LDPC-Coded Optical Communication over the Atmospheric Turbulence Channel

The free-space optical (FSO) systems that are robust in the presence of atmospheric turbulence are discussed in this invited paper: (i) coded orthogonal frequency division multiplexing (OFDM), and (ii) coded multiple-input multiple- output (i.e., multi-laser multi-detector or MIMO) concept; both employing low-density parity-check (LDPC) codes. Two MIMO concepts are discussed: (i) repetition-MIMO, and (ii) space-time coded-MIMO. The second goal of this paper is to discuss the incompatibility that arises from the bandwidth mismatch between RF/microwave and optical channels. We describe two coded modulation schemes suitable for hybrid microwave-optical communications: (i) coded-OFDM as multiplexing technique, and (ii) Q-ary bit-interleaved coded pulse-amplitude modulation. The third goal of this paper is to describe the ultimate channel capacity limits, and to see how close we can approach those limits with proposed coded-MIMO concepts.