Performance and Complexity of Turbo Equalizers for optical coherent Gbit/s Transmission with optimal and suboptimal Detection Strategies

In this paper, we investigate two different turbo equalizers with distinct detection strategies for coherent optical communication with QPSK modulated signals. The first receiver comprises a BCJR detector and a soft-in soft-out LDPC decoder. In the second receiver, the BCJR unit is replaced by a linear MMSE filter. Offering enough turbo iterations for the receiver with MMSE detection, the performance is almost the same as for the receiver with the optimal BCJR detection. As the computational complexity of the MMSE detection is considerable lower as for the BCJR detection, the turbo equalizer with MMSE detection offers a good compromise between complexity and performance.