Page 76 - Complete Wireless Design

Modulation

Modulation 75

Because of the nature of digital signals, they can maintain a relatively high
quality at the receiver—even when close to becoming unreadable as a result of
impairments. This makes the testing of a digital signal for its merits at the
receiver of little use, since the digital signal may actually be only a few dB in
signal strength from crashing the entire link. This is referred to as the cliff (or
waterfall) effect, due to the rapid degradation, or complete elimination, of the
digital signal. BER will lessen to unacceptably high levels quite rapidly (Fig.
2.32). But digital communication systems can be examined for proper opera-
tion by sending and receiving certain digital test patterns that incorporate a
recurring succession of logical 1’s and 0’s. The test then compares the impaired
received pattern to the perfect transmitted pattern. The BER can then be
established by contrasting the bits received that were incorrect with the total
number of bits received.
This degradation in digital signal quality can be caused by many things:
reflections off metallic surfaces (multipath), producing amplitude ripple with-
in the signal’s passband; inadequate signal strength at the receiver creating
decreased SNR and a corresponding blurring of the symbol points (poor SNR
can be due to transmitter power levels being too low, high receiver noise figure
(NF), or path attenuation caused by trees, weather, or Fresnel zone clearance
problems); group delay variations and amplitude ripple produced by improper
analog filtering; strong phase noise components in the frequency synthesizers
of the conversion stages; or noise and cochannel interference levels induced by
interferers of all types.
Since many communication systems live or die by their bit-error rate fig-
ures, it is therefore worthwhile to not only recapitulate what the dominant
causes of BER degradation are in a digital communications system, but also to
dig a little deeper into the reasons behind this increase in BER. Decreased sig-
nal-to-noise ratio is the main mechanism for poor BER, since noise will
smudge the symbol points, making their exact location hard to distinguish by
the receiver’s demodulator. Phase noise, another important contributor, will
cause an input signal into a radio’s frequency converter stage to be slightly
changed at its output; this phase noise is introduced by the real-world local
oscillators (LOs) of a communication system, since the LOs are not perfect sin-

Figure 2.32 A digital signal and the cliff effect.

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