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08/16/01
Link Budgets
for Cellular Networks
Presented by
Eric Johnson
08/16/01
Introduction
Overview
Link Budget Importance
Path Balance
Finding ERP
Parameters
Scenarios
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Importance of a Link Budget
What is a Link Budget?
Determines tower transmit ERP for
sufficient signal strength at the cell
boundary for a quality mobile call
Defines the cell coverage radius when
used with a path loss model
Why need a Link Budget?
Determine transmit ERP and cell radius
Ensure path balance
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Balance the uplink and downlink power
Don’t transmit more base station power than the
maximum cell phone power capability
Importance of a Link Budget
Path Balance Issue
Mobile is power limited
Stronger base station power will
“deceive” mobile into thinking there is
sufficient signal strength
Mobile can receive info but cannot send
Downlink
Uplink
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Importance of a Link Budget
Consequences
Mobile call initiations will fail and
poor handoff decisions will be made
At the cell boundary
Solution
Setting the base station power to
“match” the mobile power allows for
optimum performance
Path balance
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Path Balance
Balanced Path
Max.
Mobile
Pwr
ERP
Power
Same
Path Loss
from
tower
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Min.
Receive
Pwr
Min.
Receive
Pwr
Distance
from
mobile
Path Balance
Not path balanced
Max.
Mobile
Pwr
Current
Power
Previous
Power
Cannot Receive
Min.
Receive
Pwr
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Previous Distance
Min.
Receive
Pwr
Path Balance
Path balance limited by mobile power
IS-136
Older phone’s max. power: 3 W (35 dBm)
Current phones max. power: 0.6 W (28 dBm)
Ranges from 26 to 28 dBm
Benefit: less power consumption less recharging
Drawback: smaller cell coverage more cells
GSM
Mobile power max.: 1.0 W (30 dBm)
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Finding ERP
Link budget determines
transmit ERP
Network is limited by mobile
power
Typical transmit is 100 W ERP
Transmit ERP determines cell
radius
Radius also depends on tower
height and path loss environment
Small improvement (1 dB) in link
budget can provide large coverage
gains
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Finding ERP
Mobile to Tower
Path Loss
Max.
Mobile
Pwr
ERP?
Power
Mobile to Tower
Path Loss
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Min.
Receive
Pwr
Min.
Receive
Pwr
from
tower
Path
Loss
Distance
from
mobile
Parameters
Summary of Parameters
Thermal Noise Power
Antenna Gain
Signal to Noise (S/N)
Minimum Input Power
Simplified Example
IS-136
Thermal Noise
Antenna Gain
Cable Loss
S/N
Minimum Input Power
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-129.0 dBm
12.0 dBi
1.2 dB
15.0 dB
-124.8 dBm
A
B
C
D
E=A-B+C+D
Parameters
Noise-Limited System
Ambient temperature creates noise floor
Interference from high frequency re-use
may cause system to be interference
limited
Site measurements determine if noise or
interference limited
The following analysis assumes a noise
limited system
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Parameters
Thermal Noise Power
PN = kTB
k = boltzman’s constant
T = ambient temperature in Kelvin
B = signal bandwidth
IS-136 PN = -129 dBm
PN (1.38*1023 )(294)(30*103 ) 129dBm
GSM PN = -121 dBm
PN (1.38*1023 )(294)(200*103 ) 121dBm
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Parameters
Thermal Noise Power (cont.)
The noise floor for GSM is 8 dB
higher than IS-136 because it uses a
wider bandwidth signal
Result: IS-136 is 8 dB more
sensitive to lower power signals
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Parameters
Antenna Gain
Tower gain ranges from 6 dBd to 16 dBd
Mobile gain typically 0 dBd (-2 dBd to 0 dBd)
gain more uplink larger coverage area
gain narrower beamwidth
Gain choice depends on desired coverage area
Isotropic
Gain
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More Gain
Narrower
Beam
Less Gain
Broader
Beam
Parameters
Cable Loss
1-5/8” diameter
0.8 dB/100-ft
7/8” diameter
1.2 dB/100-ft
Tower heights range from 30
ft to 600 ft
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Parameters
Signal to Noise (S/N)
IS-136 15 dB (15 - 17 dB)
GSM 11 dB (7 - 12 dB)
GSM has a S/N advantage over IS-136
GSM has more tolerance for errors than IS-136
Wider bandwidth and different modulation scheme
Difference between GSM and IS-136
GSM noise floor is worse (higher) than IS-136
GSM S/N is better (lower) than IS-136
GSM has more uplink power available
Result: GSM and IS-136 have comparable link
budgets, so only analyze IS-136 link budget
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Scenario 1: Baseline
Site Configuration
Height: 200 ft
Antenna Gain: 12 dBd
Cable: 1-5/8” 0.8 dB/100-ft
Determine ERP
Path balance to find ERP
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Scenario 1: Baseline
Min. input power
Base
Uplink
Channel BW (kHz)
Ambient Temperature (deg F)
Thermal Noise (Kelvin)
Noise Floor (dBm)
RBS Noise Figure (dB)
Noise Floor (dBm)
Cable Length (ft)
Cable Loss per 100 ft (dB/100-ft)
Receiver Cable Loss (dB)
Effective Noise Floor (dBm)
C/N (3% BER) (dB)
Min. Radio Input (dBm)
Body Loss (dB)
Vehicle Loss (dB)
Other: in building coverage (dB)
Receiver Antenna Gain (dBd)
Receiver Diversity Gain (dB)
Effective Min. Input (dBm)
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30.0 kHz
70 deg F
294.1 K
-129.1 dBm
4.0 dB
-125.1 dBm
220.0 ft
0.8 dB
1.7 dB
-123.5 dBm
15.0 dB
-108.5 dBm
12.0 dBd
5.0 dB
-125.5 dBm
Mobile
Downlink
30.0 kHz
70 deg F
294.1 K
-129.1 dBm A
9.0 dB
B
-120.1 dBm C = A + B
D
-120.1 dBm E = C + D
15.0 dB
F
-105.1 dBm G = E + F
3.0 dB
H
5.0 dB
I
0.0 dB
J
0.0 dBd
K
L
-97.1 dBm M = G + H + I + J - K - L
Scenario 1: Baseline
Max. path loss and max. transmit power
Transmit PA (W)
Transmit PA (dBm)
Transmit Cable Loss Total (dB)
Transmit Combiner Loss (dB)
Transmit Antenna Gain (dBd)
Transmit ERP (dBm)
Transmit ERP (W)
Body Loss (dB)
Vehicle Loss (dB)
Other: in building coverage (dB)
Slow fade margin (dB)
Effective Transmit Power (dBm)
Effective Min. Input (dBm)
Max. Path Loss (dB)
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Mobile
Uplink
Base
Downlink
0.6 W
27.8 dBm
16.9 W
42.3 dBm
1.7 dB
4.5 dB
12.0 dBd
48.1 dBm
64.4 W
0.0 dBd
27.8 dBm
0.6 W
3.0 dB
5.0 dB
0.0 dB
5.4 dB
14.4 dBm
5.4 dB
42.7 dBm
-125.5 dBm
-97.1 dBm
139.8 dB
139.8 dB
A
B
C
D
E=A-B-C+D
F
G
H
I
J=E-F-G-H-I
Scenario 2: Less Antenna Gain
Less antenna gain
Wider beamwidth for broader coverage
Reduces uplink
Reduces cell radius
Site Configuration
Height: 200 ft
Antenna Gain: 8 dBd
Cable: 1-5/8” 0.8 dB/100-ft
Results
ERP: 25.7 W
Radius: 76% than with 12 dBd
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Scenario 3: TMAs
Tower-Mounted Amplifiers (TMAs)
Also called Tower-Top Amplifiers (TTAs) or
Mast Head Amplifiers (MHAs)
Essentially a Low-Noise Amplifier (LNA) mounted
most often at the top of the tower
Use TMA if high cable loss
TMA gain “eliminates” the losses due to the cable
Total system gain reduced through equation below
TMA noise figure must be lower than the cable loss
About 200 ft or taller implies 1.5 dB, so TMA useful
Fcable 1 FRBS 1
Ft FTMA
GTMA
GTMA Gcable
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Scenario 3: TMAs
Disadvantages
Intermodulation products may be
amplified causing more interference
Excessive gain amplifies intermodulation effects
more than it amplifies the desired signal
Want gain = losses, so include attenuators if
necessary
Band filters typical
Advantage: helps reduce intermodulation
interference
Disadvantage: slightly different frequency bands
replace TMA
More logistics to replace or troubleshoot
Moderately high cost
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Scenario 3: TMAs
Min. input power
Base
Uplink
Channel BW (kHz)
Ambient Temperature (deg F)
Thermal Noise (Kelvin)
Noise Floor (dBm)
RBS Noise Figure (dB)
Noise Floor (dBm)
Cable Length (ft)
Cable Loss per 100 ft (dB/100-ft)
Receiver Cable Loss (dB)
Effective Noise Floor no TMA
TMA Gain
TMA Noise Figure
System Noise Figure with TMA
Effective Gain of using TMA
Effective Noise Floor (dBm)
C/N (3% BER) (dB)
Min. Radio Input (dBm)
Body Loss (dB)
Vehicle Loss (dB)
Other: in building coverage (dB)
Receiver Antenna Gain (dBd)
Receiver Diversity Gain (dB)
Effective Min. Input (dBm)
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30.0 kHz
70 deg F
294.1 K
-129.1 dBm
4.0 dB
-125.1 dBm
220.0 ft
0.8 dB
1.7 dB
-123.5 dBm
12.0 dB
1.2 dB
5.1 dB
0.6 dB
-124.0 dBm
15.0 dB
-109.0 dBm
12.0 dBd
5.0 dB
-126.0 dBm
Mobile
Downlink
30.0 kHz
70 deg F
294.1 K
-129.1 dBm A
9.0 dB
B
-120.1 dBm C = A + B
D
AA = C + D
BB
CC
DD = C + D - BB
-120.1 dBm E = C + CC (mobile = C)
15.0 dB
F
-105.1 dBm G = E + F
3.0 dB
H
5.0 dB
I
0.0 dB
J
0.0 dBd
K
L
-97.1 dBm M = G + H + I + J - K - L
Scenario 3: TMAs
Max. path loss and max. transmit power
Transmit PA (W)
Transmit PA (dBm)
Transmit Cable Loss Total (dB)
Transmit Combiner Loss (dB)
Transmit Antenna Gain (dBd)
Transmit ERP (dBm)
Transmit ERP (W)
Body Loss (dB)
Vehicle Loss (dB)
Other: in building coverage (dB)
Slow fade margin (dB)
Effective Transmit Power (dBm)
Effective Min. Input (dBm)
Max. Path Loss (dB)
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Mobile
Uplink
Base
Downlink
0.6 W
27.8 dBm
19.3 W
42.9 dBm
1.7 dB
4.5 dB
12.0 dBd
48.7 dBm
73.6 W
0.0 dBd
27.8 dBm
0.6 W
3.0 dB
5.0 dB
0.0 dB
5.4 dB
14.4 dBm
5.4 dB
43.3 dBm
-126.0 dBm
-97.1 dBm
140.4 dB
140.4 dB
A
B
C
D
E=A-B-C+D
F
G
H
I
J=E-F-G-H-I
Summary
Scenario 1
200 ft tower, 12 dBd
No TMA
1-5/8” cable
1.7 dB cable loss
ERP: 65 W
Scenario 2
200 ft tower, 8 dBd
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No TMA
1-5/8” cable
1.7 dB cable loss
ERP: 26 W
Radius: 76% the radius
as had with 12 dBd gain
Scenario 3
200 ft tower, 12 dBd
TMA
1-5/8” cable
1.7 dB cable loss
ERP: 74 W
Uplink improved 0.6 dB
Radius 5% larger
7/8” cable
2.7 dB cable loss
ERP: 74 W
Uplink improved 1.6 dB
Radius 12% larger
Summary
Challenges in a Link
Budget
Parameters vary by user
experience
Verify interference is lower
than noise floor
Choosing antenna with as
much gain as possible that will
still adequately cover area
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Questions?
08/16/01