A scheduler for a wireless system

Allwright, David and Armour, Tristram (2007) A scheduler for a wireless system. European Study Group with Industry > 56th ESGI [Bath 3/4/2006 - 7/4/2006].

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Abstract/Summary

This problem deals with the scheduling of data packets for different users who share a common wireless channel of limited capacity and who have different service time requirements. From a system point of view it is important to maintain a high throughput, that is, a high number of packets served per unit of time. However from a user point of view it is important to minimise the service time. This poses a trade-off problem, which is one of the goals we wish to analyse.

The wireless transmission channel alternates between good and bad states over time. While the channel is in a bad state the transmission of a packet fails and the packet needs to be retransmitted. When the state of the channel is good, packets are transmitted successfully and do not require retransmission. The system transmission efficiency or throughput might be defined as the number of data packets that can be transmitted successfully in a given time.

The scheduler we wish to study is designed to consider both the wireless channel conditions and the users' quality of service requirements. For this purpose users are assigned a credit every scheduling frame, which is a function of the wireless channel conditions and the quality of service required, which depends on the traffic class.

Problem Statement

Several users, with different traffic types, share a wireless channel, and a scheduler is used to assign the order in which they are served. The overall capacity of the channel is limited by the total transmission power that is available. One possibility is for the scheduler to order the traffic by taking into account the throughput for each traffic type, together with a set of associated ‘credit functions’, which depend also on the corresponding channel quality. The Study Group was asked to consider the performance of this scheduler and how the queuing time and packet loss depend on the capacity of the system and the parameters that occur in defining throughput, credit and channel quality. An explicit version of the model was constructed and analysed, from which it can be seen how to choose the parameters so that the scheduler has the desired behaviour.

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