Uplink Multi-User MIMO Implementation in OpenAirInterface

TL;DR

This paper demonstrates a real-time 2x2 MU-MIMO testbed using OpenAirInterface, showing feasibility of uplink signal separation and decoding with commercial SDRs and modified software, advancing Cell-Free MIMO research.

Contribution

It presents the first real-time OpenAirInterface-based MU-MIMO testbed with practical channel estimation and signal decoding methods for Cell-Free MIMO systems.

Findings

Feasibility of using SRS channel estimates for uplink decoding

Successful separation of signals from two users in non-orthogonal resources

Verification of a step towards full Cell-Free MU-MIMO systems

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 non-orthogonal 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 perform downlink beamforming over multiple cells