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OWC Systems
• FSO consists of:
- building-to-building.
- satellite-to-ground.
- ground-to-satellite.
- satellite-to-satellite.
- satellite-to-airborne platforms
(unmanned aerial vehicles [UAVs] or
balloons).](https://image.slidesharecdn.com/visiblelightcommunicationvlcsystems-201226063250/85/Visible-light-communication-vlc-systems-6-320.jpg)
Visible light communication (vlc) systems
- 1. 1 Visible Light Communication (VLC) Systems MEC
- 2. 2 Contents • OWC System Classifications. • Visible Spectrum. • Introduction and Working Principle. • VLC Block Diagram. • Layer Model and Standards. • Modulation Schemes. • Advantages and Disadvantages. • VLC Challenges.
- 3. 3 OWC Systems • Two generic groups of OWC - indoor and outdoor optical wireless communications. • Unlimited bandwidth offered by OWC attributed to different bands - IR, visible (VL) and UV. • Indoor OWC uses IR/VL light for in-building wireless solution. • Indoor OWC systems - four configurations - tracked, diffused, nondirected LOS, and directed line of sight (LOS).
- 5. 5 OWC Systems • Outdoor OWC employs optical carrier to transport information from one point to another over an unguided channel. • OWC technology also known as a free- space optical (FSO) communication system. • FSO operate at near IR frequencies, classified into terrestrial and space optical links.
- 6. 6 OWC Systems • FSO consists of: - building-to-building. - satellite-to-ground. - ground-to-satellite. - satellite-to-satellite. - satellite-to-airborne platforms (unmanned aerial vehicles [UAVs] or balloons).
- 10. 10 OWC System
- 11. 11 OWC System • Wavelength ranges of 780–850 nm and 1520–1600 nm commonly used in current OWC equipments. • Wavelength ranges located in atmospheric transmission windows where molecular absorption is negligible. • Wavelength windows located in the region of four specific wavelengths - 850, 1060, 1250 and 1550 nm experience attenuation of less than 0.2 dB/km.
- 12. 12 OWC System • 850- and 1550-nm transmission windows coincide with standard transmission windows of fiber communication systems.
- 13. 13 OWC System • 1520–1600-nm wavelengths compatible with EDFA technology, helps achieve high power and high-data rate systems. • 1520–1600-nm wavelengths enable transmission of about 50–65 times more average output power than can be transmitted at 780–850 nm.
- 14. 14 VLC System • Addresses challenges such as energy efficiency, bandwidth limitation, electromagnetic radiation, and safety in wireless communications. • Operates in the wavelength range of ~390– 750 nm. • Current enhancement of LED chip design with swift nanosecond-switching times and extensive deployment of LEDs for energy efficiency paves way for visible light communication (VLC) system.
- 15. 15 VLC System • Li-Fi alternative in sensitive or hazardous environments like airplanes, hospitals, and industrial gas production plants where the employment of RF technology is not permitted. • VLC based indoor navigation services offer very high accuracy to within a few cm. • No harmful radiations, no public health concern.
- 16. 16 VLC System
- 17. 17 VLC Transmitter • LEDs and Lasers used as sources for VLC. • Use of white light based on LEDs and wavelength converters. • LED used when both communication and illumination have to be performed using a single device. • Tetra-chromatic, dichromatic and tri- chromatic modes for white light.
- 18. 18
- 20. 20 VLC Transmitter • RGB LED for white light generation - high bandwidth and high data rates. • RGB LED has high associated complexity and modulation difficulties. • Choice of LED based on the channel model.
- 21. 21 VLC Receiver • Amplification circuit, optical filter and optical concentrators. • Beam divergence due to illuminating large areas results in attenuation. • Optical concentrator to compensate for attenuation. • Light detected using a photodiode in a stationary receiver - silicon photodiode, PIN diode or avalanche photodiode used. • Converted to photo current.
- 22. 22 VLC Receiver • Imaging sensors employed instead of photodiodes in the case of mobility. • Operating imaging sensors energy expensive and slow, hence a trade-off between cost, speed and complexity. • Vulnerable to interference from other sources such as sunlight and other illumination. • Optical filters to mitigate DC noise components.
- 23. 23 VLC Receiver
- 25. 25 VLC Architecture • Two integral parts of a VLC system - transmitter and receiver. • Layered architecture of three common layers - Physical Layer, MAC Layer and Application Layer. • IEEE 802.15.7 defines only two layers (PHY and MAC) for simplicity.
- 27. 27 MAC Layer Tasks • Mobility support. • Dimming support. • Visibility support. • Security support. • Schemes for mitigation of flickering. • Color function support. • Network beacons generation if the device is a coordinator. • VPAN disassociation and association support. • Providing a reliable link between peer MAC entities.
- 29. 29 Physical Layer • Provides: - physical specification of device. - relationship between the device and the medium. System Model
- 30. 30 Physical Layer • Input bit stream passed through the channel encoder. • Linear block codes, convolutional codes and turbo codes used to enhance VLC system performance. • Channel encoded bit stream passed through line encoder to yield encoded bit stream.
- 31. 31 Physical Layer • Modulation (ON–OFF keying, PPM and PWM, etc.) performed. • Finally, data drives LED for transmission through the optical channel. • Wavelength Division Multiplexing (WDM) and Subcarrier Multiplexing (SCM) for bi- directional transmission. • Orthogonal Frequency Division Multiplexing (OFDM) and Quadrature Amplitude Modulation (QAM) to increase data rate.
- 32. 32 Modulation Schemes • Two factors to be considered in the design of the modulation scheme for VLC : (a) dimming and (b) flickering. • Non-linear relationship between measured light and perceived light.
- 33. 33 Measured vs Perceived Light by Human Eye
- 34. 34 Modulation Schemes • Changes in brightness of modulated light should not result in human-perceivable fluctuations. • IEEE 802.15.7 - switching to be done at a rate faster than 200 Hz to avoid harmful effects.
- 35. 35 Modulation Techniques • On-Off Keying. • Pulse modulation. - PWM. - PWM with Discrete Multitone. - PPM. - Multipulse PPM. - Expurgated PPM (EPPM). - Multilevel EPPM (MEPPM). • Color Shift Keying.
- 36. 36 On-Off Keying • LEDs turned off and on according to bits in the stream • LED not turned completely off in the off state, but reduction in intensity level. • Easy implementation. • Done using white LEDs (a combination of blue emitter and yellow phosphor). • Low bandwidth due to slow time response of the yellow phosphor.
- 37. 37 On-Off Keying • Data rate of upto 10Mbps using NRZ OOK with a white LED. • Combination of analogue equalization with blue filtering done to increase data rates up to 125 Mbps and 100 Mbps. • Limitation of OOK low data rates motivated researchers to develop new modulation techniques.
- 38. 38 Pulse Modulation Techniques • PWM – pulse width varied according to dimming levels. • Using high PWM frequency, different dimming levels achieved between 0% and 100%. • Limitation of PWM - low data rate upto 4.8 Kbps. • PWM combined with Discrete Multitone (DMT) for joint communication & dimming control with higher data rates.
- 39. 39 Pulse Modulation Techniques • PPM based on position of the pulse. • Division of symbol duration into equal intervals, many slots, transmission of pulse done in any of the slots. • PPM suffers from low data rate, other variants of PPM developed. • Multi-pulse PPM (MPPM) - transmission of multiple pulses in each symbol-time, more spectral efficiency.
- 40. 40 Pulse Modulation Techniques • Expurgated PPM (EPPM) - improved performance of peak-power limited M-ary communication systems. • Spectral efficiency of MPPM and EPPM less than 1. • Multilevel EPPM (MEPPM) for spectral effectiveness.
- 41. 41 Comparison of PPM Techniques
- 42. 42 Color Shift Keying (CSK) • Enhanced data rates. • Utilizes three separate LEDS - Green, Blue and Red to produce White Light. • Modulation using intensity of three colors in an RGB LED source. • CSK depends on the color space chromaticity diagram. • Maps all colors perceivable by eye into two chromaticity parameters x and y.
- 44. 44 VLC Advantages Huge Bandwidth: - unlimited and unlicensed bandwidth. - 380 nm to 780 nm. - VLC 350 THz support multi-gigabit-per- second data rates with LED arrays in a multiple-input multiple-output (MIMO) configuration. - alternative to indoor IR that operates at 780–950 nm.
- 45. 45 VLC Advantages Low Power Consumption: - provides both communication and lighting, at Gbps data rates. - consume low power compared to costly RF alternatives. Low Bandwidth: - inexpensive components, compact, light weight, amenable to dense integration, very long lifespan.
- 46. 46 VLC Advantages - large unlicensed optical spectrum. - lower power-per-bit cost compared to RF communications. - cheaper. No health concerns: - no generate radiation that leads to public health concern.
- 47. 47 VLC Advantages - lowers carbon dioxide emission. - little extra power consumption for communication. Ubiquitous Computing: - wide range of network connectivity. - may incorporate luminous devices like traffic signs, commercial displays, indoor/outdoor lamps, TVs, car head lights/tail lights.
- 48. 48 VLC Advantages Inherent security: - high security. - highly intricate for a network intruder outside to pick up the signal. - alternative technology in sensitive or hazardous environments. Indoor localization: - existing RF-based global positioning system (GPS) gives inadequate/no network coverage.
- 49. 49 VLC Advantages - high attenuation, multipath, and safety regulation, accuracy of only up to a few meters for the RF-based GPS. - VLC-based indoor positioning to attend to issues in enclosed environments. - high accuracy to within a few cm. - indoor localization system using the white LEDs.
- 50. 50 VLC Advantages - LEDs give better light source more than 400 lux. - LEDs have longer lifespan, ecological and financial benefits. - high-speed data transmission. - simultaneous employment of light sources for data communication as well as illumination.
- 51. 51 Challenges • Flicker mitigation: - Flicker: variation in the brightness of light perceived by human naked eye. result of continuous switching on and off of the light source during data transmission. can instigate negative/harmful physiological changes in humans.
- 52. 52 Challenges • Flicker prevented by making changes in brightness to be within the maximum flickering time period (MFTP). • MFTP - maximum time period within which the light intensity can be changed without any perception by the human eye. • Modulation formats for flicker mitigation. • IEEE 802.15.7 standard proposes variable pulse position modulation (VPPM) for VLC system.
- 53. 53 Challenges • Dimming support: • Variable pulse position modulation (VPPM) for VLC system for ability to control dimming. • VPPM integrates PPM and PWM to support communication with dimming control.
- 54. 54 Challenges • High path losses. • Multipath induced intersymbol interference (ISI). • Artificial light-induced interference. • Blocking. • LED electro-optic response nonlinearity. • Interference between VLC devices. • Integration with existing technologies.
- 55. 55 VLC Standardisation • Standardisation to tackle challenges. • Performed by Visible Light Communication Consortium (VLCC), Japan and IEEE. • Japan Electronics & Information Technology Industries Association (JEITA) CP-1221, JEITA CP-1222 and JEITA CP- 1223 published by VLCC. • IEEE 802.15.7 standard for physical and MAC layers - minimum benchmark for development of new products.
- 56. 56 Why Standardisation • Providing access to several hundred THz bands. • Providing immunity against EMI. • Communication that complements extra services to the existing visible light infrastructure. • Specifying FEC schemes, modulation techniques and data rates for VLC communication.
- 57. 57 Why Standardisation • Channel access mechanisms such as Contention Access Period (CAP), Contention-Free Period (CFP) and visibility support when channel access described. • PHY layer specifications, such as optical mapping, Tx-Rx turn around time, Rx-Tx turn around time and flicker and dimming mitigation explained.