1 Chapter 4 Spectrum Sensing and Identification Cognitive Radio Communications and Networks: Principles and Practice By A. M. Wyglinski, M. Nekovee, Y

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Chapter 4

Spectrum Sensing and Identification

“Cognitive Radio Communications and Networks: Principles and Practice”By A. M. Wyglinski, M. Nekovee, Y. T. Hou (Elsevier, December 2009)


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Outline

Introduction Primary Signal Detection Spectrum Opportunities Detection Performance vs. Constraint Sensing Accuracy vs. Sensing

Overhead


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Introduction

Limited supply


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Introduction

Growing demand


Current Policy & Spectrum Scarcity

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Spectrum Opportunitiesin Space, Time, & Frequency

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(Credit: DARPA XG) (Credit: ACSP Cornell)


Primary Signal Detection

Choice of detectors Criteria:

Bayesian Neyman-Pearson Parameter settings?

Energy detection Pros: easily implemented; minimal assumptions Cons: poor performance with noise uncertainty and with

multiple secondary users Performance ∼ 1/SNR2 at low SNR

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Choice of Detectors - Cyclic Detectors (2) Exploit guard bands in frequency, known carriers, data

rates, modulation type Pros:

fc, Ts easy to detect via square-law devices, or cyclic approaches

Cyclic approaches useful when σ2n is unknown (avoid SNR wall)

Easily implemented via FFTs Cons:

Timing and frequency jitter can be detrimental Requires long integration times RF non-linearities; Spectral leakage (ACI).

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Choice of Detectors: Matched Filter (3)

Exploit pilots or sync (PN) sequences in primary (WRAN 802.22)

Pros: Correlation detection is usually better than energy

detection. Performance ∼ 1/SNR at low SNR

Cons: fading may null pilot; need to cope with time and freq

sync

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Other Detectors Receiver leakage Wild-Ramachandran, Dyspan’05 Signal correlation Zeng et al, PIMRC’07 Fast fading Larson-Regnoli, CommLett’07 Multiple antennas Pandhripande-Linnartz, ICC’07 HMM classifier Kyouwoong et al, Dyspan’07 Wavelet-based Tian-Giannakis, CrownCom’06 Multi-resolution sensing Neihart-Roy-Allstot, ISCAS’07 Compressed sensing Tian-Giannakis, ICASSP’07

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Spectrum Opportunities Detection

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A channel is an opportunity for A − B if

• the transmission from A to B can succeed

• the interference power to primary is below a prescribed level


Spectrum Opportunity: Definition

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Spectrum Opportunity: Definition

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Spectrum Opportunity: Properties

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• Determined by both transmitting and receiving activities of primary users.

• Asymmetric (an opportunity for A−B may not be one for B−A).


Detection of Primary Receivers

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• rI: interference range, Rp: primary tx range, rD: detection range

• Detecting primary Rx within rI by detecting primary Tx within rD


Detecting Primary Signals (LBT)

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• rD: detection range.• H0: no primary Tx within rD, H1: alternative.• False alarms and miss detections occur due to noise and fading.


From Detecting Signal to Detecting Opportunity

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• H0: opportunity, H1: alternative.• Even with perfect ears, exposed Tx(X) ⇒ FA, hidden Rx(Y) ⇒ MD.• Adjusting detection range rD leads to different operating points.


When Is Detecting Signal = Detecting Opportunity?

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A Necessary and Sufficient Condition:

• NS condition: ∀X ∈ Ptx(A) ∩ Pctx(B), its receivers are in Prx(A)

• Perfect detection achieved when detecting Ptx(A) ∪ Ptx(B)


Miss Detection May not Lead to Collision

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• There is no primary receiver around A

• There are primary transmitters around B


Miss Detection May Lead to Success

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• There are primary receivers around A

• There is no primary transmitter around B


Correctly Identified Opportunity May Not Lead to Success

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• Successful data transmission and failed ACK


Performance vs. Constraint

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Performance Optimal under relaxed constraint on the

average number of active arms. Asymptotically optimal (N →∞ w. M/N

fixed) under certain conditions. Near optimal performance observed from

extensive numerical examples.



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Two Models Global Interference Model Local Interference Model



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Throughput comparison.


Sensing Accuracy vs. Sensing Overhead

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Optimal sensing time: efficiency η versus sensing window length n for various SNRs and PMD.


Sensing Accuracy vs. Sensing Overhead

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Optimal sensing time: efficiency η and optimal window length n∗/N versus slot length N.


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Chapter 4 Summary

The following topics have been covered: Different types of detectors for primary signal

detection Detection of spectrum opportunities based on

the detection of primary signals. The trade-off between performance and

interference constraint. The trade-off between sensing accuracy and

sensing overhead.

Documents

1 Chapter 4 Spectrum Sensing and Identification Cognitive Radio Communications and Networks: Principles and Practice By A. M. Wyglinski, M. Nekovee, Y