LDPC code design for OFDM channel: graph connectivity and information bits positioning

Abstract

Abstract-1 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 10 −4 ).

Thresholds for LDPC codes over OFDM

“…it still satisfies Equation (7). We now prove that the initial density of the LLR is given by Equation (8). The proof of the rest of the algorithm is exactly the same as in the AWGN channel case and is given in [10].…”

“…Equation (8) is no more a summation but an integral. As described earlier, the LLR distribution is now a continuous function of ω the angular frequency through its dependence on H(e jω ) the DTFT of the channel impulse response.…”

“…Mannoni proposed a linear criterion for the optimization of irregular LDPC codes for an OFDM system [7]. Baynast et al [8] have proposed a two-step optimization of irregular LDPC codes for OFDM channels. All of these previous works employ a Gaussian approximation for threshold estimation and do not completely prove the existence of thresholds.…”

LDPC Code Design for Nonuniform Power-Line Channels

“…This means that the negative tail of the density is equal to message error rate. The problem of LDPC code design for nonuniform channel has been studied in [6,18,19]. The main idea is that because different subchannels have different qualities, in order to use a single code for all bits, one should specify which bits of the codeword are sent through each subchannel.…”

A Systematic Construction of LDPC Codes for Relay Channel in Time-Division mode

“…This technique has been successfully used to design LDPC codes for various channels including additive white Gaussian noise (AWGN) channel [10], [11], Rayleigh fading channel [5], Orthogonal Frequency Division Multiplexing (OFDM) system [8], [9], [3].…”