The ns Manual (formerly ns Notes and Documentation)1 - NM Lab at ...

set prop [new Propagation/FreeSpace]$ns_ node-config -propInstance $prop18.2 Two-ray ground reflection modelA single line-of-sight path between two mobile nodes is seldom the only means of propation. The two-ray ground reflectionmodel considers both the direct path and a ground reflection path. It is shown [29] that this model gives more accurateprediction at a long distance than the free space model. The received power at distance d is predicted byP r (d) = P tG t G r h t 2 h r2d 4 L(18.2)where h t and h r are the heights of the transmit and receive antennas respectively. Note that the original equation in [29]assumes L = 1. To be consistent with the free space model, L is added here.The above equation shows a faster power loss than Eqn. (18.1) as distance increases. However, The two-ray model does notgive a good result for a short distance due to the oscillation caused by the constructive and destructive combination of the tworays. Instead, the free space model is still used when d is small.Therefore, a cross-over distance d c is calculated in this model. When d < d c , Eqn. (18.1) is used. When d > d c , Eqn. (18.2)is used. At the cross-over distance, Eqns. (18.1) and (18.2) give the same result. So d c can be calculated asd c = (4πh t h r )/λ (18.3)Similarly, the OTcl interface for utilizing the two-ray ground reflection model is as follows.$ns_ node-config -propType Propagation/TwoRayGroundAlternatively, the user can useset prop [new Propagation/TwoRayGround]$ns_ node-config -propInstance $prop18.3 Shadowing model18.3.1 BackgroudThe free space model and the two-ray model predict the received power as a deterministic function of distance. They bothrepresent the communication range as an ideal circle. In reality, the received power at certain distance is a random variabledue to multipath propagation effects, which is also known as fading effects. In fact, the above two models predicts the meanreceived power at distance d. A more general and widely-used model is called the shadowing model [29].188