Hi,
I’ve been working on this Xbee modules with the X-CTU Sofware, I recently tried the Range Test terminal and noticed a RSSI checkbox.
The test marked a RSSI of -104. What does it mean? I already know the number of good and bad packages, but how does this relate with power or troughput…
Thanks everyone for your help
Received signal strength indication.
Leon
The -104 number you mentioned is most likely a no-signal-present (noise floor) situation…
802.15.4 has “LQI” which is a number between 0 and 255 meaning link quality indicator. It has no engineering units per se.
RSSI may be in dBm or %. The latter has no engineering meaning.
The dBm is decibels relative to 1 mWatt. That is, 0dBm = 1mW.
3dBm = 2mW
-3dBm = 1/2 mW
and so on, in a log scale.
IEEE 802.14.5 provides guidelines for receiver sensitivity minimums. Of course, the end to end situation is the usual link budget
Tx Power in dBm, say, 0dBm
Tx antenna gain, say, 2dBi (or for a chip antenna, about -2dBi)
(dBi is gain relative to a perfect spherical (isotropic) antenna).
Free space (line of sight) path loss at 2.4GHz, distance x.
Additional loss for
Fresnel zone blockage. Google that
Non-line-of-sight blockage (trees, walls, terrain)
Receiver antenna gain
= received signal strength in dBm
Versus receiver sensitivity = margin.
Receiver sensitivity is about the same for all 802.15.4 products. I’d say -90dBm or so is a reasonable worst case (weakest) RSSI. Not much margin (for fades).
Lastly, since, of course, 802.15.4 is a Time Division Duplexing (TDD) system (as is 802.11), RSSI has to be measured by the receiver’s firmware as a frame has been received - and successfully. A frame is just a few miliseconds.