Carrier Sense Multiple Access (CSMA)

1. Carrier Sense Multiple Access (CSMA)
Abstract
In ad hoc networks, aggregate interference is mainly because of the concurrent transmissions
taking place in the near vicinity of receiving nodes. To deal with the interference caused due to
simultaneous transmission, some models and hard core point processes will be used which helps
to reduce the interference in the network. Broadcast nature of the channel results in interference
and in order to minimize this interference employing a scheduling technique is necessary. The
main objective of this project is to characterize the CSMA wireless networks with the help of
hard core point processes like Matern Point Process (MPP), Simple Sequential inhibition (SSI),
Node Coloring and Modified Hardcore Point process (MHCPP). By comparison we will figure
out the best process to model CSMA. Furthermore, aggregate interference modeling has also
been done for each point process via simulations.

2. Introduction
Ad hoc network is a adaptive and flexible type of network having no fixed infrastructure support
and the communicating nodes in it configure themselves on runtime. Ad hoc networks, being
infrastructure less, are robust to any minor failure as they are not completely reliant on few
critical nodes. In an ad hoc network new nodes can easily be integrated. The dynamic and self-
organizing attribute of ad hoc networks makes them extremely useful in circumstances where
rapid network deployments are required or it is prohibitively costly to deploy and manage
network infrastructure. Daily life applications of ad hoc networks include emergency services in
a natural calamity affected area, defense communication, and entertainment etc. Successful
communication in an ad hoc networks requires tackling various design challenges including but
not limited to spectral efficiency, optimized power control, and quality of service (QoS).[1]
The ad hoc networks are usually characterized by a multi-user system having no fixed
infrastructure support and using the same channel (single channel network) and therefore,
causing interference to each other when they transmit simultaneously. Thus, ad hoc networks
pose a unique challenge of managing interference which is a key to achieve performance.
Interference in a multi-user system having the luxury of fixed infrastructure can be easily
managed by orthogonalizing the users in time, frequency, space or/and codes (e.g) Cellular
Networks. While in case of ad hoc networks, orthogonalization is an issue because of lack of
central coordination [2].
In ad hoc networks, the geometry of the nodes location plays a vital role in determining the
Signal to Noise plus Interference ratio (SINR), as separation between the transmitting and
receiving node is the major contributor in aggregate interference when compared with other
fading mechanisms. Stochastic geometry provides a natural way of defining and computing
macroscopic properties of ad hoc networks. Modeling wireless networks with stochastic
geometry is especially convenient when considering relatively large number of nodes. Usually
the deployed network is considered in a snapshot of time, and the node locations are seen as
realizations of some point processes [3].
Poisson Point Process (PPP) plays a vital role in all point processes as it is a starting point of all
other point processes which also involves hardcore point process. PPP assumes that the nodes are
uncorrelated with reference to their position, but actually this is only valid for nodes modeled in
ad hoc networks in large and dense number [4]. It is observed that for spatial distribution of
nodes in CSMA networks such point processes are required which consider correlation among
node locations. Thus, PPP becomes an unrealistic process to model node locations in CSMA
networks [5]. There is another point process known as Matern Point Process (MPP) in 1960 in
which a minimum distance (exclusion) is ensured between any two concurrent transmitting
nodes. The noticeable thing here is that all these point processes we deploy is modeled for
transmitters. There was a problem observed with MPP which is intensity underestimation. To
resolve this problem, authors proposed Simple Sequential Inhibition (SSI) in [6] which partially
dealt with this problem.

3. In SSI, nodes acquire part of the network on the basis of the distance between them [7]. In this
model authors mentioned that every time at the execution the new entrants (the transmitters) try
to acquire the space in the medium. If the distance of the new entering transmitter is greater than
the R inhibition radius, only then it becomes alive (active). Else that transmitter is excluded from
the list, and will not be considered in the selection process of that system. For more accurate
results of interference dealing Modified hard core point process (MHCPP) was proposed to
model CSMA.
1.1 Problem Definition:
In this thesis, we consider different point processes like PPP, BPP, MPP,SSI, Node coloring, and
MHCPP. Following their limitations we tend to characterize the nature of interference in BPP
and MPP. We analyze their individual performance on the basis of interference.
1.2 Motivation:
In this project, MHCPP is presented which minimizes the limitation of the HCPP and perfectly
monitors the intensity of the dependently thinned Poisson Point Process. As interference is
mostly caused by the transmitter end so we will model and observe the behavior of these
different Point processes to identify the point process which can best model CSMA [8]. Our
motivation to study different point process and to propose MHCPP is to mitigate the intensity
underestimation problem.