Hybrid Constellation Modulation for Symbol-Level Precoding in RIS-Enhanced MU-MISO Systems

TL;DR

This paper introduces a hybrid-constellation modulation scheme for RIS-enhanced MU-MISO systems that improves symbol-level precoding efficiency and performance, addressing scalability and modulation limitations of traditional QAM.

Contribution

It proposes a novel hybrid modulation scheme combining QAM and ASK, and a two-stage optimization method, enhancing CI exploitation and scalability in RIS-assisted systems.

Findings

HCM achieves up to 1.5 dB SER gain over QAM 16

HCM achieves up to 1 dB SER gain over QAM 64

Improved CI region extends modulation effectiveness

Abstract

The application of symbol-level precoding (SLP) in reconfigurable intelligent surfaces (RIS) enhanced multi-user multiple-input single-output (MU-MISO) systems faces two main challenges. First, the state-of-the-art joint reflecting and SLP optimization approach requires exhaustive enumeration of all possible transmit symbol combinations, resulting in scalability issues as the modulation order and number of users increase. Second, conventional quadrature amplitude modulation (QAM) exhibits strict constructive interference (CI) regions, limiting its effectiveness for CI exploitation in SLP. To address these challenges, this paper proposes a novel modulation scheme, termed hybrid-constellation modulation (HCM), which has a structure of superposed QAM and ASK sub-constellations (SCs). HCM extends the CI regions compared to QAM. Additionally, a two-stage reflecting and SLP optimization method is developed to support HCM. The proposed methods are designed for practical RIS with discrete phase shifts and has good scalability. Simulation results show that HCM achieves up to 1.5 dB and 1 dB SER gains over QAM with modulation order 16 and 64, respectively.