Comparative study of modulation techniques for microwave digital radios

A comparative study is presented showing the relative merits of four modulation techniques for use in line-of-sight digital radio systems operating at 45 Mb/s in a bandwidth of 10 MHz. The candidate modulations are conventional 64-QAM (quadrature amplitude modulation), reduced-bandwidth 16-QAM, reduced-bandwidth 49-QPRS (quadrature partial-response signaling), and conventional 81-QPRS. Comparisons are made from three points of view: bit-error-rate performance with unfaded channels, robustness to multipath fading, and sensitivity to timing and phase errors. Extensive numerical calculations show that none of the above signaling techniques is uniformly better than the others. Conventional 64-QAM seems to be the best solution to combat selective fading. On the other hand, reduced bandwidth 16-QAM and 49-QPRS as well as conventional 81-QPRS all have the advantage of lower bit error rates and less sensitivity to synchronization errors