Focusing gain analysis of time-reversal precoding in MISO OFDM communication systems

Abstract

Emerging communication systems can benefit from time-reversal (TR) technology thanks to its good spatio-temporal signal focusing effect. The recent advances in low-cost wideband devices fabrication further leverage the use of TR wideband communication systems. The TR is generally carried out in the time domain and the focusing effect of TR comes from the use of a high transmit rate back-off factor (BOF), which is the signal up-sampling rate. In spite of the widely-used orthogonal frequency-division multiplexing (OFDM) modulation, few works have investigated the frequency-domain (FD) TR precoding in combination with OFDM communication systems. Furthermore, most existing works on FD/TR precoding rely on multiple-antenna technology at the transmitter to create the focusing effect. In this paper, we investigate the focusing gain provided by TR precoding in multiple-input single-output (MISO) OFDM systems. In particular, we compare the communication performance of such system at the intended position and that at the unintended position. Based on the analysis, we demonstrate that increasing the BOF and/or the number of transmit antennas significantly improves the focusing effect at the intended position. In contrast, the unintended positions receive less useful power. We derive approximated mean-square-error (MSE) expressions of equalized received signals at both intended and unintended positions. The subsequent focusing gain is presented as a function of the BOF and the number of antennas, enabling us to gain insights in the contributions of each parameter to the system performance. Numerical simulations with multi-path Rayleigh fading channels are carried out to validate the MSE expressions.