Cooperative Communication for Increased Reliability in Industrial Wireless Networks

Abstract: Introducing wireless networks in distributed industrial systems may enable completely new application areas and can also offer great cost reductions due to lower complexity of installation and maintenance, compared to existing wired solutions. However, signals travelling though wireless channels are more affected by pathloss, fading and shadowing, and, as a result, packet errors are both time-varying and more frequent. Communication 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 systems can be accepted for use in industrial networks only when sufficient levels of reliability and timeliness can be guaranteed.Cooperative communication, in terms of relaying, is a low-complexity 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 to increase reliability in industrial wireless networks given certain packet deadlines. Several different parameters, such as the position of the relay nodes, number of erroneous packets at the relay node and at the destination respectively, and the number of available time slots before the deadline, are evaluated to determine the best acting strategy for a relay node. Moreover, it is shown that if 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 sends them in one time slot. Given the methods proposed in this thesis, the reliability in industrial wireless networks is enhanced considerably, without increasing the delay, such that message deadlines still can be kept.