Wireless Communications Through Reconfigurable Intelligent Surfaces (1906.09490v2)

Abstract: The future of mobile communications looks exciting with the potential new use cases and challenging requirements of future 6th generation (6G) and beyond wireless networks. Since the beginning of the modern era of wireless communications, the propagation medium has been perceived as a randomly behaving entity between the transmitter and the receiver, which degrades the quality of the received signal due to the uncontrollable interactions of the transmitted radio waves with the surrounding objects. The recent advent of reconfigurable intelligent surfaces in wireless communications enables, on the other hand, network operators to control the scattering, reflection, and refraction characteristics of the radio waves, by overcoming the negative effects of natural wireless propagation. Recent results have revealed that reconfigurable intelligent surfaces can effectively control the wavefront, e.g., the phase, amplitude, frequency, and even polarization, of the impinging signals without the need of complex decoding, encoding, and radio frequency processing operations. Motivated by the potential of this emerging technology, the present article is aimed to provide the readers with a detailed overview and historical perspective on state-of-the-art solutions, and to elaborate on the fundamental differences with other technologies, the most important open research issues to tackle, and the reasons why the use of reconfigurable intelligent surfaces necessitates to rethink the communication-theoretic models currently employed in wireless networks. This article also explores theoretical performance limits of reconfigurable intelligent surface-assisted communication systems using mathematical techniques and elaborates on the potential use cases of intelligent surfaces in 6G and beyond wireless networks.

• The paper demonstrates that RISs can transform wireless propagation by converting non-line-of-sight channels into effective line-of-sight paths with significantly improved received power.
• The paper presents mathematical models showing that optimal phase adjustments yield signal-to-noise ratio gains proportional to the square of the number of RIS elements.
• The paper highlights practical benefits, including enhanced energy efficiency and reduced hardware complexity, paving the way for scalable 6G systems.

Wireless Communications Through Reconfigurable Intelligent Surfaces

The paper “Wireless Communications Through Reconfigurable Intelligent Surfaces” by Ertugrul Basar et al. presents an insightful exploration into the potential of reconfigurable intelligent surfaces (RISs) in the domain of wireless communications. This emerging technology addresses the challenges and limitations of traditional wireless propagation by allowing network operators to control the interactions of radio waves with the environment. The authors detail the capabilities, implications, and future research directions of RISs in enhancing the performance of 6G and beyond wireless networks.

Technological Context and Core Concepts

The advent of 5G has already reshaped mobile communications with use cases such as enhanced mobile broadband, ultra-reliable low-latency communications, and massive machine-type communications. However, achieving the ambitious goals of 6G necessitates radical innovations particularly at the physical layer. RISs are poised to be a cornerstone technology in this evolution. Unlike traditional wireless propagation, which is hampered by the uncontrollable scattering and reflection of signals, RISs enable the deterministic control of these interactions.

RISs, essentially smart surfaces composed of configurable meta-materials, can manipulate the wavefront of impinging signals, optimizing phase, amplitude, frequency, and polarization without complex radio frequency processing. The authors draw parallels to similar technologies like spatial modulation (SM) and media-based modulation (MBM), but emphasize the unique capability of RISs to transform the propagation environment into a programmable entity.

Numerical Results and Performance Evaluation

A key focus of the paper is the quantitative evaluation of RIS-assisted communication systems. The authors present mathematical models and performance metrics that illustrate the advantages of RISs in controlling wireless propagation.

In a one-ray system model enhanced by RISs, it is shown that the received power does not decay with the fourth power of the distance as in conventional settings but with the second power, akin to the line-of-sight (LOS) path. This significant improvement is further amplified when an RIS composed of multiple reconfigurable meta-surfaces is employed. For instance, with optimal phase adjustment of the RIS elements, the received power scales with the square of the number of reflecting elements, highlighting the promise of substantial signal strength gains.

The paper provides detailed derivations of signal-to-noise ratio (SNR) distributions and symbol error probability (SEP) for RIS-enhanced systems. For example, in a system with binary phase shift keying (BPSK), the implementation of RISs yields a received SNR proportional to the square of the number of elements, $N$. This relationship underscores the ability of RISs to remarkably reduce the SEP, demonstrating this through extensive simulations and mathematical models.

Practical and Theoretical Implications

Practical Implications

1. Enhanced Coverage and Reliability:
   • RISs can effectively mitigate coverage holes and address non-line-of-sight (NLOS) scenarios by redirecting and coherently combining reflected signals.
2. Energy Efficiency:
   • By recycling existing signals rather than generating new ones, RISs contribute to reduced energy consumption and lower electromagnetic pollution, aligning with green communication initiatives.
3. Low-Complexity Transmitters:
   • RISs enable the realization of virtual massive MIMO systems with significantly fewer RF chains, reducing hardware complexity and energy consumption in transmitters.

Theoretical Implications

1. Revised Communication Models:
   • The introduction of RISs necessitates a fundamental rethinking of communication-theoretic models. With the environment becoming a controllable entity, traditional Shannon capacity formulations require recalibration to incorporate the programmable nature of RISs.
2. New Performance Limits:
   • The deterministic control over the wireless environment afforded by RISs raises questions about achievable performance limits in terms of capacity, latency, and reliability in RIS-assisted networks.

Open Research Directions

Several open research challenges are identified, providing a fertile ground for future investigations:

1. Physics-Compliant Models:
   • Developing accurate and tractable models that faithfully represent the physical properties and operational constraints of RISs is critical.
2. Channel Estimation and Feedback:
   • Addressing the challenge of efficient channel state information acquisition and phase optimization in predominantly passive RISs remains a key hurdle.
3. Large-Scale Systems Analysis:
   • Understanding the impact of spatial configurations and scalability of RISs in dense wireless networks requires advanced modeling and analytical techniques.
4. Interdisciplinary Research:
   • Bridging the gaps between communication theory, materials science, and electromagnetism to develop holistic solutions that make full use of RIS technology is imperative.

Conclusion

The exploration of reconfigurable intelligent surfaces in wireless communications, as presented by Basar et al., provides a comprehensive overview of their potential to transform future wireless networks. With the capability to control the propagation environment, RISs promise significant improvements in coverage, energy efficiency, and system complexity. The implications of this research span both practical deployments and theoretical advancements, inviting further exploration into the integration and optimization of RISs in 6G and beyond networks.