Distributed MIMO With Over-the-Air Phase Calibration Integrated Into the TDD Flow

2025-09-03

AI Summary

In distributed MIMO systems, independent local oscillators across multiple access points (APs) induce inter-AP phase mismatches, degrading coherent transmission. To address this, we propose an over-the-air phase calibration scheme embedded within the TDD frame structure: bidirectional measurement intervals are dynamically inserted at the uplink–downlink switching point, enabling seamless, periodic phase alignment without interrupting data transmission. We introduce a scalable calibration architecture for large-scale networks and derive a theoretical trade-off model between calibration overhead and spectral efficiency. Furthermore, by jointly applying conjugate and zero-forcing beamforming, we achieve coordinated coherent downlink beamforming. Experimental results in a real-world TDD system demonstrate high-precision inter-AP phase synchronization, yielding substantial gains in spectral efficiency and multi-AP coordination performance.

Technology Category

• Search and Optimization: Distributed Search
• Machine Learning: Calibration & Uncertainty Quantification
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Data management and stream processing for Web, mobile and wireless applications
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation
• Security and Privacy: Large-scale security measurements

Abstract

Reciprocity-based, joint coherent downlink beamforming from multiple access points (APs) in distributed multiple-input multiple-output (MIMO) with independent local oscillators (LOs) requires the APs to be periodically phase-calibrated (a.k.a. phase-synchronized or phase-aligned). Such phase alignment can be accomplished by bidirectional over-the-air measurements between the APs. In this paper, we show how such over-the-air measurements can be integrated into the time-division duplexing (TDD) flow by appropriately shifting the uplink/downlink switching points of the TDD slot structure, creating short time segments during which APs can measure on one another. We also show how this technique scales to large networks. Furthermore, we analytically characterize the tradeoff between the amount of resources spent on calibration measurements and the resulting spectral efficiency of the system, when conjugate beamforming or zero-forcing beamforming is used. The results demonstrate the feasibility of distributed MIMO with phase-calibration through over-the-air inter-AP measurements integrated into the TDD flow.

Problem

• Phase calibration for distributed MIMO with independent local oscillators
• Integrating over-the-air measurements into TDD slot structure
• Tradeoff between calibration resources and spectral efficiency

Innovation

• Over-the-air phase calibration integrated into TDD
• Shifting TDD switching points for AP measurements
• Analytical tradeoff between calibration resources and efficiency