As predicted earlier, the bit error rate
performance for QPSK is theoretically identical to that for BPSK. If the carrier reference
is not perfectly phase coherent, however, not only will the wanted signal output voltage
of each detector fall, but each detector will suffer cross-talk
from the orthogonal symbols and the performance will be degraded even further. QPSK thus
has lower tolerance to phase jitter in the carrier recovery
process than BPSK.
The major attraction of QPSK is that it allows the modem designer that opportunity to
recover the bandwidth efficiency of a binary baseband
data stream in a bandpass modulation format, without
sacrificing BER performance.
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