introduction to spectrum sensing

7/26/2019 Introduction to Spectrum Sensing

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KITUniversity of the State of Baden-Wuerttemberg andNational Research Center of the Helmholtz Association

COMMUNICATIONS ENGINEERING LAB (CEL)

www.kit.edu

Cognitive Radio for Dynamic Spectrum AccessVision Meets RealityFriedrich Jondral

LStelcom SummitLichtenau, July 4, 2012


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Communications Engineering Lab (CEL)2 10.07.2012 Prof. Dr.rer.nat. Friedrich Jondral

Cognitive Radio (CR)


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Immediate Urgent Normal

ACT

OutsideWorld

NewStates

PriorStates

GenerateAlternatives

Evaluate

Alternatives

ORIENT

Establish Priority

Receive a MessageRead Buttons

Send a Message Initiate Process(es)

Register toCurrent Time

Pre-ProcessParse

Save Global States

Allocate Resources

Set Display

Infer on ContextHierarchie

Joseph Mitola III: CognitiveRadioAn Integrated AgentArchitecture for SoftwareDefined Radio. KTHStockholm, 2000

CR: Vision

OBSERVE

DECIDE

LEARN PLAN


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CR: Definition

Cognitive Radiois an intelligent wireless communication system that isaware of its surrounding environment(i.e. its outside world), and uses themethodology of understanding-by-building to learn from the environmentand adapt its internal states to statistical variations in the incoming RFstimuli by making corresponding changes in certain operating parameters

(e.g. transmit power, carrier-frequency and modulation strategy) in real-time, with two primary objectives in mind:

- highly reliable communications whenever and wherever needed;- efficient utilization of the radio spectrum.

Simon Haykin: Cognitive Radio: Brain-Empowered Wireless Communications.IEEE J. Select. Areas in Comm., vol. 23, no. 2, 2005, pp. 201-220


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Communications Engineering Lab (CEL)5 10.07.2012

Reality

Prof. Dr.rer.nat. Friedrich Jondral

CRis not a revolution in radio communications, it is merely the way ahead to

more automationand adaptation

in finding the optimum frequency and in using the optimum transmission power

With these properties

higher spectrum efficiency lower costsand more environmental acceptability

are achieved.

The CR paradigm makes sense only in networks.


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Meaning of "Spectrum"

A material quantity that may be partitioned

or an immaterial medium

that may be accessed

without regulation?


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Spectrum Utilization

M. McHenry: NSF Spectrum Occupancy Measurements.

The Shared Spectrum Company, Tech. Rep., 2005,http://sharedspectrum.com/?sectio=nsf_measurements

Fundamental Statement:Even in crowded frequency regions not more then 15 percent of the(theoretical) capacity is actually used.

However:A hundred percent usageof the transmission resourceis utopistic (interferences)

But: Struggling is promising.

Photo: The Shared Spectrum Company
http://sharedspectrum.com/?sectio=nsf_measurementshttp://sharedspectrum.com/?sectio=nsf_measurements


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Dynamic Spectrum Access (DAS)

Dynamic Spectrum Access

Dynamic

Exclusiv Use Model

Hierarchical

Access Model

Open Sharing Model

(SpectrumCommons Model)

Spectrum

PropertyRights

Dynamic

SpectrumAllocation

Spectrum

Underlay(Ultra WideBand)

SpectrumOverlay

(OpportunisticSpectrumAccess)

from: Qing Zhao, Brian M. Sadler: A Survey of Dynamic Spectrum Access.IEEE Signal Processing Magazine, May 2007, pp. 79 - 89


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DSA: Questions

What is the meaning of Spectrum Access?

To enhance the efficiency in the usage of spectrum (briefly: spectralefficiency) in a specific geographic region, CRs access spectrum holesleft by the licensed users system (primary users) as secondary users.

I.e.: Spectrum Access happens in time, frequency, and space.

What is the meaning of Dynamic?

Nobody knows

On which scale is DSA based upon? Milliseconds, seconds, minutes,? Change in primary users behavior?


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Dynamic / Detection Time

DynamicDetection

Time

Burst

TV White Space

high short

low long


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Time/Frequency Plane

GSM 1800

No. of Channels: 374

Bandwidth: 270 kHzDistance: 200 kHz

Burst Duration: 0.577 ms


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Energy Detector

s(t) Transmitter Signal

u(t) Baseband Representation of s(t)

r(t) Received Signal

v(t) Baseband Representation of r(t)

T Duration of s(t)

Radio Frontend Decision |v(t)|2dtT

0

r(t)


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Matched Filter Detector





T Duration of s(t)

Radio Frontend Decision v(t)u(T-t) dtT

0

r(t)

u(t)


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Pattern Recognition Detector





T Duration of s(t)

Radio Frontend DecisionFeature

Extraction

r(t)

u(t)

PatternRecognition

FeatureExtraction

. . .

.

.

.


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Signal Detection


DetectorA Priori

Knowledge

Detection Time/Computational

ComplexityApplicability Robustness

Energy Nothing low universal high

MatchedFilter

Signal medium specific medium

PatternRecognition

SignalFeatures

high highly specific low


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Energy Detector

Detection Time: AWGNFalse Alarm Rate: 10-4

Detection Probability: b(2: normalized noise variance)

b =0.9999n

b =0.999 b =0.99 2SNR[dB]

111 93 74 2 -3

56 47 37 1 0

28 24 19 1/2 3

14 12 10 1/4 6

7 6 5 1/8 9

4 3 3 1/16 122 2 2 1/32 15

2 2 2 1/32 15

1 1/37 15.7

1 1/47 16.7

1 1/56 17.5


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Energy Detector

D = duration for one scan over the 374 channels of GSM 1800

false alarm rate: 10-4

detection probability: 0.999

SNR: 9 dB

Monitoring of the GSM band on burst basis by one scanning energy detector with

false alarm rate 10-4and detection probability 0.999 at an SNR of 9 dB isimpossible!

And:What about the power needed in the mobile radio for permanent scanningand detection?

D = 6 x No. of Channels x = 6 x 374 x s = 8.31 ms1

Bandwidth1

270000

D = =14.4 bursts8.310.577


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Proposed Solution 1


Distributed Detection

For networks with access point:Timo Wei: OFDM-basiertes Spectrum Pooling. Dissertation, Forschungsberichte aus dem Institut frNachrichtentechnik der Universitt Karlsruhe (TH), Band 13, Karlsruhe 2004

For ad hoc networks:Ulrich Berhold: Dynamic Spectrum Access Using OFDM-based Overlay Systems. Dissertation,Forschungsberichte aus dem Institut fr Nachrichtentechnik der Universitt Karlsruhe (TH), Band 21,Karlsruhe 2009

MAC frame MAC frame MAC frame

detectionphase

boostingphase

broadcastphase

P P

2 ms


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Distributed Detection and Boosting


With Access Point Ad Hoc

b) Boosting and Collection


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Proposed Solution 2


Off-line Sensing, Data Base Query, and Instantaneous Measurement

During idle times The radio senses all potential transmission channels1) The sensing results for each channel, together with the time of the day when

the sensing took place, are stored in a data base in order to establish channel

utilization statistics depending on time and frequency

When a communications request occurs1. The radio queries the data base for a channel that is idle with highest

probability at the current time of the day and that has not been sensed yet2. The radio instantaneously senses the chosen channel

3. If the channel is idle, the radio starts operation.If not, it goes back to 1.

1)The power problem for this remains unsolved.


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Data Base Query

16:05

16:10

16:15

16:20

Time Channel Utilization Statistics

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Channel No. Priority

1 22 5

3 4

4 5

5 1

6 3

1 2 3 5 64

1 2 3 5 64

1 2 3 5 64

1 2 3 5 64

.

.

.

.

.

.


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Dontforget


Coordination

A channel idle at station A must not be idle at station B (agreement necessary).

Continuous SensingAs long as a SU station is active, it must permanently sense its channel (look

through).

Automated Frequency ChangeIf a PU signal is detected on the currently used channel, communication partnersmust identify a new usable frequency and jointly switch to it.

Hidden Stations

Multicast / Broadcast


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Summary


As of July 18, 2012 there are

8 847 papers on Cognitive Radio, 9 554 papers on Spectrum Sensing, and 2 635 papers on Dynamic Spectrum Accesslisted in the IEEE Xplore Digital Library.Many of them do not observe any constraints imposed by physics.

All notionsthat we use in communications need to be well defined.

Detection time depends on SNR, false alarm rate, detection probability, andfurther conditions imposed by wave propagation.

CRand DSAbear high potential for theoretical and practical research work.


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Communications Engineering Lab (CEL)24 10.07.2012 24

introduction to spectrum sensing

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