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08/16/01

Link Budgets for Cellular Networks

Link Budgets for Cellular Networks

Presented by Eric Johnson

08/16/01

IntroductionIntroduction

Overview Link Budget Importance Path Balance Finding ERP Parameters Scenarios

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Importance of a Link BudgetImportance 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

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Importance of a Link BudgetImportance 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

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Importance of a Link BudgetImportance 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 BalancePath Balance

Balanced Path

Distance from mobile

fromtower

Pow

er

Min. Receive Pwr

ERP Max. Mobile Pwr

Min. Receive Pwr

Same

Path Loss

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Path BalancePath Balance

Not path balanced

CurrentPower

Max. Mobile Pwr

Min. Receive Pwr

Previous Distance

Cannot Receive

PreviousPower

Min. Receive Pwr

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Path BalancePath Balance

Path balance limited by mobile powerIS-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)

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Finding ERPFinding 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

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Finding ERPFinding ERP

Distance from mobile

fromtower

Pow

erERP?

Min. Receive Pwr

Mobile to TowerPath Loss

Mobile to Tower Path Loss

PathLoss

Max. Mobile Pwr

Min. Receive Pwr

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ParametersParameters

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

Simplified Example

IS-136Thermal Noise -129.0 dBm AAntenna Gain 12.0 dBi BCable Loss 1.2 dB CS/N 15.0 dB DMinimum Input Power -124.8 dBm E = A - B + C + D

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ParametersParameters

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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ParametersParameters

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

dBm 129)10*)(294)(3010*(1.38P 323N

dBm 121)10*)(294)(20010*(1.38P 323N

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ParametersParameters

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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ParametersParameters

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

BeamIsotropicGain

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ParametersParameters

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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ParametersParameters

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

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Scenario 1: BaselineScenario 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: BaselineScenario 1: Baseline

Min. input powerBase Mobile

Uplink Downlink

Channel BW (kHz) 30.0 kHz 30.0 kHzAmbient Temperature (deg F) 70 deg F 70 deg FThermal Noise (Kelvin) 294.1 K 294.1 KNoise Floor (dBm) -129.1 dBm -129.1 dBm ARBS Noise Figure (dB) 4.0 dB 9.0 dB BNoise Floor (dBm) -125.1 dBm -120.1 dBm C = A + BCable Length (ft) 220.0 ftCable Loss per 100 ft (dB/100-ft) 0.8 dBReceiver Cable Loss (dB) 1.7 dB DEffective Noise Floor (dBm) -123.5 dBm -120.1 dBm E = C + DC/N (3% BER) (dB) 15.0 dB 15.0 dB FMin. Radio Input (dBm) -108.5 dBm -105.1 dBm G = E + FBody Loss (dB) 3.0 dB HVehicle Loss (dB) 5.0 dB IOther: in building coverage (dB) 0.0 dB JReceiver Antenna Gain (dBd) 12.0 dBd 0.0 dBd KReceiver Diversity Gain (dB) 5.0 dB LEffective Min. Input (dBm) -125.5 dBm -97.1 dBm M = G + H + I + J - K - L

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Scenario 1: BaselineScenario 1: Baseline

Max. path loss and max. transmit powerMobile BaseUplink Downlink

Transmit PA (W) 0.6 W 16.9 WTransmit PA (dBm) 27.8 dBm 42.3 dBm ATransmit Cable Loss Total (dB) 1.7 dB BTransmit Combiner Loss (dB) 4.5 dB CTransmit Antenna Gain (dBd) 0.0 dBd 12.0 dBd DTransmit ERP (dBm) 27.8 dBm 48.1 dBm E = A - B - C + DTransmit ERP (W) 0.6 W 64.4 WBody Loss (dB) 3.0 dB FVehicle Loss (dB) 5.0 dB GOther: in building coverage (dB) 0.0 dB HSlow fade margin (dB) 5.4 dB 5.4 dB IEffective Transmit Power (dBm) 14.4 dBm 42.7 dBm J = E - F - G - H - I

Effective Min. Input (dBm) -125.5 dBm -97.1 dBm

Max. Path Loss (dB) 139.8 dB 139.8 dB

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Scenario 2: Less Antenna GainScenario 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: TMAsScenario 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

cableTMA

RBS

TMA

cableTMAt GG

F

G

FFF

11

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Scenario 3: TMAsScenario 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: TMAsScenario 3: TMAs

Min. input powerBase Mobile

Uplink Downlink

Channel BW (kHz) 30.0 kHz 30.0 kHzAmbient Temperature (deg F) 70 deg F 70 deg FThermal Noise (Kelvin) 294.1 K 294.1 KNoise Floor (dBm) -129.1 dBm -129.1 dBm ARBS Noise Figure (dB) 4.0 dB 9.0 dB BNoise Floor (dBm) -125.1 dBm -120.1 dBm C = A + BCable Length (ft) 220.0 ftCable Loss per 100 ft (dB/100-ft) 0.8 dBReceiver Cable Loss (dB) 1.7 dB DEffective Noise Floor no TMA -123.5 dBm AA = C + DTMA Gain 12.0 dBTMA Noise Figure 1.2 dB BBSystem Noise Figure with TMA 5.1 dB CCEffective Gain of using TMA 0.6 dB DD = C + D - BBEffective Noise Floor (dBm) -124.0 dBm -120.1 dBm E = C + CC (mobile = C)C/N (3% BER) (dB) 15.0 dB 15.0 dB FMin. Radio Input (dBm) -109.0 dBm -105.1 dBm G = E + FBody Loss (dB) 3.0 dB HVehicle Loss (dB) 5.0 dB IOther: in building coverage (dB) 0.0 dB JReceiver Antenna Gain (dBd) 12.0 dBd 0.0 dBd KReceiver Diversity Gain (dB) 5.0 dB LEffective Min. Input (dBm) -126.0 dBm -97.1 dBm M = G + H + I + J - K - L

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Scenario 3: TMAsScenario 3: TMAs

Max. path loss and max. transmit powerMobile BaseUplink Downlink

Transmit PA (W) 0.6 W 19.3 WTransmit PA (dBm) 27.8 dBm 42.9 dBm ATransmit Cable Loss Total (dB) 1.7 dB BTransmit Combiner Loss (dB) 4.5 dB CTransmit Antenna Gain (dBd) 0.0 dBd 12.0 dBd DTransmit ERP (dBm) 27.8 dBm 48.7 dBm E = A - B - C + DTransmit ERP (W) 0.6 W 73.6 WBody Loss (dB) 3.0 dB FVehicle Loss (dB) 5.0 dB GOther: in building coverage (dB) 0.0 dB HSlow fade margin (dB) 5.4 dB 5.4 dB IEffective Transmit Power (dBm) 14.4 dBm 43.3 dBm J = E - F - G - H - I

Effective Min. Input (dBm) -126.0 dBm -97.1 dBm

Max. Path Loss (dB) 140.4 dB 140.4 dB

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SummarySummary

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

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SummarySummary

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?Questions?