Ανάλυση και σχεδίαση πυκνά δομημένων ασυρμάτων δικτύων
Analysis and design of ultra-dense wireless networks
KeywordsΑσύρματα δίκτυα ; Στοχαστική γεωμετρία ; Κυψελωτά δίκτυα ; D2D επικοινωνίες ; Cellular networks
The proliferation of wireless devices and the introduction of novel applications and services towards the realization of the internet of things (IoT) impose serious challenges for the cellular network operation. It is widely recognized that addressing these challenges will require a revolutionary (rather than evolutionary) upgrade of current cellular networks. The major enabler of this upgrade will be ultra dense networking, i.e., operation under extreme infrastructure densification levels, motivated by the need for an aggressive spatial frequency reuse towards providing the high quality of service requested by an extremely large number of user devices. At the same time, this extreme density of devices strongly motivates the introduction of new channel access methods (modes), in addition to the traditional uplink/downlink transmissions, with the most prominent example being that of direct device-todevice (D2D) communications. These two features of the future network (densification and new access methods), although, in principle, able to provide performance benefits, also introduce significant challenges that were not present in the previous cellular network generations. This thesis investigates various aspects of the operation of future ultra dense cellular networks using analytical tools from stochastic geometry. Regarding conventional downlink transmissions, the impact of an increasing infrastructure density on user performance is investigated. It is shown that infrastructure densification alone is able to provide considerable gains, however, when supported by a carefully designed intra-cell resource allocation (scheduling) scheme. Even though performance is monotonically increasing with increasing infrastructure density, densification beyond the level where the number of access points is comparable to the number of users provides only moderate gains and, therefore, may not be of interest in practice. The potential of D2D communications as a technique that is beneficial to the overall system performance is investigated based on the new notion of operational region of D2D communications, which identifies the range of system conditions (in terms of parameters such as density of access points and user devices) where D2D communications are beneficial. The operational region is identified in closed form for various combinations of mode selection, spectrum sharing, and channel access schemes. Analysis reveals that the introduction of D2D communications enhances the performance of the conventional (non-D2D-enabled) cellular network under all operational conditions as long as a distance-based mode selection scheme is adopted and D2D transmissions are performed on the same bandwidth as cellular transmissions with appropriate power control. The critical issue for future cellular networks of characterization of cross-mode interference, i.e. interference experienced by a receiver operating on a certain access mode (e.g., cellular downlink) due to transmissions corresponding to other modes (e.g., D2D) is examined. By adopting a general system model, a characterization of intercell cross-mode interference statistics is analytically obtained that is applicable for many operational scenarios of interest, ranging from conventional uplink/downlink to cross-cell D2D transmissions, with interference generated by transmissions of different mode(s). The analysis allows for an efficient computation of performance measures of interest such as coverage probability and can serve as an analytical baseline for sophisticated design of, e.g., resource allocation algorithms. As the extreme infrastructure densification motivates the management of the network by multiple competing operators, a techno-economic investigation of the system is performed under the assumption of network partitioning over multiple operators offering their services to self-interested users. By combining tools from stochastic geometry and game theory, it is shown, among others, that users enjoy maximum quality of experience in a market where multiple operators are competing with comparable networks in terms of density and transmission technology. Although increasing infrastructure density is always beneficial from an engineering perspective (e.g., higher user rates), it is shown for the tractable case of two competing operators that stable market conditions cannot be achieved above a certain density threshold, suggesting a different market model (e.g., based on partial cooperation among operators).