RAPID TRAINING OF A VOICEBAND DATA-MODEM RECEIVER EMPLOYING AN EQUALIZER WITH FRACTIONAL-T SPACED COEFFICIENTS

The parameters of a voiceband data-modem receiver must be adjusted to the characteristics of the telephone line before data transmission can begin. A voiceband data-modem design is presented which permits very short startup times. The key to rapid receiver training is the use of an adaptive transversal equalizer whose coefficient spacing is smaller than a symbol interval T. The modem receiver performs rapid gain acquisition, and monitors the equalizer delay line for the presence of a periodic training signal whose period is equal to the length of the equalizer delay line. Testing for periodicity is combined with estimating carrier-frequency offset. Once a full period of the training signal is obtained, the equalizer coefficients are computed by spectral division, and coefficient centering is performed. The receiver then proceeds in a reference-directed mode until the end of the training signal is detected. Algorithms for rapid gain acquisition and the detection of a cyclic training signal in the presence of noise and carrier-frequency offset are presented. Cyclic training of an equalizer with fractional-T spaced coefficients is derived.