[Paper Review] A Joint Design for STAR-RIS enhanced NOMA-CoMP Networks: A Simultaneous-Signal-Enhancement-and-Cancellation-based (SSECB) Design

Tianwei Hou, Jun Wang+4 more|arXiv (Cornell University)|May 2, 2021

Advanced Wireless Communication Technologies4 citations

TL;DR

This paper proposes a simultaneous-signal-enhancement-and-cancellation-based (SSECB) design for STAR-RIS-aided NOMA-CoMP networks, jointly optimizing reflection and transmission beamforming to suppress inter-cell interference while enhancing desired signals. Simulation results show that with sufficient RIS elements, inter-cell interference is perfectly canceled and spectral efficiency is significantly improved over conventional SEB and SCB designs.

ABSTRACT

In this correspondence, a novel simultaneous transmitting and reflecting (STAR) reconfigurable intelligent surfaces (RISs) design is proposed in a non-orthogonal multiple access (NOMA) enhanced coordinated multi-point transmission (CoMP) network. Based on the insights of signal-enhancement-based (SEB) and signal-cancellation-based (SCB) designs, we propose a novel simultaneous-signal-enhancement-and-cancellation-based (SSECB) design, where the inter-cell interferences and desired signals can be simultaneously eliminated and boosted. Our simulation results demonstrate that: i) the inter-cell interference can be perfectly eliminated, and the desired signals can be enhanced simultaneously with the aid of a large number of RIS elements; ii) the proposed SSECB design is capable of outperforming the conventional SEB and SCB designs.

Motivation & Objective

• To address the performance limitations of conventional SEB and SCB RIS designs in NOMA-CoMP networks.
• To enable joint signal enhancement and interference cancellation using STAR-RIS for improved spectral and energy efficiency.
• To derive the minimal required number of RIS elements for effective interference cancellation under strong LoS conditions.
• To benchmark the performance of the proposed SSECB design against existing SEB and SCB schemes.
• To provide a practical passive beamforming framework for STAR-RIS in future 6G networks.

Proposed method

• Proposes a novel SSECB beamforming design that leverages both transmission and reflection modes of STAR-RIS to simultaneously enhance desired signals and cancel inter-cell interference.
• Derives the minimal number of RIS elements required for perfect inter-cell interference cancellation under strong LoS conditions, based on path loss and channel gain parameters.
• Uses a joint passive beamforming optimization framework to control reflection and transmission coefficients of RIS elements for interference nulling and signal focusing.
• Employs power-domain NOMA for user multiplexing, with single-antenna BSs and users to isolate beamforming effects from active precoding.
• Analyzes performance using Monte Carlo simulations with realistic path loss and fading parameters, including K-factor and path loss exponents.
• Validates the design under varying RIS element counts and transmit power levels to assess rate performance and interference suppression.

Figure 1: System model of the STAR-RIS enhanced NOMA-CoMP networks.

Experimental results

Research questions

• RQ1 What is the minimal number of RIS elements required to achieve perfect inter-cell interference cancellation in a STAR-RIS enhanced NOMA-CoMP network?
• RQ2 How does the proposed SSECB design outperform conventional SEB and SCB beamforming in terms of achievable rate?
• RQ3 How do path loss exponents of different links affect the required number of RIS elements for effective interference cancellation?
• RQ4 What is the impact of RIS element count on the high-SNR slope of the achievable rate in the proposed design?
• RQ5 Can the proposed design simultaneously enhance desired signals and suppress inter-cell interference in both low- and high-SNR regimes?

Key findings

• The minimal number of RIS elements required for perfect inter-cell interference cancellation increases with higher path loss exponents on the interference links.
• With L = 27 RIS elements, the system achieves a high-SNR slope of 1, indicating perfect interference cancellation and no rate ceiling.
• When the number of RIS elements is doubled to L = 54, the achievable rate performance improves further, confirming the benefit of increased aperture gain.
• The proposed SSECB design achieves higher spectral efficiency than both SEB and SCB designs across all SNR regimes, demonstrating superior performance.
• Perfect interference cancellation is achievable in practice when the number of RIS elements is sufficiently large, due to the random nature of small-scale fading gains.
• The performance gap between the proposed SSECB design and the no-RIS baseline increases with transmit power, highlighting the design’s robustness and scalability.

Figure 2: Minimal required number of RIS elements for the proposed SSECB design in the STAR-RIS enhanced NOMA-CoMP networks, where the small-scale channel gains are normalized to all one, and $\alpha_{3,c}=\alpha_{3,e}=\alpha_{3}$ for simplicity.