Adaptive Modulation in Frequency Spreading OFDM System with Low Transmit Power Spectral Density Constraint

A power spectral density constrained adaptive modulation with frequency spreading orthogonal frequency division multiplexing (OFDM) scheme is proposed. Our objective is to improve the overall throughput under target packet-error-rate (PER) for OFDM transmission. We investigated the performance of a coded adaptive modulation scheme with and without bit interleaver and apply the results to determine the SNR thresholds of the frequency spreading OFDM scheme. Using these SNR thresholds, we derive a frequency spreading adaptive modulation scheme and obtain its performance. For comparison, we derive an algorithm which provides a tight upper bound. Simulation results indicate that our proposed scheme exhibits more than 5 dB signal-to-noise-ratio (SNR) gain relative to the constant transmit power adaptive modulation without frequency spreading in the low SNR region. When compared with the multiband UWB system, the proposed algorithm achieves up to at least 7 dB and 4 dB improvements at the high SNR region and the low SNR region respectively. The transmission ranges are improved by at least 60% for the 80Mbps data rate and 120% for the 480 Mbps.