Link budget (LB) calculates the transmitting power gains and losses that a communication signal experiences in a telecommunication system. It describes the delta between the power transmitted through a communication medium, such as air or a cable versus the received power.
Link margin (LKM) is measured in dB and describes the difference between the minimum signal power that is expected at the receiver and the minimum sensitivity of the receiver to read communication signals. The minimum sensitivity of the receiver refers to the signal power at which the receiver stops working reliably.
In a Thread system, a LKM of 20 dB, in exceptional cases 15 dB, must be planned to ensure that the system still works in case of extra signal loss.
The figure below shows the relevant power gains and losses and the resulting link margin in a Thread wireless communication.
Link budget and link margin 1 Transmission path | PTX = Transmitter power (dBm) GTX = Antenna gain of transmitter (dBi) LTX = Losses at transmitter (dB), LFS = Free space loss (db) LO = Obstacle loss (db), GRX = Antenna gain of receiver (dBi) LRX = Losses at receiver (dB), PRX = Receiver power (dBm) SRX = Receiver sensitivity (dBm) LKM = Link margin (dBm) LB = Link budget (dBm) |
Calculating link budget and link margin
The link budget helps estimate signal levels.
Perform the steps below, taking a typical receiver sensitivity (SRX) of between 90 dBm and 106 dBm for a Thread component to calculate link budget and margin in a Thread system.
- Add the transmitter and receiver output power of your Thread devices. Typically, these are values between 6 dBm and 10 dBm.
- Subtract the signal loss in free space (LFS). This can be calculated with the free space loss online calculator, see 'Related documentation'.
- Enter the distance of your device in meters.
- Enter 2400 MHz for the frequency.
- Set the transmitting antenna gain (GTx) to 0.
- Set the receiving antenna gain (GRx) to 0.
- Subtract the signal loss in obstacles. This can be calculated using the diagram in 'Damping effect of materials'.
- Subtract the signal loss due to interference (difficult to calculate). Ideally, avoid direct interference with other radio signals. For additional information, see 'Signal to noise ratio'.
The resulting number is your link margin (LKM) in dBm. Your Thread system will be reliable and stable if the link margin is higher than 20 dBm.
This is a theoretical calculation. Real world scenarios may differ.
SRX = Receiver sensitivity (dBm) PTX = Transmitter power (dBm) PRX = Receiver power (dBm) LFS = Free space loss (db) LO = Obstacle loss (db) LI = Interference loss (db) LKM = Link margin (dBm) LB = Link budget (dBm) |
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