Mobile relaying and opportunistic networking in multi-hop cellular networks

Zusammenfassung

Cellular systems have significantly evolved over the past decades through the introduction of novel radio access technologies designed to increase capacity and support higher data rates. The evolution has mainly focused around the traditional cell-centric approach where each mobile station directly communicates with the base station. Traditional cellular communications experience difficulties in providing high and homogeneous Quality of Service (QoS) levels throughout the cell (in particular at cell edges) due to the signal attenuation produced by obstacles and distance. In this context, a paradigm-shift is sought for the design of future mobile communications systems (5G networks) that will be required to efficiently support varying QoS and Quality of Experience (QoE) requirements of the exponentially increasing mobile data traffic. This thesis advocates for the need to explore and evolve from current cell-centric architectures to device-centric architectures which exploit the intelligence, communications and computing resources of smart mobile devices. One approach to do so is by exploiting Device-to-Device (D2D) communications and Multi-hop Cellular Networks (MCNs). In device-centric MCNs, smart mobile devices become prosumers of wireless connectivity and act as a bridge between the cellular infrastructure and other devices. Previous studies have demonstrated that MCNs can significantly improve the capacity and energy consumption, and provide higher and more homogeneous QoS and QoE levels. However, to date, most of these studies have remained analytical or simulation-based and hence there is the need to experimentally demonstrate the benefits of MCN technologies. In this context, one first and significant contribution of this thesis is the experimental evaluation of the performance of MCN through field tests using commercial live cellular networks. The conducted field tests are aimed at validating and quantifying the benefits that MCNs using mobile relays can provide over traditional cellular systems. To this aim, a unique hardware testbed has been designed and implemented together with the necessary software tools to monitor the operation, QoS and benefits of MCNs, and investigate the conditions under which such benefits can be obtained. Using the data obtained from the field test measurements, the thesis then proposes a unique set of empirical models that characterize the communications performance of MCNs, and that can help design, test and optimize communications and networking protocols tailored for MCNs in analytical and simulation-based studies. Complementary to these studies, this thesis proposes to improve the transmission efficiency of delay-tolerant data traffic by integrating opportunistic networking principles into MCNs. Opportunistic networking can exploit the delay tolerance characteristic of relevant data traffic services in order to search for the most efficient transmission conditions in MCNs. In this context, this thesis first derives an analytical framework for opportunistic MCNs designed to identify their optimum configuration in terms of energy efficiency. Using this reference configuration, this thesis then proposes a set of opportunistic forwarding policies that exploit context information provided by the cellular network, and that demonstrate their potential to significantly contribute towards achieving the capacity and energy-efficiency gains sought for 5G networks.