Modem for fading digital channels affected by multipath
AssigneeHughes Aircraft Company
InventorDavid N. Critchlow
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Abstract
A receiver designed for use in a mobile environment, which includes an adaptive equalizer driven by phase estimation and rotation circuitry, and which is operable in multipath fading channels. The present invention uses an AFC loop and a phase rotation circuit in front of the adaptive equalizer to improve equalizer performance in the fading channel. The present invention uses .pi./4 phase rotation circuitry before a phase quantization decision is made. This .pi./4 phase rotation circuitry allows for decisions to be made on a QPSK constellation, rather than on an input 8PSK constellation, thus improving detection performance.
Claims17
A receiver for processing a transmitted symbol received by way of at least two transmission paths, the transmission paths having at least some noise, said receiver comprising:
- an antenna for receiving the transmitted symbol;
- a downconverter for converting the received symbol into corresponding I and Q digital signals;
- a matched filter coupled to the downconverter for increasing the signal to noise ratio of the I and Q digital signals;
- phase rotation means for rotating the phase of the filtered I and Q digital signals by a computed amount to provide phase rotated signals;
- an adaptive matched filter coupled to the phase rotation means for processing the phase rotated signals and for adaptively filtering the signals to produce phase coherent I and Q signals;
- a phase extraction circuit coupled to the adaptive matched filter for converting the phase coherent I and Q signals into corresponding adaptively filtered phase signals;
- a phase detection circuit coupled to the phase extraction circuit for converting the adaptively filtered phase signals into a corresponding plural bit pattern representative of the symbol;
- and
- phase decoding means for decoding said plural bit pattern, to thereby produce the symbol.
A receiver for use with a modulator transmitter that is adapted to sequentially transmit digitally encoded data symbols having a predetermined phase advance sequentially encoded thereon at a predetermined transmit frequency, over a fading digital channel subject to noise and having multiple transmission paths, said receiver comprising:
- an antenna for receiving the digitally encoded data symbols;
- a downconverter for converting the received digitally encoded data symbols into corresponding I and Q digital signals;
- a fixed matched filter having a fixed fractional tap spacing matched to the impulse response of the transmitter, coupled to the downconverter for increasing the signal to noise ratio of the I and Q digital signals;
- phase rotation means for rotating the phase of the filtered I and Q digital signals by a predetermined amount to provide phase rotated signals;
- an adaptive matched filter coupled to the phase rotation means for adaptively filtering the phase rotated signals, to thereby produce phase coherent I and Q signals;
- a phase converter coupled to the adaptive matched filter for converting the phase coherent I and Q signals into corresponding adaptively filtered phase signals;
- and
- phase decoding means for stripping the phase advance from the adaptively filtered phase signals, to thereby produce the symbols.
In a receiver, a method of processing digital I and Q samples derived from data symbols that are transmitted by way of multiple paths from a transmitter having a predetermined impulse response, in order to produce demodulated data symbols, said method comprising the steps of:
- filtering the I and Q samples by means of a fixed tap filter that is matched to the impulse response of the transmitter;
- rotating the phase of the filtered I and Q samples to provide phase rotated signals;
- adaptively filtering the phase rotated signals;
- determining the phase associated with the phase rotated signals;
- decoding the phase of the signals to produce the demodulated data symbols.
A receiver for processing a transmitted symbol, the receiver comprising:
- a downconverter for converting a receiver transmitted symbol into corresponding I and Q digital signals;
- a phase rotator for rotating the phase of the I and Q digital signals by a computed amount;
- an adaptive matched filter coupled to the phase rotator for adaptively filtering the signals to produce phase coherent I and Q signals from the phase rotated signals;
- and
- a converter comprising a phase extraction circuit coupled to the adaptive matched filter for converting the phase coherent I and Q signals into corresponding adaptively filtered phase signals and then to a data symbol.
In a receiver, a method of processing digital I and Q samples derived from data symbols that are transmitted from a transmitter having a predetermined impulse response, in order to produce demodulated data symbols, said method comprising the steps of:
- filtering the I and Q samples by means of a fixed tap filter that is matched to the impulse response of the transmitter;
- rotating the phase of the the filtered I and Q samples to provide phase rotated signals;
- adaptively filtering the phase rotated signals;
- determining the phase associated with the phase rotated signals;
- and
- decoding the phase of the signals to produce the demodulated data symbols.
A receiver for processing a transmitted pi/4 QPSK symbol comprising:
- a downconverter for converting the pi/4 QPSK symbol into corresponding I and Q digital signals;
- a phase addition circuit for subtracting a pi/4 phase input from the I and Q signals to produce QPSK I and Q signals;
- a QPSK phase detection circuit for converting the QPSK I and Q signals into a coherent bit pattern representation of the QPSK I and Q signals;
- and
- a phase decoder for decoding the coherent bit pattern representation to produce the symbol.
Dependencies are inferred from the claim text.