RIS-Assisted Grant-Free NOMA

TL;DR

This paper proposes a RIS-assisted grant-free NOMA scheme that enhances network sum rate by joint user clustering and RIS configuration, enabling efficient, grant-free access with minimal signaling.

Contribution

It introduces a novel joint user clustering and RIS assignment method that maximizes sum rate and enables grant-free access without power control or additional signaling.

Findings

Achieves up to 20% higher sum rate than benchmark schemes

Implicit over-the-air power control reduces signaling overhead

Performance depends on UE density and RIS deployment

Abstract

This paper introduces a reconfigurable intelligent surface (RIS)-assisted grant-free non-orthogonal multiple-access (GF-NOMA) scheme. To ensure the power reception disparity required by the power domain NOMA (PD-NOMA), we propose a joint user clustering and RIS assignment/alignment approach that maximizes the network sum rate by judiciously pairing user equipments (UEs) with distinct channel gains, assigning RISs to proper clusters, and aligning RIS phase shifts to the cluster members yielding the highest cluster sum rate. Once UEs are acknowledged with the cluster index, they are allowed to access their resource blocks (RBs) at any time requiring neither further grant acquisitions from the base station (BS) nor power control as all UEs are requested to transmit at the same power. In this way, the proposed approach performs an implicit over-the-air power control with minimal control signaling between BS and UEs, which has shown to deliver up to 20% higher network sum rate than benchmark GF-NOMA and optimal grant-based PD-NOMA schemes depending on the network parameters. The given numerical results also investigate the impact of UE density, RIS deployment, and RIS hardware specifications on the overall performance of the proposed RIS-aided GF-NOMA scheme.