Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1998-04-09
2001-06-12
Cumming, William (Department: 2684)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S456500, C370S330000, C370S335000, C375S348000
Reexamination Certificate
active
06246861
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems and methods for the location of mobile telephones. BACKGROUND OF THE INVENTION
Recently, there has been a great deal of interest in systems that determine the position of cellular telephones. In 1996, the Federal Communications Commission (FCC) in the United States pass a regulation requiring all cellular telephone service providers to provide Enhanced-911 (E911) service by 2001. E911 means that when a cellular phone is used to make an emergency call, the cellular system must be able to determine the physical location of that phone to an accuracy of 125 m, 67% of the time.
A great deal of work has been done in the cellular mobile location field. There have been many solutions proposed for the problem. However, most of these solutions require some modification of the cellular mobiles or base stations. Cellular networks are used extensively around the world. Cellular manufacturers and service providers have invested a great deal of money in the existing systems and standards. Any major modifications to these systems would require a great deal of effort and would cost a considerable amount. This means a solution that will function with the existing cellular systems is much more attractive.
There are several characteristics of a radio signal that can be used to determine the location of its source. Once characteristic is the strength of the signal. The average amplitude of a radio signal decays exponentially to the distance between the source of the signal and the point of reception. If the mobile were to monitor the strengths of known signals, transmitted from several known locations, those strength measurements could be used to determine the location of the phone. There are several patents that disclose location systems based on this principle.
P. W. Dent discloses his system in U.S. Pat. No. 5,404,376. In this system, each base station transmits location information to the mobiles. This information consists of a table or contour of received strength of the signal from that base station vs. distance. The mobile measures the received signal strength from a base station and then uses the information it receives from that station to determine its distance from it. The mobiles also transmit back their signal strength measurements to the base stations who use them to update their contours. The disadvantage of this system is that it requires extensive modifications of the mobile and the base stations.
Han-Lee Song discloses a different location system in U.S. Pat. No. 5,208,756. His system uses a location device placed in a vehicle that monitors the strengths of the signals received from the surrounding base stations. The strength of a radio signal decays with the distance from the transmitter, according to an exponential factor. The system assumes that this path loss exponent and the transmit power for each base station is known before-hand from measurements or calculation. That information is stored in the memory of each location device. The device Uses the log-distance channel equation to solve for the distances from the base stations to the mobile. It then uses the distances to triangulate the mobile's location. There are three disadvantages with this system. First, additional equipment or some modification of the mobile is required. Second, the device uses the same equation to model all channels. Radio channels vary a great deal which makes characterizing them with a single equation very difficult. Finally, the path loss exponent for each base station can vary considerably, depending on location in the cell. Approximating this with a single value will result in errors in the system. Also, if there is any change in the path loss exponent values, the memory in every one of the location devices will have to be updated.
Unlike the first two systems, the system disclosed by D. Dufor in U.S. Pat. No. 5,613,205 does not require the modification of the mobile. It uses signal strength measurements taken by the mobile that are used for another function. The System forces the mobile to sequentially go into handoff with each of its neighboring cells. It then Uses the signal measurements the mobile takes of the base stations as a result of the handoffs to determine the location of the phone. The distances from each of the base stations are calculated and the mobile position is found using triangulation. The disadvantage of this system is the added complexity of forcing the cellular phone to perform several unnecessary handoffs. This method could also cause degradation in call quality and would not work if the mobile had only a small number of nearby cells. It also increases the probability of a dropped call.
The system disclosed by J. R. Doner in U.S. Pat. No. 5,657,487 also does not require modification of the mobile. It uses the signal strength readings the mobile takes of its surrounding base stations for Mobile Assisted Handoff (MAHO). It also uses transmission time advance information for each mobile to determine location. This system divides the cell into large contour regions based on signal strength measurements taken in the area. The MAHO measurements taken by the mobile are used to map it into a specific location using the contour regions. The location is further refined by constraining the position estimate to lie on known service areas, like roads. The disadvantage of this system is that it requires extensive signal strength measurements to be made in the area. Another disadvantage is that in order to constrain the position estimate to a known service area, the topology of each cell must be surveyed in detail and stored.
SUMMARY OF THE INVENTION
The invention provides a location system that uses signal strength measurements taken by a cellular mobile to determine the location of that mobile. In many cellular systems, the mobiles already measure the strength of signals received from surrounding base stations. This information can be used for operations like Mobile Assisted Handoff (MAHO). Since this feature is already incorporated into several digital cellular systems, the invention requires no modification of the mobiles or the base stations.
In order to determine the location of the mobile, an appropriate equation model is used for the channel to express the signal strength measurements taken by the mobile as a function of the mobile's (x,y) position and the path loss exponent of the channel. There are several channel equations available that are suited for specific topologies. The most appropriate equation is selected for each cell. The signal strength measurements from the mobile are used in these equations to solve for the mobile's position and the path loss exponent of the channels. Several algorithms exist for solving these signal strength equations. If the algorithm needs a guess for the initial values of the position and path loss exponent, a prediction algorithm is used to determine accurate initial values based on the previous solutions of the signal strength equations.
The power of a signal in a radio channel decays exponentially with distance from the transmitter. The exponential factor in this decay is the path loss exponent. The path loss exponent is different for different channels and will vary, depending on the location of the mobile. By solving for the path loss exponent, variation between locations is taken into account. This allows accurate position estimates to be made for a variety of different locations in the cellular system. The results are much better than if a single path loss exponent was assumed for the entire cell. This also makes detailed measurements of the cellular coverage area unnecessary. Only a brief survey of each cell is required to determine which channel model will produce the best results for that cell.
There are several other aspects of the invention that provided enhanced mobile phone location. The processing unit may be programmed to request measurements for a specific mobile from the cellular system. The effects of channel fading can be reduced by having the mobil
Fattouche Michel
Messier Geoffrey G.
Petersen Brent R.
Cumming William
Stallman & Pollock LLP
Telecommunications Research Lab.
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