3GPP Fairness Constrained Throughput Optimization for 5G NR-U and WiFi Coexistence in the Unlicensed Spectrum

5G New Radio Unlicensed (5G NR-U) and WiFi are considered to be the two most representative radio access technologies in the newly released 6 GHz unlicensed bands, and thus their efficient and fair coexistence becomes crucial. In this paper, we study the coexistence performance of 5G NR-U and WiFi by accounting the new physical layer (PHY) enhancements in 5G NR including flexible numerologies and mini-slot scheduling. Consider the 3GPP notion of fairness as the requirement, we further study how to maximize the total network effective throughput of the WiFi and NR-U coexisting network. Explicit expressions of the maximum total network effective throughput and the corresponding optimal initial backoff window sizes of WiFi and NR-U nodes are derived, and verified by simulation results. The analysis shows that if the transmission opportunity (TXOP) value of NR-U nodes exceeds a certain threshold, then a win-win coexistence can be achieved, where both the WiFi and 5G NR-U network can perform no worse than the case when two WiFi networks coexist. In this case, the maximum total network effective throughput steadily grows as the time slot length of NR-U nodes decreases, indicating the PHY enhancement in 5G NR can benefit the coexistence performance of 5G NR-U and WiFi in the unlicensed spectrum.