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

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 nearfield 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.Index Terms-Extremely large-scale array/surface (XLarray/surface), mixed near-and far-field communications, interference analysis. I. INTRODUCTIONExtremely large-scale array/surface (XL-array/surface) has emerged as a promising technology to achieve the ever-increasing performance requirements of future sixth-generation (6G) wireless networks, such as super-high spectral efficiency and spatial resolution [1], [2]. This fundamentally leads to the communication paradigm shift from the conventional far-field communications (with planar-wave propagation) to the near-field communications (with spherical-wave propagation) [3] and even the new mixed near-and far-field communications (with both planar-/spherical-wave propagation). For example, consider an XL-array communication system where a base station (BS) equipped with an antenna of diameter 0.5 meter (m) communicates with users at 30 GHz frequency. In this case, the well-known Rayleigh distance is about 50 m [4]. As such, for a typical communication scenario, it may happen that some users reside in the nearfield region, while the others locate in the far-field region, thus leading to several new design issues such as mixed-field channel estimation, joint near-/far-field beamforming, etc.In particular, consider the inter-user interference in different communication scenarios. First, if the users considered are all in the far-field region, spatial division multiple access (SDMA) [5] or beam division multiple access (BDMA) [6] can be employed to simultaneously serve multiple users with low inter-user interference. This is because different far-field directional beams pointing towards different users are asymptotically orthogonal in Y. Zhang and C. You are with the