RIS-assisted Multiuser MISO Transmission and the Impact of Imperfect Channel Estimation

TL;DR

This paper investigates RIS-assisted MU-MISO systems at mmWave frequencies, focusing on joint design, impact of imperfect channel estimation, and proposing robust pilot schemes to enhance downlink performance.

Contribution

It introduces a joint RIS and ZF precoding design for mmWave MU-MISO, analyzing the effects of channel estimation errors and proposing robust pilot transmission strategies.

Findings

RIS deployment can mitigate rank deficiency issues in MU-MISO channels.

Imperfect channel estimation degrades BER performance, but robust pilot design improves interference management.

Optimized RIS-aided ZF precoding enhances downlink wireless system reliability.

Abstract

This paper proposes the joint design of reconfigurable intelligent surfaces (RIS) and zero-forcing (ZF) precoding for the downlink (DL) multiuser multiple-input single-output (MU-MISO) setup in millimeter-wave (mmWave) bands, where ZF is particularly attractive due to its ability to suppress inter-user interference by exploiting the large antenna arrays and sparse directional channels characteristic of mmWave systems. This ensures efficient spatial multiplexing with manageable complexity, making ZF a practical and in modern 5G/6G deployments. However, a careful design is necessary to overcome potential rank deficiency in the channel matrix. For the MU-MISO case, rank deficiency may arise if users exhibit significantly different channel gains or if, being in far-field, they are aligned with the position of the transmitter. On the other hand, the deployment of a RIS introduces artificial scattering which can shape the radio environment to address those situations. We explore the joint design under perfect channel knowledge, assess the impact of imperfect channel estimation on the bit error rate (BER) and propose a robust design of pilot transmissions that equalizes multiuser interference across users in the presence of channel errors in the precoder design. This evaluation shows the advantages of optimized RIS-aided ZF MU-MISO communication for the DL of wireless systems.