Link Budgets for Cellular Networks

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Link Budgets for Cellular Networks

• Presented by Eric Johnson

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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
• Balance the uplink and downlink power
• Dont transmit more base station power than the
  maximum cell phone power capability

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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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Hardware Parameters

• Summary of Parameters
• Thermal Noise Power
• Antenna Gain
• Signal to Noise (S/N)
• Minimum (RX) Input Power
• Simplified Example

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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

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Hardware Parameters

• Thermal Noise Power
• PN kTB
• k boltzmans constant
• T ambient temperature in Kelvin
• B signal bandwidth
• IS-136 ? PN -129 dBm
• GSM ? PN -121 dBm

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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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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
• NarrowerBeam

• Less Gain
• BroaderBeam

Isotropic Gain
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Hardware 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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Hardware Requirements

• Signal to Noise (S/N) Requirement
• 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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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

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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

Same Path Loss
Power
Distance
from tower
from mobile
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Path Balance

• Not path balanced

Previous Distance
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Path Balance

• 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)

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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

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Finding ERP
Power
Distance
from tower
from mobile
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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 Receive Path
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Scenario 1 Transmit Path

• Max. path loss and max. transmit power

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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) orMast
  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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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

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Scenario 3 TMAs

• Max. path loss and max. transmit power

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Summary

• Scenario 1
• 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
• ERP 74 W
• 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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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?