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  Balance the uplink and downlink power  Don’t transmit more base station power than the maximum cell phone power capability

08/16/01 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 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 Distance from mobile from tower Power Min. Receive Pwr ERP Max. Mobile Pwr Min. Receive Pwr Same Path Loss

08/16/01 Path Balance Not path balanced Current Power Max. Mobile Pwr Min. Receive Pwr Previous Distance Cannot Receive Previous Power Min. Receive Pwr

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

08/16/01 Parameters Summary of Parameters  Thermal Noise Power  Antenna Gain  Signal to Noise (S/N)  Minimum Input Power Simplified Example

08/16/01 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  P N = kTB  k = boltzman’s constant  T = ambient temperature in Kelvin  B = signal bandwidth  IS-136  P N = -129 dBm  GSM  P N = -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 More Gain Narrower Beam Less Gain Broader Beam Isotropic Gain

08/16/01 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 ( 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 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

08/16/01 Scenario 1: Baseline Max. path loss and max. transmit power

08/16/01 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

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

08/16/01 Scenario 3: TMAs Max. path loss and max. transmit power

08/16/01 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  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

08/16/01 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?