Effects of large frequency offset in digital receivers and related algorithms

Abstract

This paper investigates the effect of large frequency offset induced (FOI) intersymbol interference (ISI) on the performance of a digital receiver and proposes corresponding remedies. General expressions are derived for the exact representation of ISI caused by an arbitrary frequency offset at the output of the receiver matched filter. Specific examples are developed for the case of a square root raised cosine (SQRT-RC) pulse shape. It is shown that when FOI ISI compensation is performed digitally after the matched filtering operation, an equalizer is necessary for efficient data demodulation, even in the case of a non - dispersive channel. Maximum likelihood (ML) frequency offset estimation is revisited to accommodate the case of large FOI ISI. Its performance is assessed via simulation examples and compared to the derived Cramer-Rao bound. A combined equalization-frequency tracking scheme is also explored, in order to cope with residual frequency estimation errors and potential phase noise.