Multirate digital filters for symbol timing synchronization in software defined radios

Abstract

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 M-stage polyphase filterbank with input data sampled at approximately N samples/symbol can be used in a loop that operates at MN samples/symbol, N 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 N times the data clock. Examples are presented to illustrate loop performance and control.