Nonlinear Precoding in the RIS-Aided MIMO Broadcast Channel

TL;DR

This paper explores the use of nonlinear precoding, specifically Tomlinson-Harashima Precoding, in RIS-aided MIMO broadcast channels, highlighting its robustness and simplified phase shift optimization over linear methods.

Contribution

It introduces the application of THP in RIS-aided MIMO channels with LOS, demonstrating advantages over linear precoding in robustness and phase shift optimization, especially with discrete phase shifts.

Findings

Nonlinear precoding offers robustness when the BS-RIS channel is not orthogonal.

THP simplifies phase shift optimization compared to linear precoding.

Advantages of THP are more significant with discrete phase shifts.

Abstract

We propose to use Tomlinson-Harashima Precoding (THP) for the reconfigurable intelligent surface (RIS)-aided multiple-input multiple-output (MIMO) broadcast channel where we assume a line of sight (LOS) connection between the base station (BS) and the RIS. In this scenario, nonlinear precoding, like THP or dirty paper coding (DPC), has certain advantages compared to linear precoding as it is more robust in case the BS-RIS channel is not orthogonal to the direct channel. Additionally, THP and DPC allow a simple phase shift optimization which is in strong contrast to linear precoding for which the solution is quite intricate. Besides being difficult to optimize, it can be shown that linear precoding has fundamental limitations for statistical and random phase shifts which do not hold for nonlinear precoding. Moreover, we show that the advantages of THP/DPC are especially pronounced for discrete phase shifts.