Interference in wireless networkscancelation, impact, practical management, and complexity

Resumo

The layered organization of traditional wired network design has continuously regarded communication links as bit pipes delivering data at some fixed rate with a certain error probability. While this modeling of the underlying physical layer may result appropriate for wired networks, it is certainly naive in the wireless domain. Unlike the fixed wired network, where the channel is time-invariant, the propagation physics of wireless channels and potential user mobility render the wireless network conditions very dynamic and time varying. Much worse, the pipe model disregards multiuser interference: by giving shared access to the same limited pool of resources to many users, the transmission rates of the communication links get coupled and the decomposition of the network into a set of independent single-user links turns out to be meaningless. While the role of multiuser interference is widely recognized in prospective systems design, its impact on network performance is diverse. In this respect, the aim of the present Ph.D. thesis is to adopt a broad approach in the study of multiuser interference in wireless networks, recognizing it as a phenomenon with many different facets out of which we concentrate on four of them: cancelation, impact, practical management, and complexity. We start studying when and how to perform partial interference cancelation, a technique that requires full statistical knowledge of the interfering signals at the receivers. We find that coding and decoding complexity can be traded whenever interference is under the control of the same source. In other words, the need for partial interference cancelation at the receivers can be alleviated through the use of appropriate coding techniques exploiting signal correlation at the transmitters. Additionally, we propose a transmission strategy based on superposition coding and aided decoding that yields an achievable region at least as large as the best long-standing region for the interference channel. Useful as it is, interference cancelation becomes infeasible in applications backed by decentralized wireless networks with uncoordinated nodes. That leaves each sender-destination pair armed only with point-to-point (single user) strategies. This motivates the study of the totally asynchronous interference channel with single-user receivers. Having a capacity region rather involved, the evaluation of achievable rates is tackled based on simpler single-letter inner and outer bounds. The study of these bounds reveals that the impact of interference on the achievable rates can be mitigated through statistical signal design. Besides, the performance losses associated to the lack of transmission synchronism and the use of single-user decoders in the low- and high-power and low- and high-interference regimes are also quantified. Next, the focus is on how to manage interference in a practical scenario where the receivers are again interference unaware but now frame- synchronous. A practical transmission scheme that allows for the design of optimal allocation policies of the limited transmission resources of the network is proposed. Giving special attention to a cellular configuration under practical conditions, efficient allocation schemes achieving Pareto and sequential optimality, respectively, are proposed and compared. The emphasis at this point is on the performance-complexity and throughput-fairness tradeoffs. While recognizing that multiuser interference and the availability of receiver information modifies the fundamental limits and the practical figures of merit of wireless networks, the thesis concludes by studying a related aspect: how multiuser interference impacts on the complexity required for the evaluation of the previous quantities. Efficient methods for the evaluation of the capacity region of multiuser channels are proposed and, unlike the single-user case, non-convexities in optimization problems need to be unavoidably faced.