One of the two keywords for the next generation wireless communications is seamless. Being involved in the essential e-Japan Plan promoted by the Japanese Government, the MIRAI (Multimedia Integrated network by Radio Access Innovation) project is responsible for the research and development on the seamless integration of various wireless access systems for practical use by the year 2005. A heterogeneous network architecture including a common tool, a common platform, and a common access is proposed in this paper. Concretely, software-defined-radio technologies are used to develop a multi-service user terminal to be used for access to different wireless networks. The common platform for various wireless networks is based on a wireless supporting IPv6 network. A basic access network, separated from other wireless access networks, is used as a means for wireless system discovery, signaling and paging. A proof-of-concept experimental demonstration system is available from March 2002
Recently, many new types of wireless networks have emerged for both civil and military applications, such as cognitive radio networks, MIMO networks. There is a strong interest in exploring the optimal performance of these new emerging networks, e.g., maximizing the network throughput, minimizing network energy consumption. Exploring the optimal performance objectives of these new types of wireless networks is both important and intellectual challenging. On one hand, it is important for a network researcher to understand the performance limits of these new wireless networks. Such performance limits are important not only for theoretical understanding, but also in that they can be used as benchmarks for the design of distributed algorithms and protocols. On the other hand, due to some unique characteristics associated with these networks, existing analytic techniques may not be applied directly to obtain the optimal performance. As a result, new theoretical results, along with new mathematical tools, need to be developed. The goal of this dissertation is to make a fundamental advance on network performance optimization via exploring a series of optimization problems. Based on the scale of the underlying wireless network, the works in this dissertation are divided into two parts. In the first part, we study the asymptotic capacity scaling laws of different types of wireless networks. By "asymptotic", we mean that the number of nodes in the network goes to infinity. Such asymptotic capacity scaling laws offer fundamental understandings on the trend of maximum user throughput behavior when the network size increases. In the second part of this dissertation, we study several optimization problems of finite-sized wireless networks. Under a given network size, we accurately characterize some performance limits (e.g., throughput, energy consumption) of wireless networks and provide solutions on how to achieve the optimal objectives. The main contributions of this dissertation can be summarized as follows, where the first three problems are on asymptotic capacity scaling laws and the last three problems are optimization problems of finite-sized wireless networks. 1. Capacity Scaling Laws of Cognitive Radio Ad Hoc Networks. We first study the capacity scaling laws for cognitive radio ad hoc networks (CRNs), i.e., how each individual node's maximum throughput scales as the number of nodes in the network increases. This effort is critical to the fundamental understanding of the scalability of such network. However, due to the heterogeneity in available frequency bands at each node, the asymptotic capacity is much more difficult to develop than prior efforts for other types of wireless networks. To overcome this difficulty, we introduce two auxiliary networks and to analyze the capacity upper and lower bounds. We derive the capacity results under both the protocol model and the physical model. Further, we show that the seminal results developed by Gupta and Kumar for the simple single-channel single-radio (SC-SR) networks are special cases under the results for CRNs. 2. Asymptotic Capacity of Multi-hop MIMO Ad Hoc Networks. Multi-input multi-output (MIMO) is a key technology to increase the capacity of wireless networks. Although there has been extensive work on MIMO at the physical and link layers, there has been limited work on MIMO at the network layer (i.e., multi-hop MIMO ad hoc network), particularly results on capacity scaling laws. In this work, we investigate capacity scaling laws for MIMO ad hoc networks. Our goal is to find the achievable throughput of each node as the number of nodes in the network increases. We employ a MIMO network model that captures spatial multiplexing (SM) and interference cancellation (IC). We show that for a MIMO network with n randomly located nodes, each equipped with antennas and a rate of W on each data stream, the achievable throughput of each node is (W/ n ln n ). 3. Toward Simple Criteria for Establishing Capacity Scaling Laws. Capacity scaling laws offer fundamental understanding on the trend of user throughput behavior when the network size increases. Since the seminal work of Gupta and Kumar, there have been tremendous efforts developing capacity scaling laws for ad hoc networks with various advanced physical layer technologies. These efforts led to different custom-designed approaches, most of which were intellectually challenging and lacked universal properties that can be extended to address scaling laws of ad hoc networks with a different physical layer technology. In this work, we present a set of simple yet powerful general criteria that one can apply to quickly determine the capacity scaling laws for various physical layer technologies under the protocol model. We prove the correctness of our proposed criteria and validate them through a number of case studies, such as ad hoc networks with directional antenna, MIMO, cognitive radio, multi-channel and multi-radio, and multiple packet reception. These simple criteria will serve as powerful tools to networking researchers to obtain throughput scaling laws of ad hoc networks under different physical layer technologies, particularly those to appear in the future. 4. Exploiting SIC forMulti-hopWireless Networks. There is a growing interest on exploiting interference (rather than avoiding it) to increase network throughput. In particular, the so-called successive interference cancellation (SIC) scheme appears very promising, due to its ability to enable concurrent receptions from multiple transmitters and interference rejection. However, due to some stringent constraints and limit, SIC alone is inadequate to handle all concurrent interference. We advocate a joint interference exploitation and avoidance approach, which combines the best of interference exploitation and interference avoidance, while avoiding each's pitfalls. We discuss the new challenges of such a new approach in a multi-hop wireless network and propose a formal optimization framework, with cross-layer formulation of physical, link, and network layers. This framework offers a rather complete design space for SIC to squeeze the most out of interference. The goal of this effort is to lay a mathematical foundation for modeling and analysis of a joint interference exploitation and avoidance scheme in a multi-hop wireless network. Through modeling and analysis, we develop a tractable model that is suitable for studying a broad class of network throughput optimization problems. To demonstrate the practical utility of our model, we conduct a case study. Our numerical results affirm the validity of our model and give insights on how SIC can optimally interact with an interference avoidance scheme. 5. Throughput Optimization with Network-wide Energy Constraint. Conserving network wide energy consumption is becoming an increasingly important concern for network operators. In this work, we study network-wide energy conservation problem which we hope will offer insights to both network operators and users. Specifically, we study how to maximize network throughput under a network-wide energy constraint for a general multi-hop wireless network. We formulate this problem as a mixed-integer nonlinear program (MINLP). We propose a novel piece-wise linear approximation to transform the nonlinear constraints into linear constraints. We prove that the solution developed under this approach is near optimal with guaranteed performance bound. 6. Bicriteria Optimization in Multi-hop Wireless Networks. Network throughput and energy consumption are two important performance metrics for a multi-hop wireless network. Current state-of-the-art is limited to either maximizing throughput under some energy constraint or minimizing energy consumption while satisfying some throughput requirement. However, the important problem of how to optimize both objectives simultaneously remains open. In this work, we take a multicriteria optimization approach to offer a systematic study on the relationship between the two performance objectives. We show that the solution to the multicriteria optimization problem characterizes the envelope of the entire throughput energy region, i.e., the so-called optimal throughput-energy curve. We prove some important properties of the optimal throughput-energy curve. For case study, we consider both linear and nonlinear throughput functions. For the linear case, we characterize the optimal throughput-energy curve precisely through parametric analysis, while for the nonlinear case, we use a piece-wise linear approximation to approximate the optimal throughput-energy curve with arbitrary accuracy. Our results offer important insights on exploiting the trade-off between the two performance metrics. ; Ph. D.
Recently, many new types of wireless networks have emerged for both civil and military applications, such as wireless sensor networks, ad hoc networks, among others. To improve the performance of these wireless networks, many advanced communication techniques have been developed at the physical layer. For both theoretical and practical purposes, it is important for a network researcher to understand the performance limits of these new wireless networks. Such performance limits are important not only for theoretical understanding, but also in that they can be used as benchmarks for the design of distributed algorithms and protocols. However, due to some unique characteristics associated with these networks, existing analytical technologies may not be applied directly. As a result, new theoretical results, along with new mathematical techniques, need to be developed. In this dissertation, we focus on the design of new algorithms and optimization techniques to study theoretical performance limits associated with these new wireless networks. In this dissertation, we mainly focus on sensor networks and ad hoc networks. Wireless sensor networks consist of battery-powered nodes that are endowed with a multitude of sensing modalities. A wireless sensor network can provide in-situ, unattended, high-precision, and real-time observation over a vast area. Wireless ad hoc networks are characterized by the absence of infrastructure support. Nodes in an ad hoc network are able to organize themselves into a multi-hop network. An ad hoc network can operate in a stand-alone fashion or could possibly be connected to a larger network such as the Internet (also known as mesh networks). For these new wireless networks, a number of advanced physical layer techniques, e.g., ultra wideband (UWB), multiple-input and multiple-output (MIMO), and cognitive radio (CR), have been employed. These new physical layer technologies have the potential to improve network performance. However, they also introduce some unique design challenges. For example, CR is capable of reconfiguring RF (on the fly) and switching to newly-selected frequency bands. It is much more advanced than the current multi-channel multi-radio (MC-MR) technology. MC-MR remains hardware-based radio technology: each radio can only operate on a single channel at a time and the number of concurrent channels that can be used at a wireless node is limited by the number of radio interfaces. While a CR can use multiple bands at the same time. In addition, an MC-MR based wireless network typically assumes there is a set of "common channels" available for all nodes in the network. While for CR networks, each node may have a different set of frequency bands based on its particular location. These important differences between MC-MR and CR warrant that the algorithmic design for a CR network is substantially more complex than that under MC-MR. Due to the unique characteristics of these new wireless networks, it is necessary to consider models and constraints at multiple layers (e.g., physical, link, and network) when we explore network performance limits. The formulations of these cross-layer problems are usually in very complex forms and are mathematically challenging. We aim to develop some novel algorithmic design and optimization techniques that provide optimal or near-optimal solutions. The main contributions of this dissertation are summarized as follows. 1. Node lifetime and rate allocation We study the sensor node lifetime problem by considering not only maximizing the time until the first node fails, but also maximizing the lifetimes for all the nodes in the network. For fairness, we maximize node lifetimes under the lexicographic max-min (LMM) criteria. Our contributions are two-fold. First, we develop a polynomial-time algorithm based on a parametric analysis (PA) technique, which has a much lower computational complexity than an existing state-of-the-art approach. We also present a polynomial-time algorithm to calculate the flow routing schedule such that the LMM-optimal node lifetime vector can be achieved. Second, we show that the same approach can be employed to address a different but related problem, called LMM rate allocation problem. More important, we discover an elegant duality relationship between the LMM node lifetime problem and the LMM rate allocation problem. We show that it is sufficient to solve only one of the two problems and that important insights can be obtained by inferring the duality results. 2. Base station placement Base station location has a significant impact on sensor network lifetime. We aim to determine the best location for the base station so as to maximize the network lifetime. For a multi-hop sensor network, this problem is particularly challenging as data routing strategies also affect the network lifetime performance. We present an approximation algorithm that can guarantee (1- )-optimal network lifetime performance with any desired error bound > 0. The key step is to divide the continuous search space into a finite number of subareas and represent each subarea with a "fictitious cost point" (FCP). We prove that the largest network lifetime achieved by one of these FCPs is (1- )-optimal. This approximation algorithm offers a significant reduction in complexity when compared to a state-of-the-art algorithm, and represents the best known result to this problem. 3. Mobile base station The benefits of using a mobile base station to prolong sensor network lifetime have been well recognized. However, due to the complexity of the problem (time-dependent network topology and traffic routing), theoretical performance limits and provably optimal algorithms remain difficult to develop. Our main result hinges upon a novel transformation of the joint base station movement and flow routing problem from the time domain to the space domain. Based on this transformation, we first show that if the base station is allowed to be present only on a set of pre-defined points, then we can find the optimal sojourn time for the base station on each of these points so that the overall network lifetime is maximized. Based on this finding, we show that when the location of the base station is un-constrained (i.e., can move to any point in the two-dimensional plane), we can develop an approximation algorithm for the joint mobile base station and flow routing problem such that the network lifetime is guaranteed to be at least (1- ) of the maximum network lifetime, where can be made arbitrarily small. This is the first theoretical result with performance guarantee on this problem. 4. Spectrum sharing in CR networks Cognitive radio is a revolution in radio technology that promises unprecedented flexibility in radio communications and is viewed as an enabling technology for dynamic spectrum access. We consider a cross-layer design of scheduling and routing with the objective of minimizing the required network-wide radio spectrum usage to support a set of user sessions. Here, scheduling considers how to use a pool of unequal size frequency bands for concurrent transmissions and routing considers how to transmit data for each user session. We develop a near-optimal algorithm based on a sequential fixing (SF) technique, where the determination of scheduling variables is performed iteratively through a sequence of linear programs (LPs). Upon completing the fixing of these scheduling variables, the value of the other variables in the optimization problem can be obtained by solving an LP. 5. Power control in CR networks We further consider the case of variable transmission power in CR networks. Now, our objective is minimizing the total required bandwidth footprint product (BFP) to support a set of user sessions. As a basis, we first develop an interference model for scheduling when power control is performed at each node. This model extends existing so-called protocol models for wireless networks where transmission power is deterministic. As a result, this model can be used for a broad range of problems where power control is part of the optimization space. An efficient solution procedure based on the branch-and-bound framework and convex hull relaxations is proposed to provide (1- )-optimal solutions. This is the first theoretical result on this important problem. ; Ph. D.
The objective of this research is to develop an adaptive cryptographic protocol, which allows users to select an optimal cryptographic strength and algorithm based upon the hardware and bandwidth available and allows users to reason about the level of security versus the system throughput. In this constantly technically-improving society, the ability to communicate via wireless technology provides an avenue for delivering information at anytime nearly anywhere. Sensitive or classified information can be transferred wirelessly across unsecured channels by using cryptographic algorithms. The research presented will focus on dynamically selecting optimal cryptographic algorithms and cryptographic strengths based upon the hardware and bandwidth available. The research will explore the performance of transferring information using various cryptographic algorithms and strengths using different CPU and bandwidths on various sized packets or files. This research will provide a foundation for dynamically selecting cryptographic algorithms and key sizes. The conclusion of the research provides a selection process for users to determine the best cryptographic algorithms and strengths to send desired information without waiting for information security personnel to determine the required method for transferring. This capability will be an important stepping stone towards the military's vision of future Net-Centric Warfare capabilities.
In 1985, the Federal Communications Commission & quot;opened" a range of radio frequencies called the Industrial Scientific and Medical band. For the first time anyone could transmit high-speed data wirelessly without a license and with minimal regulatory restrictions. Building on this opportunity, the IEEE's 802.11 protocol enabled cheap untethered access to nearby wired networks and ultimately transformed how millions of businesses, governments, and casual computer-users access the Internet. However, the designers of 802.11 did not anticipate the magnitude of their success, and thus the protocol is hard-pressed to meet many of the new demands placed on it. In particular, 802.11's decentralized management structure poses unique challenges for both security and performance. This dissertation focuses on two key instances of these problems: denial-of-service and channel efficiency. In 802.11, each wireless client implements the media access control protocol (MAC) to share the common radio medium more effectively while maintaining the decentralized design of the protocol. Additionally, each client implicitly trusts that the other clients are faithful in their MAC implementation. Under these best-case circumstances the protocol operates as designed, however, nothing prevents a client from abusing this trust. In this work I evaluate two of the more critical abuses of trust, both of which result in a denial -of-service attack where a well-behaved client is denied access to the wireless medium. In addition, I propose defense mechanisms that successfully protect the networks while preserving the decentralized nature of 802.11. The desire to efficiently use all available radio channels arises because typical deployments have many more access points and clients than they do channels. This creates a high degree of contention for a limited resource. The decentralized nature of 802.11 exosborates [sic] this issue because each access point makes a locally greedy channel decision even though the aggregate of the local decisions does not yield a global optimum. To that end I propose and evaluate a number of different automated channel selection policies and synchronization techniques using a large, real world testbed, identifying the current best practice and quantifying its performance improvement
Wireless networks are an iconic technology of today's modern era, theyare present in our daily activities as can be exemplified by cellular communications,wi-fi, bluetooth, and others. Vector measurements play an importantrole in the design, simulation, and testing of wireless networks and are usedto characterize key devices operating in the radio interface, such as amplifiers,filters, and mixers.Accurate characterization is the key for improving the capacity and efficiencyof wireless networks. As the demand for network capacity continuouslyincreases, the accuracy of vector measurements must also improve. Further,it is anticipated that such trends will continue in the years to come. Consequently,the wireless industry needs to include nonlinear behavior in theircharacterization and analysis, to assess and guaranty the operation of the devices,and to comply to the specifications from governmental regulations. Incontrast to linear behavior, nonlinear behavior presents an additional bandwidthrequirement because the signal bandwidth grows when it passes throughnonlinear devices. In this thesis, vector measurements for devices operatingin wireless networks are studied, emphasizing a synthetic approach for theinstrumentation. This approach enables the use of digital post-processing algorithms,which enhances the measurement accuracy and/or speed and canovercome hardware impairments. This thesis presents the design of a vectorialmeasurement system for wireless devices considering the aforementionedtrends and requirements. It also explores the advantages of the proposedapproach, describes its limitations, and discusses the digital signal processingalgorithms used to reach its final functionality. Finally, measurement resultsof the proposed setup are presented, analyzed and compared to those of modernindustrial instruments. ; QC 20130204
Mobile wireless networks are undoubtedly one of the main drivers of industrial development and economic growth in modern societies. This central role explains current trends which indicate unequivocally that future cellular systems must cope with enormous increase in user-capacity coupled with a reduction in energy consumption. However, as of recently, this scalability/efficiency problem has been addressed through infrastructural and topological enhancements to the existing wireless networks. This will create additional spatial domain, which will serve as a means to solve the scalability/efficiency issue. Along with this, challenges in the form of signal interference remains the main adversary of systems engineers. Compounding the problem is the current wireless framework, which predominantly consists of diverse inter-connected devices in highly dense and mobile environment, and thus, intuitively inter- and intra- interferences are guaranteed to prevail. In this thesis, despite traditional concept that identify interference as a deleterious phenomenon, we have investigated two approaches that contemplate interference as a beneficial resource. The first approach exploits the interference at the baseband level, and leads to multiuser precoding which ultimately results in improved quality of service, such as achievable rate, especially for cell-edge users. On the other hand, the second concept exploits interference as means to trick (jam) potential intruders (eavesdroppers). In the continuously evolving wireless environment, there exists broad range of � financial, political and environmental applications that require confidential and secure transmission of information, this prompt devising security measures as a paramount task. In this sequel, physical layer security (PLS) measures have been proposed as alternative to the more complex upper layer encryption and cryptographic protocols.
