Device-to-Device Communication and Multihop Transmission for Future Cellular Networks

The next generation wireless networks i.e. 5G aim to provide multi-Gbps data traffic, in order to satisfy the increasing demand for high-definition video, among other high data rate services, as well as the exponential growth in mobile subscribers. To achieve this dramatic increase in data rates, current research is focused on improving the capacity of current 4G network standards, based on Long Term Evolution (LTE), before radical changes are exploited which could include acquiring additional/new spectrum. The LTE network has a reuse factor of one; hence neighbouring cells/sectors use the same spectrum, therefore making the cell edge users vulnerable to inter-cell interference. In addition, wireless transmission is commonly hindered by fading and pathloss. In this direction, this thesis focuses on improving the performance of cell edge users in LTE and LTE-Advanced (LTE-A) networks by initially implementing a new Coordinated Multi-Point (CoMP) algorithm to mitigate cell edge user interference. Subsequently Device-to-Device (D2D) communication is investigated as the enabling technology for maximising Resource Block (RB) utilisation in current 4G and emerging 5G networks. It is demonstrated that the application, as an extension to the above, of novel power control algorithms, to reduce the required D2D TX power, and multihop transmission for relaying D2D traffic, can further enhance network performance. To be able to develop the aforementioned technologies and evaluate the performance of new algorithms in emerging network scenarios, a beyond-the-state-of-the-art LTE system-level simulator (SLS) was implemented. The new simulator includes Multiple-Input Multiple-Output (MIMO) antenna functionalities, comprehensive channel models (such as Wireless World initiative New Radio II i.e. WINNER II) and adaptive modulation and coding schemes to accurately emulate the LTE and LTE-A network standards. Additionally, a novel interference modelling scheme using the ‘wrap around’ technique was proposed and implemented that maintained the topology of flat surfaced maps, allowing for use with cell planning tools while obtaining accurate and timely results in the SLS compared to the few existing platforms. For the proposed CoMP algorithm, the adaptive beamforming technique was employed to reduce interference on the cell edge UEs by applying Coordinated Scheduling (CoSH) between cooperating cells. Simulation results show up to 2-fold improvement in terms of throughput, and also shows SINR gain for the cell edge UEs in the cooperating cells. Furthermore, D2D communication underlaying the LTE network (and future generation of wireless networks) was investigated. The technology exploits the proximity of users in a network to achieve higher data rates with maximum RB utilisation (as the technology reuses the cellular RB simultaneously), while taking some load off the Evolved Node B (eNB) i.e. by direct communication between User Equipment (UE). Simulation results show that the proximity and transmission power of D2D transmission yields high performance gains for a D2D receiver, which was demonstrated to be better than that of cellular UEs with better channel conditions or in close proximity to the eNB in the network. The impact of interference from the simultaneous transmission however impedes the achievable data rates of cellular UEs in the network, especially at the cell edge. Thus, a power control algorithm was proposed to mitigate the impact of interference in the hybrid network (network consisting of both cellular and D2D UEs). It was implemented by setting a minimum SINR threshold so that the cellular UEs achieve a minimum performance, and equally a maximum SINR threshold to establish fairness for the D2D transmission as well. Simulation results show an increase in the cell edge throughput and notable improvement in the overall SINR distribution of UEs in the hybrid network. Additionally, multihop transmission for D2D UEs was investigated in the hybrid network: traditionally, the scheme is implemented to relay cellular traffic in a homogenous network. Contrary to most current studies where D2D UEs are employed to relay cellular traffic, the use of idle nodes to relay D2D traffic was implemented uniquely in this thesis. Simulation results show improvement in D2D receiver throughput with multihop transmission, which was significantly better than that of the same UEs performance with equivalent distance between the D2D pair when using single hop transmission

Συγκριτική παρουσίαση των τεχνολογιών 5G και WiFi 6.0

Τα τελευταία χρόνια, μεγάλη σημασία έχει δοθεί στην πέμπτη γενιά ασύρματης ευρυζωνικής συνδεσιμότητας γνωστής ως 5G, η οποία υπόσχεται μια σημαντική αναβάθμιση στη ποιότητα και στη χωρητικότητα των κινητών ευρυζωνικών υπηρεσιών αλλά και ένα γενικότερο τεχνολογικό άλμα με τη παροχή μιας πληθώρας νέων δυνατοτήτων. Παραδόξως, έχει δοθεί λιγότερη προσοχή στο Wi-Fi 6, το νέο πρότυπο 802.11ax της IEEE στην οικογένεια τεχνολογιών ασύρματου τοπικού δικτύου, με χαρακτηριστικά που στοχεύουν στα ιδιωτικά, ακραία δίκτυα, υποστηρίζοντας υψηλές ταχύτητες, χαμηλή καθυστέρηση και χαμηλή ενεργειακή κατανάλωση. Αυτή η εργασία εξετάζει την καταλληλότητα των κυψελωτών και των Wi-Fi δικτύων στην παροχή υψηλής ταχύτητας ασύρματης σύνδεσης στο διαδίκτυο. Και οι δύο τεχνολογίες φιλοδοξούν να προσφέρουν σημαντικά βελτιωμένη απόδοση, πολύ πιο γρήγορη ασύρματη ευρυζωνική σύνδεση και περαιτέρω υποστήριξη για το διαδίκτυο των πραγμάτων (IoT) και τις επικοινωνίες τύπου μηχανής, τοποθετώντας τις ως τεχνικά υποκατάστατες σε πολλά σενάρια χρήσης. Και οι δύο είναι πιθανό να διαδραματίσουν σημαντικό ρόλο στο μέλλον και ταυτόχρονα να αξιοποιηθούν ως ανταγωνιστικές και συμπληρωματικές τεχνολογίες. Το 5G αναμένεται να παραμείνει η προτιμώμενη τεχνολογία για την κάλυψη μιας ευρείας περιοχής, ενώ η τεχνολογία Wi-Fi θα παραμείνει κυρίαρχη για εσωτερική χρήση, χάρη στο πολύ χαμηλότερο κόστος ανάπτυξης. Ωστόσο, τα παραδοσιακά όρια που διέκριναν τις προηγούμενες γενιές κινητών και Wi-Fi δικτύων θολώνουν, με τη παρουσία πλέον αυτών των δυο τεχνολογιών να συμβάλει στην επίτευξη του στόχου της παροχής προσιτών, αξιόπιστων, και αδιάλειπτων ασύρματων ευρυζωνικών συνδέσεων υψηλής χωρητικότητας.In recent years, significant attention has been directed toward the fifth generation of wireless broadband connectivity known as ‘5G’, currently being deployed by Mobile Network Operators. 5G promises a significant upgrade in the quality and capacity of mobile broadband services but also a more general technological leap by providing a plethora of new capabilities Surprisingly, there has been considerably less attention paid to ‘Wi-Fi 6’, the new IEEE 802.1ax standard in the family of Wireless Local Area Network technologies with features targeting private, edge-networks, supporting high speeds, low latency and low energy consumption. This work revisits the suitability of cellular and Wi-Fi in delivering high-speed wireless Internet connectivity. Both technologies aspire to deliver significantly enhanced performance, enabling each to deliver much faster wireless broadband connectivity, and provide further support for the Internet of Things and Machine-toMachine communications, positioning the two technologies as technical substitutes in many usage scenarios. We conclude that both are likely to play important roles in the future, and simultaneously serve as competitors and complements. 5G is anticipated to remain the preferred technology for wide-area coverage, while Wi-Fi 6 will remain the preferred technology for indoor use, thanks to its much lower deployment costs. However, the traditional boundaries that differentiated earlier generations of cellular and Wi-Fi are blurring. The presence of both technologies should contribute to achieving the goal of providing affordable, reliable, and seamless high-bandwidth wireless broadband connections

Energy E fficiency Oriented Full Duplex Wireless Communication Systems

Full-duplex (FD) transmission is a promising technique for fifth generation (5G) wireless communications, enabling significant spectral efficiency (SE) improvement over existing half-duplex (HD) systems. However, FD transmission consumes higher power than HD transmission, especially for millimetre wave band. Therefore, energy efficiency (EE) for FD systems is a critical yet inadequately addressed issue. This thesis addresses the critical EE challenges and demonstrates promising solutions for implementing FD systems, as detailed in the following contributions. In the first contribution, a comprehensive EE analysis of the FD and HD amplify-and-forward (AF) relay-assisted 60 GHz dual-hop indoor wireless systems is presented. An opportunistic relay mode selection scheme is developed, where FD relay with different self-interference (SIC) techniques or HD relay is opportunistically selected. Together with transmission power adaptation, EE is maximised with given channel gains. A counter-intuitive finding is shown that, with a relatively loose maximum transmission power constraint, FD relay with two-stage SIC is preferable to both FD relay with one-stage SIC and HD relay, resulting in a higher optimised EE. A full range of power consumption sources are considered to rationalise the analysis. The effects of imperfect SIC at relay, drain efficiency and static circuit power on EE are investigated. Simulation results verify the theoretical analysis. In the second contribution, EE oriented resource allocation for FD decode-of-forward (DF) relay-assisted 60 GHz multiuser systems is investigated. In contrast to the existing SE oriented designs, the proposed scheme maximises EE for FD relay systems under cross-layer constraints, addressing the typical problems at 60 GHz. A low-complexity EE-orientated resource allocation algorithm is proposed, by which the transmission power allocation, subcarrier allocation and throughput assignment are performed jointly across multiple users. Simulation results verify the analytical results and confirm that the FD relay systems with the proposed algorithm achieve a higher EE than the FD relay systems with SE oriented approaches, while offering a comparable SE. In addition, a much lower throughput outage probability is guaranteed by the proposed resource allocation algorithm, showing its robustness against channel estimation errors. In the third contribution, it is noticed that in wireless power transfer (WPT)-aided relay systems, the SE of the source-relay link plays a dominant role in the system SE due to limited transmission power at the WPT-aided relay. A novel asymmetric protocol for WPT-aided FD DF relay systems is proposed in multiuser scenario, where the time slot durations of the two hops are designed to be uneven, to enhance the degree of freedom and hence the system SE. A corresponding dynamic resource allocation algorithm is developed by jointly optimising the time slot durations, subcarriers and transmission power at the source and the relay. Simulation results con rm that, compared to the symmetric WPT-aided FD relay (Sym-WPT-FR) and the time-switching based WPT-aided FD relay (TS-WPT-FR) systems in the literature, the proposed asymmetric WPT-aided FD relay system achieves up to twice the SE and higher robustness against the relay's location and the number of users. In the final contribution, to strike the balance between high SE and low power consumption, a hybrid duplexing strategy is developed for distributed antennas (DAs) systems, where antennas are capable of working in hybrid FD, HD, and sleeping modes. To maximise the system EE with low complexity, activation/deactivation of transmit/receive chain is first performed, by a proposed channel-gain-based DA clustering algorithm, which highlights the characteristics of distributed deployment of antennas. Based on the DAs' con figuration, a novel distributed hybrid duplexing (D-HD)-based and EE oriented algorithm is proposed to further optimise the downlink beamformer and the uplink transmission power. To rationalise the system model, self-interference at DAs, co-channel interference from uplink users to downlink users, and multiuser interference in both uplink and downlink are taken into account. Simulation results confirm that the proposed system provides significant EE and SE enhancements over the colocated FD MIMO system, showing the advantages in alleviating high path loss as well as in cutting the carbon footprint. Compared to the sole-FD DA system, the proposed system shows much higher EE with marginal loss in SE. Also, the SIC operation in the proposed system is much more simplified compared to the two benchmarks