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Challenges in Enabling and Exploiting
Opportunistic Spectrum MANETs
An Industry Perspective
NSF “Beyond Cognitive Radio” Workshop
June 13-14, 2011
Ram Ramanathan
Principal Scientist
BBN Technologies
Cambridge, MA
Cognitive Radio at BBN
• DARPA neXt Generation Communications (XG) [20032005]
– Spectrum Agility: Cross-layer MANET protocols for dynamic
spectrum access
– Policy Agility: Framework for machine-readable policies and
reasoning
• DARPA Wireless Network after Next (WNaN) [2008 - ]
– Large-scale MANET using low-cost cognitive radios
– Multiple wideband (0.9 - 6 GHz) tunable transceivers per node
– Opportunistic Spectrum Access (OSA): dynamic sensing and
allocation, evacuation, reconstitution, policy conformance
– Scalable routing, Disruption tolerant network, content-based…
– 100 node mobile network demonstration last year
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OSA Model
• Primaries and Secondaries
– Secondaries can use spectrum, conforming to interference policy
• Spectrum overlay access
– Sense and transmit on unoccupied frequencies to each of transceivers
– Dynamic, distributed, frequency assignment algorithm
• Evacuate immediately to another frequency if primary is sensed
Primary on f1
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2
1
6
2
1
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2
1
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Primary on f4
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Enabling CR MANETs
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Challenges in Sensing
• Sheer number of channels (up to 1000) to sense
– Detection/classification takes time
– Need to evacuate quickly (200 ms)
– Prioritization, multiple detectors etc.
• Sensing with CSMA/CA
– Need secondary MANET nodes to be “quiet” when sensing
– No dedicated slot like in TDMA
– Detector more sensitive, so large “quiet” radius
• Collaborative sensing
– Team formation, collaboration overhead
– Linking nodes by fiber (static networks)
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Allocation and Evacuation
• Allocation and Use
– Some frequencies are inherently “clearer” than others
– Cross-channel interference cannot be ignored
• choice of frequency to use depends upon frequencies assigned to other
transceivers in same node and neighboring nodes
– Wide range of frequencies => wide range in range!
• Evacuating a frequency
– Jammers vs Primaries
– Coordinating evacuation, ripple effects
• Unified view of primaries, interference, noise
– MAC protocol that simply moves to the clearest frequency and
transmits
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Tools to help enable CR
• Multiple Hypothesis testing
– Dynamic probabilities of multiple hypothesis of primaries on each
channel
– Can be used with cooperative sensing as well
• Artificial Intelligence in CR
– Why: Human engineers cannot predict and design for all
contingencies
– Distributed planning and optimization to organize and coordinate
– Machine learning to observe prior experience and modify behavior
– Machine learning to rapidly update models
– Scenario: Network trained in a desert can learn how to perform
well under water
– BBN work showing promise
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Exploiting CR-enabled
MANETs
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E2E Throughput and Latency
• End to end throughput in excess of
per transceiver data rate via
multiple frequency-orthogonal
paths
S
D
D
– Challenge: Cross-layer solution for
assignment and multipath route
generation
• Full duplex possible
– Challenge: Picking non-interfering
channels, isolation
• Cut-through routing possible
– Can reduce E2E latency dramatically
– Challenge: New radio architecture
Proc
Network
Proc
Proc
MAC/
Link
Physical
Proc
Proc
1011111011101
Queueing
Wait for access
Backoff
Retransmission
Proc
1011011011
Packet
Forwarding
Queuing
Packet
10101101111111
CUT-THRU using
Orthogonal channels
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Make your own topology
• Sensing, allocation, etc. can be controlled by
the network layer
• Throw in power control, beamforming etc… a
general architecture for controlling the
topology with a global viewpoint
Can do
or
Can do
or (
and
if
or
Can’t do
)
Constraints
Network
Layer
Get the topology
that best matches
the need
Create
Topology
Is a controllable
parameter
Negotiate
Routes
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Summary
• Several challenges in enabling a large scale real-world
CR MANET..
– Sensing: scalability, “quieting” in CSMA, collaborative sensing
– Allocation: channel interference, no control channel, evacuation
– Multiple hypothesis and AI
• … but perhaps more challenging is to go beyond and
make sure we fully exploit what we get
– True multipath routing
– Full duplex, cut through
– Topology by design
• Acknowledgments: Chip Elliott, Karen Haigh, Jason Redi
and others at BBN
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