Transcript No Slide Title

ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Coherent Lightwave Systems
Multimedia University
Hairul Azhar Abdul Rashid, 2006
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
CONTENTS
• Principles of coherent and non-coherent
detection :
– heterodyne and
– homodyne detection;
• Modulation formats:
– ASK,PSK,FSK,PPM,DPSK;
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
CONTENTS
• Demodulation schemes :
– synchronous and
– asynchronous demodulation;
• Bit error rate performance analysis;
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
CONTENTS
• Performance degradation due to:
– laser phase noise,
– group velocity dispersion,
– self phase modulation,
– polarization mode dispersion,
– relative intensity noise,
– effect of timing jitter;
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
CONTENTS
• System design considerations:
– power budget,
– rise time budget,
– power penalty.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Modulation
Modulation process:
Switching or keying the amplitude, frequency, or phase
of the carrier in accordance with the information binary
bits.
The modulation can be either:
– Direct modulation
• Light is directly modulated inside a light source
– External modulation
• Using external modulator
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
IM-DD system
• Applied in the first generation (1970’s) intensity modulation direct
detection is still the most used for optical communications
• Information is carried only by the intensity
• not frequency or phase
Optical detection
• The received signal is applied directly to photodetector
• Photo-detection of light represents the key operation in
the optical receiver.
• Converting the collected field onto a current or voltage.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• Light is described as a stream of photons (quanta)
• The theory of quantum states that the energy of a photon is
proportional to the frequency of light
Ehf
Where the Plank constant h = 6.6261  10-34 W s2
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• Let P the optical power of a light beam, then the number of
photons per second is:
P
N
hf
For modulated optical signal with power P(t), the instantaneous
photon intensity (photon flux) varies with time:
P( t )
N p (t) 
hf
photons/s
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• For a PIN-diode photodetector, the average number of electronhole pairs generated in a time interval of T is given by

E
m   P( t ) dt 
hf 0
hf
T
Where  is the quantum efficiency of the device and E is the
energy received in a time interval T.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
The ideal receiver
• Consider an ideal OOK transmission system over an ideal
channel
• The transmitter sends light for a one
• No light for a zero
• The receiver counts N, the number of photons it receives in a bit
interval of T seconds, and zero otherwise
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• If a zero is transmitted, then there is a zero probability of
receiving zero photons.
• If a one is transmitted, then the photons arrive according to a
Poisson process with mean m
• For a ONE, the probability of receiving N photons in T seconds
is given by by the Poisson distribution.
(m ) N e  m
Pr [ N photons / ONE] 
N!
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Quantum limit
• It is possible that no photons arrive when a ONE is transmitted.
This leads to a probability of error or a Bit-error-ratio (BER), of
1
em
BER  Pr [0 photons / ONE] 
2
2
• This leads to an important lower bound on the BER called the
quantum limit
BER 
e
N
2
• It indicates a minimum signal power required by an OOK
receiver to achieve a given BER
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Example:
• Letting BER= 10-9 gives m = 20.03.
• Hence, to achieve a BER of 10-9, the pulse must have an optical
energy corresponding to an average of 20 photons.
• On average, half the signal intervals contain optical pulses, and
the average number per transmitted bit is:
m
 10 photons / bit
2
• This quantity of of 10 photons/bit is called the quantum limit for
optical detection.
• It represents a lower limit on the received power necessary in a
direct detection.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Practical receiver
Receiver configuration
IM-DD system can only be used for OOK modulation format
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Shot noise
Shot noise (from O-E counting process in PIN):
I(t)  Ip  is (t)  RPin  is (t)
is the average
photocurrent
is a stationary random process
with Poisson statistics
is(t) can be approximated by the Gaussian statistics
with its variance given by:

  i (t )   Ss (f )df  2qI p Be
2
s
2
s

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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Thermal noise
Including thermal noise (from carrier moving in any conductor):
I(t)  Ip  is (t)  iT (t)
current fluctuation induced by thermal noise
iT(t) can be modeled as a stationary Gaussian random process
with its variance given by:

2
T
 i (t) 
2
T



ST (f )df 
4k B T
Be
RL
Its spectral density (“white noise”) is given by:.
k B , T, R L :
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ST (f )  2k BT / R L
Boltzmann constant,
the absolute temperature,
and the load resistor
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Total receiver noise
Considering the dark current from PIN and the enhancement
to thermal noise from the components other than the load
resistor in the linear channel, the total noise variance is:
4k B T
      [2q(I p  I dark ) 
Fn ]Be
RL
2
Id , Fn :
2
s
2
T
the PIN dark current
and the amplifier noise figure
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Receiver signal to noise ratio
PIN receiver:
APD receiver:
R 2 Pin2
SNR 
4k T
[2q(RPin  I d )  B Fn ]Be
RL
SNR 
M 2 R 2 Pin2
4k B T
[2qM FA (RPin  I d ) 
Fn ]Be
RL
2
Where
M, FA : the APD gain and the APD excess noise factor
FA  k A M  (1  k A )(2 1 / M)
kA :
is the ionization-coefficient ratio.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
PIN and APD Noise limitations
PIN
APD
Shot noise
limited
SNR~ Pin
SNR ~ Pin / FA
Thermal
noise
limited
SNR ~ Pin2
(worse)
(large load impedance
required)
2
2
in
SNR ~ M P
(better)
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
BER Analysis for IM/Direct Detection
The bit error rate can be computed as:
BER  p(1)P(0 / 1)  p(0)P(1 / 0)
where
• Pr(0/1) is the probability that a "0" is received
when a "1" is transmitted.
• Pr(1/0) is the probability that a "1" is received
when a "0" is transmitted
• The values of Pr(0/1) and Pr(1/0) depends on the statistical
nature of the output signal in the presence of noise.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• For a binary symmetric channel, p(0)=p(1)=1/2 which
indicates equal probability of occurrence for a "1" and a "0" bit.
The output signal current is given by
i1  I1  i n
bit "1"
i 0  I0  i n
bit "0"
Where in is the noise current due to shot and thermal noise.
The probability density function of in is given by
 (i n  i mean ) 2 
p(i n ) 
exp

2
2
2

2n
n


1
• where imean=0 is the mean value of in.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Bit Error Rate
The bit error rate can be computed as:
BER  p(1)P(0 / 1)  p(0)P(1 / 0)
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• Since in is Gaussian with zero mean and variance n2 ,
the probability density function (pdf) of the receiver
output corresponding to bit "1" and bit "0" are also
Gaussian with mean I1 and I0 respectively and given by
 (i1  I1 ) 2 
p(i1 ) 
exp

2
2
2

21
1


1
 (i 0  I 0 ) 2 
p(i 0 ) 
exp

2
2
2

20
0


1
where 12 and 02 are the noise variances corresponding to
bit "1" and bit "0" respectively.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Hence
Pr(0 / 1)  Pr(i1  i th ) 
i th
 p(i ) d(i )
1
1


i th


 I1  i th 
 (i1  I1 ) 2 
1
exp

di1  erfc
2
2
2

2
2

21
1


1 

1

Pr(1 / 0)  Pr(i 0  i th )   p(i 0 ) d(i 0 )
i th



i th
 i th  I0 
 (i 0  I 0 ) 2 
1
exp

di 0  erfc
2
2
2

2
2

20
0


0 

1
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Minimum BER occurs when Pr(0/1)=Pr(1/0) which corresponds
to an optimum value of the threshold current ith and can be determined as
 i th  I 0   I1  i th 
Q



2
 2 0   21 
The optimum threshold is then given by
 0 I1  1I 0
i th 
1   0
Under the assumption that the noise current is same for bit "0" and bit "1",
1=0, then the optimum threshold is given by
i th  opt
I1  I 0

2
The above optimum threshold is applicable in absence of laser phase
noise. In the presence of laser phase noise, the optimum threshold is to be
determined numerically because 1 does not equal 0.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
The value of the parameter Q at the receiver output under optimum
threshold condition is expressed as
I1  I 0
Q
1   0
and the corresponding BER for optimum threshold is given by
 1  I1  I 0 
1
1
1
Q 1

BER  Pr(0 / 1)  Pr(1 / 0)  erfc   erfc 
2
2
2
 2 2
 2  1  0 
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
The output SNR (= signal power to noise power ratio) for a PIN-receiver
is given by
(RPin ) 2
Pin
SNR 

 N
2e(RPin  Id )Be  4kT Be Fn / R L 2hBe
where Be=Br/2. In terms of number of photons per bit N, the
BER can be expressed as
1
Q
BER  erfc 
2
 2
where
Q
I1  I 0
I
 1  SNR
1  0 1
Where we assumed that I0=0 and 0= 0 which is valid when the
receiver is dominated by shot noise.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
hence
and
Q  N
 N 
1
Q 1
BER  erfc   erfc

2
2
2
 2


 N 
1
BER  erfc

2
 2 
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Principle of Coherent Detection
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Coherent Detection
Coherent detection receiver adds light to the received
signal as part of the detection process
Beam Combiner
Electrical Signal Output
Optical Signal Input
PhotoDetector
Electronic
Circuits
Local
Optical
Oscillator
Coherent receiver model
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Detection Schemes
• Homodyne detection
– The optical signal is demodulated directly to the baseband.
– It requires a local oscillator whose frequency match
the carrier signal and whose phase is locked to the
incoming signal ( c= LO).
– Information can be transmitted through amplitude,
phase, or frequency modulation
• Heterodyne detection
– Neither optical phase locking nor frequency matching is of
the local oscillator is required ( c  LO).
– Information can be transmitted through amplitude,
phase, or frequency modulation
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Demodulation schemes in coherent
detection
• There are two basic types of demodulation in
coherent detection of optical signals
:
(a) Synchronous demodulation
(is essential for homodyne detection)
(b) Asynchronous demodulation
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
ASK, PSK, DPSK, and FSK modulation
Formats
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Optical Detection
Modulated signal:
ES  AS exp[ j(c t  S )]
Local oscillator signal:
ELO  ALO exp[ j(LOt  LO )]
The output power of the photodetector
P(t )  PS  PLO  2 PS PLO cos(IF t   )
where
AS2
A 2LO
PS 
, PLO 
, IF  c  LO ,   S  LO
2
2
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Homodyne Detection
The detector current:
I p (t )  2R PLO PS (t )
Heterodyne Detection
The detector current:
I p (t )  2 PS (t ) PLO cos(IF t  s  LO )
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Synchronous Coherent
Receiver
Optical Signal Input
Beam Combiner
PhotoDetector
BPF
Delay
LPF
Baseband Signal Output
Local
Optical
Oscillator
Carrier
Recovery
• In which the IF modulated signal is mixed with an IF carrier
recovered from the IF signal. At the output of the mixer the
baseband signal is received which is filtered by a low pass filter
and fed to the decision circuit.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• Heterodyne detection needs neither frequency
matching nor phase locking.
• The detected electrical signal is carried by the
intermediate frequency and must be demodulated
again to the baseband.
• This demodulation scheme can be used for ASK, FSK
or PSK modulation formats.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Synchronous ASK
The detector current: I p (t )  2R PS PLO cos(IF t   )
its mean square is :
id (t)
2
 2R 2 Ps PLO
The thermal noise and shot noise variances :
where
2
2  shot
 2thermal
2
shot
 2e(Is  ILO )Be  2e(RPs  RPLO )Be
2thermal  4kTBe Fn / R L
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
IF- Signal to noise ratio (IF-SNR) :
IF SNR 
id
2
2
2R 2 Ps PLO

2e(RPLO  Is )Be  2thermal
Or
IF  SNR 
id
2
2
2R 2 Ps PLO
RPs
Ps



2e(RPLO  Is )Be eBe hBe
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
If the bit rate is Br=1/T, then average signal power
Nh 
Ps 
 Nh B r
T
Let Be=Br/2, then SNR can be expressed
SNR  2N
The corresponding BER for heterodyne ASK receiver is
1
BER  erfc [ N / 4 ]
2
Syn . Het . ASK
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Receiver sensitivity
Receiver sensitivity can be defined as the minimum
required received optical power to attain a BER of 10-9
which corresponds to Q=6 or when SNR=144
or 21.6 dB.
Average received power Pr can be obtained as
Pr 
1
1
Ps  [4Q 2 hBe / ]  2Q 2 hBe /   72 hBe / 
2
2
For an ideal photodetector =1 and the number of photons
per bit required for BER=10-9 is 72 for ASK heterodyne.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Synchronous ASK in the
presence of noise
The current after photo-detection
(the output of the photodiode is passed through a BPF centered at the IF frequency and the filter out put can be written as:)
I f ( t )  2R PLOPS ( t ) cos(IF t  )
 2R PLOPS ( t )[cos cos(IF t )  sin  sin(IF t )]
The noise at the output of the filter can be expressed in
terms of its in-phase and quadrature components as :
i n (t )  i c  j is
where i c , i s
(Gaussian random variables with zero mean)
The variance are given by:
2  c2  s2
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
With noise included after BPF:
If ( t )  [2R PLOPS ( t ) cos  i c ] cos(IF t ) 
 [2R PLOPS ( t ) sin   i s ] sin(IF t )
After synchronous (coherent) demodulation and LPF
I d ( t )  {[ 2R PLOPS ( t ) cos  i c ] cos(IF t )
 [2R PLOPS ( t ) sin   i s ] sin(IF t )}cos(IF t ) 
 [2R PLOPS ( t ) cos  i c ] cos2 (IF t ) 
ic
 R PLOPS ( t ) cos 
2
It shows that only the in-phase noise component affects the
performance of synchronous heterodyne receivers.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Or
I d ( t )  R PLOPS ( t ) cos   i c 
1
I p cos   ic 
2
where
I p  2R Ps PLO
The analysis is analogous to that for direct detection receiver
and the BER is given by
1
Q
BER  erfc 
2
 2
where
Q
I1  I 0
I
1
 1 
SNR
1  0 21 2
Where we assumed that I0=0 and 0= 1 which is valid when the
receiver is dominated by shot noise at higher values of PLO.
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Using the relation SNR=2Np we get
BER 
1
erfc [ N / 4 ]
2
Syn. Het . ASK . Det .
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Homodyne Synchronous ASK
The detector current:
its mean square is :
I p (t )  2R PLO PS (t )
id (t )
2
 4R 2 Ps PLO
The thermal noise and shot noise variances :
where
2
2  shot
 2thermal
2
shot
 2e(Is  ILO )Be  2e(RPs  RPLO )Be
2thermal  4kTBe Fn / R L
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
IF- Signal to noise ratio (IF-SNR) :
IF SNR 
2
id
2
4R 2 Ps PLO

2e(RPLO  Is )Be  2thermal
Or
IF  SNR 
id
2
2
4R 2 Ps PLO
2RPs 2Ps



2e(RPLO  Is )Be
eBe
hBe
where R=e/h
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
If the bit rate is Br=1/T, then average signal power
Nh 
Ps 
 Nh B r
T
Let Be=Br/2, then SNR can be expressed
SNR  4N
The corresponding BER for homodyne ASK receiver is
BER 
1
erfc[ N / 2 ]
2
Syn . Hom . ASK
For an ideal photodetector =1 and the number of photons
per bit required for BER=10-9 is 36 for Homodyne Syn. ASK.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Homodyne Syn. ASK
Versus Heterodyne Syn.
ASK
–
ASK homodyne receiver
requires 3 dB less power
and is therefore 3-dB
more sensitive than ASK
heterodyne receiver.
Heterodyne Detection
versus IM/DD
– Sensitivity Improvement of
10 dB to 20 dB
– Frequency selectivity
– IF domain signal processing
provides better performance
Heterodyne Detection
– Receiver is more sensitive
to the phase noise of lasers
– Additional signal power is
required for the same
reliability of operation which
is called power penalty
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Synchronous PSK Detection in the
presence of noise
The detector current at the receiver output is given by
where
Id 
1
[ I p cos   ic ]
2
0

bit "1"
bit "0"
so that the output current is positive or negative
depending on the bit transmitted as:
1
I1 
I
p
2
1
I 0   I p
2
and
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Q
 ic
 ic

bit "1"
bit "0"
I1  I 0
2I
 1  SNR
1  0 21
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Using SNR=2N for heterodyne case
BER 
1
erfc [ N p ]
2
Het Syn . PSK
And using SNR=4N for homodyne case
BER 
1
erfc [ 2N ]
2
Hom . Syn . PSK
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Synchronous Dual-Filter FSK
Receiver
•
•
•
•
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FSK synchronous receiver is equivalent to two ASK asynchronous heterodyne
receivers operating in parallel. The signal is received during both binary bits, the
SNR is 3-dB higher than that for ASK heterodyne receiver.
In dual filter FSK receiver, two band-pass filters are used to pass the mark and
space frequencies separately.
The BPF are centered at (IF+) and (IF-) corresponding to "mark" and
"space" frequencies.
The output of the BPF are passed through envelope detectors and low-pass
filters. The differential signal at the output of the low-pass filter is then obtained
by subtracting the one from the other. The data decision is then made by
comparing the output samples with a threshold of zero value.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
id
SNR 
2
2
4 Rd2 Ps PLO
2 R P 2Ps

 d s 
 4N p
2e( Rd PLO  I s ) Be
eBe
hBe
The BER is then given by
BER 
1
erfc [ N p / 2 ]
2
FSK Heterodyne Synchronou s
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Asynchronous receiver
Beam Combiner
Optical Signal Input
PhotoDetector
BPF
LPF
Envelop
Detector
Baseband Signal Output
Local
Optical
Oscillator
•
•
•
•
Asynchronous demodulation does not require recovery of the microwave
carrier at the intermediate frequency
The output of the IF filter is passed through an envelope detector and is lowpass filtered.
The output of the LPF is sampled and compared with a threshold of optimum
value to make bit decisions.
This demodulation scheme can be used for ASK and FSK.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Asynchronous Detection in the
presence of noise
The current after heterodyne photo-detection
I d ( t )  2R PLOPS ( t ) cos(IF t  )
 2R PLOPS ( t )[cos cos(IF t )  sin  sin(IF t )]
The noise at the output of the filter can be expressed in
terms of its in-phase and quadrature components as :
i n (t )  i c  j is
where i c , i s
(Gaussian random variables with zero mean)
The variance are given by:
2  s2  T2
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
With noise included after BPF, envelope detector and LPF:
Id (t )  [2R PLOPS (t ) cos  i c ]2  [2R PLOPS (t ) sin   is ]2
• It shows that both the in-phase and out-of-phase noise
components affects the performance of asynchronous
(incoherent) heterodyne receivers.
• The SNR is thus degraded comparing with that of synchronous
(coherent) heterodyne receivers.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
• In case of asynchronous demodulation, the noise at the output
of the envelop detector is no longer Gaussian
– because the output of an envelop detector is square of its
input.
– So, the noise statistics are changed due to envelope
detection and hence the BER calculation becomes
complicated.
The current at the output of the envelop detector when a signal pulse is
present corresponding to bit "1" is given by

I  (I p  i c )  i
2

2 1/ 2
s
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
The probability density function (pdf) of the output current I
is given by a Rice distribution as
 I 2  I 2p   I p I 
I
p(I, I p )  2 exp
I  
2  0 2 

 2    
where I0 is the Bessel function of the first kind and 2 is
the noise variance .
The output of the envelop detector corresponding to a bit "0" is

I  ic  i
2

2 1/ 2
s
and the pdf of the output is given by a Raleigh distribution which can be
obtained by putting Ip=0 in the expression for p(I,Ip).
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
The bit error rate (BER) is then obtained as
BER  p(1)P(0 / 1)  p(0)P(1 / 0) 
1
P(0 / 1)  P(1 / 0)
2
i th
P (0 / 1)   p( I, I1 )dI
0

and
P(1 / 0)   p(I, I 0 )dI
i th
The final form of the BER is given by BER  1 1  Q I1 , i th   Q I0 , i th 

2 
  
   
The minimum BER corresponding to optimum threshold can be obtained
numerically. If I0=0 and I1>>, ith=I1/2. Under such conditions, BER is given by
 I12  1
1
 SNR 
BER  exp 2   exp

2
 8 
 8  2
Using SNR=2N for heterodyne detection, BER can be expressed as
BER 
1
exp  N / 4
2
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Asynchronous FSK- Single filter
Receiver
Beam Combiner
Optical Signal Input
PhotoDetector
BPF
Frequency
discriminator
------Envelop
Detector
LPF
Baseband Signal Output
Local
Optical
Oscillator
•
•
•
The output of the IF filter is passed through a frequency discriminator
followed by an envelope detector and is low-pass filtered.
The output of the LPF is sampled and compared with a threshold of optimum
value to make bit decisions.
The single filter FSK receiver is suitable for narrow deviation FSK
(for modulation index, <1)
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Asynchronous Dual-Filter FSK
Receiver
•
•
•
Two band-pass filters are used to pass the mark and space frequencies
separately.
The BPF are centered at (IF+) and (IF-) corresponding to "mark"
and "space" frequencies.
The data decision is then made by comparing the output samples with a
threshold of zero value.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Heterodyne Asynchronous
DPSK Delay-Demodulation
Receiver
•
•
•
In this demodulation scheme, a replica of the IF signal is delayed
by a fraction of a bit and then multiplied with the original signal.
The resulting signal is a phase modulated signal of differential
phase, =(t)-(t-) where  is delay time.
The optimum value of  is T/2.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Bit-error rate curves for various
modulation formats
Synchronous
Asynchronous
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Tutorial
• Consider a 1.55- μm heterodyne receiver with
a p–i–n photodiode of 90% quantum
efficiency connected to a 50-Ω load
resistance. How much local-oscillator power
is needed to operate in the shot-noise limit?
Assume that shot-noise limit is achieved
when the thermal-noise contribution at room
temperature to the noise power is below 1%.
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ETM 7172 OPTICAL COMMUNICATION SYSTEMS
Tutorial
• Calculate the sensitivity (in dBm units)
of a homodyne ASK receiver operating
at 1.55 μm in the shot-noise limit.
Assume that η= 0.8 and ∆f = 1 GHz.
What is the receiver sensitivity when the
PSK format is used in place of ASK?
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