Abstract:
A system for determining the location of a mobile station in a cellular communications network, comprising a plurality of locator units fixedly positioned for tracking and measuring communications between a base station and a mobile station initiating an emergency communication, each locator unit comprising a receiving means for monitoring a control channel of the base station for detecting and receiving identifying information including an assigned voice channel associated with the particular mobile station initiating said emergency communication, a storage means for storing the identifying information associated with the to emergency communication, a controller means for tuning the receiving means to the assigned voice channel associated with the emergency communication for receiving voice and control channel data between the base station and the mobile station, a transmission means operable in a first mode responsive to the receiving means and the controller means for initiating a second communication to a monitor unit based on the stored identifying information, the second communication occurring further including data indicative of the signal strength as a function of distance of the mobile station relative to the particular locator unit, wherein the monitor unit is responsive to the plurality of locator units for receiving the second communication from each locator unit and determining and tracking the position of the mobile station based on the received signal strengths and the known positions of the fixed locator units.

Description:
FIELD OF THE INVENTION 
     The invention is related to communication systems in general, and more particularly to cellular communications systems having means for determining the location of a mobile station. 
     BACKGROUND OF THE INVENTION 
     A cellular system consists of an FM radio network covering a set of geographical areas (known as Cells) inside of which mobile two-way radio units, like Cellular Telephones, can communicate. The radio network is defined by a set of base stations distributed over the area of system coverage, managed and controlled by a centralized or decentralized digital switch equipment known as MTSO, or Mobile Telephone Switching Office. A base station in its geographical placement is known as a cell site. It is composed of low powered FM transceivers, power amplifiers, control unit, and other hardware depending on the system configuration. Its function is to interface between cellular mobiles and the MTSO. It communicates with the MTSO over dedicated data links, wire or non-wire, and communicates with mobiles over the air waves. The MTSO&#39;s function is controlling call processing, call setup, and release which includes signaling, supervision, switching and allocating RF channels. MTSO also provides a centralized administration and maintenance point for the entire network. It interfaces with Public Switched Telephone Network (PSTN), over wire line voice facility, to honor services to and from conventional wire line telephones. At present, there is no way to locate wireless callers automatically when they seek emergency assistance. 
     When an emergency phone call initiates from a wireline telephone (using the PSTN), local exchange carrier switching software routes the call. This software associates the calling telephone number (ANI) with address information (ALI) stored in a location database and routes the call to-a centralized Public Safety Answering Point (PSAP). 
     When a PSAP receives an emergency call from a wireless location (using cellular, PCS, or Specialized Mobile Radio (ESMR) technology), neither ANI nor ALI information is available to the call taker. Agents must rely on the caller&#39;s ability to provide location information. Without ANI, the 911 call taker cannot re-contract the emergency caller to obtain additional information either. 
     Issued in June 1996, Docket 94-102 creates rules to govern the availability of basic 911 services and the implementation of enhanced 911 (E 911) for wireless services. 
     Phase 1 requires wireless carriers to transmit 911 emergency calls to a PSAP identifying both calling mobile unit (ANI) and cell site/cell site sector (pANI). The emergency caller must transmit a Mobile Identification Number (MIN) or its equivalent, and the local 911 district must request ANI transmission from the wireless carriers. Phase 1 compliance is required by 1998. 
     Phase 2 requires wireless carriers to relay an emergency caller&#39;s number, allow PSAP attendants to redial the caller in case of disconnection, and relay the location of the base station or cell site receiving the 911 call and its phase information. By 2001, the location of the mobile station must by provided to the PSAP in two dimensions (x,y) accurate within a radius of 125 meters in 67% of all cases. 
     Accordingly, it is highly desirable to obtain a method of locating wireless callers when they seek emergency assistance without significantly modifying the existing cellular network software and topology. 
     SUMMARY 
     A system for determining the location of a mobile station in a cellular communications network, comprising a plurality of locator units fixedly positioned for tracking and measuring communications between a base station and a mobile station initiating an emergency communication, each locator unit comprising a receiving means for monitoring a control channel of the base station for detecting and receiving identifying information including an assigned voice channel associated with the particular mobile station initiating said emergency communication, a storage means for storing the identifying information associated with the emergency communication, a controller means for tuning the receiving means to the assigned voice channel associated with the emergency communication for receiving voice and control channel data between the base station and the mobile station, a transmission means operable in a first mode responsive to the receiving means and the controller means for initiating a second communication to a monitor unit based on the stored identifying information, the second communication occurring further including data indicative of the signal strength as a function of distance of the mobile station relative to the particular locator unit, wherein the monitor unit is responsive to the plurality of locator units for receiving the second communication from each locator unit and determining and tracking the position of the mobile station based on the received signal strengths and the known positions of the fixed locator units. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 provides an illustration of a preferred embodiment of the cellular network base location system according to the present invention. 
     FIG. 2 provides a schematic illustration of the locator unit portion according to the present invention. 
     FIG. 3 is a schematic illustration of the monitor unit portion according to the present invention. 
     FIG. 4 provides a flow chart depicting the operation of the locator unit portion of the location system according to the present invention. 
     FIG. 5 provides a flow chart depicting the operation of the monitor unit portion of the location system according to the present invention. 
     FIG. 6 provides a schematic illustration of an alternative embodiment of the locator unit portion according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before embarking on a detailed discussion, the following should be understood. The novel locator system illustrated in FIG. 1 is comprised of essentially two major components: an emergency locator unit  20  having components as illustrated in FIG. 2, and a monitor unit.  100  which interacts with the locator unit to receive voice and data from each locator and through triangularization determine the position of a mobile station emergency cellular phone conversation. The LDP is controlled by a microcontroller for operation in a first stand-by mode, where the locator monitor has forward and reverse control channels for emergency 911 calls initiated from a mobile station and a second recording mode for identifying information of a mobile station upon detection of an E911 call. A third active mode is entered in response to a signal from the monitor unit requesting voice channel information from the mobile station and signal strength measurements of the signal received from the mobile station which is a function of distance of the locator unit from the mobile station. 
     Referring now to FIG. 1, there is shown an embodiment of the cellular network based location system according to the present invention. The system  10  comprises a plurality of location determination point units (LDPs)  20 ,  24 , and  28  for monitoring emergency cellular phone communications activity and transmitting such information to a monitor unit  100  located at a predetermined distance from each of the location units for calculating the position of a mobile station  40  based on the signal strength received from each of the locator units. As is well known, a typical cellular telephone network comprises a geographic area divided into a number of small neighboring cells, each containing a base station  50  as shown in FIG.  1 . The cell periphery contains each of the elements shown in FIG. 1, including the LDPs and base station. The base station  50  is assigned a number of two-way voice channels used to transmit voice signals to and from mobile station  40  and a number of set-up or control channels. Preferably, mobile station  40  is a cellular radio telephone. These channels are used for transmission of digital control information to and from the mobile station for establishing a voice communication link. The control channels assigned to each base station generally include several fields of data including a set-up call information such as a MIN number, assigned voice channel, and signal strength of the mobile station. Generally, each of the channels assigned to a particular base station operate a different frequency in order to avoid interference. Note that there is a number of limited frequencies available within a network. However, base stations remote from one another may be assigned one or more of the same frequency channels. 
     As is well known, voice information is generally transmitted over voice channels using frequency modulated (FM) analog signals, while setup and control information is transmitted over control channels effective using FM digital signals. Other methods of modulating voice and digital data onto the carrier signals may also be used including the amplitude modulation (AM), quadrature phase shift keying (QPSK), and binary phase shift keying (BPSK). 
     Mobile switching center  60  is interconnected with base station  50 , as well as with public switch telephone network (PSTN)  70 . The mobile switching center maintains network identification data for each mobile station  40  being served in certain cell sites. In general, a plurality of mobile switching centers, in communication with the base station and other switching centers, perform processing and switching functions enabling connections between mobile stations and interfacing to external PSTN network for routing signal communications over conventional telephone lines to emergency response modules  80  and  90 , such as a police station or public safety answering point. Note that when an emergency call initiates from a wireline telephone using the PSTN, local exchange carrier switching software routes the call. This software associates the calling telephone number with address information stored in the location database and routes the call to a centralized public safety answering point. Note further that, as previously mentioned, when a PSTN receives an emergency call from a wireless location using cellular PCS or specialized mobile radio technology, neither the calling telephone number nor the address information is available to the call taker. Rather, agents must rely on the caller&#39;s ability to provide this location information. Still further, without the calling telephone number, the 911 call taker is unable to recontact the emergency caller to obtain additional information. 
     The operation of the novel location determination system will now be described with reference to FIGS. 1-5 of the drawings. Referring now to FIG. 1, in conjunction with FIGS. 2 and 4, each location determination point unit  20 ,  24 , and  28  is positioned inside a particular cell at a predetermined distance between the center of the cell (i.e. base station) and the cell perimeter  55 . In the preferred embodiment, each LDP is placed at an equal distance between the base station and cell perimeter. As shown in FIG. 2, the main components of each LDP include receivers  22  and  26  for monitoring various cellular voice and/or control channels, transmitter  24 , and RAM memory  34 . Each LDP within the cell registers with the network and is activated within the particular cell using the same protocol as in conventional cellular telephone communications. Each LDP operates to monitor continuously the forward control channel (FCC) and reverse control channel (RCC) of a particular cell for detecting an emergency  911  communication from a mobile station  40  to base station  50  (Signal  1 ). Note that receiver  26  continuously monitors the FCC via Duplex filter and antenna  40 , while receiver  22  monitors the RCC, which lags the FCC by approximately 25 mega cycles. FIGS. 2-3 show a preferred embodiment of an LDP unit  20 , and monitor unit too, respectively, FIGS. 4 and 5 represent flow diagrams of a 911 LDP of unit and 911 monitor unit. Referring to this figures, the LDP operates, upon registration with the network, to monitor both the forward and reverse control channels for a signal either from the monitor unit  100  or from a mobile station where an emergency 911 call has been initiated. When the LDP detects the emergency communication signal between the mobile station and the base station, DSP  28  (FIG. 2) is operable to measure the signal strength of the received signal from the mobile station as a function of distance of the particular LDP from that mobile station. EPROM  32  and RAM  34  operate in response to the emergency signal detection to originate the communication (i.e. telephone call) to a predetermined number which has been stored in memory  34  and which is associated with the monitor unit  100 . The stored setup call information is than transmitted via transmitter  24  (FIG. 2) in addition to transmission of each LDP&#39;s unique identification number, also stored in RAM  34 . In this manner, the identity of the mobile station, the assigned voice channel, the measured signal strength, and the LDP ID associated with the transmitting LDP are communicated from the LDP to the monitor (module  60 ). Upon detecting of any 911 call (module  4 C) emergency setup request received on the RCC via receiver  22  through antenna and duplex filter  40 , each LDP  20  will baseband process the signal via DSP  28  and record in memory  34  the setup call information associated with the mobile telephone seeking to establish the telephone link and thus, communication with the base station (module  50 ). Setup call information includes the 911 ID or MIN number, the assigned voice channel over which the communication will occur, and the measured signal strength of the mobile station. The LDP then stores each of these parameters in RAM  34 . Each LDP unit then tunes its receiver  26  to the assigned voice channel for receiving voice and data from the base station. With reference to FIGS. 1,  2 ,  3 , and  5 , monitor unit  100  (FIG. 3) functions to receive a transmission (i.e. signal  3 ) from each LDP at modem  110  and calculates the position of mobile station  40  based on the signal strength values obtained from each LDP. That is, each LDP operates to measure the signal strength based on the relative distance from mobile station  40 . This measured signal strength, assigned voice channel, MIN number, and LDP ID, transmitted to monitor  100 , is received by modem  110 , and stored in memory  120 . The monitor  100  thus receives the information from each LDP sequentially by receiving information from the first calling LDP, storing the information, and terminating communication with the LDP (i.e. release LDP). The same process is repeated for each of the second and third LDPs. Memory  120  further includes prestored values of the distances of the locator units, and positions of those units in the cell (i.e. x,y coordinates). Then, based on the received signal strength indicators from each of the LDP units and the known coordinate positions of each of the LDPs, the monitor operates to use triangular location of each of the three transmitted signal strengths to obtain the relative geographic position of the mobile station within the cell via detector circuit  130 . It should be noted that every cell requires the use of a minimum of three LDPs providing signal measurements in order to perform the triangularization. However, more LDPs may be used, depending on the cell coverage area. Note that the monitor unit is also operable to receive signals from LDPs in other cells and to initiate communications with those LDPs for tracking of the mobile station and performing any cell handoff activities. Note that the monitor unit includes in its memory and database a list of each of the LDPs of neighboring cells in addition to the LDPs associated with the cell in which the monitor is located, such that the location and cell associated with a particular LDP is readily determinable via the monitor. 
     In any event, detector circuit  130  also operates to compare the received signal strength included in each of the LDP signals with one another to determine the strongest signal for voice monitoring. As previously mentioned, communication with the LDPs is sequentially terminated and the LDPs are released. In response, the contacted LDP then transmits voice and signal strength data associated with the mobile station emergency call received via receiver  22  to monitor  100 . However, upon determination of the LDP having the strongest signal, monitor  100  originates a call to that LDP to transmit voice and signal strength information associated with the particular mobile station on the assigned voice channel (module  240  of FIG.  5 ). 
     The monitor unit may be implemented as a PC computer and include a map/graphical user interface for displaying map data, as well as a cellular phone subscriber data base  140 , and a display screen  160 . The subscriber data base  140  includes user identifying data for associating the MIN number of the mobile station with the user or subscriber associated with that mobile phone. The database includes the MIN number or telephone number associated with the mobile station and the identity of the user corresponding to MIN number including the user&#39;s home telephone number, that user&#39;s name, address, and any other relevant identifying information. The monitor unit then functions to display on the display terminal  160  a map of the cell area, as well as the determined position of the mobile station on the map, and the identifying information of the user associated with the mobile station onto the display terminal. A speaker provides the received voice communication output at the monitor in an audible format. The display information includes a telephone number, name, date, time, and recording of the phone conversation. Monitor  100  maintains communication with the LDP  20  having the determined strongest signal for voice monitoring. Information regarding signal strength is periodically transmitted from locator  20  to monitor  100 . Monitor  100  further includes logic in circuit  130  for periodically sampling the signal strength associated with the transmitting LDP having the highest signal strength and comparing the signal with a threshold value stored in memory. Comparator  135  functions to compare the received signal strength with the stored value. If the received signal strength varies from the stored value by a predetermined amount, such variation is indicative of the change in position of the mobile station, and monitor  100  operates to originate a call via modem  110  to each of the locator units to perform a new signal strength measurement and to return such information to the monitor unit for calculating a new position of the 911 caller (module  250  of FIG.  5 ). In this manner, LDP locator units, in conjunction with the monitor, operate to detect, identify, and track a mobile station, making an emergency cellular phone call. FIGS. 4 and 5 provide flow charts illustrating the above described operation of the system. 
     Referring again to FIG. 2, each LDP comprises receiver  22  for monitoring and receiving RCC data, in order to detect an (E911) emergency communication, a transmitter  24  which is responsive to the detection from receiver  22  for transmitting voice and data to the monitor  100 , and a receiver  26  for receiving voice and forward control channel data from the base station via monitor  100  when the monitor requests new signal strength measurements for new position calculation using conventional cell phone call procedures and protocols. Processor  28  performs digital signal processing for baseband processing of the system for detecting the presence of an emergency signal. Microcontroller  30  controls each of the system modes via switch  38  to perform either forward control channel (FCC) reception, transmission, or RCC passive listening via filter and antenna  40 . Memory devices  32  and  34 , serve as peripherals to microcontroller  30  for storing the setup call information signal strength and additional data required for cellular protocol in communications. Power supply  36  operates to provide sufficient operating power to the unit. Alternatively, the LDP may be configured as illustrated in FIG.  6 . In this embodiment, switch  38  has been eliminated and receiver  22  includes an associated dedicated antenna  42 . This embodiment permits continuous monitoring of the appropriate RCC for a voice channel and data for response to either an emergency 911 call or in response to a call from the monitor unit requesting new signal strength measurements. In the preferred embodiment, the receiver sensitivity for each of the receiver units at 12 DB(SINAD) demodulated signal-to-noise ratio is −116 DBm. The maximum power transmitted by the transmitter is approximately 630 mW, while the received signal strength dynamic range measurement is −90 DB. The DSP includes all functions associated with control data supervision (SAT) and signal strength measurements in signaling and is incorporated into a single data processor. A microcontroller PCB80C552 operates to control and handle the function of the LDP with low power and high operation speed. Normally, capacity of the LDP unit is approximately 32 digits, while the power supply is preferably a 7.2 v Nicda with an AC/DC adapter. 
     As a result of the operation of this novel locator system, no changes are necessary to existing cellular network topology. The monitor  100  may be placed at any number of positions and a number of monitors may be used. Further, these monitors may be used in police station and PSAPs. As a result, base station, cell phone, and mobile switching center software, in addition to PSTN communication software require no modification. 
     Still further, while it has been shown that monitor unit  100  receives locator transmission via the cellular network, each locator may also include the capability to originate telephone calls via local land lines in order to notify appropriate personnel. 
     While there has been shown a preferred embodiment of the present system, it should be understood that a person may make many variations and modifications to the embodiment utilizing functionally equivalent elements to those described herein. For instance, while discussion has focused on operation within a particular cell, the system is capable of performing handoff and tracking of the mobile station across a number of cells, via conventional cellular system handoff protocol. Any and all such modifications, as well as others which may become apparent to those skilled in the art, are intended to be included within the scope of the invention as defined by the appended claims.