ASK AND FSK COHERENT LIGHTWAVE SYSTEMS - A SIMPLIFIED APPROXIMATE ANALYSIS

A simplified approximate analysis of ASK and FSK coherent optical communication systems is presented. The analysis accounts for the phase noise of the transmitter and the local oscillator lasers and for the additive Gaussian noise stemming from the shot noise and thermal noise. The analysis yields a closed-form estimate of the bit error rate (BER) and allows an immediate physical insight and appreciation of the impact of the IF filter bandwidth, laser linewidth, and bit rate on the system performance. In the case of ASK, the inaccuracy is only 0.4 dB at BER = 10-9 for optimum and nearly optimum receivers. This result is valid for a wide range of laser linewidths from 0 percent to 512 percent of the bit rate. In the case of FSK, the inaccuracy is less than 2.6 dB for linewidths up to 512 percent; the inaccuracy can be reduced to 1 dB using an empirical correction factor. Our theory also yields reasonably accurate estimates of the optimum IF band-widths (in case of ASK, our estimate is identical to the estimate obtained via accurate analysis) and of the sensitivity penalty stemming from the phase noise (for linewidths up to 100 percent, our estimate is only 0.8 dB from the estimate obtained via accurate analysis). © 1990 IEEE