L14-15

If two users want to simultaneously send different

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Unformatted text preview: (400 to 800 terahertz) range. If two users want to simultaneously send different messages over some distance using high powered lamps, they can use different colors. The first transmitter could send a message by turning on and off a red lamp, and the second transmitter could send a message by turning on an off a green lamp. Over 1 We will use the two terms interchangeably. 1 2 LECTURE 14. FREQUENCY-DOMAIN SHARING AND FOURIER SERIES Figure 14-1: A Diagram of frequency-domain sharing. time, a distant receiver will see a changing mixture of red, green, and yellow, but will be able to untangle the messages from the two transmitters by “pulling out” the frequency of interest. If there are many transmitters, all using different colors, then the receiver may see what looks like white light, but with the aid of a simple glass prism, the receiver will be able to separate out each of the colors and determine each of the messages. In our “colorful” example, one of the transmitters digitally modulates the information they wish to send over the red light by turning on an off a red lamp, where “on” can indicate a logical 1, and “off” a logical 0. The red light carries the modulated digital message and is said to be the carrier. When a receiver uses a prism to separate out the different colors, and then converts a specific color’s intensity changes back in to the digital data, we refer to the process as demodulation. Each other transmitter uses a different colored light to carry its modulated signal. Figure 14-1 summarizes, in diagram form, the process of modulation, transmission across a channel, and demodulation, for the case of P transmitters and receivers sharing a single physical channel. Spectral-domain sharing using different colors (wavelengths of light) is actually a commonly used approach to transfer data over optical fibers. The method is referred to as wavelength-division multiplexing (WDM), and modern fiber-optic communication systems often use a hundred different wavelengths, corresponding to...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.

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