A single line-of-sight path between two mobile nodes is seldom the only means
of propation. The two-ray ground reflection model considers both the direct
path and a ground reflection path. It is shown [29] that this
model gives more accurate prediction at a long distance than the free space
model. The received power at distance is predicted by
where and
are the heights of the transmit and receive antennas
respectively. Note that the original equation in [29] assumes
. To be consistent with the free space model,
is added here.
The above equation shows a faster power loss than Eqn. ()
as distance increases. However, The two-ray model does not give a good result
for a short distance due to the oscillation caused by the constructive and
destructive combination of the two rays. Instead, the free space model is
still used when
is small.
Therefore, a cross-over distance is calculated in this model. When
, Eqn. (
) is used. When
, Eqn.
(
) is used. At the cross-over distance, Eqns. (
)
and (
) give the same result. So
can be calculated as
Similarly, the OTcl interface for utilizing the two-ray ground reflection model is as follows.
$ns_ node-config -propType Propagation/TwoRayGround
Alternatively, the user can use
set prop [new Propagation/TwoRayGround] $ns_ node-config -propInstance $prop
Tom Henderson 2014-12-17