Harvard
SEAS
Theoretic Fundamentals, Regulatory
issues, Physical Limitations, and the
Future of Opportunistic Transmission
Vahid Tarokh
Harvard University
UC Irvine-CS Dept
Feb 24. 2012
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Introduction
UC Irvine-CS Dept
Feb 24. 2012
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The Goal
The Goal = Providing Wireless Services
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Services
•Traditionally voice had been the main application, but
many other services are arising.
•Emerging wireless broadband applications require both
spectrum and advanced techniques to increase bandwidth
efficiency.
Example of Services
Information
Services
Software
Distribution
Entertainment
Television
Interactive
Games
Education
Services
Electronic
Shopping
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Impact of Services
• Historically by providing services,
telecommunication engineers have had a
huge impact on society and economy.
• New services enable new non-telecom
industries and improve efficiency of
existing ones.
• They may help with development of
freedom and democracy
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A Successful Example
• Newsweek reports that cell-phones have made 5 major impact
on the world http://www.newsweek.com/2010/11/10/how-thecell-phone-is-changing-the-world.html
– Exposing Secrets - The repression and horror happening in North
Korea leaks out by cell phone.
– Advancing Democracy - Cell phones present a problem for
oppressive regimes everywhere.
– Enabling Commerce - Enabling a common method of banking using
a cell phone where there are no banks.
– Distributing Medecine - A new project in Africa, called Stop StockOuts enables activisits to report which drugs are out of stock.
– Waging War - How the Taliban have forced local cell-phone-service
providers to shut down their towers at night stopping locals from
reporting Taliban movements to Coalition forces.
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Feb 24. 2012
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Communications Help Economy
and Human/Political Development
• Kerala to send SMS alerts for vaccination of babies
• South African Students Receiving Maths Lessons by
Mobile Phones
• Kenyan farmers use SMS to beat climate-driven price
uncertainty.
• Saving Mothers' Lives With Health Tips Via Phone
Source:
http://www.textually.org/textually/archives/2011/04/028310.htm
• Mobile Internet played an important role in Arab Uprising
and the Iranian anti-government protests.
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Feb 24. 2012
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Communications Enhances
Freedom
• Videos collected by mobile phones expose government and
their brutality against citizens.
• Providing more services communications can lead to a
more “information flat world”, where everyone will access
the information that they need and contribute to information
gathering and distribution as much as they could.
– This will make suppression of the truth much harder.
• Thus providing more services is a great idea.
– But obviously this needs spectrum
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Feb 24. 2012
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Spectrum
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Scarcity of Spectrum
• Most frequency bands up to 6 GHz (and
beyond) have FCC allocations for multiple
users.
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Scarcity of Spectrum
Source
cnn.com
Feb 21,
2012
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Spectrum Crunch
Source money.cnn.com Feb 21, 2012
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Outline
• Shortage of good spectrum may appear as a
problem, but this may not be the case:
– measurements show that at anytime more than 90%
of these resources are not used.
• Idea: Intelligent radios may allow better use
(sharing) of the spectrum.
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Feb 24. 2012
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Spectrum Sharing
• This motivated a push for sharing the unused but
dedicated spectrum for providing new services.
– Is this a new idea?
• Geographical reuse of spectrum has been around for a long
time.
–
–
–
–
–
–
Is this a good idea?
How aggressively must it be pursued/allowed?
Is it technically feasible?
How much intelligence is needed in the radio?
How regulatory bodies are dealing with it?
Does it have a future?
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Existing Spectrum Sharing
Examples
• Spectrum Sharing is nothing new:
– The ISM band allows sharing of spectrum
• Many successful application exists (e.g. garage door openers, etc.)
• Perhaps the most successful application is WiFi.
– UWB
• A February 14, 2002 Report and Order by the FCC authorizes the
unlicensed use of UWB in the range of 3.1 to 10.6 GHz. The FCC
power spectral density emission limit for UWB emitters operating in
the UWB band is -41.3 dBm/MHz.
• This is the same limit that applies to unintentional emitters in the
UWB band, the so called Part 15 limit. However, the emission limit
for UWB emitters can be significantly lower (as low as -75
dBm/MHz) in other segments of the spectrum.
– UWB has not had much commercial success yet.
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Feb 24. 2012
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Wi-Fi
• Another example of wireless broadband
services is Wi-Fi.
•Wi-Fi has had
enormous success
and this is expected
to continue on into
future: please see
the old forecast 
• Range is an issue.
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Feb 24. 2012
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Existing Spectrum Sharing
Examples
• MVDDS (Multichannel Video and Data
Distribution Service)
– This terrestrial based wireless transmission
method reuses Direct Broadcast Satellite (DBS)
frequencies for distribution of multichannel video
and data over large distances.
– licensed for use in the United States by the FCC.
• Ruled that 10% increase in rain outages would not be
harmful
– The underlying spectrum is in the 12.2 - 12.7
GHz range.
UC Irvine-CS Dept
Feb 24. 2012
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Spectrum Sharing Paradigms
• Exclusive access:
– One system has exclusive access to the spectrum.
• Horizontal Sharing (Equal right access):
– All systems have the same regulatory status and may access
the spectrum on an equal footing. (e.g. usage of the ISM
bands by WLAN and Bluetooth)
• Vertical Sharing (Prioritized spectrum access):
– A primary system.
– Secondary systems can share only if they do not generate
harmful interference for the primary
• All of these schemes have existed for some time.
UC Irvine-CS Dept
Feb 24. 2012
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Spectrum Sharing Models
Spectrum sharing
Spectrum sharing with
other (legacy or other
novel) systems.
Pre-established priorities
for all involved systems.
Horizontal sharing
with coordination
All involved systems
share the spectrum
based on a set of rules
(spectrum etiquette).
Horizontal sharing
without coordination
Little can be done
To avoid interference
UC Irvine-CS Dept
Feb 24. 2012
Vertical sharing
Secondary system has
to control its emissions
to prevent interference
towards primary system.
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Cognitive Radios?
•Then What is
so new about
cognitive
radios?
•Perhaps it all
depends on
what cognitive
radio means….
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Cognitive
Radios
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Cognitive Radios
• A Cognitive radio is an intelligent
wireless communication system
that:
–is aware of its environment,
–learns from the environment, and
– adapts its internal states in real-time
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Vertical Sharing
• Following problems need to be addressed:
– need to identify the spectral “white spaces’’
– need to adapt to the restrictions identified.
– need to be smart in reducing the harmful interference to other
systems while increasing their own transmission rates
• Cognitive Radios are being discussed
from several perspectives.
• Their success will depend on:
– Fundamental Limits
– Finding methods to achieve these limits
– How conservative the regulatory rules
are, and
– Economics and business models
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Fundamental Limits
on Cognitive Radios
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Scenario: Cognitive Radio
[DMT]
Traditional
Cognitive Radios
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Potentials of Cognitive
Radios [DMT]
Can these potentials
be actually realized?
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Interference Between Cognitive
Devices
• The previous result gives some promise in potential of
secondary users not having harmful effects on the primary
user capacity.
• However, even if we can address the issue of interference
between primary and cognitive networks, we will still
have potential interference between various cognitive
networks operating on available white spaces.
• In other words, if because of availability of free spectrum
many secondary systems emerge, can these systems
support any reasonable data rate?
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Throughput Scaling
• From point of view of scaling laws (growth order), of
ad hoc networks: “cognitive networks achieve
throughput scaling of a homogeneous network”,
[WDVCT]:
– Randomly distributed n primary users, and m secondary
users with m = nβ with β > 1
– Specifically, the primary network achieves the sum
throughput of order n0.5 and, for any δ , the
secondary network achieves the sum throughput of
order m0.5- δ with an arbitrarily small fraction of
outage.
– These results are only of theoretical interest.
UC Irvine-CS Dept
Feb 24. 2012
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Co-existence of Secondary
Networks
• It has been proved [VT] that under the assumption of a
cap on the interference caused by secondary network to
primary receivers
– the secondary networks are single hop and
– transmissions transmit either (i) with constant transmit power,
and (ii) with transmit power scaled according to the distance to a
designated primary transmitter, then
as the number of secondary networks N  ∞ , the secondary
receivers can achieve at least a non-vanishing throughput.
• This shows that cognitive radios are at least scalable for
single hop networks.
• Another option is not to allow too many secondary
networks.
UC Irvine-CS Dept
Feb 24. 2012
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Co-existence of Secondary
Networks
• The FCC proposes that some form of contention protocol be
employed to reduce the interference between co-existing cognitive
networks but does not specify such a protocol.
• If the number of cognitive networks in a region grows large, this
may not be very efficient and may produce capacity losses.
• The general problem of allocation of available white spaces to
various cognitive networks in order to optimize the capacity is a.
computationally hard (NP-hard) problem.
• We will next study proposals for various secondary networks to
co-exist.
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Feb 24. 2012
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Existing Approaches
•
Methods based on Iterative Water-filling
(IW):
–
–
•
Methods based on graph coloring:
–
–
–
•
High computational complexity.
Convergence to configurations which are far from
optimal.
Computationally expensive
Too much message passing among the agents.
Complex cooperation protocols.
A Method (GADIA) inspired by Glauber
Dynamics in statistical physics [BT] that
attempt to maximize the Rosenthal potential.
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Feb 24. 2012
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Performance
• The GADIA algorithm converges
to equilibrium exponentially fast.
• The GADIA algorithm achieves
about 98% of the optimal
Shannon capacity.
• GADIA has much lower
complexity (about 3 orders of
magnitude lower) and converges
faster that the existing Iterative
Water-filling algorithm.
UC Irvine-CS Dept
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Conclusions
• Theoretical Analysis indicates that under
idealized assumptions at least for some
scenarios of interest, cognitive radios may
have some promise.
– There is a lot more to investigate particularly if
the idealized assumptions are removed
• The main question is that how much of these
gains remain
– in realistic situations, and
– under the regulatory restrictions.
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Regulatory
Issues
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Feb 24. 2012
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FCC Regulatory Issues
• The FCC has released the band 3650-3700 MHz
for cognitive transmission.
• Fixed Satellite Services and federal government
stations are currently transmitting in this band.
• Certain geographical areas around these
transmitters are not allowed for secondary
transmission.
• Otherwise secondary transmissions are allowed
(by the FCC) subject to
– 25W per 25 MHz bandwidth for fixed stations
– 1W per 25 MHz bandwidth for mobile stations
UC Irvine-CS Dept
Feb 24. 2012
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TV White Spaces
• Another band of interest is given in notice of
rule making ET Docket 04-186.
• These are TV Broadcast bands (6 MHz channels
designated channels 2 to 69 in the VHF and
UHF portions of the radio spectrum.
• 54-72 MHz, 76-88 MHz, 174-216 MHz and 470806 MHz.
• Other existing devices in some of this band
include wireless microphones.
UC Irvine-CS Dept
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TV White Spaces
• Secondary transmission in this band has
witnessed a lot of politics/resistance.
• Finally, the FCC has announced on Nov. 4, 2008
a set of rules for secondary devices to operate in
TV bands while reducing the interference to
primary users.
• This has caused some interest in network
solutions and consumer devices for these bands.
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FCC Rules
• All devices, except personal/portable devices operating
in client mode, must include a geo-location capability and
provisions to access over the Internet a database of
protected radio services and the locations and channels
that may be used by the unlicensed devices at each
location.
• The unlicensed devices must first access the database to
obtain a list of the permitted channels before operating.
• The database will be established and administered by a
third party
• The third party (Spectrum Bridge) was selected through
an open process to solicit interested parties in 2011.
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FCC Rules
• Fixed devices may operate on any channel between 2
and 51, except channels 3, 4 and 37, and subject to a
number of conditions such as a restriction against cochannel operation or operating adjacent to TV channels.
• Fixed devices may operate at up to 4 Watts EIRP.
• Personal portable devices may operate on any
unoccupied channel between 21 and 51, except channel
37.
• Personal portable devices may operate at up to 100 mW
of power, except that operation on adjacent channels
will be limited to 40 mW.
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Feb 24. 2012
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FCC Rules
• Fixed and personal/portable devices must also have a
capability to sense TV broadcasting and wireless
microphone signals as a further means to minimize potential
interference.
• Wireless microphones will be protected in a variety of ways.
The locations where wireless microphones are used, such as
entertainment venues and for sporting events, can be
registered in the database and will be protected as for other
services. In addition, channels from 2 – 20 will be restricted
to fixed devices.
• In addition, in 13 major markets where certain channels
between 14 and 20 are used for land mobile operations,
channels between 21 and 51 are left free of new unlicensed
devices.
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FCC Rules
• All fixed devices must register their locations in the database.
• In addition, fixed devices must transmit identifying
information to make it easier to identify them if they are
found to interfere. Furthermore, fixed and personal/portable
devices operating independently must provide identifying
information to the TV bands database.
• All devices must include power control so that they use the
minimum power necessary to accomplish communications.
• All white space equipment must be certified by the FCC
Laboratory.
• FCC permits applications for certification of devices that do
not include the geo-location and database access capabilities,
and instead rely on spectrum sensing to avoid causing
harmful interference, subject to a much more rigorous set of
tests by the FCC Laboratory.
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Feb 24. 2012
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Conclusions
• A fixed device must employ both geo-location,
database access and spectrum sensing capabilities
that enable the device to listen for and identify the
presence of signals from other transmitters.
• A personal/portable device must either be under the
control of a fixed device or a personal/portable
device that employs geo-location, database access
and spectrum sensing or employ geo
location/database access and spectrum sensing
itself.
• These devices will be required to sense, at levels >=
-114 dBm, signals of other services.
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Assessing The Rules
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Summary
• For reducing interference the FCC proposed
methods are based on
– Transmit power limitations/power control
– Geo-location enabled devices
– Geographic databases
– Career sensing
– Beacon detection
– Combinations of these methods
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Geo-Location Enabled Devices
• In this method of interference reduction, secondary users
must be endowed by GPS (or similar geo-location
systems) with at least 300m accuracy.
• Primary users location is known to the secondary users
(using a geographic database) and buffer regions around
the primary users are specified where secondary user
transmissions are not allowed in certain bands.
• Geo-location and also FCC power limits are safe but
conservative:
– May make more sense to allow different power limits in
various bands based on the location of the secondary user.
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Feb 24. 2012
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Career Sensing
• Secondary devices sense the channel and based
on the activity level decide if it is busy or not.
• FCC: -114 dBm power means the channel is
busy.
A
B
Failure Causes Interference
Career Sensing
C
UC Irvine-CS Dept
Feb 24. 2012
D
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Career Sensing
• It is obvious that -114 dBm is not the optimum threshold for
detecting a busy channel.
• If this threshold is not correctly selected it limits the efficiency of
cognitive devices, thus
– Optimum threshold for detection must be computed although:
• Typically the underlying ambient noise std б is not known
• The distribution of the primary signal is not known.
• The busy channel threshold must be selected based on geographic
region (and the underlying primary systems) at least for devices
using geo-location and databases.
• Similar conclusion can be made for beacon detection.
• Here we have to be also careful about transmission strategy.
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Feb 24. 2012
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Assumptions
• Assumptions:
• Ambient noise is Gaussian with zero mean and an
estimate of б can be obtained
• Primary transmission power is Pp.
• For career sensing, the primary signal is Gaussian
with mean zero and variance Pp.
– These are reasonable assumptions if each secondary
user scans and averages the channel for some
reasonable time (during both idle and busy periods).
UC Irvine-CS Dept
Feb 24. 2012
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Issues
• Computation of CCT (Clear channel threshold) for
deciding on idle channels and associated detection
strategies for both sensing and beacon based systems is a
straightforward exercise in detection theory.
• Questions:
– Given a peak power Ps and average transmit power Pav
for the secondary user, what is the best secondary user
transmission strategy that minimizes the interference
to primary receivers?
– The answer is non-trivial and is given by the following
theorem.
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Feb 24. 2012
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Results
• Theorem [KGMT]:
– The best transmit strategy (one that minimizes interference to
primary users for a fixed average and peak transmission power)
for career sensing based or beacon detection based cognitive
radios are identical:
– The cognitive radio must transmit at full power Ps when
detection reliability (LLR between clear channel and busy
channel hypotheses) is above a certain transmission threshold
(TT) [different than CCT] and refrain from transmission
otherwise. This TT depends on
• detection being beacon based or career sensing based
• average transmit power Pav
• Average idle time of primary transmitter.
• Thus even when CCT is correctly set based on location,
one should operate based on TT (again set based on
location).
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Conclusions
• Given the discussion, it seems that FCC rules
could be made less conservative.
– Not surprising given that the FCC is a political
organization.
– FCC leaders are appointed mostly for their political
ties and are not usually from the engineering
community
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Economics and
Business Models
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Feb 24. 2012
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Economist Views
• There are various economist views about
cognitive radios
– Some view spectrum sharing as less beneficial
in a long run than exclusive model. [please see
paper by Coleman Bazelon, Brattle Group in DYSPAN 2008].
– Most economists do not like the idea of sharing 
– Nevertheless spectrum sharing in some bands
has enabled Wi-Fi which has had tremendous
success
• I am personally optimistic that spectrum
sharing (in some bands) is a good idea.
UC Irvine-CS Dept
Feb 24. 2012
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Services
• What kind of services cognitive radios can enable?
• The answer is already known in the horizontal sharing
scenario.
• For vertical sharing scenarios if there is an active
primary user in the area, then quality of service may be
an issue, unless secondary signals can be spatially
separated from that of primary signals.
• If multiple cognitive radios exist, then contention can
effect their ability to provide quality of service.
• Possibly some polling of dedicated spectrum (wireless
or wired) with cognitive radio spectrum can be used to
provide some quality of service by future service
providers.
UC Irvine-CS Dept
Feb 24. 2012
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Conclusions
• Future services that will emerge on cognitive radio
spectrum will either
– separate their signals from primary signals spatially, or
– must be tolerant to delays and lack of quality of service
in the event of active primary systems,
– or must poll their cognitive spectrum with dedicated
spectrum/resources (wireless or wired) in order to
provide quality of service.
• It remain to be seen what kind of fundamentally
new services can emerge (given these constraints)
that otherwise will not be possible.
UC Irvine-CS Dept
Feb 24. 2012
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Envisioned Radios
• At present standardization efforts are under way for
cognitive Radios.
• These systems will be typically used in rural areas
where there is not a primary user.
• In England, BT wants to provide Internet services in
rural areas to consumers given that white spaces
exist.
• Similar efforts exist in USA.
• However, the underlying radios are not truly
cognitive.
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The Future of
Cognitive Radios
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Final Thoughts
• Some promising theoretical gains exist if the
wireless devices become intelligent.
– It remains to be seen if these gains can be realized given
realistic constraints.
– Co-existence limitations between cognitive networks is
not fully understood yet-- although some results exist.
• Conservative regulations (by regulatory bodies)
may not allow for a fully cognitive radio to be
realized.
– However, the FCC allows for sharing of certain bands
subject to some etiquettes and rules (a concept that
existed before)
• Quality of service is hard to obtain unless cognitive
radios poll their secondary resources with other
dedicated resources (to fall back on).
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Feb 24. 2012
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Final Thoughts
• Will cognitive radios produce services otherwise
not easily/economically feasible?
• Because some spectrum is now dedicated by the
FCC, there will be some devices and services in
these dedicated bands
– people like free spectrum
• The main question is how the communication
protocols of these devices and the services will be
fundamentally different from protocols existing in
the literature.
– will these devices be truly disruptive?
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Feb 24. 2012
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Final Thoughts
• As we can see there is a lot of uncertainty about the
amount of intelligence a future radio must have.
• There is also debates about how sharing must be
done.
• Much more analysis must be done and many
engineering issues must be resolved to answer these
questions.
• At the end, the devices may end up using the old
protocols.
• Nevertheless, however the landscape may turn out
to be, there is no doubt that intelligent radios
remains an intriguing topic.
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Thanks a lot
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