Both wireline and wireless channels share characteristics, allowing us
to use a common model for how the channel affects transmitted signals.
Both wireline and wireless channels share characteristics,
allowing us to use a common model for how the channel affects
transmitted signals.
The transmitted signal is usually not filtered by the channel.
The signal can be attenuated.
The signal propagates through the channel at a speed
equal to or less than the speed of light, which means that the
channel delays the transmission.
The channel may introduce additive interference and/or noise.
Letting represent the
attenuation introduced by the channel, the receiver's input
signal is related to the transmitted one by
This expression corresponds to the system model for the channel
shown in [link]. In this book, we
shall assume that the noise is white.
The channel component of the
fundamental model of communication has the depicted
form. The attenuation is due to propagation loss. Adding the
interference and noise is justified by the linearity property
of Maxwell's equations.
Is this model for the channel linear?
The additive-noise channel is not linear
because it does not have the zero-input-zero-output property
(even though we might transmit nothing, the receiver's
input consists of noise).
As expected, the signal that emerges from the
channel is corrupted, but does contain the transmitted
signal. Communication system design begins with detailing the
channel model, then developing the transmitter and receiver that
best compensate for the channel's corrupting behavior. We
characterize the channel's quality by the signal-to-interference
ratio (SIR) and the signal-to-noise ratio
(SNR). The ratios are computed according to
the relative power of each within the transmitted
signal's bandwidth. Assuming the signal
's spectrum spans the frequency interval
, these ratios can be expressed in terms of power
spectra.
In most cases, the interference and noise powers do not vary for a
given receiver. Variations in signal-to-interference and
signal-to-noise ratios arise from the attenuation because of
transmitter-to-receiver distance variations.