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System Operating Margin vs Fade margin-Difference between System Operating Margin(SOM) and Fade margin

This page on System Operating Margin vs Fade margin mentions difference between System Operating Margin(SOM) and Fade margin.

Both the terms are related to RF Link Budget and receiver sensitivity. Before we understand basics of SOM and Fade Margin refer RF Link Budget and RF Sensitivity.

System Operating Margin-SOM

SOM Vs Fade Margin

As we know Link Budget is the outcome of components such as gain, power loss, sensitivity and fade margin. This will be calculated taking into consideration all these factors as well as cables and connectors and also antenna. Link Budget also takes free space path loss into calculation. Link Budget yields System Operating Margin(SOM).

SOM is the amount of received signal strength relative to RF device's receiver sensitivity. In other words, System Operating Margin is the difference of sensitivity and received signal strength as shown in the figure. It is expressed as follows:

SOM (System Operating Margin) = RS - S

For Example, If RF client device has receiver sensitivity of about -95 dBm and it is picking signal of strength -65 dBm. Then SOM is about 30 dBm.
SOM = RS - S
➨SOM = -95-(-65)

SOM calculation implies that the RF Link can be maintained even after reducing the strength of the received signal by the factor of about 30. SOM calculation is ideal for outdoor conditions.

Fade margin

As the wireless link is not constant and various random factors such as rain, weather change, growth of trees and construction of buildings will result into variation or degradation of signal strength over time. This prompts RF planning engineer to add padding margin in the RF link budget calculations.

By including extra dB of signal strength in the required link budget, RF link which can last for longer period can be achieved. This extra signal strength is referred as fade margin.

For Example, if actual receiver sensitivity of wireless device is about -95dBm, then instead of this value, -80 dBm has been taken. The difference of actual and one taken for actual system installation is referred as fade margin.
➨Here Fade Margin is equal to 15 dBm.

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