HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Coherent Optical Wireless Systems for High Speed Local Area Networks with

HAROKOPIO UNIVERSITY OF ATHENS - HUA

Department of Informatics and Telematics

Coherent Optical Wireless Systems for High

Speed Local Area Networks with Increased Resilience

Katerina Margariti, HUAThomas Kamalakis, HUA



Outline• WLANs - Candidate Technologies

• Optical Wireless Technology Basic Features

• System Architecture

• System Setup & Link Types

• System Performance

• Multipath Induced Distortion

• Future Considerations



High-Speed WLAN Candidate Technologies

or POF

or POF

OWUWBWi-Fi60GHz

UWB provides data rates that are limited up to few hundred Mbps and suffers from strong interference

802.11n is limited to 600Mb/s, even when a MIMO configuration is used

60GHz systems promise license-free continuous bandwidth with a spectral space of 5-7GHz. High data rates on the order of a few Gbps



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



System ArchitectureCoherent DetectionAllows a complete representation of the optical field into the electrical domain

It provides intensity, phase, and polarization information from the incoming signal

( ) 2 ( )s LOI t Rreal E E



Tx/Rx Placement - Link TypesPosition & Orientation / Configuration A B C D

Transmitterx(m) 10 10 --- 10y(m) 10 10 --- 10z(m) 3 3 0.8 0.8elevation -900 -900 +900 +900

Receiverx(m) --- --- --- ---

y(m) --- --- --- ---

z(m) 0.3 0.3 0.3 0.3elevation +900 +900 +900 +900



Channel DC Gain Spatial Distribution

directed LOS m=2 , FOVR =600

non-directed LOS / diffuser, FOVR = 600

directed non-LOSρ=0.8, FOVR=600

diffuse ρ=0.8 FOVR=600

Channel DC gain (×10-6) for different optical link configurations



Modeling Laser Phase NoiseLaser phase noise undergoes a Brownian-motion-type process1

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.

10

( ) 2 ( ) 2t N

ii

t d

2 .2

v

Laser phase noise is modeled by summing up a number of discrete "jumps" determined by the random variables μi.

It has been shown that:



Receiver Sensitivity

•Laser phase noise causes the system performance to be degraded

•Evaluating its effect by ignoring all other noise sources

•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)

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.

BER floor as a function of the laser linewidth to system bit rate



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

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…

BER as a function of the received power for different laser linewidth values



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

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.

BER as a function of the transmission power



Multipath-induced DistortionModeling

The received optical signal envelope may be expressed:

'(t)

2

(0) k i

Njj j

t ii

S H Pe Pe

(t)(0) ,kj jtS H Pe X jY

2

2

1

2

N

ii

P

H(0): channel DC gain , Pt : transmitted optical power, Pi and φi’ : the optical power and the phase of the interfering components, respectively

where: X~N(0,σ2) and Y~N(0,σ2)

According to the CLT:

And variance given by:



Calculating σ2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-7

0

1

2

3

4

5

6

7

8

9

10

Time (sec)

Sum

Impu

lse

Res

pons

e (s

ec-1

)

Simulation Parameters for Investigating Multipath Fading Effect

PARAMETER ValuesRoom: (length, width, height) = (x,y,z) (7.5m, 5.5m, 3.5m)Source: mode 1

(length, width, height) = (x,y,z) (2m, 4m, 3.3m)

(elevation, azimuth) (-900,00)

Receiver: (Adet, FOVR) (1cm2, 700)(length, width, height) = (x,y,z) (6.6m 2.8m 0.8m)

(elevation, azimuth) (900,00)

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)

2

2

1 1( )

2 2b

N

ii T

P h



Performance Degradation due to Fading

-30 -25 -20 -15 -10 -5 0 5 1010

-4

10-3

10-2

10-1

100

Transmitted power (dBm)

Bit

erro

r ra

te

with fadingwithout fading

BER as a function of the transmitted power. The results concern LOS configurations associated or not with multipath fading effects. Laser Linewidth = 20MHz.

Introduces a small power penalty.

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.



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.



The “COWS” Project

S/PSubcarrier

Symbol Mapper

IDFT GI

D/A

D/A

Re

Im

LPF

LPF

MZM

MZM 90o

90o

-

-

A/D LPF

A/D LPF

Re

ImDFT

Data Symbol Decision

P/S

(i) DFT Window Sychronization(ii) Frequency Offset Compensation(iii) Subcarrier recovery

LD

LD

PD

PD

PD

PD

Data in

Data out

Digital OFDM transmitter Analog coherent transmitter

Analog coherent receiverDigital OFDM receiver

(Possibly) D

iffuse Channel

Coherent Optical Frequency Division Multiplexing as a means to mitigate for multipath-induced distortion in diffuse optical wireless links…



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



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



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HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Coherent Optical Wireless Systems for High Speed Local Area Networks with