On Turbo Code Decoder Performance in Optical-Fiber Communication Systems With Dominating ASE Noise

In this paper, we study the effects of different ASE noise models on the performance of turbo code (TC) decoders. A soft-decoding algorithm, the Bahl, Cocke, Jelinek, and Raviv (BCJR) decoding algorithm [1], is generally used in the TC decoders. The BCJR algorithm is a maximum a posteriori probability (MAP) algorithm, and is very sensitive to the noise statistics. The Gaussian approximation of the ASE noise is widely used in the study of optical-fiber communication systems [2]–[8], and there exist standard TCs for additive white Gaussian noise (AWGN) channels. We show that using a MAP decoding algorithm based on the Gaussian noise assumptions, however, may significantly degrade the TC decoder performance in an optical-fiber channel with non-Gaussian ASE noise. To take full advantage of TC, the accurate noise statistics in optical-fiber transmissions should be used in the MAP decoding algorithm.