Prof. M. Brandt-Pearce Lecture 6 Ultraviolet Communications

1. Optical Wireless Communications Prof. M. Brandt-Pearce Lecture 6 Ultraviolet Communications

2. Outline • Introduction • Sources and Detectors • Benefits and Challenges • Applications • Channel Modeling • Modulation Techniques

3. Ultraviolet (UV) Light • UV region is divided into three parts • UVA (315 nm - 400 nm) • UVB (280 nm - 315nm) • UVC (100 nm - 280nm) • Photons in UV region have higher energies, and therefore, large numbers of them are harmful for human health • Large fraction of the UV from the Sun is filtered by the ozone layer in upper atmosphere • The filtered UV light is from 200 nm to 280 nm

4. UV Sources • LED • LEDs are inefficient in UV and have low power (~0.5 mW). • Have to use large arrays as optical sources • Lamps • UV Lamps are cheap and can generate high power • The transmitted beam has a significantly large angle • Are appropriate for networking purposes • Fluorescent lamps without an internal phosphor coating emits UV light • Two peaks at 253.7 nm and 185 nm due to the peak emission of the mercury: 85%-90% of the produced UV is at 253.7 nm • Slow modulation

5. UV Sources • Laser • UV lasers are divided into two type: • UV light is directly generated by the lasing process: these kind of lasers have low output powers • Third or fourth harmonic generation is used to generate UV light from visible light: higher output powers can be achieved, but the size of the laser is large.

6. UV Detectors • APD • APDs are immature for UV technology • Have low gain in UV • Have small aperture (μm’s) • PMT • Have low responsivity for UV, but huge gain • Collects background light from a wide spectrum: an optical filter is required to limit the bandwidth of the incident light • Still the best option for UV communications

7. NLOS Optical Communications • In some situations direct path may not be available. • Therefore, line-of-sight (LOS) optical communication is not possible • Non-line-of-sight (NLOS) communications is the option that would be interesting for these cases. • NLOS optical communication can be easily done when refractive surfaces (buildings, clouds, sea surface) are available • But what if they are not available or reliable? • The solution is optical scattering • In FSO systems, the transmitted optical signal can be scattered in different directions using aerosols and molecules

8. UV for NLOS Communications • Why UV is suitable for NLOS communications? • It has higher atmospheric scattering compared to visible and infrared bands • The scattering is done via molecules and aerosols • The background radiation in the UV range (200-280nm) is low due to the filtered sun light by the ozone layer • Unlike the LOS communications, fog, rain, sand storm, and pollution increase the scattered power and accordingly the received power level

9. Challenges of UV Communications • Limiting Factors for Bit-Rate in NLOS UV Links • Inter-symbol interference (ISI) • The power scattered from any particle inside the common area of the transmitted signal and receiver field of view is received by detector Area in which scattering happens • Scattered energies from particles inside the common volume travels different paths to get to receiver • The energy transmitted at a certain time is received in different times • Transmitted pulses are subjected to a high temporal dispersion • This cause inter-symbol interference (ISI)

10. Ultraviolet (UV) Communications • Limiting Factors for Bit-Rate in NLOS UV Links • By increasing the range, transmitted beam angle, or receiver filed of view, the common volume become larger, and hence, the ISI becomes worse • Received power • Since the power is received via scattering, the channel has a great loss • The detector receives very weak powers • By increasing the range, received power reduces significantly • NLOS UV communication is suitable for short-range links

11. Applications of NLOS UV Communications • Used when line-of-sight communication is not possible • Used for short range (<4km) and low-rate (<5Mb/s) communications • For example: • In urban area as a backup network • For military applications in the battlefield

12. Applications of NLOS UV Communications

13. Link Geometry of NLOS UV Systems Side View Top View

14. Channel Modeling: LOS • In order to get an accurate performance analysis for a LOS UV system, the channel needs to be modeled • Loss is limiting issue • The impulse response is short – no ISI • For LOS channel: • The free space loss at distance r: • Atmospheric attenuation: • Ke: extinction factor • Receiver gain: • LOS path loss:

15. Channel Modeling: NLOS • Methods for calculating the impulse-response and link loss • Simulation methods • Analytical approaches • For small transmitted beam angle and small receiver field of view the received power can be calculated as follows • Received power density at distance r1 • Portion of the power scattered from volume V to receiver is

16. Channel Modeling • Simulation Methods for calculating the impulse-response and link loss • Monte-Carlo (impulse-response and link loss): • In this method one photon in each trial is sent and after simulating the scattering effect if it is in the receiver field of view it is counted as a received photon. • By repeating this trial for many times the ratio between the received photons and transmitted ones determine the channel gain. • Numerical integration (impulse-response and link loss): • The common volume is divided into smaller differential volumes. • The received energy and its corresponding time via each volume is calculated. • The impulse response and link loss are calculated using this differential received energies

17. Numerical Integration for Channel Modeling Transmitter gain profile: Receiver gain profile: Received energy via δV: Total received energy:

18. Numerical Integration vs. Monte Carlo • Model the channel faster than Monte-Carlo method N : Number of volumes in numerical integration P : Number of tries in Monte-Carlo simulation • Able to model the channel in the presence of shadowing

19. Experimental Results • Path loss versus distance, for different Tx and Rx elevation angles G. Chen, et. al. , “Experimental evaluation of LED-based solar blind NLOS communication links”, OPTICS EXPRESS, Vol. 16, No. 19, 2008

20. Experimental Results • Path loss versus Tx elevation angles for different Rx elevation angles G. Chen, et. al. , “Experimental evaluation of LED-based solar blind NLOS communication links”, OPTICS EXPRESS, Vol. 16, No. 19, 2008

21. Modulation Techniques • On-Off Keying (OOK) • For NLOS, UV channel is usually time-variant • Finding optimum threshold may not be easy • Pulse Position Modulation • Does not require threshold to make an optimum decision • Because of the high ISI effect in NLOS UV, is more susceptible to interference • Spectral Amplitude Coding • Can increase the data-rate by providing M-ary transmission • Symbols are spectrally encoded signals • Similar to PPM, does not require a threshold

22. OOK • Since PMTs are used for detection, the receiver is shot noise limited • For shot-noise limited system SNR is: • G : PMT gain • η : detector efficiency • h : Planck’s constant • c: light speed • Pr: received power • R: bit rate • By Gaussian approximation, the BER is

23. OOK • Gaussian approximation may not be valid • Can receive very few photons. • If no background noise or dark current, • where ns is the mean number of photons received • For ns = 11, BER= 10-5! • With background noise or dark current: • +

24. Spectral Amplitude Coding • Transmitter structure • Using a diffraction grating the spectral content of the UV LED or laser is divided into small spectral bins • An encoder mask is used to block or pass the decomposed spectral components

25. Spectral Amplitude Coding • Transmitter structure • If we use harmonic generation as UV source, the spectral encoding can be done in the following form • Encoding done in visible domain (blue or green) • Encoded signal converted to the UV range using a harmonic generation

26. Spectral Amplitude Coding • Receiver Structure • APD based receiver • Let F spectral bins be used to encode the signal • F APDs are used for detection • Each APD detects one spectral component • Decision is made using the outputs of the F APDs • Small aperture size of the APDs limit the FOV • Low gain of APDs limit the receiver SNR

27. Spectral Amplitude Coding • Receiver Structure • PMT based receiver • PMT has much higher gain compared to APD (G≈106) • PMTs have larger aperture size • Two PMTs used for symbol detection • Decoder mask changed M times in each symbol time

28. Numerical Results Maximum attainable bit rate versus the distance for BER of 3×10-5