Adaptive modulation for variable-rate OFDM systems with imperfect channel information

Abstract

Adaptive modulation has been shown to have significant benefits for high-speed wireless data transmission when orthogonal frequency division multiplexing (OFDM) is employed. However, accurate channel state information (CSI) is required at the transmitter to achieve the benefits. Imperfect CSI arises from noisy channel estimates, which may also be outdated due to a delay in getting the CSI to the transmitter. We study adaptive OFDM with both perfect and imperfect CSI for the variable bit rate case, where a target bit error rate is set. Performance results are provided for adaptive OFDM with imperfect CSI. The use of multiple estimates is shown to mitigate the effect of CSI delay. In addition, the fundamental error mechanisms which result in performance degradation are studied. Robust approaches are then proposed that are less sensitive to CSI errors.