Lorawan – Link Budget and Link Margin

 Link Budget and Link Margin 

  • A link budget is the sum of all of the gains and losses from the transmitter, through the medium (aka free space), to the receiver in a telecommunication system. It is a way of quantifying the link performance. 
  • Transmitter: The radio transmitter value must be specified in dbm, otherwise you do not know its absolute value. 
  • Gains: Antenna (Unit: dbi)  
  • Losses: cables, connectors, signal propagating thru the medium (Unit: db) 
  • When a signal propagates thru the medium, the signal loses strength. This is called the path loss or path attenuation 

 

  • A simple link budget equation looks like this:  
  • Received Power = Transmitted Power + Gains − Losses  
  • For example: Received Power = 20 – 5 + 10 – 115 + 12 – 2 = -80 dBm  
  • The receiver sensitivity is the lowest power level at which receiver can receive or demodulate the signal. For example:  
  • Receiver sensitivity = -90 dBm 
  • The link margin is the difference between the received power and receiver sensitivity.  
  • Link margin = Received power – Receiver sensitivity  
  • Link margin in dBm  
  • Received power in dBm  
  • Receiver sensitivity in dBm 

 

 

Question:  

There are two receivers: Receiver A with receiver sensitivity = -120 dBm Receiver B with receiver sensitivity = -130 dBm Which receiver is better?  

 

Answer:  

Receiver B is better because it can demodulate a RF signal at a lower power level. 

 

  • If the link margin is too big, or too small, corrective actions can be applied to ensure the system will operate satisfactorily. 
  • The link margin must be positive (Received Power > Receiver sensitivity) and should be at least a few dB for the receiver to successfully demodulate the signal. 
  • LoRa receivers are very sensitive and are offering a sensitivity down to -148 dBm [2], due to the use of Chirp Spread Spectrum. 
  • The maximum link budget can be used as a baseline value to compare one radio to the next. 
  • Maximum link budget = Maximum transmitter power – Lowest receiver sensitivity  
  • Maximum link budget in dBm  
  • Maximum transmitter power in dBm  
  • Lowest receiver sensitivity in dBm 
  • For example:  
  • Max transmitter power = 20 dBm [2], Lowest receiver sensitivity = -148 dBm [2]  
  • Max link budget = Max transmitter power – Lowest receiver sensitivity  
  • Max link budget = 20 – (-148) = 168 dBm [2] 

 

EIRP and ERP 

EIRP 

The Effective Isotropic Radiated Power (EIRP) is the total power radiated by a hypothetical isotropic antenna in a single direction. 

 

ERP 

The Effective Radiated Power (ERP) is the total power radiated by an actual antenna relative to a half-wave dipole rather than a theoretical isotropic antenna. 

 

 

 

EIRP vs ERP 

  • EIRP= Tx power (dBm) + antenna gain (dBi) – cable loss (dBm)  
  • For example: EIRP = 20 + 10 – 5 = 25 dBm  
  • ERP= Tx power (dBm) + antenna gain (dBd) – cable loss (dBm)  
  • For example: ERP = 20 + 7.85 – 5 = 22.85 dBm  
  • Relationship EIRP and ERP: EIRP (dBm) = ERP (dBm) + 2.15 

What is the purpose of ERP and EIRP? 

  • RF transmitting systems must adhere to certain rules set by the regulatory bodies such as FCC or ETSI.  
  • One of these rules: radio devices must not exceed certain ERP or EIRP values set by these regulatory bodies. 

RSSI 

  • The Received Signal Strength Indication (RSSI) is the received signal power in milliwatts and is measured in dBm. This value can be used as a measurement of how well a receiver can “hear” a signal from a sender 

 

  • The RSSI is measured in dBm and is a negative value. The closer to 0 the better the signal is. 
  • Typical LoRa RSSI values are:  
  • RSSI minimum = -120 dBm.  
  • If RSSI=-30dBm: signal is strong.  
  • If RSSI=-120dBm: signal is weak. 

SNR 

  • Signal-to-Noise Ratio (SNR) is the ratio between the received power signal and the noise floor power level. 
  • The noise floor is an area of all unwanted interfering signal sources which can corrupt the transmitted signal and therefore re-transmissions will occur 

 

  • Normally the noise floor is the physical limit of sensitivity, however LoRa works below the noise level. 

 

  • Typical LoRa SNR values are between: -20dB and +10dB A value closer to +10dB means the received signal is less corrupted. 
  • LoRa can demodulate signals which are -7.5 dB to -20 dB below the noise floor 

LoRaWAN Frequencies 

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