocessor  48  to execute the Locate? Routine  252 , whereafter the MTSO microprocessor  48  is directed back to the block of conventional code  251  for further execution. 
     FIG. 12 illustrates the Locate? Routine  252 . The purpose of the Locate? Routine  252  is to determine whether there exists a request to locate a particular mobile transceiver  18 . Such a request might be generated by any node on the mobile telephone system  10 , including the mobile transceiver  18  itself. The request would be issued in a conventional signalling manner so as to be interpreted by the MTSO  12  as such a request. Upon detecting such a request, the MTSO microprocessor  48  would be directed to set active the mobile.request element  216  if the request was issued by the mobile transceiver  18  itself or to set active the network.request element  214  if the request were issued by another node on the mobile telephone system  10 . 
     Thus, it will be appreciated that such requests may take a number of forms, and may in fact be incorporated into subscription services offered to mobile transceiver users. 
     In addition to enhanced  911  service, in which emergency personnel are summoned to the location of the mobile transceiver  18  without the mobile transceiver leaving the conversation state, a mobile transceiver user might subscribe to other services. For example, the user might cause the mobile transceiver  18  to issue an alternative subscription service signal, requesting receipt of an audible or visual indication of location. Such a subscription service would be useful in the event that the user is lost and is trying to describe his location to the party he is conversing with. 
     Similarly, network-based requests might take a number of forms. For example, police or other security personnel might issue a request to the mobile telephone system  10  to locate a particular mobile transceiver  18 . Such a request could be, satisfied without causing the mobile transceiver  18  to leave the conversation state, thus not alerting the mobile transceiver user that the location is being resolved. 
     Alternatively, a friend or family member of a mobile transceiver user might wish to locate the mobile transceiver  18  and thus the user. In this alternative, the mobile telephone switching office  12  issues a prompt signal to cause the mobile transceiver  18  to prompt its user to determine whether or not he wishes to be located by a party calling from a particular station. Desirably, such prompt signal and calling station information would be encoded within the handoff signal. After parsing and presenting such calling station information as a user-prompt and after receiving a prompt response from the user, the mobile transceiver  18  issues a prompt response signal to either enable or disable the locating process. The prompt response signal either could be a dedicated signal or alternatively could be implied by the presence or absence of the beacon signal in response to the handoff signal. 
     In greater detail, Block  402  directs the MTSO microprocessor  48  to read the network.request element  214  to determine if its contents are equal to the mnemonic constant ACTIVE  262 . If so, then a request has been received from a node on the mobile telephone system  10  to locate a particular mobile transceiver  18  and, as described above, that request is either automatically honored or has been approved by the user of the mobile transceiver  18 . Thus block  404  directs the MTSO microprocessor  48  to execute the Locate! Routine  254 , passing as a parameter the contents of the mobileID.request element  218  which uniquely identifies the mobile transceiver  18  associated with the locate request. Upon completion of the Locate! Routine  254 , block  408  directs the MTSO microprocessor  48  to return to the calling routine. 
     Alternatively, if at block  402  the contents of the network.request element  214  are not equal to the mnemonic constant. ACTIVE  262 , then block  406  directs the MTSO microprocessor  48  to read the mobile.request element  218  to determine if its contents are equal to the mnemonic constant ACTIVE  262 . If so, then a particular mobile transceiver  18  has issued a request to be located. For example, in a system compliant with IS-136, this request may take the form of a flash 911 (“*911”) signal issued to the MTSO  12 . 
     In this case, the MTSO microprocessor  48  is directed by block  404  to execute the Locate! Routine  254 , passing the contents of the mobileID.request element  218  as a parameter. Upon completion of the Locate! Routine  254 , block  408  directs the MTSO microprocessor  48  to return to the calling routine. 
     Alternatively, if at block  406  the contents of the mobile.request element  216  were not equal to the mnemonic constant ACTIVE  262 , then the MTSO microprocessor  48  is directed by block  410  to return to the calling routine, as there exists no request to locate a mobile transceiver  18 . 
     FIG. 13 illustrates the Locate! Routine  254 . The purpose of the Locate! Routine is to cause the MTSO microprocessor  48  to artificially force the mobile transceiver  18  to commence an intercellular handoff process. The mobile transceiver  18  is forced to issue a shortened burst signal to proximate cell sites  14 , ostensibly to determine the time alignment of a next cell site  14  with which it will communicate, but in actuality to generate a dataset of time of arrival samples from each of the proximate cell sites  14  on which to perform time difference of arrival calculations. 
     Block  420  directs the MTSO microprocessor  48  to store in the burst.FACCH element  220  the mnemonic constant ACTIVE  262 . Thereafter, block  422  directs the MTSO microprocessor  48  to transmit the FACCH array  202  to the mobile transceiver  18  identified by the mobileID.request element  218  via the cell site  14  with which the mobile transceiver  18  is currently in communication. 
     Thereafter, block  424  directs the MTSO microprocessor  48  to execute the Collect Dataset Routine  256 , passing as a parameter the contents of the mobileID.request element  218 . 
     Upon completion of the Collect Dataset Routine  256 , block  426  directs the MTSO microprocessor  48  to set the burst.FACCH element  220  equal to the contents of the mnemonic constant INACTIVE  264 . Thereafter, block  428  directs the MTSO microprocessor  48  to execute the Set Timealign.FACCH Routine  258 . This routine is conventional, allocating a time domain multiple access channel to the mobile transceiver  18  that provides it with the best channel characteristics currently available to it in the mobile telephone system  10 . Upon completion of the Set Timealign.FACCH Routine  258 , block  430  directs the MTSO microprocessor  48  to transmit to the mobile transceiver  18  the FACCH array  202  via the cell site  14  with which the mobile transceiver  18  is currently in communication. 
     Block  432  then directs the MTSO microprocessor  48  to execute the Time Difference of Arrival (TDOA) Routine  260  to analyze the dataset of time of arrival samples collected to determine the location of the mobile transceiver  18 . The TDOA Routine  260  is conventional. Upon completion of the TDOA routine  260 , block  434  directs the MTSO microprocessor  48  to return to the calling routine. 
     FIG. 14 illustrates in greater detail the Collect Dataset Routine  256 . The purpose of the collect dataset routine  256  is to cause the MTSO microprocessor  48  to parse the data stream arriving at the MTSO I/O module  50  to extract time of !arrival data samples submitted by cell sites  14  proximate to the mobile transceiver  18  being located. 
     Block  450  directs the MTSO microprocessor  48  to initialize the timer buffer  204  to zero and to initialize the index buffer  206  to the integer one. Block  452  then directs the MTSO microprocessor  48  to determine whether the contents of the timer buffer are greater than the mnemonic constant END_SAMPLING  266 . If so, then block  454  directs the MTSO microprocessor  48  to return to the calling routine, as the sample collecting interval has expired. 
     Alternatively, if the contents of the timer buffer  204  are not greater than the mnemonic constant END_SAMPLING  266 , then block  456  directs the MTSO microprocessor  48  to parse the next packet in the input queue  208  into the next_packet buffer  210 . 
     Block  458  then directs the MTSO microprocessor  48  to determine whether the contents of the next_packet buffer  210  includes the mnemonic constant SAMPLE_HEADER  268 . If not, then the MTSO microprocessor  48  is directed back to block  452  to determine whether there is sufficient time to conduct additional sampling. 
     Alternatively, if the contents of the next_packet buffer  210  includes the mnemonic constant SAMPLE_HEADER  268 , then block  460  directs the MTSO microprocessor  48  to determine whether the contents of the next_packet buffer  210  includes the contents of the mobileID.request element  218 , thereby identifying the sample as being associated with the mobile transceiver  18  being located. If not, then the MTSO microprocessor  48  is directed back to block  452  to determine whether there exists sufficient time to conduct further sampling. 
     Alternatively, if the contents of the next_packet buffer  210  includes the contents of the mobileID.request element  218 , then block  462  directs the MTSO microprocessor  48  to parse location and time data in the next_packet buffer  210  into the place.dataset element  224  and the time.dataset element  226  of the dataset array  212 , setting the contents of the subscript  228  equal to the contents of the index buffer  206 . It will be appreciated that the location data for all or some cell sites  14  might be stored at the mobile telephone switching office  12  instead of being received piecemeal in packets from the cell sites  14 . Thereafter, block  464  directs the MTSO microprocessor  48  to delete the contents of the next_packet buffer  210  from the input queue  208 , adjusting the read and write pointers appropriately. Block  466  then directs the MTSO microprocessor  48  to increment the contents of the index buffer  206 , and then block  452  directs the MTSO microprocessor  48  to determine whether there exists sufficient time to conduct further sampling. 
     FIG. 15 illustrates in greater detail the Cell Main Routine  300 . The cell main routine includes conventional code  301  for directing the cell microprocessor  88  to perform the functionality commonly found in a cell site. Thereafter, block  480  directs the cell microprocessor  88  to execute the Generate Data Sample Routine  302 , whereafter the cell microprocessor  88  is directed back to block  301  to re-execute the conventional code. 
     FIG. 16 illustrates in greater detail the Generate Data Sample Routine  302 . The purpose of the Generate Data Sample Routine  302  is to cause the cell microprocessor  88  to monitor the data stream arriving at the cell I/O module  90  from the multi-carrier radio transceiver  96  to determine when it has received a beacon signal—in this embodiment a shortened burst—from a mobile transceiver  18  and thereupon to create a time of arrival data sample for submission to the MTSO  12 . 
     Block  482  directs the cell microprocessor  88  to parse the next packet in the input queue  280  into the next_packet buffer  282 . Thereafter, block  484  directs the cell microprocessor  88  to determine whether the contents of the next_packet buffer  282  includes the mnemonic constant BEACON_HEADER  304 . If not, then block  486  directs the cell microprocessor  88  to return to the calling routine, no beacon signal having been received at the cell site  14 . 
     Alternatively, if the contents of the next_packet buffer  282  includes the mnemonic constant BEACON_HEADER  304 , then block  4188  directs the cell microprocessor  88  to extract from the next_packet buffer  282  into the ID buffer  284  the predetermined codes uniquely identifying the specific mobile transceiver  18  that is the source of the beacon packet. 
     Thereafter, block  490  directs the cell microprocessor  88  to read from the GPS port  98  codes representing the current time and to store those codes into the time buffer  286 . Similarly, block  492  directs the cell microprocessor  88  to read from the GPS port  98  codes representing the location of the cell site  14  and to store such codes into the place buffer  288 . Thereafter, block  494  directs the cell microprocessor  88  to store into the sample buffer  290  the mnemonic constant SAMPLE_HEADER  306 , and the respective contents of the ID buffer  284 , the time buffer  286 , and the place buffer  288 . 
     It will be appreciated that the location data for some or all cell sites  14  may be stored at the mobile telephone switching office  12  instead of being transmitted piecemeal in packets by the cell sites  14 . In fact, except in the case of mobile or portable cell sites  14 , cell site location data is generally stored at the mobile telephone switching office  12 . 
     Finally, block  496  directs the cell microprocessor  88  to transmit via the trunk  16  the contents of the sample buffer  290  encapsulated into a packet addressed to the MTSO  12 . Block  498  then directs the cell microprocessor  88  to return to the calling routine. 
     FIG. 17 illustrates the Mobile Main Routine  340  in greater detail. Block  341  represents conventional code for directing the mobile microprocessor  128  to implement the functionality conventionally found in a mobile transceiver  18 . Thereafter, block  500  directs the mobile microprocessor  128  to execute the. Beacon? Routine  342 . Upon completion of the Beacon? Routine  342 , block  502  directs the mobile microprocessor  128  to execute the Beacon! Routine  344 . Upon completion of the beacon! routine  344 , the mobile microprocessor  128  is directed back to block  341  to re-execute the conventional code. 
     FIG. 18 illustrates the Beacon? Routine  342  in greater detail. The purpose of the Beacon? Routine  342  is to cause the mobile microprocessor  128  to determine whether it is being commanded by the MTSO  12  to turn on its beacon signal or to turn off its beacon signal. 
     Block  510  directs the mobile microprocessor  128  to parse the next packet in the input queue  320  into the next_packet buffer  322 . Thereafter, block  512  directs the mobile microprocessor  128  to determine whether the contents of the next_packet buffer  322  is an FACCH array  324 . If not, then block  514  directs the mobile microprocessor  128  to return to the calling routine. 
     Alternatively, if the contents of the next_packet buffer  322  includes an FACCH array  324 , then block  516  directs the mobile microprocessor  128  to determine whether the contents of the burst.FACCH element  328  are equal to the mnemonic constant ACTIVE  346 . If so, then block  518  directs the mobile microprocessor  128  to set the contents of the beacon buffer  326  equal to the mnemonic constant ACTIVE  346 . Thereafter, block  520  directs the mobile microprocessor  128  to return to the calling routine. 
     Alternatively, if at block  516  the contents of the burst.FACCH element  328  were not equal to the mnemonic constant ACTIVE  346 , then block  522  directs the mobile microprocessor  128  to determine whether the contents of the burst.FACCH element  328  is equal to the mnemonic constant INACTIVE  348 . If not, then block  524  directs the mobile microprocessor  128  to return to the calling routine. 
     Alternatively, if the contents of the burst.FACCH element  328  are equal to the mnemonic constant INACTIVE  348 , then block  526  directs the mobile microprocessor  128  to set the contents of the beacon buffer  326  equal to the mnemonic constant INACTIVE  348 . Thereafter, block  520  directs the mobile microprocessor to return to the calling routine. 
     FIG. 19 illustrates in greater detail the Beacon! Routine  344 . The purpose of the Beacon! Routine  344  is to cause the mobile microprocessor  128  to turn on or turn off its beacon signal—in this embodiment a shortened burst—in response to the active or inactive state of the beacon buffer  326 . Thus, after the beacon buffer  326  has been loaded with the mnemonic constant ACTIVE  346 , the beacon signal will be transmitted continually until the time when the beacon buffer  326  is loaded with the mnemonic constant INACTIVE  348 . 
     Block  540  directs the mobile microprocessor  128  to determine whether the contents of the beacon buffer  326  are equal to the mnemonic constant ACTIVE  346 . If not, then block  542  directs mobile microprocessor  128  to return to the calling routine. 
     Alternatively, if at block  540 , the contents of the beacon buffer  326  were equal to the mnemonic constant ACTIVE  346 , then block  544  directs the mobile microprocessor  128  to cause a beacon packet to be transmitted from the mobile transceiver  18  via the radio transceiver  150 . Thereafter, block  546  directs the mobile microprocessor  128  to return to the calling routine. 
     Thus it will be seen that aspects of the invention provide a way for a mobile transceiver  18  in a conversation state to continually transmit a beacon signal without leaving the conversation state, the beacon signal being detectable by proximate cell sites  14  and locatable by the MTSO  12  through time difference of arrival calculations performed on time of arrival data submitted by each of the proximate cell sites  14 . 
     Those skilled in the art will appreciate that embodiments of the present invention could extend to a number of cellular technologies, including Time Division Multiple Access (TDMA), Global System for Mobile Communications (GSM), and Code Division Multiple Access (CDMA) technologies. 
     While very specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.