We have presented this comparison under the AWGN channel assumptions; a similar
comparison under the multipaths fading channel will be given in the latter chapters.
2.3.2 Power versus Bandwidth Requirements
It is not enough to compare modulation schemes only on the merit of BER performance. The communication
system designer must also consider the occupied BW required to transmit and effectively
communicate and thus obtain the BER results presented in the above subsection. Since we have
already presented various transmit PSDs of modulation schemes, in this comparison, we will use BW
to denote data rate rather than the spectral domain. A great way to demonstrate this comparison is
through the Shannon-Hartely capacity theorem [2].
This theorem is summarized as follows: system capacity perturbed by AWGN is a function of
signal power, noise power, and BW and reported as
(bps) (2.66)
(bps/Hz) (2.67)
where we can also add the following definition:
(2.68)
We can plot this relationship in what is commonly called a BW??“power efficiency plane, as
shown in Fig. 2.105 [23]. Here three modulation schemes are plotted: PSK, QAM, and FSK for a
BER 1.0E??“3.
Rb
W log2(M)
C
W log2 c1
Eb
No
# Rb
Wd
C W # log2 c1
S
Nd
MODULATION THEORY 113
Bandwidth-Efficiency Plot (BER = 1.
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