Experimental Performance of Bidirectional Phase Coherent Transmission and Sensing for mmWave Cell-free Massive MIMO Systems with Reciprocity Calibration

TL;DR

This paper presents a bidirectional calibration scheme for mmWave cell-free massive MIMO systems, enabling coherent joint transmission and high-precision sensing through reciprocity calibration and phase tracking, demonstrated via simulations and experiments.

Contribution

It introduces a novel bidirectional calibration method with reciprocity calibration and phase tracking for mmWave MIMO systems, improving coherence and sensing accuracy.

Findings

Effective reciprocal calibration with low overhead

Enhanced beamforming gain and sensing accuracy

Successful experimental validation in mmWave band

Abstract

Phase synchronization among distributed transmission reception points (TRPs) is a prerequisite for enabling coherent joint transmission and high-precision sensing in millimeter wave (mmWave) cell-free massive multiple-input and multiple-output (MIMO) systems. This paper proposes a bidirectional calibration scheme and a calibration coefficient estimation method for phase synchronization, and presents a calibration coefficient phase tracking method using unilateral uplink/downlink channel state information (CSI). Furthermore, this paper introduces the use of reciprocity calibration to eliminate non-ideal factors in sensing and leverages sensing results to achieve calibration coefficient phase tracking in dynamic scenarios, thus enabling bidirectional empowerment of both communication and sensing. Simulation results demonstrate that the proposed method can effectively implement reciprocal calibration with lower overhead, enabling coherent collaborative transmission, and resolving non-ideal factors to acquire lower sensing error in sensing applications. Experimental results show that, in the mmWave band, over-the-air (OTA) bidirectional calibration enables coherent collaborative transmission for both collaborative TRPs and collaborative user equipments (UEs), achieving beamforming gain and long-time coherent sensing capabilities.