资 源 简 介
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 (telephonelines, cable television) or free space (air) as in wireless communications(commercial radio and television, cell phones, pagers). The focus of this bookis on wireless communication systems. The transmitter broadcasts the sourceinformation through free space using radiated power from an antenna. Thispower is picked up at the receiving end by an antenna and is processed by thereceiver block.Since the information signals occupy similar frequency bands (the baseband), one cannot transmit these signals over a single communication channelwithout resorting to multiplexing the signals either in the time domainor frequency domain. Amplitude modulation (AM) and (frequency modulationFM) are two analog techniques for multiplexing signals in the frequencydomain and are used extensively in commercial radio broadcast systems, AMand FM radios, respectively. The base band signals are translated to differentlocations (channels) at higher frequencies in the frequency spectrum usingthese modulation techniques. A secondary signal called the carrier is used forthe frequency translation. Another benefit of the modulation schemes is thatthe low frequency base band signals are up converted to a higher frequencywith the aid of the carrier and this makes their transmission through freespace easier.
