Rate-Splitting Multiple Access for Downlink Communication Systems: Bridging, Generalizing and Outperforming SDMA and NOMA (1710.11018v4)

Abstract: Space-Division Multiple Access (SDMA) utilizes linear precoding to separate users in the spatial domain and relies on fully treating any residual multi-user interference as noise. Non-Orthogonal Multiple Access (NOMA) uses linearly precoded superposition coding with successive interference cancellation (SIC) and relies on user grouping and ordering to enforce some users to fully decode and cancel interference created by other users. In this paper, we argue that to efficiently cope with the high throughput, heterogeneity of Quality-of-Service (QoS), and massive connectivity requirements of future multi-antenna wireless networks, multiple access design needs to depart from SDMA and NOMA. We develop a novel multiple access framework, called Rate-Splitting Multiple Access (RSMA). RSMA is a more general and powerful multiple access for downlink multi-antenna systems that contains SDMA and NOMA as special cases. RSMA relies on linearly precoded rate-splitting with SIC to decode part of the interference and treat the remaining part of the interference as noise. This capability of RSMA to partially decode interference and partially treat interference as noise enables to softly bridge the two extremes of fully decoding interference and treating interference as noise, and provide room for rate and QoS enhancements, and complexity reduction. The three multiple access schemes are compared and extensive numerical results show that RSMA provides a smooth transition between SDMA and NOMA and outperforms them both in a wide range of network loads (underloaded and overloaded regimes) and user deployments (with a diversity of channel directions, channel strengths and qualities of Channel State Information at the Transmitter). Moreover, RSMA provides rate and QoS enhancements over NOMA at a lower computational complexity for the transmit scheduler and the receivers (number of SIC layers).

• The paper's main contribution is the introduction of RSMA, which splits messages into common and private parts to flexibly manage interference.
• It demonstrates that RSMA outperforms SDMA and NOMA across underloaded and overloaded downlink scenarios, approaching the performance of dirty paper coding.
• The paper also proposes low-complexity RSMA variants enabling efficient deployment in practical systems with diverse user capabilities.

An Overview of Rate-Splitting Multiple Access for Downlink Communication Systems

In the emerging landscape of wireless communication, the drive to accommodate massive connectivity and diverse Quality-of-Service (QoS) requirements is increasingly pressing. Traditional multiple access schemes such as Space-Division Multiple Access (SDMA) and Non-Orthogonal Multiple Access (NOMA) offer starkly contrasting approaches to interference management. SDMA treats residual multi-user interference as noise, whereas NOMA relies on successive interference cancellation (SIC) to decode and manage interference. However, both strategies fall short in flexibly addressing the growing complexities of modern networks.

This paper introduces Rate-Splitting Multiple Access (RSMA) as a comprehensive framework that broadens the design spectrum beyond the confines of treating or fully decoding interference. RSMA is shown to encompass both SDMA and NOMA as specific instances, offering a nuanced approach that dynamically blends the two strategies to better handle multi-user interference and enhance throughput and QoS.

Core Proposition of RSMA

RSMA innovatively employs linearly precoded rate-splitting whereby user messages are partitioned into common and private components. The common components are decoded by multiple users, effectively sharing part of the interference burden, while private components are decoded exclusively by their intended recipients, with any interference treated as noise. This approach grants RSMA the flexibility to operate efficiently in varying user deployments and network loads, from underloaded to overloaded regimes.

Numerical Results and Observations

The numerical analyses in the paper underscore RSMA's superior performance across diverse scenarios, notably outperforming both SDMA and NOMA. RSMA adapts to channel conditions, user deployment angles, and strength disparities, managing interference in a way that substantially increases the achievable rate region, closely paralleling the optimal, albeit complex, Dirty Paper Coding (DPC).

• Underloaded Scenarios: In scenarios with aligned channels or orthogonal user channels, RSMA exhibits rate regions substantially larger than those of SDMA or NOMA, highlighting its adeptness at bridging these strategies.
• Overloaded Scenarios: RSMA maintains robustness and efficient performance even as network loads exceed the number of available antennas, unlike SDMA which suffers significant performance drops.
• Complexity and Implementation: The paper also details low-complexity variants of RSMA, such as 1-layer RS, which retains competitive performance with reduced computational burdens at both transmitter and receiver ends—a crucial aspect for practical deployment.

Theoretical and Practical Implications

The research elucidates RSMA's theoretical underpinning, aligning it with known capacity and DoF benefits in multi-antenna BC settings—both perfect and imperfect Channel State Information at the Transmitter (CSIT). Practically, RSMA's ability to adapt to heterogeneous device capabilities, such as high-end units alongside IoT devices, positions it as a promising candidate for future network designs in 5G and beyond.

Future Directions

The work opens several avenues for further research. Exploring RSMA's application in multi-cell and massive MIMO systems, integrating modulation schemes, and extending robust optimization to broader contexts could further solidify RSMA's role in next-generation wireless networks. Additionally, RSMA holds potential for advancing multi-user communication paradigms beyond conventional frameworks, meriting deeper exploration into standardization and industry adoption.

In summation, the paper presents RSMA as a transformative approach to multiple access in wireless communication systems, advancing the discourse beyond the traditional dichotomies of SDMA and NOMA.