A Robust Design for BackCom Assisted Hybrid NOMA

TL;DR

This paper proposes a robust power minimization method for BackCom assisted H-NOMA in 6G systems, effectively handling imperfect CSI and outperforming traditional OMA in power efficiency and feasibility.

Contribution

It introduces a novel robust optimization framework for BackCom assisted H-NOMA with imperfect CSI, using conservative approximations and a convergent algorithm.

Findings

Proposed solution achieves similar power consumption as ideal cases.

Robust H-NOMA outperforms OMA in power efficiency.

High feasibility probability of the robust design under uncertainties.

Abstract

Hybrid non-orthogonal multiple access (H-NOMA) is inherently an enabler of massive machine type communications, a key use case for sixth-generation (6G) systems. Together with backscatter communication (BackCom), it seamlessly integrates with the traditional orthogonal multiple access (OMA) techniques to yield superior performance gains. In this paper, we study BackCom assisted H-NOMA uplink transmission with the aim of minimizing power with imperfect channel state information (CSI), where a generalized representation for channel estimation error models is used. The considered power minimization problem with aggregate data constraints is both non-convex and intractable. For the considered imperfect CSI models, we use Lagrange duality and the majorization-minimization principle to produce a conservative approximation of the original problem. The conservative formulation is relaxed by incorporating slack variables and a penalized objective. We solve the penalized tractable approximation using a provably convergent algorithm with polynomial complexity. Our results highlight that, despite being conservative, the proposed solution results in a similar power consumption as for the nominal power minimization problem without channel uncertainties. Additionally, robust H-NOMA is shown to almost always yield more power efficiency than the OMA case. Moreover, the robustness of the proposed solution is manifested by a high probability of feasibility of the robust design compared to the OMA and the nominal one.