Wireless Security.pdf - PDF Archive

122 Chapter 4
particularly of ASK and PSK, to put even more bits on a symbol. Quadrature amplitude
modulation, QAM, sends several signal levels on four phases of the carrier frequency
to give a high bandwidth efficiency—a high bit-rate relative to the signal bandwidth.
It seems then that there is essentially no limit to the number of bits that could be
compressed into a given bandwidth. If that were true, the 3.4-kHz telephone line could
carry millions of bits per second, and the internet bottleneck to our homes would no
longer exist. However, there is a very definite limit to the rate of information transfer over
a transmission medium where noise is present, expressed in the Hartley-Shannon law:
C Wlog( 1 S/
N)
This expression tells us that the maximum rate of information (the capacity C ) that can
be sent without errors on a communication link is a function of the bandwidth, W , and the
signal-to-noise ratio, S / N .
In investigating the ways of modulating and demodulating multiple bits per symbol, we’ll
first briefly discuss a method not commonly used in short-range applications (although
it could be). This is called M-ary FSK ( Figure 4.22b ) and in contrast to the aim we
mentioned above of increasing the number of bits per hertz, it increases the required
bandwidth as the number of bits per symbol is increased. “ M ” in “ M-FSK ” is the number
of different frequencies that may be transmitted in each symbol period. The benefit of this
method is that the required S/N per bit for a given bit error rate decreases as the number
of bits per symbol increases. This is analogous to analog FM modulation, which uses a
wideband radio channel, well in excess of the bandwidth of the source audio signal, to
increase the resultant S/N. M-ary FSK is commonly used in point-to-point microwave
transmission links for high-speed data communication where bandwidth limitation is no
problem but the power limitation is.
Most of the high-data-rate bandwidth-limited channels use multiphase PSK or QAM.
While there are various modulation schemes in use, the essentials of most of them can
be described by the block diagram in Figure 4.23 . This diagram is the basis of what may
be called vector modulation, IQ modulation, or quadrature modulation. “ I ” stands for “ in
phase ” and “ Q ” stands for “ quadrature. ”
The basis for quadrature modulation is the fact that two completely independent data
streams can be simultaneously modulated on the same frequency and carrier wave. This
is possible if each data stream modulates coherent carriers whose phases are 90 degrees
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