First, what is fade margin? It’s the difference between a radio’s sensitivity (minimum signal level at which it can decode data) and the actual received signal level. Higher fade margins can provide more reliable data radio links.
Any radio has an engineered-in receiver sensitivity, measured in dBm. (decibels relative to 1 milliwatt) This should be specified in the radio’s data sheet or brochure. The older Trio MR450 data radio, for example, has a sensitivity of -106 dBm at a 9600 bps over-the-air data rate, while the QR450 (using newer technology) has a sensitivity of -113 dB at 8 kbps.
Generally speaking, however, sensitivity goes down as data rate goes up with any specific radio type. A given radio operating at 8 kbps may have a sensitivity of -113 dBm, while at 60 kbps the sensitivity may be reduced to (e.g.) -100 dBm. So on a more difficult (long and/or heavily obstructed) a radio may need to operate at a slower data rate to allow it to use a higher sensitivity.
The link between two radios can experience an event called “fading,” in which changing atmospheric conditions causes the signal level to drop. This can be quite significant at higher frequencies. And if there is any noise (eg from atmospheric conditions or electrical equipment) or interference (from other radios nearby) the radio needs the signal to be stronger to hear it well. Fading can occur due to heavy rain or snow, or due to changing weather conditions. (eg hot & dry to cool & damp)
There can also be maintenance issues which may reduce signal levels heard by a given radio, for example old coaxial cable, or a damaged antenna or surge (lightning) arrestor. Also, interference from other radio systems may increase the background noise (the noise floor) which can effectively reduce the fade margin as well.
Under typical conditions with radios using modulation types such as CPM (continuous phase modulation) or QAM (quadrature amplitude modulation), the wireless industry has typically specified a fade margin of at least 20 dB to provide reliable paths. For example, if a system is operating at 24 kbps over-the-air, and the sensitivity at that data rate is -107 dBm, all paths must be designed such that the actual received signal level is -87 dBm or higher.
When testing wireless paths using software such as Pathloss or Radio Mobile, if the fade margin is not achieved the designer may choose any of several options:
- Increase antenna height to get the antenna above obstacles
- Select antennas with higher gain
- Reduce the radio data rate to get better receiver sensitivity
- Add a repeater between the sites
Note that with a newer modulation technology like Dynamic Radio Data Rate (available in Trio's Q radios), each radio may automatically choose to vary its data rate. When conditions are good, the radio may operate at a higher speed, with reduced sensitivity and thus reduced fade margin. And when conditions deteriorate, the radio can slow down, gaining sensitivity and thus improving the fade margin. When designing a system using Dynamic data rate, the target fade margin may be lower than the normal 20 dB. For example, it may be as low as 10 dB at the radio's highest data rate.
Background information:
Sensitivity of a new data radio is tested at the factory by pushing a constant data stream through a pair of radios in a carefully-controlled lab environment. The data stream entering one radio is compared with the data stream exiting the other radio, as signal attenuation between the two is increased. Eventually the signal will be weak enough that errors will begin to appear. The software tool tracks these errors, and the test ends when it’s found that (on average) 1 bit in a million is wrong. The signal level at this point is stated as the radio’s sensitivity at that data rate.
A “1 bit in a million” bit error rate is typically stated as a 1x10-6 BER. Note that some manufacturers have in the past stated the radio’s sensitivity at a 1x10-4 BER. This is only 1 bit in 10,000 instead of 1 in a million, so the sensitivity appears better. Not really a fair comparison. Few manufacturers try this today., but it's best to check.
A value of 0 dBm is 1 mW. Transmit power is typically specified in positive numbers eg +40 dBm is 10 watts. But receive power (coming in from the antenna) is measured in negative numbers. For example, -70 dBm is a common received signal level. For each 10 dB decrease in received signal level, the power in milliwatts is reduced by a factor of 10. So -70 dBm is a very tiny amount of power... 0.000001 of a milliwatt, or 1 picowatt !! But it's important to remember that less-negative received signal levels are better. (similar to winter in the north: -20 degrees is better than -40 degrees!)
Joel Weder
Remote Operations Specialist
Schneider Electric
