Efficient Rate-Adaptive Modulation for LDPC-Coded OFDM System

Adaptive coded modulation (ACM) is a key technique to increase spectral as well as power efficiency for orthogonal frequency division multiplexing (OFDM) systems. In this paper, an efficient rate adaptive low density parity check (LDPC) coded modulation (RALCM) algorithm is proposed, which maximizes the payload rate for OFDM systems under the constraint of the given transmit power and the target bit error rate (BER). Taking advantage of the two dimensional look up table for the signal to noise ratio (SNR) thresholds, the RALCM algorithm assigns different combinations of the LDPC code rates and quadrature amplitude modulation (QAM) modulations for subcarriers according to the channel frequency response (CFR). Both theoretical analysis and computer simulation show that the proposed algorithm not only efficiently improves the payload rate of the LDPC coded OFDM system over frequency selective fading channels, but also inherently reduces the computational complexity of the system optimization.