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
08/16/01
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


08/16/01
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
08/16/01
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
08/16/01
Path Balance
 Balanced Path
Max.
Mobile
Pwr
ERP
Power
Same
Path Loss
from
tower
08/16/01
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
08/16/01
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)
08/16/01
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
08/16/01
Finding ERP
Mobile to Tower
Path Loss
Max.
Mobile
Pwr
ERP?
Power
Mobile to Tower
Path Loss
08/16/01
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
08/16/01
-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
08/16/01
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 *10 23 )(294)(30 *103 )  129 dBm
GSM  PN = -121 dBm
PN  (1.38 *10 23 )(294)(200 *103 )  121 dBm
08/16/01
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
08/16/01
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
08/16/01
 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
08/16/01
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
08/16/01
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
08/16/01
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)
08/16/01
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)
08/16/01
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
08/16/01
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
GTMAGcable
08/16/01
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
08/16/01
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)
08/16/01
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)
08/16/01
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





08/16/01
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
08/16/01
Questions?
08/16/01