Page 70 - Complete Wireless Design

Modulation

Modulation 69

Figure 2.28 A constellation
diagram of a QPSK signal.

Figure 2.29 A constellation diagram for QAM-16.

can be subjected to damaging interference under comparatively low noise con-
ditions, since the signal’s SNR must be increased for each increase in the num-
ber of states in order to maintain the BER. For this reason satellite
communication, which is subject to a high-noise environment, will normally
use the simpler modulation schemes, such as BPSK or QPSK. However, low-
noise terrestrial radio links can use 32, 64, 128, and 256 QAM to send data at
much higher rates than possible with the older digital systems within the con-
fined bandwidth restraints of most wireless information channels.
Because of the noiselike nature of all digitally modulated signals, viewing
such signals in the frequency domain will usually not tell us much about the
complexity of the modulation—such as whether it is QPSK, QAM-16, or QAM-
256—but only the signal’s amplitude, frequency, flatness, spectral regrowth,
etc. In fact, a digital signal, when transmitting, will normally appear the same

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.