Hardware Implementation of Cell-Free MIMO and Dynamic TDD using the OAI 5G NR Codebase

The cell-edge user in a traditional cellular network experiences degraded quality of service compared to a user at the centre of the cell due to path loss and interference from neighboring base stations (BSs). Cell-free multiple input and multiple output (MIMO) is a promising technology that can improve the cell-edge performance as it offers macro-diversity and consequently improved coverage and throughput. Moreover, the network capacity in a cellular network is often underutilized due to a static time division duplex (TDD) configuration coupled with asymmetric traffic demands in neighboring cells. To handle heterogeneous traffic demands, dynamic TDD is considered to be a beneficial technology, wherein different cells can independently schedule slots in the uplink (UL)/downlink (DL) directions depending on their local traffic demands. However, dynamic TDD brings new challenges because of the introduction of cross-link interferences (CLI) between users and between BSs. Combining cell-free MIMO with dynamic TDD can effectively overcome the CLI, as there exists inbuilt co-operation between the access points (APs) in the cell-free MIMO architecture. This paper presents a hardware implementation of cell-free MIMO and dynamic TDD using the OPENAIRINTERFACE (OAI) 5G new radio (NR) software stack, with the aim to experimentally understand the role of cell-free MIMO and dynamic TDD in providing better connectivity at the cell-edge and improving network throughput.