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AM, FM, and Digital Modulated Systems Chap. 5
5
0
–5
PSD of complex envelope, g (f), in dB
–10
–15
–20
–25
–30
–35
– 40
0
Figure 5–38
5 10 15 20 25 30
Normalized frequency, fT
PSD for the complex envelope of OFDM with N = 32. (See Example5_15.m.)
In a generalization of OFDM called discrete multitone (DMT) signaling, the data rate
on each carrier can be varied, depending on the received SNR on each. That is, the data rate is
reduced (or the carrier is turned off) on carrier frequencies that have low SNR. The data rates
can be adapted as the fading conditions vary, resulting in almost error-free transmission over
multipath fading channels.
OFDM is used in the European digital broadcast systems (radio and television), some
ASDL telephone line modems, in Wi-Fi and Wi-Max wireless computer networks, and in 4G
cellular networks. (See Chapter 8 and OFDM in Wikipedia.)
5–13 SPREAD SPECTRUM SYSTEMS
In our study of communication systems, we have been concerned primarily with the performance
of communication systems in terms of bandwidth efficiency and energy efficiency (i.e.,
detected SNR or probability of bit error) with respect to natural noise. However, in some applications,
we also need to consider multiple-access capability, antijam capability, interference