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Link Budgets for Cellular Networks. Presented by Eric Johnson. Introduction. Overview Link Budget Importance Path Balance Finding ERP Parameters Scenarios. Importance of a Link Budget. What is a Link Budget?
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Link Budgets for Cellular Networks Presented by Eric Johnson
Introduction • Overview • Link Budget Importance • Path Balance • Finding ERP • Parameters • Scenarios
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
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
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
Max. Mobile Pwr ERP Min. Receive Pwr Min. Receive Pwr Path Balance • Balanced Path Same Path Loss Power Distance from tower from mobile
Max. Mobile Pwr Current Power Previous Power Cannot Receive Min. Receive Pwr Min. Receive Pwr Path Balance • Not path balanced Previous Distance
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)
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
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
Parameters • Summary of Parameters • Thermal Noise Power • Antenna Gain • Signal to Noise (S/N) • Minimum Input Power • Simplified Example
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
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
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
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 • More Gain • NarrowerBeam • Less Gain • BroaderBeam Isotropic Gain
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
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
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
Scenario 1: Baseline • Min. input power
Scenario 1: Baseline • Max. path loss and max. transmit power
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
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
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
Scenario 3: TMAs • Min. input power
Scenario 3: TMAs • Max. path loss and max. transmit power
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
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