Movable Superdirective Pairs: A Phase Shifter-Free Approach to mmWave Communications

TL;DR

This work addresses mmWave performance limitations by enabling phase-shifter-free beamforming through Movable Superdirective Pairs (MSP), which merge movable antenna elements with closely coupled superdirective pairs. An alternating-optimization framework with gradient projection and an Adam optimizer solves the non-convex joint optimization of each pair's rotation $\theta_i$ and vertical position $y_i$, exploiting mutual coupling to steer endfire superdirective gains toward multipath components. Key contributions include explicit gradient derivations for $\theta_i$ and $y_i$, a computationally efficient AO-GPM algorithm, and demonstrated spectral-efficiency gains of about $2\ \mathrm{bit/s/Hz}$ over fixed-position arrays with MRC, along with runtime advantages over PSO. The approach preserves scanning bandwidth, reduces hardware costs by avoiding phase shifters, and shows performance improvements even under realistic mmWave channel sparsity and mutual-coupling effects, highlighting practical impact for high-rate wireless links.

Abstract

In this letter, we propose a novel Movable Superdirective Pairs (MSP) approach that combines movable antennas with superdirective pair arrays to enhance the performance of millimeter-wave (mmWave) communications on the user side. By controlling the rotation angles and positions of superdirective antenna pairs, the proposed MSP approach maximizes the received signal-to-noise ratio (SNR) of multipath signals without relying on phase shifters or attenuators. This approach addresses the limitations of traditional superdirective antennas, which are typically restricted to the endfire direction and suffer from reduced scanning bandwidth and increased complexity. An efficient algorithm based on alternating optimization and the gradient projection method is developed to solve the non-convex optimization problem of antennas' joint rotating positioning. Simulation results demonstrate that the MSP approach achieves significant performance gains over fixed-position array (FPA) employing Maximum Ratio Combining (MRC), while reducing system complexity and hardware costs.

Figures (4)

• Figure 1: Illustration of the MSP communication system.
• Figure 2: Comparison of initial and optimized patterns and array configurations.
• Figure 3: Spectral efficiency comparison between the proposed MSP structure and the FPA with MRC reception under different numbers of antennas and paths.
• Figure 4: Average SNR versus normalized movement range $\gamma / \lambda$ for different numbers of antenna pairs $M$.