Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection

Research & Development

Wireless Optical Networks with Modified OFDM/OQAM and

Direct Detection

Séminaire des doctorants TELECOM Bretagne 200828/3/2008

MamdouhMamdouhMamdouhMamdouh El TabachEl TabachEl TabachEl Tabach1, 21, 21, 21, 2, Patrick Tortelier1, Ramesh Pyndiah2, Olivier Bouchet1

1111France Telecom, Orange France Telecom, Orange France Telecom, Orange France Telecom, Orange LabsLabsLabsLabs,,,, 4 rue du Clos Courtel, 35512 Cesson-Sévigné Cedex BP 52, France2222ITITITIT----TELECOM Bretagne, SC TELECOM Bretagne, SC TELECOM Bretagne, SC TELECOM Bretagne, SC DepartmentDepartmentDepartmentDepartment,,,, Technopôle Brest-Iroise, CS 83818, 29238 Brest Cedex 3, France

Email: [email protected]

Optimisation d'une Chaîne de Communication Numérique Adaptée aux Systèmes de Transmission Optique Sans Fil :

Exemple de la Modulation OFDM/OQAM Modifiée

p 2

Research & Development France Telecom GroupMamdouh El Tabach

Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection

Outline � Wireless optical networks

� Propagation types� Transmission model� Channel characteristics

� Introduction to OFDM/OQAM� Principles of OFDM/OQAM� Similarities/differences between CP-OFDM and OFDM/OQAM� OFDM/OQAM in equations

� Modified OFDM/OQAM for wireless optical networks� Hermitian symmetry� Normalized offset value� System performance

� Conclusion

p 3



Wireless Optical Networks (WON) - Why?

� Serious alternative to radio frequency transmission

� Unregulated and unlicensed optical spectrum

� No interference with existing radio systems

� Ease of home networks interface

� Potential of high bit rate

� Optical wave and walls� Transmission security� Frequency reuse

� Eye safety

p 4



WON - Propagation types� System modules:

� Transmitter• LED or Laser diode• Electrical to Optical (E/O) conversion• Divergence optics, Half Power angle (HP) • Average optical transmitted power Pt

� Receiver• Optical filter• Concentration Optics, Field Of View (FOV)• Optical to Electrical (O/E) conversion• PIN or APD Photodiode

� Optical channel • Geometrical attenuation• Multiple reflections, Inter Symbol Interference (ISI)• Noises…

p 5



WON - Propagation types

• LOS: high directivity, point-to-point systems, like IrDA• WLOS: necessity of alignment, wider divergence (HP) and Field Of View (FOV) angles,

like remote controllers• DIFF: obstructed or not, always full-duplex connection

- Multiple optical reflections

- Most user-friendly for the customer

- Need of high transmitted optical power

Line Of Sight (LOS) Wide Line Of Sight (WLOS) Diffusion (DIFF)

p 6



WON - Transmission model

� Wireless optical network model, IM/DD:

� Transmission• Intensity Modulation (IM): OOK, PPM,…• Optical power carries modulated useful information

� Reception• Output photocurrent proportional to input optical power• Direct Detection (DD)• Quadratic detection

p 7




� h(t) the optical channel impulse response� N(t) the different noise contributions� Y(t) the received photocurrent (A)

)()()(.)( tNthtXRtY +⊗=� X(t) the transmitted optical power (W)� R the receiver responsivity (A/W)

� Constraints on the transmitted signal

� Instantaneous optical transmitted power

� Average optical transmitted power

0)( ≥tX

maxd)(2

1lim PttX

TP

T

TTt ≤= ∫−+∞→

Equivalent Equivalent Equivalent Equivalent BasebandBasebandBasebandBasebandModel!Model!Model!Model!

p 8




� DC channel gain, geometrical attenuation

∫+∞

∞−= == dtthfHH f )()()0( 0

� Average optical received power

( )

( )0)()(

)(2

1lim)()()(

2

1lim

)()(2

1lim)()(

2

1lim

HPduuhPduPuhP

dudtutXT

uhdudtutXuhT

P

dtduutXuhT

dtthtXT

P

t

u

t

u

tr

u

T

TT

u

T

TT

r

T

T uT

T

TT

r

===

−=

−=

−=

⊗=

∫∫

∫ ∫∫ ∫

∫ ∫∫

+

−∞→

+

−∞→

+

−∞→

+

−∞→

( )0HPP tr =

p 9



WON – Channel characteristics

[Source: Barry & Kahn 97]

� h(t): the optical multipathchannel impulse response

� Real positive…

� Attenuation on each path

� Delay on each path� No phase component

� Absence of multipath fading, but� Presence of multipath distortion, ISI

Photodetecor diameterPhotodetecor diameterPhotodetecor diameterPhotodetecor diameter>>>>>>>>>>>>

Optical wavelengthOptical wavelengthOptical wavelengthOptical wavelength

Y(t) is a spatial mean of is a spatial mean of is a spatial mean of is a spatial mean of X(t)including all the values of fadingincluding all the values of fadingincluding all the values of fadingincluding all the values of fading

p 10



WON – Channel characteristics

� N(t) depends on:� Ambient light� Photodiode type� Receiver design� Used wavelength� Transmission environment…

Periodic Periodic Periodic Periodic electric noiseelectric noiseelectric noiseelectric noise

Thermal noiseThermal noiseThermal noiseThermal noise

Shot noiseShot noiseShot noiseShot noise

Dominant

Ambient light can be minimized by optical filtering, but it still adds shot noise!Due to its high intensity, this shot noise can be modeled as white, Gaussian, and independent of X (t) � Additive White Gaussian Noise (AWNG)

( ) ( ) ambientdambientusefuldincidentshot qIIIIqIIqDSP 222 ≅++=+=

Power spectral density:

Iambient: the ambient background currentId: the dark current

Iincident: the total current corresponding to incident powerq: the electron charge

p 11







� Conclusion

p 12



OFDM/OQAM basic principles� Aim: to increase OFDM spectral efficiency by removing the

guard interval (cyclic prefix)� How: waveform modulating sub-carriers as much localized as

possible in the time and frequency domain �robustness to both time and frequency selectivity introduced by the channel

� The waveform must guarantee orthogonality between sub-carriers and multi-carrier symbols

Orthogonality in the real domain �� OffsetQAM on each sub-carrier

� Example: IOTA waveform (quasi optimally localized in time and in frequency)

p 13



OFDM/OQAM modulation and CP-OFDM*

key features

RealComplexOrthogonality

FFT + polyphase filterFFTImplementation

Various possibilities (IOTA,…)

RectangularPrototype

T0/2T0+GISymbol duration

NoYesGuard Interval

Real (PAM)Complex (QAM)Symbols

OFDM/OQAMCP-OFDMParameters

* CP* CP--OFDM = OFDM = CyclicCyclic PrefixPrefix OFDMOFDM

p 14



OFDM/OQAM in equations

00

1

TF =

)()( 0

1

0

2,

0 nTtpectsn

M

m

tmFinm −

=∑ ∑−

=

π

∑∑−

=

=n

M

mnmnm tpcts

1

0,, )()(

� Transmitted signal in OFDM / Transmitted signal in OFDM/OQAM

where: cm,n are complex where: am,n are real

)()( 02

,0 τνπ ntfeitf tminm

nm −= +

00 2

1τ

ν =

( ) ( ) tnFinm enTtptp 02

0,π−=

( ) ( )02

,0 τνπ ntfeiats

m

tminmnm

n

−

= ∑∑ +

( ) ( )∑∑−

=

=1

0,,

M

mnmnm

n

tfats

p 15



)()( ,

1

0, tfats nm

M

m nnm∑ ∑

−

=

+∞

−∞=

=

( ) ( )( ) ( )

ceinterferen intrinsic :

*,,

,,,,

*,

0,0

00

00

0000

nmI

nmnmnmnm

nmnmnm dtffaadttfts

(

∫∑∫≠

+=

0,0 nmI(

� Demodulation

� Received signal

� Intrinsic interference is pure imaginary

is orthogonal to the real symbol00 ,nma

( ) ( )∑ ∑−

=

+∞

−∞=

=1

0,,,

M

m nnmnmnm tfahtr

� Demodulation

( ) ( )( ) ( )

ceinterferen intrinsic :

*,,

,,,,,,

*,

0,0

00

00

000000

nmI

nmnmnmnm

nmnmnmnmnm dtffahahdttftr ∫∑∫≠

+=

� Without channel:

OFDM/OQAM in equations

� With channel:� Received signal

⇒Real Orthogonality

Depends on m0,n0

p 16







� Conclusion

p 17



Modified OFDM/OQAM for WONHermitian symmetry

� In WON equivalent baseband model� X(t) is an optical power� It must be real and positive

� In OFDM/OQAM baseband model� Channel input is complex!!

OFDM/OQAM inherits OFDM/OQAM inherits OFDM/OQAM inherits OFDM/OQAM inherits from many properties from many properties from many properties from many properties

related to classical OFDM related to classical OFDM related to classical OFDM related to classical OFDM ��

HermitianHermitianHermitianHermitian Symmetry ?Symmetry ?Symmetry ?Symmetry ?

Due to Fourier transform properties, for classical OFDM, if the discrete input sequence of the modulator has its first and center coefficients null, and if it presents Hermitian symmetry with respect to its center, then the OFDM time modulated signal is real.

Hermitian symmetric conditions could be rewritten for OFDM/OQAM

p 18



Modified OFDM/OQAM for WONHermitian symmetry

� OFDM/OQAM in discrete domain

[ ] [ ]nNkfeiaksD

kN

mi

nm

n

M

mnm −=

−+

∞+

−∞=

−

=∑ ∑ 22

21

0,

πk: instant time coefficientLp: prototype filter length, D=Lp-1N=M/2 is the half number sub-carriers

For all k, s[k] is real if

( )

−

== −−

− otherwise ,1

,0 if ,0

,, nND

nmM

nma

Nma

[ ] [ ]∑ ∑∞+

−∞=

−

=

−+

−=

n

N

m

Dk

N

mi

nmnm eianNkfks

1

1

222

,Re2π

RealRealRealReal Positive ??Positive ??Positive ??Positive ??

p 19



Modified OFDM/OQAM for WONNormalized offset value

� To ensure positivity, an offset (DC) is required

� Discrete channel input will be

DCkskx += ][][

� Transmitted symbols am,n are centered

DCPt =

� We reshape the signal to normalize We reshape the signal to normalize We reshape the signal to normalize We reshape the signal to normalize Pt and and and and DC

� Photodiode discrete output is

( ) ][0][][][ knDCHkhksky ++⊗=

� Offset contribution is removed by subtracting H(0)DC� And, the OFDM/OQAM demodulation is performed

He

rmitia

nH

erm

itian

He

rmitia

nH

erm

itian

symm

etry

symm

etry

symm

etry

symm

etry

No

rma

lized

offse

t valu

eN

orm

alize

d o

ffset va

lue

No

rma

lized

offse

t valu

eN

orm

alize

d o

ffset va

lue

ModifiedModifiedModifiedModifiedOFDM/OQAMOFDM/OQAMOFDM/OQAMOFDM/OQAM

For WONFor WONFor WONFor WON

p 20



Modified OFDM/OQAM for WONSystem performance

-2 0 2 4 6 810

-4

10-3

10-2

10-1

100

Eb/No

BE

R

Modified OFDM, zero CPModified OFDM/OQAM

LOS/WLOS typologyLOS/WLOS typologyLOS/WLOS typologyLOS/WLOS typologyOne direct path…

00Cyclic prefix ∆

3232FFT size 2*N

2.5 m2.5 mDistance d

45°45°FOV

1.5 Gbaud1.5 GbaudSymbol rate

4-QAM4-QAMModulation

IOTARectangularPrototype

IM/DDIM/DDType

OFDM/OQAMOFDM(LOS/WLOS)

Modified OFDM/OQAM and OFDM� Increase the bit rate � Use complex multi level modulations

Modified OFDM/OQAM Is similar to Is similar to Is similar to Is similar to

Modified OFDM, zero CP

p 21



Modified OFDM/OQAM for WONSystem performance

-2 0 2 4 6 8 10 12 14 1610

-4

10-3

10-2

10-1

100

Eb/No

BE

R

Modified CP-OFDMModified OFDM/OQAM

Modified OFDM/OQAM and CP-OFDM

� Mitigate ISI effects � Increase the bit rate

( ) ( )

≤≤=

otherwise ,0

)cos( ,

sin

2 0032

0

FOVt

tFOVth

τττ

DIFF typology, DIFF typology, DIFF typology, DIFF typology, GfellerGfellerGfellerGfeller exampleexampleexampleexampleOne optical reflection…

minimum delay0τ

Modified OFDM/OQAM OutperformsOutperformsOutperformsOutperforms

Modified CP-OFDM

Modified CP-OFDM: CP duration: 25%Same other parameters

Modified OFDM/OQAM Optimal spectral efficiency

p 22







� Conclusion

p 23



Conclusion

� Wireless optical networks WON � A promising alternative to radio networks

� Based on direct detection and OFDM/OQAM� Modified OFDM/OQAM for WON is proposed

� Modified OFDM/OQAM scheme� Fights against ISI problems� Increases the bit rate

� Permits the use of complex QAM modulations

� Benefits from optimal spectral efficiency� Could outperform modified CP-OFDM

p 24



Thank you!

p 25



density = 1, complex orthogonality

density = 2, real orthogonality

τ0

ν0 f

t

ν0 τ0 = 1 /densityν0 τ0 = 1 /density

OFDM/OQAM time-frequency representation� Time-frequency lattice:

QAM symbol(complex)

OQAM symbol(real)

Documents

Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection