The purpose of any communication system is the transmission of information
(speech, video, data) from a source to a receiver through a medium or channel.
The channel for transmission can be wires as in wired communications (telephone
lines, cable television) or free space (air) as in wireless communications
(commercial radio and television, cell phones, pagers). The focus of this book
is on wireless communication systems. The transmitter broadcasts the source
information through free space using radiated power from an antenna. This
power is picked up at the receiving end by an antenna and is processed by the
receiver block.
Since the information signals occupy similar frequency bands (the base
band), one cannot transmit these signals over a single communication channel
without resorting to multiplexing the signals either in the time domain
or frequency domain. Amplitude modulation (AM) and (frequency modulation
FM) are two analog techniques for multiplexing signals in the frequency
domain and are used extensively in commercial radio broadcast systems, AM
and FM radios, respectively. The base band signals are translated to different
locations (channels) at higher frequencies in the frequency spectrum using
these modulation techniques. A secondary signal called the carrier is used for
the frequency translation. Another benefit of the modulation schemes is that
the low frequency base band signals are up converted to a higher frequency
with the aid of the carrier and this makes their transmission through free
space easier.