How does veins calculate RSSI in a Simple Path Loss Model?

Question

We are working on an application based on Veins framework which needs RSSI value of received signal and the distance between sender and receiver.

We referred to the VeReMi project which also calculates RSSI value and sends it to upper level.

We compared our simulation result (RSSI vs Distance) with the VeReMi dataset and they look quite different. Can you help us to explain how RSSI is calculated and whether our result is normal?

In our application, we obtain the distance and rssi value by

auto distance = sender.getPosition().distance(receiverPos);
auto senderRSSI = sender.getRssi(); 

In the lower level, the rssi is set in the Decider80211p::processSignalEnd(AirFrame* msg) method as in the VeReMi project.

if (result->isSignalCorrect()) {
    DBG_D11P << "packet was received correctly, it is now handed to upper layer...\n";
    // go on with processing this AirFrame, send it to the Mac-Layer

    WaveShortMessage* decap = dynamic_cast<WaveShortMessage*>(static_cast<Mac80211Pkt*>(frame->decapsulate())->decapsulate());

    simtime_t start = frame->getSignal().getReceptionStart();
    simtime_t end = frame->getSignal().getReceptionEnd();
    double rssiValue = calcChannelSenseRSSI(start, end);
    decap->setRSSI(rssiValue);

    phy->sendUp(frame, result);
}

Regarding the simulation configuration, our config.xml differs from VeReMi and there is no the following lines in our case.

<AnalogueModel type="VehicleObstacleShadowing">
        <parameter name="carrierFrequency" type="double" value="5.890e+9"/>
</AnalogueModel>.

The 11p specific parameters and NIP settings in the omnetpp.ini are the same.

Also, our simulation is based on Boston map.

The scatter plot of our simulation result of RSSI_vs_Distance is shown in the following figure.

RSSI vs Distance from our simulation shows that even at distance beyond 1000 meters we still have received signal with strong RSSI values

In comparison, we extract data from VeReMi dataset and plot the RSSI vs Distance which is shown in following pic.

VeReMi dataset RSSI vs Distance is what we were expecting where RSSI decreases as distance increases

Can you help us explain whether our result is normal and what may cause the issue we have now? Thanks!

Answer 1

I am not familiar with the VeReMi project, so I do not know what value it is referring to as "the RSSI" when a frame is received. The accompanying ArXiV paper paper mentions no more details than that "the RSSI of the receiver" is logged on frame receptions.

Cursory inspection of the code for logging the dataset you mentioned shows that, on every reception of a frame, a method is called that sums up the power levels of all transmissions currently present at the receiver.

From this, it appears quite straightforward that (a) how far a frame traveled when it arrives at the receiver has only little relation to (b) the total amount of power experienced by the receiver at this time.

If you are interested in the Received Signal Strength (RSS) of every frame received, there is a much simpler path you can follow: Taking Veins version 5 alpha 1 as an example, your application layer can access the ControlInfo of a frame and, from there, its RSS, e.g., as follows: check_and_cast<DeciderResult80211*>(check_and_cast<PhyToMacControlInfo*>(wsm->getControlInfo())->getDeciderResult())->getRecvPower_dBm(). The same approach should work for Veins 4.6 (which, I believe, the VeReMi dataset you are referring to is based) as well.

In simulations that only use SimplePathlossModel, Veins' version of a free space path loss model, this will result in the familiar curve: