Cooperative interference and radio resource management in self-organizing small cell networks

Abstract The aim of this thesis is to devise novel cooperative paradigms that allow small base stations (SBSs) to perform joint optimizations in the field of interference and spectral resource management, in an automated, self-organizing way. Fostering cooperation requires a careful negotiation process and incurs additional operational costs that can ultimately limit the scalability of such an approach. Hence, we design cooperative models around the self-organizing functions of small cell networks, through which each SBS dynamically adapts its own cooperative strategy to the current network status. In the first part of the thesis, we study the co-tier interference in the downlink of an underlay small cell tier. Showing that mutual interference is a limiting factor, we propose a cooperative scheme based on the concept of interference alignment. Thereby, the SBSs autonomously devise their cooperative strategy, select partner small base stations and suppress the mutual interference via alignment. In the second part, we extend the above problem by considering additional constraints posed by non-cooperative macro cell users sharing the spectrum with the SBSs. Due to the dimension of such a problem, we show that interference alignment solutions do not often exist, hence, we propose a distributed solution based on the concept of interference draining for solving the interfering signals in the space-frequency domain. Finally, we focus on inter-tier cooperation and propose an adaptive hybrid access policy based on spectrum leasing, which jointly enhances the SBSs' transmission capacity and the macro cell users' QoS . In the studied scenarios, we highlight how SBSs' individual performance is network-dependent, due to the shared nature of the spectrum and the network infrastructure. For addressing such issues, we use novel concepts from coalitional games in partition form in which the strategic decisions are optimized by accounting for external effects, such as interference or dynamic spectrum allocation. Based on the properties of optimality of coalitional games in partition form, the proposed cooperative solutions show two key features. First, by leveraging on decentralized strategic decisions, complex interference management techniques, such as interference alignment or draining, can be implemented in practical, scalable way. Second, although notable cooperative solutions exists for certain problem dimensions, the proposed algorithms are shown to be effective for a wide range of network sizes, by achieving significant data rate enhancement and low overhead.Tiivistelmä Työssä käsitellään merkittäviä häiriön ja radioresurssien hallintaongelmia ja ehdotetaan käytännöllisiä yhteistoiminnallisuusmalleja, jotka tuottavat merkittäviä suorituskykyparannuksia, ja samaan aikaan ottavat huomioon yhteistyön kustannukset ja strategisten päätösten vaikutuksen piensolujen keskinäiseen suorituskykyyn. Työn ensimmäisessa osassa tutkitaan makro- ja piensoluverkon välistä häiriötä laskevalla siirtosuunnalla. Koska keskinäishäiriö osoittautuu merkittäväksi rajoittavaksi tekijäksi, ehdotetaan yhteistyömenetelmää, joka perustuu ns. häiriön laskostamiseen. Tällaisessa menetelmässä pientukiasemat päättävät itsenäisesti yhteistyöstrategiansa ja valitsevat ne tukiasemat, joiden kanssa tehdään yhteistyötä ja vaimennetaan keskinäishäiriötä laskostamalla. Työn toisessa osassa edellämainittua ongelmaa laajennataan ottamalla huomioon rajoitteet, jotka aiheutuvat samoja taajuusresursseja jakavista, mutta piensolutukiasemien kanssa yhteistyötä tekemättömistä makrosoluista. Ongelman laajuudesta havaitaan, että yleensä häiriön laskostusratkaisua ei löydy tällaisessa ongelmassa. Niinpä ehdotetaan hajautettua häiriön kohdennukseen perustuvaa ratkaisua, jossa häiriösignaalit lomitellaan tila-taajuustasossa. Työn viimeisessä osassa keskitytään eri tyyppisten tukiasemasolujen kesken käytävään yhteistyöhön ja ehdotetaan mukautuvaa taajuusspektrin vuokraamiseen perustuvaa hybridi-käyttöoikeusmenetelmää, joka parantaa piensolujen kapasiteettia ja makrosolukäyttäjien palvelun laatua. Kaikissa tutkituissa menetelmissä korostetaan kuinka yksittäisen pientukiaseman suorituskyky on verkosta riippuvainen johtuen jaetusta taajuuskaistasta ja verkkoinfrastuktuurista. Näissä skenaarioissa käytetään uusia ositukseen perustuvia ns. liittoumapelikonsepteja. Tämän tyyppisissä peleissä strategiset päätökset on optimoitu ottamalla huomioon ulkoiset tekijät kuten häiriö ja dynaaminen taajuuden allokointi. Ositukseen perustuvien liitoumapelien optimaalisuus- ja stabiilisuusominaisuuksien pohjalta ehdotetaan hajautettuja algoritmeja tutkittuihin ongelmiin. Ehdotetut yhteistoiminnalliset ratkaisut osoittavat kaksi avaintekijää. Ensinnäkin käyttämällä hajautettuja strategisia päätöksiä kompleksiset häiriönhallintatekniikat, kuten häiriön lomittelu ja kohdennus, voidaan toteuttaa käytännöllisesti siten, että hyöty-kustannussuhde on tasapainossa. Toiseksi, vaikka merkittäviä yhteistyöratkaisuja on olemassa tietyn dimensioisiin ongelmiin, ehdotetut algoritmit ovat osoittautuneet tehokkaiksi hyvin erikokoisissa verkoissa. Samalla on saavutettu merkittäviä datanopeusparannuksia ja hyvä skaalautuvuus

On the Dynamic Formation of Cooperative Multipoint Transmissions in Small Cell Networks

The ever-growing data rate requirements of next-generation mobile networks mandate a highly efficient exploitation of the spectral and energy resources. In this respect, a key technology which can allow high quality-of-service provisioning with reasonable energy expenditures is given by the deployment of Coordinated Multi-point (CoMP) transmissions for small cells. While the benefits of CoMP have been explored in the literature, little has been done to study how a network can adaptively decide on whether or not to adopt CoMP. To this end, in this paper, we propose a cooperative framework for small cell networks, in which groups of small cells dynamically decide when to perform CoMP transmission while optimizing the tradeoff between rate improvement and energy consumption. We formulate the problem as a dynamic coalitional game with the small base stations as players, and we provide a distributed algorithm that allows the small cells to self-organize into CoMP-based coalitions. The resulting network partition represents the \u3f5-core of the game, which is a key solution concept for dynamic coalition games. Simulation results show that the proposed dynamic coalition formation algorithm leads to a significant gain of up to 41% sum rate to transmit power ratio in the small cell tier

"A Mode Identification System For a Reconfigurable Terminal Using Wigner Distribution and Non-parametric Classifiers"

In this work, a mode identification system for superimposed signals in the same band is presented. More precisely, a pattern recognition approach, based on Wigner Distribution for features extraction and non parametric classifiers (k-Nearest Neighbors and Parzen) is proposed for identifying the transmission modes in an indoor wireless environment A reconfigurable terminal based on Software Defined Radio technology is considered aiming at the identification of the presence of two co-existent communication modes such as Bluetooth, based on Frequency Hopping - Code Division Multiple Access, and IEEE WLAN 802.11b, based on Direct Sequence - Code Division Multiple Access. Results in terms of error classification probability, expressed as relative error frequency, will be provided with a comparison between the use of the two classifiers

A self-organizing solution for interference avoidance in TDD underlay femtocells

In this paper, we draw our attention to the problem of interference avoidance in a macrocell-femtocell network. Further, macro user equipments operate in a Frequency Division Duplexing mode while all femtocells employ the macrocellular uplink spectrum in a Time Division Duplexing mode. We propose a self-adapting algorithm to tackle highly interfering MUEs through handover procedure. Results reveal that the proposed solution provides lower outage probability than closed and open access policies, without significant loss in terms of signal-to-noise-and-interference ratio

Enabling Macrocell-Femtocell Coexistence through Interference Draining

Underlay femtocells promise to enhance the coverage and rate performance of next-generation wireless networks. Nevertheless, many concerns still remain in the context of shared-spectrum operations and quality of service (QoS) provisioning at the macrocell users. In this paper, we introduce a novel approach for cooperative femtocell-to-femtocell interference management based on interference draining. Accordingly, a group of femtocells can decide to cooperate and improve their downlink rate, by exploiting the available frequency-spatial directions, and still guarantee a minimum target QoS at the closest MUEs. To address this problem, we use tools from cooperative game theory that enable the femtocells to decide, in a distributed manner, on whether to cooperate or not. In the proposed framework, each femtocell access point individually decides its own cooperative strategy, and maximizes a utility function that captures the cooperative gains and the limitations due to the macrocell users QoS targets. We show that, using the proposed approach, the femtocells can self-organize into a network partition composed of disjoint groups of femtocells which form the recursive core of the cooperative game. Simulation results show significant gains in terms of average payoff per femtocell, reaching up to 23%, for a network of K = 140 FBSs, relative to the non-cooperative approach

Coalition Formation Games for Femtocell Interference Management: A Recursive Core Approach

Overlaying low-power, low-cost, femtocells, over existing wireless networks has recently emerged as a means to significantly improve the coverage and performance of next-generation wireless networks. While most existing literature focuses on spectrum sharing and interference management among non-cooperative femtocells, in this paper, we propose a novel cooperative model that enables the femtocells to improve their performance by sharing spectral resources, minimizing the number of collisions, and maximizing the spatial reuse. We model the femtocell spectrum sharing problem as a coalitional game in partition form and we propose a distributed algorithm for coalition formation. Using the proposed algorithm, the femtocells can take autonomous decisions to cooperate and self-organize into a network partition composed of disjoint femtocell coalitions and that constitutes a stable partition which lies in the recursive core of the considered game. Whenever a coalition forms, the femtocells inside this coalition can cooperatively pool the occupied spectral resources. Additionally, the members of any given coalition jointly schedule their transmissions in order to avoid collisions, in a distributed way. Simulation results show that the proposed coalition formation algorithm yields a performance advantage, in terms of the average payoff (rate) per femtocell reaching up to 380% relative to the non-cooperative case