OPE3 – Operational & Experimental Insights
Thursday, 4 June 2026, 11:00-12:30, room Sala 12 (1st floor)
Session Chair: Aloizio Da Silva (Virginia Tech, USA & Commonwealth Cyber Initiative, US)
Uplink Multi-User MIMO Testbed Implementation in OpenAirInterface
Utku Uçak (Fraunhofer Heinrich Hertz Institute, Germany); Fariba Armandoust (Fraunhofer HHI, Germany); Matthias Mehlhose (Fraunhofer HHI & Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute, Germany); Daniel Schäufele, Jochen Fink and Renato L. G. Cavalcante (Fraunhofer Heinrich Hertz Institute, Germany); Slawomir Stanczak (Technische Universität Berlin & Fraunhofer Heinrich Hertz Institute, Germany)
Cell-Free Multiple-Input Multiple-Output (MIMO) and Open Radio Access Network (O-RAN) have been active research topics in the wireless communication community in recent years. As an open-source software implementation of the 3rd Generation Partnership Project (3GPP) 5th Generation (5G) protocol stack, OpenAirInterface (OAI) has become a valuable tool for deploying and testing new ideas in wireless communication systems. In this paper, we present our OAI based real-time uplink Multi-User MIMO (MU-MIMO) testbed developed at Fraunhofer HHI. As a part of our Cell-Free MIMO testbed development, we built a 2×2 MU-MIMO system using general purpose computers and commercially available software defined radios (SDRs). Using a modified OAI next-Generation Node-B (gNB) and two unmodified OAI user equipment (UE), we show that it is feasible to use Sounding Reference Signal (SRS) channel estimates to compute uplink combiners. Our results verify that this method can be used to separate and decode signals from two users transmitting in nonorthogonal time-frequency resources. This work serves as an important verification step to build a complete Cell-Free MU-MIMO system that leverages time domain duplexing (TDD) reciprocity to do downlink beamforming over multiple cells.
Experimental Evaluation of Beam Steering with Phased MIMO Radar for OFDM-Based ISAC
Georgios Andreadis and Anastasios Kleniatis (WINLAB, USA); Prasanthi Maddala (Rutgers University, USA); Ivan Seskar (WINLAB, Rutgers University, USA); Aggelos Bletsas (Rutgers University & WINLAB, USA); John Sahalos (Aristotle University of Thessaloniki, GR, Thessaloniki, Greece & University of Nicosia, CY, Nicosia, Cyprus)
In upcoming 6G networks, energy efficiency and integrated sensing and communication (ISAC) are key priorities. Deploying massive antenna arrays remains challenging, as increased number of RF chains compromises energy efficiency. This work explores the utilization of current phased-array systems more effectively in MIMO settings. It is demonstrated that in a multi-target environment, strategically steering the array beams towards the target of interest suppresses interference from others and offers better individual angle estimation accuracy. This technique can enhance sensing performance without increasing the number of RF chains. Additionally, a compressed sensing method is proposed that achieves reduced angle estimation errors compared to a conventional DFT-based method, although with a tradeoff in terms of probability of target detection. All comparisons were conducted with real-world experimental measurements from a phased 2 × 2 MIMO radar and OFDM waveforms.
Experimental Demonstration of Multi-Target Tracking in Integrated Sensing and Communication
Maximilian Bauhofer (University of Stuttgart, Germany); Marcus Henninger (Nokia Bell Labs, Germany); Meik Kottkamp (Rohde-Schwarz, Germany); Lucas Giroto (Nokia Bell Labs, Germany); Philip Grill (Rohde & Schwarz, Germany); Alexander Felix (University of Stuttgart, Germany & Nokia Bell Labs, Germany); Thorsten Wild (Nokia Bell Labs, Germany); Stephan ten Brink (University of Stuttgart, Germany); Silvio Mandelli (Nokia Bell Labs, Germany)
For a wide range of envisioned integrated sensing and communication (ISAC) use cases, it is necessary to incorporate tracking techniques into cellular communication systems. While numerous multi-target tracking (MTT) algorithms exist, they have not yet been applied to real-world ISAC, with its challenges such as clutter and non-optimal hardware with design emphasis on communication instead of sensing. In this work, we showcase MTT based on the probability hypothesis density (PHD) filter in the range and radial speed domain. The measurements are taken with a 5G compliant ISAC proof-of-concept in a real factory environment, where the pedestrian-like targets are generated by a radar target emulator. We detail the complete pipeline, from measurement acquisition to evaluation, with a focus on the post-processing of the raw captured data and the tracking itself. Our end-to-end evaluation and comparison to simulations show good MTT performance with mean absolute ranging error <1.5m and detection rates >91% for realistic but challenging scenarios.
Proof of Concept: Local TX Real-Time Phase Calibration in MIMO Systems
Carl R. Collmann (TU Dresden, Germany); Ahmad Nimr and Gerhard P. Fettweis (Technische Universität Dresden, Germany)
Channel measurements in multiple-input multiple-output (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 root mean square (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 software-defined radio (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.
Measurement-Based Validation of Geometry-Driven RIS Beam Steering in Industrial Environments
Adam Umra (Ruhr University Bochum, Germany); Simon Tewes (Ruhr-University Bochum, Germany); Niklas Beckmann and Niels König (Fraunhofer Institute for Production Technology IPT, Germany); Aydin Sezgin (RUB, Germany); Robert Schmitt (RWTH Aachen University & Fraunhofer Institute for Production Technology IPT, Germany)
Reconfigurable intelligent surfaces (RISs) offer programmable control of radio propagation for future wireless systems. For configuration, geometry-driven analytical approaches are appealing for their simplicity and real-time operation, but their performance in challenging environments such as industrial halls with dense multipath and metallic scattering is not well established. To this end, we present a measurement-based evaluation of geometry-driven RIS beam steering in a large industrial hall using a 5 GHz RIS prototype. A novel RIS configuration is proposed in which four patch antennas are mounted in close proximity in front of the RIS to steer the incident field and enable controlled reflection. For this setup, analytically computed, quantized configurations are implemented. Two-dimensional received power maps from two measurement areas reveal consistent, spatially selective focusing. Configurations optimized near the receiver produce clear power maxima, while steering to offset locations triggers a rapid 20-30 dB reduction. With increasing RIS–receiver distance, elevation selectivity broadens due to finite-aperture and geometric constraints, while azimuth steering remains robust. These results confirm the practical viability of geometry-driven RIS beam steering in industrial environments and support its use for spatial field control and localization under non-ideal propagation.
