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From: bando on 17 Jun 2010 17:24
John made a really good point. QPSK and BPSK have the same BER performance
for identical Eb/N0, not for identical Es/N0. Since each symbol carries
two bits in QPSK, and each symbol carries one bit in BPSK, the amount of
energy per symbol (Es) required to achieve the same BER in QPSK as BPSK is
3dB higher. Since changing modulation does not normally change the
requirement on BER, it will therefore require higher transmit power (3dB),
all other things being equal, to double your datarate by switching to QPSK
from BPSK.

But hey, that's really good for a few reasons. You can double your
datarate without increasing your bandwidth, and you don't need a bigger
transmitter than it would take to double your datarate staying with BPSK.

The disadvantages are that phase noise (and some other signal distortions)
impact QPSK more severely than BPSK. Phase noise is a nonlinear effect
found in many real-world systems that acts like a complex noise source
mixed with (multiplied by) your signal.

Since the distance between symbols in phase angle is smaller for QPSK than
for BPSK (regardless of Eb), QPSK will be more sensitive to this type of
distortion.

~Bando

>I understand the confusion, but remember that there's no such thing as
>a free lunch. QPSK requires twice the power to achieve the same
>distance between symbols.
>
>John


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