Digital Transmission on Fading Multipath Channels
We have discussed thus far digital modulation and demodulation methods for the transmission of information over two types of channels, namely, an additive noise channel and a linear filter channel.While these channel models are appropriate for a large variety of physical channels, they become inadequate in characterizing signal transmission over radio channels whose transmission characteristics varywith time. Time-varying behavior of the channel is exhibited by the following:
• Signal transmission via ionospheric propagation in the HF band in which signal fading is a result of multipath signal propagation that makes the signal arrive at the receiver via different propagation paths with different delays.
• Mobile cellular transmission, between a base station and a telephone-equipped automobile, in which the signal transmitted by the base station to the automobile is reflected from surrounding buildings, hills, and other obstructions.
• Line-of-sight microwave radio transmission, in which signals may be reflected from the ground to the receiving antenna due to tall obstructions or hilly terrain in the path of propagation.
• Airplane-to-airplane radio communications, in which secondary signal components may be received from ground reflections.
Such channels may be treated as linear systems with time-varying impulse response by adopting a statistical characterization. Models for time-variant multipath channels will not be considered here.