Page 170 - Fundamentals of Communications Systems

5.10 Chapter Five

mt() A rt() ∑ Yt () H () ˆ m t ()
f
b
R

Noise
Figure 5.9 The baseline standard for comparison in analog modulation.

This transmission efficiency is a measure of how effectively the modulation and
demodulation algorithms process the message signal and the corrupting noise
in comparison to the case where no modulation is used.
The spectral efficiency of a communication system is typically a measure of
how well the system is using the bandwidth resource. Bandwidth costs money
to acquire. Examples are licenses to broadcast radio signals or the installation
of copper wires to connect two points. Hence spectral efficiency is very impor-
tant for people who try to make money selling communication resources. For
instance, if one communication system has a spectral efficiency that is twice
the spectral efficiency of a second system then the first system can support
twice the users on the same bandwidth. Twice the users implies twice the rev-
enue. The measure of spectral efficiency that we will use in this class is called
bandwidth efficiency and is defined as
W
E B =
B T
where W is the message bandwidth and B T is the transmission bandwidth.
Bandwidth efficiency is a measure of how effectively the modulation uses band-
width in comparison to the case where no modulation is used in sending the
message (as in Figure 5.9).

EXAMPLE 5.6
The most common example of voice transmission in the United States is AM broadcast-
ing. AM broadcasting in the United States usually refers to transmissions confined to
a band from 535 kHz to 1,700 kHz. The channels are set up to have center frequencies
spaced at 10 kHz spacings and the US Federal Communications Comission (FCC) allows
each station to use about 8 kHz of bandwidth (B T = 8 kHz). Since each voice band signal
has a bandwidth of around W = 4 kHz, AM broadcast in the United States achieves a
bandwidth efficiency of
4
E B = = 50% (5.12)
8

EXAMPLE 5.7
The most common example of high fidelity audio transmission in the United States is
FM broadcasting. FM broadcasting in the United States usually refers to transmissions
confined to a band from 88 MHz to 108 MHz. The channels are set up to have center