0.1 A telecommunication system

The next two chapters provide more depth and detail by outlining a complete telecommunicationsystem. When the transmitted signal is passed through the air using electromagnetic waves, it must takethe form of a continuous (analog) waveform. A good way to understand such analog signals is via the Fourier transform, and this is reviewedbriefly in  [link] . The five basic elements of the receiver will be familiar to many readers, and they are presented in [link] in a form that will be directly useful when creating M atlab implementations of the various parts of the communications system. By the end of the second layer, the basic system architecture isfixed; the ordering of the blocks in the system diagram has stabilized.

The reason digital radio is so reliable is because it employs a smart receiver. Inside each digital radio receiver, there is a tiny computer: acomputer capable of sorting through the myriad of reflected and atmospherically distorted transmissions and reconstructing a solid,usable signal for the set to process.

—from http://radioworks.cbc.ca/radio/digital-radio/drri.html (2/2/03)

Telecommunications technologies using electromagnetic transmission surround us: televisionimages flicker, radios chatter, cell phones (and telephones) ring, allowing us to see and heareach other anywhere on the planet. E-mail and the Internet link us via our computers, and a large varietyof common devices such as CDs, DVDs, and hard disks augment the traditional pencil and paper storage andtransmittal of information. People have always wished tocommunicate over long distances: to speak with someone in another country, to watch a distant sporting event, to listen to musicperformed in another place or another time, to send and receive data remotely using a personal computer.In order to implement these desires, a signal (a sound wave, a signal from a TV camera, or a sequence of computer bits)needs to be encoded, stored, transmitted, received, and decoded. Why? Consider the problem of voice or music transmission.Sending sound directly is futile because sound waves dissipate very quickly in air. But if the sound is first transformedinto electromagnetic waves, then they can be beamed over great distances very efficiently. Similarly, the TV signaland computer data can be transformed into electromagnetic waves.

Electromagnetic transmission of analog waveforms

There are some experimental (physical) facts that cause transmission systems to be constructed as they are. First,for efficient wireless broadcasting of electromagnetic energy, an antenna needs to be longer than about1/10 of a wavelength of the frequency being transmitted. The antenna at the receiver should also be proportionally sized.

The wavelength $\lambda$ and the frequency $f$ of a sinusoid are inversely proportional. For an electrical signal travelling at thespeed of light $c$ ( $=3\times {10}^8$ meters/second), the relationship between wavelength and frequency is

For instance, if the frequency of an electromagnetic wave is $f=10$ KHz, then the length of each wave is