In space, radiation particles can introduce temporary or permanent errors in memory systems. To protect against potential memory faults, either thick shielding or error correcting codes (ECC) are used. Thick shielding translates into increased mass and conventional ECCs designed for memories are typically capable of only correcting a single error and detecting a double error. Decoding is usually performed through hard-decisions where bits are treated as either correct or flipped in polarity. In this work, we demonstrate that low-density parity-check (LDPC) codes that are already prevalent in many communication applications can also be used to protect memories in space. We develop a channel that models memory error events in a space radiation environment. We describe how to compute soft symbol reliabilities on our channel and compare the performance of soft-decision decoding LDPC codes against conventional hard-decision decoding of Reed-Solomon (RS) codes and Bose-Chaudhuri-Hoquenghem (BCH) codes for a specific memory structure.

Luby transform (LT) codes provide efficient erasure protection for information distributed across a multiuser channel with unknown characteristics. For real-time applications, however, the random selection of information symbols within a large message can increase encoding and decoding latency beyond allowable parameters. By using a smaller message size, the encoder can generate a codeword at a faster rate, leading to a higher decoder throughput. We decrease decoding latency by optimizing various parameters of the Robust Soliton distribution. This optimization in terms of delay does not lead to minimizing overhead. However, if a small increase in overhead can be tolerated, particularly for a smaller message size, then the overall encoding and decoding delay can be greatly reduced.

Significant technological advances were made toward utilizing the Hale telescope for receiving the faint laser communication signals transmitted from an optical transceiver on a spacecraft orbiting Mars. The so-called Palomar receive terminal design, which would have supported nominal downlink data rates of 1-30 Mbps, is described. Testing to validate technologies for near-Sun (3deg from edge of solar disc) daytime operations is also discussed. Finally, a laboratory end-to-end link utilizing a 64-ary pulse-position modulated photon-counting receiver and decoder that achieved predicted near-capacity (within 1.4 dB) performance is described.

We present an efficient implementation of a coded modulation for the deep space optical channel. NASA designed this so called serially concatenated pulse position modulation (SCPPM) code to provide an optical link that can operate within one dB signal energy of the Shannon capacity during a nominal mission condition from Mars. Here, we describe some of the challenges in realizing the SCPPM decoder on a field-programmable gate array (FPGA). Through various architectural optimizations, we achieve a 6 Mbps decoder on a single FPGA. Moreover, we demonstrate that it is possible to communicate reliably on an efficient bits-per-photon count in an end-to-end SCPPM coded system.

We describe novel interleaver and deinterleaver architectures that support bandwidth efficient memory access for decoders of turbo-like codes that are used in conjunction with high order modulations. The presentation focuses on a decoder for serially concatenated pulse-position modulation (SCPPM), which is a forward-error-correction code designed by NASA to support laser communications from Mars at mega-bits-per-second (Mbps) rates. For 64-ary PPM, the new architectures effectively triple the fan-in of the interleaver and fan-out of the deinterleaver, enabling parallelization that doubles the overall throughput. The techniques described here can be readily modified for other PPM orders

An (n,k) Reed-Solomon (RS) code is used in a magnetic recording system to help reduce the word failure rate (WFR). If the channel signal-to-noise ratio (SNR) exceeds a certain value, the full power of a given RS code may be needed only on a few occasions to guarantee a target WFR. When this occurs, a parity-sharing scheme can be used to group a number of RS codewords into a larger codeword, block. The target WFR can, therefore, be achieved at a higher code rate. An efficient list-decoding technique has recently been developed by Guruswami and Sudan (G-S) that allows error correction beyond the classical "half-the-minimum-distance" bound. Koetter and Vardy (K-V) have further extended the G-S algorithm to perform soft-decision list-decoding. This work will show that G-S hard-decision and K-V soft-decision list-decoding of parity-sharing codes are both effective and computationally manageable schemes on the discrete memoryless and partial response channels.

Since the discovery of turbo codes, iterative decoding has gained enormous momentum. The idea of repeatedly passing information between components of a receiver or decoder to increase the overall system performance has attracted much research effort. In this work, we present an iterative soft-decision decoding architecture for Reed-Solomon (RS) codes on a partial-response (PR) channel. The architecture incorporates a symbol-based a-posteriori probability (APP) detector for the channel, and an enhanced soft-decision RS decoder based upon the recently introduced Koetter-Vardy (KY) algorithm. From the list of candidate RS codewords generated by the KV decoder, we calculate output symbol reliabilities that can be fed back to the APP detector as extrinsic information to be used in a subsequent decoding iteration. Through simulations, we show the efficacy of this approach, especially when the initial KV list size is large. We will also propose ways to modify the decoding scheme in order to beneficially increase the size of the candidate codeword list and thereby improve the overall system performance.

We compare the performance of list-type soft-decision Reed-Solomon (RS) decoding algorithms to that of classical hard-decision RS decoding on partial response (PR) channels. The two soft-decision RS decoding approaches that we consider, the list-GMD and the Koetter-Vardy (see Proc. IEEE Int. Symp. Information Theory, (Sorrento, Italy), IEEE, June 2000) algorithm, are both based on Sudan's (1997) (polynomial) interpolation approach to polynomial-time list-decoding. The soft-decision RS decoders take as input symbol-based reliabilities which can be provided by a symbol-based BCJR algorithm. The symbol-based BCJR algorithm is an extension of the original BCJR algorithm and calculates the a posteriori probability (APP) of a block of l-bits. The complexity of the algorithm can be reduced when applied to a PR channel. We present a hybrid Viterbi-BCJR approach that can be used when only the reliability of the most-likely symbol is desired. The hybrid approach calculates the APP of the most reliable symbol without the need to run the complete forward and backward algorithm. We demonstrate through simulations that soft-decision RS decoding will lead to a lower probability of decoding error. Moreover, we show that using the symbol-based APPs will yield a lower symbol error rate (SER) than using the measures obtained by multiplying the bit reliabilities.