USING TIMES-4 CARRIER RECOVERY IN M-QAM DIGITAL RADIO RECEIVERS

An investigation was made of the performance of the times-four method of carrier recovery in a digital radio link transmitting M-level quadrature amplitude modulation (M-QAM), where M can be 16, 64, 256, etc. With the help of a recently built multipath fade simulator, the authors measured rms phase jitter for various combinations of fade notch frequency and fade depth. A report is presented on the experiment and the associated analysis. The times-four method has the virtues that it does not depend upon accurate data decisions, and it does not interact with the receiver's adaptive equalizer. Results show that the rms phase jitter using this method, for all but the most severe and improbable fades, could be held to a few degrees. Although the measurements were done only for 16 QAM, the analysis predicts only modest increases for higher number of levels. The analysis also predicts, and the measurements verify, that the recovered carrier amplitude shrinks to zero for four distinct combinations of fade notch frequency and fade depth. It is believed that this phenomenon has not been reported previously in the literature. The authors show that, from a statistical viewpoint, the damage associated with the method (measured in terms of outage time) should be quite small.