Towards Zero-Power Wireless Machine-to-Machine Networks

This thesis aims at contributing to overcome two of the main challenges for the deployment of highly dense wireless M2M networks in data collection scenarios for the Internet of Things: the management of massive numbers of end-devices that attempt to get access to the wireless channel; and the need to extend the network lifetime to reduce maintenance costs. In order to solve these challenges, two complementary strategies are considered. Firstly, the thesis focuses on the design, analysis and performance evaluation of random and hybrid access protocols that can handle abrupt transitions in the traffic load and minimize the energy consumption devoted to communications. And secondly, the use of energy harvesting (EH) is considered in order to provide the network with unlimited lifetime. To this end, the second part of the thesis focuses on the design and analysis of EH-aware MAC protocols, and proposes new performance metrics that take the variability and the unpredictability of the harvested energy into account. While the Frame Slotted-ALOHA (FSA) protocol has been traditionally adopted in star topology networks for data collection, results show that FSA-based protocols lack of scalability and present synchronization problems as the network density increases. Indeed, the frame length of FSA must be adjusted to the number of contenders, which may be complex to attain in dense networks with large and dynamic number of end-devices. In order to overcome these issues, a tree splitting-based random access protocol, referred to as Low Power Contention Tree-based Access (LP-CTA is proposed in the first part of this thesis. In LP-CTA, the frame length can be very short and fixed, which facilitates synchronization and provides better network scalability than FSA. While LP-CTA uses data slots for contention, it is possible to use short access requests in minislots, where collisions are resolved using tree splitting, and avoid the contention in data. Since these minislots can be much shorter than the duration of a data packet, the performance can be improved. The Low Power Distributed Queuing (LP-DQ) protocol proposed in this thesis is based on this idea. LP-DQ combines tree splitting with the logic of two distributed queues that manage the contention resolution and the collision-free data transmission. Results show that LP-DQ outperforms LP-CTA and FSA in terms of delay and energy efficiency, and it relaxes the need to know the size of the network and adapts smoothly to any change in the number of end-devices. The approach of LP-DQ is convenient when the messages transmitted by each end-device fit in one single slot, however, if the end-devices generate long messages that have to be fragmented, it is better to add a reservation mechanism in order to boost the performance. In this sense, the LPR-DQ protocol is proposed as an extension of LP-DQ where the concept of reservation is integrated to allow the end-devices reserve as many collision-free slots as needed. The second part of the thesis is devoted to the integration of the MAC layer with the use of energy harvesting. While energy harvesters can theoretically provide infinite lifetime, the variability and fluctuations of the harvested energy may cause the end-devices to enter temporarily in energy shortage. This fact is considered for the design of EH-aware MAC protocols and three performance metrics are proposed: the probability of delivery, the data delivery ratio and the time efficiency. While previous research works on data collection networks with energy harvesting focus on Dynamic FSA (DFSA), the EH-CTA protocol is proposed in this thesis as an adaptation of LP-CTA that takes the energy harvesting process into account. Results show that EH-CTA outperforms DFSA if the energy threshold for an end-device to become active is properly configured. In addition, while DFSA needs to adapt the frame length dynamically, EH-CTA uses a fixed frame length, thus facilitating scalability and synchronization. Finally, the EH-RDFSA and EH-DQ protocols are proposed for scenarios where data must be fragmented. EH-RDFSA is a combination of RFSA and DFSA, and EH-DQ is an extension of LPR-DQ. Results show that EH-DQ uses a short and fixed frame length, regardless of the number of contenders, and outperforms EH-RDFSA. Taking that into account, EH-DQ is a promising alternative for highly dense data collection networks with EH where end-devices generate bursts of data fragmented in multiple packets.