Method and apparatus for echo cancellation of a multicarrier signal

Claim 1

A method of cancelling the echoes of a signal transmitted via a two-wire line in a data modem comprising a multicarrier transmitter which transforms blocks of encoded frequency-domain samples into blocks of time-domain samples and a multicarrier receiver which performs the inverse operation, said transmitter and receiver being both connected to said two-wire line, which echoes are reflected back into said receiver by an imperfect match between the output impedance of said modem and the input impedance of said two-wire line, said method comprising the steps of: (a) forming a first estimate of the frequency-domain characteristics of the path traversed by said echo; (b) transforming said estimated frequency-domain characteristics of step (a) into the time-domain to form an estimate of the impulse response parameters of said echo path; (c) calculating the expected time-domain samples of the echo of said transmitted signal based upon said estimated impulse response parameters of step (b); (d) subtracting said expected echo signal of step (c) from the actual signal received from said two-wire line in order to calculate a modified received signal; (e) transforming a first block of time-domain samples of said modified received signal of step (d) into the frequency domain; (f) correlating said transformed modified received signal of step (e) with said encoded frequency-domain samples of said transmitted signal; (g) calculating a new set of estimated frequency-domain echo path characteristics by decreasing each of said first estimated frequency-domain characteristics by a small quantity that is proportional to said correlation of step (f); and (h) mproving the estimate of said frequency-domain characteristics by repeating steps (a) - (g) using subsequent samples of said transmitted and received signals.

Claim 4

The method of one of the preceding claims in which said estimates of the characteristics of the echo path comprise just one complex number for each of the carriers used in said transmitted signal.

Claim 5

The method of one of the preceding claims in which said encoded frequency-domain samples occupy a first frequency band, and are augmented by some other samples that are not used to convey data information, so that the composite set, which is transformed into said set of time-domain samples and is also correlated with said modified received signal, occupies a second frequency band that is wider than said first frequency band.

Claim 8

A method of cancelling a far echo of a transmitted signal in a data modem comprising a multicarrier transmitter which transforms blocks of encoded frequency-domain samples into blocks of time-domain samples and a multicarrier receiver which performs the inverse operation, said transmitter and receiver being both connected to a communication link that comprises, in turn, a first two-wire line, one or more four-wire terrestrial carrier systems, and a second two-wire line which echo is reflected back to said receiver from said second two-wire line, and which may be offset in frequency, said method comprising the steps of: (a) forming first estimates of the round-trip delay of said far echo, and of the frequency-domain characteristics and frequency offset of the path traversed by said far echo; (b) transforming said estimated frequency-domain characteristics of step (a) into the time-domain to form an estimate of the impulse response parameters of said far echo path; (c) calculating the expected time-domain samples of the echo of said transmitted signal based upon said estimated impulse response parameters of step (b); (d) rotating said expected time-domain samples of step (c) by an angle that increases linearly with time in proportion to said estimate of the frequency offset; (e) subtracting said rotated expected echo signal of step (d) from the actual signal received from said two-wire line in order to calculate a modified received signal; (f) delaying said samples of said modified received signal of step (e) by a time that is equal to the difference between an integer number of periods of said blocks and said round-trip delay of step (a); (g) transforming a first block of time-domain samples of said modified and delayed received signal of step (f) into the frequency domain; (h) delaying said blocks of said encoded frequency-domain samples by a time that is an integer multiple of the period of said blocks; (i) correlating said transformed modified received signal of step (g) with said encoded and delayed frequency-domain samples of step (h); (j) calculatng a new set of estimated frequency-domain echo path characteristics by decreasing each of said characteristics by a small quantity that is proportional to said correlation of step (i); (k) calculating a set of estimates of the phase angles by which each of said new frequency characteristics has rotated; (l) averaging said changes of phase angles of step (k) over the set of said characteristics; (m) filtering said average phase angle of step (l) to generate an improved estimate of the frequency offset of said far echo; and (n) repeating steps (b) through (m) in order to continually improve said estimates of step (a).

Claim 9

A method of initially estimating the parameters of near and far echoes of a transmitted signal in a data modem comprising a multicarrier transmitter which transforms blcoks of encoded frequency-domain samples into blocks of time-domain samples and a multicarrier receiver which performs the inverse operation, said transmitter and receiver being both connected to a terrestrial communication link that comprises, in turn, said first two-wire line, one or more four-wire terrestrial carrier systems, and said second two-wire line, which echoes are reflected back into said receiver by said first and second two-wire lines respectively, said method comprising the steps of: (a) repeatedly transmitting a cyclic signal that has a period that is equal to the duration of said sets of encoded frequency-domain samples, and longer than the delay of said far echo; (b) transforming into the frequency domain sets of samples of the signals received via said echo paths; (c) dividing said transform of said sets of samples of step (b) by the transform of said cyclic signal of step (a) to form a first estimate of the combined frequency-domain characteristics of both of said near and far echo paths; (d) calculating the time-domain parameters of said near echo path by transforming said first estimate of step (c) into the time domain, and setting to zero all samples outside the span of said near echo; (e) calculating a first set of time-domain parameters of said far echo by setting to zero all samples of said transformed first estimate of step (d) which are outside the span of said far echo; (f) generating a second set of time-domain parameters of said far echo by rotating said first set of time-domain parameters of step (e) by the angles needed to compensate for the estimated frequency offset of said far echo path; (g) transforming said second set of time-domain parameters of step (f) into the frequency domain; and (h) for all sets of samples of said actual received signal after the first, averaging the change of phase angle of said frequency-domain parameters and thereby determining an update of the estimated frequency offset.

Claim 10

A method of initially estimating the parameters of near and far echoes of a transmitted signal in a data modem comprising a multicarrier transmitter which transforms blocks of encoded frequency-domain samples into blocks of time-domain samples and a multicarrier receiver which performs the inverse operation, said transmitter and receiver being both connected to a communication link that comprises, in turn, a first two-wire line, one or more four-wire carrier systems transmitted via an orbiting satellite, and a second two-wire line, which echoes are reflected back into said receiver by said first and second two-wire lines respectively, said method comprising the steps of: (a) repeatedly transmitting a cyclic signal that has a period that is equal to the duration of said sets of encoded frequency-domain samples; (b) transforming into the frequency domain those first sets of samples of the signals which are received via said near echo path before said far echo has arrived at the receiver of said modem; (c) dividing the transform of said first sets of samples of step (b) by the transform of said cyclic signal to form a first estimate of the frequency-domain characteristics of said near echo path; (d) calculating the time-domain parameters of said near echo path by transforming said first estimate of step (c) into the time domain, and setting to zero all samples outside the span of said near echo; (e) calculating a smoothed set of frequency-domain parameters of said near echo path by transforming said time-domain parameters of step (d) into the frequency domain; (f) averaging all sets of said time-domain and frequency-domain parameters of steps (d) and (e) respectively that are received before said far echo arrives at said receiver; (g) calculating the time-domain samples of the estimated near echo using said averaged sets of parameters of step (f); (h) generating a first set of samples of said far echo by subtracting said samples of step (g) from those samples of said actual received signal that are received after said far echo arrives at said receiver; (i) generating a second set of time-domain samples of said far echo signal by rotating said first set of samples of step (h) by the angles needed to compensate for the estimated frequency offset of said far echo path; (j) transforming said second set of samples of step (i) into the frequency domain; and (k) for all sets of samples of said received signal that comprise both near and far echoes after the first, averaging the change of phase angle of said frequency-domain samples and thereby calculating an update of said estimated frequency offset.

Claim 11

A method of initially estimating the parameters of near and far echoes of a transmitted signal in a data modem comprising a multicarrier transmitter which transforms blocks of encoded frequency-domain samples into blocks of time-domain samples and a multicarrier receiver which performs the inverse operation, said transmitter and receiver being both connected to a communication link that comprises, in turn, a first two-wire line, one or more four-wire carrier systems transmitted via an orbiting satellite, and a second two-wire line, which echoes are reflected back into said receiver by said first and second two-wire lines respectively, said method comprising the steps of: (a) repeatedly transmitting a cyclic signal that has a period that is equal to the duration of said sets of encoded frequency-domain samples; (b) transforming into the frequency domain those first sets of samples of the signals which are received via said near echo path before said far echo has arrived at the receiver of said modem; (c) dividing the transform of said first sets of samples of step (b) by the transform of said cyclic signal to form a first estimate of the frequency-domain characteristics of said near echo path; (d) calculating the time-domain parameters of said near echo path by transforming said first estimate of step (c) into the time domain, and setting to zero all samples outside the span of said near echo; (e) calculating a smoothed set of frequency-domain parameters of said near echo path by transforming said time-domain parameters of step (d) into the frequency domain; (f) averaging all sets of said time-domain and frequency-domain parameters of steps (d) and (e) repectively that are received before said far echo arrives at said recever; (g) calculating the frequency-domain paramters of the combined near and far echo paths by transforming into the frequency domain sets of those samples of said actual received signal that are received after said far echo arrives at said receiver; (h) generating a first estimate of the frequency-domain paramters of said far echo path by subtracting from said frequency-domain parameters of said combined echo paths of step (g) said smoothed and averaged frequency-domain parameters of said near echo path of step (f); (i) transforming said first estimate of step (h) into the time domain; (j) generating a second set of time-domain parameters of said far echo signal by rotating said first set of parameters of step (i) by the angles needed to compensate for the estimated frequency offset of said far echo path; (k) transforming said second set of parameters of step (h) into the frequency domain; and (l) for all sets of samples of said received signal that comprise both near and far echoes after the first, averaging the change of phase angle of said frequency-domain parameters and thereby calculating an update of said estimated frequency offset.

Claim 16

The apparatus of ome of claims 12 to 15 further comprising means for smoothing said frequency-domain parameters, said means comprising: means for changing said frequency-domain characteristics in order to generate a first set of new characteristics; means for transforming said first set of new characteristics into the time-domain, and truncating the time-domain impulse response in order to generate a new set of parameters of said estimated impulse response; and means for transforming said new, truncated estimated impulse response back into the frequency domain to generate a second set of new frequency-domain characteristics.