1. 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, HUA Thomas Kamalakis, HUA

2. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Outline WLANs - Candidate Technologies Optical Wireless Technology Basic Features System Architecture System Setup & Link Types System Performance Multipath Induced Distortion Future Considerations

3. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics High-Speed WLAN Candidate Technologies or POF OW UWB Wi-Fi 60GHz 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

4. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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

5. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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

6. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Tx/Rx Placement - Link Types Position & Orientation / Configuration ABCD Transmitter x(m)10 ---10 y(m)10 ---10 z(m)330.8 elevation-90 0 +90 0 Receiver x(m) --- y(m) --- z(m)0.3 elevation+90 0

7. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Channel DC Gain Spatial Distribution directed LOS m=2, FOV R =60 0 non-directed LOS / diffuser, FOV R = 60 0 directed non-LOS ρ=0.8, FOVR=60 0 diffuse ρ=0.8 FOV R =60 0 Channel DC gain (×10 -6 ) for different optical link configurations

8. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Modeling Laser Phase Noise  Laser phase noise undergoes a Brownian-motion-type process 1 1 G. Einarsson et. al., “Error Probability Evaluation of Optical Systems Disturbed by Phase Noise and Additive Noise”, IEEE Journal of Lightwave Technology, vol. 13, 1995. Laser phase noise is modeled by summing up a number of discrete "jumps" determined by the random variables μ i.  It has been shown that:

9. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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 10 7 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

10. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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

11. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Transmitted Power Requirements  The results involve 1Gbps data transmission with laser linewidth = 20MHz and FOV R = 60 0  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. 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

12. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Multipath-induced Distortion Modeling The received optical signal envelope may be expressed: H(0): channel DC gain, P t : transmitted optical power, P i and φ i ’ : the optical power and the phase of the interfering components, respectively where: X~N(0,σ 2 ) and Y~N(0,σ 2 ) According to the CLT: And variance given by:

13. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Calculating σ 2 Simulation Parameters for Investigating Multipath Fading Effect P ARAMETER Values Room:(length, width, height) = (x,y,z)(7.5m, 5.5m, 3.5m) Source:mode1 (length, width, height) = (x,y,z)(2m, 4m, 3.3m) (elevation, azimuth)(-90 0,0 0 ) Receiver:(A det, FOV R )(1cm 2, 70 0 ) (length, width, height) = (x,y,z)(6.6m 2.8m 0.8m) (elevation, azimuth)(90 0,0 0 ) 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)

14. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Performance Degradation due to 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 σ 2 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.

15. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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.

16. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics The “COWS” Project Coherent Optical Frequency Division Multiplexing as a means to mitigate for multipath-induced distortion in diffuse optical wireless links…

17. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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

18. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics 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

19. HAROKOPIO UNIVERSITY OF ATHENS - HUA Department of Informatics and Telematics Thanks for Listening!