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 from the transmitter .
where is the transmitted signal power. and are the antenna gains of the transmitter and the receiver respectively. is the system loss, and is the wavelength. It is common to select and 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