Robust Transmission in Downlink Multiuser MISO Systems: A Rate-Splitting Approach (1602.04345v3)

Abstract: We consider a downlink multiuser MISO system with bounded errors in the Channel State Information at the Transmitter (CSIT). We first look at the robust design problem of achieving max-min fairness amongst users (in the worst-case sense). Contrary to the conventional approach adopted in literature, we propose a rather unorthodox design based on a Rate-Splitting (RS) strategy. Each user's message is split into two parts, a common part and a private part. All common parts are packed into one super common message encoded using a public codebook, while private parts are independently encoded. The resulting symbol streams are linearly precoded and simultaneously transmitted, and each receiver retrieves its intended message by decoding both the common stream and its corresponding private stream. For CSIT uncertainty regions that scale with SNR (e.g. by scaling the number of feedback bits), we prove that a RS-based design achieves higher max-min (symmetric) Degrees of Freedom (DoF) compared to conventional designs (NoRS). For the special case of non-scaling CSIT (e.g. fixed number of feedback bits), and contrary to NoRS, RS can achieve a non-saturating max-min rate. We propose a robust algorithm based on the cutting-set method coupled with the Weighted Minimum Mean Square Error (WMMSE) approach, and we demonstrate its performance gains over state-of-the art designs. Finally, we extend the RS strategy to address the Quality of Service (QoS) constrained power minimization problem, and we demonstrate significant gains over NoRS-based designs.

• The paper proposes a novel Rate-Splitting strategy splitting user messages for enhanced robust transmission in multiuser MISO systems facing imperfect CSIT.
• The Rate-Splitting strategy achieves higher optimum DoF and guarantees non-saturating max-min rates, providing a theoretical enhancement over existing robust methods.
• Simulations demonstrate that the Rate-Splitting strategy consistently outperforms traditional methods, and the proposed algorithm effectively solves the optimization problem for improved performance.

Robust Transmission in Downlink Multiuser MISO Systems: A Rate-Splitting Approach

This research paper presents an advanced analysis within the field of multiuser MISO (Multiple-Input Single-Output) systems, focusing specifically on robust transmission strategies when the Channel State Information at the Transmitter (CSIT) is subject to bounded errors. Traditionally, robust designs in this space aim to sustain performance amidst CSIT imperfections, typically employing linear precoding strategies. However, this paper proposes an unconventional method grounded in the Rate-Splitting (RS) approach, fundamentally challenging conventional designs.

Core Findings and Methodology

The authors, Joudeh and Clerckx, introduce a novel RS strategy, which involves splitting each user's message into a common and a private part. The common parts consolidate into a super common message, while private parts are independently encoded and precoded using linear methods. This strategy allows for enhanced performance under CSIT uncertainty, as demonstrated through a thorough mathematical framework and derivation.

Theoretical Contributions

1. Optimum Degrees of Freedom (DoF): The paper mathematically derives that the RS strategy can achieve a higher maximum minimum DoF compared to traditional NoRS strategies, which are limited by multiple limitations under imperfect CSIT. Specifically, RS designs can guarantee non-saturating max-min rates even when CSIT errors are non-scaling, signifying a substantial theoretical enhancement over existing methods.
2. Algorithmic Development: The authors develop a robust algorithm employing the cutting-set method combined with the Weighted Minimum Mean Square Error (WMMSE) approach. This algorithm effectively solves the problem by alternating between optimization and pessimization steps, showing significant improvements over previous methods.
3. Quality of Service (QoS) Power Minimization Extension: The RS approach is extended to solve QoS-constrained power minimization problems, demonstrating substantial gains over the NoRS-based designs. This extension highlights the practical applicability and versatility of the proposed RS strategy in real-world scenarios where power efficiency and service quality are pivotal.

Numerical Results and Performance Analysis

Through comprehensive simulations, the RS strategy consistently outperforms NoRS methods. Even when constrained by scaling CSIT errors, RS shows marked rate improvements. Furthermore, the cutting-set method exhibits superior performance in terms of convergence and solution optimality compared to conservative algorithms, aligning closely with the theoretically predicted DoF.

Implications and Future Work

This paper underscores the limitations of conventional robust designs in multiuser MISO systems and introduces Rate-Splitting as an effective alternative. The consistent performance gains and theoretical advancements suggest significant implications for future wireless communication systems, particularly in scenarios heavily burdened by interference. Potential future work could explore more sophistic deployment frameworks for RS strategies, integrating them into massive MIMO systems, and further analyzing their impact on energy-efficient transmissions and cognitive radio settings.

The groundbreaking analysis presented prompts further investigation into RS's capabilities and invites a broader rethinking of interference management and robust design strategies across various communication domains. As practical deployments of MISO systems continue to expand, embracing advanced methodologies such as RS could be pivotal in overcoming prevailing CSIT uncertainty challenges.