RF Link Budgeting - Commagility
Introduction toRF Link BudgetingSeptember 2019www.commagility.com
The Radio ChannelFind the signal-to-noise ratio (SNR) at the receiverSignal Transmitterpower Antenna gain Path loss Fading Other channel effectsNoise Receiver gain Thermal noise Receiver noise figure
Combining Gains and Losses When dealing with power in Watts, multiply and divideSignal x Gain / LossSNR Signal / Noise When dealing with power in dB, add and subtractdBm 10log10(mW)SignaldB GaindB β LossdBSNRdB SignaldB - NoisedB
The Transmitter
Transmitter PowerOther considerations:PA max powerPA linearityCrest factor-3 dBM gain-1 dBAntenna-1dB -1 nnelfilterlAnosstennacablingloss0dBm 27dB10dBi Transmitter EIRP31dBm in this example
The Channel
Free-space Path LossAn isotropic antenna radiates a perfect sphereTransmitted energy is spread over the surface of the sphereSurface area of a sphere: π¨ ππ π π(d is Tx-Rx distance)Energy density decreases with the square of distance
Free-space Path LossThe size of an isotropic antenna is related to wavelengthAs the wavelength gets shorter, the antenna gets smallerCaptures less energyThe βsizeβ of the receiving antenna is its apertureAperture of an isotropic antenna: π¨π ππππ Energy captured increases with the square of wavelength
Free-space Path LossDue to the βexpanding sphereβ of energy with distance:Free space path loss ππ π πDue to the increasing receiver aperture with wavelength:Free space path loss ππ ππFree space path loss 4ππ -./01ππ π π π
Free-space Path LossFree space path loss ππ π ππ ππ π π ππExpressed in decibels, 10log10(FSPL) : 20log9: 4π π 20log9: π 20log9: π πππ₯π¨π ππ π πππ₯π¨π ππ π πππ. ππ
Antenna GainFree space path loss 20πππ9: π 20πππ9: π 147.55Does not include antenna gain β directivityShaped beam, rather than sphericalChanges the radiation pattern in TxChanges the effective aperture in Rx
Antenna GainUnits are dBi β dB relative to the isotropic casee.g. 3-sector cell tower, 120 propagationπππ₯π¨π ππ π π. πdBi(additional gain in elevation gives up to 15dBi)Power at transmitter port antenna gain EIRP- equivalent isotropic radiated powerGain (log) terms that simply add
Other Sources of LossMultipath fading Fading due to multipath (fast fading) Shadowing (slow fading) due to no LOS Empirical channel models attempt to take these into account.Examples include: OkumuraβHata model COST Hata model COST231-Walfish-Ikegami model Interferers Highly deployment-dependent Margin added to allow for these based on modelling or fieldmeasurements 180 phase0 phaseShadowing
Other Sources of Loss Atmospheric absorption β negligible vs. FSPL in most cases
The Receiver
Calculating Noise at the Receiverπ πππ΅Noise power (Watts)Bandwidth (Hz)Receiver temperature (Kelvin)Boltzmannβs constant (1.38e-23)-174dBm/Hz at room temperature 10log10(18.015e6) -101dBm for a 20MHz LTE carrier
Calculating Noise at the ReceiverFriis formula for noiseSignalThermal BlossesLNAnoiseG1Antenna F2G2 Receiver noise figureSubtracts from SNRe.g. 5dB for an LTE eNB 9dB for an LTE UE
SNR and Throughput
Overall SNR Calculation β LTEExample Sum all gains and losses to get absolute power at receiver Subtract noise power to get SNRπππ₯π¨π ππ ππππ πππ₯π¨π ππ π. π πππ πππ. ππSignal at receiverTransmitter powerTx antenna gainFree space path lossRx antenna gainSignal at receiverGainUnits24 dBm5 dBi-103 dB0 dBi-74 dBmπππ₯π¨π ππ π. ππ ππYππ πππ ππ. πππ πππNoise at receiverReceiver thermal noiseRx noise figureNoise at receiverGainUnits-101 dBm9 dB-92 dBmSignal β Noise SNR 18dB
Overall SNR Calculation β 5G FR2Example Sum all gains and losses to get absolute power at receiver Subtract noise power to get SNRπππ₯π¨π ππ ππππ πππ₯π¨π ππ ππ πππ πππ. ππSignal at receiverTransmitter powerTx antenna gainFree space path lossRx antenna gainSignal at receiverGainUnits24 dBm5 dBi-121 dB0 dBi-92 dBmπππ₯π¨π ππ π. ππ ππYππ πππ πππ πππNoise at receiverReceiver thermal noiseRx noise figureNoise at receiverGainUnits-91 dBm9 dB-82 dBmSignal β Noise SNR -10dB
Overall SNR Calculation β 5G FR2Example Sum all gains and losses to get absolute power at receiver Subtract noise power to get SNRπππ₯π¨π ππ ππππ πππ₯π¨π ππ ππ πππ πππ. ππSignal at receiverGainUnitsTransmitter power24 dBmTx antenna gain18 dBiFree space path lossRx antenna gainSignal at receiver-121 dB18 dBi-61 dBmπππ₯π¨π ππ π. ππ ππYππ πππ πππ πππNoise at receiverReceiver thermal noiseRx noise figureNoise at receiverGainUnits-91 dBm9 dB-82 dBmSignal β Noise SNR 21dB
Calculating Throughput β Shannon LimitπΆ π΅ \ log Channel capacity(bps)π1 πBandwidth (Hz)Signal-to-noise ratio (linear!) Capacity increases linearly with bandwidth(but so does noise) Capacity increases logarithmically with transmitter power Includes channel coding, equalisation etc. Diversity (e.g. MIMO) effectively creates extra channels
Calculating Throughput β Mobile StandardsStandardMax spectralEfficiency(bit/sec/Hz) (SISO)2G (GSM)0.523G (CDMA2000 EVDO)2.504G (LTE)4.085G (256-QAM)5.84WiFi (802.11ac)5.42Each standard has its own requirements for SNR
Calculating Throughput β LTE Example3GPP 36.213 rel10 Table 7.2.3-1 Consider 18dB SNR from earlier example Estimate Channel Quality Indicator CQI 12 Spectral efficiency 3.9b/s/Hz Bandwidth 18.015MHz Throughput 70.3MbpsCQI M64QAMcode rate x 1024out of 61.91412.40632.73053.32233.90234.52345.11525.5547 Shannon limit:18 10 \ log - 1 109 /9: 108MbpsDownlink SNR to CQI Mapping for Different Multiple Antenna Techniques in LTE, Kawser et al 2012
Summary Transmitter power plus antenna gain gives Tx EIRP Calculate path losses based on distance, frequency etc. Add up all the gains and losses in the system to get Rx power Calculate Rx noise Subtract noise to get SNR Calculate throughput based on SNR
Rx noise figure 9dB Noise at receiver -82dBm Signal at receiver Gain Units Transmitter power 24dBm Tx antenna gain 18dBi Free space path loss -121dB Rx antenna gain 18dBi Signal at receiver -61dBm Signal βNoise SNR 21dB. Calculating Throughput βShannon Limit [ V\log-1 ] S
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