Contents
2.1 Interference Modeling . . . . 16 2.2 Asynchronous Interference Modeling and Analysis . . . . 23 2.3 Interference Analysis and Management in Heterogeneous/ Overlaid
Wireless Network . . . . 30 2.4 Summary . . . . 40
In last few decades, tremendous growth in wireless networks and devices can be witnessed.
Different applications of wireless communications and networks are becoming ubiquitous and so- phisticated. The number of users of wireless systems are also continuing to increase at higher rate.
With the current spectrum allocation strategies as well as due to the fact that the communications at higher frequencies are still challenging, the spectrum is presently scarce. Therefore, current and future wireless systems are expected to operate in a challenging scenario of spectrum availability in order to fulfill the various kind of needs of users.
Keeping in view the limited spectrum availability, frequency reuse comes as a viable mean to manage the scarcity of spectrum. It allows distant users to use the same frequency. But, this scheme of frequency scarcity management does not come at free of cost due to the broadcast nature of wireless medium with omnidirectional antennas†. Reuse of frequency results in CCI that can not be avoided completely due to broadcast nature of wireless medium. In order to have a proper avoidance/management of CCI, which arises due to the frequency reuse, general and accurate models are necessary to get an insight of interference mechanism. Some models are available in the literature to quantify different interference scenarios. But, most of the available models and approaches are developed by assuming that the network is centrally synchronized. This assumption is not tenable with infrastructure-less ad-hoc kind of networks having limited power and processing capability constraints. Processes, like achieving and maintaining synchronization around the network requires a lot of overhead resulting into additional power consumption and reduces the life span of the device (network). Taking recent interest of research community in ad- hoc networks into account, it will be meaningful to analyze the networks under practical scenarios in which power consuming processes are not accounted. Out of many consequences of taking practical scenarios into account, one possibility is the asynchronized communication in a network.
As discussed in the chapter 1, models/approachs used for the synchronized networks may not be suitable for asynchronized scenario. Asynchronized scenario requires models which are general enough to take into account various situations. This chapter further elaborates some aspects of modeling and analytical approaches for the interference in an asynchronized environment.
In this chapter, first, we elaborate on different models for interference based on various classifica- tions. Some of the problems associated with the interference modeling are also discussed. Issues re- lated to asynchronized interference are also pointed out. Finally, concept of overlaid/heterogeneous
†Antennas are assumed to be omnidirectional for this work. Directional antennas are not being considered here.
2.1 Interference Modeling
networks along with interference management approaches are discussed from user’s different QoS related requirements point of view.
2.1 Interference Modeling
CCI is one of the primary bottle-neck in wireless networks which employ frequency reuse.
Broadcast nature of wireless channel hinders the complete removal of the detrimental effects of CCI but those effects can be compensated upto certain extent with proper insight of interference mechanism happening in the network.
An accurate modeling of interference provides such valuable insights to the system designer. A lot of interference models, with different intended applications, level of details and form of describing interference are proposed in literature to quantify the effects of interference in wireless networks.
The main objective of this thesis work is to model and analyze the asynchronous interference.
Before delving deeper into that topic, first a brief summary of available models is presented to get a feel of existing approaches for interference analysis.
Interference models
For infrastructured
networks
For
infrastructure-less networks Synchronous case
Asynchronous case
Synchronous case
Asynchronous case
Figure 2.1: Classification of various available models of interference.
As shown in Figure 2.1, various available models of interference for wireless network can be
2.1 Interference Modeling
(i) models for infrastructured networks (ii) models for infrastructure-less networks
The fundamental differences between the two kind of networks justifies the classification. As mentioned before, the major difference between infrastructured networks and infrastructure-less networks is the absence of any central entity in later class of networks. For infrastructure-less networks, the absence of central entity makes the synchronization/co-ordination difficult to achieve.
In the absence of proper co-ordination, the level of synchronization among user nodes of same network or different wireless networks can be negligible. Consequently, distributed implementation of several network protocols and algorithms in infrastructure-less networks remains useful since most of the time, nodes in such networks are restricted to operate using limited or no information on the status of whole network. Nodes in the infrastructured networks, on the other side, often have sufficient information about the network through the central node (e.g. base station in cellular networks). The presence of central entity in infrastructure based networks, makes it easy to achieve and maintain synchronization/co-ordination among user nodes of (different) networks. Moreover, due to absence of any central entity, nodes operate directly on a peer to peer basis in infrastructure- less networks, while the presence of central entity makes the direct communication possible among nodes and base station most of the time. These characteristic differences indicate the necessity of different treatment for interference analysis/modeling in these two kind of networks.
The models, developed for these networks, can be further divided into two classes for this thesis work:
(a) for synchronized scenario (b) for asynchronized scenario
Most of the modeling approaches in literature assume a centrally synchronized scenario for both infrastructure based as well as infrastructure-less networks. As discussed in chapter 1, for infras- tructure based networks, this assumption is reasonable due to the presence of a governing node but for infrastructure-less networks, this assumption is not tenable and if used, lead to impractical representation of the real systems. Achieving global synchronization, without any central node is a difficult task to perform. This process is also highly energy consuming due to large number of packet exchanges and therefore can not be termed as efficient for energy/power constrained
2.1 Interference Modeling
ad-hoc and sensor kind of networks. However, works are available that points out the problem of asynchronization in both kind of networks. But, they do not address various interference re- lated issues adequately up to the bit level [2]. Various issues related to asynchronous interference modeling along with inapplicability of models/approaches developed for synchronous scenario in asynchronous scenario are discussed in Sec. 2.2.1. Here, a brief introduction of some widely used interference models is given. There exist a fair degree of trade-off among interference models in terms of simplicity and accuracy. These models differ in the way they treat the interfering signal while evaluating the system performance. Some of the important models available in literature to quantify the interference are as follows:
u Protocol model : Let {Mi;i∈ k} be the subset of nodes concurrently transmitting over a particular sub channel. Given that the transmitter node is in reception range of receiver node, i.e. |Mi−Mj| ≤RC, protocol model gives the condition for a transmission, originated by node Mi towards another (receiver) node Mj, j ∈ k, to be successful in frequency reuse scenario as
|Mk−Mj| ≥(1 + ∆)|Mi−Mj| (2.1) where Mk, k∈kcan be any other node of the network transmitting on the same frequency and RC denotes the reception range. The parameter ∆>0 incorporates the effects of guard