According to the Internet Society, over 80% of the world will be connected to the Internet by 2020. Mobile and application services are the future of the Internet. 3G: 2 Mb/s 4G: designed for 1Gb/s 4G speed in ATT and Verizon is 10 Mb/s Demand for High-speed Communications
5 Demand for High-speed Communications
6 Optical Communications: The Backbone of Telecommunications Optical fibers around the world
7 Free Space Optical (FSO) Communications
8 History of FSO Communications Has been used for thousands of years in various forms Around 800 BC, ancients Greeks and Romans used fire beacons for signaling In 1880 Alexander Graham Bell created the Photophone by modulating the sun radiation with voice signal German troops used Heliograph telegraphy transmitters to send optical Morse signals for distances of up to 4 km at daylight (up to 8 km at night) during the 1904/05 The invention of lasers in the 1960s revolutionized FSO communications Transmission of television signal over a 30-mile using GaAs LED by researchers working in the MIT Lincolns Laboratory in 1962 The first laser link to handle commercial traffic was built in Japan by Nippon Electric Company (NEC) around 1970
9 History of FSO Communications Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
10 Spectrum is scarce and low bandwidth Spectrum is regulated Suffers from multi-path fading Susceptible to eavesdropping Large components Why Free Space Optics (FSO)? FSO vs Radio-Frequency (RF) RF A single FSO channel can offers Tb/s throughput Spectrum is large and license free (very dense reuse) Small components Secure Transmission range limited by weather condition Are very difficult to intercept FSO
11 High cost Requires permits for digging (Rights of Way) Trenching Time consuming installation Mobility impossible FSO vs Fiber Optic No permits (especially through the window) No digging No fees Faster installation Mobility/reconfigurability possible Fiber Optic FSO Why Free Space Optics (FSO)?
12 Access Network Bottleneck Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
13 Bandwidth capabilities for a range of optical and RF technologies Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
14 1010 DATA IN LED/LD DRIVER PHOTO DETECTOR SIGNAL PROCESSO R DATA OUT ATMOSPHERIC CHANNEL TRANSMITTER RECEIVER FSO Block-Diagram 1 Network traffic converted into pulses of invisible light representing 1’s and 0’s 2 Transmitter projects the carefully aimed light pulses into the air 5 Reverse direction data transported the same way. Full duplex 3 A receiver at the other end of the link collects the light using lenses and/or mirrors 4 Received signal converted back into fiber or copper and connected to the network
15 Challenges Sunlight Building Motion Alignment Window Attenuation Fog Scintillation Range Obstructions Low Clouds
16 850 nm1550 nm Challenges Visible range
17 Power Spectra of Ambient Light Sources Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
19 Uncoated glass attenuates 4% per surface due to reflection Tinted or insulated windows can have much greater attenuation Possible to trade high altitude rooftop weather losses vs. window attenuation Window Attenuation
20 Small Angles - Divergence and Spot Size 1 mrad 1 km 1 m Small angle approximation: Angle (in milliradians) * Range (km)= Spot Size (m) DivergenceRangeSpot Diameter 0.5 mrad2.0 km~1 m (~40 in) 2.0 mrad1.0 km~2.0 m (~6.5 ft) 4.0 mrad (~ ¼ deg)1.0 km~4.0 m (~13.0 ft) 1° ≈ 17 mrad → 1 mrad ≈ 0.0573° Alignment
Building Motion TypeCause(s)MagnitudeFrequency Tip/tiltThermal expansionHighOnce per day SwayWindMediumOnce every several seconds VibrationEquipment, door slamming, etc.LowMany times per second Building Motion Due to the Thermal Expansion 15% of buildings move more than 4 mrad 5% of buildings move more than 6 mrad 1% of buildings move more than 10 mrad 21 Alignment Challenges
22 1.Automatic Pointing and Tracking – Allows narrow divergence beams for greater link margin – System is always optimally aligned for maximum link margin – Additional cost and complexity 2.Large Divergence and Field of View – Beam spread is larger than expected building motion – Reduces link margin due to reduced energy density – Low cost Compensating for Building Motion – Two Methods 0.2 – 1 mrad divergence = 0.2 to 1 meter spread at 1 km 2 – 10 mrad divergence =2 to 10 meter spread at 1 km Alignment
23 Modulation Method
24 Noise in FSO Systems Background Radiation (e.g. sun light) Shot Noise (Poisson distributed) Thermal Noise (Gaussian distributed) Scintillation Noise