Full-Duplex Time-Domain Contention for WiFi Networks: Opportunities and Challenges

For over two decades, commercial WiFi networks have evolved from WiFi 1 to 7. Yet they are built on half-duplex (HD) radio, and retain simple and robust contention mechanisms to coordinate channel access among nodes. Unfortunately, the contention mechanisms are time-consuming since their designs are based on HD. As the physical-layer data rate increases, the dominant contention, and resulting collision overheads, increasingly degrade data throughput efficiency. Wireless full-duplex (FD) radio can radically change the design foundation of WiFi networks, and has great potential to improve their throughput efficiency. Existing FD-based WiFi protocols have been proposed to improve the throughput efficiency, but most of them inherit the time-consuming HD-based contention mechanisms and hence achieve limited throughput-efficiency improvement. This article presents the first FD-based time-domain contention mechanism that makes the contention, and resulting collision overheads, almost negligible. This mainly benefits from a fine-grained slotwise arbitration policy, and a crucial insight that an FD-based contending node can simultaneously transmit a contention signal and sense the channel states as in today's WiFi networks. We then demonstrate the effectiveness of the proposed contention mechanism. Finally, we discuss open research issues in addressing well-known problems such as security and hidden nodes, and integration with core WiFi features such as channel bonding and multi-link operation. The proposed contention mechanism breaks the limit of the conventional ones and is expected to make future WiFi networks more efficient and powerful. It is also helpful for designing better full-duplex cellular, relay, and cognitive radio systems.