Comparison of coded and uncoded QPSK intradyne and heterodyne receivers

Without the requirement for optical phase synchronization, the QPSK intradyne receiver provides a way to minimize the processing bandwidth of the optical receiver by reducing the intermediate frequency (IF) to a level that it is lower than the symbol frequency of the QPSK signal. As a result, with a low processing bandwidth, the QPSK intradyne scheme allows the use of a digital demodulating circuit at the receiver. An intradyne receiver is investigated, and convolutional coding is applied to the intradyne and heterodyne receivers using the Viterbi decoding algorithm. Due to the limitation of the digital PLL used in the intradyne receiver, it has generally a poor performance as compared to the heterodyne receiver. On the other hand, as expected, the application of convolution codes on both systems largely relaxes the requirement for laser linewidths. The improved performance is mainly the result of the error correcting capability of the codes and the reduction of the laser linewidth to data bit rate ratio. With BER fixed at 10^-10, by using a (2, 1) code with constraint length 11, the required laser linewidth can be relaxed by a relaxing factor 7 for the intradyne receiver and 1000 for the heterodyne receiver. For a fixed laser linewidth, the receiver sensitivities for both systems can be improved by 20 dB