RSSI and Communication Range
Radio Signal Strength Indication (RSSI)
All Raveon data radio modems have a built in RSSI indicator to help determine the signal strength of a received radio modem signal. Raveon GPS tracking products also include location (latitude/Longitude) with the RSSI information which makes RF propagation analysis very simple.
RSSI is usually expressed in dBm which is decibels relative to milliwatts of received power. 0dBm = 1 mW. Every time the received power drops in half, the RSSI will change by 3dB. .5mW = -3dBm. .25mW = -6dBm and so on. Everytime the received power drops by 1/10th, the RSSI will drop by 10dB. 1/10 of a mW is -10dBm. 1/100th of a mW is -20dBm, 1/1000th of a mW is -30dBm and so on.
Radio Signal Strength (RSSI) will drop the further away a receiver is from the transmitter it is receiving. This is a square-law phenemoena, so the received signal will drop by at least 1/4 every time the distance from the transmitter doubles. 1/4power is a 6dB drop in RSSI, so when a receiver with a given RSSI moves twice the distance from a transmitter, the RSSI will drop at least 6dB. For quick range calculations, assume 6-10dB drop for every doubling of distance.
For example, in the above image, if the center dark-blue circle were 1/2 mile across at -70dBm signal, the -80dBm range would be about 1 mile, the -90dBm circle 2 miles and the -100dBm circle 4 miles across.
But, remember that most all communication systems on earth are limited by terrain not line-of-sight distance. A 5 watt radio modem transmitting 5 watts of RF can be received thousands of miles away in free-space. On earth, it may only be 1 to 100 miles. Terrain, antenna heights, foliage, buildings, interference, and antenna gains play a huge role in determining how far a radio can communicate. Also due to multipath, moving objects, and varying antenna positions, the RSSI at a typical location in the fringe area of reception will often vary 10dB – 20dB over a short amount of time (seconds). This may cause sporadic reception, but with a good communication protocol, the 20dB drop-outs will not be noticed.
Signal strengths near a base station are typically in the -30 to -60dBm range. Most Raveon radios can measure an RSSI as large as -60dBm. Above that, they will report some maximum value such as -58. The upper limit varies by model. Here is a rough summary of RSSI signal implications.
RSSI Level |
Comments |
| -30 to -60 |
Very strong. The receiver is very near the transmitter or base station. |
| -60 to -90 | Excellent signal strength. Usually the close to 100% coverage and reception. |
| -90 to -105 | Good reception, but occasional missed data. |
| -105 to -115 | Reception can be 100% but often will have drop-outs/blind spots when the average signal is this weak. |
| -105 to -120 | Signal reception will be sporadic. Don’t design a reliable system to work with this weak of a signal unless you utilize an error-correcting and lost-message protocol. |
