Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-06-03
2003-02-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S326000, C375S340000
Reexamination Certificate
active
06519296
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to modulation and demodulation in communications systems and, more particularly, to pilot symbol aided modulation techniques and associated receiver signal processing.
Pilot symbol aided modulation has been investigated for mobile communications applications since it facilitates estimation of the carrier signal, which is needed for coherent demodulation at the receiver. One technique is to periodically insert known pilot symbols (i.e., known symbols) into the sequence of modulated data symbols comprising the signal to be transmitted. These pilot symbols essentially sample the channel process. The receiver recovers the pilot symbols from the sequence of symbols received from the channel and uses a standard interpolation technique to reconstruct a coherent carrier for use in demodulation.
In the mobile application, pilot symbol aided modulation's main drawback is that it requires relatively high overhead because the carrier's frequency and phase can vary quite rapidly due to the Doppler effect. However, in a geosynchronous satellite communications system with stationary terminals, the apparent carrier frequency changes much more slowly. Therefore, although it is tempting to utilize pilot symbol aided modulation in order to simplify the terminal's receiver, the required overhead is still quite high when the channel process is sampled above the Nyquist rate based on the initial carrier frequency uncertainty. One cause of this carrier frequency uncertainty is the satellite's motion. Second, the use of low-cost oscillators in the system can result in unacceptable shorter-term frequency stability and significant longer-term frequency drift. Thus, while the receiver becomes less expensive to build, the high overhead required reduces system capacity and, therefore, nullifies any cost advantage in the system.
Accordingly, it is desirable to provide a pilot symbol aided modulation and demodulation signal processing which overcome the shortcomings described hereinabove. It is furthermore desirable to provide such modulation and demodulation techniques particularly as applicable to very small aperture satellite (VSAT) communications terminals.
BRIEF SUMMARY OF THE INVENTION
In accordance with preferred embodiments of the invention described herein, variable-interval pilot symbol aided modulation and demodulation techniques significantly reduce the signaling overhead required for pilot symbol aided modulation. Variable-interval pilot symbol aided modulation involves partitioning a transmitted message burst into at least two segments, generally of unequal length, such that the interval between pilot symbols differs; and corresponding demodulation signal processing involves reducing the channel process sampling rate (i.e., reducing the frequency of pilot symbols) after an initial carrier frequency estimate is made during an initial signal interval. A receiver recovers all of the pilot symbols in the received burst's shorter pilot inter-symbol interval segment and reconstructs an estimate of the received signal's carrier therefrom, the shorter pilot inter-symbol interval segment of the received burst being demodulated such that the estimated carrier is derived as in standard pilot symbol aided modulation techniques. The receiver calculates an estimate of the received signal's carrier frequency and phase from the pilot symbols in the shorter pilot inter-symbol interval segment of the received burst. In the longer pilot inter-symbol interval segment of the burst, the received carrier process is sampled by the pilot symbols at a lower rate such that frequency-shifted pilot symbols are used to obtain a local carrier for demodulation of the shorter pilot inter-symbol interval segment of the received burst. Switching circuitry is used to alternate between demodulating the shorter and longer pilot inter-symbol interval segments of the burst.
REFERENCES:
patent: 5381449 (1995-01-01), Jasper et al.
patent: 5875215 (1999-02-01), Dobrica
“An Analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels,” James K. Cavers, IEEE Transactions on Vehicular Technology, vol. 40, No. 4, Nov. 1991.
Breedlove Jill M.
Cabou Christian G.
Chin Stephen
General Electric Company
Lugo David B.
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