Ο αριθμός των συνδρομητών κινητής τηλεφωνίας έχει αυξηθεί σημαντικά τα τελευταία χρόνια. Σε μεγάλο βαθμό οι φωνητικές υπηρεσίες εξυπηρετούνται από κινητά δίκτυα, ενώ παράλληλα αυξήθηκε σε μεγάλο βαθμό η χρήση των δεδομένων στα δίκτυα εκείνα που εφαρμόστηκε το 3GPP High Speed Packet Access (HSPA), αποδεικνύοντας ότι οι χρήστες επιδοκιμάζουν τη χρήση δεδομένων που παρέχονται ασύρματα με ευρυζωνικές ταχύτητες. Ο μέσος αριθμός χρήσης δεδομένων ξεπερνά τα εκατοντάδες megabytes ανά χρήστη κάθε μήνα. Τα ασύρματα δίκτυα πρέπει να αυξήσουν τις ταχύτητες μετάδοσης δεδομένων έτσι ώστε να πλησιάσουν εκείνες της ενσύρματης επικοινωνίας. Οι χρήστες είναι συνηθισμένοι να χρησιμοποιούν ενσύρματα δίκτυα και έτσι περιμένουν από τα ασύρματα δίκτυα να προσφέρουν συγκρίσιμες αποδόσεις με χαμηλό κόστος μετάδοσης δεδομένων. Το 3GPP Long Term Evolution (LTE) είναι σχεδιασμένο να επιτύχει αυτούς τους στόχους. Η τεχνολογία LTE προσφέρει κλιμακωτό εύρος ζώνης (απο 1.25 έως 20 MHz) με ρυθμούς μετάδοσης στα 100 Mbps για τον κατερχόμενο σύνδεσμο και στα 50 Mbps για τον ανερχόμενο. Αυτές οι πτυχές συνοδευόμενες από την τεχνολογία πρόσβασης που χρησιμοποιείται, η οποία είναι η OFDM (Orthogonal frequency division multiplexing), βελτιώνουν τη ρυθμαπόδοση του χρήστη και τη χωρητικότητα, ενώ μειώνουν τις καθυστερήσεις προσφέροντας παράλληλα βελτιωμένες συνθήκες κατά την κινητικότητα του χρήστη. Η OFDM προσφέρει επίσης μεγαλύτερη ανέχεια σε φαινόμενα όπως η εξασθένιση και το multipath σε σύγκριση με τεχνολογίες που εφαρμόζονταν σε προηγούμενα δίκτυα. Επίσης, είναι σημαντικό να αναφερθεί ότι το LTE χρησιμοποιεί τη μεταγωγή πακέτων και χρησιμοποιεί την τεχνολογία πολλαπλών κεραιών καθώς επίσης πολυπλεξία στο επίπεδο του χρόνου και της συχνότητας. Τέλος, υποστηρίζει unicast και multicast μετάδοση, τόσο σε microcell (κελιά μικρότερου εύρους) όσο και macrocell (μεγαλύτερα κελιά) περιβάλλον. Το αντικείμενο που μελετάται σε αυτή τη διπλωματική είναι το πρόβλημα του περιορισμού των παρεμβολών οι οποίες επηρεάζουν σε μεγάλο βαθμό την απόδοση των LTE συστημάτων. Ιδιαίερη μελέτη γίνεται όσο αφορά στην Inter Cell παρεμβολή και στις Inter Cell Interference Coordination τεχνικές. Συγκεκριμένα, η έρευνα εστιάζει στη βελτίωση της απόδοσης των χρηστών μειώνοντας την παρεμβολή μέσω διαφόρων σχημάτων επαναχρησιμοποίησης συχνότητας. / The number of mobile subscribers has increased tremendously in recent years. Voice communication has become mobile in a massive way and the mobile is the preferred way for voice communication. At the same time the data usage has grown fast in those networks where 3GPP High Speed Packet Access (HSPA) was introduced indicating that the users find value in broadband wireless data. The average data consumption exceeds hundreds of Megabytes per subscriber per month. Wireless networks must make data rates higher in order to match the user experience provided by wireline networks. When customers are used to wireline performance, they expect the wireless network to offer comparable performance with low cost of data delivery. 3GPP Long Term Evolution (LTE) is designed to meet those targets. LTE technology offers scalable bandwidth (from 1.25 up to 20 MHz), with transmission rates of 100 Mbps in downlink and 50 Mbps in uplink. These aspects accompanied with the access technology used, which is the OFDM (Orthogonal frequency division multiplexing), improves end-user throughputs, sector capacity and reduces user plane latency, bringing significantly improved user experience with full mobility. OFDM also offers bigger tolerance in phenomena such as multipath and fading compared to technologies used in previous mobile networks. It is also important to be mentioned that LTE is fully packet switched and uses multiple antenna techniques along with FDD and TDD duplexing. Finally, it supports unicast and multicast transmission, in both microcell and macrocell environment. The subject studied in this thesis is the problem of mitigating Interferences which dramatically affects the performance of LTE system. Extensive study is done concerning Inter Cell Interference and Inter Cell Interference Coordination techniques. In particular, research focuses in enhancing users’ performance by reducing interference via varius schemes of frequency reuse.

Modern cellular systems feature increasingly dense base station deployments, augmented by multiple tiers of access points, in an effort to provide higher network capacity as user traffic, especially data traffic, increases. The primary limitation of these dense networks is co-channel interference. The primary source of interference is inter-cell and cross-tier interference, which is especially limiting for users near the boundary of the cells. Inter-cell interference coordination (ICIC) is a broad umbrella term for strategies to improve the performance of the network by having each cell allocate its resources such that the interference experienced in the network is minimized, while maximizing spatial reuse. Fractional frequency reuse (FFR) has been proposed as an ICIC technique in modern wireless networks. The basic idea of FFR is to partition the cell’s bandwidth so that (i) cell-edge users of adjacent cells do not interfere with each other and (ii) interference received by (and created by) cell-interior users is reduced, while (iii) improving spectral reuse compared to conventional frequency reuse. It is attractive for its intuitive implementation and relatively low network coordination requirements compared to other ICIC strategies including interference cancellation, network MIMO, and opportunistic scheduling. There are two common FFR deployment modes: Strict FFR and Soft Frequency Reuse (SFR). This dissertation identifies and addresses key technical challenges associated with fractional frequency reuse in modern cellular networks by utilizing an accurate yet tractable model of both the downlink (base station to mobile) and uplink (mobile to base station) based on the Poisson point process for modeling base station locations. The resulting expressions allow for the development of system design guidelines as a function of FFR parameters and show their impact on important metrics of coverage, rate, power control, and spectral efficiency. This new complete analytical framework addresses system design and performance differences in the uplink and downlink. Also, this model can be applied to cellular networks with multiple tiers of access points, often called heterogeneous cellular networks. The model allows for analysis as a function of system design parameters for users under Strict FFR and SFR with closed and open access between tiers. / text

There has been a rapid growth in the data rate carried by cellular services, and this increase along with the emergence of new multimedia applications have motivated the 3rd Generation Partnership (3GPP) Project to launch Long-Term Evolution (LTE) [1]. LTE is the latest standard in the mobile network technology and is designed to meet the ubiquitous demands of next-generation mobile networks. LTE assures significant spectral and energy efficiency gains in both the uplink and down- link with low latency. Multiple access schemes such as Orthogonal Frequency Division Aultiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) which is a modified version of OFDMA have been recently adopted in 3GPP LTE downlink and uplink, respectively [1]. A typical feature of OFDMA is the decomposition of available bandwidth into multiple narrow orthogonal subcarriers. The orthogonality among subcarriers causes minimal intra-cell interference, however, the inter-cell interference (ICI) incurred on a given subcarrier is relatively impulsive and poses a fundamental challenge for the network designers. Moreover, as the number of interferers on a given subcarrier can be relatively limited it may not be accurate to model ICI as a Gaussian random variable by invoking the central limit theorem. The nature of ICI relies on a variety of indeterministic parameters which include frequency reuse factor, channel conditions, scheduling decisions, transmit power, and location of the interferers. This thesis presents a combination of algorithmic and theoretical studies for efficient modeling and management of ICI via radio resource management. In the preliminary phase, we focus on developing and analyzing the performance of several centralized and distributed interference mitigation and rate maximization algorithms. These algorithms relies on optimizing the spectrum allocation and user’s transmission powers to maximize the system capacity. Even though, the developed algorithms possesses low complexity, the simulation run-time may become challenging in the practical scenarios with very large number of users and subcarriers. Motivated by this fact, we then develop several statistical models that can accurately capture the dynamics of interference with distinct applications in the performance analysis of single carrier and multicarrier future wireless networks. The developed models can be customized for (i) various state-of-the-art coordinated and uncoordinated scheduling algorithms; (ii) slow and fast power control mechanisms; (iii) partial and fractional frequency reuse systems; and (iv) various composite fading distributions. The developed framework is useful in evaluating important system performance metrics such as outage probability, ergodic capacity, and average fairness numerically without the need of time consuming Monte-Carlo simulations. The theoretical framework is expected to enhance the planning tools for OFDMA based wireless networks by providing fast estimates of the typical performance metrics. Finally, we investigate and quantify the spectral and energy efficiency of two tier heterogeneous networks (HetNets) by employing power-control based interference mitigation technique. In particular, we analyze the performance of two tier HetNets deployment by deriving the theoretical bounds on the area spectral efficiency and exact analytical expressions for the energy efficiency by considering slow and fast power control mechanisms. The derived expressions are expected to be useful in providing insights for the design of efficient HetNet deployments.

Indoor parasitic cellular systems are in-building stand alone cellular networks that use the concept of simultaneously reusing the frequencies of cellular systems outside the building for wireless communications inside the building. The objective of this thesis is to provide an analysis to determine the frequency reuse possible between in-building and outside cellular systems. The amount of frequency reuse currently available for an urban office building is presented based on field strength measurements made inside the building. In addition, this thesis describes the simulation code written which models a growing cellular system for the purpose of analyzing the effect that a growing cellular system will have on in-building frequency reuse. Future in-building frequency reuse is predicted in three month intervals for a time period of six years based on the results of the simulation code. / Master of Science

LTE-A在靜態頻率重用的情形下，雖能有效減少干擾，但在UE (User Equipment)較為集中情形下，會有頻譜使用不足的情況。在傳統的傳輸模式中，若UE之間欲進行傳輸，通常需由傳輸端發送訊號給基地台，基地台再發送訊號給接收者，需要進行兩次的無線傳輸。若UE在彼此距離相近的環境中，D2D (Device-to-Device)傳輸可讓UE之間直接利用LTE-A頻譜進行傳輸，進而達到節省頻譜資源的效果。 本研究探討靜態頻率重用的缺點與位置相近的D2D傳輸模式，提出DFRDD (Dynamic Frequency Reuse for D2D transmission)方法，使用動態頻率重用與D2D傳輸技術。我們將一個細胞(cell)劃分為中央區域及外圍區域，外圍區域又劃分為三個扇形區，使用動態頻率重用的方法調整頻譜，在頻譜不足時，中央區域可使用外圍區域的頻譜，外圍區域最多可使用中央區域三分之一的頻譜。在使用D2D技術時，利用D2D UE與BS/RS (Base Station/Relay Station)的距離，計算出對基地台UE干擾較少的頻譜，進而提升傳輸效率。 實驗結果顯示，DFRDD利用動態頻率重用與D2D選擇頻譜的方法、在吞吐量方面表現得較H. S.Chae [18]、Bao [19]、Zhang [20]所提出的方法為佳。 / In the case of LTE-A static frequency reuse, although it can effectively reduce the interference, however, in the case of more dense UEs (User Equipment) environment, there will be problem of insufficient spectrum. In the traditional transmission method, if a pair of UEs want to communicate with each other, the transmitter sends a signal to the base station, the base station then sends a signal to the receiver, the signal need to to be wirelessly transmitted twice. If a pair of UEs are within a close distance, D2D (device-to-device) transmission allows users to communicate with each other directly using the same LTE-A spectrum to save spectrum resource. In order to improve LTE-A system performance, this paper proposes a DFRDD (Dynamic Frequency Reuse for D2D transmission) method. Using dynamic frequency reuse and D2D transmission, we divide a cell into center region and outer area, where the outer area is divided into three sectors. We use dynamic frequency reuse method to allocate spectrum. When the spectrum is insufficient, the center region can use the spectrum of the outer region. On the other hand, the outer area can use up to one third of the spectrum of the center region. When using D2D technic, we calculate the distance between D2D UE and the BS / RS (Base Station / Relay Station), choose the frequency that may reduce the interference of cellular UE and improve transmission efficiency. The experimental results show that DFRDD uses the method of dynamic frequency reuse and D2D to select the spectrum, which has better performance than those methods proposed by Chae [18], Bao [19] and Zhang [20].

Increasing energy consumption is a result of the rapid growth in cellular communication technologies and a massive increase in the number of mobile terminals (MTs) and communication sites. In cellular communication networks, energy efficiency (EE) and spectral efficiency (SE) are two of the most important criteria employed to evaluate the performance of networks. A compromise between these two conflicting criteria is therefore required, in order to achieve the best cellular network performance. Fractional frequency reuse (FFR), classed as either strict FFR or soft frequency reuse (SFR), is an intercell interference coordination (ICIC) technique applied to manage interference when more spectrum is used, and to enhance the EE. A conventional cellular model's downlink is designed as a reference in the presence of inter-cell interference (ICI) and a general fading environment. Energy-efficient cellular models,such as cell zooming, cooperative BSs and relaying models are designed, analysed and compared with the reference model, in order to reduce network energy consumption without degrading the SE. New mathematical models are derived herein to design a distributed antenna system (DAS), in order to enhance the system's EE and SE. DAS is designed in the presence of ICI and composite fading and shadowing with FFR. A coordinate multi-point (CoMP) technique is applied, using maximum ratio transmission (MRT) to serve the mobile terminal (MT), with all distributed antenna elements (DAEs), transmit antenna selection (TAS) being applied to select the best DAE and general selection combining (GSC) being applied to select more than one DAE. Furthermore, a Cloud radio access network (C-RAN) is designed and analysed with two different schemes, using the high-power node (HPN) and a remote radio head (RRH), in order to improve the EE and SE of the system. Finally, a trade-off between the two conflicting criteria, EE and SE, is handled carefully in this thesis, in order to ensure a green cellular communication network.

Multimedia services over wireless networks are getting popular. With multicast many mobile stations can join the same video multicast group and share the same radio resource to increase frequency utilization efficiently. However users may locate at different positions so as to suffer different path loss, interference and receive different signal to interference and noise ratio (SINR). Users at the cell-edge receiving lower SINR may degrade the multicast efficiency. In this thesis we propose four schemes considering fractional frequency reuse (FFR) over relay networks to reuse frequency in multi-cells. With fractional frequency reuse, users close to the base station (BS) have more resources to improve the total frequency utilization. A resource allocation scheme is also proposed to efficiently allocate wireless resources. Compared to the conventional relay scheme, the proposed schemes can provide more than 10% video layers for all users and give better video quality for users near BS.

With the popularity of wireless networks, it needs to support user¡¦s mobility cross different base stations, hence, the handover mechanism becomes an important issue. When the user frequently moves between two cells, it will occur the Ping-Pong effect that increases the delay time and reduces the efficiency of system. In this thesis, we proposed a new handover mechanism by considering the fractional frequency reuse (FFR) over relay networks to reuse frequency in multi-cells. The proposed method can reduce the unnecessary handover caused by the interference in the system of FFR. It uses the value of signal to interference and noise ratio (SINR) and the parameter of distance to make handover decision. The simulation results indicate the proposed handover mechanism can reduce more than 8% of the handover number in average in comparison to the competing method in the best case.

The thesis is concerned with capacity and coverage enhancement of OFDMA heterogeneous cellular systems with a specific focus on fractional frequency reuse (FFR), femtocells and amplify-and-forward (AF) relay systems. The main aim of the thesis is to develop new mathematical analysis for the spectral efficiency and outage probability of multi-cells multi-tier systems in diverse traffic, interference and fading scenarios. In the first part of the thesis, a new unified mathematical framework for performance analysis of FFR and soft frequency reuse (SFR) schemes is developed. This leads to new exact expressions of FFR and SFR area spectral efficiency in downlink and uplink scenarios which account for a mixture of frequency reuse factors in a homogeneous cellular system. The mathematical framework is extended to include modelling and performance analysis of FFR systems with elastic data traffic. Further analysis is carried out in relation to the performance of FFR and/or SFR schemes, in terms of energy efficiency and base station cooperation. The new proposed analytical framework can lead to a better understanding and computationally efficient performance analysis of next generation heterogeneous cellular systems. Next generation cellular systems are characterized by an increase in the spatial node density to improve the spectral efficiency and coverage, especially for users at home and at the cell edges. In this regard, relays and femtocells play a major role. Therefore, relays and femocells are the focus of the second part of the thesis. Firstly, we present a new and unified spectral efficiency analysis in dual-hop fixed-gain AF relay systems over generalised interferences models. The generalised interference models are either based on the Nakagami-m fading with arbitrary distance or on spatial Poisson Point Process in case of randomly deployed heterogeneous interferers. The models have been considered separately in the open literature due to the complexity of the mathematical analysis. Secondly, the outage probability is utilised to deduce the femtocell exclusion region for FFR system and a new static resource allocation scheme is proposed for femtocells which improve the capacity. The work presented in the thesis has resulted in the publication of seven scientific papers in prestigious IEEE journals and conferences.

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