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Proof of Concept: Local TX Real-Time Phase Calibration in MIMO Systems

Carl Collmann, Ahmad Nimr, Gerhard Fettweis

TL;DR

This work tackles the challenge of sustain­ing real-time phase coherence across transmit RF chains in a fully digital MIMO array. It proposes a local calibration scheme using a dedicated reference TX chain and a TDMA-based synchronization to estimate per-chain phase and pre-compensate data, comparing instantaneous and smoothed averaging strategies. Hardware measurements on six USRP X310 chains demonstrate substantial reductions in cycle-to-cycle jitter, with residual phase errors becoming Gaussian after calibration, and smoothed calibration consistently delivering the best performance, including jitter as low as $\sim$124 fs for PLL-based chains. Simulations corroborate that the method approaches fundamental PLL jitter limits and significantly improves beamforming gains, validating a practical, low-complexity solution for coherent transmission on SDR testbeds and enabling more reliable MIMO experiments in real-world conditions.

Abstract

Channel measurements in MIMO systems hinge on precise synchronization. While methods for time and frequency synchronization are well established, maintaining real-time phase coherence remains an open requirement for many MIMO systems. Phase coherence in MIMO systems is crucial for beamforming in digital arrays and enables precise parameter estimates such as Angle-of-Arrival/Departure. This work presents and validates a simple local real-time phase calibration method for a digital array. We compare two different approaches, instantaneous and smoothed calibration, to determine the optimal interval between synchronization procedures. To quantitatively assess calibration performance, we use two metrics: the average beamforming power loss and the RMS cycle-to-cycle jitter. Our results indicate that both approaches for phase calibration are effective and yield RMS of jitter in the 2.1 ps to 124 fs range for different SDR models. This level of precision enables coherent transmission on commonly available SDR platforms, allowing investigation on advanced MIMO techniques and transmit beamforming in practical testbeds.

Proof of Concept: Local TX Real-Time Phase Calibration in MIMO Systems

TL;DR

This work tackles the challenge of sustain­ing real-time phase coherence across transmit RF chains in a fully digital MIMO array. It proposes a local calibration scheme using a dedicated reference TX chain and a TDMA-based synchronization to estimate per-chain phase and pre-compensate data, comparing instantaneous and smoothed averaging strategies. Hardware measurements on six USRP X310 chains demonstrate substantial reductions in cycle-to-cycle jitter, with residual phase errors becoming Gaussian after calibration, and smoothed calibration consistently delivering the best performance, including jitter as low as 124 fs for PLL-based chains. Simulations corroborate that the method approaches fundamental PLL jitter limits and significantly improves beamforming gains, validating a practical, low-complexity solution for coherent transmission on SDR testbeds and enabling more reliable MIMO experiments in real-world conditions.

Abstract

Channel measurements in MIMO systems hinge on precise synchronization. While methods for time and frequency synchronization are well established, maintaining real-time phase coherence remains an open requirement for many MIMO systems. Phase coherence in MIMO systems is crucial for beamforming in digital arrays and enables precise parameter estimates such as Angle-of-Arrival/Departure. This work presents and validates a simple local real-time phase calibration method for a digital array. We compare two different approaches, instantaneous and smoothed calibration, to determine the optimal interval between synchronization procedures. To quantitatively assess calibration performance, we use two metrics: the average beamforming power loss and the RMS cycle-to-cycle jitter. Our results indicate that both approaches for phase calibration are effective and yield RMS of jitter in the 2.1 ps to 124 fs range for different SDR models. This level of precision enables coherent transmission on commonly available SDR platforms, allowing investigation on advanced MIMO techniques and transmit beamforming in practical testbeds.
Paper Structure (16 sections, 14 equations, 12 figures, 3 tables)

This paper contains 16 sections, 14 equations, 12 figures, 3 tables.

Table of Contents

  1. Introduction
  2. System Model
  3. Local Calibration of TX Array
  4. Phase Noise Model
  5. Measurement Results
  6. Measurement Setup
  7. Jitter distribution
  8. RF Chains with Negligible Drift
  9. RF Chains with Significant Drift
  10. Quantile-Quantile Plot
  11. Simulation Results
  12. Effect of Calibration in Time Domain
  13. Free-Running VCO
  14. Phase-Locked-Loop
  15. Effect of Calibration on Beamforming
  16. ...and 1 more sections

Figures (12)

  • Figure 1: Setup for phase calibration of TX array, comparison to reference.
  • Figure 2: Phase calibration scheme, periodic transmission of synchronization preamble, observed signal at transmitter reference.
  • Figure 3: System setup for transceiver with $4$ TX and real-time calibrationColl202501_2.
  • Figure 4: Estimated phases for $6$ transmit chains vnn4-cz93-26.
  • Figure 5: PDF for TX1 before/after calibration
  • ...and 7 more figures