Wireless telephones include cellular, cordless, mobile and PCS (Personal Communication Systems) telephones while radio tags include passive and active RF identification tags.
The need for wireless location finding and tracking of wireless telephones and radio tags is ever increasing. Some of the possible services for locating radio tags are for monitoring children, herds, valuable goods, toxic waste, fleet management, etc. Some of the possible services for locating wireless telephones are:
Enhanced Emergency Services:
In recent years pressure has been mounting for the development of technology to position cellular telephones. The primary driving force has been enhanced 911 (E911) services for wireless telephone subscribers. E911 services provide the 911 operator with information such as calling number, address, and the primary subscriber's name. This enables the operator to dispatch emergency response teams without waiting for the caller to provide their position verbally. Due to the nature of wireless services, the location of the caller is currently only available via verbal communication. The only positional information that may be derived from the current cellular infrastructure is the cell site with which the cellular caller is communicating. For cell sizes in the order of kilometers, this is not sufficient.
A notice of proposed rule making concerning E911 emergency calling systems was released by the FCC on Oct. 19, 1994 (CC Docket No. 94-102). In the document the FCC proposes to require that within five years of the rules being adopted, a wireless 911 caller be located in a three-dimensional environment within a radius of no more than 125 m with probability of 100%. Since this accuracy requirement is somewhat unreasonable, on Dec. 19, 1995 a technology working group composed of representatives of the Cellular Telecommunications Industry Association (CTIA), the National Emergency Number Association (NENA), and the National Association of State Nine-One-One Administrators (NASNA), reached an agreement to recommend an alternative. The recommendation is to require a wireless location technology capable of a horizontal accuracy of 125 m RMS within five years of the rule making. A vertical component is not required at this time. A horizontal accuracy of 125 m DRMS (Distance Root Mean Squared) equates to a 63% probability circle of radius 125 m.
Any location technology adopted should be capable of the above accuracy at the lowest possible cost and with minimal impact on the network and subscriber equipment. The use of GPS to determine position at the cellular telephone has been proposed in Grimes, U.S. Pat. No. 5,479,482 issued Dec. 26, 1995 and as in Bird, U.S. Pat. No. 5,418,537 issued May 23, 1995. However, a positioning system based on GPS equipped telephones would make all current telephones obsolete and would require a change to the signal standards in order to accommodate the positional information such as Sood, U.S. Pat. No. 5,293,645 issued Mar. 8, 1994. In addition, such a system would suffer the same line of sight availability limitations as GPS. It would suffer tremendously wherever the GPS signals are blocked such as in buildings, vehicles, tunnels etc., areas considered critical from an emergency point of view.
Smith, W. W. Jr.,"Passive Location of Mobile Cellular Telephone Terminals," Proceedings 25th annual IEEE International Carnahan Conference on Security Technology, Taipei, Taiwan, Oct 1-3, 1991, describes techniques for locating AMPs-based cellular telephones.
Tracking of Fraudulent Calls:
There is a general agreement that fraudulent calls cause a major revenue loss to the cellular service providers due to congestion and increased blockage of cellular calls, particularly during peak hours.
Tracking of Stolen Vehicles:
Conventionally, to track stolen vehicles requires installing an RF tag such as in Bird, U.S. Pat. No. 5,418,537 issued May, 23, 1995, leaving it permanently on in each vehicle to be tracked and a new infrastructure for the MSs throughout the desired service area.
Fleet Management for Courier and Transportation Businesses:
To manage fleet one may instal an RF tag such as in Song, U.S. Pat. No. 5,208,756 issued May, 4, 1993, and Sheffer et al., U.S. Pat. No. 5,218,367 issued Jun. 8, 1993, but this requires additional equipment to be added to the fleet.
Location Finding of Wireless Telephones:
This is envisioned to be a 1-900 service where the person dialing pays for the service. Basically, to know the location of any wireless telephone, one dials 1-900-TELFIND and gets prompted for a password which is provided to subscribers to the service (for security purposes). Upon validation of the telephone number and password, in about 20 seconds the telephone could be located. The coordinates could be given in one of many formats:
5.1 Longitude and latitude; PA1 5.2 Street address; PA1 5.3 Location on maps provided when subscribing to the service (e.g. page 54 G-4) which then pinpoints to a certain box on a map; and PA1 5.4 Electronic for inputting into other databases. PA1 1) the bandwidth (BW) of the transmitted radio signal, and PA1 2) the signal-to-noise ratio (SNR) of the received radio signal. PA1 1) multipath, PA1 2) clock error, PA1 3) frequency offsets, PA1 4) time synchronization and PA1 5) geographical geometry of the location acquisition stations. PA1 1) the time to post-process the correlation lobe, and PA1 2) the type of signal monitored. PA1 (1) AMPs, the North American analog standard for cellular telephones, is widely used in North America with about 26 million subscribers; PA1 (2) The environment where AMPs-based cellular telephones exist makes a horizontal accuracy of 125 m DRMS extremely difficult to attain without use of the current invention. The inventors have implemented a system based on the current invention to locate AMPs-based cellular telephone with a horizontal accuracy approaching 115 m. PA1 1) the Reverse Analog Control Channel (RECC), and PA1 2) the Reverse Analog Voice Channel (RVC). PA1 1. Alert (forces the phone to ring); PA1 2. Release (forces the phone to drop the call); PA1 3. Stop Alert; PA1 4. Audit (confirms the message sent to the mobile); PA1 5. Send Called-address; PA1 6. Intercept; PA1 7. Maintenance; PA1 8. Change Power to Power level 0-to-7; PA1 9. Directed Retry; PA1 10. Registration (forces another registration on the RECC). PA1 Step 1) the time required for monitoring the transmitted radio signal on either the RECC channel or the RVC channel, followed by the time for processing the received signal in order to estimate its TOA relative to a time reference; (an appropriate time reference could be the GPS Time); PA1 Step 2) the time required for downloading each TOA estimate from its corresponding Monitoring Station (MS) to a central site, followed by the time for processing all TOA estimates through hyperbolic (differential) trilateration in order to estimate the location of the mobile terminal; (an appropriate central site could be the Mobile Switching Center (MSC) and an appropriate trilateration method could be least squares-based); PA1 Step 3) the time required for overlaying the mobile subscriber's geographical location with individual Public Safety Answering Point (PSAP) coverage regions for determining appropriate PSAP routing.
Pursuit of Criminals:
To allow law enforcement agencies to find and track wanted criminals who use wireless telephones.
It is clear therefore that there are many applications for a wireless location system. The wireless location system, however, faces technological challenges. Two of these challenges are location accuracy, and processing time (to provide a location reading).
Location Accuracy:
Theoretically, the location accuracy performance for a wireless location technology is lower-bounded by the Cramer-Rao lower bound on the rms location error which depends directly on two factors:
Practically, in a wireless location system, many other factors affect the performance depending on the technology used:
4) interference (co-channel and adjacent),
In AMPs for example, the radio frequency (RF) channels are spaced by 30 KHz which is a relatively small BW compared to systems designed primarily for location such as GPS with a BW of 1 MHz over C/A (Coarse Acquisition) channels and ISM-based location systems with a typical bandwidth of 10 MHz. In GPS, the location system uses initially a conventional sliding correlator to obtain a set of pseudo-ranges (one pseudo-range per satellite). The pseudo-ranges are then used in trilateration to obtain a position fix of the GPS receiver. A typical accuracy for a commercial one point (i.e. no differential reception) GPS receiver with C/A code is around 30 m rms without Selective Availability (SA). In direct proportions, an AMPs land-based location system which uses initially a conventional sliding correlator at each location acquisition station to obtain a TOA estimate of the transmitted radio signal followed by a hyperbolic (differential) trilateration of all the TOA estimates (at some central site) should offer a location accuracy of around an unacceptable 900 m rms assuming no multipath.
When multipath is considered, the accuracy of the AMPs land-based TDOA location system could potentially degrade even further. The cellular frequency band is between 800 and 900 MHz and the propagation characteristics at these UHF frequencies will have a significant impact on positioning by trilateration as shown in Parsons D., "The Mobile Radio Propagation Channel," John Wiley & Sons, New York, 1992. That the ranges measured correspond to Line Of Sight (LOS) distances is a major assumption made when estimating position by trilateration. Although the dominant transmission mode in this band is LOS, reflections from natural and man-made objects as well as diffraction around said objects are also possibilities. Multipath and diffraction allow the cellular signal to propagate in heavily built up areas as well as indoors. However, they also cause the measured ranges to be longer than the true LOS distance which introduces error into the trilateration process. In addition, the propagation distance at UHF is relatively short. This allows frequency reuse in the cellular system but limits the number of observables in the trilateration process. For instance, in a dense urban environment with a delay spread of 3 microseconds (as shown in Hata, M., "Empirical Formula for Radio Propagation Loss in Land Mobile Radio Services," IEEE Transactions on Vehicular Technology, Vol. VT-29, No. 3, August 1980) multipath causes the location accuracy to degrade to more than 1400 m rms.
From the above one might conclude that a GPS system offers a much better accuracy of the TOA estimate than a land-based AMPs system using a conventional correlator. Correlation is an effective method of estimating TOA when the signal is known. The resolution of TOA estimation by correlation is a function of the bit rate and hence the bandwidth. In GPS receivers, correlators are an integral part of the delay lock loop (DLL) mechanism used to track satellite signals as shown in Spilker, J. J., "GPS Signal Structure and Performance Characteristics," Global Positioning System, Volume I, The Institute of Navigation, Washington D.C., 1980. Spread spectrum codes used to spread the satellite signals serve two primary purposes:
The first purpose is to enable multiple access to the L1 and L2 carrier frequencies. This allows each satellite to transmit data over a common frequency channel.
The second purpose is to allow for pseudoranging. The time delay between the received satellite code and the code replica within the receiver is a measure of the range between the satellite and receiver. Since the satellite signal is continuous a delay lock loop is able to track the signal. This enables the signal to be despread and the data demodulated.
GPS, however, requires additional equipment to be installed with the wireless transceivers.
In terms of time for the location information to be available, GPS receivers require several minutes from a cold start to attain an acceptable reading. By contrast, in an AMPs land-based location system, for example, the time for the location information to be available depends on two factors:
In Lo, U.S. Pat. No. 5,293,642 issued Mar. 8, 1994 and in Kennedy et al., U.S. Pat. No. 5,317,323 issued May 31, 1994, the post-processing time is relatively long due to its complexity. For example, in Kennedy et al., U.S. Pat. No. 5,317,323 issued May 31, 1994, the patent improves the location accuracy of a WT using a beam-former to reject multipath. Such an algorithm requires a number of antennas (typically eigth), a receiver following each antenna and an extremely powerful processor to combine all received signals in such a way to be able to form a beam in a desired direction.
One alternative to GPS is to use the cellular signals themselves. There are essentially two types of cellular signals to monitor: the signal on the reverse control channel and the signal on the reverse voice channel. Although treated as a spread spectrum code for the purpose of pseudoranging, both signals are not continuous and do not consist entirely of Pseudo Random Noise (PRN) codes. Therefore, it is not necessary to employ a DLL to track them for the purpose of despreading. Instead, one can use convolution to estimate TOA. Convolution of the received signal with a stored replica of the transmitted signal results in a correlation peak at the delay between the two signals. Rather than convolve in the time domain, it is sometimes more convenient to multiply in the frequency domain. Both the received signal and its replica are first transformed to the frequency domain where they are multiplied and the result inverse transformed to the time domain. Although the result of this process will give a correlation function from which a TOA may be derived, the resolution is limited to that of the Fourier transform. The traditional resolution bound on Fourier-based methods is the Rayleigh resolution criterion as shown in Haykin, S., "Adaptive Filter Theory," 2nd Edition, Prentice Hall, Englewood Cliffs, N.J., 1991, wherein the Rayleigh resolution is the inverse of the sampling period. Thus, conventionally, the resolution of a system based on the TOA of cellular signals does not approach the resolution of GPS based wireless location systems.