Automatic frequency control under low signal-to-noise conditions

Abstract

Carrier frequency offset is determined by sample-to-sample phase shifts of a digital radio signal. The CFO range is divided into a number of intervals and creates as many parallel derived streams as there are interval endpoints by pre-compensating ('back rotating') the input by the sample-to-sample phase shift corresponding to the particular endpoint. Magnitude and phase values are computed of the correlation of a preamble pattern period with the preamble segment of each derived stream in parallel. The largest resulting magnitude value(s) are used to zoom in on the actual CFO present in the input stream. Improved accuracy in the presence of noise may be obtained by repeating the search for a shorter interval centered on the prior CFO value. Final CFO and phase values from corresponding correlation computations then determine the actual CFO and corresponding sample-to-sample phase shift to be applied for pre-compensation ('back rotation') in an open-loop AFC.