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Max-Min Fairness and PHY-Layer Design of Uplink MIMO Rate-Splitting Multiple Access with Finite Blocklength

Jiawei Xu, Bruno Clerckx

TL;DR

The paper tackles the challenge of reliable, fair uplink communication in multi-antenna networks under finite blocklength by formulating a general $K$-user uplink MIMO RSMA framework with perfect CSIT/CSIR and MMF objectives. It develops an alternating optimization framework that uses successive convex approximation to jointly design precoders and combiners under FBL constraints, and introduces a low-complexity receiver decoding order. A comprehensive PHY-layer design is proposed and validated through link-level simulations incorporating finite constellation modulation, polar codes, and adaptive modulation and coding, demonstrating that RSMA yields substantial max-min throughput gains over SDMA and NOMA, and remains robust under varying network loads. The study also provides practical insight by detailing the transceiver architecture and showing that the benefits translate from theory to realistic hardware-oriented simulations. Overall, RSMA emerges as a strong uplink MIMO strategy for achieving fair and reliable communications with reduced latency in high-demand scenarios.

Abstract

Rate-Splitting Multiple Access (RSMA) has emerged as a potent and reliable multiple access and interference management technique in wireless communications. While downlink Multiple-Input Multiple-Ouput (MIMO) RSMA has been widely investigated, uplink MIMO RSMA has not been fully explored. In this paper, we investigate the performance of uplink RSMA in short-packet communications with perfect Channel State Information at Transmitter (CSIT) and Channel State Information at Receiver (CSIR). We propose an uplink MIMO RSMA framework and optimize both precoders and combiners with Max-Min Fairness (MMF) metric and Finite Blocklength (FBL) constraints. Due to the high coupling between precoders and combiners, we apply the Alternating Optimization (AO) to decompose the optimization problem into two subproblems. To tackle these subproblems, we propose a Successive Convex Approximation (SCA)-based approach. Additionally, we introduce a low-complexity scheme to design the decoding order at the receiver. Subsequently, the Physical (PHY)-layer of the uplink MIMO RSMA architecture is designed and evaluated using multi-user Link-Level Simulations (LLS), accounting for finite constellation modulation, finite length polar codes, message splitting, adaptive modulation and coding, and Successive Interference Cancellation (SIC) at the receiver. Numerical results demonstrate that applying RSMA in uplink MIMO with FBL constraints not only achieves MMF gains over conventional transmission schemes such as Space Division Multiple Access (SDMA) and Non-orthogonal Multiple Access (NOMA) but also exhibits robustness to network loads. The benefits of splitting messages from multiple users are also illustrated. LLS results confirm the improved max-min throughput benefits of RSMA over SDMA and NOMA.

Max-Min Fairness and PHY-Layer Design of Uplink MIMO Rate-Splitting Multiple Access with Finite Blocklength

TL;DR

The paper tackles the challenge of reliable, fair uplink communication in multi-antenna networks under finite blocklength by formulating a general -user uplink MIMO RSMA framework with perfect CSIT/CSIR and MMF objectives. It develops an alternating optimization framework that uses successive convex approximation to jointly design precoders and combiners under FBL constraints, and introduces a low-complexity receiver decoding order. A comprehensive PHY-layer design is proposed and validated through link-level simulations incorporating finite constellation modulation, polar codes, and adaptive modulation and coding, demonstrating that RSMA yields substantial max-min throughput gains over SDMA and NOMA, and remains robust under varying network loads. The study also provides practical insight by detailing the transceiver architecture and showing that the benefits translate from theory to realistic hardware-oriented simulations. Overall, RSMA emerges as a strong uplink MIMO strategy for achieving fair and reliable communications with reduced latency in high-demand scenarios.

Abstract

Rate-Splitting Multiple Access (RSMA) has emerged as a potent and reliable multiple access and interference management technique in wireless communications. While downlink Multiple-Input Multiple-Ouput (MIMO) RSMA has been widely investigated, uplink MIMO RSMA has not been fully explored. In this paper, we investigate the performance of uplink RSMA in short-packet communications with perfect Channel State Information at Transmitter (CSIT) and Channel State Information at Receiver (CSIR). We propose an uplink MIMO RSMA framework and optimize both precoders and combiners with Max-Min Fairness (MMF) metric and Finite Blocklength (FBL) constraints. Due to the high coupling between precoders and combiners, we apply the Alternating Optimization (AO) to decompose the optimization problem into two subproblems. To tackle these subproblems, we propose a Successive Convex Approximation (SCA)-based approach. Additionally, we introduce a low-complexity scheme to design the decoding order at the receiver. Subsequently, the Physical (PHY)-layer of the uplink MIMO RSMA architecture is designed and evaluated using multi-user Link-Level Simulations (LLS), accounting for finite constellation modulation, finite length polar codes, message splitting, adaptive modulation and coding, and Successive Interference Cancellation (SIC) at the receiver. Numerical results demonstrate that applying RSMA in uplink MIMO with FBL constraints not only achieves MMF gains over conventional transmission schemes such as Space Division Multiple Access (SDMA) and Non-orthogonal Multiple Access (NOMA) but also exhibits robustness to network loads. The benefits of splitting messages from multiple users are also illustrated. LLS results confirm the improved max-min throughput benefits of RSMA over SDMA and NOMA.
Paper Structure (20 sections, 31 equations, 7 figures, 1 algorithm)

This paper contains 20 sections, 31 equations, 7 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. Motivations and Contributions
  3. Organisation
  4. Notations
  5. System Model
  6. MIMO RSMA
  7. Transmitter
  8. Receiver
  9. Rate with FBL constraints
  10. MIMO NOMA
  11. Problem Formulation and Optimization
  12. Problem Formulation
  13. Precoders Optimization
  14. Combiners Optimization
  15. Proposed Algorithm, Convergence and Complexity
  16. ...and 5 more sections

Figures (7)

  • Figure 1: Transmission Model for uplink MU-MIMO adopting RSMA at the user side. Users are classified into splitting users (depicted in orange block) and non-splitting users (depicted in yellow block), with each user in the former group splitting their respective messages to experience different decoding priorities at the BS, leveraging the benefit of rate-splitting.
  • Figure 2: The schematic illustration of the two-user rate-region. The highlighted points (RSMA marked by a square and NOMA marked by a circle) represent the best fairness achieved by RSMA and NOMA respectively.
  • Figure 3: The Proposed Transceiver Architecture of uplink MIMO RSMA for Link-Level Simulations.
  • Figure 4: The MMF performance of RSMA, NOMA and SDMA versus blocklengths with three different transmit SNR averaged over 100 random channel realizations. $K=2$. $N_{t}=2$. (a) is the underloaded scenario; (b) is the overloaded scenario.
  • Figure 5: The MMF performance of RSMA, NOMA and SDMA in the underloaded network settings averaged over 100 random channel realizations. Blocklength is 250 bits. $K=4$. $N_{t}=2$. (a) is the MMF versus transmit SNR; (b) is the relative gain of RSMA at the transmit SNR of 20 dB.
  • ...and 2 more figures

Theorems & Definitions (1)

  • Remark 1