Adaptive OFDM systems with imperfect channel state information

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 (CST) is required at the transmitter to achieve the benefits. Imperfect CST arises from noisy channel estimates, which may also be outdated due to a delay in getting the CST to the transmitter. In this paper, we study adaptive OFDM with imperfect CST for the uncoded variable bit rate case, where a target bit error rate is set. A loading algorithm based on the statistics of the real channel is proposed. Performance results in terms of the average spectral efficiency are provided for adaptive OFDM systems when there is noisy channel estimation or CST delay. The use of multiple estimates is then proposed to improve the performance. It is shown that multiple estimates from different frequencies or times can enhance the performance significantly, which enables the system to tolerate larger errors in channel estimation or longer delay in CST.