54 research outputs found

    Spatial spectrum reuse in heterogeneous wireless networks: interference management and access control

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    Διαχρονικά, η κυρίαρχη σχεδιαστική επιλογή για την βελτίωση της φασματικής απόδοσης των ασύρματων δικτύων κινητών επικοινωνιών είναι η χωρική επαναχρησιμοποίηση φάσματος, η δυνατότητα δηλαδή να επαναχρησιμοποιείται το ίδιο κομμάτι φάσματος πολλές φορές στο χώρο με την προϋπόθεση ότι διατηρούνται χαμηλά τα επίπεδα των παρεμβολών. Στα σύγχρονα δίκτυα κινητών επικοινωνιών μελετώνται δύο νέοι τρόποι χωρικής επαναχρησιμοποίησης φάσματος: α) η ανάπτυξη φεμτοκυψελών (femtocells), η ανάπτυξη δηλαδή μικρών κυψελών για εξυπηρέτηση κυρίως εσωτερικών χώρων στην ευρύτερη περιοχή κάλυψης μία κύριας κυψέλης, και β) η ενεργοποίηση επικοινωνιών συσκευής-σε-συσκευή (Device-to-Device – D2D), απευθείας δηλαδή επικοινωνιών χωρίς την διαμεσολάβηση του σταθμού βάσης της κυψέλης. Σκοπός της παρούσας διατριβής είναι να μελετηθούν και να αντιμετωπιστούν οι προκλήσεις που προκύπτουν από την εισαγωγή φεμτοκυψελών και την υιοθέτηση επικοινωνιών συσκευής-σε-συσκευή σε κυψελωτά δίκτυα προτυποποιημένα από την 3GPP (3rd Generation Partnership Project). Πιο συγκεκριμένα, μελετώνται τα προβλήματα της διαχείρισης του φάσματος και των παρεμβολών, καθώς και θέματα πρόσβασης στο φάσμα για Long Term Evolution (LTE) και LTE-Advanced (LTE-A) δίκτυα με φεμτοκυψέλες και με επικοινωνίες συσκευής-σε- συσκευή. Για το σκοπό αυτό, αρχικά μελετήθηκαν τα LTE/LTE-A κυψελωτά δίκτυα ως προς το φυσικό επίπεδο, την αρχιτεκτονική και τις παρεμβολές, αποτυπώνοντας και την τρέχουσα κατάσταση στο τομέα της προτυποποίησης των φεμτοκυψελών και των επικοινωνιών συσκευής-σε-συσκευή. Ακολούθησε μια συγκριτική μελέτη μηχανισμών διαχείρισης παρεμβολών σε κανάλια ελέγχου ενός LTE/LTE-A δικτύου με φεμτοκυψέλες και ένας καινοτόμος μηχανισμός ελέγχου ισχύος για μεταδόσεις φεμτοκυψελών, βασισμένος στην ποιότητα εμπειρίας στο τελικό χρήστη. Η δουλειά αυτή άνοιξε νέους ερευνητικούς ορίζοντες, όπου το επίπεδο ικανοποίησης του τελικού χρήστη παίζει ενεργό ρόλο στη διαχείριση του δικτύου και την παροχή των υπηρεσιών. Παρόλα αυτά, η περεταίρω μελέτη προς την κατεύθυνση αυτή είναι εκτός του σκοπού της παρούσας διατριβής. Στην συνέχεια, το κύριο βάρος της μελέτης μεταφέρθηκε στο πρόβλημα της διαχείρισης του φάσματος και των παρεμβολών στο πολύ πιο δυναμικό περιβάλλον ενός κυψελωτού δικτύου όπου επιτρέπονται οι επικοινωνίες συσκευής-σε-συσκευή. Σε πρώτη φάση, θεωρήθηκε ένα σύνολο από ζεύγη συσκευών που επικοινωνούν μεταξύ τους με επικοινωνίες συσκευής-σε-συσκευή και προτάθηκε ένας μηχανισμός συλλογής πληροφορίας παρεμβολών και ένα σχήμα ανάθεσης πόρων βασισμένο στη θεωρία γράφων. Το κύριο αποτέλεσμα της μελέτης αυτής ήταν πως αν και υψηλά επίπεδα χωρικής επαναχρησιμοποίησης μπορούν να επιτευχθούν, η συλλογή και η επεξεργασία πληροφορίας παρεμβολών είναι ένα πολύπλοκο πρόβλημα το οποίο απαιτεί και επιπλέον πόρους σηματοδοσίας. Έτσι, προτάθηκε και αναλύθηκε μίας λύση βασισμένη στον ανταγωνισμό. Πρακτικά οι χρήστες των επικοινωνιών συσκευής-σε-συσκευή εφαρμόζουν ένα σχήμα ανταγωνισμού όμοιο με αυτό που χρησιμοποιείται στα δίκτυα WiFi (Wireless Fidelity), προσαρμοσμένο όμως στο φυσικό επίπεδο των LTE/LTE-A δικτύων. Μαθηματική ανάλυση του σχήματος έδειξε ισχυρή εξάρτηση των επιδόσεων από το πλήθος των χρηστών που ανταγωνίζονται για το φάσμα. Σε μια προσπάθεια περιορισμού του πλήθους των ανταγωνιζόμενων χρηστών μόνο σε αυτούς που βρίσκονται σε γειτνίαση, και άρα μονό σε αυτούς που η άμεση επικοινωνία τους είναι εφικτή, μελετήθηκε το πρόβλημα της ανίχνευσης γειτονικής συσκευής. Με βάση τις τρέχουσες προδιαγραφές της 3GPP, για την επίλυση του προβλήματος ανίχνευσης γειτονικής συσκευής, μία συσκευή είτε ανακοινώνει με μετάδοση περιοδικών μηνυμάτων την παρουσία της σε μια συγκεκριμένη περιοχή, είτε αιτείται από κάποια συγκεκριμένη συσκευή πληροφορία ανίχνευσης. Υιοθετώντας τη δεύτερη περίπτωση, προτάθηκαν βελτιώσεις στο LTE/LTE-A δίκτυο πρόσβασης ώστε να επιτρέπεται η ανάθεση φάσματος για μεταδόσεις ανίχνευσης γειτονικών συσκευών. Παράλληλα, δεδομένου ότι και για τις μεταδόσεις αυτές απαιτείται η κατανάλωση φάσματος, σχεδιάστηκε και αξιολογήθηκε μία λύση βασισμένη στη χωρική επαναχρησιμοποίηση φάσματος. Το βασικό συμπέρασμα ήταν ότι λόγω των χαμηλών απαιτήσεων ποιότητας των μηνυμάτων ανίχνευσης, κάτω από ορισμένες συνθήκες πυκνότητας του δικτύου, μπορεί να επιτραπεί η χωρική επαναχρησιμοποίηση του κυψελωτού φάσματος για μεταδόσεις ανίχνευσης συσκευής.Historically, the spatial spectrum reuse has been the most efficient approach for improving cellular system capacity. Based on this observation, the 3rd Generation Partnership Project (3GPP) has proposed new spatial spectrum reuse schemes, towards fulfilling the International Mobile Telecommunications-Advanced (IMT-Advanced) requirements for the 4G networks. In this direction, a major shift is realized from wide-range cells with high transmit power (macrocells) to low-power small-sized cells (femtocells), while a lot of effort is allocated to the spatial spectrum reuse by enabling Device-to-Device (D2D) communications, i.e., direct communications in a cellular network, without the intervention of the base station. The scope of this thesis is to deal with challenges arising from the introduction of femtocells and D2D communications in cellular networks standardized by 3GPP Release 8 and beyond, i.e., Long Term Evolution (LTE) and LTE-Advanced (LTE-A). More specifically, for the case of femtocells, the interference management problem is studied, while for the D2D communications the radio resource management and the spectrum access challenges are addressed. First, a comprehensive description of the physical layer and architecture of the LTE/LTE-A networks is provided, and the current standardization efforts for the introduction of femtocells and D2D communications are described. Subsequently, different control channel interference management schemes for femtocell-overlaid LTE/LTE-A networks are studied, while an innovative power control scheme for the femtocell downlink transmissions is proposed, utilizing the end user’s quality of experience. This work brings to the surface new research challenges, where the end user’s satisfaction level plays an active role in network management and service provisioning. However, the further investigation of these challenges is out of this thesis’ scope. Considering the much more dynamic environment defined by the D2D communications in a cellular network, the major research effort is then shifted to the resource and interference management problem for D2D communications. Assuming a predefined set of D2D pairs in a cellular network, an interference information collection mechanism and a D2D resource allocation scheme, based on the graph-coloring theory, are proposed. Evaluation results showed that even high spatial spectrum reuse levels can be achieved, the interference collection and processing problem is quite complex, while additional signaling is needed. Taking this into account, a contention-based approach is proposed. Under this approach, the D2D devices compete for accessing the spectrum following a procedure similar with that used in WiFi (Wireless Fidelity) networks. Performance analysis shows that the efficiency of the proposed scheme depends on the number of competing devices. Towards restricting the number of competing devices, only to those that are in proximity and, thus, in valid positions for D2D communication, the device discovery problem is studied. According to the 3GPP standardization efforts, the solution of the device discovery problem requires frequent transmission of discovery signals from each device, either announcing its presence in a specific area, or requesting discovery information from a target device. Adopting the second option, enhancements in the 3GPP standardized access network are proposed, enabling a resource request / allocation procedure for device discovery transmissions. In parallel, a spatial spectrum reuse scheme is designed and evaluated, as an effort to reduce the consumption of radio resources for discovery transmissions. Analytical and simulation results show that, under certain conditions for the network density, a number of discovery transmissions can be enabled in a multi-cellular network even if no interference information is available

    Cognition-inspired 5G cellular networks: a review and the road ahead

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    Despite the evolution of cellular networks, spectrum scarcity and the lack of intelligent and autonomous capabilities remain a cause for concern. These problems have resulted in low network capacity, high signaling overhead, inefficient data forwarding, and low scalability, which are expected to persist as the stumbling blocks to deploy, support and scale next-generation applications, including smart city and virtual reality. Fifth-generation (5G) cellular networking, along with its salient operational characteristics - including the cognitive and cooperative capabilities, network virtualization, and traffic offload - can address these limitations to cater to future scenarios characterized by highly heterogeneous, ultra-dense, and highly variable environments. Cognitive radio (CR) and cognition cycle (CC) are key enabling technologies for 5G. CR enables nodes to explore and use underutilized licensed channels; while CC has been embedded in CR nodes to learn new knowledge and adapt to network dynamics. CR and CC have brought advantages to a cognition-inspired 5G cellular network, including addressing the spectrum scarcity problem, promoting interoperation among heterogeneous entities, and providing intelligence and autonomous capabilities to support 5G core operations, such as smart beamforming. In this paper, we present the attributes of 5G and existing state of the art focusing on how CR and CC have been adopted in 5G to provide spectral efficiency, energy efficiency, improved quality of service and experience, and cost efficiency. This main contribution of this paper is to complement recent work by focusing on the networking aspect of CR and CC applied to 5G due to the urgent need to investigate, as well as to further enhance, CR and CC as core mechanisms to support 5G. This paper is aspired to establish a foundation and to spark new research interest in this topic. Open research opportunities and platform implementation are also presented to stimulate new research initiatives in this exciting area

    Opportunistic device-to-device communication in cellular networks: from theory to practice

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    Mención Internacional en el título de doctorCellular service providers have been struggling with users’ demand since the emergence of mobile Internet. As a result, each generation of cellular network prevailed over its predecessors mainly in terms of connection speed. However, the fifth generation (5G) of cellular network promises to go beyond this trend by revolutionizing the network architecture. Device-to-Device (D2D) communication is one of the revolutionary changes that enables mobile users to communicate directly without traversing a base station. This feature is being actively studied in 3GPP with special focus on public safety as it allows mobiles to operate in adhoc mode. Although under the (partial) control of the network, D2D communications open the door to many other use-cases. This dissertation studies different aspects of D2D communications and its impact on the key performance indicators of the network. We design an architecture for the collaboration of cellular users by means of timely exploited D2D opportunities. We begin by presenting the analytical study on opportunistic outband D2D communications. The study reveals the great potential of opportunistic outband D2D communications for enhancing energy efficiency, fairness, and capacity of cellular networks when groups of D2D users can be form and managed in the cellular network. Then we introduce a protocol that is compatible with the latest release of IEEE and 3GPP standards and allows for implementation of our proposal in a today’s cellular network. To validate our analytical findings, we use our experimental Software Defined Radio (SDR)-based testbed to further study our proposal in a real world scenario. The experimental results confirm the outstanding potential of opportunistic outband D2D communications. Finally, we investigate the performance merits and disadvantages of different D2D “modes”. Our investigation reveals, despite the common belief, that all D2D modes are complementary and their merits are scenario based.This work has been supported by IMDEA Networks Institute.Programa Oficial de Doctorado en Ingeniería TelemáticaPresidente: Douglas Leith.- Secretario: Albert Banchs Roca.- Vocal: Carla Fabiana Chiasserin

    Enabling Secure Direct Connectivity Under Intermittent Cellular Network Assistance

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    This work targets at investigating direct communications as a promising technology for the next-generation 5G wireless ecosystem that improves the degrees of spatial reuse and creates new opportunities for users in proximity. While direct connectivity has originally emerged as a technology enabler for public safety services, it is likely to remain in the heart of the 5G ecosystem by spawning a wide diversity of proximate applications and services. Direct communications couples together the centralized and the distributed network architectures, and as such requires respective enablers for secure, private, and trusted data exchange especially when cellular control link is not available at all times. Within the research group, the author was tasked to provide the state-of-the-art technology overview and to propose a novel algorithm for maintaining security functions of proximate devices in case of unreliable cellular connectivity, whenever a new device joins the secure group of users or an existing device leaves it. The proposed solution and its rigorous practical implementation detailed in this work open door to a new generation of secure proximity-based services and applications in future wireless communications systems

    QoE Driven Multimedia Service Schemes in Wireless Networks Resource Allocation: Evolution from Optimization, Game Theory, to Economics

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    In order to deal with the Quality of Experience (QoE) improvement issue in the wireless networks services. In this dissertation we first investigated the Device to Device (D2D) relaying approach in the conventional Base Station (BS) to User Equipment (UE) two entities multimedia service system. In this part, the Multiple Input Multiple Output (MIMO) technology will be implemented in the D2D communication. Furthermore, factors such as the multimedia content distribution (i.e., Quad-tree fractal image compression method), the power allocation strategy, and modulation size are jointly considered to improve the QoE performance and energy efficiency. In addition, the emerging Non-Orthogonal Multiple Access (NOMA) transmission method is becoming very popular and being considered as one of the most potential technologies for the next generation of wireless networks. For the purpose of improving the QoE of UE in the wireless multimedia service, the power allocation method and the corresponding limitations are studied in detail in the wireless system where the traditional Orthogonal Multiple Access (OMA) technology and the promising NOMA technology are compared. At last, facing the real business model in the wireless network services, where the Content Provider (CP), Wireless Carrier (WC), and UE are included, we extend on work from the conventional BS-UE two entities research model to the CP-WC-UE three entities model. More specifically, a generalized best response Smart Media Pricing (SMP) method is studied in this dissertation. In our work, the CP and WC are treated as the service provider alliance. The SMP approach and the game theory are utilized to determine the data length of UE and the data price rate determined by the CP-WC union. It is worth pointing out that the concavity of utility function is no longer necessary for seeking the game equilibrium under the proposed best response game solution. Numerical simulation results also validate the system performance improvement of our proposed transmission schemes

    Game Theory for Multi-Access Edge Computing:Survey, Use Cases, and Future Trends

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    Game theory (GT) has been used with significant success to formulate, and either design or optimize, the operation of many representative communications and networking scenarios. The games in these scenarios involve, as usual, diverse players with conflicting goals. This paper primarily surveys the literature that has applied theoretical games to wireless networks, emphasizing use cases of upcoming multiaccess edge computing (MEC). MEC is relatively new and offers cloud services at the network periphery, aiming to reduce service latency backhaul load, and enhance relevant operational aspects such as quality of experience or security. Our presentation of GT is focused on the major challenges imposed by MEC services over the wireless resources. The survey is divided into classical and evolutionary games. Then, our discussion proceeds to more specific aspects which have a considerable impact on the game's usefulness, namely, rational versus evolving strategies, cooperation among players, available game information, the way the game is played (single turn, repeated), the game's model evaluation, and how the model results can be applied for both optimizing resource-constrained resources and balancing diverse tradeoffs in real edge networking scenarios. Finally, we reflect on lessons learned, highlighting future trends and research directions for applying theoretical model games in upcoming MEC services, considering both network design issues and usage scenarios
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