A simple analysis technique for heterodyne ASK and FSK lightwave systems

An analysis technique for amplitude-shift-keyed (ASK) and frequency-shift-keyed (FSK) coherent optical communication systems is presented. The analysis accounts for the phase noise of the transmitter and the local oscillator (LO) lasers, and for additive Gaussian noise stemming from the shot noise and thermal noise. The analysis yields a closed-form estimate of the bit error rate (BER) as a function of the IF filter bandwidth, laser linewidth, and bit rate. The accuracy of the analysis is within 0.4 dB of the sensitivity predicted by more accurate techniques for ASK, and 0.2 dB for FSK, for optimum and nearly optimum receivers. This result is valid for a wide range of laser linewidths from 0% to 256% of the bit rate. The Gaussian approximation technique is shown to yield reasonably accurate estimates of the optimum IF bandwidth and of the sensitivity penalty stemming from the phase noise