Rayleigh fading computation

[dragos-bth]

Hi,

I am looking into the source code for computing Rayleigh fading in INET/INETMANET

inet/src/inet/physicallayer/wireless/common/pathloss/RayleighFading.cc

Lines 32 to 39 in d93b7bc

	double RayleighFading::computePathLoss(mps propagationSpeed, Hz frequency, m distance) const
	{
	m waveLength = propagationSpeed / frequency;
	double freeSpacePathLoss = computeFreeSpacePathLoss(waveLength, distance, alpha, systemLoss);
	double x = normal(0, 1);
	double y = normal(0, 1);
	return freeSpacePathLoss * 0.5 * (x * x + y * y);
	}

double RayleighFading::computePathLoss(mps propagationSpeed, Hz frequency, m distance) const
{
    m waveLength = propagationSpeed / frequency;
    double freeSpacePathLoss = computeFreeSpacePathLoss(waveLength, distance, alpha, systemLoss);
    double x = normal(0, 1);
    double y = normal(0, 1);
    return freeSpacePathLoss * 0.5 * (x * x + y * y);
}

I would like to understand where 0.5 term comes from on the line with the return statement. As far as I know, $\sqrt{x^2 + y^2}$ has a Rayleigh density when $x$ and $y$ are normal and independent random variables with zero mean and equal variance. Therefore I was expecting that line to look like

return freeSpacePathLoss * sqrt((x * x + y * y));

This has also been reported in the OMNeT++ discussions: omnetpp/omnetpp#1119.

[rhornig]

This is definitely a bug. This code is inherited from the original Mobility framework back from 2008. It was merged into MiXiM and the eventually landed in INET framework. Here is the original paper