Chapter 6 - Scheduling, Link Adaptation, and Hybrid ARQ

Abstract

This chapter presents an introduction to some of the techniques for handling variations in the instantaneous radio-link quality. Channel-dependent scheduling in a mobile communication system deals with the question of how to share, between different users, the radio resources available in the system to achieve as efficient resource utilization as possible. Closely related to scheduling is link adaptation, which deals with how to set the transmission parameters of a radio link to handle variations of the radio-link quality. Both channel-dependent scheduling and link adaptation try to exploit the channel variations through appropriate processing prior to transmission of the data. However, due to the random nature of the variations in the radio-link quality, perfect adaptation to the instantaneous radio-link quality is never possible. Hybrid ARQ, which requests retransmission of erroneously received data packets, is therefore useful. This can be seen as a mechanism for handling variations in the instantaneous radio-link quality after transmission and nicely complements channel-dependent scheduling and link adaptation. Hybrid ARQ also serves the purpose of handling random errors due to noise in the receiver. This chapter discusses the basic idea of downlink scheduling and uplink scheduling. Concepts of link adaptation and channel-dependent scheduling in the frequency domain are also summarized. The chapter concludes with a discussion on advanced retransmission schemes.