chapter  8
28 Pages

Wireless Communications

In modern communication systems, coded signals are used for transmitting information, which consists of a bit stream of ones and zeros. The bits are suitably coded into identifi able waveforms; for example “1” may be coded into a sinusoid of frequency f1 and “0” is coded into another sinusoid of frequency f0, as in frequency shift keying (FSK) modulation. The bits thus coded after mixing with a carrier are sequentially transmitted. Since the physical channel is a shielded cable (including optical cable), there is much less cross-channel interference. But, in a radio communication scenario, both transmitter and receiver are in the open space. Naturally, a sensor will receive signals from more than one source. It is therefore of great interest to minimize this co-channel interference, a problem unique to radio communication. To overcome the problem of co-channel interference, modern cellular radio communication has been devised. A user needs to communicate to the nearest base station, which, in turn, is connected to a central exchange. Thus, it is possible to communicate with a distant user without having to radiate a lot of power causing a drain on the battery (in case of a mobile transmitter) and creating interference to other users. Even this system seems to fail when many users in the same cell are trying to reach the base station. To further mitigate the problem of co-channel interference, it is proposed to use a highly directional antenna at the base station. This would enable the base station to separate the users having different bearings and reduce the co-channel interference among them.