Performance of Binary Turbo-Coded 256-ary Pulse-Position Modulation

This article compares the performance of binary turbo codes and Reed–Solomon (RS) codes in an optical communications system employing high-order pulseposition modulation (PPM) and an avalanche photodiode (APD) detector. Despite restriction of the turbo codes to binary alphabets and very simple 4or 8-state encoders, they have coding gains 0.5-dB to 0.7-dB higher than equivalent-rate RS codes when using 256-PPM, employing a typical APD detector, and operating at an end-to-end bit-error rate (BER) of 10−5. BER curves are given for code rates of 1/3 and 1/2, using a variety of turbo and RS coding methods. The simulations use an accurate modeling of APD output statistics instead of typical but less accurate Poisson or Gaussian approximations.