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Abstract-This paper presents a strategy for the design of bandwidth-efficient LDPC codes with unequal error protection. Bandwidth efficiency is obtained by appropriately designing the codes for higher order constellations, assuming an AWGN channel. The irregularities of the LDPC code are designed, using the Gaussian approximation of the density evolution, to enhance the unequal error protection property of the code as well as account for the different bit error probabilities given by the higher order constellation. The proposed code design algorithm is flexible in terms of the number and proportions of protection classes. It also allows arbitrary modulation schemes. Our method combines the design of unequal error protection LDPC codes for the binary input AWGN channel with the code design for higher order constellations by dividing the variable node degree distribution into sub-degree distributions for each protection class and each level of protection from the modulation. The results show that appropriate code design for higher order constellations reduces the overall bit-error rate significantly. Furthermore, the unequal error protection capability of the code is increased, especially for high SNR.Index Terms-LDPC, unequal error protection, higher order constellations, bandwidth efficiency

This paper analyzes construction algorithms for low-density parity-check (LDPC) codes with respect to their unequal error protection (UEP) capabilities. We show that the choice of code construction algorithm highly affects the performance and UEP properties of LDPC codes with identical degree distributions. Our results provide an explanation to disagreements in earlier research.

We present an optimization method for unequal error protection (UEP)-LDPC codes with higher order constellations. By modifying the density evolution algorithm under the Gaussian approximation, we propose a flexible code design algorithm for a variable number of protection classes and arbitrary modulation schemes with Gray mapping. Our results show that appropriate code design for higher order constellations reduces the overall bit-error rate. Furthermore, the influence on the UEP capability of the code, that is, the difference in bit-error rate between the protection classes, is investigated.

First unequal error protection (UEP) proposals date back to the 1960's (Masnick and Wolf; 1967), but now with the introduction of scalable video, UEP develops to a key concept for the transport of multimedia data. The paper presents an overview of some new approaches realizing UEP properties in physical transport, especially multicarrier modulation, or with LDPC and Turbo codes. For multicarrier modulation, UEP bit-loading together with hierarchical modulation is described allowing for an arbitrary number of classes, arbitrary SNR margins between the classes, and arbitrary number of bits per class. In Turbo coding, pruning, as a counterpart of puncturing is presented for flexible bit-rate adaptations, including tables with optimized pruning patterns. Bit- and/or check-irregular LDPC codes may be designed to provide UEP to its code bits. However, irregular degree distributions alone do not ensure UEP, and other necessary properties of the parity-check matrix for providing UEP are also pointed out. Pruning is also the means for constructing variable-rate LDPC codes for UEP, especially controlling the check-node profile.

This paper addresses the design of low-density parity-check (LDPC) codes for half-duplex co-operative relay networks. Structured rate-compatible codes with unequal error protection (UEP) are designed through joint optimization of the codes for the channels between source and relay and source and destination. The proposed codes clearly outperform simpler LDPC codes which are not optimized for relay channels and puncturing-based rate-compatible LDPC codes, and they show a significant performance improvement over the direct link communication depending on the position of relay. The optimization algorithm for the proposed codes is based on density evolution using the Gaussian approximation and optimal variable node degree distributions are found through iterative linear programming. Interestingly, they anyhow show performance which is almost comparable to the performance of codes optimized through a more complex non-linear optimization algorithm. We analyze the performance of our proposed codes with short, medium and long block lengths, and with low and high rates under realistic assumptions, i.e., imperfect decoding of the codeword at relay and variant signal-to-noise ratio within a single codeword.

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