# A Reliable Handoff Mechanism for Mobile Industrial Wireless Sensor Networks
## 1 Introduction  
Nowadays, growing demands to improve process efficiencies, comply with environmental regulations, and meet corporate financial objectives faced by a number of companies, which makes the industry marketplace competitive [1]. With a growing number of intelligent devices, mobile devices have a major role to play in industrial production. The roadmap of Factories of the Future indicates that mobility support is one of the main requirements in the wireless scenario [2]. Meanwhile, it is necessary to integrate such characteristics into the key technology of Industry 4.0 [3]. As an emerging class of wireless sensor networks (WSNs), IWSNs are indispensable in industrial production which aim to increase productivity and profit, decrease cost and provide convenience. However, the issued IWSNs standards, e.g., WirelessHART [4] and ISA100.11a [5], introduce mechanisms such as time division multiple access (TDMA) to focus on monitoring for fixed devices. Research on mobility support in IWSNs has been limited because of the specific constraints linked to the specific requirements of the industry. Since the applicability of IWSN protocols is significantly limited to monitoring and control systems by the lack of mobility support [6], this paper presents a new mechanism to deploy a reliable IWSN for applications involved in the mobility of devices, workers or robots. The common solution for mobility in traditional WSNs is a periodic reception of messages from coordinators or access points to the node. However, this kind of solution works best with the star topology, which lacks scalability, and the contention-based MAC protocol will deteriorate the performance of mobility support. Compared with traditional WSNs, supporting mobility in IWSNs involves more challenges that the adaptation of the complicated resource scheduling needs to be dynamic for the movement [7]. Moreover, there are more interference and unstable factors affecting the performance of wireless communication, such as metallic shelter or electromagnetism. They also affect the accuracy of handoff triggering and easily cause the ping-pong effect while supporting mobility. Generally, the ping-pong effect during handoff and registration is a critical problem for all mobile wireless networks, especially in centralized, TDMA-based IWSNs. Moreover, the effect of harsh and multivariate environments means these challenges are difficult to be solved [8]. For instance, the accuracy of triggering and the time of registration are the two most important aspects of the handoff mechanism. Traditional approaches to the metrics of triggering are based on setting a threshold for radio signal strength indication (RSSI) such as [9; 10]. Though the RSSI can evaluate the possible state of the channel [11; 12], the large number of lost packets and varied environment are not taken into consideration, which causes false triggering or longer handoff delay [13]. There is also the disputation that RSSI can only indicate some possible anomalous behaviour of devices instead of the packets delivery estimation [14; 15]. False triggering not only impacts the timeliness of industrial data but makes the network unstable, which results in a vicious spiral. Such a reaction also increases energy consumption due to unnecessary handoff. However, the problem of frequent registration stems from the fact that the mobile node immediately registers a new link once handoff is triggered. The difference between mobility awareness and the link estimation during network formation is the coherence time for measurements collection. The measurements should be easily obtained and it is necessary to consider the changes of slot scheduling. Generally, both bad channel condition and actual movement may trigger handoff. To find a correct solution to the problem on mobile handoff, the actual movement must be identified. A few solutions were introduced in [16], which can overcome this problem based on the limited communication range of radio and its traditional handoff mechanism.