Tuesday, January 1, 2008

ns2 radio propagation models

ns2 radio propagation models


ns2 implements
three different propagation models to simulate the wireless channel:
the Free space model, the Two-ray ground model and the Shadowing model.

The propagation models are used to compute the received power.
When a packet is received, the propagation model determines the
attenuation between transmitter and receiver and computes the received
signal strength. If the signal strength is lower than the Carrier
Sensing Threshold, CSThresh_, the packet is discarded by the physical
layer. This threshold simulates the effect of the receiver sensibility.

If the level is higher than this threshold, the signal strength is
then compared with the receiver threshold, RxThresh_. This threshold
determines if the packet is received successfully or with errors; in
the first case the packet is passed to the MAC layer, in the second
case the packet is marked as erroneous and passed to the MAC layer that
will provide to discard it.

Erroneous packets are delivered to the MAC layer so that it can
detect a packet collision where multi-packets are received
simultaneously. In this case the MAC layer determines the ratio between
the strongest received signal strength and the sum of the other signal
levels. The ratio is then compared with the CPThresh_ threshold that determines if the packet has been destroyed by a collision or it captured the channel.

It is worth noticing that ns2
uses a threshold to determine if a packet is received correctly or not,
without considering a more correct bit error rate computation.


The Free space model is the simplest one. It assumes ideal
propagation conditions and a single line-of-sight path between the
transmitter and receiver.

The Two-ray ground reflection model considers both the direct path
and a ground reflection path. As with the free space model, both
transmitter and receiver node are assumed to be in line of sight. It
has been shown that this model is more accurate than free space model
in case of long distance line of sight path.

Both the Free space model and the Two-ray model predict the
received power as a deterministic function of the distance between the
transmitter and receiver.


A more realistic model is the Shadowing model. It adds to the
deterministic path loss, a random component to the received power that
attempts to reproduce random variability typical of wireless links
(e.g. fading). The shadowing model consists, in fact, of two parts: the
first one is the deterministic path loss that predicts the received
power from the distance between the receiver and transmitter nodes; the
second part of the shadowing model reflects the variation of the
received power at certain distance. The shadowing is simulated as a
log-normal random variable, with zero mean. The effect of shadowing
correlation is not considered.


A detailed description of the ns2 propagation models and an accurate parameter setting description can be found in the ns2 manual: http://www.isi.edu/nsnam/ns/doc/node215.html.


Moreover, it is worth to pinpoint that there exist some extensions to default ns2 propagation model that are not included into the ns2 official distribution, but can be downloaded from the ns2 contributed code web page: http://nsnam.isi.edu/nsnam/index.php/Contributed_Code.


Among several extensions the more relevant are:

• realistic channel propagation by Wu Xiuchao,

• ricean propagation model by Ratish J. Punnoose.


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2 comments:

Basma Bejaoui said...

Hi every body , i'm trying to recover RSS value in a handover procedure,
I use this instruction defined in mac/wirelss-phy.cc
p->txinfo_.RxPr
but the value is always null ??
why ?
and how can I resolve this problem??
thx in advance

Anu said...

RSS value is very very small. Instead of printing with %lf try printing with %e. It will work.