NR MIMO Feature Implementation into OpenAirInterface

In this paper, we introduce the design and implementation steps of a 5G multiple-input multiple-output (MIMO) feature into OpenAirInterface (OAI). We start with the MIMOOFDM system model of the new radio (NR) physical downlink shared channel (PDSCH) implemented in the transmitting procedures of the gNB. Herewith, we adopt a demodulation reference signal (DM-RS) with configuration type 2 for transmitting pilot symbols, which can provide up to 6 channel-layer estimates in the case of single DM-RS symbols. In our current implementation, we limit the number of layers to 4, since our real-time setup uses the USRP N310, which supports up to 4 x 4 MIMO TX/RX channels. Afterwards, we provide the extension of NR receive PDSCH procedures to support MIMO reception of the NR MIMO feature. This extension is implemented and tested in OAI to support up to 4 layers for the 4 x 4 MIMO system. Our real-time system can support up to 2 layers with a 2 x 4 MIMO system, which was limited by the processing capabilities of this current implementation. Finally, we evaluate our design and implementation in OAI physical (PHY) layer simulator, RF-simulator, and in real-time systems. We provide the results of MIMO channel estimation, QAM demodulation, and log likelihood ratio (LLR) computations for the 2 x 4 MIMO system supporting 2 layers.