A joint precoding framework for wideband reconfigurable intelligent surface-aided cell-free network (2002.03744v3)

Abstract: Thanks to the strong ability against the inter-cell interference, cell-free network is considered as a promising technique to improve network capacity. However, further capacity improvement requires to deploy more base stations (BSs) with high cost and power consumption. To address this issue, inspired by the recently developed reconfigurable intelligent surface (RIS) technique, we propose the concept of RIS-aided cell-free network to improve the capacity with low cost and power consumption. The key idea is to replace some of the required BSs by low-cost and energy-efficient RISs. Then, in a wideband RIS-aided cell-free network, we formulate the problem of joint precoding design at BSs and RISs to maximize the network capacity. Due to the non-convexity and high complexity of the formulated problem, we develop an alternating optimization framework to solve this challenging problem. In particular, we decouple this problem via fractional programming, and solve the subproblems alternatively. Note that most of the scenarios considered in existing works are special cases of the general scenario studied in this paper, and the proposed joint precoding framework can serve as a general solution to maximize the capacity in most existing RIS-aided scenarios. Finally, simulation results demonstrate that, compared with the conventional cell-free network, the network capacity under the proposed scheme can be improved significantly.

• The paper proposes a joint active and passive precoding framework using alternating optimization and fractional programming to maximize network capacity in wideband RIS-aided cell-free networks.
• Simulation results show significant capacity improvements and demonstrate that strategic RIS deployment can effectively replace costly base station alternatives.
• The research highlights the potential of integrating RIS with advanced network management techniques for future wireless systems, opening avenues for further research in areas like mmWave/MIMO integration and handling mobility.

Overview of a Joint Precoding Framework for Wideband Reconfigurable Intelligent Surface-Aided Cell-Free Network

Zijian Zhang and Linglong Dai present an investigation into improving network capacity through the use of Reconfigurable Intelligent Surfaces (RIS) in wideband cell-free networks. This paper discusses replacing certain base stations (BS) with RIS to enhance the network's efficiency and capacity while keeping costs and power consumption low. The central concept is an RIS-aided cell-free network that allows the deployment of RIS to manipulate wireless channels constructively and thereby improve capacity by avoiding the traditional network constraints of additional BSs.

Technical Contributions

The paper formulates an optimization problem focusing on joint precoding design at BSs and RISs aiming to maximize network capacity. Since precoding design in wideband scenarios involves high-dimensional and non-convex optimization, the authors employ an alternating optimization framework to address these challenges. The framework is constructed using fractional programming to decouple the original problem and tackle subproblems iteratively. Each iteration involves updating auxiliary variables and solving Quadratically Constrained Quadratic Programs (QCQP). This meticulous design enables effective joint active and passive precoding, which enhances the overall network capacity compared to conventional networks not utilizing RIS.

Results and Implications

Simulation results demonstrate considerable improvements in network capacity under the proposed scheme. One key finding is the ability of RIS to replace costly BS deployment alternatives effectively. Moreover, the framework's extension to two-timescale optimization shows practical relevance, reducing the channels’ state information requirement. This extension advocates matching users with well-performing RIS on broader timescales, thus reducing overhead.

Future Developments

The introduction and application of RIS in cell-free networks illustrate not only the potential of RIS technology but also open up numerous avenues for further research. There is significant interest in broader RIS integration with other technologies, such as mmWave and MIMO, to fully realize the potential of RIS in wireless networks.

Furthermore, translating these qualitative gains into quantitative gains across various network scenarios remains a pivotal future effort. Research could investigate new ways of distributing computational loads or exploring RIS design improvements to handle different environmental conditions such as mobility and harsh propagation conditions typical in some wireless networks.

Conclusion

"Joint Precoding for RIS-Aided Cell-Free Network" highlights a viable path for enhancing network architecture by integrating intelligent reflecting surfaces. The results suggest that strategic use of RIS can significantly boost network capacity efficiently, bringing forward a promising approach for next-generation wireless systems like 6G. The framework and methodologies presented provide vital insights and tools for other researchers aiming to explore the combined potential of RIS and advanced network-management techniques.