wireless optical networks with modified ofdm/oqam and direct detection
TRANSCRIPT
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
WON - Transmission model
� 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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
WON - Transmission model
� 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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
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 12
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
)()( ,
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
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 17
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
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 23
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
Thank you!
p 25
Research & Development France Telecom GroupMamdouh El Tabach
Wireless Optical Networks with Modified OFDM/OQAM and Direct Detection
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)