Link Margin (also known as Fade Margin) is the difference between the received RF signal power and the minimum signal level required for the receiver to demodulate a signal.
The received signal power (PRx) can be estimated from the Link Budget analysis.
Enter the numbers for each (in dBm) below and the tool will provide the Link Margin in dB.
Formula
Link Margin = Received Signal Power – Receiver Sensitivity
Background
What is Fading?
In wireless communication systems, fading refers to the variation in signal strength as a wireless signal travels from the transmitter to the receiver.
This variation is caused by several factors including the path between the transmitting and receiving antennas, multipath propagation, and atmospheric conditions.
These factors can result in loss or attenuation of the transmitted signal, leading to a decrease in signal strength at the receiver.
Fading can be classified into two types:
- Fast fading and
- Slow fading
Fast fading occurs when there are rapid changes in signal strength over short periods of time, while Slow fading occurs when the variations happen over longer periods.
Fading can lead to poor signal quality and increased bit error rate (BER). To combat fading, techniques such as diversity reception, equalization, amplification and power control are used to improve the received signal quality and mitigate the effects of interference and noise.
By continuously monitoring the signal strength and maintaining it above the receiver’s threshold, reliable and efficient communication can be achieved even in fading conditions.
What is Fade Margin?
Fade or Link margin is an important concept in data communication systems that refers to the difference between the minimum signal level required by a receiver and the actual received signal strength.
It is crucial for ensuring a reliable link between radios and a successful data transmission. Fade margin accounts for various factors that affect the signal strength, such as the distance between the transmitting and receiving antennas, the path the radio signal takes, and potential interference from other sources.
Link or Fade margin provides an estimate of how much signal attenuation can be tolerated before the wireless communication link fails.
To design a radio system with sufficient fade margin, Radio Engineers need to consider the receiver sensitivity, which is the minimum signal level required for proper operation. Additionally, factors like frequency, weather conditions, and the presence of obstacles in the signal path can cause signal attenuation or interference.
The Free space path loss calculation is a simple estimator that accounts for the effect of distance, frequency of operation and antenna gains. However, the signal can also be attenuated due to multipath propagation, weather and shadowing.
Designing the radio system to have a higher fade margin helps to compensate for these factors, ensuring a reliable and consistent signal even in adverse conditions.
Other considerations, such as lightning protection, proper cable management, reducing receiver noise and interference are also important to minimize signal degradation and maximize performance.
Example Calculation
In a Wi-Fi system, the receiver sensitivity for 256-QAM and 20 MHz bandwidth is -57 dBm. This is the minimum power required for successful demodulation and decoding of the signal.
For a received signal power of -40 dBm, the Link Margin is 17 dB.
Reducing the Impact of Fading
Typically the effects of fading are countered with the use of
- Antenna diversity
- Coding schemes
- Error correction
- Modulation
- Receiver design
- Transmit signal power
The higher the value of the fade margin and the greater the channel protection using these techniques, the more resilient a wireless communication link will be.
How much fade margin is required?
In general, at least 10 dB of fade/link margin is required in a wireless communication system. A good number to design for is 30 dB.
In the above example with Wi-Fi receiver sensitivity of -57 dBm, the required received signal power is -57 dBm + 30 dB = -27 dBm.
References
[1] Fade Margin
[2] Signal Fading on Wikipedia
[3] IEEE Std 802.11-2020 IEEE Standard for Information Technology – Telecommunications and Information Exchange between Systems – Local and Metropolitan Area Networks – Specific Requirements – Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.
[4] Fading, Shadowing and Link Budgets by Thomas Schwengler