Re: [Tinyos-help] RSSI and TOSSIM
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 ___ Tinyos-help mailing list Tinyos-help@millennium.berkeley.edu https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
Re: [Tinyos-help] RSSI and TOSSIM
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. Thanks in advance for your attention, Iñigo On Wed, Dec 19, 2007 at 11:40 AM, Philip Levis <[EMAIL PROTECTED]> wrote: > > On Dec 19, 2007, at 6:09 AM, Tal Rusak wrote: > > > Hi, > > I noticed that TOSSIM uses the following formula to compute RSSI: > > rcv->strength = (int8_t)(floor(10.0 * log(pow(10.0, power/10.0) + > > pow(10.0, noiseStr/10.0)) / log(10.0))); > > (in CpmModelC.nc) > > I notice that this is adding the signal power (gain) and noise > > levels. I am wondering if this is a true reflection of the physical > > phenomenon--i.e., in the CC2420, when RSSI is computed > > (CC2420Packet.getRssi(packet)), do signal and noise strength > > contribute in an additive fashion? Are there any paper or documents > > that support this notion? > > I appreciate any response in advance. > > It's basic wave physics. This is a very simplistic way to compose the > signals. If the signals are completely in phase, it will be additive. > In practice, they're not, so it's less. But trying to capture this > more accurately becomes strange, especially when the two signals are > not precisely the same frequency. > > Phil > > > ___ > Tinyos-help mailing list > Tinyos-help@millennium.berkeley.edu > https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help > ___ Tinyos-help mailing list Tinyos-help@millennium.berkeley.edu https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
Re: [Tinyos-help] RSSI and TOSSIM
On Dec 19, 2007, at 6:09 AM, Tal Rusak wrote: Hi, I noticed that TOSSIM uses the following formula to compute RSSI: rcv->strength = (int8_t)(floor(10.0 * log(pow(10.0, power/10.0) + pow(10.0, noiseStr/10.0)) / log(10.0))); (in CpmModelC.nc) I notice that this is adding the signal power (gain) and noise levels. I am wondering if this is a true reflection of the physical phenomenon--i.e., in the CC2420, when RSSI is computed (CC2420Packet.getRssi(packet)), do signal and noise strength contribute in an additive fashion? Are there any paper or documents that support this notion? I appreciate any response in advance. It's basic wave physics. This is a very simplistic way to compose the signals. If the signals are completely in phase, it will be additive. In practice, they're not, so it's less. But trying to capture this more accurately becomes strange, especially when the two signals are not precisely the same frequency. Phil ___ Tinyos-help mailing list Tinyos-help@millennium.berkeley.edu https://www.millennium.berkeley.edu/cgi-bin/mailman/listinfo/tinyos-help
