[Tinyos-devel] Lqi and CC2420

[Philip Levis]

On Mar 3, 2008, at 1:00 AM, Andreas Koepke wrote:

>>>
>>>  Pragmatically: something that delivers a comparable performance.
>> Phil can elaborate more because he has looked at RSSI and LQI in
>> detail but it is not clear if one can get comparable performance  
>> using
>> RSSI vs LQI.
>> - om_p
>
> Well, his published results afaik lack a QQ-Plot using the packet  
> delivery ratio PDR of RSSI vs. LQI. There may exist a cost function  
> such that PDR = cost(RSSI) = cost'(LQI). Theoretically, RSSI and  
> LQI must be strongly correlated in the absence interferers (both in  
> system and out of system like microwave ovens). So I believe that  
> there is such a function, and I would not be surprised if it is  
> linear at least for the relevant interval. Excluding interferers is  
> the difficult part here...

I'm not sure why a QQ plot would help. The two functions are very  
different: while they fact that they measure the same thing means  
they are correlated, that does not mean they have similar probability  
distributions, as the LQI/RSSI plots show. Much of the data from the  
published results is online, though, so someone can do a QQ plot if  
they want.

There is no accurate function f() such that f(RSSI) = LQI. Two  
reasons. First, the quantization of RSSI readings. LQI readings are  
essentially lower-order bits on the SNR curve. A one dB change in  
RSSI can lead to a 40+ unit change in LQI. Second, LQI represents  
SINR, while RSSI is independent of the Interference and Noise.

Basically, the CC2420 chip correlation indicator provides fine- 
grained information on the part of the SINR curve where the slope is  
significant. This is why the values show much larger temporal  
variation: a tiny shift in SINR causes a change in LQI. Also, LQI,  
due to its measurement, is more greatly affected by statistical  
sampling. Even with a very stable RSSI and SINR, you'll see LQI  
variations.

Phil