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Reconfigurable Intelligent Surface-Enabled Downlink NOMA

Ali Tugberk Dogukan, Emre Arslan, Ertugrul Basar

TL;DR

This work introduces a reconfigurable intelligent surface (RIS) enabled downlink non-orthogonal multiple access (NOMA) scheme in which NOMA is performed over-the-air by partitioned RIS groups, with each partition serving a distinct user. The BS transmits an unmodulated carrier to the RIS, and each RIS partition coherently modulates the reflected signal for its designated user via a phase shift, enabling per-user constructive combining without requiring successive interference cancellation at the receivers. The paper provides an end-to-end system model, theoretical outage probability analysis using CLT and Gil-Pelaez inversion, and BER/SEP analysis via gamma distribution fitting and MGF methods, supplemented by extensive simulations that compare against RIS-OMA benchmarks under perfect and imperfect CSI. Results show that increasing the RIS size and optimizing partitioning yields substantial improvements in outage, BER, and sum-rate, while avoiding the SIC-related complexity of conventional DL-NOMA. The findings highlight the practical potential of RIS partitioning to enable efficient, low-SIC DL-NOMA in future 6G networks, and point to future work on active RIS, element optimization, and learning-based design approaches.

Abstract

Reconfigurable intelligent surfaces (RISs) bring great potential to the advancement of 6G and beyond wireless communication technologies. RISs introduce a great degree of flexibility, allowing some sort of virtual control over the wireless channel. Exploiting the flexibility introduced by RISs, we propose a novel RIS-enabled downlink (DL) non-orthogonal multiple access (NOMA) scheme where NOMA is enabled over-the-air rather than at the base station (BS) or the receiver (Rx). Here, the RIS is partitioned into distinctive groups where each part of the RIS serves a different user equipment (UE) to perform multiple accessing. The BS transmits an unmodulated signal to the RIS, and each partition modulates the impinging signal over-the-air by introducing a phase shift according to the incoming information bits to serve the corresponding UE. First, the end-to-end system model for the proposed system is presented. Furthermore, outage probability calculations, theoretical error probability analysis, and bit error rate (BER) derivations are discussed and reinforced with comprehensive computer simulation results.

Reconfigurable Intelligent Surface-Enabled Downlink NOMA

TL;DR

This work introduces a reconfigurable intelligent surface (RIS) enabled downlink non-orthogonal multiple access (NOMA) scheme in which NOMA is performed over-the-air by partitioned RIS groups, with each partition serving a distinct user. The BS transmits an unmodulated carrier to the RIS, and each RIS partition coherently modulates the reflected signal for its designated user via a phase shift, enabling per-user constructive combining without requiring successive interference cancellation at the receivers. The paper provides an end-to-end system model, theoretical outage probability analysis using CLT and Gil-Pelaez inversion, and BER/SEP analysis via gamma distribution fitting and MGF methods, supplemented by extensive simulations that compare against RIS-OMA benchmarks under perfect and imperfect CSI. Results show that increasing the RIS size and optimizing partitioning yields substantial improvements in outage, BER, and sum-rate, while avoiding the SIC-related complexity of conventional DL-NOMA. The findings highlight the practical potential of RIS partitioning to enable efficient, low-SIC DL-NOMA in future 6G networks, and point to future work on active RIS, element optimization, and learning-based design approaches.

Abstract

Reconfigurable intelligent surfaces (RISs) bring great potential to the advancement of 6G and beyond wireless communication technologies. RISs introduce a great degree of flexibility, allowing some sort of virtual control over the wireless channel. Exploiting the flexibility introduced by RISs, we propose a novel RIS-enabled downlink (DL) non-orthogonal multiple access (NOMA) scheme where NOMA is enabled over-the-air rather than at the base station (BS) or the receiver (Rx). Here, the RIS is partitioned into distinctive groups where each part of the RIS serves a different user equipment (UE) to perform multiple accessing. The BS transmits an unmodulated signal to the RIS, and each partition modulates the impinging signal over-the-air by introducing a phase shift according to the incoming information bits to serve the corresponding UE. First, the end-to-end system model for the proposed system is presented. Furthermore, outage probability calculations, theoretical error probability analysis, and bit error rate (BER) derivations are discussed and reinforced with comprehensive computer simulation results.
Paper Structure (18 sections, 2 theorems, 26 equations, 11 figures, 2 tables)

This paper contains 18 sections, 2 theorems, 26 equations, 11 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Related Work
  3. Studies on RIS
  4. Studies on NOMA
  5. Studies on RIS+NOMA
  6. Main Contributions
  7. Paper Organization
  8. System Model
  9. Performance Analysis
  10. Theoretical Outage Probability Analysis
  11. Theoretical Bit Error Rate Analysis
  12. Simulation Results And Comparisons
  13. Benchmark Schemes
  14. Simulation Setup Parameters
  15. Computer Simulations and Discussions
  16. ...and 3 more sections

Key Result

Lemma 1

The mean and variance values of $\mathcal{A}$ can be given as $\mu_\mathcal{A}=N_g \sigma_h \sigma_{g_k} \frac{\pi}{4}$ and $\sigma_\mathcal{A}=N_g \sigma^2_h \sigma^2_{g_k} (1-\frac{\pi^2}{16} )$, respectively.

Figures (11)

  • Figure 1: The system model of the proposed DL NOMA scheme that serves $K$ UEs.
  • Figure 2: Distribution fit of SINR for varying UEs.
  • Figure 3: Comparison of the outage probability performance of the proposed DL-NOMA scheme and TDMA for $R_k=2$ bps/Hz, $N=512$ and $K=2$, $4$.
  • Figure 4: Comparison of the outage probability performance of the proposed DL-NOMA scheme and TDMA for $R_k=2$ bps/Hz, $N=129$, $255$, $513$ and $K=3$.
  • Figure 5: Outage probability vs QoS performance comparison of the proposed DL-NOMA and OMA schemes for $K=2$, $N=256$, $512$, $1024$, $P_t = 5$ dBm.
  • ...and 6 more figures

Theorems & Definitions (4)

  • Lemma 1
  • proof
  • Lemma 2
  • proof