8/3/2019 Bit Error Rate (BER)

1/23

Mahesh B.S

Roll No: 09

8/3/2019 Bit Error Rate (BER)

2/23

FUNDAMENTAL RECEIVER

OPERATION

An optical receiver consists of

Photo detector

Amplifier

Processing circuitry

8/3/2019 Bit Error Rate (BER)

3/23

OPTICAL RECEIVER

8/3/2019 Bit Error Rate (BER)

4/23

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

8/3/2019 Bit Error Rate (BER)

5/23

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

8/3/2019 Bit Error Rate (BER)

6/23

Cntd

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

8/3/2019 Bit Error Rate (BER)

7/23

Cntd

To find the bit boundaries clock is used

Which has the periodicity equal to the time

interval It is called timing recovery

8/3/2019 Bit Error Rate (BER)

8/23

ERROR SOURCES

8/3/2019 Bit Error Rate (BER)

9/23

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

8/3/2019 Bit Error Rate (BER)

10/23

PROBABILITY OF ERROR

Divide number of errors over a certain timeinterval by number of pulses transmitted

- No of errors

- No. of pulses transmitted

8/3/2019 Bit Error Rate (BER)

11/23

Cntd

Bit error rate is the chance that a bit is

corrupted and received in error

Bit error rate is abbreviated as BER

8/3/2019 Bit Error Rate (BER)

12/23

COMPUTATION

We have to know probability distribution

Because the decision is made as to whether a 0 or

1 is sent

8/3/2019 Bit Error Rate (BER)

13/23

Cntd

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

8/3/2019 Bit Error Rate (BER)

14/23

Cntd

Pe =aP1(vth) + bP0(vth)

Pe - ErrorProbability Vth- Threshold Voltage

a, bP

robabilities either a 1 or 0 receives

respectively

8/3/2019 Bit Error Rate (BER)

15/23

GAUSSIAN APPROXIMATION

f(s) ds=

f(s)- Probability density function

- Noise variance

8/3/2019 Bit Error Rate (BER)

16/23

Gaussian Noise Statistics

8/3/2019 Bit Error Rate (BER)

17/23

0 pulse sent

8/3/2019 Bit Error Rate (BER)

18/23

1 pulse sent

8/3/2019 Bit Error Rate (BER)

19/23

BER

P0(vth)= P1(vth )= Pe

8/3/2019 Bit Error Rate (BER)

20/23

Cntd

8/3/2019 Bit Error Rate (BER)

21/23

Cntd

Q used to specify receiver performance

Typical value=6

8/3/2019 Bit Error Rate (BER)

22/23

BERV/s Q

8/3/2019 Bit Error Rate (BER)

23/23

THANK YOU