The attached narrated power point presentation attempts to explain the various digital communication techniques as applied to optical communications. The material will be useful for KTU final year B tech students who prepare for the subject EC 405, Optical Communications.

1. 1
Digital Transmission Systems
MEC

2. 2
Contents
• System Components.
• Digital Transmission.
• Quantization and Encoding.
• Pulse Code Modulation and IM/DD.
• Duobinary System.
• Binary Transmission.
• Channel Losses and Temporal Response.
• Line Coding.

3. 3
OFC System Components

4. 4
Digital Transmission Systems
• Superior performance over analog
counterparts.
• Ideal channel for data communications.
• Compatible with digital computing and
storage techniques.
• Optical fiber communication suited for
baseband digital transmission.

5. 5
Digital Transmission Systems
• Acceptable SNR at optical fiber receiver
over analog transmission by 20 to 30 dB.
• Use of baseband digital signaling reduces
problems with optical source.
• Nonlinearities & temperature dependence
may severely affect analog transmission.
• Convey digital information in the baseband
using intensity modulation of the optical
source.

6. 6
Digital Transmission Systems
• Pulse Code Modulation (PCM) – encoding
analog signal into digital bit pattern by initially
sampling the analog signal in excess of
Nyquist rate, Eg: for 3.4 kHz sampling rate is
8 kHz.
• Amplitude of constant width sampling pulses
varied in proportion to sample values of
analog signal, gives a discrete signal - pulse
amplitude modulation (PAM) - quantized to
discrete levels - PCM.

7. 7
Quantization and Encoding

8. 8
Digital Transmission Systems
• Digitized analog signal transmitted as a
baseband signal or be modulated by
amplitude, frequency or phase shift keying.
• Greater bandwidth required for PCM
transmission – optical channels are
wideband.
• Nonlinear encoding through companding -
input signal compressed before transmission,
expanded at the receive terminal after
decoding.

9. 9
Companding
• Companding reduces quantization error on
small-amplitude analog signal levels when
encoded from PAM to PCM.
• Quantization error (rounding off to nearest
discrete level) exhibited as distortion or noise
on the signal (quantization noise).
• Companding tapers step size, reduces
distance between levels for small-amplitude
signals, increases distance between levels
for higher amplitude signals.

10. 10
Companding
• Reduces quantization noise on small
amplitude signals at the expense of
slightly increased quantization noise for
larger signals.
• SNR improvement for small amplitude
signals reduces overall signal degradation.
• Converting continuous analog waveform
into discrete PCM signals permit time
division multiplexing.

11. 11
Companding

12. 12
Simplex Baseband PCM
Transmission
TDM
Simplex PCM

13. 13
Digital Transmission Systems
• Received PCM waveform decoded back to
PAM, and then simply passed through a
low-pass filter, recovers original analog
signal.
• Encoded samples from different channels
interleaved within multiplexer to give a
single composite signal, transmitted over
the optical channel.

14. 14
Digital Transmission Systems
• At receive terminal, interleaved samples
separated by synchronous switch or
demultiplexer.
• Analog signal reconstructed from the set of
samples.
• Time slots from channels interleaved
(multiplexed) into a frame, say of 32 time
slots.
• 2 additional time slots for signaling and
synchronization information, no encoded
speech.

15. 15
Timing for Line Signalling
Bits per time slot
Time slots per frame
Frames per multiframe

16. 16
Optical Transmitter and Modulation
Formats
• Average optical power launched into fiber
from the transmitter depends on type of
source used and required system bit rate.
• Laser launches around 1 mW, LED limited
to about 100 μW - both devices emit less
power at higher bit rates.
• LED gives reduced output at modulation
bandwidths in excess of 50 MHz, laser
output unaffected below 40 GHz.

17. 17
Optical Transmitter and Modulation
Formats
• Source signal to be modulated in the
transmitter before transmission.
• Two major modulation formats in IM/DD*
based digital optical communication
systems - nonreturn-to-zero (NRZ) and
return-to-zero (RZ).
• RZ pulses produced using two intensity
modulators / intensity and phase
modulator cascade.

18. 18
RZ Signalling Format Transmitter
*

19. 19
RZ Signalling Merits
• Higher Peak Power.
• Greater Noise Immunity.
• Better BER Performance.
• Less subjected to fiber non-linear effects.
• Eye diagram for RZ format displays larger
vertical eye opening against NRZ.
• Narrow vertical eye opening (eye closure) -
intersymbol interference due to fiber
nonlinear effects.
• Greater tolerance to ISI for WDM signals.

20. 20
RZ Signalling Merits
Maximum 9.6 dB, 400 km
Maximum 9.6 dB, 700 km

21. 21
Return to Zero Signalling
• Chirped return-to-zero (CRZ) - prechirping
of the pulse with sign of chirp opposite to
that introduced by fiber dispersion.
• CRZ - Enhanced system performance due
to pulse compression effect through
prechirping, combats fiber link dispersion.
• Carrier-suppressed return-to-zero (CSRZ)
- alternate bit phase inversion process
removes or suppresses carrier component
from power spectral density of RZ signal –
longer transmission distances.

22. 22
VSB-CSRZ
• Vestigal Side Band Carrier Suppressed –
partial removal of one sideband spectra
using optical filter.
• Increased spectral efficiency, decreased
channel spectrum requirements, reduced
channel spacing.
• Complete information of a VSB channel
contained in only half of its spectrum, other
half is redundant.
• Redundant information ignored/reproduced
from the other half.

23. 23
Optical Fiber Multiplexed
Transmission/WDM
• Spectral efficiency - ratio of average
channel capacity to average channel
spacing - determines overall density of a
WDM system, Eg : WDM system 40
Gbit/s, channel spacing 100 GHz -
spectral efficiency for a conventional
binary signal will be 0.4 bit/s/Hz.
• Decrease channel spacing to increase
spectral efficiency.

24. 24
Optical Fiber Multiplexed
Transmission/WDM
• If channel spacing decreased beyond
specific limit - overlapping of adjacent
channel information, degradation of data
signals.
• Use of efficient modulation formats -
alternate mark inversion (AMI) / duobinary
(DB) to decrease optical spectral band
occupied by a channel without decreasing
amount of information / data carried.

25. 25
Duobinary Transmitter and
Receiver
XOR gate + adder + 1 bit delay

26. 26
Duobinary Transmitter and
Receiver
• Transmitter - electrical duobinary
encoder, Mach–Zehnder modulator.
• Duobinary encoder consists of XOR gate,
adder and 1 bit delay circuit.
• Electrical duobinary data converted to
optical signal using both ON/OFF and 0/π
phase values.
• ON state optical signal with 0 phase
represents binary 1, ON state with π
phase indicates minus one level
corresponding to electrical duobinary
signal.

27. 27
Duobinary Transmitter and
Receiver
• Zero level of electrical duobinary produced
by not transmitting an optical signal (i.e.
OFF).
• Binary data recovered by simply inverting
optical intensity modulated signal.
• Electrical signal recovered by direct
detection at photodiode then an electrical
signal inversion, no need to determine /
recover phase of the optical signal.

28. 28
Mach-Zehnder Modulator
• Interferometric structure made from
material with strong electro-optic effect
(LiNbO3, GaAs, InP etc.).
• Applying electric fields to arms changes
optical path lengths, results in phase
modulation.
• Combining two arms with different phase
modulation converts phase modulation
into intensity modulation.

29. 29
Mach-Zehnder Modulator
• Optical input Ein split
into upper & lower
modulator arms,
phase modulated
with two phase
shifters driven by
electrical signals V1
& V2.
• Recombined into the
optical output Eout.

30. 30
Optical Duobinary Signal
• More tolerance to chromatic dispersion than
conventional binary signaling.
• Occupies only around half the bandwidth of
an optical NRZ signal.
• Twice dispersion tolerance to chromatic
dispersion.
• Narrow bandwidth enables reduced channel
spacings when combined with WDM.
• Employed with dense WDM over long
distance single-mode fiber links.

31. 31
Optical Receiver
• Input optical power required at the receiver
a function of detector and the electrical
components within the receiver structure.
• Strongly dependent upon noise (quantum,
dark current, thermal) associated with
optical fiber receiver.
• Apprx. 21 incident photons at an ideal
photodetector for a binary 1 with BER of
10−9 - cannot be achieved !
• Estimates of minimum required optical input
power made in relation to practical devices
and components.

32. 32
Binary Transmission
Binary signal with additive noise
Probability of falsely identifying a binary 1
Probability of falsely identifying a binary 0
Total probability of error

33. 33
Binary Transmission
• Signals greater than decision threshold (D)
are registered as a one and those less
than D as a zero.
• Noise current (or voltage) sufficiently large
can either decrease a binary 1 to a 0 or
increase a binary 0 to a 1.
• P(e) = P(1)P(0|1) + P(0)P(1|0).

34. 34
Channel Losses
• Total channel loss (dB) = installed fiber
cable loss + fiber–fiber jointing losses +
coupling losses of optical source and
detector.
• Fiber cable loss αfc (dB/km) specified by
manufacturer/measured.
• Loss due to joints αj (generally splices) on
the link specified in terms of equivalent
loss in dB/km.

35. 35
Channel Losses
• Loss contribution due to connectors αcr
(dB) used for coupling optical source and
detector to the fiber included in the overall
channel loss.
• Total channel loss CL = (αfc + αj)L + αcr dB,
(L – fiber length in km) in the absence of
any pulse broadening due to dispersion
mechanisms.

36. 36
Temporal Response
• System design considerations must take
into account temporal response of system
components.
• Finite bandwidth of optical system result in
overlapping of received pulses or ISI*,
reduces receiver sensitivity.
• BER** to be tolerated / ISI* to be
compensated by equalization within the
receiver.

37. 37
Temporal Response
• Loss Penalty - increase in optical power at
the receiver - dispersion–equalization / ISI
penalty.
• Dispersion–equalization penalty:
• τe - 1/e full width pulse broadening due to
dispersion on the link, τ - bit interval / period.
DL significant in wide band systems.

38. 38
Temporal Response
• For Gaussian-shaped pulses,
σ - rms pulse width.
• Bit rate BT is the reciprocal of bit interval τ,
• Total channel loss with dispersion –
equalization penalty:

39. 39
Line Coding
• Efficient timing recovery & synchronization
(frame alignment), error detection and
correction at the receiver.
• Suitable shaping of transmitted signal
power spectral density.
• Binary codes insert extra symbols into the
information data stream.
• Two-level block codes (nBmB) convert
blocks of n bits to blocks of m bits, m > n.

40. 40
Line Coding
• Biphase or Manchester encoding - 1B2B
code - 0 transmitted as 01, 1 as 10 – no
more than two consecutive identical
symbols.
• Coded mark inversion (CMI) code - 1B2B
code - 0 transmitted as 01, 1 alternately as
00 or 11.
• Error monitoring - parity check, disparity
between numbers of 1s and 0s, forward
error correction etc.

41. 41
Line Coding

42. 42
Thank You