Telecommunications – Transmitter and receiver at separate stations – With control signal
Reexamination Certificate
1998-04-28
2001-05-29
Hunter, Daniel S. (Department: 2689)
Telecommunications
Transmitter and receiver at separate stations
With control signal
C455S502000, C342S358000
Reexamination Certificate
active
06240276
ABSTRACT:
This invention generally relates to the field of radio frequency (RF) communication, more particularly, to communicating position information using a Global Positioning Satellite (GPS) receiver.
The GPS system, which has 24 orbiting satellites, is gaining wide spread use in many positioning applications. In cellular communication systems that offer positioning services, GPS receivers are being incorporated in mobile stations to provide position information. The position information so provided can be used, for example, to locate the user of a mobile station during an emergency situation.
The GPS satellites transmit satellite signals periodically, every 1 msec, on a known radio frequency (RF) carrier. The satellite signals, which are direct sequence coded signals, specify the transmission time and the identity of the orbiting satellite. After receiving the satellite signals, a GPS receiver uses well known correlation technique to recover the coded information. By processing the coded information from at least four orbiting satellites, the GPS receiver determines the position information, which include latitude, longitude, and amplitude coordinates of a point where the satellite signals are received.
An important feature of a GPS receiver is the speed by which it determines the position information. This speed is directly related to the speed by which the GPS receiver locks onto the RF carrier of the satellite signals. As with other propagating waves, the satellite signals are subject to doppler frequency shifts. It is well known that if the doppler frequency shifts are compensated for, the GPS receiver can lock onto the RF carrier at a faster speed. In this way, weak satellite signals may be detected more quickly.
In order to quickly lock onto the RF carrier, the GPS receiver must be tuned by an accurate reference frequency. For most positioning applications, a reference frequency accuracy of about +/−0.1 ppm is desired. Some conventional solutions rely on expensive local oscillator arrangements for providing such an accurate reference frequency.
In existing cellular systems, a mobile station generates a local reference frequency by locking onto RF signals that are received from a base station. Some of the base stations of existing cellular systems, for example, AMPS systems, however, do not have an accurate reference frequency. Although very stable, the reference frequency of the base station may have +/−1.5 ppm inaccuracy. Therefore, the local reference frequency of the base station, which is generated based on the reference frequency, becomes inaccurate. Because of such inaccuracy, a mobile station's GPS receiver is tuned with a tuning frequency error, which increases the time needed to lock onto the RF carrier. Consequently, the inaccuracy of the reference frequency reduces the speed by which the position information are determined.
Also, in order to perform longer coherent correlations of the GPS signal, for example, when receiving weaker signals (such as indoors), it is required that the doppler frequency be compensated more accurately. Therefore, there exists a need for providing position information more quickly, despite inaccuracies associated with the reference frequency of a cellular system.
SUMMARY
Briefly, the present invention is exemplified in a communication system that compensates for tuning frequency error of a positioning receiver by transmitting a measured reference frequency error from a base station to a mobile station. According to the invention, the positioning receiver, which is incorporated in the mobile station, is responsive to the transmitted reference frequency error, to quickly lock onto the RF carrier of a positioning signal.
The base station includes a reference frequency source that generates a reference frequency for communicating RF signals with the mobile station. A base station transmitter transmits the reference frequency error, which is measured based on the reference frequency and a known frequency, to the mobile unit. The reference frequency error transmission may be periodic or based on a request from the mobile station. The mobile station includes a receiver that receives the reference frequency error. The mobile station adjusts the tuning frequency of the positioning receiver according to the reference frequency from the base station RF signal and the reference frequency error, thereby compensating for frequency error in the base station reference frequency.
According to some of the more detailed features of the invention, the base station includes an external frequency source that generates the known frequency using an accurate frequency source. Under this arrangement, the base station includes a discriminator that generates the frequency error based on the difference between the reference frequency and the known frequency. In an exemplary arrangement, the external frequency source is another positioning receiver, preferably located at the base station, that is tuned using the reference frequency. The mobile station includes a local reference frequency source that generates a local reference frequency, which is adjusted based on the reference frequency error. Alternatively, the mobile unit includes a phase lock loop (PLL) having a reference clock for generating the local reference frequency. Under this arrangement, the reference clock is adjusted based on the reference frequency error.
According to another aspect of the invention, a method for receiving position information in a positioning receiver includes measuring a reference frequency error relative to a known frequency. In an exemplary embodiment, the reference frequency error may be measured using a frequency counter at the base station. Then, the reference frequency error is transmitted from the base station to the mobile station. The positioning receiver is tuned based on the reference frequency error to quickly lock onto positioning signals.
A further alternative simply locks the frequency of the positioning receiver to a reference signal from the mobile station that in turn has been phase locked, to the base station signal. The positioning receiver uses the received reference frequency error to adjust doppler shifts according to the amount of reference frequency error.
Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
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Burns Doane Swecker & Mathis L.L.P.
Corsaro Nick
Ericsson Inc.
Hunter Daniel S.
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