Single Carrier FDMA: A New Air Interface for Long Term Evolution

5

Channel Dependent Scheduling

5.1 Introduction

Frequency division techniques were originally proposed for broadband wireless transmission systems to avoid the severe inter-symbol interference that would be present in time division or code division transmissions. This chapter presents another advantage of frequency division transmissions – the opportunity to apply channel dependent scheduling in the process of subcarrier mapping and thereby obtain performance improvements due to multi-user diversity. The benefits of channel dependent scheduling are consequences of the frequency selective nature of broadband channel transfer functions. When terminals are dispersed spatially, each one has a different channel transfer function.

Figure 5.1 shows the frequency responses of channels assigned to two terminals, represented as the square of the channel gain at each of 256 subcarriers. The two curves conform to the TU6 propagation model [1]. For the most part the two terminals (labeled User 1 and User 2) have distinctly different channel gains in most regions of the frequency band spanned by the 256 subcarriers. As the terminals move, the frequency responses change. As a consequence, a practical system would have to monitor periodically the frequency response of each terminal and devise a new schedule matched to the current frequency responses of all the terminals sharing the frequency band.

Channel dependent scheduling is a form of subcarrier mapping that can be employed in SC-FDMA and ...