OFDM Without CP in Massive MIMO

TL;DR

This paper explores removing the cyclic prefix in OFDM for massive MIMO systems to improve spectral efficiency, proposing time reversal and zero-forcing techniques to mitigate interference and outperform conventional methods.

Contribution

It introduces a novel approach combining time reversal and zero-forcing to enable CP-free OFDM in massive MIMO, enhancing spectral efficiency and reducing complexity.

Findings

CP removal increases spectral efficiency in massive MIMO.

Time reversal and zero-forcing mitigate residual interference.

Proposed system outperforms conventional CP-OFDM in realistic channels.

Abstract

We study the possibility of removing the cyclic prefix (CP) overhead from orthogonal frequency division multiplexing (OFDM) in massive multiple-input multiple-output (MIMO) systems. We consider the uplink transmission while our results are applicable to the downlink as well. The absence of CP increases the spectral efficiency in expense of intersymbol interference (ISI) and intercarrier interference (ICI). It is known that in massive MIMO, the effects of uncorrelated noise and multiuser interference vanish as the number of base station (BS) antennas tends to infinity. To investigate if the channel distortions in the absence of CP fade away, we study the performance of the standard maximum ratio combining (MRC) receiver. Our analysis reveals that in this receiver, there always remains some residual interference leading to saturation of signal-to-interference-plus-noise ratio (SINR). To resolve this problem, we propose to use the time reversal (TR) technique. Moreover, in order to further reduce the multiuser interference, we propose a zero-forcing equalization to be deployed after the TR combining. We compare the achievable rate of the proposed system with that of the conventional CP-OFDM. We show that in realistic channels, a higher spectral efficiency is achieved by removing the CP from OFDM, while reducing the computational complexity.