Link Budget
A link budget is the summing in dB of all gains and losses from transmitter to receiver in order to find the level arriving at the receiver. It is written as: received power = Tx power + antenna gains - cable losses - free space path loss - obstacle losses. As long as the result stays above the receiver sensitivity, the link is established.
A worked example shows where the numbers come from. Assume a transmitter with 14 dBm output, 2 dBi antennas at both ends, 2 dB of cable loss on the receiver side and 65.2 dB free space loss at 433 MHz over 100 metres:
- 14 + 2 + 2 - 2 - 65.2 = -49.2 dBm received level.
- If receiver sensitivity is -110 dBm, the 60 dB in between is the safety margin.
- That margin is spent on obstacles, shadowing and a rising noise floor; on an indoor site 40-50 dB of it goes easily.
The benefit of the budget is being able to see in advance how much each measure will gain you. Cutting a 5 metre cable down to 2 metres gains roughly 1.3 dB, taking the antenna out of the enclosure can gain tens of dB. It is no coincidence that in AXI's IP67 sets the antenna and cable gland are part of the set contents: the largest line in the budget is most of the time where the antenna is placed.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety