Using Outage Probability to Tailor Communication Protocols Suitable for Cyber-Physical Systems Operating in Confined Areas

Sammanfattning: Some types of Cyber-Physical System (CPS) applications found within e.g., quarrying, mining, harbors, and construction sites, operate within confined areas. To guarantee that communications between nodes in such systems operate smoothly, the adopted wireless communication protocols must meet stringent requirements, even in the presence of interference and/or jamming. This thesis work targets finding performance metrics suitable for designing and evaluating wireless networks when used for CPSs in confined areas. Given the shortage of spectrum and the increased risk of cyber-attacks experienced lately, the protocols must also be adjusted to perform well when subject to strong interference and/or adversary jamming. Hence, to be useful in practice, metrics that can be recalculated during runtime are needed, such that they can be used not only to propose, select and tailor the communication protocols to fulfill the stringent CPS requirements but also to optimize their performance during runtime. The literature survey conducted in this thesis shows that a reliability metric in terms of thresholding the so-called outage probability is valuable for all design steps as well as for continuously optimizing the communications. However, since the outage probability is defined as the probability that an individual wireless link will experience outage, it does not address the reliability of the entire network within the confined area. The thesis work shows that it is possible to consider overall outage probabilities and evaluates the probability of outage for each individual packet from a specific source node to a final destination, including all possible links in the network. This way, it is possible to consider the overall outage for all packets within the network in the confined area. Outage probability measures are defined for different system models and closed-form expressions for the outage are derived for the Individual Outage Probability (IOP), the Overall Outage Probability (OOP), the IOP for one packet (IOPP), and the OOP for all packets (OOPP), with and without the presence of attackers and/or strong interference. These reliability metrics can be adopted to analyze the effects of a wide range of parameters such as power allocation, node positions, the use of relay nodes or retransmissions, etc. on the individual link, individual source packet, and overall network reliability. The analysis thereby makes it possible to provide a set of general guidelines for tailoring the protocols and enhance the communication reliability of all legitimate nodes in the CPS. Having the closed-form expressions readily available also enables recalculating and adjusting parameters faster in order to find the best solution to improve the communication reliability during runtime. The thesis work demonstrates how outage probability can be used to enhance system performance in several example scenarios, including a multiple access scheme, pairwise Non-Orthogonal Multiple Access (NOMA), mobile access points and/or relay nodes. The outage analysis can thereby be applied to an existing CPS application to enhance reliability, robustness, and flexibility while maintaining a low delay.