A Closed-Loop $2 \times 4$ Downlink MIMO Framework for 5G New Radio Using OpenAirInterface

We present the first-of-a-kind closed-loop <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \times 4$</tex> MIMO implementation for the downlink of 5G Open RAN using OpenAirInterface (OAI), which is capable of transmitting up to two transmission layers. Our implementation is a fully functional 5G New Radio (5G NR) system, including the 5G Core Network (5G CN), 5G Radio Access Network (5G RAN), as well as 5G NR User Equipment (UEs). This serves as a foundational framework for further advancements in the context of emerging Open RAN (O-RAN) development. A key feature of our implementation is the enhanced Channel State Information (CSI) reporting procedure at the UE, which includes Rank Indicator (RI), Precoding Matrix Indicator (PMI), and Channel Quality Indicator (CQI). It is adjusted for the extended configuration to maximize data rates. To demonstrate the performance of our implementation, we measure the downlink data rates using iperf3 in two scenarios: (i) fixed channels to assess two-layer data transmission and (ii) Rice1 channels for general transmission analysis. The obtained simulation results demonstrate that, compared to the existing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2 \times 2$</tex> MIMO configuration in the OAI, our implementation improves the data rates in almost all scenarios, especially at the high Signal-to-Noise-Ratios (SNRs).