Multirate digital filters for symbol timing synchronization in software defined radios

This paper describes the use of a polyphase filterbank to perform the interpolations required for symbol timing synchronization in a sampled-data receiver. The polyphase filterbank possesses advantages over architectures based on separate matched and interpolation filters. Interpolations are realized by filterbank index selection and a separate interpolating filter following the matched filter is not required. Maximum likelihood timing synchronization techniques can be easily incorporated into the polyphase filter bank in a natural way. An -stage polyphase filterbank with input data sampled at approximately samples/symbol can be used in a loop that operates at samples/symbol, samples/symbol, or 1 sample/symbol. When operating at 1 sample/symbol, auxiliary control must also be included to adjust the clocking of data into the filter bank to account for small differences in the sample clock and times the data clock. Examples are presented to illustrate loop performance and control.