Flexible backhaul design with cooperative transmission in cellular interference networks

Interference is an important factor that limits the rates that can be achieved by mobile users in a cellular network. Interference management through cooperation has emerged as a major consideration for next-generation cellular networks. In this thesis, we focus on the downlink of a sectored hexagonal cellular network, under the assumption of local interference i.e., the interference at each user is only due to transmitters in neighboring sectors. We explore the potential degrees of freedom (DoF) gain in this network under constraints on the cooperation between base-stations. The constraints that we consider are the cooperation order M, and the average backhaul load B, which denote the maximum and the average number of transmitters, respectively, that jointly transmit any message. We first study the DoF gains in a scenario where mobile receivers can be associated to any neighboring cell but no cooperative transmission is allowed, and derive bounds on the maximum achievable per user DoF for orthogonal schemes. We then show that by combining cooperative transmission with flexible message assignment to the transmitters, it is possible to achieve a per user DoF strictly greater than that without cooperation. The proposed cooperative transmission scheme does not require extra backhaul capacity, as it uses a smart assignment of messages to transmitters to meet an average backhaul load constraint of one message per transmitter. The schemes presented are simple zero-forcing beamforming schemes that require linear precoding over a single time/frequency slot (one-shot). Similar schemes are proposed which achieve a per user DoF greater than half with a minimal increase in the backhaul load. These results are derived for networks with intra-cell interference and networks without intra-cell interference

Interference management in wireless networks

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.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste

DoF analysis in a two-layered heterogeneous wireless interference network

Abstract Degrees of freedom (DoF) is studied in the downlink of a heterogenous wireless network modeled as a two-layered interference network. The first layer of the interference network is the backhaul layer between macro base stations (MBs) and small cell base stations (SBs), which is modeled as a Wyner type linear network. The second layer is the transmission layer between SBs and mobile terminals (MTs), which is modeled as a linear Wyner LT network. The SBs are assumed to be half-duplex, thus restricting the per user degrees of freedom (puDoF) in the system to 1/2. It is established that the optimal puDoF of 1/2 can be achieved in the linear network with sufficient number of antennas using only interference avoidance schemes. For the case of higher connectivity in the transmission layer, it is shown that the optimal puDoF is achieved by sending an appropriate linear combination to the SB to zero-force interference at the intended user. These results are also extended to a more realistic hexagonal cellular model