Movable-Antenna Enhanced Multiuser Communication via Antenna Position Optimization

Movable antenna (MA) is a promising technology to improve wireless communication performance by varying the antenna position in a given finite area at the transceivers to create more favorable channel conditions. In this paper, we investigate the MA-enhanced multiple-access channel (MAC) for the uplink transmission from multiple users each equipped with a single MA to a base station (BS) with a fixed-position antenna (FPA) array. A field-response based channel model is used to characterize the multi-path channel between the antenna array of the BS and each user’s MA with a flexible position. To evaluate the MAC performance gain provided by MAs, we formulate an optimization problem for minimizing the total transmit power of users, subject to a minimum-achievable-rate requirement for each user, where the positions of MAs and the transmit powers of users, as well as the receive combining matrix of the BS are jointly optimized. To solve this non-convex optimization problem involving intricately coupled variables, we develop two algorithms based on zero-forcing (ZF) and minimum mean square error (MMSE) combining methods, respectively. Specifically, for each algorithm, the combining matrix of the BS and the total transmit power of users are expressed as a function of the MAs’ position vectors, which are then optimized by using the proposed multi-directional descent (MDD) framework. It is shown that the proposed ZF-based and MMSE-based MDD algorithms can converge to high-quality suboptimal solutions with low computational complexities. Simulation results demonstrate that the proposed solutions for MA-enhanced multiple access systems can significantly decrease the total transmit power of users as compared to conventional FPA systems employing antenna selection under both perfect and imperfect field-response information.

I. Introduction

With advances in the multiple-input multiple-output (MIMO) technologies, the capacity of today’s wireless communication systems has been dramatically increased by exploiting the new degrees of freedom (DoFs) in the spatial domain. By leveraging the independent/quasi-independent channel fading caused by the random superposition of multipath components, MIMO systems can support parallel transmissions of multiple data streams in the same time-frequency resource block [1], [2], [3]. Thus, MIMO and/or massive MIMO technologies can improve the spectral efficiency manifold compared to single-antenna systems. However, conventional MIMO and/or massive MIMO systems usually adopt fixed-position antennas (FPAs) with spacing no smaller than a half-wavelength at the transceivers [4], [5], [6], [7], [8]. Such fixed and discrete deployment of antennas limits the diversity and spatial multiplexing performance of MIMO systems because the channel variation in the continuous spatial field is not fully utilized.