techniques for cognitive radio

Vermelding onderdeel organisatie

1

Instituto Superior Tecnico, Lisbon, Portugal, 27 September 2007.

TECHNIQUES FOR COGNITIVE RADIO

International Research Centre for Telecommunications and Radar (IRCTR) Delft University of Technology The Netherlands

H. NIKOOKAR

2

Cognitive Radio

Cognitive Radio=an intelligent wireless communication system aware of its surrounding environment.

CR uses the methodology of understanding by building to learn from the environment and adapt to statistical variations in the radio channel.

Changing operating parameters in real time and efficient utilization of the radio spectrum.

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Cognitive Radio: A radio (to generate, transmit, receive and process of wireless signals) and a cognitive brain which learns from the environment and performs rational processes and predicts probable consequences and remembers past successes and failures.

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Talk Outline• Spectrum Overview• Spectrum Sensing• Adaptive OFDM and Spectrum Pooling• Waveshaping

Transfer Domain Communication System (TDCS)Wavelet Domain Communication System (WDCS)Multi Carrier Wavelet Packet Modulation withInterference Mitigation

• Beamforming• Summary

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Spectrum Overview

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Spectrum Sensing

• Energy Detector

• Cyclostationary feature detector

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OFDM

8

Adaptive OFDM and Spectrum Pooling (1)

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Reasons for using of OFDM as modulation scheme in Cognitive Radio : • Many Parameters to be altered based on channel and spectrum

information• FFT for OFDM and its use for spectrum sensing

Adaptive OFDM :• Adaptive bit loading

ChowFischerSimple Blockwise Loading Algorithm (SBLA)Fischer Groupwise

• Adaptive power loading• Combination of adaptive bit and power loading

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Adaptive Bit Loading :•Fischer-Huber Algorithm : Minimizing the probability of error

with the constraints of limited total power and total number of bits allocation.

•Chow Algorithm : Maximizing allowable additional amount of noise that the system can tolerate, with the constraints of limited total power and total number of bits allocation.

•Simple Blockwise Algorithm : Grouping of subcarriers and modulation modes based on the mean SNR per group.

•Blockwise bit loading with Fischer - Huber algorithm : Bit loading (per group) using the information of mean channel gain per group of subcarriers.

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Adaptive OFDM and Spectrum Pooling (4)• Adaptive Bit loading :1. Fischer-Huber algorithm , Minimizing subject to

and where

Bit allocation :

Kn: Number of nearest neighbourdn: Minimum distance between constellation pointspn: Error rate in subcarrier nRn : Number of allocated bits to subcarrier nRQn : Quantized number of allocated bits (0,1,2,4,6) based on constellation size to

subcarrier nRT: Number of bits per OFDM symbolK : Number of active carriersPn : Allocated power to subcarrier nPT: Total available power for an OFDM symbolHn: Complex channel gain at subcarrier nNn: Noise variance at subcarrier n C : Constant

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2. Chow algorithmbit allocation :

Rn : Transmission rate of carrier n Pn: Power allocation in subcarrier nRtarget : Target allocation bits per OFDM s Pbudget : Total available power / OFDM Sn : Signal power of carrier n symbolNn : Equivalent noise power of carrier nΓ : SNR gap is a constant, where γmargin : Noise margin

664-QAM2QPSK

416-QAM1BPSK

Number of bitsModulation modeNumber of bitsModulation mode

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3. Blockwise loading algorithm

Initial bits / modulation mode allocationIf the total number of bits is not equal to the desired rate, addition/ subtraction is applied with priority is given to the highest/lowest SNR per group until the total number of bits fulfills the desired rate.The grid is shifted according on the average SNR

and average number of bits per carrier

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Adaptive OFDM and Spectrum Pooling (7)4. Groupwise bit loading with Fischer Algorithm, subcarriers are divi-

ded into G groups, error probability is minimized,bit allocation :

subject to

Ri : Transmission rate of carrier group i Rtarget : Target allocation bits per OFDM symbol G ‘ : Number of active groups Hi : Average channel power on carrier group iψ : subset of active groups

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Adaptive OFDM and Spectrum Pooling (8)Adaptive Power Allocation: Power is allocated to each carrier to minimize error probability

subject to

Pn : Power allocated on the n’th carrierPT : Total powerM : Constellation sizeW(.) : Lambert function (inverse function of f(w)=wew )фk = |Hk|2 , where Hk is the estimated channel gain on carrier kη : constantσk

2 : noise variance on carrier k

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Spectrum Pooling : Public access to the spectrum without sacrificing the transmission quality of license owners.

OFDM use in Spectrum Pooling- Deactivation of subcarriers. Non Contigiuous OFDM

(important parameters are b=# deactivated carriers, LU/Δf = a;pool occupancy = b/N)

- Interference to licensed users can be mitigated by windowing of OFDM signal.Different windows : Bartlett

Raised CosineBetter than Raised Cosine

- Combination of windowing and carriers deactivation

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• Spectrum Pooling with dynamic deactivation of subcarriers adjacent to licensed user’s band

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Raised Cosine window

g(t) : window functionTu : OFDM useful symbol duration

: roll off factor

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Spectrum of Raised Cosine window with Variation of roll off factor ( )

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Bartlett window

Better than Raised Cosine Window⎪⎪⎪

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21


Window design with influence on the transmitted signal

Window design without influence on the transmitted signal

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Adaptive OFDM and Spectrum Pooling(15)

ωm = 2πfm, fm frequency on subcarrier mA(ωm) the symbol from constellation PSK or QAM on

subcarrier mg(t) window functionp(ωm ) power allocation to subcarrier mθ (ωm ) phase of carrier mT OFDM useful symbol duration

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Adaptive OFDM and Spectrum Pooling (16)Windowing effect

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Waveshaping (1)

•Shaping the spectrum of transmitted signal in such a way not to interfere to licensed users

Transform Domain Communication System (TDCS)

Wavelet Domain Communication System (WDCS)

Wavelet based interferece mitigation

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Waveshaping (2)

Transmitter ReceiverTransform Domain Communication System (TDCS)

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Waveshaping (3)Wavelet domain communication system (WDCS)

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Multi Carrier Wavelet Packet Modulation

Waveshaping (4)

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Analysis

Synthesis

H 2:1↓ 1:2↑ H’

G 2:1↓ 1:2↑ G’

+

Analysis Filter Banks Synthesis Filter Banks

Waveshaping (5)

Wavelet design for LUInterference rejection

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Wavelet Packet Decomposition of Signal

Wavelet Packet Reconstruction of Signal

Waveshaping (6)

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Subband Removal according to Gray code position

Waveshaping (7)

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Maximally frequency selective wavelet according to Remez Algorithm

Ripple

Stop bandAttenuation

Transition BandPass Band Stop Band

Waveshaping (8)

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0 2 4 6 8 10 12 1410

-4

10-3

10-2

10-1

100

Ex/No [dB]

Bit E

rror

Rat

e

AWGN AloneAWGN and 802.11aDaub-15Frequency Selective WaveletCoif-5Symlet-15

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Beamforming (1)

CR With BeamformingCR Without Beamforming

• Interference avoidance to LU by not directing signal to LU while occupying LU freq

Communicating CR nodes without interfering each other

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Beamforming (2)

Baseband model of a linear equally spaced arrayA linear equally spaced array antenna

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Summary• Spectrum sensing is the critical ingredient of CR. All the

transmission techniques for CR rely on the accurate informationfrom the spectrum sensing module.

• Multicarrier Non contiguous OFDM has been introduced as a CR technique. Proper window will further decrease the level of interference to LUs. Loss of bitrate due to carriers deactivation and long window duration is compensated by applying higher modulation modes to the carriers with good channel condition.

• Single carrier TDCS as an alternative CR technique was reviewed.• Wavelet basis function as replacement of Fourier transform in OFDM

and TDCS for CR system is a research issue for further investigation.• Beamforming as a spatial interference avoidance technique in CR

has been discussed.

techniques for cognitive radio

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