Transcript Coherent Optical Wireless Systems for High Speed Local

HAROKOPIO UNIVERSITY OF ATHENS - HUA
Coherent Optical Wireless Systems for High
Speed Local Area Networks with Increased
Resilience
Katerina Margariti, HUA
Thomas Kamalakis, HUA
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Outline
• WLANs - Candidate Technologies
• Optical Wireless Technology Basic Features
• System Architecture
• System Setup & Link Types
•
System Performance
• Multipath Induced Distortion
• Future Considerations
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
High-Speed WLAN Candidate Technologies
UWB provides data rates that are
limited up to few hundred Mbps and
suffers from strong interference
or POF
802.11n is limited to 600Mb/s, even
when a MIMO configuration is used
or POF
OW
UWB
Wi-Fi
60GHz
60GHz systems promise license-free
continuous bandwidth with a spectral
space of 5-7GHz. High data rates on
the order of a few Gbps
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HAROKOPIO UNIVERSITY OF ATHENS - HUA
Optical Wireless Becomes
“Best Practice”
Networking's future appears to stay focused on achieving higher speeds
Unregulated optical spectrum
Up to 40Gbps transmission rates, reported in literature (experimentally wise) for intensity
modulated signals
Inherently secure LANs (unlike radio signals, do not penetrate walls thus providing a degree
of privacy) Minimizes the requirements for data encryption
IR transmission does not interfere with existing RF systems
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HAROKOPIO UNIVERSITY OF ATHENS - HUA
System Architecture
Coherent Detection
Allows a complete representation of the optical
field into the electrical domain
It provides intensity, phase, and polarization
information from the incoming signal
I (t )  2 Rreal ( Es ELO )
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Tx/Rx Placement - Link Types
Position & Orientation /
Configuration
Transmitter
x(m)
y(m)
z(m)
elevation
Receiver
x(m)
y(m)
z(m)
elevation
A
B
C
D
10
10
3
-900
10
10
3
-900
----0.8
+900
10
10
0.8
+900
---
---
---
---
---
---
---
---
0.3
+900
0.3
+900
0.3
+900
0.3
+900
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HAROKOPIO UNIVERSITY OF ATHENS - HUA
Channel DC Gain Spatial
Distribution
Channel DC
gain (×10-6) for
different optical
link
configurations
non-directed LOS /
diffuser, FOVR = 600
directed LOS
m=2 , FOVR =600
directed non-LOS
ρ=0.8, FOVR=600
Department of Informatics and Telematics
diffuse ρ=0.8
FOVR=600
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Modeling Laser Phase Noise
Laser phase noise undergoes a Brownian-motion-type process1
t
N
0
i 1
 (t )  2   ( )d  2  i
It has been shown that:
2 
v
.
2
Laser phase noise is modeled by summing up
a number of discrete "jumps" determined by
the random variables μi.
1G.
Einarsson et. al., “Error Probability Evaluation of Optical Systems Disturbed by Phase Noise and Additive Noise”, IEEE Journal of Lightwave Technology, vol. 13,
1995.
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Receiver Sensitivity
•Laser phase noise causes the system
performance to be degraded
•Evaluating its effect by ignoring all
other noise sources
BER floor as a function of the laser linewidth to system bit
rate
Acceptable BER performance of 10-3 is obtained for
laser linewidths up to 40 MHz, that are certainly
within the range of commercially available lasers.
•The phase noise BER floor over a range
of laser linewidth to system bit rate
values calculated by our numerical
model and out of 107 transmitted bits
•The results are in good agreement with
those obtained in (Kaiser et al. 1995) for
the standard binary DPSK receiver
Kaiser, C. P., Smith, P. J. and Shafi, M.: An Improved Optical Heterodyne DPSK Receiver to Combat Laser Phase Noise. IEEE Journal of Lightwave Technology 13.
525 – 533 (1995)
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Receiver Sensitivity
Laser linewidths up to 40MHz
can be used for a BER value below
10-3 at -50dBm received power
The same BER performance is
obtained at -60dBm and laser
linewidth of 20MHz
BER as a function of the received power for different laser
linewidth values
An increase in the received
power at -50dBm will allow
almost optimal performance for
the same value of laser linewidth
Low receiver sensitivities can be obtained at 1Gbps
data rates significantly relaxing link budget
limitations…
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Transmitted Power
Requirements
The results involve 1Gbps data
transmission with laser linewidth = 20MHz
and FOVR = 600
Horizontal distance between the
transmitter (or the reflection point for the
hybrid configuration) and the receiver =
1m
BER as a function of the transmission power
As expected, the diffuse topology indicates the worst performance.
For BER = 10-3, 5dB power penalty is observed between the hybrid and the diffuse arrangements.
At higher levels of emitted power the different arrangements exhibit a quite similar BER performance.
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Multipath-induced Distortion
Modeling
The received optical signal envelope may be expressed:
S  H (0) Pt e
jk  j (t)
N
  Pi e
ji'
i 2
H(0): channel DC gain , Pt : transmitted optical power, Pi and φi’ : the optical power
and the phase of the interfering components, respectively
According to the CLT:
S  H (0) Pt e jk  j (t)  X  jY ,
And variance given by:
where: X~N(0,σ2) and Y~N(0,σ2)
1 N
   Pi
2 i 2
2
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HAROKOPIO UNIVERSITY OF ATHENS - HUA
Calculating σ2
Simulation Parameters for Investigating Multipath Fading Effect
Room:
Source:
Receiver:
PARAMETER
(length, width, height) = (x,y,z)
mode
(length, width, height) = (x,y,z)
Values
(7.5m, 5.5m, 3.5m)
1
(2m, 4m, 3.3m)
(elevation, azimuth)
(-900,00)
(Adet, FOVR)
(length, width, height) = (x,y,z)
(1cm2, 700)
(6.6m 2.8m 0.8m)
(elevation, azimuth)
(900,00)
10
9

1 N
1
   Pi   h( )
2 i 2
2 Tb
-1
Sum Impulse Response (sec )
8
2
7
6
5
4
3
2
1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
Time (sec)
0.7
0.8
0.9
1
-7
x 10
Channel impulse response for the examined configuration
(no LOS signal component)
Barry, J.R., Kahn, J.M., Krause, W.J., Lee, E.A.,
Messerschmitt, D.G.: Simulation of multipath
impulse response for indoor wireless optical
channels. IEEE J. Sel. Areas Commun. 11, 367–379
(1993)
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Performance Degradation due
to Fading
0
10
Introduces a small power penalty.
-1
Bit error rate
10
In situations where either the diffuse
component σ2 is more significant or
there is no LOS component measures
should be taken in order to mitigate the
effect of fading , i.e., OFDM or diversity
schemes.
-2
10
-3
10
with fading
without fading
-4
10
-30
-25
-20
-15
-10
-5
0
5
10
Transmitted power (dBm)
BER as a function of the transmitted power. The results
concern LOS configurations associated or not with
multipath fading effects. Laser Linewidth = 20MHz.
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Aknowledgement
The research reported is supported by the “ARISTEIA ΙΙ” Action (“COWS”
program) of the “Operational programme Education and Life Long Learning”
and is co-funded by the European Social Fund (ESF) and the Greek state.
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
The “COWS” Project
Digital OFDM transmitter
Analog coherent transmitter
Re
D/A
Subcarrier
Symbol
Mapper
MZM
GI
IDFT
LD
MZM
D/A
Digital OFDM receiver
90o
LPF
Analog coherent receiver
ns
Le
Re
PD
A/D
Data out
P/S
Data
Symbol
Decision
LPF
-
(Possibly) Diffuse Channel
Im
Le
ns
Data in
S/P
LPF
LD
PD
DFT
Im
PD
A/D
LPF
90o
PD
(i) DFT Window
Sychronization
(ii) Frequency Offset
Compensation
(iii) Subcarrier
recovery
Coherent Optical Frequency Division Multiplexing as a means to
mitigate for multipath-induced distortion in diffuse optical wireless
links…
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
The “COWS” Project
The project aims:
 to provide valuable proof-of-concept on the applicability of coherent optical
detection
 to investigate the performance of various equalization methods such as
OFDM as a means to mitigate multipath dispersion and increase the
transmission rate
 to undertake a thorough investigation of design parameters at a component
and a system level
 to investigate MIMO techniques as a means to improve the overall link
capacity and coverage
 to implement a coherent optical wireless testbed in order to ascertain the
applicability of this technology in real world conditions
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Some Future Considerations…
Modeling the diffuse indoor optical wireless channel
Investigating whether orthogonal frequency modulation (OFDM) will improve the
system performance and mitigate ISI, as expected
Investigating the performance of other equalization schemes
Department of Informatics and Telematics
HAROKOPIO UNIVERSITY OF ATHENS - HUA
Thanks for Listening!
Department of Informatics and Telematics