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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.

Radio Range

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.

 

 

 

 

Filed under: System Design | Posted on September 24th, 2014 by John Sonnenberg

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