[ns] The "Friss" propagation model

[Leonard Miller <lmiller@antd.nist.gov>]

A search of the ns-users archives doesn't turn up with the following
observation, so here goes what I hope is a helpful comment:

The "Friss propagation model" or "Friss free space propagation model"
used in the new wireless modes of ns (propagation.cc) is actually an
application of the standard "free space propagation model," which gives
the median path loss in dB (exclusive of antenna gains and other losses)
as

Path loss in dB = 32.45dB + 20*log10(f_MHz * d_km)

The name "Friis" (not "Friss") is associated with the formula when using
antenna gains to model the transference of power between finite antennas
in free space [see "Friis transmission formula" in _Antennas_ by Kraus].

See http://ece.gmu.edu/~pceperle/projs513/Nishanian/RadioP.htm for a
nice summary of propagation.  The student here uses the name "Friis" in
connection with the free-space model and references Rappaport's book. 
Most other places where free-space propagation is discussed, including
the popular paper by Bullington, don't associate Friis with the formula,
but maybe it's overdue [K. Bullington, IEEE Trans. VT 11/1977].  But for
ns purposes, Fri*s, whatever..., I suppose.

In addition to the antenna gains, the formula for median link loss in
propagation.cc includes an implementation loss, L [called "sysLoss" in
shadowing.cc].  The value of L is set equal to 1.0, for example, as a
property of Phy/WirelessPhy in the example files ~ns/tcl/ex/wireless.tcl
and ~ns/tcl/ex/wireless-test.tcl.

Say when... I'm going to look further into the link budget matters for
the wireless models in ns, since my background has me coming to this
project from that direction.  I will give more comments as I go if
you-all find it helpful.  Let me know if it's not appropriate in this
forum, please.

--Sincerely, 
   Len Miller 
________________________________________________________
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Leonard E. Miller, Wireless Commun. Technologies Group
    National Institute of Standards & Technology (NIST)
    100 Bureau Drive Stop 8920, Bldg. 820, Room 445
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