Modulation formats for digital fiber transmission Eric Tell 050329.

Modulation formats for digital fiber transmission

Eric Tell 050329

Outline• Fiber performance limitations• WDM• Optical vs. radio communication• Optical modulators• Modulation formats

– Amplitude shift keying

– Duo-binary signalling

– Optical single sideband signalling

• Simulation/experimental results• Summary

Fiber performance limitations

• Fiber Loss

• Chromatic dispersion– different refractive index for different

wavelengths

• Fiber non-linearities

Chromatic dispersion

• Distance limit ~1/(bit rate)²– Example: Single mode fiber @1550nm

• chromatic dispersion: 17ps/km-nm

• dispersion limited distance: ~100km @10Gbit/s

• comparable to loss limit

• EDFA => increased loss-limited distance– Chromatic dispersion becomes the limiting factor in

single mode long-haul fibers!

• We want to decrease the bandwidth for a given datarate!

Wave Division Multiplexing

• Decreased channel spacing leads to interchannel interference and makes it difficult to compensate for fiber nonlinearities

• Narrower subchannels would be nice...

WDM (cont'd)

• In a high capacity link the whole EDFA spectrum is filled with subchannels

• The bandwidth of each subchannel is proportional to its bit rate

• Total fiber capacity is given by the spectral efficency: (bitrate per channel)/(channel spacing)

WDM (cont'd)

• In a practical case using NRZ a spectral efficiency of 40% can be reached

Power spectral density of NRZ

WDM (cont'd

• More GB/s per channel does not increase total bandwith, however– It results in fewer channels to manage– Increased channel spacing decreases

some non-linear distortions• BUT to reach higher spectral efficiency a

format with narrower spectrum for a given bandwidth is needed (while at the same time not increasing other impairments)

How can this be achieved?

• M-ary Amplitude Shift Keying (ASK)

• Duo-binary signaling• Optical Single Sideband (OSSB)

Comparison to radio systems

• Much of the same theory can be applied, except– Carrier frequency is different

• 1550 nm => 194 Thz

– The available components are different• no coherent detection (no PLLs)

– The channel is different

Component imperfections

• Modulators are nonlinear– difficult to achieve pure AM

• PIN photo detectors responds to optical power rather than electrical field amplitude (“square envelope”)

• Dispersion introduces a frequency dependent phase shift

• “intensity-modulated” approaches are used

Optical Modulators

• Direct modulation– directly modulate the drive current of a semiconductor

laser

• Absorbtion modulation– Modulate the absorption spectrum of reverse-biased

diod placed in front of the laser

– Faster and more linear than direct modulation (60 GHz)

• The Mach-Zender (MZ) modulator– modulation my adding phase shifted signals

Optical modulators (cont'd)

• Direct modulators and absorption modulators directly modulates the optical power, but will also generate phase modulation

• The MZ modulator is more flexible and can generate different kinds if modulation other than NRZ/RZ/ASK

The MZ modulator

V1(t)

V2(t)

Ein

Ein/2

γEin/2

Eout

LiNbO3

waveguidecontacts

VtvjVtvjintjtjinout ee

Eee

EE /)(/)()()( 2121

22

MZ modulator transfer function VtvjVtvj

in

out eeE

tEtvtv /)(/)(

2121

2

1)())(),((

VtvtvjetvtvV

tvtv 2/))()((2121

21))()((2

cos())(),((

))(cos( 1 tvV

EE inout

With γ=1 this can be rewritten as:

Amplitude modulation Phase modulation (chirp)

With v1(t)=-v2(t) we remove the phase modulation and get:

))((cos 12 tv

VPP inout

MZ modulator biasing)()( 12 tvVtv dc

VVjdctdc

dceVvV

Vtv 2/)(11 )2(

2cos()),((

2/VVdc

VVdc

)(1

2

1

4

1)(cos)2/),(( 1

1 tvVV

tvVta

)()(sin2

1)(cos)),(( 11

1 tvV

tvVV

tvVta

“Normal bias”:

“Bias at extinction”:

MZ modulators - observations

• These modulators are only linear in a small region– A problem for other than RZ/NRZ signaling

• There must normally be an unmodulated carrier in order to use non-coherent detection

M-ASK• Less bandwidth

• More power needed for a given BER• non-linearities become limiting in

long-haul DWDM systems• More complicated (analog and

digital) electrical circuits• Possibly useful in multi-mode

dispersion limited systems e.g. 10 Gbit/s Ethernet

levels

bandwidth

2 ±B

4 ±B/2

8 ±B/3

16 ±B/4

32 ±B/5

64 ±B/6

Duo-binary signaling

• Introduce correlation between consecutive symbols

• A special case of partial response signaling:

Duo-binary signaling

• Add consecutive symbols => three signal levels

-1,1,1,-1

-2,0,2,0MZ modulator

AM-PSK Duo-binary

• Problem: Normally impractical to handle three levels

• Solution: Use 0,E,-E– The detector will detect two levels 0 and E²– By precoding these two levels will correspond to

0 and 1– a.k.a Amplitude Modulated Phase Shift Keying

(AM-PSK) duo-binary signaling

AM-PSK duo-binary system

1,1,0,1,0

MZ modulator biased at extinction

xor map

|x|2

0,1,1,0,0

1,-1,-1,1,1

0,0,-2,0,2 0,0,-E,0,E

1

0,0,1,0,1

0,0,E2,0,E2

(fiber)

Photo detector

Precoder

Optical Single Sideband (OSSB)

• Observation: The frequency spectrum is symmetrical

• Implication: Half of it can be filtered out to save bandwidth => Single Sideband Transmission!

• Used e.g. in TV

Subcarrier OSSB

• In conventional subcarrier modulation the subcarrier appears on both sides of the optical carrier

• Dispersion causes a phase shift between the two signals, which depends on the distance

• At certain points the entire signal is canceled out!

Subcarrier OSSB (cont'd)(decided to skip the equations: Optical fiber communications IVB, eq.16.30-16.36)

Creating an SSB signal

• Two ways– Use a filter (half the energy is lost)– Use the Hilbert transform

• known as a Hartley modulator

Hartley modulator)2sin()(ˆ)2cos()()( tftatftats ccsSSB

)(ˆ)()( tajtats sSSB SSB signal:

Baseband signal:

Optical SSB modulator)(ˆ)()( tajetatg “Approximation” of SSB signal:

MZAmplitudemodulator

Hilberttransform

Phasemodulator

a(t)

Optical carrier

â(t)

OSSB signal

Simulation results: ASK/duo-binary

Dispersion induced receiver sensitivity degradation for Gbit/s signalling

More practical issues…

• ASK– Nees more power =>

non-linearities limiting• Duo-binary

– Needs extra filtering– Optical dispersion

compensation could be an alternative

– 225 km @10Gbit/s 1550 nm has been reached

Experimental results: OSSB

Experimental receiver sensitivity degradation vs. fiber length @ 10Gbit/s, BER=10 -9

DWDM

• “Normal” NRZ– 40% spectral efficiency over 150 km

• Duo-binary AM-PSK– 100% over 100 km

• OSSB– 66% over 300 km

Summary

• Distance between repeaters is limited by either of– Fiber loss– Chromatic dispersion– Fiber non-linearities

• With the advent of EDFA chromatic dispersion has become the limiting factor in long-haul systems

Summary (cont’d)

• We want to limit the bandwidth in order too– Reduce the effects of chromatic dispersion

– Reach higher spectral efficiency in DWDM systems

• Two potential methods:– Duo-binary signaling

– Optical single sideband

• Both methods could potentially halve the bandwidth• None of the methods are currently used in commercial

systems, but there are some promising experimental results