Mixed Near- and Far-Field Communications for Extremely Large-Scale Array: An Interference Perspective

TL;DR

This paper investigates interference in mixed near- and far-field communications with XL-arrays, deriving interference expressions and analyzing how system parameters affect user rates and interference levels.

Contribution

It introduces a new mixed near- and far-field communication scenario and provides a closed-form interference expression using Fresnel functions, analyzing key parameter effects.

Findings

Strong interference occurs with small antenna numbers and close near-field users.

Interference is significant when near- and far-field user angles are similar.

The achievable rate of near-field users is quantified and affected by interference.

Abstract

Extremely large-scale array (XL-array) is envisioned to achieve super-high spectral efficiency in future wireless networks. Different from the existing works that mostly focus on the near-field communications, we consider in this paper a new and practical scenario, called mixed near- and far-field communications, where there exist both near- and far-field users in the network. For this scenario, we first obtain a closed-form expression for the inter-user interference at the near-field user caused by the far-field beam by using Fresnel functions, based on which the effects of the number of BS antennas, far-field user (FU) angle, near-field user (NU) angle and distance are analyzed. We show that the strong interference exists when the number of the BS antennas and the NU distance are relatively small, and/or the NU and FU angle-difference is small. Then, we further obtain the achievable rate of the NU as well as its rate loss caused by the FU interference. Last, numerical results are provided to corroborate our analytical results.