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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.
- Fall '13