www.cs.helsinki.fi overview of cognitive radio basics and spectrum sensing cn-s2013 faculty of...
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www.cs.helsinki.fi
Overview of Cognitive Radio Basics and Spectrum Sensing
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Faculty of ScienceDepartment of Computer Science 1
Jan.29, 2013
Suzan Bayhan
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Cognitive radio: What, why, and how
Spectrum Sensing: Basics and challenges
Summary of Today’s Class
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Joseph Mitola III and Gerald Q. Maguire, Jr. (KTH, Sweden), Aug.1999 IEEE Personal Communications, Cognitive Radio: Making Software Radios More PersonalSimon Haykin, Feb. 2005, IEEE Journal on Selected Areas in Communications, Cognitive Radio: Brain-Empowered Wireless Communications
“an intelligent wireless communication system that is aware of its environment and uses the methodology of understanding-by-building to learn from the environment and adapt to statistical variations in the input stimuli, with two primary objectives in mind: (1) highly reliable communication whenever and wherever needed; (2) efficient utilization of the radio spectrum”
Cognitive Radio: Definition and History
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Cisco Report: http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-520862.html
Wireless data consumption increases (from Cisco’s report)
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By 2012, the number of mobile-connected devices will exceed the world's population.
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Radio spectrum: 3kHz to 300 GHz
The use of radio spectrum for communication dates back to
How is the wireless spectrum is managed?
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Image from http://kids.britannica.com/elementary/art-87886/Guglielmo-Marconi-is-pictured-with-his-telegraph-equipment
1895: Guglielmo Marconi, radio signal transmission using telegraph codes over 1,25 mile distance
Static Spectrum Access
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Use of Radio Frequencies in Finland (www.ficora.fi)
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License for a large region, usually country-wide
Large chunk of licensed spectrum (expensive licenses)
Barriers to new ideas
Prohibited spectrum access by unlicensed users
ISM bands are unlicensed WLAN bands at 2.4 GHz, 5 GHz
Temporary short range licenses
Shortcomings of current spectrum management
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The Finnish Communications Regulatory Authority (FICORA)
International Telecommunication Union (ITU)
European Telecommunications Standards Institute (ETSI)
Radio Spectrum Use in Finland
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Ficora allocates spectrum in Finland
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How much is this frequency? Calculate the fee for frequency!
http://www.ficora.fi/en/index/luvat/taajuusmaksut/laskentakaavatjakertoimet.html
You can check from this document:
http://www.ficora.fi/attachments/englantiav/673vb43bJ/TJTen_20042012.pdf
You can find radio spectrum regulations in Finland here:
http://www.ficora.fi/en/index/palvelut/palvelutaiheittain/radiotaajuudet.html
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Spectrum Measurements
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Image from RWTH http://www.inets.rwth-aachen.de/static-spectrum.html
Image from http://www.cmpe.boun.edu.tr/WiCo/doku.php?id=research#cognitive_radio
Measurement campaigns have shown that there is plenty of unused spectrum!
Working time vs. night time usage
City-center to suburb usage
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Cognitive Radio (CR)
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There is a huge demand for spectrum, but there is unused spectrum Radio spectrum is inefficiently used.
Change in ownership; a resource is owned by the one who uses it. Sharing for sustainability.
Static spectrum management since 1900s.
Imagine a world with no-lane-changing.
Smarter schemes: Dynamic spectrum access (DSA)
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Primary User, Secondary User
Licensed, primary, incumbent, higher-priority user: PU
Secondary, cognitive, unlicensed user: SU, CR
Spectrum hole, white space, white spectrum, idle frequency/channel/band
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Hardware: Static, once designed at the factory, never changed
SDR: Reconfigurable radio (e.g. operation frequency, modulation type)
Multiple standards
Multiple bands
Software Defined Radio (SDR)
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SDR is the building block of the CR.
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How does cognitive radio work?
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SPECTRUM SENSING
Cognitive Cycle
Image from http://pgcoaching.nl
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Reading Material:
- T. Yucek and H. Arslan A survey of spectrum sensing algorithms for cognitive radio applications, IEEE Communications Surveys and Tutorials, vol. 11, no. 1, pp. 116-130, 2009. - Ghasemi, Amir, and Elvino S. Sousa. Spectrum sensing in cognitive radio networks: requirements, challenges and design trade-offs. IEEE Communications Magazine, 46.4 (2008): 32-39.
Spectrum Sensing Reading Material
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What is spectrum sensing?
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Time
Time1- Sense:There is PU
2- Sense: IDLE3- Sense: PUPU collision: Interference or harmful interference
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1- Sense for vacating the band if PU arrives. CR must not harm PUs
2- Sense for finding unused spectrum
How to measure quality of sensing?
•Probability of detection (Pd) Higher is better
•Probability of false alarm (Pf) Lower is better
Spectrum Sensing
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Various aspects of spectrum sensing
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Sensing: PHY and MAC Layer Issues
PHY SensingSpectrum Sensor at PHY
MAC SensingSensing and access strategy
CR SENSING DESIGN = SENSOR + SENSING STRATEGY + ACCESS
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Energy Detector: Measures the energy received on a primary band during an observation interval and declares a white space if the measured energy is less than a properly set threshold. (2) Do not differentiate PU and CR signals (3) Low complexityWaveform-based Sensing: (1) Preambles, midambles can be used to detect PU signals. (2) Short measurement time; Susceptible to synchronization errorsMatch Filtering MF: (1) If transmitted signal is known, test using filters. (2) Dedicated circuitry for each primary licenseeRadio Identification: Identifying the transmission technologies used by PUs, channel bandwidth, coverage etc.Cyclostationary: PU signal differentiated from noise
PHY Sensing
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Energy Detector:Binary Hypothesis Test
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H0: The frequency is idle, there is no PU signalH1: The frequency is occupied, there is PU signalw(n): Noise, s(n): PU signal, y(n): Measured signal, N number of
samples
H0 or H1?
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Effect of Signal to Noise Ratio (SNR)
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Decibel: 10log10(P2/P1)
Generally, sensing performance increases under increasing SNR.
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Comparison of Sensing Schemes
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1. Energy Detector2. Waveform-based Sensing3. Match Filtering4. Radio Identification5. Cyclostationary
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Types of Spectrum Sensing
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Proactive
Reactive
Local
Cooperative
Distributed Centralized
In-band
Out-of-band
Synchronious
Asynchronious
SequentialParallel
SPECTRUM SENSING
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Parallel
Sequential
Proactive
Reactive
Local
Cooperative
Centralized
Distributed
Synchronous
Asynchron.
In-band
Out-of-band
Sense channels 1 to N at the same time (parallel) requires N sensing device!
If there are N frequency channels
Sequential: Sense channels one by one. Which order? May take too long to find an empty channel.
Parallel vs. Sequential Sensing
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Proactive
Reactive
Local
Cooperative
Centralized
Distributed
Synchronous
Asynchron.
In-band
Out-of-band
Parallel
Sequential Proactive Sensing:CR senses even if it will not transmit immediately, e.g. periodic sensing.
Trade-offcollected information about the channels vs. sensing cost
Reactive Sensing:CR senses only if it will transmit or receive
Energy-efficient, time to find an idle channel may be longer than Proactive Sensing.
Proactive vs. Reactive Sensing
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Proactive
Reactive
Local
Cooperative
Centralized
Distributed
Synchronous
Asynchron.
In-band
Out-of-band
Parallel
Sequential Local Sensing:Each CR senses itself and uses its sensing data to give a decision on channel state, i.e. idle or busyWhat if hidden node or bad channel conditions?
Cooperative Sensing:CR shares its sensing data with others and utilize the sensing outcomes of others to give a decision
Robust to sensing errors due to hidden node or fading channels.
Cost of cooperation?
Cooperative vs. Non-cooperative Sensing
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Cooperative Sensing
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More robust to sensing errors.
Hidden node problem
PU is hidden to the CR. CR’s transmission will result in interference at the PU receiver.
Cooperate with this user!
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Proactive
Reactive
Local
Cooperative
Centralized
Distributed
Synchronous
Asynchron.
In-band
Out-of-band
Parallel
SequentialCentralizedA Central Manager (BS or AP) collects CR sensing data and makes a decision on channel state, i.e. idle or busyCost of transmission sensing data? What if the Central Manager fails? Single
Point of Failure.
Distributed (Decentralized)Each CR makes decision itself.
Centralized vs. Distributed Sensing
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Centralized/Distributed Cooperative Sensing
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Decision Fusion Center
Increased sensing reliability at the expense of increased communication overhead
How to communicate: Common control channels (CCC)
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Decision Fusion: How to decide?
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Yes, there is PU
No, it is IDLE
Yes Yes No
How to decide? (DECISION FUSION LOGIC) AND OR MAJORITY K-of-N
Soft or Hard Decision Combining: Yes or No answers (0-1), or Received Signal Strength
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Number of Cooperating Users vs. Sensing Time
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Amir Ghasemi and Elvino S. Sousa, Spectrum Sensing in Cognitive Radio Networks: Requirements,Challenges and Design Trade-offs
Cooperation overhead generally increases with the number of cooperating
Optimal number of cooperating users
Single CR or 5 CRs
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Proactive
Reactive
Local
Cooperative
Centralized
Distributed
Synchronous
Asynchron.
In-band
Out-of-band
Parallel
SequentialSynchronousAll CRs have the same sensing schedule to sense a channel.
How to synchronize?Stop transmission and sense the medium.
AsynchronousEach CR has its own schedule to sense a channel.
If other CRs are transmitting while this CR is sensing, how to distinguish between SU and PU signal.
Synchronous vs. Asynchronous Sensing
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Proactive
Reactive
Local
Cooperative
Centralized
Distributed
Synchronous
Asynchron.
In-band
Out-of-band
Parallel
Sequential In-bandCR senses the channel that it is already transmitting- To detect if a PU appears
Out-of-bandCR senses channels other than the channel it is in
To find other spectrum holes To find another channel to switch since a PU has
already appeared.
In-band vs. Out-of-band Sensing
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Hardware requirements: High speed processing units (DSPs or FPGAs) performing
computationally demanding signal processing tasks with relatively low delay.
Operation in a wide spectrum range
Sensing-Transmission Tradeoff
Security: a selfish or malicious user can modify its air interface to mimic a primary user.
Challenges of Spectrum Sensing
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Static spectrum access is cumbersome!
CR facilitates unused spectrum to be used opportunistically.
Spectrum sensing facilitates discovery of unoccupied spectrum.
The spectrum sensing can be designed considering various criteria at MAC and PHY layer.
The longer is the sensing duration, generally the higher is the sensing reliability.
Cooperation increases sensing performance but has higher overhead.
Summary
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References
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T. Yucek and H. Arslan, A survey of spectrum sensing algorithms for cognitive radio applications, IEEE Communications Surveys and Tutorials, vol. 11, no. 1, pp. 116-130, 2009.
Ghasemi, Amir, and Elvino S. Sousa. Spectrum sensing in cognitive radio networks: requirements, challenges and design trade-offs. IEEE Communications Magazine, 46.4 (2008): 32-39.
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Questions?
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Self-Study: Make sure you know all the terms below
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Primary User Secondary User Cognitive Radio Spectrum Hole Spectrum Sensing Harmful Interference SNR Cooperative Sensing Dynamic Spectrum Access Static Spectrum Access Spectrum Underutilization Sensing-transmission trade-off Decision fusion logic
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Presentation Schedule
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Next week
2-Minute Madness Session: In two minutes present your topic’s basic idea, questions, etc! Only 2 minutes.
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