Multilevel QAM Modulation Techniques for Digital Microwave Radios

A pilot carrier injection method is described together with feedback balance coding which reduces spectral power near the carrier. Robustness of carrier recovery using the pilot carrier injection method is theoretically estimated. The estimation suggests that recovered carrier SNR higher than 40 dB can be expected even under muitipath fading with notch depth of 45 dB located just at the carrier frequency. Signatures for multipath fading are estimated for a 64-QAM system with transversal equalizers as a countermeasure. Measured signatures agree reasonably well with the calculated ones. Dependences of signatures on modulation level, transversal equalizer tap number, and rolloff rate are also shown.