Abstract
With the introduction of the CubeSat standard, the number of small-satellite missions has increased dramatically over the last two decades.Initially developed by universities and research centres for technology validation and academic experiments, these low-cost platforms currently allow to perform a variety of advanced, novel applications.In this thesis we are interested in the use of small satellites for global data collection and, more generally, for Internet of Things (IoT) and machine-to-machine (M2M) applications.Since both the space and ground segments are subject to stringent constraints in terms of size and mass, the overall capacity of the communications channel is highly limited, specially that of the uplink, which is a multi-access channel.These systems are also characterised by bursty, short messages, meaning that any protocol overhead may have a significant impact on the bandwidth efficiency. Hence, a random access approach is usually adopted for the uplink.Facing these challenges requires to optimize the communication system by taking an holistic approach. In particular, a joint design of both the physical (PHY) and Medium Access Control (MAC) layers is needed.The main contributions of this thesis are related to the study of Time-- and Frequency--Asynchronous ALOHA (TFAA), a random access approach adopted in terrestrial ultra narrowband (UNB) networks. By trading data rate for communication range or transmission power, TFAA is particularly attractive in power constrained applications such as low power wide area networks and M2M over satellite. First, we evaluate its MAC performance (i.e., its throughput and packet error rate) under three different reception models: the collision channel, the capture channel and a more detailed model that takes into account the PHY layer design.Then, we study the impact of PHY layer parameters, such as forward error correction (FEC), pulse shaping filter and modulation order, on the MAC performance.We show that, due to the characteristics of the multiple access interference, significant improvements can be obtained by applying low-rate FEC.To further improve TFAA's performance, we propose Contention Resolution Time-- and Frequency--Asynchronous ALOHA (CR-TFAA), a more advanced design which is in line with recent developments such as Asynchronous Contention Resolution Diversity ALOHA (ACRDA).Under the same set of hypothesis, we see that CR-TFAA provides similar and even better performance than ACRDA, with a decrease in the packet error rate of at least one order of magnitude.Finally, we study the benefits that can be obtained by trading delay for MAC performance and energy efficiency, using simple techniques such as transmission control and packet-layer erasure coding.