Communication Modes with Large Intelligent Surfaces in the Near Field

TL;DR

This paper introduces a practical method for defining communication modes in near-field large antenna systems, leveraging focusing beams to enhance channel capacity without complex signal processing.

Contribution

It proposes a beamspace modeling approach for near-field communication modes, deriving their number and basis sets beyond traditional approximations, enabling simpler yet effective capacity enhancement.

Findings

Quasi-optimal communication achievable with focusing functions

Significant channel capacity improvement in line-of-sight conditions

Revises traditional paraxial approximation results

Abstract

This paper proposes a practical method for the definition of communication modes when antennas operate in the near-field region, by realizing ad-hoc beams exploiting the focusing capability of large antennas. The beamspace modeling proposed to define the communication modes is then exploited to derive expressions for their number (i.e., the degrees of freedom) in a generic setup, beyond the traditional paraxial scenario, together with closed-form definitions for the basis set at the transmitting and receiving antennas for several cases of interest, such as for the communication between a large antenna and a small antenna. Numerical results show that quasi-optimal communication can be obtained starting from focusing functions. This translates into the possibility of a significant enhancement of the channel capacity even in line-of-sight channel condition, without the need of implementing optimal but complex phase/amplitude profiles on transmitting/receiving antennas as well as resorting to intensive numerical solutions. Traditional results valid under paraxial approximation are revised in light of the proposed modeling, showing that similar conclusions can be obtained from different perspectives.