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February 17 Wireless CommunicationsThe Maryland Optics Group
HYBRID FSO/RF LINKS AND NETWORKS WITH DIVERSITY
CONTROL Christopher C. Davis
The Maryland Optics Group Department of Electrical and Computer Engineering
University of Maryland, College Park, MD 20742
February 17 Wireless CommunicationsThe Maryland Optics Group
RESOLUTION IN NEAR-FIELD
ACKNOWLEDGEMENTS
• Dr. Stuart D. Milner – Department of Civil and Environmental Engineering
• Dr. Igor Smolyaninov, Department of Electrical and Computer Engineering
• Dr Quirino Balzano, Department of Electrical and Computer Engineering
• Professor Kyuman Cho (Sogang University, Seoul, KOREA)
• Pam Clark, ITT• Linda Wasiczko, Sugianto Trisno, Jaime Llorca, Tzung-
Hsien Ho, Heba El-Erian, Aniket Desai, Clint Edwards, (graduate students)
• AFOSR, DARPA,NSA, ARL, Army CECOM
February 17 Wireless CommunicationsThe Maryland Optics Group
WI-FI
• The current “hot topic”
• Its growing popularity will cause its demise– Spectral overcrowding– Lack of security– Interference with other users and equipment– Remember CB radio?
• But… if you are mobile you can’t be connected by wires
February 17 Wireless CommunicationsThe Maryland Optics Group
Dynamic, Reconfigurable
Hybrid FSO/RF Wireless Networks
Modified from a TeraBeam picture
February 17 Wireless CommunicationsThe Maryland Optics Group
Hybrid FSO/RF Wireless Networks – WHY?
• RF wireless networks– Broadcast RF networks are not scaleable– RF cannot provide very high data rates– RF is not physically secure
• High probability of detection/intercept
– Not badly affected by fog and snow, affected by rain
• Optical wireless networks– Very high data rates
• 2.5Gb/s commercially available
• 1Tb/s demonstrated
– Almost zero probability of detection/intercept– Affected by fog and snow
February 17 Wireless CommunicationsThe Maryland Optics Group
Hybrid FSO/RF Wireless Networks – WHY?• Deal with the non-acceptance of optical wireless
alone• High availability (>99.99%)• Much higher goodput than RF alone• Last/First Mile Solution• FSO is not regulated by the FCC
– must be eyesafe
• For greatest flexibility need unlicensed RF band• Installed optical fiber – up to $1M/mile
February 17 Wireless CommunicationsThe Maryland Optics Group
0 10 20 30 40 50 60 70 80 90 100
FSO LINK AVAILABILITY (%)
0
1
2
3
AVERAGE DATA RATE (Gb/s)
AVERAGE DATA TRANSFER RATE OF HYBRID FSO/RF LINK
FSO 2.5Gb/s
RF 10Mb/s
A Hybrid FSO/RF Link Handles Weather
A Hybrid FSO/RF Network Involves Disparate Data Rates
February 17 Wireless CommunicationsThe Maryland Optics Group
Challenges and Developments
• FSO is available commercially– has not been widely accepted– most systems do not do pointing, acquisition,
and tracking (PAT)– most systems are not FSO/RF Hybrids
• FSO/RF Hybrid networks are in the R&D stage
• High performance PAT must be developed
February 17 Wireless CommunicationsThe Maryland Optics Group
Challenges and Developments (2)• Many applications of FSO/RF networks
involve dynamic situations– Reconfigurability (topology control) is required– Diversity of links (transmitter and receivers)– Changeover algorithms– Network optimization
• DoD applications
February 17 Wireless CommunicationsThe Maryland Optics Group
TerrestrialNear-Surface,Surface, and
Subsurface User Systems
Optical Links+
Peer-to-Peerand
Base-Station RF “subnets”
Airborne Free Space Optical
Backbone
Figure 1. Deployable THOR NetworkFigure 1. Deployable THOR Network
February 17 Wireless CommunicationsThe Maryland Optics Group
DYNAMIC AND VOLATILE ATMOSPHERICAND PLATFORM EFFECTS
February 17 Wireless CommunicationsThe Maryland Optics Group
OPTICAL WIRELESS TRANSCEIVER
February 17 Wireless CommunicationsThe Maryland Optics Group
OMNIDIRECTIONAL OPTICAL WIRELESS TRANSCEIVER
February 17 Wireless CommunicationsThe Maryland Optics Group
Topology Control
in Optical Wireless Networks
Network Layer
Link Layer
Physical Layer
Topology Control•Autonomous Backbone Reconfiguration•Pointing, Acquisition and Tracking
February 17 Wireless CommunicationsThe Maryland Optics Group
Pointing, Acquisition, and Tracking
in Optical Wireless Networks
• Allows wireless links to be established and maintained between moving platforms
• Maintains alignment of optical wireless links
• Required for autonomous reconfiguration and topology control in optical wireless networks
February 17 Wireless CommunicationsThe Maryland Optics Group
Agile Optical Wireless Transceiver and Motorized Platform
Data rate: 155Mb/s High speed (800K
steps per second), resolution and pointing accuracy up to 0.00072° per step
Fish-eye lens (180°) used to identify and track neighbor nodes (beacons)
February 17 Wireless CommunicationsThe Maryland Optics Group
Mono-static
Advantages: Reduces the complexity of PAT process
Disadvantages: Power isolation problem (TX/RX feedback)
Bi-static
Advantages: No power isolation problem
Disadvantages:
1. Extra alignment process required to obtain parallel axes
2. Potential misalignment in short-distance application
Bi-Static Transceiver Design
February 17 Wireless CommunicationsThe Maryland Optics Group
Link Failures between 2 Transceivers
For large application distance
For short application distance
February 17 Wireless CommunicationsThe Maryland Optics Group
PAT Process
Object disappears
Acquisition Process
Tracking Process
TransceiverAxis
AlignmentStep
Select the desired target from the CCD
image
SystemScanning
Step
Using TCP/IP socket to check link availability
Link TableUpdate
Record the current [θ,φ] into the link table
Object still exists
TrackingProcess
Motion Prediction Analysis(Track beacon)
February 17 Wireless CommunicationsThe Maryland Optics Group
Experimental Setup
1. Study the performance of the link with respect to link closure latency for different motor parameters
2. To investigate the effects of larger FOV of our system
February 17 Wireless CommunicationsThe Maryland Optics Group
FEATURES OF OUR CURRENT OPTICAL WIRELESS SYSTEMS
• Bistatic TX/RX systems• 1.3m and 1.55m transmitters• CPC and lens based receivers• Fast aspheric lens receivers• Cassegrain and Fresnel lens receivers• Rugged alignment stages• Topology control
February 17 Wireless CommunicationsThe Maryland Optics Group
OUR NEW CONCEPTS AND THEIR IMPACT
• Maximally efficient use of high data rate FSO and RF communication modes
• Network and link recovery everywhere through communication mode diversity and autonomous Physical and logical reconfigurability
• Reduced GTT due to instantaneous network recovery • Physical reconfigurability assures > 99% availability
– Higher optical availability increases MDR• Seamless diversity control between optical and RF
communication• Internet-like software fully portable to DoD systems• Network software is independent of terminal design
specifics
February 17 Wireless CommunicationsThe Maryland Optics Group
INNOVATION
• Intelligent Aperture Diversity and Media Controller – “Smart” identification of RF/FSO availability at each RX/TX– Dynamic allocation of FSO/RF
• Autonomous physical and logical reconfiguration – “Make before break” dissemination of topologies using high availability
RF control channel • Enhanced TCP/IP protocol suite for Hybrid FSO/RF Networks
– Multi-Protocol Label Switching (Traffic Engineering) exploits media diversity
– Proxy software provides instantaneous reaction to physical change in topology
– Autonomous reconfigurability integrated with TCP/IP suite • Comprehensive network modeling and simulation
– Advanced atmospheric propagation modeling (turbulence, aerosols, obscuration)
– Discrete Event Simulation for Hybrid Networks to aid implementation planning
February 17 Wireless CommunicationsThe Maryland Optics Group
BACKUP SLIDES
February 17 Wireless CommunicationsThe Maryland Optics Group
TX TX
RXRX
Bistatic optical wireless link
February 17 Wireless CommunicationsThe Maryland Optics Group
The DARPA ORCLE PROGRAM(formerly THOR Program)
• Long range (up to 100km) high altitude (10km) laser communication links
• Rytov variance is 2
lnI Ranges from 10 to 100
• Small Cn2, but long range makes this a strong
turbulence situation• May be strong boundary layer turbulence at
transmitter and receivers
6/116/722ln 23.1 LkCnI =
February 17 Wireless CommunicationsThe Maryland Optics Group
Many Link Physics and Engineering Issues
• Turbulence– Variations with height
• Obscuration– Optical depth– Spatial distribution
• Aerosols
• Aperture averaging
• Transceiver optimization
February 17 Wireless CommunicationsThe Maryland Optics Group
1.00e-18 1.00e-17 1.00e-16 1.00e-152 3 4 5 6 2 3 4 5 6 2 3 4 5 6
Cn2 m-2/3
100
101
102
2
3
4567
2
3
4567
2
3
4567
Rytov Variance
RYTOV VARIANCE FOR A 100km LINK
1.3micrometer laser