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