[Tinyos-help] RSSI and TOSSIM

Philip Levis pal at cs.stanford.edu
Thu Feb 14 18:25:24 PST 2008


On Feb 14, 2008, at 1:37 PM, Iñigo Urteaga wrote:

> Hi all,
>
> I found this previous post from Dec. 19th in tynios-help and I would
> like to have some points clarified.
>
> First, I agree with Phil that the received signal strength will be
> (assuming waves are in phase) the addition of both desired signal's
> power and existing noise power level.
>
> As far as I understand TOSSIM's code, no radio stack is simulated. All
> the radio communications are simulated by TossimActiveMessageC, which
> relies in CPMModel. CPMModel computes the packet's received power with
> it's corresponding noise level using CPM gain/noise model. At this
> point is calculated the "strength" value as mentioned by Tal, which is
> included in the metadata of the packet. This "strength" field can then
> be accessed by TossimPacket interface.
>
> So, having this scenario, I understand the goal of TossimPacket
> interface is to give the application a chance to access radiolink
> quality information, am I right? Accessing received message's strength
> value, it is possible to have a rough idea of the link quality between
> the sender and the receiver.
>
> However, I am not completely comfortable with using signal strength as
> link quality indicator. Although I agree received signal strength is
> the sum of both the original signal and noise, I don't think this
> truly reflects link quality. It would be possible to have really noisy
> links (with low desired signal power level) that would have as good
> strength values as really good links (where desired signal power is
> much bigger than noise level). Instead, since signal's power level and
> noise level are accessible by the CPMModel component, why not use
> Signal To Noise Ratio?
>
> That is why I would like to know if there is any reason that makes
> necessary to have the strength value included in the metadata. Would
> not be possible to include the SNR value in that field? My point here
> is that, since no radio stack is simulated in TOSSIM, it would be more
> useful to offer a SNR indicator than the "strength" field.

Of course SNR is a better indicator than S+N. But all most radios let  
you do is sample the RF power, which is inherently S+N. The CC2420  
provides an indirect measure of SNR through its chip correlation  
indicator (commonly but incorrectly called LQI, for link quality  
indicator). This measures the soft chip correlation across a couple  
of symbols. LQI is pretty good as a very lightweight way to see if a  
link has a good SNR, but as you point out, if the link has varying  
SNR then you don't get it on bad periods. So it turns out that a  
cross-layer approach -- uses layer 1 feedback as well as layer 2  
delivery -- generally works better.[1]

Phil

[1] Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, and Philip  
Levis. "Four-Bit Wireless Link Estimation." In Proceedings of the  
Sixth Workshop on Hot Topics in Networks (HotNets VI), 2007.  http:// 
sing.stanford.edu/pubs/hotnets07-4b.pdf


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