Pulse or digital communications – Spread spectrum
Reexamination Certificate
1999-09-09
2003-12-16
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
C455S456500
Reexamination Certificate
active
06665332
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to mobile radio geolocation systems. More particularly, the present invention relates to CDMA geolocation systems capable of determining the location of a mobile CDMA transceiver operating in a CDMA communication system.
BACKGROUND OF THE INVENTION
Mobile radio communication systems are well known in the art. Such systems typically use a plurality of base stations for broadcasting signals to, and maintaining voice or data communications with, mobile radio equipment. Most such mobile radio equipment, which usually are actually transceivers in that they are typically enabled to both broadcast and receive, are individually identified by a code or call number. Once turned on, each radio transmits an identifying signal that includes the code or call number so that the radio may be contacted if a call is directed to it. That is, the user of the radio does not have to be actively engaged in communication on the radio for an identifying signal of that radio to be received by the base stations.
Over time, it has become increasingly apparent that the ability to locate the source of a mobile radio would be advantageous for a variety of reasons. Of paramount importance is the ability to locate an injured person or law enforcement officer in distress. Other advantageous reasons relate to locating the origin or source of an illegally operated mobile radio.
In most mobile radio communication systems there is a high likelihood that the mobile radio signal can be received at multiple dispersed sites, such as dispersed base stations, allowing triangulation through lines of bearing or time difference of arrival (TDOA) hyperbolas. However, certain characteristics of code-division multiple access (CDMA) communication systems present special problems when implementing a geolocation system for determining the location of a mobile CDMA transceiver.
One main difference between CDMA communication systems and other mobile radio communication systems, such as FDMA and TDMA, is the received signal strength of the mobile radio emission at multiple base stations, which is critical for a triangulation-based geolocation system. Conventional communication systems typically use higher transmission power from the mobile radios, thus increasing the chances that the emission will be received and recoverable at multiple base stations. In the CDMA standard, mobile transceiver transmission power is kept at the minimum level required to ensure reliable communication with one base station. This is done in order to maximize the capacity of the CDMA communication system.
As a result of power control in a CDMA communication system, mobile CDMA transceivers are often transmitting at very low power levels. This is especially apparent as they move closer to a base station. Due to what is termed the “near-far” affect, mobile CDMA transceivers close to a base station need only transmit at low power. Those that are far away from a base station need to transmit at higher levels such that all emissions are optimally at equal power when they arrive at the base station.
In terms of the geolocation of CDMA signals, this power control severely limits the applicability of conventional geolocation methods, since mobile CDMA transceiver emissions are often only easily recoverable at one base station. Thus, new techniques for receiving mobile CDMA transceiver emissions at multiple base stations are needed in order to use conventional triangulation techniques for systems utilizing emerging CDMA air standards such as IS-95.
Unlike the air standards applicable to FDMA and TDMA systems, IS-95 allows multiple users to share a common frequency spectrum simultaneously, by assigning each system user a unique spread spectrum spreading code. A spread spectrum system makes use of a sequential noise-like signal structure, for example P.N. (pseudo-noise) codes, to spread the normally narrow band information signal over a relatively wide band of frequencies. The receiver correlates these signals to retrieve the original information signal.
A variety of triangulation-based CDMA geolocation systems have been proposed in U.S. Pat. No. 5,508,708, issued to Ghosh et al, U.S. Pat. No. 5,736,964, issued to Ghosh et al., U.S. Pat. No. 5,675,344, issued to Tong et al., U.S. Pat. No. 5,365,544, issued to Schilling, U.S. Pat. No. 5,506,864, issued to Schilling, and U.S. Pat. No. 5,228,056, issued to Schilling. However, these systems do not address how to overcome the low power characteristic of the received mobile CDMA transceiver emissions without requiring system intervention, such as transmission of special sequences or use of power up functions by the mobile CDMA transceivers. In addition, these systems to not account for frequency offsets between the transmitting and receiving hardware due to tuning error and doppler frequency shifts imposed by transmitter motion. Even slight differences in frequency will render these systems largely ineffective.
Accordingly, it would be advantageous to provide means for geolocating the position of a spread spectrum coded radio frequency emission, such as a CDMA radio signal, which addresses problems resulting from the low power characteristics of received mobile CDMA transceiver emissions without requiring special intervention by the communication system.
SUMMARY OF THE INVENTION
In accordance with the present invention, a CDMA geolocation system capable of determining the location of a mobile CDMA transceiver operating in a CDMA communication system includes at least one mobile transceiver capable of spread spectrum coded radio frequency emissions and communication with a plurality of base stations. Each base station is capable of receiving the spread spectrum coded radio frequency emissions from the mobile transceiver.
The system also includes means for synchronizing the base stations to the mobile transceiver in time and means for determining the geolocation of the mobile transceiver based on times of arrival of the spread spectrum coded radio frequency emission received at the base stations.
Each of the base stations preferably comprises means for identifying information bits of interest from orthogonal sequences of information bits, such as Walsh codes, in the spread spectrum coded radio frequency emission received at the base station and means for determining a time of arrival of the spread spectrum coded radio frequency emission received at base station.
The means for determining the time of arrival of the spread spectrum coded radio frequency emission received at the base station comprises means for dividing the spread spectrum coded radio frequency emission received at the base station into a plurality of stages, despreader/demodulator means for despreading/demodulating each of the stages into a plurality of Walsh codes, and calculating means for calculating the fast Fourier transform of each of the stages to produce a time-frequency cross ambiguity function. The purpose of which is to compensate for frequency offsets imposed by motion induced doppler frequency shifts, or tuning frequency mismatches between the transmitter and receiver hardware. A means for interpolating a selected peak in the time-frequency cross ambiguity function for determining the time of arrival is desirably also included.
The geolocation system further includes a geolocation control unit for determining a primary base station from the plurality of base stations. The primary base station is the base station in active communication with the mobile transceiver.
The geolocation control system includes means for routing the identified Walsh codes of interest from the primary base station to the secondary base stations, which include all of the base stations except the primary base station. Upon receiving the routed identified Walsh codes of interest, each of the secondary base stations identifies the Walsh codes of interest in the spread spectrum coded radio frequency emission it receives based on the routed identified Walsh codes of interest. The geolocation control system also includes mean
Carlson John P.
Gravely Thomas B.
Sullivan Mark C.
Allen Telecom Inc.
Chin Stephen
Michael Best & Friedrich LLC
Yeh Edith
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