NOMA-based Improper Signaling for MIMO STAR-RIS-assisted Broadcast Channels with Hardware Impairments

TL;DR

This paper introduces a novel combination of improper Gaussian signaling, NOMA, and STAR-RIS to enhance spectral efficiency in MIMO broadcast channels with hardware impairments like IQI, demonstrating significant performance improvements.

Contribution

It proposes new schemes integrating IGS, NOMA, and STAR-RIS to mitigate hardware impairments and improve system performance in MIMO broadcast channels.

Findings

IGS and NOMA increase minimum user rates

STAR-RIS significantly enhances system performance

Ignoring IQI degrades system efficiency

Abstract

This paper proposes schemes to improve the spectral efficiency of a multiple-input multiple-output (MIMO) broadcast channel (BC) with I/Q imbalance (IQI) at transceivers by employing a combination of improper Gaussian signaling (IGS), non-orthogonal multiple access (NOMA) and simultaneously transmit and reflect (STAR) reconfigurable intelligent surface (RIS). When there exists IQI, the output RF signal is a widely linear transformation of the input signal, which may make the output signal improper. To compensate for IQI, we employ IGS, thus generating a transmit improper signal. We show that IGS alongside with NOMA can highly increase the minimum rate of the users. Moreover, we propose schemes for different operational modes of STAR-RIS and show that STAR-RIS can significantly improve the system performance. Additionally, we show that IQI can highly degrade the performance especially if it is overlooked in the design.