Abstract
Wireless ad hoc networks provide a new dimension to computing by allowing a group of portable devices to talk to each other without any additional infrastructure and have been the focus of research in the area of wireless networks. CDMA has been gaining popularity as the multiaccess technique and is part of many wireless standards now. In this project we present techniques to improve performance of the CDMA based wireless ad hoc systems and present results based on extensive simulations. The resource utilization of network in a CDMA based system depends largely on the code allocation scheme. Using a graph theoretic formulation of the problem, we explore solutions for code allocation problem under different requirements. Index Terms—Ad hoc networks, MANET, CDMA, OVSF codes, Graph Coloring, Range Sum
I. INTRODUCTION
MOBILE ad hoc network (MANET) is an autonomous system of mobile nodes connected to each other through a fully mobile infrastructure. These networks are attractive due to the ease and speed of deployment. Although they haven’t been really deployed but their use has been envisioned in wide variety of areas like military applications, conferences and rescue missions. Since these networks are in an incipient stage, there are no established standards for them. The IEEE802.11 series which has been gaining acceptance as the standard for wireless communication networks, also has provision of using Code Division Multiple Access(CDMA) for MANETs. The scheme proposed is a very simple one and does not try to maximize the use of available network resources. Also the support for MANETs is provided for the sake of completeness and is not truely based on an “infrastructureless” network. In this project, we studied some of the issues in such CDMA based networks. We focused mainly on the OVSF code allocation schemes and looked at the performance of the algorithms suggested in the literature through simulations done partly in NS-2 and partly on our own simulator. A. Organization of the report We start this report by a brief introduction of CDMA in section II. This is accompanied by an overview of OVSF codes and the MC-CDMA system. In section III we discuss some issues concerning resource allocation in wireless ad-hoc network and motivate the need for optimal code allocation strategies, and hence this work. Section IV gives a formulation of the problem of code allocation to maximize aggregate throughput in terms of the concept of Range Coloring of a graph and some previous relevant results including the NPCompleteness of the problem. This is followed by a discussion on centralized code allocation schemes in section V. We start by presenting an approximation scheme proposed in an earlier work and the approximation bound for that algorithm. A more careful analysis of the algorithm gives a better approximation bound for the algorithm.We then present heuristics to improve the performance of the algorithm and a comparison of their performance based on simulations. Finally we conclude by presenting the conclusions in section VI.

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