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The National Aeronautics and Space Administration has developed a capacity approaching modulation and coding scheme that comprises a serial concatenation of an inner accumulate pulse-position modulation (PPM) and an outer convolutional code [or serially concatenated PPM (SCPPM)] for deep-space optical communications. Decoding of this code uses the turbo principle. However, due to the nonbinary property of SCPPM, a straightforward application of classical turbo decoding is very inefficient. Here, we present various optimizations applicable in hardware implementation of the SCPPM decoder. More specifically, we feature a Super Gamma computation to efficiently handle parallel trellis edges, a pipeline-friendly 'maxstar top-2' circuit that reduces the max-only approximation penalty, a low-latency cyclic redundancy check circuit for window-based decoders, and a high-speed algorithmic polynomial interleaver that leads to memory savings. Using the featured optimizations, we implement a 6.72 megabits-per-second (Mbps) SCPPM decoder on a single field-programmable gate array (FPGA). Compared to the current data rate of 256 kilobits per second from Mars, the SCPPM coded scheme represents a throughput increase of more than twenty-six fold. Extension to a 50-Mbps decoder on a board with multiple FPGAs follows naturally. We show through hardware simulations that the SCPPM coded system can operate within 1 dB of the Shannon capacity at nominal operating conditions. Index Terms--Cyclic redundancy check (CRC), field-programmable gate array (FPGA) implementation, optical communications, quadratic polynomial interleaver, turbo decoding.

We review recent progress in the design of high-rate, distance-enhancing codes that achieve the matched-filter-bound (MFB) on the [E.sup.2]PR4 channel, h(D) = (1 - D)[(1 + D).sup.3], including a rate 8/9 block code satisfying a time-varying maximum-transition-run (TMTR) constraint. We then prove that these TMTR codes also achieve the MFB on the [E.sup.3]PR4 channel, h(D) = (1 - D)[(1 + D).sup.4]. Finally, we describe a TMTR constraint that achieves the MFB on the PR2 channel, h(D) = [(1 + D).sup.2], and the EPR2 channel, h(D) = [(1 + D).sup.3], both of which are of interest as channel models in optical recording, and we present a new, rate 3/4 block code that satisfies this constraint. Computer simulation results confirm that the codes provide substantial performance improvement in additive, white, Gaussian noise (AWGN).