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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
MSIT 413: Wireless TechnologiesWeek 3
Michael L. HonigDepartment of EECS
Northwestern University
January 2011 1
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Cellular Frequency Assignments
ED
F
CA
EB
GD
C
E
AB
C
DF
AG
BA
GD
C
BA
FG
CGD
AD
BC
FE
G
B
cell cluster(contains all channels) co-channel cells
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Cellular Terminology
• Cell cluster: group of N neighboring cells which use the complete set of available frequencies.
• Cell cluster size: N
• Frequency reuse factor: 1/N
• Uplink or reverse link: Mobiles Base station
• Downlink or forward link: Base station mobiles
• Co-channel cells: cells which are assigned the same frequencies
CD
FA
BAF
GCG
D
ADB
CF
EGB
cell cluster
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Performance Measure:Signal-to-Interference-Plus-Noise Ratio (SINR)
• Expressed in dB (10 log (SINR))
• Typically, the interference power dominates (ignore noise)– SINR Signal-to-Interference Ratio (SIR or S/I)
• Total interference power is the sum over all interferers:– More co-channel users more interference
Power Noise Power ceInterferenPower Signal Received
+=SINR
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Voice Capacity• Defined as
– Example: SE = 0.1 Erlang/MHz/km2
10 MHz needed to support 1 Erlang/km2
(Note that 1 km2 may correspond to a cell.)
– To convert Erlangs to users, must estimate traffic per user, e.g., if on average, each user is active 1/10 of the time, then 1 Erlang corresponds to 10 users.
• Traffic per cell depends on:– Number of channels– Grade of Service (e.g., typically 2%)– S/I requirement (determined by cluster size N)
)(kmarea Total(MHz)] bandwidth [Total(Erlangs) carried trafficTotal
2×
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Channel Allocation
• Consider GSM: 25 MHz (simplex), 200 kHz channels125 channels (1 is used for control signaling)
• How to allocate channels to cells?
Suburb(lightly loaded)
Train stationShopping mall
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Channel Allocation
• Consider GSM: 25 MHz (simplex), 200 kHz channels125 channels (1 is used for control signaling)
• How to allocate channels to cells?– Objective: equalize blocking probability
(target is around 2%)
• Fixed channel allocation assignsfixed set of channels to each cell– Drawback?
Suburb(lightly loaded)
Train station
mall
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Channel Allocation• Consider GSM: 25 MHz (simplex), 200 kHz channels
125 channels (1 is used for control signaling)
• How to allocate channels to cells?– Objective: equalize blocking probability
(target is around 2%)
• Fixed channel allocation assignsfixed set of channels to each cell.– Drawback: traffic is time-varying
• Dynamic channel allocation varies the number of channels per cell, depending on the load.
Suburb(lightly loaded)
Train station
mall
8
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation
2,61,3
Channels already in use
5
1,3,4new call: search for channel
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation
• Channels assigned from complete set of available channels
• Each user searches for a channel with high SINR (low interference)
• No frequency planning!
2,61,3
Channels already in use
5
3,4new call: search for channel
4
10
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
802.11b/g Channels
• 14 overlapping (staggered) channels (11 in the U.S.)• Center frequencies are separated by 5 MHz• Bandwidth/interference controlled by “spectral mask”
– 30 dB attenuation 11 MHz from center frequency– 50 dB attenuation 22 MHz from center frequency
Channels: 1 6 11
Comparison table
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation: 802.11b/g
Channel 6 Channel 1Channel 11
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation: 802.11b/g
Channel 6 Channel 1Channel 11
Channel 6
Add new router
interference
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation: 802.11b/g
Switches to channel 1 Channel 1Channel 11
Channel 6
interference
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation: 802.11b/g
channel 1 Switches to channel 6Channel 11
Channel 6
interference
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dynamic Channel Allocation: 802.11b/g
channel 1 channel 6Channel 11
Switches to channel 1
interference
Dynamic channel assignment becomes unstable! 16
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Overlapping Channel Assignment
channel 1 channel 6Channel 11
Channel 3
Interference(less than before) Interference
(less than before)power
frequencyChannels: 1 3 6 17
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
FCA vs. DCA
• Moderate/High complexity – Must monitor channel occupancy,
traffic distribution, S/I (centralized)• Better under light/moderate traffic• Insensitive to changes in traffic• Stable grade of service• Low probability of outage
(call termination)• Suitable for micro-cellular systems
(e.g., cordless)• Moderate/high call setup delay• No frequency planning• Assignment can be centralized or
distributed
• Low complexity
• Better under heavy traffic • Sensitive to changes in traffic• Variable grade of service• Higher probability of outage
• Suitable for macro-cellular systems(e.g., cellular)
• Low call setup delay• Requires careful frequency planning• Centralized assignment
DCAFCA
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Why Study Radio Propagation?• To determine coverage
• Must determine path loss– Function of
• Frequency• Distance• Terrain (office building, urban, hilly, rural, etc.)
Need “large-scale” models
Can we use the samechannels?
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Why Study Radio Propagation?
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Why Study Radio Propagation?• To enable robust communications (MODEM design)
• How can we guarantee reliable communications?• What data rate can we provide?• Must determine signal statistics:
– Probability of outage– Duration of outage
Need “small-scale” models
Received Power
time
Deep fades may cause an outage
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Will provide answers to…
• What are the major causes of attenuation and fading?• Why does the achievable data rate decrease with
mobility?• Why are wireless systems evolving to wider
bandwidths (spread spectrum and OFDM)?• Why does the accuracy of location tracking methods
increase with wider bandwidths?
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Propagation Key Words• Large-scale effects
– Path-loss exponent– Shadow fading
• Small-scale effects– Rayleigh fading– Doppler shift and Doppler spectrum– Coherence time / fast vs slow fading
• Narrowband vs wideband signals• Multipath delay spread and coherence bandwidth• Frequency-selective fading and frequency diversity 23
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Propagation Mechanisms:1. Free Space
reference distance d0=1
distance d
Reference power at reference distance d0 Path loss exponent=2
In dB: Pr = P0 (dB) – 20 log (d)
Pr (dB)
log (d)
slope = -20 dB per decade
P0
0
P0 = Gt Gr (λ/4π)2
antenna gains wavelength24
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Wavelength
• Wavelength >> size of object signal penetrates object.
• Wavelength << size of object signal is absorbed and/or reflected by object.
• Large-scale effects refers to propagation over distances of many wavelengths.
Small-scale effects refers to propagation over a distances of a fraction of a wavelength.
λ (meters) = c (speed of light) / frequency
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Dipole Antenna
cable from transmitter
wire (radiator)
802.11 dipole antenna
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Radiation Pattern: Dipole Antenna
Dipole axisDipole axis
Electromagnetic wave radiates outfrom the dipole axis.
Cross-section of doughnut pattern 27
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Antenna Gain Pattern
Dipole pattern (close to isotropic)
Red curve shows the antenna gain versusangle relative to an isotropic pattern (perfect circle) in dB.
Often referred to as dBi, dB “isotropic”.
-5 dB (factor of about 1/3) relative to isotropicpattern
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Antenna Gain Pattern
Dipole pattern (vertical) 90 degree sector 29
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Attenuation: Wireless vs. Wired
• Path loss ~ 13 dB / 100 meters or 130 dB / 1 km– Increases linearly with
distance
• Requires repeaters for long distances
• Path loss ~ 30 dB for the first meter + 20 dB / decade– 70 dB / 100 meters
(2 decades)– 90 dB / 1 km
(3 decades)– 130 dB / 100 km!– Increases as log (distance)
• Repeaters are infeasible for satellites
Short distance Wired has less path loss.Large distance Wireless has less path loss.
Unshielded Twisted Pair 1 GHz Radio (free space)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Propagation Mechanisms
2. Reflection
3. Diffraction
4. Scattering
Incident E-M waveθ θ
reflected wave
transmitted waveLength of boundary >> wavelength λ
Hill
Signal loss dependson geometry
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Why Use > 500 MHz?
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Why Use > 500 MHz?• There is more spectrum available above 500 MHz.• Lower frequencies require larger antennas
– Antenna dimension is on the order of awavelength = (speed of light/frequency) = 0.6 M @ 500 MHz
• Path loss increases with frequency for the first meter– 10’s of GHz: signals are confined locally– More than 60 GHz: attenuation is too large
(oxygen absorbs signal)33
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
700 MHz Auction• Broadcast TV channels 52-69 relocated in Feb. 2009.
– 6 MHz channels occupying 698 – 806 MHz
• Different bands were auctioned separately:– “A” and “B” bands: for exclusive use (like cellular bands)– “C” band (11 MHz): must support open handsets, software apps– “D” band (5 MHz): shared with public safety (has priority)
• Commenced January 24, 2008, ended in March
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Why all the Hubbub?• This band has excellent propagation characteristics for cellular
types of services (“beach-front property”).• Carriers must decide on technologies: 3G, LTE, WiMax,…• Rules for spectrum sharing can be redefined…
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
36
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
C Band Debate• Currently service providers in the U.S. do not allow any services,
applications, or handsets from unauthorized 3rd party vendors.
• Google asked the FCC to stipulate that whoever wins the spectrum must support open applications, open devices, open services, open networks (net neutrality for wireless).
• Verizon wants to maintain “walled-garden”.
• FCC stipulated open applications and devices, but not open services and networks: spectrum owner must allow devices or applications to connect to the network as long as they do not cause harm to the network
• Aggressive build-out requirements:– Significant coverage requirement in four years, which continues to
grow throughout the 10-year term of the license. 37
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Sold to…
• Verizon
• Other winners: AT&T (B block), Qualcomm (B, E blocks)
• Total revenue: $19.6 B– $9.6 B from Verizon, $6.6 B from AT&T
• Implications for open access, competition?
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
D Band Rules• Winner gets to use both D band and adjacent public service band
(additional 12 MHz!), but service can be preempted by public safety in emergencies.
• Winner must build out public safety network:must provide service to 75% of the population in 4 years, 95% in 7 years, 99.3% in 10 years
• Minimum bid: $1.3 B; estimated cost to deploy network: $10-12 B
• Any takers? …
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
D Band Rules• Winner gets to use both D band and adjacent public service band
(additional 12 MHz!), but service can be preempted by public safety in emergencies.
• Winner must build out public safety network:must provide service to 75% of the population in 4 years, 95% in 7 years, 99.3% in 10 years
• Minimum bid: $1.3 B; estimated cost to deploy network: $10-12 B
• Any takers? … Nope! Highest bid was well below reserve…
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Radio Channels
Troposcatter Microwave LOS
Mobile radio Indoor radio
T
T
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Sinusoidal Signal
Amplitude A=1
Time delay = 12,Phase shift θ = 12/50 cycle = 86.4 degrees
Period= 50 sec, frequency f = 1/50 cycle/sec
Time t (seconds)
Electromagnetic wave s(t) = A sin (2 π f t + θ)s(
t)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Two Signal Paths
s1(t)
s2(t)
Received signal r(t) = s1(t) + s2(t)
Suppose s1(t) = sin 2πf t.Then s2(t) = h s1(t - τ) = h sin 2πf (t - τ)
attenuation(e.g., h could be ½)
delay(e.g., τ could be 1 microsec.)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Sinusoid Addition (Constructive)
0 20 40 60 80 100 120 140 160 180 200−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
time
sin(
2*pi
*t −
pi/2
) +
0.7
07*s
in(2
*pi*
t)
0 20 40 60 80 100 120 140 160 180 200−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
time
0.70
7*si
n(2*
pi*t
)
+ =
s1(t)
s2(t)
r(t)
Adding two sinusoids with the same frequency gives another sinusoid with the same frequency! 44
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Sinusoid Addition (Destructive)
0 20 40 60 80 100 120 140 160 180 200−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
time
sin(
2*pi
*t −
pi/2
) +
0.7
07*s
in(2
*pi*
t)
+ =
0 20 40 60 80 100 120 140 160 180 200−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
time
sin(
2*pi
*t −
3*p
i/2)
Signal is faded.
s1(t)
s2(t)
r(t)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Indoor Propagation Measurements
Hypothetical large indoor environment
Ceiling
Normalized received power vs. distance46
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Power Attenuation
In dB: Pr = P0 (dB) – 10 n log (d)
reference distance d0=1
distance d
Reference power at reference distance d0 Path loss exponent
Pr (dB)
log (d)
slope (n=2) = -20 dB per decadeP0
0
slope = -40(n=4)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Path Loss Exponents
ENVIRONMENT PATH LOSS EXPONENT, n
Free space 2
Urban cellular radio 2.7 to 3.5
Shadowed urban cellular radio 3 to 5
In building line-of-site 1.6 to 1.8
Obstructed in building 4 to 6
Obstructed in factories 2 to 3
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Large-Scale Path Loss (Scatter Plot)
Aver
age
Rec
eive
d P
ower
(dB
m)
Distance (meters) 49
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Shadow Fading
• Random variations in path loss as mobile moves around buildings, trees, etc.
• Modeled as an additional random variable:
Pr = P0 – 10 n log d + X
“log-normal” random variablestandard deviation
σ−σ received power in dB
“normal” (Gaussian)probability distribution
For cellular: σ is about 8 dB 50
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Large-Scale Path Loss (Scatter Plot)
Most points are less thanσ dB from the mean
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Empirical Path Loss Models• Propagation studies must take into account:
– Environment (rural, suburban, urban)– Building characteristics (high-rise, houses, shopping malls)– Vegetation density– Terrain (mountainous, hilly, flat)
• Okumura’s model (based on measurements in and around Tokyo)– Median path loss =
free-space loss + urban loss + antenna gains + corrections– Obtained from graphs– Additional corrections for street orientation, irregular terrain
• Numerous indoor propagation studies for 802.11 52
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
SINR Measurements: 1xEV-DO
drive test plots
53
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Link BudgetHow much power is required to achieve target S/I?
• dBs add:
Target S/I (dB) + path loss (dB) + other losses (components) (dB) - antenna gains (dB)
Total Power needed at transmitter (dB)
• Actual power depends on noise level.– Given 1 microwatt noise power, if the transmit power is 60 dB
above the noise level then the transmit power is 1 Watt.54
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Example
• Recall that dBm measures the signal power relative to 1 mW (milliwatt) = 0.001 Watt. To convert from S Watts to dBm, useS (dBm) = 10 log (S / 0.001)
• Transmitted power (dBm) = -30 + 40 = 10 dBm = 10 mW
• What if the received signal-to-noise ratio must be 5 dB, and the noise power is -45 dBm?
Transmitter Receiverwireless channel
40 dB attenuationReceived powermust be > -30 dBm
What is the requiredTransmit power?
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Urban Multipath
• No direct Line of Sight between mobile and base• Radio wave scatters off of buildings, cars, etc.• Severe multipath 56
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Narrowband vs. Wideband
• Narrowband means that the bandwidth of the transmitted signal is small (e.g., < 100 kHz for cellular). It therefore looks “almost” like a sinusoid.– Multipath changes the amplitude and phase.
• Wideband means that the transmitted signal has a large bandwidth (e.g., > 1 MHz for cellular).– Multipath causes “self-interference”.
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Narrowband FadingReceived signal r(t) = h1 s(t - τ1 ) + h2 s(t - τ2) + h3 s(t - τ3 ) + …
attenuationfor path 1 (random) delay for path 1 (random)
If the transmitted signal is sinusoidal (narrowband), s(t) = sin 2πf t,
then the received signal is also sinusoidal, but with a different(random) amplitude and (random) phase:
r(t) = A sin (2πf t + θ)
A, θ depend on environment, location of transmitter/receiver
Transmitted s(t) Received r(t)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Rayleigh FadingCan show:
A has a “Rayleigh” distributionθ has a “uniform” distribution
(all phase shifts are equally likely)
Probability (A < a) = 1 – e-a2/P0
where P0 is the average received power (averaged over different locations)
Ex: P0 =1, a=1: Pr(A<1) = 1 – e-1 = 0.63 (probability that signal is faded)
P0 = 1, a=0.1: Pr(A<0.1) = 1 – e-1/100 ≈ 0.01 (prob that signal is severely faded)
a
Prob(A < a)1
1-e-a2/P0
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Small-Scale Fading
a = 0.1
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Small-Scale Fading
Fade rate depends on• Mobile speed• Speed of surrounding objects• Frequency
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Short- vs. Long-Term Fading
Long-term (large-scale) fading:• Distance attenuation
• Shadowing (blocked Line of Sight (LOS))
• Variations of signal strength over distances on the order of a wavelength
Time (t)
Sig
nal S
treng
th (d
B)
T T
Short-term fading
Long-term fading
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Combined Fading and Attenuation
Time (mobile is moving away from base)
Received powerPr (dB)
distance attenuation
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Combined Fading and Attenuation
Time (mobile is moving away from base)
shadowing
Received powerPr (dB) distance attenuation
64
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Combined Fading and Attenuation
Time (mobile is moving away from base)
shadowing
Received powerPr (dB)
distance attenuation
Rayleigh fading
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Example Diagnostic Measurements: 1XEV-DO
drive test measurements
drive path
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Time Variations: Doppler Shift
Audio clip (train station)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Time Variations: Doppler Shift
Propagation delay = distance d / speed of light c = vt/c
Received signal r(t) = sin 2πf (t- vt/c) = sin 2π(f – fv/c) t
velocity v
distance d = v t
transmittedsignal s(t)
receivedsignal r(t)
propagationdelay
received frequencyDoppler shift fd = -fv/c
delay increases
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Doppler Shift (Ex)Mobile moving away from base v > 0, Doppler shift < 0Mobile moving towards base v < 0, Doppler shift > 0
Carrier frequency f = 900 MHz, v = 60 miles/hour = 26.82 meters/sec
Mobile Base: fd = fv/c = (900 × 106) × 26.82 / (3 × 108) ≈ 80 Hzmeters/sec
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Doppler (Frequency) Shift
Frequency= 1/50 Frequency= 1/45
in phase out of phase
½ Doppler “cycle”
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Doppler Shift (Ex)Mobile moving away from base v > 0, Doppler shift < 0Mobile moving towards base v < 0, Doppler shift > 0
Carrier frequency f = 900 MHz, v = 60 miles/hour = 26.82 meters/sec
Mobile Base: fd = fv/c = (900 × 106) × 26.82 / (3 × 108) ≈ 80 Hz
Suppose the data rate is 9600 bits/sec, 80 Hz Doppler shiftphase inversion every (9600/80)/2 = 60 bits!
As the data rate increases, Doppler shift becomes less significant, i.e., channel is stable over more transmitted bits.
IS-136 data rate: 48.6 kbps GSM data rate 270 kbps
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Application of Doppler Shift: Astronomy
Doppler shift determinesrelative velocity ofdistant objects(e.g., stars, galaxies…)
“red shift”: objectis moving away
“blue shift” objectis moving closer
sun light spectrum spectrum of galaxy supercluster
Observed “spectral lines”(radiation is emittedat discrete frequencies)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Application of Doppler Shift: Police Radar
Doppler shift can be used to compute relative speed.
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Scattering: Doppler Spectrum
distance d = v t
transmittedsignal s(t)
• Received signal is the sum of all scattered waves
• Doppler shift for each path depends on angle (vf cos θ/c )
• Typically assume that the received energy is the same from all directions (uniform scattering)
receivedsignal ??
freq.
power
frequency of s(t)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Scattering: Doppler Spectrum
distance d = v t
transmittedsignal s(t)
frequency
power
frequency of s(t) frequency
power
frequency of s(t) + Doppler shift fd
Doppler shift fd2fd
Doppler Spectrum(shows relative strengths of Doppler shifts)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Scattering: Doppler Spectrum
distance d = v t
transmittedsignal s(t)
frequency
power
frequency of s(t)
power
frequency of s(t) + Doppler shift fd
Doppler spectrum
2fd
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Rayleigh Fading
phase shiftdeep fade
Received waveform Amplitude (dB)77
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Channel Coherence Time
Coherence Time: Amplitude and phaseare nearly constant.
• Rate of time variations depends on Doppler shift:(velocity X carrier frequency)/(speed of light)
• Coherence Time varies as 1/(Doppler shift).
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Fast vs. Slow Fading
Fast fading: channel changes every few symbols. Coherence timeis less than roughly 100 symbols.
Slow fading: Coherence timelasts more than a few 100 symbols.
timetime
rece
ived
am
plitu
de
transmitted bits
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Fade Rate (Ex)• fc = 900 MHz, v = 60 miles/hour
Doppler shift ≈ 80 Hz.Coherence time is roughly 1/80, or 10 msec
• Data rate (voice): 10 kbps or 0.1 msec/bit100 bits within a coherence time (fast fading)
• GSM data rate: 270 kbps about 3000 bits within a coherence time (slow fading)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Channel Characterizations:Time vs. Frequency
• Frequency-domain description
• Time-domain description
Multipathchannel
input s(t) is a sinusoid“narrowband” signal
Amplitude attenuation,Delay (phase shift)
Multipathchannel
input s(t) is an impulse (very short pulse)“wideband” signal(Note: an impulse has zero duration and infinite bandwidth!)
time t
s(t)time t
r(t)
multipath components
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Pulse Width vs. Bandwidth
frequency
Powerbandwidth = 1/T
frequency
Powerbandwidth = 1/T
time
signal pulse
time
signal pulse
T
T
Narrowband
Wideband
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Power-Delay Profile
delay spreadτ
Received power vs. time in response to a transmitted short pulse.
For cellular systems (outdoors), the delay spread is typically afew microseconds. 83
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Two-Ray Impulse Response
direct path (path 1)
reflection (path 2)
time t
s(t)
time t
r(t) reflection is attenuated
τ
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Two-Ray Impulse Response
direct path (path 1)
reflection (path 2)
τ = [(length of path 2) – (length of path 1)]/c
time t
s(t)
time t
r(t) reflection is attenuated
τ
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Urban Multipath
time t time t
s(t)r(t)
time t
r(t)different locationfor receiver
Spacing and attenuation of multipath components depend onlocation and environment. 86
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Delay Spread and Intersymbol Interference
Multipathchanneltime t
s(t)
time t
r(t)
Multipathchannel
r(t)
Time between pulses is >> delay spread, therefore the receivedpulses do not interfere.
Time between pulses is < delay spread, which causesintersymbol interference. The rate at which symbols can betransmitted without intersymbol interference is 1 / delay spread.
s(t)time t
87
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Coherence Bandwidth
frequency
channelgain
The channel gain is approximately constant within a coherence bandwidth Bc. Frequencies f1 and f2 fade independently if |f1 – f2 | >> Bc.
coherence bandwidth Bc
f1 f2
Frequencies far outside the coherencebandwidth are affected differently by multipath.
88
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Coherence Bandwidth and Delay Spread
delay spread τ
frequency
channelgain
coherence bandwidth Bc
Coherence bandwidth is inversely proportional to delay spread:Bc ≈ 1/τ.
frequency
channelgain
coherence bandwidth Bc
delay spread τ
89
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Narrowband Signal
frequency
channelgain
• The signal power is confined within a coherence band.• Flat fading: all signal frequencies are affected the same way.
coherence bandwidth Bc
f1 f2
Frequencies far outside the coherencebandwidth are affected differently by multipath.
signal power(narrowband)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Wideband Signals
frequency
channelgain
• A wideband signal spans many coherence bands.
• Frequency-selective fading: different parts of the signal (in frequency)are affected differently by the channel.
coherence bandwidth Bc
f1 f2
Frequencies far outside the coherencebandwidth are affected differently by multipath.
signal power(wideband)
91
N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Frequency Diversity
frequency
channelgain
Wideband signals exploit frequency diversity. Spreading power across many coherence bands reduces the chances of severe fading.
Wideband signals are distorted by the channel fading (distortion causes intersymbol interference).
coherence bandwidth Bc
f1 f2
Frequencies far outside the coherencebandwidth are affected differently by multipath.
signal power(wideband)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Coherence Bandwidth for Cellular
frequency
channelgain
For the cellular band, Bc is around 100 to 300 kHz.How does this compare with the bandwidth of cellular systems?
coherence bandwidth Bc
f1 f2
Frequencies far outside the coherencebandwidth are affected differently by multipath.
signal power(wideband)
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N O R T H W E S T E R NU N I V E R S I T Y
MSIT | Master of Science in Information Technology
Fading Experienced by Wireless Systems
Standard Bandwidth Fade rate
AMPS 30 kHz (NB) FastIS-136 30 kHz FastGSM 200 kHz SlowIS-95 (CDMA) 1.25 MHz (WB) Fast3G 1.25-5 MHz Slow to Fast
(depends on rate)LTE up to 20 MHz Slow802.11 > 20 MHz SlowBluetooth > 5 MHz (?) Slow
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