A Radio Frequency (RF) Link Budget is a summary of all the wireless signal power gains and losses in a communication system.
Formula
The link budget equation gives the receive signal power in dBm,
Received power = Transmitted power + Gain (dB) − Loss (dB)
where
Gain is due to antennas and can be calculated from antenna factor and wavelength.
Gain = GTx + GRx
Loss is due to antenna cable, free space path loss and insertion loss due to adapters and connectors.
Loss = LTx + LRx + LFS
A more detailed equation can be written as
PRx = PTx – LTx – LRx – LFS + GTx + GRx
where,
- PRx = Receive signal power (dBm)
- PTx = Transmit signal power (dBm)
- LTx = Loss from transmitter antenna adapters, cables (dB)
- LRx = Loss from transmitter antenna adapters, cables (dB)
- GTx = Transmit antenna gain (dBi)
- GRx = Receive antenna gain (dBi)
- LFS = Free space path loss (dB)
Background
What is Link Budget?
A wireless or radio frequency link budget refers to the calculation of the various factors that determine the performance of a wireless communication system.
It takes into account the power output of the transmitter, antenna gains, adapter, cable and connector losses and propagation loss.
The goal of a link budget calculation is to determine whether the signal power at the receiver is sufficient for reliable communication.
This calculation can take into consideration factors such as the distance between the transmitter and receiver, the frequency being used, and any obstacles or interference in the transmission path. In the calculator on this page the free space path loss is used (although it does not take interference or obstructions into account).
By considering these factors, the link budget can estimate the received power at the receiver.
The next step is to calculate the noise floor and determine whether it meets the minimum SNR required for a successful wireless connection.
???????????????????? The link budget helps determine the overall quality and range of a wireless connection, and it is used by RF Engineers to design and optimize wireless communication systems
What is the link budget Calculator?
The link budget calculator is a useful tool that gives the signal level in dBm or Watt at the receiver. This is useful in predicting whether a signal can be demodulated.
???? Knowing the signal level alone is not enough to predict whether it can be demodulated. It’s also important to know the noise level at the receiver. With this the signal-to-noise ratio (SNR) can be computed.
As the SNR is increased, the signal quality improves. In other words, more bits can be transmitted across the wireless link. Use this calculator to estimate the throughput as a function of SNR. For a fixed bandwidth, increase the SNR to see how the throughput (bps/kbps/Mbps) increases.
Example Link Budget Calculation
Here is an example for wireless communication.
Assume the following parameters:
- PTx = +30 dBm (1 Watt)
- LTx = 1 dB
- LRx = 1 dB
- GTx = 0 dBi (for an Omni-directional antenna)
- GRx = 11 dBi (for a directional Yagi-Uda antenna)
- LFS = 80 dB
Enter these numbers into the calculator gives
- PRx = -41 dBm
???? Is this a good enough received signal power?
To answer this question, we have to find the receiver sensitivity. There are three relevant parameters:
- Receiver noise figure
- Bandwidth
- SNR requirement for a specified performance level
Assuming NF = 30 dB and a bandwidth of 100 MHz, and SNR requirement of 12 dB, the receiver sensitivity is -59 dBm. This is the minimum signal level required to demodulate the signal.
✅ Since the received signal level is -41 dBm, there should be no problem processing this signal.
Additional Notes
This is a similar type of analysis that goes into the planning of wireless networks such as 5G and Wi-Fi. Use this tool for example to find the Wi-Fi received signal strength PRx in an indoor environment.
According to Wi-Fi hardware manufacturers, a signal level greater than -65 dBm is good enough. For high bandwidth applications like streaming however, our calculation indicates that it’s a good idea to target -50 dBm for moderate internet connectivity speed.
References
[1] Link Budget on Wikipedia