Coded Modulation at Low-SNR – Comparison With 4D-BPSK Constellation Modulation

Transmitting high capacity under low SNR conditions is increasingly a challenge in many wireless and free-space communications applications. E.g. in free space optical satellite links signals have to overcome a large loss budget due to free space path losses and atmospheric turbulence while maintaining a highcapacity link. In this context, polarization switched (PS) QPSK or coded modulation using block code were proposed as powerefficient modulation formats to serve low SNR conditions. Recently, the four-dimensional BPSK (4D-BPSK) modulation format, which is a four-dimensional constellation modulation of BPSK has been proposed and demonstrated to achieve a higher power efficiency when compared with PS-QPSK. In this paper 4DBPSK and other power-efficient modulation formats, including coded modulation with extended Hamming, Golay or Reed-Muller code as well as probabilistically shaped 16QAM, are compared in simulation using generalized mutual information as a metric. We also discuss the complexities of the log-likelihood ratio calculations. The simulation results were validated in experiments. The comparison results provide guidelines for selecting a power efficient modulation format under constraints of bandwidth and complexity, and show that 4D-BPSK is an attractive option.