Communicating with Large Intelligent Surfaces: Fundamental Limits and Models (1912.01719v2)

Abstract: This paper analyzes the optimal communication involving large intelligent surfaces (LIS) starting from electromagnetic arguments. Since the numerical solution of the corresponding eigenfunctions problem is in general computationally prohibitive, simple but accurate analytical expressions for the link gain and available spatial degrees-of-freedom (DoF) are derived. It is shown that the achievable DoF and gain offered by the wireless link are determined only by geometric factors, and that the classical Friis' formula is no longer valid in this scenario where the transmitter and receiver could operate in the near-field regime. Furthermore, results indicate that, contrarily to classical MIMO systems, when using LIS-based antennas DoF larger than 1 can be exploited even in strong line-of-sight (LOS) channel conditions, which corresponds to a significant increase in spatial capacity density, especially when working at millimeter waves.

• The paper presents novel analytical methods that model coupling efficiency and spatial degrees of freedom in near-field LIS communication.
• It demonstrates that spatial degrees of freedom can exceed unity under LOS conditions, enhancing multiplexing capabilities compared to traditional MIMO.
• The study reveals that normalized coupling gains depend on geometric parameters rather than absolute size or distance, redefining classical far-field assumptions.

Communicating with Large Intelligent Surfaces: An Exposition on Fundamental Limits and Models

This paper under review addresses the complex problem of optimal communication facilitated by Large Intelligent Surfaces (LIS), discussing boundary conditions and enhancements specifically in scenarios where current models fall short, such as near-field conditions. The research explores the fundamental aspects of LIS-based wireless communication systems, deriving analytical expressions that transcend the traditional Friis' formula for far-field communication, which becomes obsolete in near-field scenarios.

Key Findings

The paper asserts that in contexts where the near-field regime is applicable, characterized by the proximity between transmitters and receivers relative to the size of the LIS, traditional models fail to provide accurate power gain estimates. Instead, the paper proposes novel analytical solutions by solving eigenfunction problems linked to the electromagnetic wave equations. These solutions model the coupling efficiency and degrees of freedom (DoF) between the LIS and a transmission source, which are crucial indicators of communication performance.

Several significant results are highlighted in this exposition:

1. Degrees of Freedom: The paper reveals that the spatial DoF available in LIS-based setups can exceed one, even under line-of-sight (LOS) conditions. This departs starkly from conventional multiple-input multiple-output (MIMO) systems where LOS typically constrains the DoF to unity. Such a finding implies a potential increase in spatial capacity, as DoF directly influences multiplexing capabilities.
2. Normalized Coupling Gain: The coupling gain, or the link gain between communication counterparts, is shown to be predominantly dependent on geometric parameters, normalized to wavelength, rather than on absolute size or distance as suggested by Friis' transmission equation. This gain could saturate at a particular limit which cannot be enhanced merely by increasing the size of the surfaces involved in communication.
3. Near-field and Far-field Distinctions: The research delineates explicit conditions under which the gains from the LIS reflect near-field characteristics such as beamforming capabilities and energy focusing, challenging existing paradigms in antenna theory that primarily cater to far-field scenarios.

Practical and Theoretical Implications

The paper underscores the substantial benefits of deploying LIS in environments where millimeter-wave (mmWave) or high-frequency transmissions dominate, given their potential to exploit additional spatial DoFs under LOS conditions—scenarios typical in urban environments where path diversity can be limited. Furthermore, by focusing on geometric considerations, the research offers a direct line of inquiry into optimizing LIS placements and configurations to maximize communication channels without supplementary power consumption or bandwidth allocation.

On a theoretical level, this research prompts a reevaluation of classical electromagnetic theory applications, advocating for the development of more flexible models that integrate the high-dimensional state-space facilitated by LIS technologies.

Future Prospects

This paper opens several avenues for future research, particularly in extending these principles to truly holographic MIMO communications, where surfaces act not only as reflective/absorptive instruments but as active field manipulators. The implications could materialize in fostering advancements in smart environments—where infrastructure, working in tandem with LIS, optimizes propagation characteristics automatically, aligning with the paradigms of reconfigurable intelligent surfaces.

Such insights could extensively inform the development strategies of next-generation wireless networks, including 6G, enabling practical applications such as high fidelity urban data transmission, augmented spatial communication in smart buildings, and beyond.

The innovative implications for information theory and electromagnetic processing proposed in this paper provide a cornerstone for extensive research into the vast potential introduced by Large Intelligent Surfaces in modern-day communications.