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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
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0. 1
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0 . 1 8
1 2 3 4
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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
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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
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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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Optimal s har ing
Gr ee dy
Matrix Number
Power per Channel
Powe
r(W)
N = 6, K = 8