Mishra Vangreunen.slides

8/10/2019 Mishra Vangreunen.slides

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Shridhar Mubaraq Mishra

Jana van Greunen

May 13th, 2004

* Adapting behavior based on external factors


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Time (min)

Freq

uency(Hz)

Existing spectrum policy forces

spectrum to behave like a fragmented

disk

Bandwidth is expensive and goodfrequencies are taken which seems toimply spectrum scarcity!

Recent measurements by theFCC in the US show 70% of theallocated spectrum is not utilized

Time scale of the spectrumoccupancy varies from msecs tohours


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Definition: A cognitive radio (CR) is a radio that can change its transmitter

parameters based on interaction with the environment in which is operates -[FCC NPRM - 03-322]

Cognitive radio properties

Sensing: RF technology that "listens" to huge swaths ofspectrum

Cognition: Ability to identify Primary Users

Adaptability : Ability to change parameters to best use whitespaces:

Power levels

Frequency bands of operation

Modulation parameters


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Bluetooth

Cordless phone

CR2

AP

CR3

Dyn

amic

Freque

ncySelection

Centralized system model with AccessPoint

CR system functions:

1. Sensing

2. Reporting

3. Channel Sounding

4. Channel Allocation

Primaryusers

CR1


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Nusers

Kchannels (K >> N)

Channel gains for user (i), : [||hi1||2 ||hi2||2 ||hiK||2]

Power allocated for user (i) : [Pi1Pi2PiK]

Optional:requested rates : [R1 R2RN]

Goal: minimize total power P, more formally:


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Existing channel allocation framework

What is the solution space?

Are there low complexity algorithms for one user per channel? How do user rates affect minimum power?

What is different for CRs?

What does the channel gain matrix look like with PUs?

How do different AP allocation strategies change interference toPUs?


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If no rate constraints but sum capacity constraint then:

Dual of problem in lecture!

Optimal solution is no sharing (Tse98)

If rate constraints then :

Optimal solution is sharing

Successive decoding required

If one user per channel then :

Constraint optimization problem (integer programming)

NP Complete!


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Channel Gain Matrix (H):

U1

U2

C1

C2

C3

C4

Users Channels Parameters:N= 2,K = 4,Csum= 2 bits/s/Hz,Rreq1 = Rreq2= 1 bits/s/Hz


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0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0. 1

0 . 1 2

0 . 1 4

0 . 1 6

0 . 1 8

1 2 3 4

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0. 1

0 . 1 2

0 . 1 4

0 . 1 6

0 . 1 8

1 2 3 4

Optimal:P* = 0.4344 W,

R1 = 1.5832 bits/s/Hz,R2= 0.4168 bits/s/Hz,

Optimal sharing:Ps= 0.4467 W

Optimal no sharing:

Pnos= 0.4534 W

0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0. 1

0 . 1 2

0 . 1 4

0 . 1 6

0 . 1 8

1 2 3 4

1 2 3 4

Channel Number

Power per Channel

Pow

er(W)

User 2

User 1


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Two step approach for one user per channel

Channel allocation

Power allocation waterfilling

Channel allocation algorithm:

while (there are unassigned channels)

Perform max weighted matching (Hungarian

method)

Remove allocated channels from graph

for all users (i)

if Ci> Ri/Sum(Ri), then remove user

end for

end while

Complexity O(K(N+K)2) O(K3)

U1

U2

C1

C2

C3

C4

U1

U2

C1

C3

U1 -> C2

U2 -> C4

4

4

4.5

U1 -> C3

U2 -> C1

2

3

1

3.5


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Optimal power

algorithm is quick &

easy but R*wont

meet requested rates

Estimate performance

for non-optimal withrequested rate R from

optimal algorithm

Question: How doesRreqR*influenceminimum power?

It is polynomial in ||R-R*||


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What is different for Cognitive Radios?


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Presence of Primary Users in

channels introduces 0s.

Number of zeros depends onthe frequency range

CR detecting primary usersdepends on spectral

environment/location

Used measurements at BWRCto estimate zeros per channel

Developed Matlab model togenerate Channel Gain matrixin accordance with the data

0 0.5 1 1.5 2 2.5

x 109

-90

-85

-80

-75

-70

-65

-60

-55

-50

-45

-40

Frequency (Hz)

SignalStrength(d

B)

TV bands

Cell

PCS

Spectrum usage in (0, 2.5) GHz


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Two extreme approaches:

Aggressive approach: Allocate the channel even if subset of CRssee Primary User (PU) => Might cause interference to PU

Conservative Approach: If any CR detects a Primary User, dontuse that channel => CR system might need more power for samerequested rates

CR power penalty and PU interference tradeoff:

Use threshold to determine channel availability

Find optimal threshold based on interference specification ofPrimary User


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Power penalty to CRsInterference to PUKey:

Threshold

Pow

er(Ratio

)

Power penalty and Interference to PU (additional power required)vs. threshold


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Contributions:

1. Developed a O(K3) complexity algorithm for one user perchannel allocation

2. Developed model to estimate power penalty from R* givenrequested rates R

3. Modeled channel gain matrix for CRs consistent with real data

4. Analyzed the effect of various thresholds on power penalty and

interference to PUs


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Future Work (we plan to do before May 24th)

In a real system power assignment per channel is a function ofsensing radios sensitivity

Highly sensitive radios can allocate more power (Pik

sensitivity)

Investigate tradeoff between CR sensitivity and power requirement

Future Work (Maybe for EE 224B, Spring 05 !!) Explore channel allocation algorithms in ad-hoc networks

Develop lower-complexity [O(K)/O(K2)] allocation algorithms

Incremental channel allocation for channels with different coherence

times


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Wireless Overload !!


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Backup slides


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0

0. 5

1

1. 5

2

2. 5

3

3 . 5

1 2 3 4 5

Optimal s har ing

Gr ee dy

Matrix Number

Power per Channel

Powe

r(W)

N = 6, K = 8

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