Channel Shortening by Large Multiantenna Precoding in OFDM

Abstract

A channel delay spread larger than the cyclic prefix (CP) creates self-interference (ISI/ICI) in orthogonal frequency-division multiplexing (OFDM). Recent interests in low-latency application has motivated usage of shorter OFDM symbols. In turn, one can either downscale the CP at the cost of interference, or maintain the CP but with increased overhead. To simultaneously maintain low overhead and interference, this paper studies channel shortening methods exploiting the properties of large multiantenna precoding in OFDM. It is shown that ISI/ICI can be asymptotically canceled out by subcarrierlevel precoding with infinite number of antennas. The method, coined time-frequency (TF) precoding, is based on introducing time-delay selectivity inside conventional frequency-selective precoders in order to remove undesired delayed signals. This leads to an optimization trade-off in the precoder between interference mitigation and multi-path combining gain. Time-reversal (TR) filtering, where an OFDM signal without CP is filtered according to the multi-antenna channels, is considered as a benchmark since it provides asymptotically the optimal rate, having no CP overhead, and both full interferencecancellation and maximum muli-path combining gain. Meanwhile, finite-size analysis shows that TFprecoding converges faster to its asymptotic rate than TR-filtering, so that TF-precoding can outperform TR-filtering in the high-SNR regime with not-so-large number of antennas.