A Generalized "Polarity-Type" Costas Loop for Tracking MPSK Signals

A generalized tracking loop is proposed for tracking MPSK signals at high SNR. It is similar in form to the polarity-type Costas loop used for tracking QPSK signals, in which limiters in the arms provide the data estimates needed for data wipeoff in forming the loop error signal. For the MPSK signal, the hard limiters are replaced by "multivalued limiters" with positive and negative values. The outputs of the multivalued limiters in each arm are the and projections of the data. The curve is computed for this MPSK loop, and plotted for different SNR\´s. A comparison is made to the curve for the th power loop (or -phase Costas loop), which is optimum for low SNR. From this comparison, a rough value of the "boundary" SNR can be obtained; this value may be used by the designer to determine which loop structure should be used. The advantage of the high SNR structure proposed here is that it is much simpler to implement than the corresponding low SNR structures.