Link Budget Calculation

Yaser Rahmati | یاسر رحمتی

Link Budget Calculation is an essential process in planning a wireless communication link. It involves determining the total gain and loss in the signal strength from the transmitter to the receiver. This helps ensure that the signal at the receiver is strong enough for reliable communication.

Here’s a step-by-step guide to performing a Link Budget Calculation using MikroTik RouterBoard, along with a practical example.

1. Understanding Link Budget Components

Before diving into the calculation, it's essential to understand the components involved:

• Transmit Power (Tx Power): The power at which the wireless device transmits the signal, usually measured in dBm.

• Antenna Gain (Transmit and Receive): The increase in signal strength provided by the antenna, measured in dBi.

• Free Space Path Loss (FSPL): The loss of signal strength as it travels through space, measured in dB.

• Cable and Connector Loss: The loss incurred due to cables and connectors, measured in dB.

• Receiver Sensitivity: The minimum signal strength required by the receiver to successfully decode the signal, measured in dBm.

• Fade Margin: The additional signal strength required to account for environmental factors and ensure a reliable link, measured in dB.

2. Practical Example Setup

Let's set up a practical scenario:

• Distance between devices: 5 km

• Frequency: 5 GHz

• Transmit Power (Tx Power): 20 dBm

• Transmit Antenna Gain: 15 dBi

• Receive Antenna Gain: 15 dBi

• Cable and Connector Loss: 2 dB (both for transmit and receive)

• Receiver Sensitivity: -80 dBm

• Fade Margin: 10 dB

3. Step-by-Step Link Budget Calculation

Step 1: Calculate Free Space Path Loss (FSPL)

The formula for FSPL is:

$$FSPL (dB)=20log_{10}(d)+20log_{10}(f)+32.44$$

• d is the distance between devices in kilometers.

• f is the frequency in MHz.

For our example:

• d=5 km

• f=5000 MHz (since 5 GHz = 5000 MHz)

Let's calculate FSPL:

$$FSPL (dB)=20log_{10}(5)+20log_{10}(5000)+32.44=120.4dB$$

Step 2: Calculate Total Gains and Losses

Total Transmit Gain (Tx):

• Transmit Power (Tx Power): 20 dBm

• Transmit Antenna Gain: 15 dBi

• Cable and Connector Loss (Tx): -2 dB

$$Total\; Tx\; Gain=20+15−2=33dB$$

Total Receive Gain (Rx):

• Receive Antenna Gain: 15 dBi

• Cable and Connector Loss (Rx): -2 dB

$$Total\;Rx\;Gain=15−2=13dB$$

Step 3: Calculate Received Signal Strength (RSSI)

The formula for Received Signal Strength (RSSI) is:

$$RSSI (dBm)=Total\;Tx\;Gain+Total\;Rx\;Gain−FSPL$$

For our example:

$$RSSI=33+13−120.4=−74.4dBm$$

Step 4: Calculate Link Margin

The link margin is the difference between the received signal strength (RSSI) and the receiver sensitivity.

$$Link\;Margin (dB)=RSSI−Receiver\;Sensitivity$$

For our example:

• Receiver Sensitivity = -80 dBm

$$Link\;Margin=−74.4−(−80)=5.6dB$$

Step 5: Evaluate the Link

• Link Margin: The link margin of 5.6 dB is positive, which indicates that the link will work. However, we need to account for the fade margin (10 dB in our example).

• Fade Margin: A link margin of 5.6 dB is less than the desired fade margin of 10 dB, meaning the link may not be reliable under all conditions.

4. Implementing in MikroTik RouterOS

In a practical scenario, you would now adjust the settings on the MikroTik devices to optimize the link. Here’s how to apply some of these settings:

Step 1: Adjust Transmit Power

• Open Winbox or SSH into the MikroTik device.

• Go to Wireless -> Interfaces -> Double-click on your wireless interface.

• Under the Tx Power tab, set the transmit power. For instance, you may set it to 20 dBm.

Step 2: Set Frequency

• Under the Wireless tab, set the frequency to 5 GHz (5000 MHz).

• Adjust the Channel Width as needed (e.g., 20 MHz or 40 MHz).

Step 3: Monitor Signal Strength

• Go to Wireless -> Registration to view connected devices.

• Check the signal strength (RSSI) and ensure it is within acceptable levels based on your link budget calculation.

Step 4: Fine-Tuning

• You may need to adjust the antenna alignment, transmit power, or antenna gain to achieve a better link margin.

5. Summary

By following these steps, you can calculate the link budget for a wireless connection between two MikroTik devices and adjust your network settings accordingly. The example provided shows how each component of the link budget contributes to determining whether a wireless link will be reliable. Fine-tuning these parameters helps ensure that your wireless network is stable and performs well in various conditions.

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