Μελέτη και κατηγοριοποίηση μηχανισμών ελέγχου ισχύος σε LTE/LTE-A δίκτυα με φεμτοκυψέλες

Η εργασία αυτή επικεντρώνεται στα Long Term Evolution (LTE)/LTE-Advanced (LTE-A) δίκτυα κινητών επικοινωνιών με φεμτοκυψέλες και παρέχει μια ολοκληρωμένη μελέτη τεχνικών ελέγχου ισχύος που έχουν προταθεί στη βιβλιογραφία για μείωση των παρεμβολών. Στο πρώτο κεφάλαιο γίνεται μια αναδρομή στα κυψελωτά δίκτυα επικοινωνιών. Πιο συγκεκριμένα, περιγράφεται η σταδιακή εξέλιξη τους μέχρι και το σύστημα LTE, και παρουσιάζονται τα κυριότερα πρότυπα και τα βασικά χαρακτηριστικά κάθε γενιάς. Στο δεύτερο κεφάλαιο, αρχικά, παρουσιάζεται η αρχιτεκτονική του συστήματος LTE-A και οι βασικές της οντότητες. Στη συνέχεια περιγράφεται το φυσικό επίπεδο του LTE-A και παρουσιάζονται οι τεχνικές πολλαπλής πρόσβασης για την ανερχόμενη και την κατερχόμενη ζεύξη, ενώ αναλύονται τα σήματα αναφοράς και οι ακολουθίες συγχρονισμού και αναφέρονται τα φυσικά κανάλια μετάδοσης και ελέγχου Στο τρίτο κεφάλαιο αναλύονται οι τύποι παρεμβολών που προκαλούνται σε ένα ετερογενές LTE/LTE-A δίκτυο και παρουσιάζονται οι κυριότερες τεχνικές διαχείρισης τους. Στο τέταρτο κεφάλαιο παρουσιάζεται η προτυποποιημένη από την 3GPP (3rd Generation Partnership Project) μέθοδος ελέγχου ισχύος που χρησιμοποιείται για τη διαχείριση των παρεμβολών σε ανερχόμενη και κατερχόμενη ζεύξη. Στο πέμπτο κεφάλαιο παρουσιάζονται εκτενώς μια σειρά από λύσεις που έχουν προταθεί στη βιβλιογραφία για τον έλεγχο ισχύος στα LTE δίκτυα. Στο έκτο και τελευταίο κεφάλαιο γίνεται μια κατηγοριοποίηση των υπαρχόντων σχημάτων ελέγχου ισχύος και παρέχονται κάποια συμπεράσματα για τα πλεονεκτήματα και τα προβλήματα που παρουσιάζει κάθε κατηγορία.This paper focuses on Long Term Evolution (LTE)/LTE-Advanced (LTE-A) mobile networks and provides a comprehensive survey of the power control techniques that have been proposed to reduce the interference in those networks. The first chapter describes the evolution of the cellular communication networks up to the LTE system, providing the key characteristics of each generation and the standard. The second chapter presents the architecture and the physical layer characteristics of the LTE-A system. More specifically, we present the multiple access techniques used in the uplink and downlink, we analyze the reference signals and synchronization sequences and we list the physical transport and control channels. The third chapter analyzes the types of interference caused to a heterogeneous LTE network, and presents the main techniques which are used to mitigate interference. The fourth chapter presents the standardized power control method used for interference management as proposed by the 3GPP (3rd Generation Partnership Project). The fifth chapter presents an extensive range of solutions that have been proposed in the literature for power control in LTE networks. In the sixth and final chapter we categorize the existing power control schemes and provide some conclusions on the advantages and disadvantages of each category

Low complexity radio resource management for energy efficient wireless networks

Energy consumption has become a major research topic from both environmental and economical perspectives. The telecommunications industry is currently responsible for 0.7% of the total global carbon emissions, a figure which is increasing at rapid rate. By 2020, it is desired that CO2 emissions can be reduced by 50%. Thus, reducing the energy consumption in order to lower carbon emissions and operational expenses has become a major design constraint for future communication systems. Therefore, in this thesis energy efficient resource allocation methods have been studied taking the Long Term Evolution (LTE) standard as an example. Firstly, a theoretical analysis, that shows how improvements in energy efficiency can directly be related with improvements in fairness, is provided using a Shannon theory analysis. The traditional uplink power control challenge is re-evaluated and investigated from the view point of interference mitigation rather than power minimization. Thus, a low complexity distributed resource allocation scheme for reducing the uplink co-channel interference (CCI) is presented. Improvements in energy efficiency are obtained by controlling the level of CCI affecting vulnerable mobile stations (MSs). This is done with a combined scheduler and a two layer power allocation scheme, which is based on non-cooperative game theory. Simulation results show that the proposed low complexity method provides similar performance in terms of fairness and energy efficiency when compared to a centralized signal interference noise ratio balancing scheme. Apart from using interference management techniques, by using efficiently the spare resources in the system such as bandwidth and available infrastructure, the energy expenditure in wireless networks can also be reduced. For example, during low network load periods spare resource blocks (RBs) can be allocated to mobile users for transmission in the uplink. Thereby, the user rate demands are split among its allocated RBs in order to transmit in each of them by using a simpler and more energy efficient modulation scheme. In addition, virtual Multiple-input Multiple-output (MIMO) coalitions can be formed by allowing single antenna MSs and available relay stations to cooperate between each other to obtain power savings by implementing the concepts of spatial multiplexing and spatial diversity. Resource block allocation and virtual MIMO coalition formation are modeled by a game theoretic approach derived from two different concepts of stable marriage with incomplete lists (SMI) and the college admission framework (CAF) respectively. These distributed approaches focus on optimizing the overall consumed power of the single antenna devices rather than on the transmitted power. Moreover, it is shown that when overall power consumption is optimized the energy efficiency of the users experiencing good propagation conditions in the uplink is not always improved by transmitting in more than one RB or by forming a virtual MIMO link. Finally, it is shown that the proposed distributed schemes achieve a similar performance in bits per Joule when compared to much more complex centralized resource allocation methods