System Operating Margin (SOM) vs. Fade Margin: Key Differences

This article explains the difference between System Operating Margin (SOM) and Fade Margin. Both terms are crucial in RF link budget analysis and understanding receiver sensitivity.

Before diving into SOM and Fade Margin, it’s helpful to understand the basics of RF Link Budgets and RF Sensitivity.

What is System Operating Margin (SOM)?

Link budget calculations involve considering various components like gain, power loss, sensitivity, and fade margin. These calculations also factor in cables, connectors, and antennas, as well as free space path loss. The ultimate result of a link budget is the System Operating Margin (SOM).

SOM represents the amount of received signal strength relative to the RF device’s receiver sensitivity. In simpler terms, it’s the difference between the received signal strength and the receiver sensitivity.

Mathematically, SOM is expressed as:

$SOM = RS - S$

Where:

• RS = Received Signal Strength
• S = Receiver Sensitivity

Example:

Let’s say an RF client device has a receiver sensitivity of -95 dBm and is receiving a signal with a strength of -65 dBm. Then, the SOM would be:

$SOM = -65 - (-95) = 30 \text{ dBm}$

This SOM calculation implies that the RF link can still be maintained even if the received signal strength decreases by 30 dB. SOM calculations are particularly valuable in outdoor environments.

What is Fade Margin?

Wireless links aren’t constant. Factors like rain, weather changes, tree growth, and building construction can cause variations or degradation in signal strength over time.

To account for these unpredictable changes, RF planning engineers incorporate a “padding margin” in their RF link budget calculations. This extra dB of signal strength ensures the RF link remains viable for a longer period. This added signal strength is known as the fade margin.

Example:

Imagine a wireless device has an actual receiver sensitivity of -95 dBm. Instead of using this value directly in the link budget, the engineer uses -80 dBm. The difference between the actual sensitivity and the value used for system planning is the fade margin.

In this case, the fade margin is:

$-80 \text{ dBm} - (-95 \text{ dBm}) = 15 \text{ dBm}$

Therefore, the fade margin is 15 dBm.