Coded Modulation for the Deep-Space Optical Channel: Serially Concatenated Pulse-Position Modulation

We present an error-control coding technique for optical communications. It consists of the serial concatenation of an outer convolutional code, an interleaver, a bit-accumulator, and pulse-position modulation (PPM). We refer to the coded modulation as serially concatenated PPM, or SCPPM. The encoding is accomplished with simple shift register operations and a table look-up to map code bits to PPM symbols. The code is decoded with an iterative demodulator–decoder, using standard turbo-decoding techniques. For system constraints typical of the Mars Laser Communications Demonstration, simulations indicate operation within 1 dB of capacity. We show that the standard decoder can be simplified by precomputing certain edge likelihoods on a reduced-edge trellis, without approximation or degradation, and that an M -input ∗ max function may be distributed and pipelined. A further simplification allows one to discard many of the channel observables, with negligible degradation.