Page 248 - Anatomy of a Robot

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COMMUNICATIONS 233
the signal itself. For an RS232 signal at 100 Kbps, the signal has a waveform with
about 10 microseconds per bit. Light travels 3,000 meters, about 2 miles, in 10
microseconds. We’d need an antenna two miles long to transmit such a signal effi-
ciently into the impedance of space. Clearly, this won’t work well. It’s one of the
primary reasons almost no baseband wireless communication systems exist. They
almost all use modulation.
Sometimes the channel is so noisy that special techniques must be used to encode
the signal prior to transmission.
The FCC and other organizations regulate the use of transmission spectra.
Communication links must be sandwiched between other communication links in
the legal communication bands. To keep these competing communication links
separate, precision modulation is used.
Modulation generally involves the use of a carrier signal. The information signal (I)
is mixed (multiplied by) the carrier signal (C), and the modulated signal (M) is broad-
cast through the communication channel:

M I C

Although many different signals can be used as the carrier C, the type of signal most
often used is the sine wave. Although the operation x can be just about any type of oper-
ation, the most common type of mixing involves multiplication.
A sine wave only has a few parameters in its equation. Thus, modulating a carrier
sine wave can only involve a few different operations:

C A sin 1v t u2

where A is the amplitude, v is the frequency, and u is the phase.
Any modulation of this carrier wave by the data must involve a modification of one
or more of these three parameters. One or more of the parameters (A, v, or u) may take
on one or more values based on the data. As the data input, I, takes on one of n differ-
ent values, the modulated carrier wave takes on one of n different shapes to represent
the data I. The following 3 discussions describe modulating A, v, and u in that order.

Amplitude Shift Keying (ASK) sets

M1n2 An sin 1v t u2

where A is one of n different amplitudes, v is the fixed frequency, and u is the
fixed phase. In the simplest form, n 2, and the waveform M looks like a sine
wave that vanishes to zero whenever the data is zero (A 0 or 1).