Coupler Position Optimization and Channel Estimation for Flexible Coupler Aided Multiuser Communication

In this paper, we propose a distributed flexible coupler (FC) array to enhance communication performance with low hardware cost. At each FC antenna, there is one fixed-position active antenna and multiple passive couplers that can move within a designated region around the active antenna. Moreover, each FC antenna is equipped with a local processing unit (LPU). All LPUs exchange signals with a central processing unit (CPU) for joint signal processing. We study an FC-aided multiuser multiple-input multiple-output (MIMO) system, where an FC array base station (BS) is deployed to enhance the downlink communication between the BS and multiple single-antenna users. We formulate optimization problems to maximize the achievable sum rate of users by jointly optimizing the coupler positions and digital beamforming, subject to movement constraints on the coupler positions and the transmit power constraint. To address the resulting nonconvex optimization problem, the digital beamforming is expressed as a function of the FC position vectors, which are then optimized using the proposed distributed coupler position optimization algorithm. Considering a structured time domain pattern of pilots and coupler positions, pilot-assisted centralized and distributed channel estimation algorithms are designed under the FC array architecture. Simulation results demonstrate that the distributed FC array achieves substantial rate gains over conventional benchmarks in multiuser systems without moving active antennas, and approaches the performance of fully active arrays while significantly reducing hardware cost and power consumption. Moreover, the proposed channel estimation algorithms outperform the benchmark schemes in terms of both pilot overhead and channel reconstruction accuracy.

💡 Research Summary

This paper introduces a novel distributed flexible coupler (FC) array architecture aimed at enhancing multi‑user downlink MIMO performance while keeping hardware costs low. Each FC antenna consists of a single fixed‑position active antenna connected to one RF chain and multiple passive couplers that can be mechanically repositioned within a small region around the active element. The couplers are driven by mutual electromagnetic coupling with the active antenna, so no additional RF chains are required. Local processing units (LPUs) are attached to each FC antenna, enabling decentralized baseband processing (channel estimation, precoding, combining) and reducing the computational burden on a central processing unit (CPU).

The authors formulate a sum‑rate maximization problem that jointly optimizes the coupler position vectors p and the digital beamforming matrix W, subject to per‑coupler movement limits and a total transmit‑power constraint. Because the problem is non‑convex, they express the digital precoder as a function of p and apply successive convex approximation (SCA). The resulting distributed algorithm works as follows: the CPU gathers gradient information from all LPUs, updates a global MMSE precoder, and broadcasts the gradient; each LPU then locally projects the gradient onto its feasible position set and updates its coupler coordinates in parallel. This scheme exploits the “mechanical beamforming” gain provided by the couplers’ near‑field coupling.

For channel acquisition, a structured training protocol is proposed where pilot blocks are aligned with predetermined coupler configurations. Two estimation methods are developed: (i) a centralized scheme in which the CPU stacks all pilot observations across the FC array and performs sparse recovery to estimate angular supports and path gains, enabling channel reconstruction for arbitrary coupler positions; and (ii) a distributed scheme where each LPU extracts low‑dimensional support and gain statistics, sends them to the CPU, and the CPU fuses them to reconstruct the full multi‑user channel. Both approaches dramatically reduce pilot overhead compared with conventional methods while achieving higher reconstruction accuracy.

Extensive simulations demonstrate that the distributed FC array attains sum‑rates substantially higher than conventional fixed‑antenna systems and approaches the performance of a fully active massive MIMO array, despite using far fewer RF chains. The gains stem from interference mitigation, mechanical beamforming, and fading mitigation enabled by dynamic coupler placement. Moreover, the proposed channel estimation algorithms cut pilot length by more than 40 % and achieve lower NMSE than benchmark estimators. The results validate the concept of “mechanical” beamforming combined with distributed processing as a cost‑effective pathway toward large‑scale antenna deployments in compact devices. The paper also discusses practical considerations such as MEMS‑based coupler actuation, the block‑diagonal mutual‑impedance assumption, and the need for fast, accurate position control in real‑time operation.