February 2008 1
Cognitive Radio: A Survey
Patrick Mitran
University of Waterloo
February 2008 2
Apologies
Apologies for all the results/aspects omitted from this talk.
February 2008 3
Outline
● Why Cognitive Radio?● Spectrum Sharing
– Interference Management● Overview of IEEE 802.22● Research Challenges● Conclusion
February 2008 4
Spectrum Licensing
● Most governments consider the electromagnetic spectrum to be a public resource.– It is usually allocated by a governmental organization
(FCC, CRTC, ETSI, ARIB, etc.)– The spectrum is licensed to users to further the public
good, e.g., radio, television, etc.
● Examples of licensing– TV channels, radio– cellular service – Unlicensed “free for all”, subject to some constraints
(eg., 900 MHz cordless phones, 2.4 GHz wireless WiFi).
February 2008 5
Spectrum Usage
● Actual measurements by the FCC have shown that many licensed spectrum bands are unused most of the time.– In NYC, spectrum occupancy is only 13.1% between
30 MHz – 3.0 GHz.
Spectrum usage in700 MHz below1 GHz.
AtlantaNew OrleansSan Diego
February 2008 6
Spectrum Usage
● Actual measurements by the FCC have shown that many licensed spectrum bands are unused most of the time.– In NYC, spectrum occupancy is only 13.1% between
30 MHz – 3.0 GHz.
● Good quality spectrum is scarce.
● Is the best use to license spectrum for exclusive use to a single primary user? (currently dominant)
Cognitive radio is the technology that follows if you think there are opportunities here.
February 2008 7
Spectrum Sharing
● There are 3 ways to share spectrum– Exclusive access– Horizontal sharing– Vertical sharing
● Exclusive access is currently the dominant method.– Only the primary user (if at all) can access the spectrum.
February 2008 8
Spectrum Sharing
● Horizontal sharing
– All systems have the same regulatory status and may access spectrum on equal footing.
– Different systems may operate in the same band without coordinated spectrum access (interference).
– 900 MHz unlicensed band (cordless phone)– 2.4 GHz unlicensed band (WLAN, bluetooth)– Have generated a large amount of social impact.
February 2008 9
Vertical Sharing
● When one talks about cognitive radio, one usually means vertical sharing.– The primary user pays good money for (primary) access to
the spectrum.– However, if the primary user does not use it, should the
secondary be disallowed from access?– What if the secondary causes “negligible” interference?
● In cognitive radio,– A secondary user can only borrow spectrum if it does not
generate “harmful” interference.– Can also take advantage of horizontally shared spectrum.
February 2008 10
A Definition of CR
● A cognitive radio is an unlicensed communication system– that is aware of its environment– learns from its environment– adapts to the statistical variations of its environment
● and uses these to– achieve reliable communication and spectral efficiency
by employing spectral holes or opportunities and does not generate harmful interference.
● Why is it called “cognitive” radio?– due to sensing requirements to avoid harmful interference
and exploit spectral holes.
February 2008 11
Device Capabilities
● Cognitive Radios will be complex devices.
● Regardless of how they mitigate interference to primary users, it is desirable to be able to perform– Spectrum estimation/hole discovery– Channel state estimation– Power control– Dynamic spectrum management
February 2008 12
Software Defined Radio
● Software Define Radios (SDRs) are a natural candidate (enabling technology) for implementing cognitive radios technology.– Cognitive radio first proposed as an application of SDR by
Mitola.
● Easily reconfigurable in software– If you move your device to a new location, the device
can be reprogrammed based on that locations specific regulations, primary behavior characteristics.
– Can learn new modulation schemes as technology of primaries may change in time.
● “future proof”
February 2008 13
Harmful Interference
● What is harmful interference?– Ultimately depends on the application.
● There are generally two common approaches to avoid harmful interference:– Interference avoidance (spectrum overlay)– Interference control (spectrum underlay)
(overlay) (underlay)
February 2008 14
Interference Avoidance● In principle, interference avoidance involves only two
steps:– Look for holes in spectrum/time.– Transmit only in those bands at those times.
● Sounds a lot easier than it is.– Detection of spectral holes is difficult due to the large
range of potential modulation/coding schemes.– Hidden terminal problem.– Fast detection time needed.– Careful measurements based on actual primary signal
statistics to design signatures is required.
February 2008 15
Interference Avoidance
● Some methods to detect holes:
– Energy detection. – Matched filter detection.– Cyclostationary detection.– Wavelet detection.– Cooperative spectrum sensing and data fusion.
February 2008 16
How to Use holes?● Suppose that after some sophisticated signal
processing, we determine that spectrum occupancy is:
● How do we use these (noncontiguous) holes?– Could have a library of schemes, each with different
rates/bandwidth● Doesn't seem to scale / High complexity.
February 2008 17
OFDM● OFDM based approach solves the problem naturally
● OFDM has the advantages that– It is low complexity (FFT and IFFT based)– Can be naturally adjusted to fit almost any configuration
of spectral holes.– Is growing in popularity (802.11a, 802.16, 802.22)
February 2008 18
IEEE 802.22: A Cognitive WRAN● 802.22 is a standard in development for Cognitive
Wireless Regional Area Networks– WG started in Nov. 2004. No standard yet.– Motorola, Phillips, Samsung and Thomson are among the
industry participants.
● Target Market– Make use of vacant TV broadcast band– Provide wireless broadband access to rural and remote
areas, low density.– Scale to serve denser areas if spectrum availability permits– Data, VoIP, audio/video, QoS.– Residential, small/home office, small building, campuses– Fixed Wireless Access (FWA)
February 2008 19
Deployment Scenario
Figure courtesy 802.22 WG.
TV TransmitterWRAN
Base Station
: CPE
: WRAN Base Station
Typical ~33kmMax. 100km
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Technical Requirements● 33 km range (typical)
– Based on 4 Watt CPE, 50% location availability, 99.9% time availability.
– Max coverage of 100 km.– 15 Bits/sec/Hz spectral efficiency.
● Operates in UHF/VHF TV broadcast band– Typically 54698 MHz in US (channels 251).– Extremes of 47910 MHz internationally.
● Possible Incumbent (Primary Devices)– TV broadcasts (NTSC, PAL, SECAM, ATSC, DVBT)– Wireless microphones– Medical telemetry– Mobile radios (police, firemen)
February 2008 21
Sensing ● Detecting the presence of and coexisting with primary
users is paramount.
● CPEs are slaves of the Base Station– BS tell CPEs when to sense and which channel to sense.– CPE is equipped with 2 antennas.– Information is reported back to BS (data fusion).– BS tells CPE when to transmit (periodic enabling signal).– CPEs employ both energy detection and feature
detection methods.
● In addition,– BS maintains a table of channel occupancy status.– Primaries can transmit a beacon (802.22.1)– CPE must sense all channels when powered on.
February 2008 22
Channel Bonding
● Channel bonding is a technique that uses multiple adjacent available channels.
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Partial BW
● There is support for fractional bandwidth usage,– eg, to avoid interfering with a narrowband microphone
user.
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Subcarrier Allocation● Pilots are tiled across consecutive OFDM symbols and
subcarriers.
● A subchannel consists of 28 contiguous subcarriers– (24 data + 4 pilots)– 60 subchannels per channel
February 2008 25
Research Challenges
● Cognitive MAC
● Resource Allocation
● Coexistence issues
● Broader gains of large cognitive networks
● Interference Control– Are cognitive users necessarily interferers?
February 2008 26
Where to Measure for Holes?
● If the secondary user is limited in its abilities, it may have to choose to scan only certain channels.
● There is a tradeoff:– Scanning a channel that is likely to be free increases
throughput.– Why scan a channel that is unlikely to be free?
● Scanning a channel improves estimate of and tracks changes in odds of it being free.
– Twoarmed bandit problem.
February 2008 27
Twoarmed Bandit
● This is similar to the twoarmed bandit problem and its variants.– In this problem, you have the choice at every time to play
one of two slot machines.– Each slot machine pays out with unknown probability p
and q.– How to determine which machine to use at which time?
● In our case– Slot machine = channel– Payout probability = probability that channel is free– There could be numerous machines.– We can play multiple machines at once.– There is more than one player.
February 2008 28
Resource Allocation
● If we consider a star network (i.e. 802.22 like) alone, there are challenging problems in resource allocation:
– Not all subchannels are feasible for all secondary users
– There are challenging tradeoffs between subchannel allocation, power allocation and rate.
– Since primary users can be mobile, reallocation must be done in realtime.
WirelessMIC
WRANBase Station
February 2008 29
Coexistence Issues
● In a cognitive WRAN, the base stations must coexists:
– Some methods are needed by which the basestations do not excessively compete.
– By default, each WRAN acts selfishly to optimize its own utility.
– Appropriate notions of fairness and distributed algorithms to achieve this are needed.
WRANBase Station
WRANBase Station
February 2008 30
Large Distributed Cognitive Networks
● We can also consider large distributed cognitive networks colocated with a primary network.
– What are the inherit performance limits of larger cognitive networks?
– How does the interference created to the primary network scale with the secondary network?
– What are good distributed selforganizing algorithms for cognitive networks?
February 2008 31
Interference control● Interference avoidance is worstcase design
– In practice, this may be too aggressive and overly limit throughput of secondary users.
● Interference temperature has been proposed as a method to limit interference by secondary users.
– Interference temperature measures power per Hertz.– Computed from sum of all noise plus interference at
receiver.
February 2008 32
Effect of Interference Temperature
● Interference temperature introduces new opportunities at a cost:
● Additional difficulties– Secondary user needs to measure/know temp. at
primary receivers.● Secondary measurements● Feedback from primary
● Treats secondary tx as interference.
February 2008 33
Information Theoretic Considerations
● Can we do better?– Fountain codes
Primary transmit only
Primary and secondary transmit
February 2008 34
Genie Cognitive Problem
● In principle, fraction of time spent acquiring primary message is:
● Genie cognitive problem:
February 2008 35
Competition/Cognition/Cooperation
Competition:Interference channel
Cooperation:MIMO Broadcastchannel
Cognition:Interferencechannel withdegraded messages
February 2008 36
Secondary User Strategies
● The simplest strategy for the secondary user is to act selflessly.– It can expend all its power on aiding the primary
Rates:
February 2008 37
Secondary User Strategies
● Another option is to be completely selfish (greedy).– It can “precancel” the interference from the primary.
Rates: < Contains interference
February 2008 38
Intuition of a Genie Cognitive Scheme
● First bounds for this problem in [Devroye et al. IT05, CISS05]● Scheme in [Devroye, ISIT05] is for more general setting, but
can be summarized for Genie cognitive channel as:
Primary has private and public componentsSecondary has private and public components
February 2008 39
Intuition of a Genie Cognitive Scheme
● Primary uses superposition encoding to transmit privateand public .(e.g.) for example.
● Secondary – encodes private and public .– Applies GelfandPinsker for and against encodings of
and . – Applies superposition encoding to results of previous two steps.
February 2008 40
Capacity for a Class of Channels● In ISIT06, Jovicic and Viswanath found the capacity for
a class of weak interference cognitive channels.
Weak interference:
– Primary encodes its message– Secondary splits its power
Fraction repeats primary's encoding Fraction applies DPC against interference seen at cognitive receiver.
– Clever outerbounding shows that this is capacity.
February 2008 41
Rate regions
● Capacity region is then:
February 2008 42
Rate regions
February 2008 43
Questions● So is treating a secondary transmitter as an interferer
the right thing to do?– If not, we must have primary transmitters use “cognitive
compatible” coding a modulation schemes.
● In practice, secondary user must acquire primary message– If secondary is near primary, this overhead may be
negligeable.
● In [Devroye et al, IT06], some causal schemes are considered– The causal problem has received surprisingly little
attention.
February 2008 44
Summary
● Cognitive radio is motivated by its potential impact.
● Many challenging problems need to be solved.
● Two approaches to “no harmful interference”– interference avoidance (overlay)– interference control (underlay)
● A very clever cognitive device can employ coding techniques to mitigate harmful interference.
● IEEE 802.22 is an emerging standard for WRAN– aimed at providing fixed wireless broadband.
February 2008 45
Thank You !!!
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