Adaptive Channel Shortening Equalization For Coherent OFDM Doubly Selective Channels

Abstract

In this work we propose a new adaptive channel shortening technique for doubly selective (time-varying frequency-selective) orthogonal frequency division multiplexing (OFDM) channels. OFDM is considered to be a very bandwidth efficient wireless transmission technique (due to its tightly packed orthogonal subcarrier structure) for multi-path channels. Transmissions in multi-path channels exhibiting long delay spread channels, however, require an excessively long cyclic prefix (CP) to prevent intersymbol interference (ISI) with OFDM, and thus undesirably reduce the bandwidth and power efficiency of the information transmission. Channel shortening equalization (CSE) is used to shorten the channel delay spread to a certain length (less than the CP length) so that ISI is minimized. Our technique adapts to channel variation and provides a significant bit error rate (BER) improvement in performance over non-adaptive techniques. We show that the adaptive technique improves the demodulated BER by greater than a factor of 10, with diversity order 1, in a mobile long delay spread (frequency-selective) channel, while the non-adaptive method exhibits a BER floor at 10-1