The term mobile channel refers to the transfer function of adio link when one or both of the terminals are moving. The moving terminal is typically in a vehicle such as a car, or a personal communications terminal
such as a cellphone. Normally one end of the radio link is fixed, and this is referred to as the base station. 

In the link, there is usually multipath radiowave propagation, which is changing with time, or more specifically, as a function of position of the moving terminal. The effects of this multipath propagation dominate the behavior and characterization of the mobile channel.

The radio frequency of the link ranges from hundreds of kilohertz, as in broadcast AMradio, to microwave
frequencies, as in cellphone communications. Indeed, even optical frequencies are used, as in an infrared link
used for indoor computer communications. 

The kind of channel most often referred to as “mobile,” however, is that using microwave frequencies, and this article concentrates on the characteristics of a mobile microwave radio link. Much of the channel behavior can be scaled by the carrier frequency and by the speed of the mobile terminal.

Current spectral usage is a result of many different historical developments, so the bands used by mobile
radio channels have evolved to be at many frequencies. For example, current vehicular and personal communications terminals mostly use frequencies around 400 MHz, 900 MHz, and 1.8 GHz. 

In the future, higher frequencies will be used. The frequency has a definitive bearing on the rate at which the channel changes.

Some examples of mobile channels include: domestic cordless telephones; cellular telephones and radiotelephones; pagers; satellite communication terminals, including navigational services such as Global Positioning System (GPS) reception; and radio networks for local data communications. 

Finally, the reception by portable receivers of broadcast radio, at frequencies of a few hundred kilohertz (AM radio) are common forms of the mobile radio channel.

The use of mobile channels has grown very quickly in the last decade. This growth will continue. It is
driven by a combination of consumer demand for mobile voice and data services and advances in electronic
technology. A limiting factor to the growth is that many users must share the radio spectrum, which is a finite

The spectral sharing is not only local, it is also international, and so spectral regulatory issues have
also become formidable. The increasing pressure to use the spectrum more efficiently is also a driving force in regulatory and technical developments.

To a user, amobile or personal communications system is simple: it is a terminal, such as a telephone, that
uses a radio link instead of a wire link. The conspicuous result is that the terminal is compact for portability,
and it has an antenna, although for personal communications the antenna is often no longer visible. 

To the communication engineer, however, the mobile terminal is just a component in a vast, complex circuit. The mobile channel is one link in the circuit, but this link is the most complex, owing to its use of radio waves in complicated propagation environments and of radio signal-processing technology needed to facilitate wireless transmission among multiple users.

In mobile channels, efficient spectral utilization is a function of the basic limitations on controlling radiowave behavior in complicated physical environments, including the launching and gathering of the waves.
Thus antennas and propagation are key topics, and their roles characterize the channel behavior.

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