RT Dissertation/Thesis
T1 Blind interference alignment for cellular networks
A1 Morales Céspedes, Máximo
AB Managing the interference is the main challenge in cellular networks. Multiple-InputMultiple-Output (MIMO) schemes have emerged as a means of achieving high-capacityin wireless communications. The most efficient MIMO techniques are based on managingthe interference instead of avoiding it by employing orthogonal resource allocationschemes. These transmission schemes require the knowledge of the Channel State Information at the Transmitter (CSIT) to achieve the optimal Degrees of Freedom (DoF),also known as multiplexing gain. Providing an accurate CSIT in cellular environmentsinvolves high-capacity backhaul links and accurate synchronization, which imply the useof a large amount of network resources. Recently, a Blind Interference Alignment (BIA)scheme was devised as a means of achieving a growth in DoF regarding the amount ofusers served without the need for CSIT in the Multiple-Input Single-Output (MISO)Broadcast Channel (BC). It is demonstrated that BIA achieves the optimal DoF inthe BC without CSIT. However, the implementation of BIA in cellular networks is notstraightforward. This dissertation investigates the DoF and the corresponding sum-rateof cellular networks in absence of CSIT and their achievability by using BIA schemes.First, this dissertation derives the DoF-region of homogenous cellular networks withpartial connectivity. Assuming that all the Base Stations (BSs) cooperate in order totransmit to all users in the network, we proposed an extension of the BIA scheme for theMISO BC where the set of BSs transmits as in a network MIMO. It is shown that thecooperation between BSs results futile because of the lack of full connectivity in cellularnetworks. After that, this dissertation presents several transmission schemes based onthe network topology. By differentiating between users that can treat this interferenceoptimally as noise and those who need to manage the interference from neighbouringBSs, a network BIA scheme is devised to achieve the optimal DoF in homogeneouscellular networks. Second, the use of BIA schemes is analyzed for heterogeneous cellular networks. It is demonstrated that the previous BIA schemes based on the network topology result nonoptimal in DoF because of the particular features of the heterogenous cellular networks. More specifically, assuming a macro-femto network, cooperation between both tiers leads to a penalty for macro users while femto users do not exploit the particular topology of this kind of network. In this dissertation, the optimal linear DoF (lDoF) in a two-tier network are derived subject to optimality in DoF for the upper tier. It is demonstratedthat, without CSIT or any cooperation between tiers, the lower tier can achieve nonzeroDoF while the upper tier attains the optimal DoF by transmitting independently ofthe lower tier deployment. After that, a cognitive BIA scheme that achieves this outerbound is devised for macro-femto cellular networks.The third part of this dissertation is focused on the implementation of BIA in practicalscenarios. It is shown that transmission at limited SNR and coherence time are themain hurdles to overcome for practical implementations of BIA. With aim of managingboth constraints, the use of BIA together with orthogonal approaches is proposed in thiswork. An improvement on the inherent noise increase of BIA and the required coherencetime is achieved at expenses of losing DoF. Therefore, there exists a trade-off betweenmultiplexing gain, sum-rate at finite SNR and coherence time in practical scenarios. Theoptimal resource allocation for orthogonal transmission is obtained after solving a veryspecific optimization problem. To complete the characterization of the performance ofBIA in realistic scenarios a experimental evaluation based on a hardware implementationis presented at the end of this work. It is shown that BIA outperforms the sum-rateof schemes based on CSIT such as LZFB because of the hardware impairments and thecosts of providing CSIT in a realist implementation.
YR 2015
FD 2015-11
LK https://hdl.handle.net/10016/22599
UL https://hdl.handle.net/10016/22599
LA eng
NO Mención Internacional en el título de doctor
DS e-Archivo
RD 6 ago. 2024