Path Loss, Link Budget & System Operating Margins

Path and Free-Space Loss

There are many factors that affect the acceptable maximum range of your Wireless Network with the main culprit being the Path Loss of your wireless signal. Path Loss is a culmination of other factors relating to the signal that include free-space loss, wave refraction, diffraction and reflection, aperture-medium coupling loss and absorption but is also influenced by factors such as geographical contours, the local, natural environment (i.e. foliage) and the propagation medium (i.e. air humidity levels).

Path Loss (attenuation of signal path) is the drop in the density of power (attenuation) of the wireless transmission's wave as it propagates (travels) through a medium over a distance. It is an important element when it comes to the design and planning of a Wireless Network and is a key factor when calculating the link budget.

The Path Loss due to Free-Space Loss will probably be the most important factor when planning Outdoor, long range links with the signal strength being reduced by 4 times if the distance between devices is doubled. Free-Space Loss does not include factors such as the gain of the antennas used at the transmitter and receiver, nor any loss associated with hardware imperfections; it is solely based on the LoS (Line of Sight) path through free space air.

Free-Space Loss can be calculated using the following equations:

Where,

 Free-Space Loss

 Signal’s wavelength (m)

 Distance between transmitter and receiver (m)

Through some further derivation, the following, more practical equation can be used to show the Free-Space Loss in dB (decibels):

Where,

 Free-Space Loss (dB)

 Distance between transmitter and receiver (km)

 Signal's frequency (GHz)

Example 1:

If calculating the Free-Space Loss of a 2.4GHz signal, on Channel 6 (2.437GHz) over a distance of 250m:

Example 2:

If calculating the Free-Space Loss of a 70GHz signal (72,000MHz) over a distance of 250m:

You can see from the two examples above, as the frequency increases, the Free-Space Loss (reduction in signal power) increases with regard to distance.

Link Budget

For Wireless Networking connections a Link Budget can be calculated that takes into account all of the gains and losses throughout the link, from the transmitter, to the air and then through the receiver. It looks at signal propagation, antenna gains and other losses from factors such as terrain effects and humidity etc.

A very simplified version of the Link Budget equation can be written as follows:

These simple areas can be broken further into smaller effect variables giving a more practical equation:

Where,

PRX = Received Power (dBm)

PTX = Transmitter Power Output (dBm)

GTX = Transmitter Antenna Gain (dBi)

LTX = Losses from Transmitter (cable, connectors etc.) (dB)

LFS = Free-Space Loss (dB)... this is covered in the chapter above.

LM = Misc. Losses (fade margin, polarization misalignment etc.) (dB)

GRX = Receiver Antenna Gain (dBi)

LRX = Losses from Receiver (cable, connectors etc.) (dB)

In office environments, it is said that LoS propagation only accounts for the first 3m, beyond that indoor losses can increase at up to 30dB per 30m.

Example 3:

If calculating the Link Budget of two identical transceivers operating on a 5GHz signal (5765MHz) over a distance of 500m with transmitter output powers of 21dBm, antenna gains of 14.6dBi, device losses (cable, connecters etc.) of 1.2dB, miscellaneous losses of 0dB and a path loss of 101.5dB:

From this your receiver would have to have a receive sensitivity of -53.7dBm or lower (higher negative value) for your link to operate at all.

System Operating Margin

SOM (System Operating Margin) is a way of calculating the difference between the signal strength the receiver’s radio is actually receiving as opposed to what it actually needs for good data recovery. This is calculated by subtracting the receiver sensitivity value (dB) from the Link Budget figure that was explained in the chapter above.

Where,

SOM = System Operating Margin (dBm)

PRX = Received Power (dBm)

SRX = Receiver Sensitivity (dBm)

For optimum operation, a System Operating Margin of 20dBM or more would be ideal however these values are not always achievable but a value lower than 10dBm is usually considered unacceptable. Shown below is a table showing the relationship between SOM and Availability. 20dBm accounts for a downtime of around 70 hours per year.