Recent advances in VLSI technology, and wireless communication have enabled the development of tiny, low-cost sensor nodes that communicate over short distances. These sensor nodes, which consist of sensing, data processing, and wireless communication capabilities, suggest the idea of sensor networks based on collaborative effort of a large number of sensor nodes. Sensor networks hold the promise for numerous applications such as intrusion detection, weather monitoring, security and tactical surveillance, distributed computing, and disaster management. Several new protocols and algorithms have been proposed in the recent past in order to realize these applications. In this thesis, we consider the problem of routing in Wireless Sensor Networks (WSNs). Routing is a challenging problem in WSNs due to the inherent characteristics which distinguish these networks from the others. Several routing algorithms have been proposed for WSNs, each considering a specific network performance objective such as long network lifetime (ChangandTassiulas,2004), end-to-end delay guarantees (T.Heetal,2003), and data fusion (RazvanCristescuetal,2005) etc. In this thesis, we utilize the Potential-based Routing Paradigm to develop routing algorithms for different performance objectives of interest in WSNs. The basic idea behind the proposed approach is to assign a scalar called the potential to every sensor node in the network. Data is then forwarded to the neighbor with highest potential. Potentials cause the data to flow along certain paths. By defining potential fields appropriately, one can cause data to flow along preferred paths, so that the given performance objective is achieved. We have demonstrated the usefulness of this approach by considering three performance objectives, and defining potentials appropriately in each case. The performance objectives that we have considered are (i) maximizing the time to network partition, (ii) maximizing the packet delivery ratio, and (iii) Data fusion. In an operational sensor network, sensor nodes’ energy levels gradually deplete, leading eventually to network partition. A natural objective is to route packets in such a way that the time to network partition is maximized. We have developed a potential function for this objective. We analyzed simple network cases and used the insight to develop a potential function applicable to any network. Simulation results showed that considerable improvements in time to network partition can be obtained compared to popular approaches such as maximum lifetime routing, and shortest hop count routing. In the next step, we designed a potential function that leads to routes with high packet delivery ratios. We proposed a “channel-state aware” potential definition for a simple 2-relay network and performed a Markov-chain based analysis to obtain the packet delivery ratio. Considerable improvement was observed compared to a channel-state-oblivious policy. This motivated us to define a channel-state-dependent potential function for a general network. Simulation results showed that for a relatively slowly changing wireless network, our approach can provide up to 20% better performance than the commonly-used shortest-hop-count routing. Finally, we considered the problem of correlated data gathering in sensor networks. The routing approach followed in literature is to construct a spanning tree rooted at the sink. Every node in the tree aggregates its data with the data from its children in order to reduce the number of transmitted bits. Due to this fact, the total energy cost of the data collection task is a function of the underlying tree structure. Noting that potential based routing schemes also result in a tree structure, we present a potential definition that results in the minimum energy cost tree under some special conditions. Specifically, we consider a scenario in which sensor nodes’ measurements are quantized to K values. The task at the sink is to construct a histogram of measurements of all sensor nodes. Sensor nodes do not directly send their measurements to sink. Instead, they construct a temporary histogram using the data from its children and forward it to its parent node in the tree. We present a potential definition that results in the minimum energy cost tree under some conditions on sensor nodes’ measurements. We include both the transmission energy cost as well as the energy cost associated with the aggregation process.

The objective of this thesis is to study of optical effects, arising in the form of non-uniform waveguide structure, complicated refractive index profiles or due to pulse propagation in dense wavelength division multiplexing (DWDM) optical communication systems. These effects are important and critically influence the performance of DWDM optical systems. A comprehensive survey of current literature on optical effects due to nonuniform optical structure and nonlinear optical effects is first done, showing their advantages and disadvantage in optical communication systems. A survey on methods of optical waveguide analysis is also done. The main contribution has been made to three main aspects of the problem : Accurate analysis of uniform/non-uniform optical waveguides with arbitary refractive index profiles Pulse propagation and distortion in DWDM Raman amplification systems. Use of non-uniform FBG to compensate for pulse distortion We study several existing analytical techniques developed so far for analyzing the mode of non-uniform optical waveguide structures. Later, we verify the analytical results by finite element method (FEM). The convergence study is also carried out. A new computational technique is proposed modifying the finite element method to analyze complex refractive index profiles required for the analysis, namely single mode step index profile, multi clad fiber, W -profile, chirp profile etc. An accuracy of 10−4 in the calculation of propagation constant/eigen-value is demonstrated. Dispersion characteristics of optical fibers w.r.t. different profile parameters is evaluated. A modification to scalar BPM is proposed and applied to study the effects of inhomogeneities along the propagation direction. The applicability and accuracy of the method is tested using integrated optic waveguide devices, namely, graded index slab waveguide. The proposed BPM uses Fourier decomposition of the transverse field. Coupled mode theory (CMT) of optical waveguides in non-homogeneous optical medium is applied to study the interaction of lightwaves propagation together such as in a DWDM system. The BPM results is verified by CMT. The inhomogeneous waveguide theory is extended to study pulse propagation in DWDM optical communication system. Nonlinear optical effects are an important aspects of DWDM systems with fiber Raman amplifier. Finite difference time domain (FDTD) method is necessary to study these nonlinear optical effects as other conventional methods are not suitable here. Here, we discuss DWDM optical communication systems due to nonlinearity in the form of SRS effect. In case of FRA, we study the various kinds of fiber profile design parameters, for the purpose to achieve and extend the flat gain bandwidth over the EDFA window. We also propose and study, a new bi-directional optical fiber transmission scheme with various constraints, using Raman amplification process with and without pump depletion. Our scheme, provides an advantage like high SNR, low pump induced noise, for long-haul communication link. We find that, there is a quite significant crosstalk and power coupling among the dense DWDM channels but earlier discussed BPM fails to account for possible interference effects among the channels. To reduce the harmful nonlinear optical effects like four wave mixing (FWM), we need to deploy a high chromatic dispersion fiber, which will ultimately lead to high pulse walk-off rate among the DWDM channels; hence for high bit rate long haul systems, walk-off effect can not be ignored. Application of FDTD provided an improved insight into the effect of GVD on stimulated Raman scattering crosstalk than different modulation techniques and line codes. It is shown through analysis that pulse walk-off phenomena may distort the data asymmetrically; especially for case of wide-band DWDM transmission system. Hence, the pulse walk-off effect should be considered in future systems containing optical amplifier. It is shown, that large walk-off rate may reduce the crosstalk among DWDM channels but tends to increase the asymmetric pulse distortion. Data may lose due to high walk-off effect. We also investigate channel addition/removal process in DWDM fiber Raman amplifier. We also demonstrate that the pulse walk-off effect tends to lead significantly to positive chirp for higher frequency channels. This feature can be exploited to overcome the chromatic dispersion effects in DWDM transmission systems. Pulse walk-off induced chirp, can be compensated by using the nonuniform fiber Bragg grating (NUFBG). The CMT due to periodic perturbation of the circular cylindrical waveguide structures is applied here. Here, we discuss the function of fiber Bragg grating as a transmission versus reflecting grating filter. We also discuss, FBG application to gain flattening of an EDFA window as well as how the group velocity dispersion (GVD) will be affected with bandwidth and coupling coefficient. We develop a new analytical technique to estimate the bandwidth of FBG based optical system. Finally, we investigate the dispersion compensation properties, pulse distortion, peak reflectivity analysis in uniform/non-uniform FBG due to an uniform/non-uniform incoming signal. More complicated refractive index profile can significantly reduce the GVD as well as side lobes intensity. Dispersion characteristic due to an arbitrary refractive index profile is discussed in details for the case of non-uniform FBG. Thus, we concluded that wide band DWDM optical communication system need to closely take into account various inhomogeneities and nonlinearities of optical fibers w.r.t. wave and pulse propagation.

In this thesis we address the problem of providing end-to-end quality of service (QoS) to real-time and data connections in a third generation (3G) cellular network based on the Universal Mobile Telecommunication System (UMTS) standard. Data applications usually use TCP (Transmission Control Protocol) and the QoS is a minimum guaranteed mean throughput. For this one first needs to compute the throughput of a TCP connection sending its traffic through the UMTS network (possibly also through the wired part of the Internet). Thus we obtain closed form expressions for a TCP throughput in a UMTS environment. For downloading data at a mobile terminal, the packets of each TCP connection are stored in separate queues at the base station (node B). These are fragmented into Protocol Data Units (PDU). The link layer uses ARQ (Automatic Repeat Request). Thus there can be significant random transmission/queueing delays of TCP packets at the node B. On the other hand the link may not be fully utilized due to the delays of the TCP packets in the rest of the network. In such a scenario the existing models of TCP may not be sufficient. Thus we provide new approximate models for TCP and also obtain new closed form expressions of mean window size. Using these we obtain the throughput of a TCP connection for the scenario where the queueing delays are non-negligible compared to the overall Round Trip Time (RTT) and also the link utilization is less than one. Our approximate models can be useful not only in the UMTS context but also else where. In the second half of the thesis, we use these approximate models of TCP to provide minimum mean throughput to data connections in UMTS. We also consider real-time applications such as voice and video. These can tolerate a little packet loss (~1%) but require an upper Bound on the delay and delay jitter (≤ 150 ms). Thus if the network provides a constant bandwidth and the received SINR is above a specified threshold ( with a certain probability), QoS for the real-time traffic will be satisfied. The 3G cellular systems are interference limited. Thus wise allocation of power is critical in these systems. Hence we consider the problem of providing end-to-end QoS to different users along with the minimization of the downlink power allocation.

We study the problem of joint congestion control, routing and MAC layer scheduling in multi-hop wireless mesh networks, where the nodes in the network are subjected to energy expenditure rate constraints. As wireless scenario does not allow all the links to be active all the time, only a subset of given links can be active simultaneously. We model the inter-link interference using the link contention graph. All the nodes in the network are power-constrained and we model this constraint using energy expenditure rate matrix. Then we formulate the problem as a network utility maximization (NUM) problem. We notice that this is a convex optimization problem with affine constraints. We apply duality theory and decompose the problem into two sub-problems namely, network layer congestion control and routing problem, and MAC layer scheduling problem. The source adjusts its rate based on the cost of the least cost path to the destination where the cost of the path includes not only the prices of the links in it but also the prices associated with the nodes on the path. The MAC layer scheduling of the links is carried out based on the prices of the links. The optimal scheduler selects that set of non-interfering links, for which the sum of link prices is maximum. We study the effects of energy expenditure rate constraints of the nodes on the maximum possible network utility. It turns out that the dominant of the two constraints namely, the link capacity constraint and the node energy expenditure rate constraint affects the network utility most. Also we notice the fact that the energy expenditure rate constraints do not affect the nature of optimal link scheduling problem. Following this fact, we study the problem of distributed link scheduling. Optimal scheduling requires selecting independent set of maximum aggregate price, but this problem is known to be NP-hard. We first show that as long as scheduling policy selects the set of non-interfering links, it can not go unboundedly away from the optimal solution of network utility maximization problem. Then we proceed and evaluate a simple greedy scheduling algorithm. Analytical bounds on performance are provided and simulations indicate that the greedy heuristic performs well in practice.

A Mobile Ad Hoc Network (MANETs) is a collection of wireless hosts that can be rapidly deployed as a multi hop packet radio network without the aid of any established infrastructure or centralized administration. Such networks can be used to enable next generation of battlefield applications envisioned by the military, including situation awareness systems for maneuvering war fighters, and remotely deployed unmanned microsensor networks. Ad Hoc networks can also provide solutions for civilian applications such as disaster recovery and message exchanges among safety and security personnel involved in rescue missions. Existing solutions for wired network Intrusion Detection Systems (IDSs) do not suit wireless Ad Hoc networks. To utilize either misuse detection or anomaly detection to monitor any possible compromises, the IDS must be able to distinguish normal from anomaly activities. To enable intrusion detection in wireless Ad Hoc networks, the research problems are: • How to efficiently collect normal and anomaly patterns of Ad Hoc networks? The lifetime of the hosts is short and Ad Hoc networks do not have traffic concentration points (router, switch). • How to detect anomalies? The loss could be caused by host movement instead of attacks. Unexpectedly long delay could be caused by unreliable channel instead of malicious discard. In this thesis, we have proposed a novel architecture that uses specification based intrusion detection techniques to detect active attacks against the routing protocols of mobile Ad Hoc networks. Our work analyzes some of the vulnerabilities and discuss the attacks against the AODV protocol. Our approach involves the use of an FSM (Finite State Machine) for specifying the AODV routing behavior and the distributed network monitors for detecting the sequence number attack. Our method can detect most of the bad nodes with low false positive rate and the packet delivery ratio can also be increased with high detection rate. For packet dropping attack, we present a distributed technique to detect this attack in wireless Ad Hoc networks. A bad node can forward packets but in fact it fails to do so. In our technique, every node in the network will check the neighboring nodes to detect if any of them fail to forward the packets. Our technique can detect most of the bad nodes with low false positive rate and the packet delivery ratio can also be increased. The proposed solution can be applied to identify multiple malicious nodes cooperating with each other in MANETs and discover secure routes from source to destination by avoiding malicious nodes acting in cooperation. Our technique will detect the sequence number and Packet Dropping attacks in real time within its radio range with no extra overhead. For resource consumption attack, the proposed scheme incurs no extra overhead, as it makes minimal modifications to the existing data structures and functions related to bad listing a node in the existing version of pure AODV. The proposed scheme is more efficient in terms of the resultant routes established, resource reservations, and computational complexity. If multiple malicious nodes collaborate, they in turn will be restricted and isolated by their neighbors, because they monitor and exercise control over forwarding RREQs by nodes. Hence, the scheme successfully prevents Distributed attacks. The proposed scheme shifts the responsibility of monitoring this parameter to the node's neighbor, ensuring compliance of this restriction. This technique solves all of the problems caused due to unnecessary RREQs from a compromised node. Instead of self-control, the control exercised by a node's neighbor results in preventing this attack. Experiments show that the tool provides effective intrusion detection functionality while using only a limited amount of resources. The loop freedom property has been reduced to an invariant on pairs of nodes. Each node decides & transmits its decision to a control center. Robustness to Threats, Robustness to nodes destruction: Loss of Performance (in terms of ratio) is least for Distributed Option and highest for Centralized Option and Robustness to observations deletion. All the proposed schemes were analyzed and tested under different topologies and conditions with varying number of nodes .The proposed algorithms for improving the robustness of the wireless Ad Hoc networks using AODV protocol against Packet Dropping Attack, Sequence Number attack and resource consumption attack have been simulated for an illustrative network of about 30 nodes. Our experiments have shown that the pattern extracted through simulation can be used to detect attacks effectively. The patterns could also be applied to detect similar attacks on other protocols.

This thesis consists of two parts addressing two different problems in fading channels. The first part deals with asynchronous cooperative relay communication. The assumption of nodes in a cooperative communication relay network operating in synchronous fashion is often unrealistic. In this work we consider two different models of asynchronous operation in cooperative-diversity networks experiencing slow fading and examine the corresponding Diversity-Multiplexing Tradeoffs (DMT). For both models, we propose protocols and distributed space-time codes that asymptotically achieve the transmit diversity bound on DMT for all multiplexing gains and for number of relays N ≥ 2. The distributed space-time codes for all the protocols considered are based on Cyclic Division Algebras (CDA). The second part of the work addresses the DMT analysis of diversity embedded codes for MIMO channels. Diversity embedded codes are high rate codes that are designed so that they have a high diversity code embedded within them. This allows a form of opportunistic communication depending on the channel conditions. The high diversity code ensures that at least a part of the information is received reliably, whereas the embedded high rate code allows additional information to be transferred if the channel is good. This can be thought of coding the data into two streams: high priority and low priority streams so that the high priority stream gets a better reliability than the lower priority stream. We show that superposition based diversity embedded codes in conjunction with naive single stream decoding is sub-optimal in terms of the DM tradeoff. We then construct explicit diversity embedded codes by the superposition of approximately universal space-time codes from CDAs. The relationship between broadcast channels and the diversity embedded setting is then utilized to provide some achievable Diversity Gain Region (DGR) for MIMO broadcast Channels.

Network management plays a vital role to keep a network and its application work e ciently. The network management in MANETs is a crucial and the challenging task, as these networks are characterized by dynamic environment and the scarcity of resources. There are various existing approaches for network management in MANETs. The Ad hoc Network Management Protocol (ANMP) has been one of the rst e orts and introduced an SNMP-based solution for MANETs. An alternative SNMP-based solu-tion is proposed by GUERRILLA Management Architecture (GMA). Due to self-organizing characteristic feature of MANETs, the management task has to be distributed. Policy-based network management relatively o ers this feature, by executing and applying policies pre-viously de ned by network manager. Otherwise, the complexity of realization and control becomes di cult Most of the works address the current status of the MANET to take the network man-agement decisions. Currently, MANETs addresses the dynamic and intelligent decisions by considering the present situation and all related history information of nodes into consid-eration. In this connection we have proposed a network management system using agents (NMSA) for MANETs, resolving major issues like, node monitoring, location management, resource management and QoS management. Solutions to these issues are discussed as inde-pendent protocols, and are nally combined into a single network management system, i.e., NMSA. Agents are autonomous, problem-solving computational entities capable of performing e ective operation in dynamic environments. Agents have cooperation, intelligence, and mobility characteristics as advantages. The agent platforms provide the di erent services to agents, like execution, mobility, communication, security, tracking, persistence and directory etc. The platform execution environment allows the agents to run, and mobility service allows them to travel among the di erent execution environments. The entire management task will be delegated to agents, which then executes the management logic in a distributed and autonomous fashion. In our work we used the static and mobile agents to nd some solutions to the management issues in a MANET. We have proposed a node monitoring protocol for MANETs, which uses both static agent (SA) and mobile agents (MA), to monitor the nodes status in the network. It monitors the gradational energy loss, bu er, bandwidth, and the mobility of nodes running with low to high load of mobile applications. Protocol assumes the MANET is divided into zones and sectors. The functioning of the protocol is divided into two segments, The NMP main segment, which runs at the chosen resource rich node (RRN) at the center of a MANET, makes use of SA which resides at same RRN, and the NMP subsegment which runs in the migrated MAs at the other nodes. Initially SA creates MAs and dispatches one MA to each zone, in order to monitor health conditions and mobility of nodes of the network. MAs carrying NMP subsegment migrates into the sector of a respective zone, and monitors the resources such as bandwidth, bu er, energy level and mobility of nodes. After collecting the nodes information and before moving to next sector they transfer collected information to SA respectively. SA in turn coordinates with other modules to analyze the nodes status information. We have validated the protocol by performing the conformance testing of the proposed node monitoring protocol (NMP) for MANETs. We used SDL to obtain MSCs, that repre-sents the scenario descriptions by sequence diagrams, which in turn generate test cases and test sequences. Then TTCN-3 is used to execute the test cases with respect to generated test sequences to know the conformance of protocol against the given speci cation. We have proposed a location management protocol for locating the nodes of a MANET, to maintain uninterrupted high-quality service for distributed applications by intelligently anticipating the change of location of its nodes by chosen neighborhood nodes. The LMP main segment of the protocol, which runs at the chosen RRN located at the center of a MANET, uses SA to coordinate with other modules and MA to predict the nodes with abrupt movement, and does the replacement with the chosen nodes nearby which have less mobility. We have proposed a resource management protocol for MANETs, The protocol makes use of SA and MA for fair allocation of resources among the nodes of a MANET. The RMP main segment of the protocol, which runs at the chosen RRN located at the center of a MANET, uses SA to coordinate with other modules and MA to allocate the resources among the nodes running di erent applications based on priority. The protocol does the distribution and parallelism of message propagation (mobile agent with information) in an e cient way in order to minimize the number of message passing with reduction in usage of network resources and improving the scalability of the network. We have proposed a QoS management protocol for MANETs, The QMP main segment of the protocol, which runs at the chosen RRN located at the center of a MANET, uses SA to coordinate with other modules and MA to allocate the resources among the nodes running di erent applications based on priority over QoS. Later, to reallocate the resources among the priority applications based on negotiation and renegotiation for varying QoS requirements. The performance testing of the protocol is carried out using TTCN-3. The generated test cases for the de ned QoS requirements are executed with TTCN-3, for testing of the associated QoS parameters, which leads to performance testing of proposed QoS management protocol for MANETs. We have combined the developed independent protocols for node monitoring, location management, resource management, and QoS management, into one single network management system called Network Management System using Agents (NMSA) for MANETs and tested in di erent environments. We have implemented NMSA on Java Agent development environment (JADE) Platform. Our developed network management system is a distributed system. It is basically divided into two parts, the Network Management Main Segment and other is Network Management Subsegment. A resource rich node (RRN) which is chosen at the center of a MANET where the Main segment of NMSA is located, and it controls the management activities. The other mobile nodes in the network will run MA which has the subsegments of NMSA. The network management system, i.e., the developed NMSA, has Network manage-ment main (NMSA main), Zones and sector segregation scheme, NMP, LMP, RMP, QMP main segments at the RRN along with SA deployed. The migrated MA at mobile node has subsegments of NMP, LMP, RMP, and QMP respectively. NMSA uses two databases, namely, Zones and sectors database and Node history database. Implementation of the proposed work is carried out in a con ned environment with, JDK and JADE installed on network nodes. The launched platform will have AMS and DF automatically generated along with MTP for exchange of message over the channel. Since only one JVM, which is installed, will executes on many hosts in order to provide the containers for agents on those hosts. It is the environment which o ered, for execution of agents. Many agents can be executed in parallel. The main container, is the one which has AMS and DF, and RMI registry are part of JADE environment which o ers complete run time environment for execution of agents. The distribution of the platform on many containers of nodes is shown in Fig. 1. The NMSA is based on Linux platform which provides distributed environment, and the container of JADE could run on various platforms. JAVA is the language used for code development. A middle layer, i.e., JDBC (java database connection) with SQL provides connectivity to the database and the application. The results of experiments suggest that the proposed protocols are e ective and will bring, dynamism and adaptiveness to the applied system and also reduction in terms network overhead (less bandwidth consumption) and response time.

We consider optimal power allocation policies for a single server, multiuser wireless communication system. The transmission channel may experience multipath fading. We obtain very efficient, low computational complexity algorithms which minimize power and ensure stability of the data queues. We also obtain policies when the users may have mean delay constraints. If the power required is a linear function of rate then we exploit linearity and obtain linear programs with low complexity. We also provide closed-form optimal power policies when there is a hard deadline delay constraint. Later on, we also extend single hop results to multihop networks. First we consider the case, when the transmission rate is a linear function of power. We provide low complexity algorithms for joint routing, scheduling and power control which ensure stability of the queues, certain minimum rates, end-to-end hard deadlines, and/or upper bounds on the end-to-end mean delays. Further we extend these results to the multihop networks where the power is a general monotonically increasing function of rate. For our algorithms, we also provide rates of convergence to the stationary distributions for the queue length process and also approximate end-to-end mean delays. Finally, we provide computationally efficient algorithms that minimize the total power when there is a end-to-end hard deadline delay constraint.

Error-control codes, which are being extensively used in communication systems, have found themselves very useful in data storage as well during the past decade. This thesis deals with two types of codes for data storage, one pertaining to the issue of privacy and the other to reliability. In many scenarios, user accessing some critical data from a server would not want the server to learn the identity of data retrieved. This problem, called Private Information Retrieval (PIR) was rst formally introduced by Chor et al and they gave protocols for PIR in the case where multiple copies of the same data is stored in non-communicating servers. The PIR protocols that came up later also followed this replication model. The problem with data replication is the high storage overhead involved, which will lead to large storage costs. Later, Fazeli, Vardy and Yaakobi, came up with the notion of PIR code that enables information-theoretic PIR with low storage overhead. In the rst part of this thesis, construction of PIR codes for certain parameter values is presented. These constructions are based on a variant of conventional Reed-Muller (RM) codes called binary Projective Reed-Muller (PRM) codes. A lower bound on block length of systematic PIR codes is derived and the PRM based PIR codes are shown to be optimal with respect to this bound in some special cases. The codes constructed here have smaller block lengths than the short block length PIR codes known in the literature. The generalized Hamming weights of binary PRM codes are also studied. Another work described here is the implementation and evaluation of an erasure code called Coupled Layer (CL) code in Ceph distributed storage system. Erasure codes are used in distributed storage to ensure reliability. An additional desirable feature required for codes used in this setting is the ability to handle node repair efficiently. The Minimum Storage Regenerating (MSR) version of CL code downloads optimal amount of data from other nodes during repair of a failed node and even disk reads during this process is optimum, for that storage overhead. The CL-Near-MSR code, which is a variant of CL-MSR, can efficiently handle a restricted set of multiple node failures also. Four example CL codes were evaluated using a 26 node Amazon cluster and performance metrics like network bandwidth, disk read and repair time were measured. Repair time reduction of the order of 3 was observed for one of those codes, in comparison with Reed Solomon code having same parameters. To the best of our knowledge, such large gains in repair performance have never been demonstrated before.

Opportunistic use of unused spectrum could efficiently be carried out using the paradigm of Cognitive Radio (CR). A spectrum remains idle when the primary user (licensee) is not using it. The secondary nodes detect this spectral hole quickly and make use of it for data transmission during this interval and stop transmitting once the primary starts transmitting. Detection of spectral holes by the secondary is called spectrum sensing in the CR scenario. Spectrum Sensing is formulated as a hypothesis testing problem wherein under H0 the spectrum is free and under H1, occupied. The samples will have different probability distributions, P0 and P1, under H0 and H1 respectively. In the first part of the thesis, a new algorithm - entropy test is presented, which performs better than the available algorithms when P0 is known but not P1. This is extended to a distributed setting as well, in which different secondary nodes collect samples independently and send their decisions to a Fusion Centre (FC) over a noisy MAC which then makes the final decision. The asymptotic optimality of the algorithm is also shown. In the second part, the spectrum sensing problem under impediments such as fading, electromagnetic interference and outliers is tackled. Here the detector does not possess full knowledge of either P0 or P1. This is a more general and practically relevant setting. It is found that a recently developed algorithm (which we call random walk test) under suitable modifications works well. The performance of the algorithm theoretically and via simulations is shown. The same algorithm is extended to the distributed setting as above.