Modern communication standards such as WiMAX and UMTS-LTE rely on multiple signaling bandwidths and modulation schemes to provide the user with a variety of services. At times, these services require high data rate communication utilizing complex modulation schemes for video and file transfer applications, while at other times the user simply desires to establish a simple phone callusing a voice application. This flexibility implies that the radio has to be capable of processing signals at various data rates. Multirate digital signal processing techniques provide the tools that enable the SDR to process the data signals with varying bandwidths. Interpolation, for example, can be used to increase the data rate of a given signal. Decimation is concerned with lowering the data rate of a given signal to obtain a new signal with a smaller data rate. Filtering and fractional delay processing are also powerful tools that are used by the SDR to alter the data rate of a signal.

This chapter is divided into three main sections. In Section 10.1, the basics of interpolation and decimation in the context of integer or rational fractions are presented. The underlying theory of multistage decimation and interpolation is discussed in detail. Section 10.2 delves into the details of polyphase structures, half-band filters, and cascaded-integrator comb filters. The pros and cons of each approach are discussed. Finally, Section 10.3 presents certain popular approaches to rational and irrational sampling rate conversions, namely using Lagrange interpolation and the Farrow filter.