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The steep growth in mobile data traffic has gained a lot of attention in recent years. With current infrastructure deployments and radio resources, operators will not be able to cope with the upcoming demands. Consequently, discussions of the next generation of mobile networks, referred to as the fifth generation (5G), have started in both academia and industry. In addition to more capacity, stringent requirements for improving energy efficiency, decreasing delays, and increasing reliability have been envisioned in 5G. Many solutions have been put forward, one of them being device-to-device (D2D) communications where users in close proximity can transmit directly to one another bypassing the base station (BS). In this thesis, we identify trade-offs and challenges of integrating D2D communications into cellular networks and propose potential solutions. To maximize gains from such integration, resource allocation and interference management are key factors. We start by introducing cooperation between D2D and cellular users in order to minimize any interference between the two user types and identifying the scenarios where this cooperation can be beneficial. It is shown that an increase in the number of cellular users within the coverage area and in the size of the cell is associated with a higher probability of cooperation. With this cooperation, we can potentially increase the number of connected devices, reduce the delay, increase the cell sum rate, and offload an overloaded cell. Next, we consider D2D communications underlaying the uplink of cellular networks. In such a scenario, any potential gain from resource sharing (time, frequency, or space) is determined by how the interference is managed. The quality and performance of the interference management techniques depend on the availability of the channel state information (CSI) and the location of nodes as well as the frequency of updates regarding such information. The more information is required, the more signaling is needed, which results in higher power consumption by the users. We investigate the trade-off between the availability of full CSI, which necessitates instantaneous information, and that of limited CSI, which requires infrequent updates. Our results show that with limited CSI, a good performance (in terms of the sum rate of both user types) can be achieved if a small performance loss is tolerated by cellular users. In addition, we propose a novel approach for interference management which only requires the information on the number of D2D users without any knowledge about their CSI. This blind approach can achieve a small outage probability with very low computational complexity when the number of scheduled D2D users is small. We then study the problem of mode selection, i.e., if a user should transmit in the D2D mode or in the conventional cellular mode. We identify the decision criteria for both overlay and underlay scenarios with two different objectives. We find out that the D2D communication is beneficial in macro cells or at cell boundaries. The area in which D2D mode is optimal varies with the objective of the network, transmit power, required quality-of-service, and the number of BS antennas. In the second part of this thesis, we study the effects of integration and coexistence of underlay D2D communications with another promising technology proposed for 5G, namely massive multiple-input-multiple-output (MIMO). Potential benefits of both technologies are known individually, but the possibility of and performance gains from their coexistence are not adequately addressed. We evaluate the performance of this hybrid network in terms of energy efficiency and the average sum rate. Comprehensive analysis reveals that the performance highly depends on the D2D user density. We conclude that underlay D2D communications can only coexist with massive MIMO systems in the regime of low D2D user density. By introducing a high number of D2D users, gains from the massive MIMO technology degrade rapidly, and therefore in this case, the D2D communications should use the overlay approach rather than the underlay, or the network should only allow a subset of D2D transmissions to be active at a time. Den stora ökningen i mobildatatrafik de senaste åren har tilldragit sig mycket intresse. Med nuvarande infrastruktur och radioresurser kommer inte mobiloperatörerna att kunna hantera de kommande kraven. Därför har diskussioner kring den femte generationens (5G) mobila nätverk startat inom både akademin och industrin. Utöver högre kapacitet så kommer strikta krav på ökad energieffektivitet, minskad fördröjning samt ökad tillförlitlighet att planeras för 5G. En av många lösningar som har föreslagits är enhet-till-enhetskommunikation (device-to-device communications, D2D, på engelska), vilket innebär att närliggande mobilanvändare kan sända direkt till varandra utan att gå genom basstationen. I denna avhandling identifierar vi kompromisser och problem kring, samt föreslår lösningar för, integrering av D2D-kommunikation i cellulära nätverk. Viktiga faktorer för att maximera vinsten av sådan integrering är resursallokering och störningshantering. Avhandlingen börjar med att beskriva samarbetet mellan D2D- och cellulära användare för att minska störningen mellan de två användartyperna, samt för att identifiera scenarier där denna typ av samarbete kan vara fördelaktigt. Vi visar att samarbetssannolikheten ökar med antalet cellulära användare i täckningsområdet, samt när cellstorleken ökar. Denna typ av samarbete kan användas för att öka antalet ansluta enheter, minska fördröjningen, öka cellsummadatataken eller avlasta överlastade celler. Härnäst studerar vi D2D-kommunikation underliggande upplänken i cellulära nätverk. I ett sådant scenario bestäms eventuell vinst från resursdelning (t.ex. i tid, frekvens eller rymd) av hur störningen hanteras. Kvaliteten och prestandan hos störningshanteringen beror på tillgängligheten av kanalkännedom och information om nodernas position, samt uppdateringsfrekvensen för dessa. Ju mer information som behövs, desto mer signalering krävs, vilket leder till högre effektförbrukning hos användarna. Vi undersöker kompromissen mellan fullt tillgänglig kanalkännedom, vilket kräver momentan information, och ett scenario där kanalkännedomen är begränsad, vilket enbart kräver uppdatering med låg frekvens. Våra resultat visar att god summadatatakt kan uppnås när enbart begränsad kanalkännedom är tillgänglig, om en liten prestandaförlust tillåts för cellulära användare. Vi föreslår dessutom en ny metod för störningshantering som enbart kräver information om antalet D2D-användare, utan vetskap om deras kanalkännedom. Denna blinda metod kan uppnå hög täckningssannolikhet med låg beräkningskomplexitet när antalet schemalagda D2D-användare är lågt. Vi studerar även lägesvalsproblemet, dvs. om en användare ska sända i D2D-läge eller i konventionellt cellulärt läge. Vi karaktäriserar beslutskriterierna för både överliggande och underliggande scenarier med två olika objektivfunktioner och visar att D2D-kommunikation är fördelaktig i makroceller samt vid cellkanterna. Området för D2D-optimalitet varierar med objektivfunktionen för nätverket, sändeffekten, servicekvalitetskraven och antalet basstationsantenner. I den andra delen av avhandlingen så studerar vi effekter kring integrering och samexistens av underliggande D2D-kommunikation med en annan lovande teknologi för 5G, nämligen massiv multiple input-multiple output (massiv MIMO). De individuella fördelarna för de två teknologierna är välkända, men eventuella prestandavinster när teknologierna samexisterar har inte studeras tillräckligt. Vi undersöker prestanda i detta hybridnätverk i termer av energieffektivitet och genomsnittlig summadatatakt. En noggrann analys visar att prestandan beror på tätheten av D2D-användare. Vi drar slutsatsen att underliggande D2D-kommunikation bara kan samexistera med massiv MIMO när tätheten av D2D-användare är låg. När det existerar många D2D-användare minskas prestandavinsten från massiv MIMO snabbt och därför bör D2D-kommunikationen ske i överliggande läge istället för underliggande läge. Alternativt kan nätverket tillåta att enbart en delmängd av D2D-sändningar är aktiva samtidigt. QC 20150529

The steep growth in the mobile data traffic has gained a lot of attention in recent years. This growth is mainly the result of emerging applications, multimedia services, and revolutions in the device technology. With current deployments and radio resources, operators will not be able to cope with the growing demands. Consequently, there is a need to either provide new resources or increase the efficiency of what is available. Proposed solutions for accommodating growing data traffic are based on improvements in three dimensions: efficient use of radio resources especially the spectrum, technology advancements, and densifying the current infrastructure. In this thesis, we focus on the spectrum dimension. Providing more spectrum is a long-term process. However, increasing the spectrum usage and efficiency can be put rapidly in practice. We discuss potential solutions in the area of spectrum sharing. Among enabling technologies to facilitate spectrum sharing, we consider the cognitive radio and device-to-device (D2D) communications. In order to gain from sharing the spectrum, systems need to somehow deal with extra sources of interference. In the first part of the thesis, we consider a primary-secondary sharing model in cognitive radio networks. We employ the cooperative communication method in order to facilitate the access of the secondary system to the licensed spectrum of the primary system, and therefore increase the spectrum usage. The cooperation between the two systems is formed provided that it is beneficial for the primary system. In this way, the primary users' quality-of-service can be preserved while at the same time the secondary users can access the spectrum. This cooperative approach prevents both systems from concurrent transmissions. As a consequence, the need for interference control techniques are eliminated. We evaluate different models and transmission schemes and optimize the corresponding parameters to quantify the gain resulting from cooperative spectrum sharing. In the second part of the thesis, we consider spectrum sharing within one system between different types of users. This is done in the context of D2D communications where close proximity users can transmit directly to each other. For this type of communications, either dedicated resources are allocated or resources of the cellular users are reused. We first study the feasibility of cooperation between D2D and cellular users and identify the scenarios where it can be beneficial. Then we take on a challenging problem which guarantees the gain from the D2D communication, namely the mode selection. For this problem, we characterize the decision criteria that determines if D2D communication is gainful. Next, we focus on the problem of interference in D2D communications underlaying cellular networks, where the same spectrum is reused in the spatial domain. In such scenarios, the potential gain is determined by how the interference is managed, which in turn depends on the amount of available information at the base station. The more information is required, the more signaling is needed. In this part of the thesis, we address the trade-off between the signaling overhead and the performance of the system and propose a novel approach for interference control which requires very little information on the D2D users. QC 20140509