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This paper describes experimental results of simulated 12000 and 14400 bits/s quadrature amplitude-modulated (QAM) systems (which were described in detail in Part I 1 ) operating on real and simulated voiceband channels. These experimental results show that both Viterbi algorithm detection and decision feedback equalization allow digital data transmission at symbol rates exceeding the nominal bandwidth capabilities of typical telephone channels. It is demonstrated in these experiments that the Viterbi algorithm receiver structure tested is less affected by linear distortion than the decision feedback receiver but exhibits greater sensitivity to phase jitter. On conditioned voiceband channels where there may be substantial phase jitter, a reasonable compromise between sensitivity to distortion and noise and sensitivity to phase jitter is made by a modem employing decision feedback equalization and decision-directed carrier phase tracking.
Falconer et al. (Mon,) studied this question.