The Symbiosis of Cognitive Radio and
WMNs
from “Guide to WMNs” by Sudip Misra and et al,
2009
Myungchul Kim
[email protected]
Atstract
– CR provides WMNs with additional bandwidth and
improved efficiency
Introduction
• Unlicensed spectrum: cordless phones, remote controls, and
even microwave ovens
• Obtaining additional spectrum is very difficult
• More efficient use of available bandwidth?
Background
• Radio communication
– The wave attenuates, reflects, and refracts.
– Signal-to-noise ration (SNR), signal-to-interference-plusnoise (SINR)
– The radio spectrum can be shared along three dimensions
– frequency, time, and space
• Spectrum allocation
– Frequency division
• FCC, CRTC, KCC
• ITU, ITR-R
– Spectrum auction
Background
• Spectrum usage
– new spectrum increasingly scarce
– Spectrum is vastly under-used: 5.2% usage
• Change
– “command and control” approach to spectrum regulation
Cognitive radio
• What is Cognitive Radio?
– By J. Mitola, 1999
– Describe a device that used its awareness of its
environment to intelligently choose the best parameters to
use for its own communications
– Key factors
• An incredible boom in wireless networks and devices
• Increased interest in wireless research
• Software Defined Radio (SDR) allows the behavior of the
radio to be controlled by software rather than in fixed
hardware and allows a device to switch between different
network technologies using a single physical radio. SDR
focuses on specifying architectures and the wired interface,
an important component for building CR devices.
Cognitive radio
• Key characteristics of a Cognitive Radio
– Advanced interoperability
• Antenna technologies: antenna arrays, MIMO, and
adaptive beam-forming
• UWB using the medium without adding significantly to
overall interference levels
• At least the first generation of CR devices will have to
coexist with existing noncognitive wireless technologies
– Frequency agility
• Being able to dynamically adjust the frequencies and
bandwidth of their transmissions
• A CR will require a much better ability to detect different
types of transmissions, including those spread over a range
of frequencies.
Cognitive radio
• Key characteristics of a Cognitive Radio?
– Awareness
• The ability to detect transmissions
• The ability to sense and measure channel conditions
throughout the spectrum
• Awareness of its hardware, applications, user
characteristics and its goals
– Cognition
• The CR must decide what transmission must occur – to
whom, on what network – and when and how the
transmission will occur.
– Collaboration
• The CR must consider the interactions between not only
different nodes in ins network, but in all networks.
Cognitive radio
• How CR changes spectrum management
– Open Spectrum Policy
– Researchers have proposed different methods for CR
devices to use the same licensed frequencies
– IEEE 802.22
• Share the frequencies occupied by broadcast television
– A CR uses Guard bands of TV bands
– CR can communicate at transmission powers and ranges that
are low enough to avoid interfering with TVs.
– If a CR can determine when and where there are no users of
the primary technology, it may be able to make full use of
spectrum
Cognitive radio
• How CR changes spectrum management
– Shorter-term licenses (and more frequent auction),
licenses allowing for secondary cognitive use while
maintaining primary user rights and priority, and a fully
dynamic spectrum market.
– Ability to buy, sell, trade, or lease spectrum rights
Applying CR to a WMN
• WMN characteristics
– The access and transit links to operate over separate
wireless interface and on different channels
– Transit network
•
•
•
•
Openly shared wireless medium
Multihop forwarding
To or from the gateway
Directional antennas and multiple channels -> multiple
links
Applying CR to a WMN
• Benefits of CR to WMNs
– Providing additional bandwidth
• Transient frequency holes could be used by the WMN
– Rebalancing the access and transit network bandwidth
• If both the access and transit links use the same
technology?
• Multichannel MAC
• Multiple interfaces
– Changing the nature of gateways
• WiMAX as a gateway
• Multiple gateways
– Multiple user technologies
• Legacy devices
CR research
– Find ways to allow intermediate technological advances
to enter mainstream use before the completion of a fully
capable CR
• Transmission
– How much usable information can be encoded on a
channel using different techniques
– UWB, MIMO, directional antennas, frequency agility
• Awareness
– A huge volume of environmental data collected and
shared
– Spectrum sensing is difficult when wide spectrum is
considered
– Dynamic nature of the wireless medium
CR research
• Awareness
– Radio Knowledge Representation Language (RKRL)
– Interference Temperature: a metric for estimating the
cumulative interference energy at a receiver -> “not a
workable concept” by FCC
– Virtual cubes with time, frequency and power
– Active and passive (receiver only): user locations and
characteristics
– Network loads and application conditions
CR research
• Sharing information
– Different nodes will have a different set of collected
knowledge in location, configuration, and history.
– The system must be able to control or limit the exchanged
information
– Cf. sensor networks: filtering information
• Decision making
– Conventional wireless interface: linear, modeled on
protocol stack
– CR interface: nonlinear, all parts of the communication
must be decided through cross-layer protocols
CR research
• Decision making
– CR utilizes all available information in all
communication-related decisions
– What to send
• Communicate now or wait later
• Adjust the traffic according to the network condition
– Where to send it
• Vertical handoff
• Different networks for different traffic types
• Routing in multihop networks
CR research
• Decision making
– How to send
• After where to send, choose the appropriate parameters
based on the current environment
– When to send
• The MAC determines when to access the medium
• Choose to transmit now or delay
Thoughts for practitioners
• Regulatory agencies
– On what time-scale should spectrum be allocated
– Should a centralized or a distributed approach to be used
• For very short leases, the fully centralized system of a
single regulatory body
• Real-time spectrum auction
• Spectrum could be allocated for single flows or even single
packets
– Should access be scheduled or contention-based
– Secondary spectrum market
• Cellular provider
– Dynamic spectrum necessitates an integrated enforcement
solution
Directions for future research
• Static core topology
– For CR, the fixed network changes the problem of
collecting awareness of the network’s surroundings
• Spectrum information collection
– The WMN presents a distributed infrastructure to collect
spectrum data at a large number of locations
– CR devices act as sensors to gauge interference levels
• Traffic awareness
– Fairly easy to obtain from the gateway
Directions for future research
• Data distribution and decision making
– Within the mesh itself
• Spectrum monitoring and policing
– Primary spectrum rights should be protected
– The WMN may be able to collaborate to detect users and
determine the location of illegal transmissions