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Optical Wireless Communications

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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 nm) UVB (280 nm nm) UVC (100 nm nm) 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 nm and 185 nm due to the peak emission of the mercury: 85%-90% of the produced UV is at 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 PMT 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 ( nm) 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
Numerical Integration vs. Monte Carlo Model the channel faster than Monte-Carlo method Numerical Integration Monte-Carlo Complexity O (N) O (P) Error Bound O (N -2/3) O (P -1/2) 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, 𝐵𝐸𝑅= 1 2 𝑒 − 𝑛 𝑠 where ns is the mean number of photons received For ns = 11, BER= 10-5! With background noise or dark current: 𝐵𝐸𝑅= 1 2 𝑛=0 𝜏 𝑒 − (𝑛 𝑠 + 𝑛 𝑏 ) ( 𝑛 𝑠 + 𝑛 𝑏 ) 𝑛 𝑛! 𝑛=𝜏+1 ∞ 𝑒 − 𝑛 𝑏 ( 𝑛 𝑏 ) 𝑛 𝑛!

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

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