Bug 1064: Correct Friis propagation loss equation

The FrissSpectrumPropagationLoss model doesn't implement Friis formula
correctly - the whole numerator needs to be squared, rather than just
the last term.

RELEASE_NOTES

@@ -29,6 +29,7 @@ since ns-3.10, in many cases referencing the Bugzilla bug number.
 
    - Fixed the UanPhyGen::IsStateBusy method, error with logical OR
    - CsmaNetDevice ReceiveErrorModel was not dropping the packet
+   - bug 1064 - Correct Friis propagation loss equation in spectrum module
 
 Known issues
 ------------

src/spectrum/model/friis-spectrum-propagation-loss.cc

@@ -86,7 +86,8 @@ FriisSpectrumPropagationLossModel::CalculateLoss (double f, double d) const
     }
 
   NS_ASSERT (f > 0);
-  double loss = ( d * f * f) / ((3e8 * 3e8) / (4 * M_PI)) ;
+  double loss_sqrt = (4 * M_PI * f * d) / 3e8;
+  double loss = loss_sqrt * loss_sqrt;
 
   if (loss < 1)
     {

src/spectrum/model/friis-spectrum-propagation-loss.h

@@ -49,7 +49,7 @@ public:
    * Return the propagation loss L according to a simplified version of Friis'
    * formula in which antenna gains are unitary:
    *
-   *         4 * pi * d * f^2
+   *      (4 * pi * d * f) ^ 2
    * L = ----------------------
    *               C^2
    *