When designing a wireless system, the ultimate goal is to broadcast a signal between two devices, each with their own antennas and unique equipment for collecting and conditioning signals. The portion of signal transmission over the air and across the interconnect leading to the transmission system will limit the total transmission distance. In RF design, an important design goal is to balance a distance limitation with the complexity of the signal conditioning chain in the transmitting and receiving systems.
A useful metric used to examine the effectiveness of an interconnect for transferring signal in a wireless system is the RF link budget. This metric gives the amount of power a receiving component can expect to see given all the possible gains and losses encountered by a wireless signal as it propagates between two devices. In this article, we’ll show how to calculate the important values that go into a link budget calculation and the over-the-air propagation losses in a wireless channel.
What is a Wireless Link Budget?
The central concept in a wireless link budget is simple: the goal is to calculate the expected power observed at the receiver in a wireless channel given a transmitter’s power output and interconnect characteristics. The interconnect characteristics include losses in components and cables, losses on the PCB, antenna gain, and losses while the signal propagates through the air. Since you’re calculating a power value, a link budget is not a budget at all, but simply an estimate of received power at a receiver.
RF link budget model
The link budget in a wireless channel should not be confused with a loss budget as defined in fiber optic channels. In fiber optics, we typically specify the minimum required receiver sensitivity, and the transmitter output, and then we use this to determine the total allowed loss. From this we can get a maximum allowed distance in the presence of any splices or bends.
An RF link budget calculation can be used in the same way, as will be outlined below. You can approach the design problem for a wireless channel in two ways:
- Determine the minimum receiver sensitivity for a prescribed propagation distance
- Determine the maximum propagation distance for for a prescribed receiver sensitivity
To get started, we’ll take approach #1 to determine the expected power seen at a receiving component. Then we’ll show how to use the same model to get the maximum propagation distance for a channel.
To undertake this approach, we need to first calculate the free space path loss, or the losses the signal will experience while it propagates between two devices. Once that has been calculated, the link budget for the signal can be calculated.
Free Space Path Loss
The free space path loss is the total point-to-point loss that a signal will experience as it travels between the transmitting antenna and the receiving antenna. The free space path loss is defined as the loss experienced while in a straight line between the transmitting and receiving antennas.
In terms of the distance and the transmitting frequency, the free space path loss is defined below:
RF link budget model
In this formula, we have FSPL in dB, d measured in km, and f measured in GHz. If the frequency were measured in another unit (MHz for example), then the constant on the right-hand side would take a different value due to a unit conversion.
This formula does not account for multipath effects, losses due to weather, losses due to terrain, or losses due to buildings. If these values are known, they can be included as a miscellaneous loss term when calculating the expected power (see below).
Expected Receiver Power
Next, using the free space path loss, the expected power to be observed at the receiver can be calculated. The receiver in this case is located after the antenna, any cables or connectors, transmission lines on the PCB, filters, and any other components like an impedance matching network. The expected receiver power is defined as:
In this equation, there are several terms that are defined below:
- PT = Transmitter power output (dBm)
- GT = Transmitting antenna gain (dBi)
- LT = Transmitter interconnect losses (dB)
- FSPL = Free space path loss (dB)
- GR = Receiver antenna gain (dBi)
- LR = Receiver interconnect losses (dB)
- LMisc = Any other losses (dB)
In this approach, the designer specifies the transmitter power and the receiver sensitivity, and the maximum propagation distance is determined for a given frequency and interconnect losses. The distance is calculated using the free space path loss formula shown above. To do this, we solve for FSPL in receiver power equation, plug this into the FSPL formula, and solve for the distance to get the following result:
Wireless link distance for a specified set of losses and receiver sensitivity.
Here again we have the frequency f in GHz and the distance d in km. The distance here is also a straight line distance not accounting for any multipath effects or other obstacles that could create additional losses. In such a case, we would just take LMisc to be zero. However, we can then model what happens as some other source of loss begins to exist in the wireless link (such as bad weather) to see how the distance can be limited by this additional loss.
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