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Interference Effects of Multi-User Ultra-wideband Systems
Anup DoshiCarnegie Mellon University July 31, 2003
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Outline
Intro
Models
Observations
Summary
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What is an Ultra-wideband Signal? Short impulses in
succession
FCC Definition – Bandwidth > 25% of
center frequency 0 200 400 600 800 1000-1
-0.5
0
0.5
1
ns
Impulse Signal
0 2000 4000 6000 8000 10000-100
-80
-60
-40
-20
0
MHzPow
er S
pect
rum
Mag
nitu
de (d
B)
PSD
-0.5
0
0.5
1
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Advantages of UWB
Low power levels spread over large spectrum Operates below noise floor of narrowband devices
Possibility of >500Mbps short range
GP
S
Frequency (Ghz)1.6 1.9 5
802.11a
-41 dBm/Mhz“Part 15 Limit”
UWB Spectrum 10.63.1
Source: Intel
PC
S
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Potential Applications are Numerous
Personal Area Network Interconnect Computers, Devices, PDAs, Printers Entertainment...TV, Camcorder, DVD Music…MP3, Audio Systems, etc
Safety Through-wall Imaging Sensor Network
Lots of other exciting applicationsBroadband
UWBUWB
LAN/WLAN
UWB
UWB
UWB
Image Sources: Intel, AetherWire
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Why Only Now?
Started as impulse radar, 1960’s Primitive forms, simple communication Studied & used by military
New technology enables digital comm., 1990’s Commercial applications seen by several companies 1998 - Petitioned FCC to review potential uses 2003 - FCC approves development of conservative
applications
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Problem…
What happens when lots of UWB devices are transmitting in close proximity?
Will the combined noise level be too much for a victim narrowband receiver?
Existing studies claim minimal effects Done by various agencies and companies
Those studies do not examine all cases…
This is my job!
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VICTIM
Constant-Distance DistributionMultiple UWB devices located three meters from a victim
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Units turn on and off in a 2-state Markov Process
Switching times are Exponential Random Variables Time until on ~ Exponential(λ) => mean 1/λ sec Time until off ~ Exponential(µ) => mean 1/µ sec
Rho=ρ= λ/µ
Characterizing the Transmitters
UnitOff
UnitOn
λ
µ
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Characterizing the Transmitters
Total Number on modeled as a Markov Chain
Steady-state probabilities:
0 1 2 N-1 N
Nλ (N-1)λ … λ
µ … Nµ2µ
N
n
n n
Np
)1(
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1
2
3
4
5
6
7
8
9
10
0 0.5 1 1.5 2 2.5 3 3.5 4
How Does the System Act Over Time?
λ =1, µ=2
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1
2
3
4
5
6
7
8
9
10
0 0.5 1 1.5 2 2.5 3 3.5 4
How Does the System Act Over Time?
Total Number of Units Onλ =1, µ=2
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Noise Level in Victim Receiver
Each UWB signal modeled as White Noise Total Noise= N0+M(t)*N1
Ambient Noise Floor(=kTw)
Number of Transmitters On(Markov Chain)
Power Received at Victim from UWB Signal
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Some Properties of This Model
Autocorrelation Spectral Density
-100 -50 0 50 1000
1
2
3
4
5
6
7
8
Hz
Ma
gn
itu
de
PSD = fft(Rx(t))
-5 0 50
0.5
1
1.5
2
2.5
tau (sec)
Ma
gn
itu
de
Rx(tau) = E(M(t)M(t+tau))
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Probability of Error in Receiver
On Average:
101 *
2*))((
NiN
EQitMPP
bk
ierr
0 0.5 1 1.5 2 2.5 30
0.05
0.1
0.15
0.2
0.25
0.3
0.35Average Probabilty of Error
rho
Pe
rr
20 units
10 units
(µ=1)
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Other Ways to Describe Model
Probability of Outage P(outage)=Probability( Perr > Pe* )
Pe*=.1, .01
Perr=
Expected Time of Outage E(T10) = T1+aN,1E(T20)
E(T20)=T2+aN,2E(T10)+bN,2E(T30) … E(TN0)=TN+E(T(N-1)0)
10 )(
2
NtMN
EQ
b
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P(outage), Expected Time of Outage
0 0.5 1 1.5 2 2.5 30
2
4
6
8
10
12E(outage)
t (s
ec
)
rho
12 units
10 units
3m radius
0 0.5 1 1.5 2 2.5 30
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
rho
P(o
uta
ge
)
P(outage)
10 units
12 units
3m radius(µ=1) (µ=10)
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VICTIM
Random-Distance DistributionUWB devices distributed uniformly in a circular area around victim
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Properties of Random-Distance Model
Moved to a computer simulation
Experimentally calculated: P(outage), Expected Time of Outage, Max and mean power levels over time
Done on Matlab – Monte Carlo simulation
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Example Simulation Run
0 0.5 1 1.5 2 2.5 3 3.5 4-52
-50
-48
-46
-44
-42
-40
-38
-36
-34
time
dB
m
Interference Level
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0 1 2 3 4 5 6 7 8 9
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
time
dB
m
Interference Level
Example Simulation Run
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Example Simulation Run
0 5 10 15 20-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
time
dB
m
Interference Level
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0 5 10 15 20 25 30 35 40
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
time
dB
m
Interference Level
Example Simulation Run
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P(outage) & Expected Time of Outage
0 0.5 1 1.5 2 2.5 30
0.2
0.4
0.6
0.8
1
Probability of Outage
rho
Po
uta
ge
Random-distanceradius=3m
Constant-distanceradius=3m
10 units
0 0.5 1 1.5 2 2.5 30
1
2
3
4
5
6
7
8
9Expected Time of Outage
tim
e (s
ec)
rho
Random-distanceradius=3m
Constant-distanceradius=3m
10 units(µ=1) (µ=10)
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Max/Mean Power Levels
0 5 10 15 20 25 30 35 40-90
-80
-70
-60
-50
-40
-30
-20
Number of Transmitters in 10m radius
dB
m
Average Max and Mean Power Levels over 500 simulations
Average Max Power
Average Mean Power
Allowed Interference Level
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Observations
Multiple transmitters will cause major problems in worst cases
Such situations may soon arise in real-life situations
Important to consider every possible case in testing
Broadband
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Future Work
Need to consider many more variables Receiver type Frequency, PRR Different distributions
Once 802 Standard comes out, incorporate into model Possibly Multi-Band OFDM (TI, Intel) Possibly Dual-Band (Time Domain, Motorola)
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Summary
Characterized an aggregate of UWB transmitters
Realized various methods of measuring effect on victim receiver
Concluded that as number of UWB transmitters increase, performance of victim receiver attenuates
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Acknowledgments
Prof Baum
Prof Noneaker, Prof Xu
ECE Faculty and Grads
NSF