# A Reliable Handoff Mechanism for Mobile Industrial Wireless Sensor Networks
## 2 Related Work  
Solutions such as [17; 18; 19; 20] have been proposed to support mobility in WSNs. Unfortunately, few of these protocols cater to the demand of reliability and timeliness in industry since their contention-based media access control (MAC) protocols cannot provide deterministic packet delivery. Moreover, the harsh and varied environments may deteriorate the stability if applied them into industrial field. Therefore, this paper mainly concentrates on TDMA-based and centralized solutions which can provide the deterministic communication. In some solutions, the transmission scheduling is designed to make up the communication of mobile nodes. In [21], [PERSON] et al., present a cross layer architecture which combines the MAC and routing layers to achieve energy efficient communication. The costly ability of the multi-channel transceiver is used to allow the cluster-heads to schedule traffic dynamically. A mobility-aware scheduling algorithm is proposed in [22], where the mobile nodes in the algorithm can dynamically associate with the infrastructure nodes by reserving bandwidth for potential communication paths. However, the number of mobile nodes and their moving path are so unpredictable that it cannot be used to design the transmission scheduling. Thus, the complexity of the mobility solution is greatly increased when supporting multi-hop, large, and mesh networks is the basic requirements. As previously mentioned, the accuracy of triggering and the time of registration are two important aspects to consider. For the former aspect, the triggering rule of most solutions is that if the RSSI of the communication link between the mobile node and the current parent is below a predefined threshold then the mobile node will trigger a handoff. However, the suitability of the sole measurement of RSSI in an industrial environment was argued in [23; 24]. The theory of fuzzy logic which is used to predict and support mobility in wireless network is not rare either such as [25; 26]. In [27], it proposes a reliable handoff procedure, where nodes periodically send a probe message to their access point and expect acknowledgement. The triggering accuracy is improved by using the fuzzy logic rule to combine the velocity and radio signal strength. The problem is that the deterministic delivery of periodic message and acknowledgement cannot be guaranteed. Meanwhile, a fuzzy logic-based mobility controller is designed to describe the degree of triggering in [28] by obtaining the linguistic variables. Many fuzzy rules should be predefined on each node, and more importantly, they do not involve the reliable handoff procedure to avoid the key problem of ping-pong effect. The periodic characteristics is not considered in its controller. As for the time of registration, the state of the mobile node should be evaluated or predicted during its movement. In [29], it uses historical data of radio connectivity between users and static sensor nodes to predict future paths, and another similar solution presents a mobility aware technique of low-latency deterministic network by analyzing and minimizing the dissociation time of the mobile node [30]. Though these solutions can decrease the handoff latency within an over four-hop network, they have not solved the ping-pong effect problem well. In this paper, we employ the ordered weighted average operator to improve the accuracy of handoff triggering, which relates to the theory of fuzzy logic, and more importantly integrate it into the resource scheduling to provide the more reliable communication during handoff. Moreover, the evaluation of actual mobile state is added after triggering handoff to avoid the ping-pong effect and frequent registration.