Phase noise effects on coding gain in forward error control coded communications

Abstract

Forward Error Control (FEC) coding has become common in digital communications systems in recent years due to technology advances. This paper illustrates coding concepts by presenting analysis techniques and resulting performance of an LSB coded 8PSK modulation scheme under the influence of phase noise; two different types of FEC for coding the LSB stream are examined: 1) ten-error correcting Reed-Solomon code over GF(64), and 2) three-error correcting binary BCH code. The difference in impact of phase noise for the two coding schemes is compared. Importantly, performance degrades more rapidly with phase noise using the Reed-Solomon code than with the BCH code. Therefore, even with a small amount of phase noise the benefits offered by the more complex Reed-Solomon code are negligible. Finally, the benefit of introducing an interleaver is analyzed for one of the selected codes. With sufficient interleaving, BER degradation due to phase noise is relatively small. On the other hand, with phase noise, degradation becomes severe above 2° RMS. Clearly, if more than 2° RMS phase noise is expected, it is critical to 1) characterize the time-varying nature of the phase noise rather than accepting the worst-case slowly varying assumption, and 2) consider the practicality of interleaving.