Scheduling for Source Relaying With Packet Aggregation in Industrial Wireless Networks

The interest in wireless communication systems for industrial applications has grown significantly over the last years. More flexible, easier to install and maintain, wireless networks present a promising alternative to the currently used wired systems. However, reliability and timeliness requirements at present met by wired networks also need to be fulfilled by wireless solutions. Packet errors introduced when packets travel through wireless channels imply a significant challenge to fulfill these requirements. Relaying has been recognized to improve the reliability in industrial wireless networks without causing additional delay. Furthermore, the recent results have shown that relaying combined with packet aggregation significantly outperforms simple relaying. However, it is not always cost efficient to introduce additional relay nodes into an industrial network and hence, in this paper, we propose using a combination of relaying and packet aggregation at the source nodes. The results show that when relaying and aggregation are used at the source nodes, the transmission schedule plays a crucial role. A schedule adapting to the varying channel conditions improves performance substantially. By carefully choosing which packet to aggregate, even further improvements can be achieved.

I. Introduction

Great benefits can be achieved by replacing currently used wired networks with wireless systems. Not requiring many meters of wires makes the tasks of installation and maintenance of a network much easier. Additionally, wireless networks give an opportunity to enable new application areas, such as, e.g., controlling of moving robots. Moreover, wireless solutions are more flexible and their usage can lead to significant cost reductions. Several new standards for Industrial Wireless Sensor Networks (IWSNs) like, e.g., WirelessHART [1], [2], WIA-PA [3], ISA100.11a [4], and IEEE 802.15.4e [5] were developed and investigated over the past years [6]–[10]. However, in wireless systems, transmitted packets are affected by the channels they propagate through and thus the number of packet errors in wireless networks is much higher compared to their wired counterparts. There are several traditional techniques, such as, e.g., retransmissions and error control codes, which can be used to reduce the number of errors in wireless systems. However, error control codes only work for channels with bursts of errors shorter than the correction capability of the code and retransmissions can, in some situations, lead to extensive delays and thus deadline misses. Too many missed deadlines or packet errors in industrial systems will lead to financial losses due to damage of expensive equipment, stop of production or, depending on the application, even danger to humans. Thus, to be suitable for industrial use, wireless systems need robust techniques to deal with errors.