Further Results on Adaptive Equalizer Improvements for 16 QAM and 64 QAM Digital Radio

Abstract
Channel distortions due to multipath fading are a principal source of outage for high-capacity digital radio. Time domain equalization is increasingly becoming an effective means to compensate for fading induced ISI. This paper compares the performance of transversal and decision feedback equalizers used for 16 QAM and 64 QAM digital radio systems. The effect of tap spacing is examined by comparing the results of baud period spacing with fractionally spaced equalizers. The performance measure used is peak distortion. This measure is plotted against various fade parameters for the different equalizers studied, the two modulation schemes, forms of timing recovery, and the effects of joint operation of demodulation and symbol synchronization circuitry. Results clearly indicate, in terms of better signature curves, the superiority of fractionally spaced equalizers over synchronous equalizers. An additional advantage is the robustness of FSE's to timing phase errors. A decision feedback equalizer with fractional spacing on the feedforward part offers the best performance. It is shown that the performance of a DFE synchronized to the first precursor is significantly improved as compared to the conventional case of synchronizing with the main sample of the received pulse. With the new position of reference (first precursor), fractional DFE's benefit more than theirT-spaced counterpart.