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Uplink Multi-User MIMO Implementation in OpenAirInterface for a Cell-Free O-RAN Testbed

Utku Uçak, Fariba Armandoust, Matthias Mehlhose, Daniel Schäufele, Jochen Fink, Renato L. G. Cavalcante, Sławomir Stańczak

TL;DR

This work addresses enabling real-time uplink MU-MIMO and cell-free operation within an open-source O-RAN stack by implementing SRS-based channel estimation to compute uplink combiners and enabling nonorthogonal PUSCH scheduling. The approach integrates a 2x2 MU-MIMO testbed on OpenAirInterface with a private gNB branch, leveraging RZF detectors and a two-step channel equalization process to mitigate residual CFO, validated on a hardware setup using USRPs and general-purpose PCs. Key contributions include the end-to-end PHY/MAC alterations for MU-MIMO on OAI, SRS-centric scheduling strategies, and demonstration of nearly doubled per-user throughput under MU-MIMO, establishing a concrete step toward a scalable cell-free MU-MIMO/O-RAN testbed. The work has practical impact by providing an open, extensible platform for researchers to explore uplink/downlink MU-MIMO, channel reciprocity calibration, and distributed RU deployments without vendor lock-in.

Abstract

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 2x2 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.

Uplink Multi-User MIMO Implementation in OpenAirInterface for a Cell-Free O-RAN Testbed

TL;DR

This work addresses enabling real-time uplink MU-MIMO and cell-free operation within an open-source O-RAN stack by implementing SRS-based channel estimation to compute uplink combiners and enabling nonorthogonal PUSCH scheduling. The approach integrates a 2x2 MU-MIMO testbed on OpenAirInterface with a private gNB branch, leveraging RZF detectors and a two-step channel equalization process to mitigate residual CFO, validated on a hardware setup using USRPs and general-purpose PCs. Key contributions include the end-to-end PHY/MAC alterations for MU-MIMO on OAI, SRS-centric scheduling strategies, and demonstration of nearly doubled per-user throughput under MU-MIMO, establishing a concrete step toward a scalable cell-free MU-MIMO/O-RAN testbed. The work has practical impact by providing an open, extensible platform for researchers to explore uplink/downlink MU-MIMO, channel reciprocity calibration, and distributed RU deployments without vendor lock-in.

Abstract

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 2x2 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.
Paper Structure (18 sections, 4 equations, 7 figures, 1 table, 2 algorithms)

This paper contains 18 sections, 4 equations, 7 figures, 1 table, 2 algorithms.

Table of Contents

  1. Introduction
  2. System Description
  3. Theoretical Background
  4. MU-MIMO and Linear Detection
  5. Two Step Channel Equalization
  6. Physical Layer Implementation
  7. Open Source RAN Stack: Open Air Interface
  8. SRS
  9. PUSCH Processing
  10. Why SRS Instead of DMRS?
  11. Fixed vs Floating Point Arithmetic
  12. User Equipment and Frequency Synchronization
  13. MAC Implementation
  14. SRS Scheduling
  15. Nonorthogonal PUSCH Scheduling
  16. ...and 3 more sections

Figures (7)

  • Figure 1: Block diagram of main system components.
  • Figure 2: A simplified OFDM grid showing three uplink slots with physical channel allocation when MU-MIMO scheduling is disabled (left) and enabled (right).
  • Figure 3: A picture of the setup in our lab.
  • Figure 4: Channel frequency domain response from SRS pilots.
  • Figure 5: Received (left) and MRC equalized (right) signal constellation of one of the users when MU-MIMO scheduler is switched off.
  • ...and 2 more figures