Transmission Strategies For Interfering Networks With Finite Rate And Outdated Channel Feedback

The emergence of very capable mobile terminals, e.g. smartphones or tablets, has dramatically increased the demand of wireless data traffic in recent years. Current growth forecasts elucidate that wireless communication standards will not be able to afford future traffic demands, thus many research efforts have been oriented towards increasing the efficiency of wireless networks. MIMO technologies, entailing the use of multiple antennas, stand as one of the candidates. This solution allows increasing not only the reliability and robustness (diversity gain), but also the efficiency of the communication (multiplexing gain or degrees of freedom (DoF)). The DoF describe the slope of channel capacity at very high signal-to-noise-ratio (SNR) regime, and for the point-to-point (P2P) channel are equal to the minimum between the number of antennas at the transmitter and the receiver. Consequently, the throughput may be scaled in a promising way. However, the DoF behavior in case of having interference is still an open problem in general. This thesis studies the DoF of interference networks. The most trivial way of tackling this problem is by means of orthogonalization, either in time, frequency or space. However, it does not allow that the scaling of DoF with the number of users. For example, if transmissions are orthogonalized in time each user is served only a fraction of time inversely proportional to the number of users. Likewise, if transmissions are orthogonalized in space, transmitters must be equipped with a large number of antennas, which is costly and not practical. For dimensionally-limited systems, the interference alignment (IA) principle proposes that instead of forcing the design to null the interference terms at the receivers, make the receiver observe them overlapped. This way the number of dimensions occupied by interference is reduced, thus allowing the allocation of more desired signals, thus symbols per user, and also relaxing the constraint on the number of required antennas. Following IA allows that "each user achieves half the cake independently of the number of users", where the cake represents the DoF of the P2P channel. At first, full channel state information was assumed to be available at the transmitter side (full CSIT), i.e. the information is acquired instantaneously, and with perfect quality. The first part of this thesis studies this case and completes the DoF characterization of the 3-user MIMO interference channel for some antenna configurations when channel coefficients are assumed constant. In practice, CSIT should be obtained from channel feedback, thus incurring delays and errors. In this context, and especially intended to scenarios with high mobility, IA concepts were extended to networks where only outdated information of the channels is available, a framework known as delayed CSIT where the channel feedback delay may be larger than the channel coherence time. This form of IA is denoted as retrospective interference alignment, since the transmission is carried out in multiple phases, and signals may be aligned along space and the different phases. The second part of the thesis deepens into the DoF of two network topologies with delayed CSIT, for which linear precoding strategies are proposed. Moreover, it is shown that the proposed strategies are better than state-of-the-art in terms of DoF-delay trade-off, which is relevant as most strategies based on delayed CSIT entail long communication delays. The concluding part of the thesis analyses how one of schemes proposed in the second part performs in terms of DoF with delayed CSIT with errors, and net DoF. This last metric describes the DoF as a function of the coherence time, and taking into account all issues related to channel acquisition at both the transmitter and receiver side: consumption of resources for channel training, for feedback transmission, and feedback waits.

In an interference network, multiple transmitters communicate with multiple receivers using the same communication channel. The capacity region of an interference network is defined as the set of data rates that can be simultaneously achieved by the users of the network. One of the most important example of an interference network is the wireless network, where the communication channel is the wireless channel. Wireless interference networks are known to be interference limited rather than noise limited since the interference power level at the receivers (caused by other user's transmissions) is much higher than the noise power level. Most wireless communication systems deployed today employ transmission strategies where the interfering signals are treated in the same manner as thermal noise. Such strategies are known to be suboptimal (in terms of achieving higher data rates), because the interfering signals generated by other transmitters have a structure to them that is very different from that of random thermal noise. Hence, there is a need to design transmission strategies that exploit this structure of the interfering signals to achieve higher data rates. However, determining optimal strategies for mitigating interference has been a long standing open problem. In fact, even for the simplest interference network with just two users, the capacity region is unknown. In this dissertation, we will investigate the capacity region of several models of interference channels. We will derive limits on achievable data rates and design effective transmission strategies that come close to achieving the limits. We will investigate two kinds of networks - "small" (usually characterized by two transmitters and two receivers) and "large" where the number of users is large.

In response to a request from the Defense Advanced Research Projects Agency, the committee studied a range of issues to help identify what strategies the Department of Defense might follow to meet its need for flexible, rapidly deployable communications systems. Taking into account the military's particular requirements for security, interoperability, and other capabilities as well as the extent to which commercial technology development can be expected to support these and related needs, the book recommends systems and component research as well as organizational changes to help the DOD field state-of-the-art, cost-effective untethered communications systems. In addition to advising DARPA on where its investment in information technology for mobile wireless communications systems can have the greatest impact, the book explores the evolution of wireless technology, the often fruitful synergy between commercial and military research and development efforts, and the technical challenges still to be overcome in making the dream of "anytime, anywhere" communications a reality.

This textbook takes a unified view of the fundamentals of wireless communication and explains cutting-edge concepts in a simple and intuitive way. An abundant supply of exercises make it ideal for graduate courses in electrical and computer engineering and it will also be of great interest to practising engineers.

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.

Massive MIMO Networks is the first book on the subject to cover the spatial channel correlation and consider rigorous signal processing design essential for the complete understanding by the students, practicing engineers and researchers working on modern day communication systems.

Physical Layer Security in Wireless Communications supplies a systematic overview of the basic concepts, recent advancements, and open issues in providing communication security at the physical layer. It introduces the key concepts, design issues, and solutions to physical layer security in single-user and multi-user communication systems, as well as large-scale wireless networks. Presenting high-level discussions along with specific examples, and illustrations, this is an ideal reference for anyone that needs to obtain a macro-level understanding of physical layer security and its role in future wireless communication systems.

The Bulletin of the Atomic Scientists is the premier public resource on scientific and technological developments that impact global security. Founded by Manhattan Project Scientists, the Bulletin's iconic "Doomsday Clock" stimulates solutions for a safer world.

Multi-point Cooperative Communication Systems: Theory and Applications mainly discusses multi-point cooperative communication technologies which are used to overcome the long-standing problem of limited transmission rate caused by the inter-point interference. Instead of combating the interference, recent progress in both academia and industrial standardizations has evolved to adopt the philosophy of “exploiting” the interference to improve the transmission rate by cooperating among multiple points. This book addresses the multi-point cooperative communication system systematically giving the readers a clear picture of the technology map and where the discussed schemes may fit. This book includes not only the theories of the paradigm-shifting multi-point cooperative communication, but also the designs of sub-optimal cooperative communication schemes for practical systems. Ming Ding is a senior researcher at Sharp Laboratories of China; Hanwen Luo is a professor at Shanghai Jiao Tong University.