LDPC Code Design for OFDM Channel: Graph Connectivity and Information Bits Positioning
de Baynast, Alexandre
LDPC codes; OFDM system; Factor Graph; Density Evolution Algorithm
We present an optimized channel coding scheme for OFDM transmission. In the future wireless standards such IEEE 802.11n, frame error rates (FER) as low as 0.0001 are targeting. Irregular LDPC codes have excellent performance at moderate computational complexity: they are strong candidates for channel coding in such systems. In the context of an OFDM transmission, the authors in [Mannoni, 2002] proposed to optimize the graph connectivity of the irregular LDPC code accordingly to the channel spectrum. However, their codes did not have good performance for a FER range of 0.001 - 0.0001. In this paper, we propose to optimize the LDPC code in two steps: first, we optimize the graph connectivity in order to get a minimum operational average SNR and therefore excellent FER; then we optimize the information bits placement accordingly to the channel. By simulation, we show that our approach provides substantial performance gain in term of FER over the existing methods (1 dB or more for FER equal to 10e-4.
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