Adaptive receiver for multiplex/multi-access communications
A received communication signal in which input data has been differentially encoded using a number of nominal symbols is filtered according to variable filter coefficients. A symbol is predicted based on the filtered signal, as if the input data had been encoded non-differentially in the communication signal. The filter coefficients are updated in response to the predicted symbol. An example of the differential encoding is differential phase shift keying (DPSK) modulation.
1. A method comprising:
filtering, according to a plurality of variable filter coefficients, a received communication signal in which input data has been encoded according to differential phase shift keying (PSK) modulation;
predicting a symbol based on the filtered signal by performing non-differential PSK demodulation upon the filtered signal and non-differential PSK modulation upon the demodulated filtered signal; and
updating the plurality of filter coefficients in response to the predicted symbol.
2. The method of claim 1 wherein the predicted symbol is a nominal symbol point that is closest to a sample of the filtered signal when plotted in a symbol constellation space.
3. The method of claim 2 wherein the filter coefficients are updated in response to an error signal being a difference between the predicted symbol and the a sample of the filtered signal.
4. The method of claim 1 wherein the received communication signal is digitized and digitally filtered.
5. The method of claim 1 further comprising performing a training process during which the plurality of variable filter coefficients are repeatedly updated in response to training symbols; and then performing said filtering, predicting and updating during decision direct mode; and
differentially decoding the input data from the filtered signal.
6. The method of claim 1 wherein the received communication signal is a multi-access spread spectrum communication signal, and the filtering includes despreading the signal.
7. An apparatus comprising:
an adaptive filter whose filtering characteristics are controlled by a plurality of variable filter coefficients whose values are updated in response to a signal at an error input, a further input of the filter being coupled to receive a communication signal in which input data has been differentially encoded;
an estimator having an input coupled to an output of the adaptive filter, the estimator having a non-differential phase shift keying (PSK) demodulator having an input that is coupled to the output of the adaptive filter, and a non-differential PSK modulator having an input that is coupled to an output of the non-differential PSK demodulator and an output that is coupled to an output of the estimator; and
a subtractor having an output coupled to the error input of the adaptive filter and inputs coupled to the outputs of the estimator and the adaptive filter.
8. The apparatus of claim 7 wherein the adaptive filter is a discrete time filter.
9. The apparatus of claim 7 further comprising a differential decoder having an input coupled to the output of the adaptive filter, to recover the input data that has been differentially encoded in the received communication signal.
10. The apparatus of claim 9 wherein the adaptive filter and the differential decoder together implement an autocorrelator to non-coherently detect the differentially encoded input data.
11. The apparatus of claim 7 further comprising a spread spectrum de-spreader coupled in a signal path before the adaptive filter.
12. The apparatus of claim 10 wherein the adaptive filter further implements a de-spreader.
13. An apparatus comprising:
means for correcting a received, noise-corrupted communication signal in which a plurality of transmitted symbols represent differentially encoded input data and have been corrupted by one of intra-cell and inter-cell interference;
means for demodulating the corrected communication signal and modulating the demodulated corrected communication signal each according to non-differential phase shift keying to obtain a plurality of generated symbols; and
means for adjusting the correction means in response to the corrected communication signal and in response to the plurality of the generated symbols.
14. The apparatus of claim 13 further comprising means for recovering the input data that has been differentially encoded in the received communication signal.
15. The apparatus of claim 14 wherein the input data recovery means and the correcting means comprise means for non-coherently detecting the input data from the corrected communication signal.