18.1 Free space model

The free space propagation model assumes the ideal propagation condition that there is only one clear line-of-sight path between the transmitter and receiver. H. T. Friis presented the following equation to calculate the received signal power in free space at distance $d$ from the transmitter [12].

P_r (d) = \frac{P_t G_t G_r \lambda^2}{(4\pi)^2 d^2 L}
\end{displaymath} (18.1)

where $P_t$ is the transmitted signal power. $G_t$ and $G_r$ are the antenna gains of the transmitter and the receiver respectively. $L (L\ge1)$ is the system loss, and $\lambda$ is the wavelength. It is common to select $G_t = G_r = 1$ and $L = 1$ in ns  simulations.

The free space model basically represents the communication range as a circle around the transmitter. If a receiver is within the circle, it receives all packets. Otherwise, it loses all packets

The OTcl interface for utilizing a propagation model is the node-config command. One way to use it here is

$ns_ node-config -propType Propagation/FreeSpace

Another way is

set prop [new Propagation/FreeSpace]
$ns_ node-config -propInstance $prop

Tom Henderson 2011-11-05