08/16/01

Link Budgets for Cellular Networks Presented by Eric Johnson 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

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) 08/16/01

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

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

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

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 08/16/01

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

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

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

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

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

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

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

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 08/16/01

Scenario 1: Receive Path 08/16/01

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

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

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 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

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

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 08/16/01

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 08/16/01

Questions? 08/16/01