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Link Budgets for Cellular Networks

Link Budgets for Cellular Networks. Presented by Eric Johnson. 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

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Link Budgets for Cellular Networks

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  1. Link Budgets for Cellular Networks Presented by Eric Johnson

  2. 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

  3. 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)

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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. Downlink Uplink 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

  12. 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

  13. Max. Mobile Pwr ERP Min. Receive Pwr Min. Receive Pwr Path Balance • Balanced Path Same Path Loss Power Distance from tower from mobile

  14. Max. Mobile Pwr Current Power Previous Power Cannot Receive Min. Receive Pwr Min. Receive Pwr Path Balance • Not path balanced Previous Distance

  15. 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)

  16. 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

  17. Mobile to Tower Path Loss Max. Mobile Pwr ERP? Mobile to Tower Path Loss Path Loss Min. Receive Pwr Min. Receive Pwr Finding ERP Power Distance from tower from mobile

  18. 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

  19. Scenario 1: Receive Path

  20. Scenario 1: Transmit Path • Max. path loss and max. transmit power

  21. 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

  22. 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

  23. 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

  24. Scenario 3: TMAs • Min. input power

  25. Scenario 3: TMAs • Max. path loss and max. transmit power

  26. Summary • 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 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 • No TMA • 1-5/8” cable • 1.7 dB cable loss • ERP: 26 W • Radius: 76% the radius as had with 12 dBd gain

  27. 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

  28. Questions?

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