Statistical adaptive modulation for QAM-OFDM systems

Certain OFDM channnels can be shown to exhibit different fading severity amongst the subcarriers. For such channels, we propose novel adaptive power control and bit loading algorithms to maximize the spectrual efficiency of the OFDM system while minimizing the required transmit power at some specified BER level. Unlike its existing counterparts, our adaptive modulation algorithm adapts the subcarrier transmission parameters based on the subcarrier fading statistics rather than the instantaneous channel conditions. A new closed-form expression with good inversion property for QAM BER in Rician fading channel is also developed to facilitate the computation of the optimum adaptive modulation settings. Results show that the proposed scheme can achieve considerable performance enhancement over non-adaptive systems.