1. 08/16/01

2. Link Budgets for Cellular Networks
Presented by Eric Johnson
08/16/01

3. 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
Balance the uplink and downlink power
Don’t transmit more base station power than the maximum cell phone power capability
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4. Link Budget and Cell Design Process
Determine Hardware Information
Gains, Losses, Reflection Coefficients, Power output, noise sources
Power input required, SNR required
Calculate Path Loss (for a given cell radius) and all other system losses.
“Balance” the UPlink and DOWNlink
Cell spacing and topology will be determined by adjacent channel interference (D/R)
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5. Hardware Parameters Summary of Parameters Simplified Example
Thermal Noise Power
Antenna Gain
Signal to Noise (S/N)
Minimum (RX) Input Power
Simplified Example
08/16/01

6. Hardware Noise and Interference
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

7. Hardware Parameters Thermal Noise Power PN = kTB
k = boltzman’s constant
T = ambient temperature in Kelvin
B = signal bandwidth
IS-136  PN = -129 dBm
GSM  PN = -121 dBm
08/16/01

8. Thermal Noise Power (cont.)
Hardware 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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9. Hardware Parameters Antenna Gain
Tower gain ranges from 6 dBd to 16 dBd
Mobile gain typically 0 dBd (-2 dBd to 0 dBd)
dBd = dB relative to a DIPOLE antenna
gain  more uplink  larger coverage area
gain  narrower beamwidth
Gain choice depends on desired coverage area
More Gain
Narrower Beam
Less Gain
Broader Beam
Isotropic
Gain
08/16/01

10. Hardware Parameters Cable Loss 1-5/8” diameter 7/8” 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

11. Hardware Requirements
Signal to Noise (S/N) Requirement
IS-136  15 dB ( dB)
GSM  11 dB ( 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

12. 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
Uplink
Downlink
08/16/01

13. 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

14. Path Balance Balanced Path Same Path Loss Power Distance Max. Mobile
Pwr
ERP
Same
Path Loss
Power
Min.
Receive
Pwr
Min.
Receive
Pwr
Distance
from
tower
from
mobile
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15. Path Balance Not path balanced Max. Mobile Current Pwr Power Previous
Min.
Receive
Pwr
Min.
Receive
Pwr
Cannot Receive
Previous Distance
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16. Path balance limited by mobile power IS-136
Analog Phone (older) max. power: 3 W (35 dBm)
Digital phones (current) 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

17. Finding Base Station Effective Radiated Power (ERP)
Link budget determines transmit ERP
Network is limited by mobile power
Typical base station 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

18. Finding ERP Power Distance Mobile to Tower Path Loss Max. Mobile ERP?
Pwr
Min.
Receive
ERP?
Min.
Receive
Pwr
Mobile to Tower
Path Loss
Power
Distance
from
tower
from
mobile
08/16/01

19. Scenario 1: Baseline Site Configuration Determine ERP 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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20. Scenario 1: Receive Path
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21. Scenario 1: Transmit Path
Max. path loss and max. transmit power
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22. Scenario 2: 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

23. Tower-Mounted Amplifiers (TMAs)
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
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24. 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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25. Scenario 3: TMAs
Min. input power
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26. Max. path loss and max. transmit power
Scenario 3: TMAs
Max. path loss and max. transmit power
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27. Summary Scenario 1 Scenario 3 Scenario 2 200 ft tower, 12 dBd
No TMA
1-5/8” cable
1.7 dB cable loss
ERP: 65 W
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
Uplink improved 1.6 dB
Radius 12% larger
Scenario 2
200 ft tower, 8 dBd
No TMA
1-5/8” cable
1.7 dB cable loss
ERP: 26 W
Radius: 76% the radius as had with 12 dBd gain
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28. Challenges in a Link Budget
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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29. Questions?
08/16/01