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Mahesh B.S
Roll No: 09
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FUNDAMENTAL RECEIVER
OPERATION
An optical receiver consists of
Photo detector
Amplifier
Processing circuitry
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OPTICAL RECEIVER
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Cntd
Photo detector produces an electric current
proportional to the received power level
Photo detectors can be PIN or APD
Since the electric current is weak an amplifierboosts it to a level that can be used
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Cntd
It is then passed through a low pass filter to
reduce the noise
The filter can reshape the pulses that have
been distorted due to ISI
This filter is called equalizer since it equalizes
or cancels pulse spreading effects
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A decision circuit samples the signal level at
the midpoint of each time slot and compares it
with
th
resh
old level If the received signal level is greater than the
threshold level,1 is said to be received
If the received signal level is less than the
threshold level,0 is assumed to have been
received
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To find the bit boundaries clock is used
Which has the periodicity equal to the time
interval It is called timing recovery
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ERROR SOURCES
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NOISE Un wanted signals
2 types external & internal
External electric power lines, radio
transmission, lightning Internal- shot, thermal
Shot noise- random fluctuations in voltage or
current Thermal noise-random motion of electrons in a
conductor
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PROBABILITY OF ERROR
Divide number of errors over a certain timeinterval by number of pulses transmitted
- No of errors
- No. of pulses transmitted
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Bit error rate is the chance that a bit is
corrupted and received in error
Bit error rate is abbreviated as BER
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COMPUTATION
We have to know probability distribution
Because the decision is made as to whether a 0 or
1 is sent
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P1(v)=
It is the probability when o/p voltage less than
v when 1 sent
P0(v)=
It is the probability when o/p voltage exceeds vwhen 0 sent
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Pe =aP1(vth) + bP0(vth)
Pe - ErrorProbability Vth- Threshold Voltage
a, bP
robabilities either a 1 or 0 receives
respectively
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GAUSSIAN APPROXIMATION
f(s) ds=
f(s)- Probability density function
- Noise variance
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Gaussian Noise Statistics
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0 pulse sent
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1 pulse sent
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BER
P0(vth)= P1(vth )= Pe
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Cntd
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Q used to specify receiver performance
Typical value=6
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BERV/s Q
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THANK YOU