Performance of low-density parity-check (LDPC) coded OFDM systems

Orthogonal frequency division multiplexing (OFDM) is a very attractive technique for high-bit-rate data transmission in multipath environments. Many error-correcting codes have been applied to OFDM. Recently, LDPC codes have attracted much attention. The performance of LDPC codes is very close to the Shannon limit, with practical decoding complexity. We proposed LDPC coded OFDM (LDPC-COFDM) systems with BPSK and showed that the LDPC codes are effective in improving the bit error rate (BER) of OFDM in multipath environments (see Futaki, H. and Ohtsuki, T., IEEE VTC2001 fall, vol.1, p.82-6, 2001). LDPC codes can be decoded using a probability propagation algorithm known as the sum-product algorithm or belief propagation. To clarify iterative decoding properties in LDPC-COFDM systems, we first investigate the distribution of the number of iterations where the decoding algorithm stops. In mobile communications, multilevel modulation is preferred for high bandwidth efficiency. However, it has not been clarified how to apply LDPC codes to OFDM systems with multilevel modulation. We propose a decoding algorithm for the LDPC-COFDM systems with M-PSK. By simulation, we show that LDPC-COFDM systems achieve good error rate performance with a small number of iterations on both AWGN and frequency-selective fading channels. We confirm that the algorithm for LDPC-COFDM systems with M-PSK work correctly.