Downlink Channel Access Performance of NR-U: Impact of Scheduling Flexibility on Coexistence with Wi-Fi in the 5 GHz Band

Coexistence between cellular systems and Wi-Fi gained the attention of the research community when LTE License Assisted Access (LAA) entered the unlicensed band. The recent introduction of NR-U as part of 5G introduces new coexistence opportunities because it implements scalable subcarrier spacing, flexible OFDM symbol lengths, and non-slot based scheduling, which considerably impact channel access. This paper analyses the impact of NR-U settings on its coexistence with Wi-Fi networks and compares it with LAA operation using modeling and experiments. First, we propose a downlink channel access model, which addresses the problem of the dependency and non-uniformity of transmission attempts of different nodes, as a result of the synchronization mechanism introduced by NR-U. Second, we validate the accuracy of the proposed model using FPGA-based LAA, NR-U, and Wi-Fi prototypes. Additionally, we show that replacing LAA with NR-U would not only allow to overcome the problem of bandwidth wastage caused by reservation signals but also, in some cases, to preserve fairness in channel access for both scheduled and random-access systems. Finally, we conclude that the introduction of novel mechanisms is necessary to provide fully fair coexistence of such systems in the unlicensed bands.