We use analog communication techniques for analog message
signals, like music, speech, and television. Transmission and
reception of analog signals using analog results in an
inherently noisy received signal (assuming the channel adds
noise, which it almost certainly does).
The simplest form of analog communication is baseband
communication.
We use analog communication
techniques for analog message signals, like music, speech, and
television. Transmission and reception of analog signals using
analog results in an inherently noisy received signal
(assuming the channel adds noise, which it almost certainly
does).
Here, the transmitted signal equals the message times a
transmitter gain.
An example, which is somewhat out of date, is the wireline
telephone system. You don't use baseband communication in
wireless systems simply because low-frequency signals do not
radiate well. The receiver in a baseband system can't do much
more than filter the received signal to remove out-of-band noise
(interference is small in wireline channels). Assuming the
signal occupies a bandwidth of
WW
Hz (the signal's spectrum extends from zero to
WW), the receiver applies a lowpass
filter having the same bandwidth, as shown in
Figure 1.
We use the signal-to-noise ratio of the
receiver's output
m
^
t
m
^
t
to evaluate any analog-message communication system. Assume that
the channel introduces an attenuation
αα and white noise of
spectral height
N02
N0
2
. The filter does not affect the signal component—we
assume its gain is unity—but does filter the noise,
removing frequency components above
WW Hz. In the filter's output, the
received signal power equals
α2G2powerm
α
2
G
2
powerm
and the noise power
N0W
N0
W
,
which gives a signal-to-noise ratio of
SNRbaseband=α2G2powermN0W
SNRbaseband
α
2
G
2
powerm
N0
W
(2)
The signal power
powermpowerm will be
proportional to the bandwidth
WW;
thus, in baseband communication the signal-to-noise ratio varies
only with transmitter gain and channel attenuation and noise
level.
"Electrical Engineering Digital Processing Systems in Braille."