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[ns] Recommendation to change wireless propagation equations



A previous message discussed the calculation of the crossover distance
that is found in ~ns/tworayground.cc:

http://www.isi.edu/nsnam/archive/ns-users/webarch/2000/msg04473.html

(with an attachment now available at the following URL:
http://w3.antd.nist.gov/wctg/proploss.pdf )

The crossover distance is where the free space propagation loss equals
the fourth-power law propagation loss that is associated with the
two-ray plane-earth model.  It was noted that the equation for
crossover_dist in tworayground.cc includes the system loss parameter L,
which it should not, and the equation for the fourth-power law
propagation loss does not include the system loss, which it should in
order to be consistent with the definition of system loss.

Thus, I am offering two recommendations regarding tworayground.cc:
(1) Instead of the equation

crossover_dist = sqrt((16 * PI * PI * L * ht * ht * hr * hr) / (lambda *
lambda))

the calculation should be without L included:

crossover_dist = 4 * PI * ht * hr / lambda

(2) The function TwoRay in tworayground.cc should include the system
loss, L, as an additional argument.  That is, the code for TwoRay should
be replaced by the following:

TwoRay(double Pt, double Gt, double Gr, double ht, double hr, double L,
double d)
{
	/*  Two-ray ground reflection model with 
	 *  antenna gains and system loss.
	 *
	 *        Pt * Gt * Gr * (ht^2 * hr^2)
	 *  Pr =  ----------------------------
	 *                  L * d^4
	 */
return Pt * Gt * Gr *(hr * hr * ht * ht) / (L * d * d * d * d)
}

[Other references to the function TwoRay in tworayground.cc and
tworayground.h need to be modified accordingly.]

In the following graphs, using dx to denote the crossover distance, the
present propagation model as it is affected by the system loss is
compared to the recommended changed propagation model, also as affected
by the system loss:

	http://w3.antd.nist.gov/wctg/propcomp.pdf

The graphs show that the current version does not correctly utilize the
system loss parameter, L, since the system loss only affects the
propagation for the shorter distances in the current version of
tworayground.cc.  When the changes that are recommended here are
implemented, the system loss will affect the propagation at all
distances, just as the antenna gains do.

--Len Miller 
________________________________________________________
*._..*.*_.***__*..*._..*._..*.*._.***._*_***_.*..*...*_*
--------------------------------------------------------
Leonard E. Miller, Wireless Commun. Technologies Group
    National Institute of Standards & Technology (NIST)
    100 Bureau Drive Stop 8920, Bldg. 820, Room 445
    Gaithersburg, Maryland 20899-8920
Phone: 301-975-8018        Fax: 301-590-0932
email: [email protected], [email protected]
http://w3.antd.nist.gov/wctg/