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Modulation
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Two Maximum Bits/Symbol for Common Modulation Schemes Modulation MSK BPSK QPSK QAM-16 QAM-32 QAM-64 QAM-256 Bits/symbol 1 1 2 4 5 6 8
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TABLE 22
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TABLE 23
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SNR for Various Modulation Formats Signal-to-noise ratio, dB
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BER 10 10 10 10 10 10 10
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4 5 5 7 8 9 10
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QPSK 8 10 11 12 125 13 1325
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QAM-16 13 14 15 1575 1625 165 1675
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QAM-32 15 16 17 18 185 19 1925
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QAM-64 17 18 19 20 21 215 2125
QAM-128 19 205 215 225 235 24 2425
QAM-256 215 23 24 25 2575 2625 265
TABLE 24
Common Modulation Schemes and Their Properties Bits/symbol (h) 1 2 3 4 5 6 States 2 4 8 16 32 64 Amplitudes 1 1 1 3 5 9 Phases 2 4 8 12 28 52
type BPSK QPSK PSK-8 QAM-16 QAM-32 QAM-64
Adaptive equalization will correct certain signal impairments in real time, such as group delay variations (GDV), amplitude tilt, ripple, and notches Adaptive equalization, however, will not improve impairments created by a nonlinear amplifier, noise, or interference, but it will mitigate the sometimes massive multipath effects that would normally render a digitally modulated signal unreadable because of the high BER caused by the resultant amplitude variations Adaptive equalization basically uses a dynamically varying adaptive filter that corrects the received signal in amplitude, phase, and delay, making highdensity modulations possible Virtually all terrestrial microwave communication systems employ some form of adaptive equalization, located right after the receiver s demodulator
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Modulation
Modulation
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 232) But digital communication systems can be examined for proper operation 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 within 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 figures, 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 signal-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-
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