Prior knowledge of the typical signal spectrum at the equalizer input can be used to speed up the convergence of a least mean-square error nonrecursivc equalizer by placing a weighting matrix in the path of the coefficient (tap gain) corrections. When the training sequence is periodic with period equal to the time-spread of the nonrecursive equalizer, the weighting matrix is not only symmetric and Toeplitz, but also circulant. Thus the fast update algorithm can be implemented by inserting a single nonrecursive filter in the path of the periodic input sequence before it is used for computing coefficient correction terms. The stability and convergence of the fast start-up algorithm with "stochastic" update in the presence of noise are examined. In the absence of prior information about the channel characteristics, this fast converging training procedure may be easily extended to a one-shot method of equalizer setup. The one-shot method, which involves estimation of the inverse filter from one period of the received signal, proves to be robust in the presence of noise. Simulation resuits are given for class I partial-response and Nyquist (20 percent roll-off raised cosine) data transmission systems with quadrature amplitude modulation over a dial-up telephone connection.

Fast start-up equalization with periodic training sequences

This invention relates to an echo canceller, for a transmission line, including a shift register having an input for receiving an input signal from a signal source and having a plurality of output taps for providing a plurality of equally spaced delayed signals. A tap coefficient storage memory stores a plurality of tap coefficients. A multiplier multiplies certain of the delayed signals by certain tap coefficients to produce multiplier outputs. A tap allocation circuit determines, on a tap by tap basis, how many and which of the taps are to be turned ON so that its corresponding tap coefficients are multiplied by the multiplier. A subtracter cancels echoes by subtracting the multiplier outputs from signals received on the transmission line. Taps which are turned OFF may be assigned a status of STANDBY and these taps may be activated to compensate for changes in the characteristics of the transmission line.

Adaptive echo canceller with sparse dynamically positioned taps

In a high speed QAM data communications receiver of the type adapted to receive bursts of signals having a bandwidth B Hz and sent at a predetermined rate of 1/T signals per second over a channel, wherein the receiver includes an automatic adaptive equalizer having taps spaced equally apart, tap coefficient circuitry for repeatedly multiplying the output of each tap by a respective tap coefficient, and adjustment circuitry for adjusting the tap coefficients, and output circuitry responsive to the equalizer for providing output signals at times kT + τ, k = 0, 1, . . . , where τ is a timing epoch, that improvement wherein the taps are spaced apart by T/n seconds, where n is greater than TB, and control circuitry is provided for early actuation of the adjustment circuitry to begin adjustment of the tap coefficients and hence training of the equalizer regardless of the initial value of the timing epoch, whereby the data capacity of the receiver is increased by reduction of the time required for set up of the receiver.

QAM receiver having automatic adaptive equalizer

Embodiments of the invention comprise a continuous-time equalizer for reducing ISI in data received from a communication channel, and methods and circuitry for tuning or calibrating that equalizer. Selected coefficients for a transfer function of the equalizer circuit are fixed, while other coefficients are tuned by an adaptive algorithm. The adaptive algorithm minimizes errors associated with the tunable coefficients based on one or more training signals sent by the transmitter and received by the equalizer circuit at the receiver. The training signals allow for a variety of error terms to be calculated, from which the tunable coefficients are updated so as to iteratively minimize the error terms and simultaneously tune the equalizer to more accurately compensate for the degrading effects of the channel.

Self-calibrating continuous-time equalization

Coin telephone measurement circuitry

Adaptive linear transversal filter is trained with a periodic training sequence having period exactly equal to the number of variable parameters of the filter to be set in the training mode. After training, tap coefficients may be cycled in a closed loop to a preferred position.

G Forney

Papers

Training adaptive linear filters