Phase correction and phase cancelling networks in QPSK modems

At bit rates higher than 100 Mbit/sec, the multiply-by-four process in a QPSK modem deteriorates the SNR by more than 15 dB, and the synchronization SNR at the input to a synchronous oscillator (SO), drops below zero. The reduction in the SNR reduces the tracking band of the SO, which in turn increases the phase shift per unit frequency offset, resulting in a need of phase correction network (PCN) or a phase cancelling network. A new design for a PCN is propoed for the 120-Mbit QPSK-TDMA modem which limits the BER degradation to less than 0.2 dB. This PCN is a universal one and can be used with any SO recovery technique.