A QPSK demodulator is designed to put out a null signal, called an erasure, when the decision is ambivalent. Specifically, the decision regions are modified as shown in the accompanying diagram.
When the received sample lies in the hatched region, the demodulator output is a special symbol denoting an erasure. When the received sample lies in one of the other regions, the appropriate two bits of demodulated data are the output of the demodulator. Use the union bound to set up approximate expressions for the probability of erasure and the probability of symbol error.