Random and Hybrid Medium Access for M2M Communication : Scalability and Energy Analysis
Sammanfattning: The term machine-to-machine (M2M) communication identifies any fully automated communication between intelligent devices, autonomous from human intervention. M2M communication is a key enabling technology for the Internet of Things (IoT), where it is used to provide ubiquitous connectivity between a large number of intelligent devices. M2M technologies find applications in numerous emerging use cases, such as smart metering, smart cities, intelligent transportation systems, eHealth monitoring, and surveillance/security. The service requirements placed onM2M communication can vary greatly depending on the intended area of application. In general, M2M applications are characterized by the high number of devices communicating with one another through sporadic and short transmissions. The devices are generally distributed over wide areas without easy access to the power grid, relying for their energy supply on batteries and energy harvesting. Therefore, the design of M2M communication technologies should meet the goal of supporting a large number of connected devices while retaining low energy consumption. One of the obstacles to achieving this goal is the high level of interference that can be present on the channel if a large number of M2M devices decide to transmit within a short period of time. To understand how to overcome this obstacle, it is necessary to explore new and old design options available in the channel access of M2M communication. The aim of this work is to study the performance and propose improvements to the channel access mechanisms of M2M communication technologies operating in the unlicensed frequency spectrum. The two technologies discussed in this thesis are IEEE 802.11ah and LoRaWAN. The performance metrics that have been considered consistently throughout this work are the scalability and energy efficiency of the investigated channel access mechanisms, which are especially critical to massive M2M.The first part of the thesis focuses on the IEEE 802.11ah standard and its medium access mechanism with station grouping. An analytical model of the grouping mechanism of IEEE 802.11ah combined with enhanced distributed channel access (EDCA) is presented to assess the quality of service (QoS) differentiation available in IEEE 802.11ah. The throughput and delay of the access categories of EDCA are investigated for different group size and composition. The results reveal that grouping is effective at increasing the throughput of both high and low priority access categories up to 40% compared to the case without groups. A redesign of the access mechanism of IEEE 802.11ah is proposed to realize a hybrid channel access for energy efficient uplink data transmission. The numerical results show that fora wide range of contending M2M devices and even for the relatively small frame size of 256 bytes, the use of an hybrid channel access can help reducing the average energy consumption of the devices per successful uplink frame transmission. In the considered scenarios, the proposed MAC mechanism was able to reduce the average energy consumption per successful transmission up to 55% compared to standard approach. The second part of the thesis focuses on LoRa, with an investigation on the performance of alternative random channel access mechanisms in LoRaWAN. The connection between the channel access mechanism and the intensity of interference in LoRa networks is characterized for pure Aloha, slotted Aloha, and CSMA channel access. The results reveal several assisting guidelines on the design and selection of a medium access solution within LoRa’s parameter space: device density, service area, and spreading factor allocation. An out-of-band synchronization mechanism based on FM-Radio Data System (FM-RDS) is proposed to achieve synchronous channel access in LoRa. The throughput and fairness results for the proposed communication show the clear advantages of synchronous communication in LoRa, meanwhile, the use of out-of-band synchronization reduces the usage of LoRa channels, improving the scalability. The timing errors of FM-RDS are evaluated combining experimental approach and analytical methods. The observations reveal that despite the poor absolute synchronization, FM-RDS can effectively be used to realize time-slotted communication in LoRa, with performance similar to those obtained by more accurate but expensive time-dissemination technologies. Finally, a comprehensive model of the interference in neighboring clusters of LoRa devices is proposed, highlights the disruptive effects of the inter-cluster interference on the transmissions success probability, particularly for the devices using the largest spreading factors.
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