Network Interference Management Via Interference Alignment

Currently, we are witnessing a veritable explosion in the number of mobile devices with network connectivity. This explosion in the number of mobile devices which guzzle data is resulting in bandwidth becoming an increasingly scarce resource. The surge in the demand for data calls for new techniques to understand and improve the capacity (data rates) of wireless networks. In this thesis, I will describe and explore the benefits of interference alignment - a recently discovered technique to manage interference, which is the primary bottleneck of rates of communication in wireless communication networks. A primary object of study of this thesis is a communication network with K wireless transmitter-receiver pairs mutually interfering with each other, also known as the K user interference network. In this thesis, we study a high SNR approximation to its capacity known as the degrees of freedom. A widely held belief that influences design of most, if not all wireless networks is the following: in the K user interference network it is optimal from a network degrees of freedom perspective to divide the spectrum among the users like cutting a cake. This cake cutting view of spectrum access also known as orthogonalization enables each user in the interference network to get a fraction of 1/K degrees of freedom, i.e., 1/K of the spectrum free of interference. In this thesis, we will show that, from a degrees of freedom perspective, the belief in the optimality of the cake cutting view of spectrum access (i.e., orthogonalization) is flawed. We show that if the network is frequency-selective or time-varying, then each of the K users of an interference network can essentially get half the degrees of freedom of a single user (i.e., half the spectrum at high signal-to-noise ratios) simultaneously. In other words, each user can get ``half the cake'' rather than merely a fraction 1/K. The key to achieving this is the powerful interference management strategy of interference alignment. The thesis will study and develop various aspects of interference alignment. First, we develop an asymptotic alignment scheme to achieve ``half the cake'' in frequency-selective/time-varying interference channels. We then extend the idea of interference alignment to channels that are not frequency-selective or time-varying (i.e., channels which are constant) via three approaches: asymmetric complex signaling, a deterministic approach, and a distributed (numerical) alignment algorithm. In each of these cases, we will demonstrate degrees of freedom and capacity benefits of interference alignment in wireless interference networks. We also demonstrate practical benefits of the third approach - distributed alignment - in terms of rates at moderate signal-to-noise ratios and distributed implementations. Finally, we show that the impact of interference alignment extends beyond the context of just wireless systems. In particular, we explore an alternate application of the idea of alignment - erasure codes for distributed storage systems.

In wireless communication networks, competition among users for channel resources can result in severe mutual interference. This is a bottleneck for obtaining higher communication rates. Recent advances in the network information theory, such as the idea of interference alignment, have greatly facilitated our understanding of signal dimensions or even exact capacity of wireless networks and produced a number of new transmission schemes to achieve higher rates. Usually, we are interested in the fundamental questions -- what is the channel capacity of multiuser networks, and how to achieve higher communication rates, attractive for both theoretical researchers and engineers. Since finding the exact capacity of multiuser wireless networks is quite challenging, if not impossible, we are interested in the degrees of freedom (DoF) characterization, i.e., a coarse capacity approximation, of wireless networks. The number of DoF of a communication network is a metric of great significance as it provides a lens into the most essential aspects of the communication problem. DoF investigations have motivated many fundamental ideas such as interference alignment. In this dissertation, we investigate the DoF of a number of multiuser wireless networks using the idea of interference alignment. In particular, we start from the classical interference channels with global channel knowledge at each node. A number of scenarios will be studied, including networks with single antenna or multiple antennas at each node. Next, we consider the interference channel with local cooperation and local connectivity. Then we go beyond one-hop to multihop wireless networks where we find the DoF of multiple unicast for 2-source 2-sink layered networks with arbitrary topologies. Finally, we weaken the global channel knowledge assumption, to study broadcast channels with no channel state information at the transmitter. Several interesting tools, insights and surprising results are obtained in this work -- including phase alignment, asymmetric complex signaling, subspace alignment chains, genie chains, the observation that removing interference-carrying links can reduce the channel capacity, and blind interference alignment.

Interference Alignment: A New Look at Signal Dimensions in a Communication Network provides both a tutorial and a survey of the state-of-art on the topic.

Learn about a new, information-theoretic approach to minimizing interference in 5G wireless networks.

This authoritative resource offers a comprehensive overview of heterogeneous wireless networks, small cells, and device-to-device (D2D) communications. The book provides insight into network modeling and performance analysis of heterogeneous wireless networks. Interference management framework and design issues are covered as well as details about resource mobility, channel models, and typical and statistical interference modeling. This resource explains leveraging resource heterogeneity in interference mitigation and presents the challenges and feasible solutions for concurrent transmission. Moreover, complete coverage of interference alignment in MIMO heterogeneous networks for both downlink and uplink is presented. This book provides performance results for an ideal partially connected interference network as well as a practical heterogeneous network. Readers find practical guidance for LTE and LTE-Advanced as well as 5G in this resource. New techniques and designs for heterogeneous wireless networks are included.

Cognitive Radio Communications and Networks gives comprehensive and balanced coverage of the principles of cognitive radio communications, cognitive networks, and details of their implementation, including the latest developments in the standards and spectrum policy. Case studies, end-of-chapter questions, and descriptions of various platforms and test beds, together with sample code, give hands-on knowledge of how cognitive radio systems can be implemented in practice. Extensive treatment is given to several standards, including IEEE 802.22 for TV White Spaces and IEEE SCC41 Written by leading people in the field, both at universities and major industrial research laboratories, this tutorial text gives communications engineers, R&D engineers, researchers, undergraduate and post graduate students a complete reference on the application of wireless communications and network theory for the design and implementation of cognitive radio systems and networks Each chapter is written by internationally renowned experts, giving complete and balanced treatment of the fundamentals of both cognitive radio communications and cognitive networks, together with implementation details Extensive treatment of the latest standards and spectrum policy developments enables the development of compliant cognitive systems Strong practical orientation – through case studies and descriptions of cognitive radio platforms and testbeds – shows how real world cognitive radio systems and network architectures have been built Alexander M. Wyglinski is an Assistant Professor of Electrical and Computer Engineering at Worcester Polytechnic Institute (WPI), Director of the WPI Limerick Project Center, and Director of the Wireless Innovation Laboratory (WI Lab) Each chapter is written by internationally renowned experts, giving complete and balanced treatment of the fundamentals of both cognitive radio communications and cognitive networks, together with implementation details Extensive treatment of the latest standards and spectrum policy developments enables the development of compliant cognitive systems Strong practical orientation – through case studies and descriptions of cognitive radio platforms and testbeds – shows how "real world" cognitive radio systems and network architectures have been built

Interference is a key property of wireless communications due to the broadcasting nature of wireless links. The design of wireless networks needs to put interference management into consideration. Traditionally, interference management is done by partitioning the whole network into orthogonal non-interfering channels via time- or frequency-division multiplexing. While orthogonalization significantly reduces the complexity of the design and implementation of wireless networks, it also introduces artificial restriction and leads to suboptimal performance. This thesis is devoted to the design and analysis of interference management from a cross-layer perspective. The key to increase spectrum efficiency of a wireless network is to treat the entire network as a channel rather than viewing them as a set of separate links. Based on this idea, we propose three interference management schemes and evaluate the fundamental limits associated with them. We use the notions of both conventional and generalized degrees of freedom (DOF), which are two widely-used approximations of channel capacity, as merits to evaluate and compare the performance improvement brought by the interference management schemes. The thesis consists of four main results. First, we consider a multiple-input-multiple-output (MIMO) 2-suer cognitive radio system in an information theoretic setting where some messages are made available, by a genie, to some nodes (other than the intended nodes) non-causally, noiselessly, and for free. We find the DOF region of this system and show that this region is larger than the one without cognitive message sharing. Our results also show that in general it may be more beneficial, in terms of sum DOF, for a user to have a cognitive transmitter than to have cognitive receiver. Second, we consider a MIMO Gaussian interference channel with user cooperation, including cooperation at transmitters only, at receivers only, and at transmitters as well as receivers. We find the DOF region of this system and obtain a negative result that allowing users to cooperate does not enlarge the DOF region of this channel. Third, we explore the capacity and generalized degrees of freedom (GDOF) of a 2-user Gaussian X channel, i.e. a generalization of the 2-user interference channel where there is an independent message from each transmitter to each receiver. We provide the GDOF characterization of the channel under a symmetric setting. We also identify the regime where interference alignment is helpful so that the X channel has a higher capacity than the underlying symmetric interference channel. We further extend the noisy interference capacity characterization previously obtained for the interference channel to the X channel. Lastly, we study the effect of the absence of channel knowledge for MIMO networks. In particular, we assume perfect channel state information at the receivers and no channel state information at the transmitter(s). We provide the characterization of the DOF region for a 2-user MIMO broadcast channel. We then use the result of the broadcast channel to find the DOF region for some special cases of a 2-user MIMO interference channel.

Learn about an information-theoretic approach to managing interference in future generation wireless networks. Focusing on cooperative schemes motivated by Coordinated Multi-Point (CoMP) technology, the book develops a robust theoretical framework for interference management that uses recent advancements in backhaul design, and practical pre-coding schemes based on local cooperation, to deliver the increased speed and reliability promised by interference alignment. Gain insight into how simple, zero-forcing pre-coding schemes are optimal in locally connected interference networks, and discover how significant rate gains can be obtained by making cell association decisions and allocating backhaul resources based on centralized (cloud) processing and knowledge of network topology. Providing a link between information-theoretic analyses and interference management schemes that are easy to implement, this is an invaluable resource for researchers, graduate students and practicing engineers in wireless communications.

The increasing complexity of communication networks in size and density provides us enormous opportunities to exploit interaction among multiple nodes, thus enabling higher date rate of data streams. On the flip side, however, this complexity comes with challenges in managing interference that multiple source-destination pairs in the network may cause to each other. In this dissertation, we make progress on how we exploit the opportunities, as well as how we overcome the challenges. In the first part, we find that feedback - one of the common ways to enable interaction in networks - has a promising role in improving the capacity performance of networks. Earlier results on feedback capacity were somewhat discouraging. This is mainly due to Shannon's original result on feedback capacity where he showed that in point-to-point communication, feedback does not increase capacity. Hence, traditionally it is believed that feedback has had little impact on increasing capacity of communication links. Therefore, the use of feedback has been limited to improving the reliability of communications, usually in the form of ARQ. In this dissertation, we show that in stark contrast to the point-to-point case, feedback can improve the capacity of interference-limited network. In fact, the improvement can be unbounded. This result shows that feedback can have a potentially significant role to play in mitigating interference. Also in the process of deriving this conclusion, we characterize the feedback capacity of the two-user Gaussian interference channel to within 2 bits, one of the longstanding open problems in network information theory. In the second part, we propose a new interference management technique for widely deployed cellular networks. Inspired by a recent breakthrough, the concept of interference alignment, we develop an interference alignment technique for cellular networks. Our technique promises almost interference-free communication with the increase of the number of clients in cellular networks. It shows substantial gain (around 30% to 60%) as compared to one of the interference management techniques in current cellular systems. In addition, it comes with implementation benefits: it can actually be implemented with small changes to emerging 4G cellular standards and architectures at the base-stations and clients. In particular, the required signal-processing circuitry, software control, and channel-state feedback mechanisms are extensions of existing implementations and standards. Lastly, we extend the interference alignment principle, developed in the context of wireless networks, into other fields of network research such as storage networks. In an effort to protect information against node failures, storage networks employ coding techniques, such as maximum distance separable (MDS) erasure codes, known as optimal codes in reliability with respect to redundancy. However, these MDS codes come with prohibitive maintenance cost when it comes to repairing failed storage nodes. While only partial information stored in the failed node needs to be recovered, the conventional MDS codes focus on the complete data recovery (including unwanted data, corresponding to interference) by downloading too much information from survivor storage encoded nodes, thus causing the high repair cost. Building on the connection between wireless and wireline networks, we leverage the interference alignment principle to develop a new class of MDS codes that significantly reduces the repair cost over the conventional MDS codes and also achieves information-theoretic optimal bound on the repair cost for all admissible code parameters.

Interference creates a fundamental barrier in attempting to improve throughput in wireless networks, especially when multiple concurrent transmissions share the wireless medium. In recent years, significant progress has been made on characterizing the capacity limits of wireless networks under the premise of global and instantaneous channel state information at transmitter (CSIT). In practice, however, the acquisition of such instantaneous and global CSIT as a means toward cooperation is highly challenging due to the distributed nature of transmitters and dynamic wireless propagation environments. In many limited CSIT scenarios, the promising gains from interference management strategies using instantaneous and global CSIT disappear, often providing the same result as cases where there is no CSIT. Is it possible to obtain substantial performance gains with limited CSIT in wireless networks, given previous evidence that there is marginal or no gain over the case with no CSIT? To shed light on the answer to this question, in this dissertation, I present several achievable sum of degrees of freedom (sum-DoF) characterizations of wireless networks. The sum-DoF is a coarse sum-capacity approximation of the networks, deemphasizing noise effects. These characterizations rely on a set of proposed and existing interference management strategies that exploit limited CSIT. I begin with the classical multi-user multiple-input-single-output (MISO) broadcast channel with delayed CSIT and show how CSI feedback delays change sum-capacity scaling law by proposing an innovative interference alignment technique called space-time interference alignment. Next, I consider interference networks with distributed and delayed CSIT and show how to optimally use distributed and moderately-delayed CSIT to yield the same sum-DoF as instantaneous and global CSIT using the idea of distributed space-time interference alignment. I also consider a two-hop layered multiple-input-multiple-output (MIMO) interference channel, where I show that two cascaded interfering links can be decomposed into two independent parallel relay channels without using CSIT at source nodes through the proposed interference-free relaying technique. Then I go beyond one-way and layered to multi-way and fully-connected wireless networks where I characterize the achievable sum-DoF of networks where no CSIT is available at source nodes using the proposed space-time physical-layer network coding. Lastly, I characterize analytical expressions for the sum spectral efficiency in a large-scale single-input-multiple- output (SIMO) interference network where the spatial locations of nodes are modeled by means of stochastic geometry. I derive analytical expressions for the ergodic sum spectral efficiency and the scaling laws as functions of relevant system parameters depending on different channel knowledge assumptions at receivers.