Analysis of Throughput Distribution in IEEE 802.16 Distributed Scheduling

Abstract

IEEE 802.16 offers mesh mode of operation to cover outage areas outside the range of Base Station (BS). The standard defines three scheduling mecha- nisms for mesh mode: coordinated centralized, coordinated distributed and uncoordinated distributed. In coordinated distributed scheduling, all nodes are treated equally and coordinate their transmissions within their two-hop neighbourhood. Since no central authority is involved, throughput distribu- tion among nodes is unfair. Some nodes may undergo starvation and some may enjoy highest achievable throughput. In this thesis, we analyse the dis- tributed scheduling mechanism of WiMAX mesh mode to study the fairness issue. We model the behaviour of a single node under distributed scheduling mechanism using 2-D Markov Chain where a node can be in one of four states of holdoff, election, wait and transmission. Analytical results of the model show that time spent by a node to fulfil its data demand is considerably small (maximum upto 10%). Our analysis shows that for most of the time a node is either in holdoff (at least 24%) or competing for mesh election(on average 40%). Moreover, some allowable high values of holdoff exponent (i.e. 4 to 7) by standard produce very unrealistic scenarios with incredibly scarce chances of data transmission. To control unfairness and impracticality of distributed scheduler, optimal values of holdoff exponent are found to be between 0 and 3. Handshake probability value greater than 0.1 yields more chances for data transmission. The gap from current time till the arrival of requested data slot does not have any significant impact on time duration spent in different states.