Abstract:
A method for determining the location of a mobile unit within a cellular system. A synchronized signal at a common phase is generated at each base station in a system. The mobile unit transmits a signal tone. Each base station compares the phase of the signal tone to the common phase of the synchronized signal to produce a phase offset. A system controller compares the difference between the phase offset of a first base station and the phase offset of a second base station and determines the difference in distance between the first base station and the mobile unit and the second base station and the mobile unit defining a hyperbolic curve of locations. The system controller determines the intersection of the first and second hyperbolic curves thus determining the location of the mobile unit. The mobile station transmits the signal tone on a second channel that is distinct from a first channel over which user information is transmitted. The mobile station switches to the second channel in response to a request by a base station to the mobile station.

Description:
This application is a divisional application of pending Serial No. 08/568,441, filed Dec. 7,1995, which is a file wrapper continuation of 08/218,197, filed Mar. 25,1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The following disclosure describes a method and system for determining the position of a mobile radio operating in the cellular radio service. A position service would have many desirable applications in the cellular radio service such as, location service for emergency callers (911), child locators, dispatch services, and fleet monitoring systems. Also, cellular system operators could use such methods to customize service parameters based on accurate knowledge of mobile telephone location such as lower cost services for limited mobility customers. Such a service would also be of use in locating stolen cellular phones and for investigating fraudulent use of cellular services. 
     Methods for radio position determination make use of techniques for measuring the propagation delay of a radio signal which is assumed to travel in a straight line from a transmitter to a receiver. A radio delay measurement in combination with an angle measurement provided by a directive antenna is the fundamental principle of radar. Radar location is frequently augmented by use of a transponder in the mobile vehicle rather than relying entirely on the signal reflected by the mobile vehicle. 
     Alternatively, multiple time delay measurements can be made using multiple transmitters and/or receivers to form a so-called tri-ateration system. The Loran system is an example of a system which transmits a series of coded pulses from base stations at know and fixed locations to mobile receivers. The mobile receiver compares the times of arrival of signals from the different transmitters to determine hyperbolic lines of position. Similarly, the global positioning system (GPS) provides transmission from a set of 24 earth orbiting satellites. Mobile receivers can determine their position by using knowledge of the satellites&#39;locations and the time delay differences between signals received from four or more satellites. 
     From the above examples, it can be seen that radio position location systems can be divided into two types, those which allow a mobile user to determine its own position and those which allow another party to determine the position of a mobile transponder such as radar systems. The system herein disclosed is of the second type where the fixed portion of a cellular telephone system determines the location of a mobile cellular telephone. Generally, such systems require that the mobile user transmit a radio signal (except in the case of passive radar.) 
     Methods of radio location such as disclosed in “Dual Satellite Navigation Method and System.” U.S. Pat. No. 5,126,748, issued June, 1992 require the mobile terminal to both transmit and to receive which allows round trip timing measurements defining circular lines of position to be performed using fewer transmitter sites than required for the Loran and GPS systems in which the mobile terminals contain only receiving capability. In other systems, the mobile terminal may contain only a transmitter and the remaining system elements perform direction finding or multiple receptions of the signal from different locations to determine the position. An example of this is the SARSAT system for locating downed aircraft. In this system, the downed aircraft transmits a signal on the international distress frequency 121.5 MHz (and 273 MHz). An earth orbiting satellite relays the signal to an earth terminal. As the satellite passes overhead, the change in Doppler shift can be detected and a line of position can be determined. Multiple overhead passes by the same or similar satellites can determine a set of lines of position, the intersection of which determines the location of the downed aircraft. 
     In the disclosed system, we wish to make use of the existing capabilities of mobile cellular telephones operating in the AMPS service (or similar service) to provide a new service of position location without modifying the millions of already existing AMPS mobile cellular telephones. In the AMPS service, the mobiles transmit at UHF frequencies between 824-849 MHz and base stations transmit at frequencies between 869-894 MHz. The frequency bands are divided into two sets of 833 channel pairs evenly spaced 30 kHz apart. One set of 833 channel pairs is licensed to each of two service providers in a given area. 
     The AMPS system uses analog FM modulation to transmit telephone speech. The mobile and base stations transmit simultaneously using full duplex techniques so that the user perceives a continuous link in both directions at all times. 
     Normally, each base station in a large cellular system serving a metropolitan area will be assigned a set of 57 channel pairs for providing telephone service. Additionally, one channel is assigned for signaling and paging. Calls are initiated at the mobile station by transmitting a digital message to the nearest base station on its control channel. The base station will respond on its corresponding control channel with a channel assignment to be used by the mobile while the call is in progress within the confines of this cell. If the call continues while the mobile moves into another cell, a control message from the base station will command the mobile to change channels to one assigned to the cell the mobile is moving into in a process called handoff. 
     The AMPS system includes a technique called supervisory audio tone (SAT) to insure that calls are being handled by the proper base stations. In this system, each base station adds a high frequency audio tone to the telephone audio of each call in progress. This tone will either be transmitted at 5980, 6000, or 6030 Hz. The mobile station will detect and filter this tone and transmit it back to the base station by adding it to the telephone audio. The base station then filters and detects the SAT tone and insures that the received tone is the same frequency as the tone it transmits. A pattern of SAT tone assignment to different neighboring base stations allows instances of incorrect connections to be detected and corrected. 
     When the AMPS system was originally being defined, it was contemplated that the mobile stations&#39;positions could be located by measuring the phase difference between the forward link SAT tone and the SAT tone received by the base station from the mobile. This would permit a round trip time delay measurement which would locate the mobile on a circle around the base station. It was seen that this technique would introduce the need for a specification controlling the phase shift of the returned SAT tone in order to provide consistent measurements. Because of this added complexity, this approach was dropped from the specification. 
     SUMMARY OF THE INVENTION 
     When the system desires to locate a particular mobile station, the mobile station is commanded to go to a predetermined and dedicated channel and transmit an audio tone over it&#39;s FM transmitter for a short interval, say one to ten milliseconds. The audio tone&#39;s frequency should be above the speech spectrum, e.g. greater than 4 kHz. At the end of the tone burst, the mobile returns to whatever it was doing previously, e.g., continuing its call, idle mode, etc. The channel frequency used for the position determination service would normally be dedicated to this purpose throughout the system and the controller would insure that only one mobile at a time transmits a positioning signal. 
     At the same time that the control message is sent to the mobile, the base stations are sent a control message indicating that a mobile is about to transmit a tone burst. The base stations are equipped with GPS receivers allowing accurate time and frequency references to be available at each base station. The base stations produce a tone reference signal at the same frequency and with synchronized phase based on the GPS receiver. The base station measures the phase difference between the tone reference signal and the signal tone (if any) received from the mobile station. The measurement interval used is the same as the transmission time, normally about one to ten milliseconds. The phase difference measurements are reported to the control station along with a measurement of S/N of the measurement. 
     The mobile station&#39;s position is calculated by computing phase differences between the tone burst phases reported by adjacent base stations. For example, if two adjacent base stations report the same phase difference relative to the reference phase, then the mobile is known to be somewhere on the perpendicular bisector between the two stations. If the phases are unequal, then the mobile is known to be on a hyperbola which is the locus of points having the same measured phase difference. If a third base station reports a phase measurement, then two more hyperbolas are determined. The intersection of the hyperbolas determines a solution for the mobile&#39;s location. 
     The precision required for the phase measurement is on the order of one degree. For example, 100 meter precision (300 ft) requires about 300 nsec precision. If the tone burst were 6 KHz in frequency, then about one-half degree resolution in the phase measurement would yield the desired precision. Note that this order of precision should be easily obtainable if the S/N is high enough. 
     It is possible to use the SAT signal (supervisory audio tones) for the above purposes. In the AMPS system, each base station transmits on the forward channel a tone of either 5980, 6000, or 6030 Hz. The mobile station receives this tone and retransmits it on the reverse channel. 
     One could perform position determination of any standard AMPS phone by the following method: 
     1) a call is established with the mobile station in the normal fashion; a command is sent to the mobile station ordering it to change channels to the above predetermined positioning channel; 
     2) the base station the mobile was connected to transmits on the channel with a predetermined SAT tone assignment, usually 6000 Hz; 
     3) the mobile station receives and retransmits the SAT tone in the normal way; the surrounding base stations measure the phase difference between the returned SAT tone and the reference tone derived from a received GPS timing signal; 
     4) the measurements are collected in a central point and the position computed; 
     5) the mobile station is commanded (by transmitting a normal control message) to return to its previous frequency to continue any call it may have in progress. 
     It is possible, using the SAT tone method, to track the position of the mobile as the call continues. The connected base station and the neighbor base stations can continue to measure the SAT tone phase difference relative to the synchronized reference tones on the normal cellular communication signals. However, the SAT tone frequency (the so-called SAT color code) now varies from one base station to another and the proper measurement must be made for each case. Also, the possibility of co-channel interference from mobiles in nearby co-channel cells can degrade the measurement accuracy. If, at any time during a call, a more accurate position is needed, the mobile can be commanded to the dedicated measurement frequency. 
     The signal to noise ratio (SNR) required to achieve the desired accuracy is determined by the received SNR and the averaging time. A half-degree resolution corresponds to seven bits of resolution. Each bit of resolution requires an additional 6 dB of SNR so that the total SNR is required to be 42 dB. If the received signal has a 20 dB SNR in a 4 kHz bandwidth, then the bandwidth must be narrowed 22 dB. This would appear to require a four millisecond measuring time. 
     Note that mobile terminal motion should not significantly affect the measurement. Consider that in four milliseconds, a mobile moving at 100 ft/sec will travel only 25 feet, significantly less than the measurement resolution. 
     Note also that the SAT tone frequency of 6 KHz is adequate to support unambiguous positioning in typically sized cellular telephone systems. The ambiguity distance for this tone frequency is about 50 kilometers corresponding to one complete cycle of the waveform or 166.7 microseconds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an overall block diagram of the system. 
     FIG. 2 shows a block diagram of the processing operations performed by the base stations to support the position determination process. 
     FIG. 3 shows a block diagram of the phase detector circuit. 
     FIG. 4 shows hyperbolic lines of position used to determine the location of the mobile unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An overall block diagram of the system is shown in FIG.  1 . The cellular system is comprised of base stations  100 ,  101 ,  102  and possibly many additional base stations and system controller &amp; switch  20 , also known in the cellular industry as the Mobile Telephone Switching Office (MTSO). The system subscriber mobile telephones are depicted as both car telephones  10  and  11  and pocket sized portable cellular telephones  12  and  13 . 
     The disclosed position determination system also utilizes earth orbiting satellites of the Global Positioning System (GPS)  200 ,  201 ,  202  and  203  as a means of precise synchronization. The GPS system is comprised of a constellation of 24 (plus spares) satellites arranged in orbits so that four or more satellites are visible any place on earth at all times. These satellites transmit signals that allow precise differences in time of arrival of signals from the satellites in view to be measured by GPS system receivers. The satellites are synchronized to Universal Coordinated Time (UTC) which allows precise time of day and frequencies to be obtained at GPS receivers. This capability of the GPS is utilized as a means for generated precisely synchronized reference tones at each base station. 
     The cellular system can provide up to about fifty-seven simultaneous telephone calls to mobile stations within each base station&#39;s coverage area. Normally, each base station is assigned 57 radio frequency channel pairs which are not used in any surrounding base station. Base stations located farther away may reuse the channels in a technique known as frequency reuse. Frequency reuse allows a city to be covered by many hundreds of base stations without running out of channels even though each system has a total of only 416 channels at its disposal. 
     In the disclosed system it is contemplated that one or more of the 416 channels would be dedicated to the position determination system and not used for telephone calls. This will allow a “clear channel” without co-channel interference from other mobile phones, resulting in higher signal-to-noise ratios, shorter measurement times and greater accuracy of position determination. The single channel for position determination would be shared by all the mobile stations in the system under control of the system controller. The system controller would determine which mobile stations require positions to be determined and transmit the commands to the mobile to be measured and the surrounding base stations. If a large number of position measurements must be made, more than one RF channel can be dedicated to this purpose. 
     It is also possible to use the disclosed techniques to monitor the position of each mobile telephone call in the system on a continuous basis rather than a sampled basis by making the disclosed measurements on each call in progress. 
     FIG. 2 shows a block diagram of the processing operations performed by the base stations to support the position determination process. The GPS antenna  300  and GPS receiver  301  provide a means for distribution of a common frequency and time reference to the base stations in the network. The Global Positioning System (GPS) consists of a network of 24 (plus spares) satellites in circular inclined orbits around the earth so that every point on the earth will have four or more of the satellites continuously in view. The satellites transmit a radio signal modulated by a spread spectrum waveform that allows very precise time measurements to be performed on the received signals. The GPS receivers acquire and synchronize to four or more of the downlink signals from the satellites. Then, the differences in time of arrival between the signals from the different satellites are measured. Together with knowledge of the orbital parameters, which are continuously transmitted on a data link from the satellites, the GPS receiver can solve for its position in three dimensional space and, as a by product, obtains very accurate time of day which is synchronized to universal coordinated time (UTC). If each base station in the system is so equipped, then they can all be synchronized very accurately. Note that for the purpose of the position determination functions, synchronization to UTC is not required, merely synchronization of all base stations to a common reference is required. 
     The GPS receiver will provide a one pulse-per-second (1 pps) signal together with a 1 MHz signal. The 1 MHz signal is used to synchronize a stable oscillator. The stabilized 1 MHz and the 1 pss signals are used to generate a 6000 Hz signal which is synchronized (that is, in phase) at each of the base stations. 
     The synthesizer  302  can be quite simple. For example, the 1 MHz signal is multiplied by 12 to produce a 12 MHz signal. The 12 MHz signal is then divided by 2000 in a digital divider. The result is a 6000 Hz square wave signal. The divide circuit is reset by the 1 pps signal at the beginning of each second. This insures that the divide by 2000 circuits in all of the base stations are in the same state at the same time. 
     The 6 kHz reference signal is summed with the transmit audio from the MTSO in summer  318 . The 6 kHz signal is used as the SAT tone for this channel. The resulting signal is converted into a FM transmit signal by cellular transmitter  317 . A predetermined 30 kHz channel in the 869-894 MHz band is dedicated for the purpose of position determination. In principle, any of the channels that can be used for a normal telephone call can be used for this purpose. The resulting FM signal is then radiated by the cellular transmit antenna  316 . Cellular transmitter  317  with summer  318  is an ordinary cellular base station transmitter with only the additional function of accepting the reference 6 kHz signal from synthesizer  302  for the SAT tone. 
     Only one of the base stations in the system will transmit to the mobile station at one time. The surrounding base stations will, however, normally be capable of receiving the signal and making measurements on it. 
     The mobile station will receive the signal, demodulate the SAT tone and sum it with its own baseband transmit audio and transmit it back to the base station. These functions of the mobile station are the ordinary functions of an AMPS mobile station during the course of an ordinary cellular telephone call. Thus, no special mobile station equipment is required to provide the desired services of position determination. Added equipment for position location is installed only in base stations and only in those base stations where the position location service is desired to be provided. 
     The base station will receive the signal from the mobile station with cellular receive antenna  310 . This signal is then amplified and demodulated by cellular receiver  311 . The receiver will accept control signals from the base station controller  320  and provide a receive signal strength indicator (RSSI) signal to the controller  320 . The controller  320  will relay the RSSI signal along with other signals to the MTSO for use in the calculation of the mobile station&#39;s position. The receive audio is delivered to the MTSO in the normal manner of an AMPS base station. The only additional function required of receiver  311  beyond that of an ordinary AMPS base station receiver is that the SAT tone signal be output to bandpass filter  312 . 
     The SAT tone signal added to the mobile station&#39;s transmitted signal is demodulated along with the speech signals in receiver  311 &#39;s FM demodulator circuit, typically a limiter-discriminator but many other circuits for FM demodulation are known in the art. The SAT tone signal at 6 kHz is separated from the speech signal by bandpass filter  312 . This filter might normally have a bandwidth of several hundred Hz. Generally, the narrower the bandwidth of this filter, the higher will be the signal-to-noise ratio of the SAT tone but a longer time will be required for the measurement of the phase of the output signal as it will exhibit a time response proportional to the bandwidth of the filter. 
     The output of the filter  312  is input to phase detector  314  along with the reference 6 kHz signal from frequency synthesizer  302 . The phase detector measures the difference in phase between the reference 6 kHz signal and the received SAT tone. The phase difference will be proportional to the distance between the mobile station being received and the base station. There are certain phase shifts in the SAT tone which are fixed and may be determined in advance such as the effective phase shift between the input to cellular transmitter  317  and the transmit antenna  316 . Phase shift is caused both by the response of filters and equalization circuits in the transmitter as well as physical delays of transporting the signal from one place to another. Likewise, there will be additional phase shifts in the cellular receiver  311  and bandpass filter  312 . These phase shifts can be measured and calibrated out of the system as they are fixed. 
     An additional source of phase shift is caused by the circuits in the mobile cellular telephone. The precise value of this phase shift will vary from telephone to telephone, however the general range of phase shift can be determined and calibrated out. Fortunately, the positioning accuracy of the disclosed system does not depend on the phase shift caused by the mobile&#39;s circuits. 
     The final source of phase shift, and the most important one, is due to the physical delay of transporting the signal through the air between the mobile and the base station. Only the phase shift of transmitting from the mobile to the base stations is important to positioning because the relative phase shift is measured in each of three or more base stations. Only one of the base stations is transmitting to the mobile so this phase shift is not important, except to establish the operating range of phase detector  314 . 
     A block diagram of a phase detector circuit is shown in FIG.  3 . The input signal is fed to multiplier circuits  330  and  331 . The reference 6 kHz signal is fed to the second input of multiplier  330  and to the input of phase shifter circuit  332 . The phase shifter circuit shifts the phase of the 6 kHz reference by 90 degrees (or pi/2). The output of phase shifter  332  is fed to the second input of multiplier  331 . Circuits for shifting the phase of a sinusoid by 90 degrees are well known in the art. It may be preferable in some cases for the frequency synthesizer  302  to produce two outputs of the reference 6 kHz signal that are 90 degrees shifted in phase. The multiplier circuit forms the arithmetic product of the two input signals. That is, if the two inputs are signals x and y then the output is x*y. The outputs of the two multiplier circuits  330  and  331  are inputs a and b to combiner circuit  333 . The combiner&#39;s output signal is equal to the arc tangent function of the ratio of inputs a and b. This phase detector circuit will function over a phase difference range of 180 degrees. Note that ratio a/b becomes undefined when input b is equal to zero which will happen at two points 180 degrees apart in phase difference. Normally, the fixed values of phase shifts will be adjusted so that the phase detector circuits active range will lie between these two undefined values. The operating range of 180 degrees will correspond to a range variation of 25 km or 15.5 statue miles. This will be adequate for most applications of the technique to AMPS cellular telephone systems. 
     The output of the phase detector circuit  314  is averaged by low pass filter  315  to produce the desired signal to noise ratio. The filtered output is then digitized and passed on to the controller  320 . The controller forwards the phase difference measurement along with the RSSI measurement to the MTSO where the position of the mobile station is calculated. 
     A desirable enhancement for the filters would be to reset the filters  312  and  315  at the beginning of the measurement process so as to prevent noise signals received prior to the measurement from affecting the result. 
     There are many different methods of implementing phase detector circuits which will be apparent to those skilled in the arts. An attractive implementation would be to digitize the SAT tone signal output from cellular receiver  311  and then to perform all of the functions of bandpass filter  312 , phase detector  314  and low pass filter  315  in digital circuits. This would allow the synthesizer  302  output to be used directly without first converting this signal to analog form. These processing steps could also be performed by a suitable digital signal processor (DSP) circuit. 
     The position determination process is begun at the MTSO. If we assume that a given mobile station is already in communication with the system and that it is determined that a position determination should be made for this mobile, the MTSO will control the following steps: 
     1) a command is sent to the mobile station ordering it to change channels to the above predetermined positioning channel; 
     2) the base station the mobile was connected to transmits on the channel with a predetermined SAT tone assignment of 6000 Hz; 
     3) the mobile station receives and retransmits the SAT tone in the normal way; all the surrounding base stations measure the phase difference between the returned SAT tone and the reference tone derived from a received GPS timing signal; 
     4) the measurements are collected at the MTSO and the position computed; 
     5) the mobile station is commanded (by transmitting a normal control message) to return to its previous frequency to continue any call in may have in progress. 
     The MTSO will signal all the base stations in the system that the measurement is taking place and all base stations will attempt to make a phase measurement at the indicated time. Most of the base stations will be out of range of the mobile station and will not make a meaningful measurement. The RSSI signal from such receivers will indicate a weak signal and the MTSO will ignore phase measurements from these base stations. Normally, only the closest neighbor base stations to the base station communicating with the mobile will make a meaningful measurement. The MTSO processing may in addition to ignoring phase measurements from receivers with low RSSI may also choose to ignore measurements from base stations not close to the communicating base station. 
     The position is determined as follows: phase measurements from pairs of base stations will define hyperbolic lines of position between the stations. For example, suppose the phase difference measurements for base stations  100  and  101  of FIG. 4 differ by 10 degrees, e.g., base station  100  measures −49 degrees relative to the reference and base station  101  measures −59 degrees. Thus, the portable unit  12  is about 1 mile closer to base station  100  than base station  101 . Furthermore, it is on a hyperbola which is the locus of points one mile closer to  100  than to  101 . Suppose we also find that base station  102  measures the portable unit as −39 degrees relative to the reference. Thus, the mobile is 10 degrees, or about 1 mile closer to  102  than to  100 . Also, the mobile is about 2 miles closer to  102  than to  101 . We have now defined three hyperbolas. Where they intersect on a map is the location of the mobile. Algorithms for computing position based on time differences given the Be knowledge of the base station locations is well known in the art. In fact the computation process is identical to that used in the Loran positioning system except that the measurements and computations are performed at the base stations and MTSO instead of at the mobile station. 
     On completion of the measurement process, the MTSO will command the mobile and base station to return to a regular (non-positioning) channel of the system to free up the positioning channel for another mobile to use. 
     Clearly, if all base station receivers are equipped to make the disclosed relative phase measurements then the position of all mobiles with calls in progress can be determined at the same time. The only significant additional problem with this is that there will likely be co-channel interference present due to the frequency reuse process normally employed by cellular systems.