Decision-directed delay-lock loop using fast Fourier-transform crosscorrelation

A decision-directed delay-lock loop is described using the fast Fourier transform (f.f.t.) for cross-correlation, together with decisions based on maximum likelihood. The f.f.t. produces a complete set of lagged products of the received echo and the delayed replica of the transmitted signal. This enables the loop to realise a wide dynamic range. Another useful feature of the loop is that it has the ability to detect unlocking and to switch automatically between the modes of acquisition, tracking, and detection of unlocking and reacquisition.