Communications: directive radio wave systems and devices (e.g. – Directive – Position indicating
Reexamination Certificate
2002-06-25
2004-06-15
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Position indicating
C342S451000, C342S457000
Reexamination Certificate
active
06750818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus that enables the computation of the geolocation of a communication device, and particularly to the use of such a method and apparatus to compute the location of a mobile transmitter such as a mobile telephone (cell-phone) using either a handset-based or a network-based solution.
2. Description of the Prior Art
It is important to be able to geolocate a mobile telephone unit within a given service area both for security and for commercial reasons. As the number of wireless subscribers increase, an increasing number of emergency calls (911 calls) are expected to originate from mobile telephones. A study in the state of New Jersey indicated that about 43% of E-911 calls originated from mobile telephones (‘State of New Jersey, Report on the New Jersey enhanced 911 system trial: The first 100 days’, Jun. 16, 1997). Additionally, the Federal Communication Commission (FCC) has mandated that all mobile units should have a built-in location feature, such that mobile units are able to compute and transmit their location (to an accuracy of within 125 m) to a Public Safety Answering Point (PSAP) at least 67% of the time. This would facilitate emergency response to be dispatched to the site since the user may be incapacitated, or not be aware of their precise location. Besides emergency services, commercial applications involving location specific services such as advertising, location specific billing, fleet location monitoring, navigation, etc. are also envisioned in the near future.
The prior art for computing the location of a mobile telephone can be split into two broad categories; handset-based solutions and network-based solutions. Handset-based solutions are implemented such that the handset has the capability, using a Global Positioning System (GPS) or, alternatively, the time of arrival of signals (forward-link base station to mobile signals) from different base stations (at least 2 base stations are needed), the signal to noise ratio (SNR) of the signals, or a combination of GPS and time of arrival techniques, to ascertain its relative location with respect to the known, fixed locations of the Base Stations, and thereby its exact location, which it then can transmit to the network or to the PSAP.
There are significant drawbacks to using GPS because the use of a GPS involves additional hardware. Therefore, this handset-based discussion will focus on the use of a forward-link solution. In addition, GPS-alone solutions for mobile stations involve long acquisition times for the GPS signal and require a lot of power in the receiver of the mobile station. Server assisted GPS solutions overcome these limitations, but have poor reception deep inside buildings.
The basic principle of location computation, for a forward-link handset-based solution, is to receive signals from two or more base stations, and use the signal parameters (like SNR or times of arrival) to estimate the location of the handset from each of the base stations. Since the position of the base stations is known, knowledge of the relative position between the two or more fixed locations leads to an estimate of the location of the mobile station.
Network-based solutions involve two or more base stations simultaneously ascertaining the location of the mobile phone from which a location query has originated, relative to the base-stations, using the angle of arrival of the signals (a reverse-link mobile-to-base-station signal), the time(s) of arrival of the signals, or the signal-to-noise ratio of the signals actually received. The signal to noise ratios of these signals are compared to the pre-computed signal-to-noise ratios expected for different locations computed using drive test data. Thus, in a network based solution, two or more base stations receive the signal from the mobile station, i.e., on the reverse link, and those signal parameters (SNR, the time of arrival or the angle of arrival) are used by each base station to estimate the relative location of the mobile, and consequently, its actual location.
There are some deficiencies in each of the methods described above. Methods based on angle of arrival are susceptible to large errors when the base station and mobile station are close to one another; reflections of buildings can also cause huge changes in angles of arrival respect to the line of sight angle and induce errors. In addition, interference from ‘strong’ interfering sources may prevent adequate signal sources from being received and cause error in computation of location.
Given the importance of location determination, many approaches to computing the location of a mobile telephone exist in the patent literature. For example, U.S. Pat. No. 6,275,186, by Seung-Hyun Kong, teaches a solution where a dedicated searcher uses a combination of Signal-to-Interference ratios and times of arrival of the signal from a handset at multiple base stations to estimate its location. Both network- and handset-based solutions that depend on times of arrival and angle of arrival techniques share the problem that, in many environments, the three base stations needed to accurately locate the mobile transmitter are not ‘seen’ by the mobile transmitter because of the near-far problem. This happens because, on the forward link, the interference from a ‘strong’ base station contributes to the interference seen when the receiver is in the process of detecting and processing ‘weaker’ base stations.
In the case of network based solutions, interference from other mobile transmitters often prevents three base stations from being able to receive, detect and process the signal from a single mobile transmitter. This occurs because, in the reverse link between the mobile station and base station, especially for a base station that is not the serving base station for the mobile station in question, or for one that is not in handoff, the signals from the other mobiles sharing the bandwidth and being served by the base station cause interference in that base station's ability to detect and process the signal from the mobile station that needs the location service.
U.S. Pat. No. 6,263,208, by Chang et al., proposes a solution that uses a pre-computed probability map of pilot strengths for a given cellular area, and uses the actual received pilot strengths to estimate the location. They do not specifically address the issue of how a solution may be obtained if an adequate number of pilot signals are not visible to the mobile unit. Thus, there is no teaching of how to solve the problems of angle of arrival, reflections, and strong signals.
U.S. Pat. No. 6,163,696, by Qi Bi and Wen-Yi Kuo, proposes a network based solution where fake handoff messages are used, i.e., the mobile station increases its power until the primary base station and two or more other surrounding base stations are able to receive its signal. This solution, however, will cause increased interference to the signals from the other served mobiles in the area for the base stations in question. Thus, this solution compounds the strong signal problem.
U.S. Pat. No. 5,812,086 to Bertiger et al. proposes a central transceiver and a re-transmitter in order to service users within buildings. Such a system would be very costly and would not permit a user to use a communication device for geolocation purposes in a building that was not fitted with such a system.
The use of power control on the reverse and forward links is designed to limit interference and to ensure that the cellular network is balanced and at its optimal capacity. But even with the use of power control, say, when a mobile phone is very close to a particular base station, such interference is unavoidable because the strength of the signal from that base station contributes to the interference when the mobile phone is in the process of receiving, detecting and processing the signal from base stations
2
and
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, which may be further away. Similarly, in the case of the reverse link, since the servin
Narayan Anand P.
Thomas John K.
Blum Theodore M.
TensorComm, Inc.
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