Access and Resource Allocation Strategies for Machine-to-Machine Communications

Machine-to-Machine (M2M) communications represent the cornerstone technology enabling the pervasive deployment of automated applications connecting billions of devices or objects without the need for human intervention. Within the past few years, the number of M2M-based services has dramatically grown, spurred by remarkable benefits provided by the development of smart and cost-effective applications in a wide range of areas, including remote sensing, health monitoring and Intelligent Transportation Systems (ITS). Therefore, great research effort has been invested to the design of novel solutions or to the optimization of existing communication systems in order to accommodate M2M traffic with diverse QoS requirements. This thesis introduces novel approaches to enhance M2M communications based on the adoption of Device-to-Device (D2D) connectivity among the devices, in conjunction with existing techniques widely implemented, such as clustering, packet aggregation and trunking. D2D communications are an attractive and intelligent solution to reduce the power consumption and the latency due to the short-range connectivity employed by two devices operating within the licensed spectrum. In the first approach, we envision a centralized scheme to allocate cellular resources in a scenario where the machine devices can exploit D2D connectivity, realizing a multi-hop D2D network overlaying an LTE-A system. The main challenges involving the D2D link establishment are covered, such as proximity discovery and device pairing, and a routing mechanism is proposed in order to efficiently forward packets to a collector device through multi-hop transmissions. To address the RAN overload issue originated from the mass access, machine devices can communicate with a cellular user in the proximity through low power D2D connections a cellular user can collect and combine the traffic generated by machine devices through low power D2D connections and then forward the aggregated data to the attached base station, along with the traffic originated from higher layers. As a result, enormous benefits can be obtained in terms of energy efficiency and throughput, as the cellular acts as a relay and is responsible of the data transmission on the trunked cellular uplink. We further enhance this mechanism by introducing a backoff mechanism to mitigate collisions occurring during the access reservation phase, performed by the machine devices before transmitting a packet. In addition, we evaluate the advantages in terms of trunking gains provided by a multi-cluster configuration, where multiple cellular users forward traffic sent by devices belonging to different clusters, without generating interference. In the context of vehicular networking, M2M communications aim to enable the internet-working among connected vehicles, which can autonomously exchange information and make critical decisions in diverse situations. The main challenge associated with Vehicular Ad-hoc Networks (VANET) is how to provide reliable connectivity between vehicles in scenarios with highly dynamic topologies and unpredictable channel conditions. We choose to overcome these issues by developing a clustering algorithm based on the mobility correlation degree of the vehicles in a typical highway scenario, along with a relaying scheme aiming at supporting the communication between different clusters. The proposed protocol is then evaluated by implementing a realistic mobility model, taking into account interactions between approaching vehicles and assuming that the communications are established according to the IEEE 802.11p/WAVE standard.