[Tinyos-help] About Sensitivity and RSSI.

[Philip Levis]

On Feb 10, 2007, at 3:43 PM, Madhu Mudigonda wrote:

> We have a question on monitoring of environment by reading RSSI  
> values continuously from RSSI register. We ran an experiment, where  
> two nodes exchange messages between them and continuously read the  
> RSSI values from both the received packet and RSSI register. We  
> could not observe any change in RSSI values, when there is packet  
> loss between motes. Sometimes, there was not much variation in the  
> RSSI values, but still there was some packet-loss.
> RSSI Values obtained by us are like this:
>
> -99,-99,-99,-99,-98,-65,-56,-86,-99,-99,-98..........
>
> According to our understanding, as the sensitivity threshold of  
> CC2420 radio is -95dBm, mote should lose packets, if the RSSI is  
> less than -95dBm ( i.e, -96dBm,-97dBm,...........). But, in reality  
> though the RSSI values are less than the -95dBm, packets are being  
> received by the receiver successfully and sometimes though the RSSI  
> value is something like -76dBm, the packets are getting lost. We  
> couldnot understand why the RSSI register values are less than  
> -95dBm even when we are receiving packets?
>
> Any suggestions on how to estimate the channel quality using the  
> RSSI values?
>

The -95 dBm number on the datasheet is an approximation. Packet  
reception is based on the signal-to-noise ratio. If you noise floor  
was say, -94dBm, then in the case of the CC2420 you wouldn't see any  
packets  at -95dBm.

Phil