Four-Dimensional Trellis Coded Modulation for Flexible Optical Communications

We experimentally investigate four-dimensional (4-D) Trellis coded modulation (TCM) based on polarization-division multiplexed (PDM) 16-ary quadrature amplitude modulation (QAM), PDM-32QAM, and PDM-64QAM formats. A multirate optical transponder is enabled by only a single-encoder/decoder structure. The scheme is resilient to cycle-slip events due to its 90° phase rotation invariant property. In addition, the performance of 4-D turbo TCM (TTCM) is investigated. The superior performance of TTCM over TCM is experimentally validated. Furthermore, the performance of 4-D TCM/TTCM is compared with standard PDM-mQAM formats combined with the soft-decision (SD) forward error correction (FEC) codes. It is shown that 4-D TCM concatenated with a low-complexity hard-decision FEC could be an interesting candidate for complexity-performance tradeoff with SD-FEC.