Adaptive modulation with imperfect channel information in OFDM

Adaptive modulation in OFDM is a means by which the OFDM transmitter adapts the subchannel bit and power allocation to the amplitude response of the frequency selective channel for improved performance in high data rate wireless communications. Previous studies demonstrating the performance gains of adaptive modulation have assumed the availability of perfect channel state information. This paper examines the impact on performance of an adaptive OFDM system due to imperfect channel information arising from realistic channel estimators and time-varying channels. The latter is of particular relevance to mobile wireless applications, for which OFDM has received much attention. The performance of a typical channel estimator is found to be sufficient to justify the use of adaptive modulation in wireless OFDM systems, but in mobile situations the effect of outdated channel information in the presence of quickly time-varying channels can be significant. The use of channel prediction is shown to mitigate the impact of outdated channel information and improve performance when the Doppler spread, or mobile velocity, is high