Adaptive equalization of digital modulating signal recovered from amplitude-modulated signal subject to multipath
First Claim
1. A method for adapting adjustable weighting coefficients of a feed-forward finite-impulse-response (FIR) digital filter that provides for performing at least a part of the channel-equalization and echo-suppression filtering of successive samples of a demodulation signal resulting from demodulation of an amplitude-modulation signal descriptive of digital symbols occurring at a baud rate, which demodulation signal is subject to being accompanied by unwanted echoes because of said amplitude-modulation signal being received via a channel that at times has more than one length of propagation path therethrough, which unwanted echoes are to be suppressed in successive baud-rate samples of a baseband response of said channel-equalization and echo-suppression filtering to said successive samples of said demodulation signal, said method comprising steps of:
- initially measuring the echo content of samples of said demodulation signal to determine an initial channel impulse response (CIR) that characterizes the reception channel in the time-domain;
measuring reception error in said baseband response of said channel-equalization and echo-suppression filtering to said successive samples of said demodulation signal;
continuously updating the CIR responsive to said reception error;
periodically, normalizing the CIR with respect to the strength of a cursor component thereof to generate a normalized CIR;
convolving the normalized CIR with at least a portion thereof to generate a synthetic normalized CIR; and
determining the weighting coefficients of said feed-forward FIR digital filter from corresponding terms of said synthetic normalized CIR.
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Abstract
An adaptive equalizer for amplitude-modulation (AM) signal received by over-the-air radio transmission comprises feed-forward finite-impulse-response (FIR) filtering and further comprises infinite-impulse-response (IIR) filtering for feeding back decisions. The echo content of baseband signal demodulated from the AM signal is measured for initially determining the reception channel impulse response (CIR) in the time-domain. Thereafter, the CIR is updated by decision feedback. Periodically, the CIR is normalized with respect to the strength of a cursor component thereof. The strengths of longer-delay post-echoes in the normalized CIR are used to determine the weighting coefficients of the IIR filtering, which suppresses the longer-delay post-echoes. The pre-echo and short-delay post-echo portion of the normalized CIR is convolved with the normalized CIR to generate a synthetic normalized CIR. The strengths of pre-echoes and short-delay post-echoes in the synthetic normalized CIR are used to determine the weighting coefficients of the feed-forward FIR filtering, which suppresses the pre-echoes and short-delay post-echoes.
31 Citations
20 Claims
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1. A method for adapting adjustable weighting coefficients of a feed-forward finite-impulse-response (FIR) digital filter that provides for performing at least a part of the channel-equalization and echo-suppression filtering of successive samples of a demodulation signal resulting from demodulation of an amplitude-modulation signal descriptive of digital symbols occurring at a baud rate, which demodulation signal is subject to being accompanied by unwanted echoes because of said amplitude-modulation signal being received via a channel that at times has more than one length of propagation path therethrough, which unwanted echoes are to be suppressed in successive baud-rate samples of a baseband response of said channel-equalization and echo-suppression filtering to said successive samples of said demodulation signal, said method comprising steps of:
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initially measuring the echo content of samples of said demodulation signal to determine an initial channel impulse response (CIR) that characterizes the reception channel in the time-domain; measuring reception error in said baseband response of said channel-equalization and echo-suppression filtering to said successive samples of said demodulation signal; continuously updating the CIR responsive to said reception error; periodically, normalizing the CIR with respect to the strength of a cursor component thereof to generate a normalized CIR; convolving the normalized CIR with at least a portion thereof to generate a synthetic normalized CIR; and determining the weighting coefficients of said feed-forward FIR digital filter from corresponding terms of said synthetic normalized CIR. - View Dependent Claims (2, 3)
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4. Channel-equalization and echo-suppression filtering apparatus connected for processing successive samples of a demodulation signal resulting from demodulation of an amplitude-modulation signal descriptive of digital symbols occurring at a baud rate, which demodulation signal is subject to being accompanied by unwanted echoes because of said amplitude-modulation signal being received via a channel that at times has more than one length of propagation path therethrough, which unwanted echoes are to be suppressed in successive baud-rate samples of a baseband response of said channel-equalization and echo-suppression filtering apparatus to said successive samples of said demodulation signal, said channel-equalization and echo-suppression filtering apparatus comprising:
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adaptive digital transversal filtering apparatus with adjustable weighting coefficients, said adaptive digital transversal filtering apparatus connected for receiving as an input signal thereof said successive samples of said demodulation signal and for supplying an output signal from which is derived said response of said channel-equalization and echo-suppression filtering apparatus to said successive samples of said demodulation signal; apparatus for determining an initial channel impulse response (CIR) that characterizes the reception channel in the time-domain; apparatus for generating estimates of the digital modulation of the amplitude-modulation signal as originally transmitted; a reception error detector connected for generating a reception error signal responsive to the amount by which the baseband response of said channel-equalization and echo-suppression filtering apparatus differs from estimates of the digital modulation of the amplitude-modulation signal at the transmitter thereof; a filter coefficients computer connected for receiving said CIR and said reception error signal, operable for continually updating the CIR responsive to said reception error signal, operable for periodically normalizing the CIR with respect to a principal component thereof to generate a periodically-updated normalized CIR, operable for computing an update of the adjustable weighting coefficients of said adaptive digital transversal filtering apparatus from each periodically-updated normalized CIR. - View Dependent Claims (5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
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Specification