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Title:Interference management in wireless networks
Author(s):Bande, Meghana
Director of Research:Veeravalli, Venugopal
Doctoral Committee Chair(s):Veeravalli, Venugopal
Doctoral Committee Member(s):Srikant, Rayadurgam; Viswanath, Pramod; ElGamal, Aly
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):degrees of freedom, COMP, heterogeneous, multi-armed bandit, dynamic spectrum access
Abstract:Interference management in wireless networks has emerged as an important task in order to meet the increased demand for data. In this thesis, interference management through cooperative transmission in the downlink is studied for a cellular network. Degrees of freedom (DoF) gains are first studied in a hexagonal sectored cellular network with cooperative transmission under a backhaul load constraint that limits the average number of messages that can be delivered from a centralized controller to basestation transmitters. The backhaul load is defined as the sum of all the messages available at all the transmitters per channel use, normalized by the number of users. Using insights from the analysis of Wyner’s linear interference network, the results are extended to the more practical hexagonal sectored cellular net- work, and coding schemes based on cooperative zero-forcing are shown to deliver significant DoF gains. It is established that by allowing for cooperative transmission and a flexible message assignment that is constrained only by an average backhaul load, one can deliver the rate gains promised by information-theoretic upper bounds with practical one-shot schemes that incur little or no additional load on the backhaul. Finally, useful upper bounds on the per user DoF for schemes based on cooperative zero-forcing are presented for the average backhaul load constraint, and an optimization framework is formulated for the general converse problem. Degrees of freedom (DoF) gains through cooperative transmission are then studied in the downlink of a two-layered heterogeneous network with macro basestations (MBs), small-cell basestations (SBs) that act as half-duplex analog relays, and mobile terminals (MTs). The first layer is a wireless back- haul layer between MBs and SBs, and the second layer is the transmission layer between SBs and MTs. The two layers use the same time/frequency resources for communication, limiting the maximum per user degrees of freedom (puDoF) to half, due to the half-duplex nature of the SBs. A linear network is first considered, and it is established that the optimal puDoF can be achieved by cooperation with sufficient antennas. The proposed schemes are simple zero-forcing schemes that achieve cooperation without overloading the backhaul. Cooperation is implemented by sending an appropriate linear combination of users’ messages from the MBs to the SBs that zero-force interference at the MTs. The achievable schemes exploit the half-duplexity of the SBs and schedule the SBs and MTs to be active in different time-slots in a smart manner to reduce interference. These results are then extended to a more realistic hexagonal network, and it is shown that the optimal puDoF of half can be approached using only zero-forcing schemes, without using interference alignment. Interference management is then considered through the design of an efficient algorithm in a decentralized uncoordinated spectrum sharing system. A multi-user multi-armed bandit (MAB) framework is used to develop algorithms for uncoordinated spectrum access. The number of users is assumed to be unknown to each user. A stochastic setting is first considered, where the rewards on a channel are the same for each user. In contrast to prior work, it is assumed that the number of users can possibly exceed the number of channels, and that rewards can be non-zero even under collisions. The proposed algorithm consists of an estimation phase and an allocation phase. It is shown that if every user adopts the algorithm, the systemwide regret is sub-linear over a horizon of time T . The regret guarantees hold for any number of users and channels; in particular, they hold even when the number of users is less than the number of channels. Next, an adversarial multi-user MAB framework is considered, where the rewards on the channels are user-dependent. It is assumed that the number of users is less than the number of channels, and that the users receive zero reward on collision. The proposed algorithm combines the Exp3.P algorithm developed in prior work for single-user adversarial bandits with a collision resolution mechanism to achieve sub-linear regret. It is shown that if every user employs the proposed algorithm, the systemwide regret is O(T^{3/4}) over a horizon of time T . The algorithms in both stochastic and adversarial scenarios are extended to the dynamic case where the number of users in the system evolves over time and are shown to lead to sub-linear regret.
Issue Date:2019-03-22
Rights Information:Copyright 2019 Meghana Bande
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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