6.8 Channel models

Both wireline and wireless channels share characteristics, allowing us to use a common model for how the channel affectstransmitted 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 thechannel delays the transmission.
• The channel may introduce additive interference and/or noise.

As expected, the signal that emerges from the channel is corrupted, but does contain the transmittedsignal. Communication system design begins with detailing the channel model, then developing the transmitter and receiver thatbest compensate for the channel's corrupting behavior. We characterize the channel's quality by the signal-to-interferenceratio ( 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 $x(t)$ 's spectrum spans the frequency interval $\left[f_l , f_u\right]$ , these ratios can be expressed in terms of power spectra.