Rate-Splitting Multiple Access: Fundamentals, Survey, and Future Research Trends (2201.03192v3)

Abstract: Rate-splitting multiple access (RSMA) has emerged as a novel, general, and powerful framework for the design and optimization of non-orthogonal transmission, multiple access (MA), and interference management strategies for future wireless networks. Through information and communication theoretic analysis, RSMA has been shown to be optimal (from a Degrees-of-Freedom region perspective) in several transmission scenarios. Compared to the conventional MA strategies used in 5G, RSMA enables spectral efficiency (SE), energy efficiency (EE), coverage, user fairness, reliability, and quality of service (QoS) enhancements for a wide range of network loads (including both underloaded and overloaded regimes) and user channel conditions. Furthermore, it enjoys a higher robustness against imperfect channel state information at the transmitter (CSIT) and entails lower feedback overhead and complexity. Despite its great potential to fundamentally change the physical (PHY) layer and media access control (MAC) layer of wireless communication networks, RSMA is still confronted with many challenges on the road towards standardization. In this paper, we present the first comprehensive overview on RSMA by providing a survey of the pertinent state-of-the-art research, detailing its architecture, taxonomy, and various appealing applications, as well as comparing with existing MA schemes in terms of their overall frameworks, performance, and complexities. An in-depth discussion of future RSMA research challenges is also provided to inspire future research on RSMA-aided wireless communication for beyond 5G systems.

• The paper demonstrates that RSMA improves DoF and spectral efficiency by flexibly managing interference through common and private message splitting.
• The paper compares multiple RSMA schemes, including 1-layer and 2-layer hierarchical RS, highlighting performance gains over SDMA, NOMA, and OMA.
• The paper outlines future research directions such as advanced precoding, multi-cell integration, and AI-driven network adaptations for next-generation wireless systems.

Rate-Splitting Multiple Access: Fundamentals, Survey, and Future Research Trends

The paper "Rate-Splitting Multiple Access: Fundamentals, Survey, and Future Research Trends" provides an extensive overview of Rate-Splitting Multiple Access (RSMA) as an emerging multiple access (MA) strategy for wireless communication networks beyond 5G. The paper propounds RSMA as an innovative and comprehensive framework that not only addresses current challenges in non-orthogonal transmission and interference management but also bridges the gap between various existing MA schemes such as SDMA, NOMA, and OMA.

Overview and Key Insights

RSMA entails splitting user messages for more efficient management of interference through encoding common and private message parts. This strategy flexibly accommodates scenarios by transitioning between treating interference as noise and fully decoding it, thus providing a broader and more adaptable interference management mechanism.

The paper elaborates on different RSMA schemes—1-layer RS, 2-layer hierarchical RS, generalized RS, THPRS, and RS-CMD—detailing their architectures, mathematical formulations, and distinct transmission mechanisms. Among these, 1-layer RS serves as a fundamental building block, demonstrating that even simple RSMA schemes can achieve significant performance gains over existing MA schemes in terms of Degrees-of-Freedom (DoF), spectral efficiency (SE), and robustness to CSIT imperfections.

Numerical and Theoretical Benefits

Empirical evaluations further corroborate RSMA’s superiority. Its ability to manage co-channel interference efficiently leads to an increased DoF, thereby enhancing network throughput, energy efficiency, coverage, and fairness across both loaded and underloaded network regimes. RSMA shows remarkable robustness against imperfect CSIT, a perennial issue in MIMO systems, thus qualifiedly supporting wide-ranging network topologies and user conditions. Particularly, its DoF region outperforms SDMA and NOMA, especially in systems with heterogeneous CSIT and user disparities.

Implications for Wireless Networks and AI Developments

The practical implications of RSMA are profound. The framework not only promises improved performance metrics and reliability for current wireless systems but also presents a platform facilitating the transition to the next-generation 6G networks. RSMA's adaptability and efficiency align well with the multifaceted demands for massive connectivity, ultra-reliable low-latency communication (URLLC), and enhanced mobile broadband (eMBB), thus enabling seamless integration of future AI-driven applications into wireless frameworks.

Moreover, the paper envisions RSMA's potential to transform physical (PHY) and media access control (MAC) layer architectures, contributing to energy-efficient designs particularly significant in AI-powered IoT ecosystems. Future research may explore leveraging RSMA for cognitive and cooperative communication paradigms where adaptability and intelligence are pivotal.

Future Research Directions

While acknowledging RSMA's extensive potential, the paper outlines several avenues for future research. These include further exploration of its applicability in multi-cell and cooperative networks, refined precoding techniques for complex deployment scenarios, and integration with heterogeneous network technologies such as UAVs, satellite communications, and multi-beam systems. Additionally, the need for standardization and practical adoption remains paramount, calling for comprehensive evaluations of RSMA's system-level performance, signaling protocols, and cross-layer design considerations.

In summary, RSMA stands at the frontier of wireless communication, offering a versatile, capable, and robust framework poised to meet the demanding specifications of future wireless systems. As research and deployments advance, RSMA will undeniably play a critical role in shaping the trajectory of wireless network innovations.