Asynchronous Random Access Systems With Immediate Collision Resolution for Low Power Wide Area Networks.

Low power wide area networks (LPWAN) have adopted pure ALOHA systems as medium access control (MAC) protocol. Since pure ALOHA works asynchronously, it can reduce the cost of the system and devices and make the devices small by eliminating the time-synchronization hardware. However, its maximum throughput is limited only up to 0.183 (packets/sec). To overcome such a low throughput limit, this work proposes a novel asynchronous random access (RA) system that can yield the maximum throughput as good as slotted ALOHA (S-ALOHA). In the proposed system, the users transmit their packets as in pure ALOHA but trigger their feedback timer upon (re)transmission. When a collision occurs, the feedback timers enable the users who have transmitted the first and last packets in the collision to retransmit one after another right after the collision. Under some conditions, one or two users can retransmit successfully right after the collision, which improves the throughput significantly. This work analyzes the saturation throughput of the proposed system and proposes an optimal online backoff algorithm to maximize the throughput of the unsaturated traffic system.