Wireless communication systems

This thesis addresses issues faced in the practical implementation of a wireless ad hoc network (WAHN) protocol for data transmission. This study focuses on: (1) Evaluating existing hardware and software options available for the WAHN implementation. (2) Appraising the issues faced while implementing a practical wireless ad hoc protocol. (3) Applying a set of MAC protocol specifications developed for a wireless ad hoc data network to a practical test network. Specific to the above topics of interest, the following research tasks are performed: (1) An elaborate survey and relevant discussions on wireless MAC protocols. (2) A comprehensive study comparing various wireless transceivers is performed. Range, data rate, frequency, interfacing method and cost are the factors compared. (3) A simple, low-cost and low baud-rate transceiver is modified with appropriate interface circuits to support wireless communications. A more advanced transceiver is also considered and used for the software foundation of a practical implementation of the ad hoc and MAC protocols. The studies enable assessing the problems faced during the implementation and suggest solutions to resolve these problems. Further areas for study are also discussed.

This thesis is concerned with the performance analysis of mobile cellular systems under various distributions of portable users. The performance measure used is the average outage probability. Performance analysis is performed for macrocellular as well as microcellular systems, for different distributions of mobile users such as uniform, ring, and bell distribution. The outage probability is evaluated for systems with hexagonal, triangular, and square grid layouts. The effect of macroscopic diversity on system performance is also considered. Finally, computer simulations are used to verify the evaluated results.

The performance of a multicarrier DS CDMA system is evaluated over a Nakagami multipath fading channel. After the spreading process, the data sequence is applied to multiple carrier rather than a single carrier. Each carrier is provided a corralator in the receiver, and the maximal-ratio combiner is used for the corralator outputs. A bandlimited spreading waveforms are used for self-interference prevention. Robustness to multipath fading and narrow band interference suppression are achieved by this type of configuration. A comparison is made with a single carrier system in the presence of interference.

Capture effect has shown considerable improvement on performance of slotted ALOHA systems. Further, improvement is expected by increasing the number of base stations. The performance of such slotted ALOHA systems is analyzed with the aid of Equilibrium point analysis. Packet dropping due to finite number of retransmissions is taken into account. The numerical results indicate that the finite number of retransmission trials mainly contribute to the improvement of the packet dropping probability in the range of light input traffic. The use of multiple base stations improves the overall throughput and the average transmission delay in the range of heavy input traffic.

Indexed Resource Auction Multiple Access (I-RAMA), a new medium access protocol for wireless cellular networks based on Resource Auction Multiple Access (RAMA) is presented. I-RAMA relies in variable length resource auctions, whose length depends on the time it takes the Base Station to uniquely identify the Mobile Station. This identification is done by using dynamic Base Station information about the users present in the cell at any moment. I-RAMA effectively reduces the amount of time spent in the resource auctions without introducing contention or excessive complexity at the Base Station. The effects of introducing data users in the system are investigated using a simulation, and it is shown that I-RAMA guarantees Quality of Service for isochronous users while maintaining a bounded delay for data users at much higher loads than RAMA.

In a mobile ad hoc network, node cooperation in packet forwarding is required for the network to function properly. However, since nodes in this network usually have limited resources, some selfish nodes might intend not to forward packets to save resources for their own use. To discourage such behavior, we propose RMS, a reputation-based system, to detect selfish nodes and respond to them by showing that being cooperative will benefit there more than being selfish. We also detect, to some degree, nodes who forward only the necessary amount of packets to avoid being detected as selfish. We introduce the use of a state model to decide what we should do or respond to nodes in each state. In addition, we introduce the use of a timing period to control when the reputation should be updated and to use as a timeout for each state. The simulation results show that RMS can identify selfish nodes and punish them accordingly, which provide selfish nodes with an incentive to behave more cooperatively.

In recent years, advances in wireless technologies have enabled novel applications for wireless devices. Sensor network is one such application that consists of large number of battery-operated nodes. To simulate such networks with large number of nodes, a wireless sensor network simulator that is highly scalable is vital. JiST/SWANS is one such simulator that is highly scalable. However, the JiST/SWANS MAC layer implementation of 802.11b is not suitable for sensor networks, which are energy-constrained. Hence, our main focus is to implement the S-MAC protocol in JiST/SWANS. The S-MAC protocol allows the nodes to go to sleep and thereby it helps conserve energy. This subsequently helps the nodes to extend their effective lifetime. We validate our S-MAC protocol implementation in the JiST/SWANS through simulations.

IP address autoconfiguration poses a challenge for mobile ad-hoc networks (MANETs) because it has to be done to ensure correct routing. An IP autoconfiguration protocol that is based on quorum voting is proposed. Nodes are distributed configured when a write quorum can be collected. Making the compromise between message overhead and data consistency, quorum voting enforces data consistency by ensuring fresh read on every access so that each node is configured with a unique IP address. The protocol is scalable since the configuration information is maintained locally and no central server is involved. Extensive experiments are carried out comparing the configuration latency, message overhead and address reclamation cost between our protocol and existing stateful protocols. The simulation results show that nodes are configured in lower latency and the message overhead for maintaining the network is fairly low. Moreover, the proposed protocol greatly enhances the address availability by keeping proper redundancy.

We consider a heterogeneous wireless sensor network, which has several supernodes for data relay and a large number of energy-constrained sensor nodes that are deployed randomly to cover certain targets. Since targets are covered by many sensors, we create several cover sets that are active successively to save power. We introduce the Heterogeneous Connected Set Covers (HCSC) which aims to find at least one cover set that covers all the targets and is connected to a data-relaying supernode. A sensor node can participate in different set covers but the sum of energy spent in all sets is constrained by the initial energy resources of that sensor node. This is the first solution proposed for the target coverage in heterogeneous wireless sensor networks. We show that the HCSC is an NP-Complete problem and propose three distributed algorithms for it and showing simulation results to verify the proposed approaches.

Compared to the traditional wireless network, the multi-hop ad hoc wireless network (simply called ad hoc networks) is self-configurable, dynamic, and distributed. During the past few years, many routing protocols have been proposed for this particular network environment. While in wired and optical networks, multi-protocol label switching (MPLS) has clearly shown its advantages in routing and switching such as flexibility, high efficiency, scalability, and low cost, however MPLS is complex and does not consider the mobility issue for wireless networks, especially for ad hoc networks. This thesis migrates the label concept into the ad hoc network and provides a framework for the efficient Label Routing Protocol (LRP) in such a network. The MAC layer is also optimized with LRP for shorter delay, power saving, and higher efficiency. The simulation results show that the delay is improved significantly with this cross-layer routing protocol.