link budgets for cellular networks

08/16/01

08/16/01

Link Budgets for Cellular Networks

Link Budgets for Cellular Networks

Presented by Eric Johnson

08/16/01

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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Link Budget and Cell Design ProcessLink 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)

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Hardware ParametersHardware Parameters

Summary of Parameters Thermal Noise Power Antenna Gain Signal to Noise (S/N) Minimum (RX) 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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Hardware Noise and Interference 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

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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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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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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

BeamIsotropicGain

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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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Hardware Requirements 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

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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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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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Not path balanced

CurrentPower

Max. Mobile Pwr

Min. Receive Pwr

Previous Distance

Cannot Receive

PreviousPower

Min. Receive Pwr

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Path balance limited by mobile powerIS-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)

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Finding Base Station Effective Radiated Power (ERP)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

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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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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: Receive PathScenario 1: Receive PathScenerio 1 Base Mobile

Uplink DownlinkNoisechannel BW 30000 30000 HzAmbient Temperature 294 294Boltzman 1.38E-23 1.38E-23Noise Figure (F) 4 9 dB Noise Floor -125 -120 dBm NoiseLossesCable Length 220.00 ftCable Loss / 100 ft 0.80 dBReceiver Cable Loss 1.76 dB Body Loss 3.00 dBVehicle Loss 5.00 dBBuilding Loss 0.00 dBTotal Losses 1.76 8.00 dB LossGains Antenna Gain 12 dBd

14.15 dB Diversity Gain 5.00 dBTotal Gains 19.15 0 dB GainHardwareSignal / Noise (req) 15 15 dB SNRMinimum Power -128 -97 P=SNR-Gain+Loss+NoiseThat must be received

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Scenario 1: Transmit PathScenario 1: Transmit Path

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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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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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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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


No TMA 1-5/8” cable 1.7 dB cable loss ERP: 26 W Radius: 76% the radius

as had with 12 dBd gain


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

08/16/01

Questions?Questions?

link budgets for cellular networks

Documents

gsm noise floor

noise floorinterference

gsmthermal noise

dbmathermal noise

mobile power

noise sourcespower input

power output

noise sn requirementis