Intersymbol interference effects on BPSK and QPSK carrier tracking loops

The tracking performance of three conventional carrier tracking loops in the presence of intersymbol interference (ISI) is analyzed. The closed-form expressions for the squaring losses of BPSK (binary phase-shift-keyed) low-SNR (signal-to-noise ratio), BPSK high-SNR, and QPSK (quaternary phase-shift-keyed) low-SNR loops are derived. ISI is shown to affect the BPSK high-SNR loop as much as the I-Q loop despite the nonlinear effect of the hard-limiter. For the QPSK carrier tracking loop, the degradation is shown to be greater due to the modulation of both components of the carrier. In this case, ISI, in addition to modifying the coefficients in the squaring loss, adds a new term not present in wideband channels. In all cases, a unity squaring loss could not be achieved because of the irreducible error due to pattern noise. This noise is only present in band-limited channels and is dependent on the data model used