Tektronix TSG4102A, Руководство пользователя

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Digital communications
Susceptibility to noise As mentioned previously, digital constellations have a fi nite number of allowed
states. A BPSK constellation, for instance, has only two allowed states: 0° and
180°. This property greatly enhances the robustness of digital communications in
the face of noise. Since a BPSK constellation contains only two allowed states,
any transmission which includes a deviation from these two states must be the
result of noise. If the noise deviations are small, the receiver can recover the actual
transmission with 100% accuracy by assuming the nearest allowed constellation
point was the intended transmission. This is in stark contrast to analog
communications, where any noise in the bandwidth of the channel will degrade
the
fi delity of the transmitted signal. Digital transmissions suffer no degradation
until the noise becomes so great that the nearest neighbor principle is not always
true. Even then, errors can often be corrected by the receiver if the protocol makes
use of Gray code and suf
fi cient redundancy has been built into the transmission.
Intersymbol interference
Pulse shaping fi lters fi x the frequency domain problems by fi ltering out the high
frequency components that would interfere with neighboring users. Unfortunately,
they introduce a new problem in the time domain, intersymbol interference (ISI).
The problem can be understood by observing the impulse response of the pulse
shaping
fi lter as a function of time. Generally speaking, pulse shaping fi lters with
low bandwidth have long response times. Conversely, fi lters with relatively high
bandwidth have short response times. Low bandwidth is good, but long response
times create a problem.
A digital communications receiver must make a decision about which symbol was
transmitted after every symbol period. The decision is usually made when the
impulse response for that symbol is at its peak. Intersymbol interference occurs
when the response of adjacent symbols interferes with the response of the current
symbol at the moment the decision is made.
The following fi gure shows the impulse responses of three symbols superimposed
on each other. At time t = 0, the receiver must decide which symbol was
transmitted. Notice that the responses from both the previous symbol and
succeeding symbols are nonzero. The full response at time t = 0 is the
superposition of all three responses. The residual responses of the adjacent
symbols will add or subtract to the symbol under question, thus, interfering with
the decision about what was transmitted.
40 TSG4100A Series RF Signal Generators User Manual