Backoff Algorithms for Fading Wireless Channels
Seon Yeong Han and Nael B. Abu-Ghazaleh
We consider the impact of wireless channel fading on the backoff algorithm behavior in the IEEE 802.11 protocol. Specifically, since the backoff algorithm assumes that all packet losses are due to collisions, it unnecessarily backs off when a packet is lost due to fading. Two performance problems arise as a result: (1) low throughput, due to unnecessary loss of transmission time; and (2) unfairness when two competing links have different link qualities. In this paper, we characterize this problem and propose three solutions to it. The solutions aim to provide discrimination between transmission errors and collisions such that the sender can back off appropriately. The first algorithm relies on receiver discrimination and feedback; the receiving radio can in many instances differentiate between collisions and transmission errors. The second algorithm estimates the clear channel quality, and backs off if the observed quality deviates from the clear channel quality (indicating collisions). The third algorithm develops the probability of collision as a function of the number of observed idle slots during contention, and uses this probability to control the backoff algorithm. We show via simulation that the techniques significantly improve both performance and fairness of IEEE 802.11 in the presence of fading.
Keywords: IEEE802.11, backoff algorithm, fading channel.