Interference alignment (IA) has been shown to provide all users of an interference channel with half the capacity achievable in an interference free point-to-point link resulting in linear sum capacity scaling with the number of users in the high SNR regime. The linear scaling is achieved by precoding transmitted signals to align interference subspaces at the receivers, given channel knowledge of all transmit-receive pairs, effectively reducing the number of discernible interferers. The theory of IA was derived under assumptions about the richness of the propagation channel; practical channels do not guarantee such ideal characteristics. This paper presents the first experimental study of IA in measured multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) interference channels. We show that IA achieves the claimed scaling factors in a wide variety of measured channel settings for a 3 user, 2 antennas per node setup. In addition to verifying the claimed performance, we characterize the effect of several realistic system imperfections such as channel estimation error, feedback delay, and channel spatial correlation, on sum rate performance. / text

In this dissertation, we study the state-dependent two-user interference channel, where the state information is non-causally known at both transmitters but unknown to either of the receivers. We first propose two coding schemes for the discrete memoryless case: simultaneous encoding for the sub-messages in the first one and super-position encoding in the second one, both with rate splitting and Gel'fand-Pinsker coding. The corresponding achievable rate regions are established. Moreover, for the Gaussian case, we focus on the simultaneous encoding scheme and propose an active interference cancellation mechanism, which is a generalized dirty-paper coding technique, to partially eliminate the state effect at the receivers. The corresponding achievable rate region is then derived. We also propose several heuristic schemes for some special cases: the strong interference case, the mixed interference case, and the weak interference case. For the strong and mixed interference case, numerical results are provided to show that active interference cancellation significantly enlarges the achievable rate region. For the weak interference case, flexible power splitting instead of active interference cancellation improves the performance significantly. Moreover, we focus on the simplest symmetric case, where both direct link gains are the same with each other, and both interfering link gains are the same with each other. We apply the above coding scheme with different dirty paper coding parameters. When the state is additive and symmetric at both receivers, we study both strong and weak interference scenarios and characterize the theoretical gap between the achievable symmetric rate and the upper bound, which is shown to be less than 1/4 bit for the strong interference case and less than 3/4 bit for the weak interference case. Then we provide numerical evaluations of the achievable rates against the upper bound, which validates the theoretical analysis for both strong and weak interference scenarios. Finally, we define the generalized degrees of freedom for the symmetric Gaussian case, and compare the lower bounds against the upper bounds for both strong and weak interference cases. We also show that our achievable schemes can obtain the exact optimal values of the generalized degrees of freedom, i.e., the lower bounds meet the upper bounds for both strong and weak interference cases.

A network of n transmitters and n receivers is considered. We assume that transmitter i aims to send data to its designated destination, receiver i. Communications occur in a single-hop fashion and destination nodes are simple linear receivers without multi-user detection. Therefore, in each time slot every source node can only talk to one other destination node. Thus, there is a total of n communication links. An important question now arises. How many links can be active in such a network so that each of them supports a minimum rate Rmin? This dissertation is devoted to this problem and tries to solve it in two di erent settings, dense and extended networks. In both settings our approach is asymptotic, meaning, we only examine the behaviour of the network when the number of nodes tends to in nity. We are also interested in the events that occur asymptotically almost surely (a.a.s.), i.e., events that have probabilities approaching one as the size of the networks gets large. In the rst part of the thesis, we consider a dense network where fading is the dominant factor a ecting the quality of transmissions. Rayliegh channels are used to model the impact of fading. It is shown that a.a.s. log(n)^2 links can simultaneously maintain Rmin and thus be active. In the second part, an extended network is considered where nodes are distant from each other and thus, a more complete model must take internode distances into account. We will show that in this case, almost all of the links can be active while maintaining the minimum rate.

In this thesis, the problem of quantifying the Pareto optimal boundary of the achievable rate region is considered over multiple-input single-output(MISO)interference channels, where the problem boils down to solving a sequence of convex feasibility problems after certain transformations. The feasibility problem is solved by two new distributed optimal beam forming algorithms, where the first one is to parallelize the computation based on the method of alternating projections, and the second one is to localize the computation based on the method of cyclic projections. Convergence proofs are established for both algorithms.

In wireless channels and many other channels, interference is a central phenomenon. Mitigating interference is a key to improving system performance. To find the limit of the achievable rates for these channels in the presence of interference, the two-user Gaussian interference channel has been the subject of intensive study in network information theory. However, most current results have been obtained by assuming Gaussian input distributions. While optimal in single-user Gaussian channels, the issue with this assumption is that the Gaussian noise becomes the worst noise when the input distribution is also Gaussian. In this thesis, we propose a class of discrete constellations. We show that this class of constellations can automatically achieve the same sum rates as schemes that treat interference as noise or perform time sharing.

In wireless channels and many other channels, interference is a central phenomenon. Mitigating interference is a key to improving system performance. To find the limit of the achievable rates for these channels in the presence of interference, the two-user Gaussian interference channel has been the subject of intensive study in network information theory. However, most current results have been obtained by assuming Gaussian input distributions. While optimal in single-user Gaussian channels, the issue with this assumption is that the Gaussian noise becomes the worst noise when the input distribution is also Gaussian. In this thesis, we propose a class of discrete constellations. We show that this class of constellations can automatically achieve the same sum rates as schemes that treat interference as noise or perform time sharing.

A network of n transmitters and n receivers is considered. We assume that transmitter i aims to send data to its designated destination, receiver i. Communications occur in a single-hop fashion and destination nodes are simple linear receivers without multi-user detection. Therefore, in each time slot every source node can only talk to one other destination node. Thus, there is a total of n communication links. An important question now arises. How many links can be active in such a network so that each of them supports a minimum rate Rmin? This dissertation is devoted to this problem and tries to solve it in two di erent settings, dense and extended networks. In both settings our approach is asymptotic, meaning, we only examine the behaviour of the network when the number of nodes tends to in nity. We are also interested in the events that occur asymptotically almost surely (a.a.s.), i.e., events that have probabilities approaching one as the size of the networks gets large. In the rst part of the thesis, we consider a dense network where fading is the dominant factor a ecting the quality of transmissions. Rayliegh channels are used to model the impact of fading. It is shown that a.a.s. log(n)^2 links can simultaneously maintain Rmin and thus be active. In the second part, an extended network is considered where nodes are distant from each other and thus, a more complete model must take internode distances into account. We will show that in this case, almost all of the links can be active while maintaining the minimum rate.

The storage of frequently requested multimedia content at small-cell base stations (BSs) can reduce the load of macro-BSs without relying on high-speed backhaul links. In this work, the optimal operation of a system consisting of a cache-aided small-cell BS and a macro-BS is investigated for both offline and online caching settings. In particular, a binary fading one-sided interference channel is considered in which the small-cell BS, whose transmission is interfered by the macro-BS, has a limited-capacity cache. The delivery time per bit (DTB) is adopted as a measure of the coding latency, that is, the duration of the transmission block, required for reliable delivery. For offline caching, assuming a static set of popular contents, the minimum achievable DTB is characterized through information-theoretic achievability and converse arguments as a function of the cache capacity and of the capacity of the backhaul link connecting cloud and small-cell BS. For online caching, under a time-varying set of popular contents, the long-term (average) DTB is evaluated for both proactive and reactive caching policies. Furthermore, a converse argument is developed to characterize the minimum achievable long-term DTB for online caching in terms of the minimum achievable DTB for offline caching. The performance of both online and offline caching is finally compared using numerical results.

Dans de nombreux scénarios de communication sans fil multiutilisateurs, une bonne rétroaction est un ingrédient essentiel qui facilite l'amélioration des performances. Bien qu'étant utile, une rétroaction parfaite reste difficile et fastidieuse à obtenir. En considérant ce défi comme point de départ, le principal objet de cette thèse s'applique à adresser la question simple et pourtant insaisissable et fondamentale suivante: "Quel niveau de qualité de la rétroaction doit-on rechercher, et à quel moment faut-il effectuer un envoi pour atteindre une certaine performance en degrés de liberté (DoF)". La présente étude réussit à décrire de manière concise les régions DoF dans un cadre très général, correspondant à un processus général de rétroaction qui, à tout moment, peut ou non fournir des informations sur l'état du canal au niveau de l'émetteur (CSIT) - d'une qualité arbitraire - pour toute réalisation passée, actuelle ou future du canal. Sous des hypothèses standard , et en supposant par ailleurs que l'on dispose d'un CSIT suffisamment bien retardé, l'effet de la qualité du CSIT offert à tout moment, pour presque tout type de canaux est étudié de manière précise. Ceci est réalisé dans le cadre MISO-BC à deux utilisateurs, puis est directement étendu aux cas des MIMO-BC et MIMO-IC. En outre différents aspects de communication avec rétroaction limitée sont considérés, ainsi que l'aspect CSI global au niveau des récepteurs, et l'aspect diversité. En plus de fournir des limites théoriques et des nouveaux encodeurs et décodeurs, l'étude s'applique à obtenir une meilleure comprehension sur plusieurs questions pratiques d'intérêt capital dans le domaine. / In many multiuser wireless communications scenarios, good feedback is a crucial ingredient that facilitates improved performance. While being useful, perfect feedback is also hard and time-consuming to obtain. With this challenge as a starting point, the main work of thesis seeks to address the simple yet elusive and fundamental question of ``HOW MUCH QUALITY of feedback, AND WHEN, must one send to achieve a certain degrees-of-freedom (DoF) performance''. The work manages to concisely describe the DoF region in a very broad setting corresponding to a general feedback process that, at any point in time, may or may not provide channel state information at the transmitter (CSIT) - of some arbitrary quality - for any past, current or future channel (fading) realization. Under standard assumptions, and under the assumption of sufficiently good delayed CSIT, the work concisely captures the effect of the quality of CSIT offered at any time, about any channel. This was achieved for the two user MISO-BC, and was then immediately extended to the MIMO BC and MIMO IC settings. Further work also considers different aspects of communicating with limited feedback, such as the aspect of global CSI at receivers, and the aspect of diversity. In addition to the theoretical limits and novel encoders and decoders, the work applies towards gaining insights on many practical questions.

In multiuser wireless communications, interference not only limits the performance of the system, but also allows users to eavesdrop on other users’ messages. Hence, interference management in multiuser wireless communication has received significant attention in the last decade, both in the academia and industry. The interference channel (IC) is one of the simplest information theoretic models to analyze the effect of interference on the throughput and secrecy of individual messages in a multiuser setup. In this thesis, the IC is studied under different settings with and without the secrecy constraint. The main contributions of the thesis are as follows: • The generalized degrees of freedom (GDOF) has emerged as a useful approximate measure of the potential throughput of a multiuser wireless system. Also, multiple antennas at the transmitter and receiver can provide additional dimension for signaling, which can in turn improve the GDOF performance of the IC. In the initial part of the thesis, a K-user MIMO Gaussian IC (GIC) is studied from an achievable GDOF perspective. An inner bound on GDOF is derived using a combination of techniques such as treating interference as noise, zero-forcing receiving, interference alignment (IA), and extending the Han-Kobayashi (HK) scheme to K users. Also, outer bounds on the sum rate of the K-user MIMO GIC are derived, under different assumptions of cooperation and providing side information to the receivers. The derived outer bounds are simplified to obtain outer bounds on the GDOF. The relative performance of these bounds yields insight into the performance limits of the multiuser MIMO GIC and the relative merits of different schemes for interference management. • Then, the problem of designing the precoding and receive filtering matrices for IA is explored for K-user MIMO (M × N) GIC. Two algorithms for designing the precoding and receive filtering matrices for IA in the block fading or constant MIMO IC with a finite number of symbol extensions are proposed. The first algorithm for IA is based on aligning a subset of the interfering signal streams at each receiver. As the first algorithm requires global channel knowledge at each node, a distributed algorithm is proposed which requires only limited channel knowledge at each node. A new performance metric is proposed, that captures the possible loss in signal dimension while designing the precoders. The performance of the algorithms are evaluated by comparing them with existing algorithms for IA precoder design. • In the later part of the thesis, a 2-user IC with limited-rate transmitter cooperation is studied, to investigate the role of cooperation in managing interference and ensuring secrecy. First, the problem is studied in the deterministic setting, and achievable schemes are proposed, which use a combination of interference cancelation, relaying of the other user’s data bits, time sharing, and transmission of random bits, depending on the rate of the cooperative link and the relative strengths of the signal and the interference. Outer bounds on the secrecy rate are derived, under different assumptions of providing side information to receivers and partitioning the encoded message/output depending on the relative strength of the signal and the interference. The achievable schemes and outer bounds are extended to the Gaussian case. For example, while obtaining outer bounds, for the Gaussian case, it is not possible to partition the encoded message or output as performed in the deterministic case, and the novelty lies in finding the analogous quantities for the Gaussian case. The proposed achievable scheme for the Gaussian case uses a combination of cooperative and stochastic encoding along with dummy message transmission. For both the models, one of the key techniques used in the achievable scheme is interference cancelation, which has two benefits: it cancels interference and ensures secrecy simultaneously. The results show that limited-rate transmitter cooperation can greatly facilitate secure communications over 2-user ICs.