A commonly used reference is the BER of QPSK in an AWGN channel with no ISI present. The
equation used is
(2.60)
where
(2.61)
and is plotted in Fig. 2.103, for sake of discussion, where we have highlighted two operating points
A and B.
First, in order to satisfy our first goal, all communication system designers strive to have this performance
curve move to the left as much as theoretically and practically possible.
Operating point A shows we can have an average BER1E-2 for a given received Eb/No4.2 dB.
The average BER can be lowered to 1E-3 for operating point B; however, we require a receiver Eb/No6.8
dB. Since there is no ISI present in the system (transmitreceive), increasing the transmission power
will directly have a positive impact on the performance against noise. Recall earlier, we had called
this antipodal signaling performance. Moreover, if we plot the BER versus Eb/No, we will have the
performance observations (assuming coherent detection).
Now let us add the BER performance of DPSK and FSK for coherent detection and noncoherent
detection for further comparisons. Below we present their theoretical equations for sake of discussion,
for binary signaling (see [2] and [23]).
Q(x)
1 22p 3
`
x
e
z2
2 dz
Pe Qa?…2Eb
No b
MODULATION THEORY 111
Coherent Detection of BPSK/QPSK
1.
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