LoRa/LoRaWAN tutorial 8: Link Budget and Link Margin

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This is part 8 of the LoRa/LoRaWAN tutorial.

In this video series different topics will be explained which will help you to understand LoRa/LoRaWAN.
It is recommended to watch each video sequentially as I may refer to certain LoRa/LoRaWAN topics explained earlier.

In this video I will explain what link budgets, maximum link budgets and link margins are.

If an input signal (=message) is imposed onto a carrier signal this process is called modulation.
The modulated signal is broadcasted to the receiver.
Demodulation is the opposite, where the original signal (= message) is recovered from the modulated carrier wave.

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

For example:
Received power = -80dBm
Receiver sensitivity = -90
dBmLink margin = -80 - (-90) = 10 dBm = 10 mW

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 must be greater than the 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.
More information:
https://www.semtech.com/products/wire...

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]

Check out all my other LoRa/LoRaWAN tutorial videos:
   • LoRa/LoRaWAN tutorials  

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   / @mobilefish  

The presentation used in this video tutorial can be found at:
https://www.mobilefish.com/developer/...

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