Opportunistic Link Scheduling for Multihop Wireless Networks

One of the reasons that cause low transmission throughput in IEEE 802.11-based multihop networks is packet losses. Compared to transmitting a new packet, retransmitting a packet in 802.11-based networks has a lower priority in accessing the channel and requires a longer channel idle time for backoffs. When the number of retransmissions exceeds a certain threshold, a packet is dropped at the link layer. For TCP traffic, this will eventually result in TCP timeout, and the lost packet will be retransmitted at the transport layer, causing end-to-end throughput degradation. Furthermore, the mechanism that TCP adjusts its congestion window size negatively affects the transmission throughput in 802.11-based multihop networks. In this paper we propose an opportunistic link scheduling (OLS) protocol, which schedules transmissions of the links based on their channel conditions, including both channel fading and co-channel interference. Links with good channel conditions are given a higher priority to access the channel and allowed to transmit a limited number of packets consecutively without repeatedly competing the channel. OLS also includes mechanisms to avoid buffer overflow and prevent starving links with poor channel conditions. Our results show that OLS can significantly improve the end-to-end transmission throughput, while keeping reasonably low transmission delay. The protocol is easy to implement, and requires minor changes to the 802.11 protocol.