Reconfigurable Intelligent Surfaces assisted Communications with Limited Phase Shifts: How Many Phase Shifts Are Enough? (1912.01477v2)

Abstract: Reconfigurable intelligent surface~(RIS) has drawn a great attention worldwide as it can create favorable propagation conditions by controlling the phase shifts of the reflected signals at the surface to enhance the communication quality. However, the practical RIS only has limited phase shifts, which will lead to the performance degradation. In this letter, we evaluate the performance of an uplink RIS assisted communication system by giving an approximation of the achievable data rate, and investigate the effect of limited phase shifts on the data rate. In particular, we derive the required number of phase shifts under a data rate degradation constraint. Numerical results verify our analysis.

• The paper shows that aligning discrete phase shifts with the LoS component optimizes the achievable uplink data rate.
• It introduces a detailed system model using a Rician channel to quantify the SNR improvements from phase shift optimization.
• It concludes that as the number of RIS elements increases, fewer phase shifts—sometimes as little as one-bit—are sufficient for near-optimal performance.

Reconfigurable Intelligent Surfaces Assisted Communications with Limited Phase Shifts: How Many Phase Shifts Are Enough?

The paper entitled "Reconfigurable Intelligent Surfaces assisted Communications with Limited Phase Shifts: How Many Phase Shifts Are Enough?" explores the potential of Reconfigurable Intelligent Surfaces (RIS) to enhance wireless communication systems. It focuses on mitigating the constraint of limited phase shifts available in practical RIS implementations.

Key Contributions and Methodology

The authors provide an analysis of an uplink RIS-assisted communication system, emphasizing the degradation in data rate due to limited phase shifts. The RIS is proposed as a cost-effective method to achieve high spectral and energy efficiency by manipulating the phase shifts of numerous low-cost reflecting elements. Unlike earlier studies that assumed continuous phase shifts, this paper considers the practical constraints of discrete phase shifts.

A detailed system model is presented, where the RIS is deployed to improve communication quality between a base station (BS) and a user when the direct link suffers from deep fading. The channel model incorporates the RIS-mediated reflections, distinguishing between the Line-of-Sight (LoS) and non-LoS (NLoS) components using a Rician channel model.

Achievable Data Rate and Phase Shift Design

The authors derive an expression for the achievable data rate, considering both continuous and discrete phase shifts. The relationship between phase shifts and the received signal power reveals that optimizing phase shifts can significantly enhance the SNR and, consequently, the data rate.

For systems using continuous phase shifts, it is demonstrated that the data rate increases with the Rician factor $\kappa$, indicating that a stronger LoS component yields better performance. The paper presents an optimal phase shift strategy that maximizes the achievable data rate by aligning the phase shifts with the LoS path.

Limited Phase Shifts and Data Rate Degradation

The impact of finite phase shifts is quantitatively evaluated by defining the phase error relative to the optimal phase shifts. The paper provides the conditions under which the data rate degradation is kept below a certain threshold despite the limitation on phase shift granularity. Through mathematical analysis and numerical simulation, it is concluded that fewer phase shifts are sufficient as the number of RIS elements increases, with one bit being adequate when the RIS size approaches infinity.

Implications and Future Directions

This study offers actionable insights for the deployment of RIS in real-world communication systems where phase shift implementations are inherently discrete. The provision of bounds on the required number of phase shifts ensures that system designers can balance complexity and performance effectively.

For future research, the paper suggests exploring RIS-assisted communications in more complex network configurations, including Device-to-Device (D2D) scenarios and heterogeneous networks. It also opens avenues for integrating energy cooperation features and RF sensing capabilities, leveraging the unique attributes of RIS.

Conclusion

The analysis underscores the significance of RIS in enhancing wireless communication reliability and performance, especially under practical constraints. By taking into account the limitations of phase shift implementations, this research paves the way for the development of robust and efficient RIS-enabled communication networks. Further exploration of RIS applications across various network settings can lead to more comprehensive and versatile communication solutions.