Introducing wireless networks into distributed industrial systems may enable new or improved application areas and also great cost reductions due to lower complexity of installation and maintenance, compared to existing wired solutions. However, signals travelling through wireless channels are affected by pathloss, fading and shadowing, and, as a result, packet errors are both time-varying and more frequent than in e.g., existing wired fieldbuses. Packet errors or delays occurring in industrial systems can, in critical situations, lead to damage of expensive equipment or even danger to human life. Thus, wireless networks can be accepted for use in industrial networks only when sufficient levels of reliability and timeliness can be guaranteed.
Relaying is a technique that has the potential to increase reliability with maintained delay. Having different geographical locations, and thus different wireless channel qualities, some nodes may overhear transmitted packets even in cases when the intended receiver did not, and then cooperate by relaying these packets to their final destination.
This thesis deals with design of relaying strategies aiming to increase reliability in deadline-constrained industrial applications using wireless networks. The influence of several different parameters, such as positions of relay nodes, number of erroneous packets at the relay node and at the destination respectively, as well as the number of available time slots before the deadline, are evaluated to determine the best acting strategy for each relay node. Moreover, it is shown that when a specific relay node has the opportunity to aid more than one source node, performance can be improved even further if the relay node combines several packets, using Luby coding or packet aggregation, and instead relays such combined packets. Given the methods proposed in this thesis, the reliability in industrial wireless networks can be enhanced considerably, without increasing the delay, such that message deadlines still are kept.
Västerås: Mälardalen University , 2013.