Saddle point bit error rate computations for optical communication systems incorporating equalizers

Bit error rate computations for optical communication systems incorporating equalizers and under both noise and intersymbol interference (ISI) are discussed. An accurate method based on saddle point approximation is used. Previous work using saddle point approximation considered only basic integration-and-dump detection. The use of equalizers other than integration-and-dump complicates the computation because of the complexity of the moment generating function involved. When ISI is strong, use of an equalizer such as raised-cosine filtering is important for ISI reduction. To help optical receiver design, this paper explains the use of saddle point approximation when an equalizer is used. In addition to integration-and-dump, cosine and raised-cosine equalizers are considered for ISI reduction. To study the interdependence between the received waveform and equalizer used, two different input pulses are also considered. To verify the computation accuracy, exact contour integration based on the numerical quadrature method is performed. Results show that saddle point approximation can provide a fast and accurate computation. From the bit error rate (BER) performance computed, it is found that the traditional use of matched filtering has a poor performance when either ISI is strong or the APD gain is large, and the use of a proper equalizer can significantly improve the performance. In other words, the best choice of an equalizer design depends on the input waveform, amount of ISI, and the APD gain.<
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