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.

翻译：可移动天线（MA）是一项有前景的技术，它通过调整收发器在给定有限区域内的天线位置来创造更有利的信道条件，从而提升无线通信性能。本文研究了MA增强的多址接入信道（MAC），用于多个各配备单个MA的用户到配备固定位置天线（FPA）阵列的基站（BS）的上行传输。采用基于场响应的信道模型来表征BS天线阵列与各用户位置灵活MA之间的多径信道。为评估MA带来的MAC性能增益，我们构建了一个优化问题，以最小化用户的总发射功率为目标，同时满足每个用户的最小可达速率要求，其中联合优化了MA的位置、用户的发射功率以及BS的接收合并矩阵。为求解这一包含复杂耦合变量的非凸优化问题，我们分别基于迫零（ZF）和最小均方误差（MMSE）合并方法开发了两种算法。具体而言，对于每种算法，BS的合并矩阵和用户的总发射功率均表示为MA位置向量的函数，随后通过提出的多方向下降（MDD）框架对这些位置向量进行优化。结果表明，所提出的基于ZF和基于MMSE的MDD算法能够以较低的计算复杂度收敛到高质量的次优解。仿真结果证明，与采用天线选择的传统FPA系统相比，本文提出的MA增强多址接入系统解决方案在完美和不完美场响应信息下均能显著降低用户的总发射功率。