## 6 CMR-WSN Mobility Management Utilizing Fuzzy Logic  
Mobility is one of the main advantages of CMR-WSN over classical WSN, as mobile nodes (robots in this case) can move within the AoI to perform extended sensing mechanisms for areas not reachable by static sensors. However, mobility management protocol is required to maintain the connectivity of the mobile nodes (the robots in this case) during the nodes' movement. Therefore, a proper mobility management protocol must consider all the necessary actions to support the movement of mobile users without losing connectivity. The major process of a mobility management protocol is called handoff. The handoff process involves three phases: (a) triggering, (b) decision, and (c) execution. In the literature, several handoff-triggering techniques have been proposed. Most of them rely on the following criteria:  
* Received signal strength indicator (RSSI) threshold--when the RSSI is below a predefined value, then the handoff is triggered. * Link loss--if the link loss of the mobile node is above a predefined value, then the handoff is initiated. * RSSI moving average--if the average value of the RSSI over a period is below than a predefined value, then the handoff is triggered. * Exponential weighted moving average--if the moving average of the RSSI over a period is below a predefined value, then the handoff is triggered. Link loss denotes the ability of the mobile node to successfully communicate directly with the parent node while also considering any retransmissions at the Medium Access Control (MAC) layer. Figure 14 shows the behavior of RSSI during a random walk in an industrial environment and, more specifically, in a refinery [57]. It is obvious that RSSI is used as a single parameter to decide whether the handoff triggering will produce unnecessary overhead in terms of energy consumption and computation due to the scanning period for searching new attachment points, and this is due to its unpredictable behavior. On the other hand, using the link quality as a metric to initiate the handoff poses another drawback, which is the number of packets needed to achieve reasonable accuracy. Averaging techniques such as moving average and exponential weighted moving average can provide solutions to some of the drawbacks of using a single value of RSSI or link loss. Single metrics, such as RSSI, are not suitable to trigger the handoff procedure, especially when considering wireless sensor networks in dynamic environments [58; 59; 60; 61]. To solve these performance issues (increased energy consumption and a large number of unnecessary triggers) introduced by the usage of solely metrics such as RSSI and link loss, researchers started investigating different techniques to control the handoff procedure. One such technique is to use fuzzy logic. The reason for using fuzzy logic resides on the fact that:  
Figure 14: Received signal strength indicator (RSSI) behavior in a refinery environment [57]. * It can control nonlinear systems based on observable phenomena. * It is flexible due to the option to easily modify the expert-defined rules and tune the membership functions so that it can achieve the desired performance. * It can work without training and learning as other solutions such as neural network do. A fuzzy-logic system can be built on the basis of the experience of people who already understand the system. In what follows, the most related fuzzy logic-based mobility management mechanisms proposed in the literature are summarized. In [62], the authors introduced MoMoRo, a mobility layer that can be easily integrated into existing systems enabling the mobility of the nodes. MoMoRo uses a fuzzy estimator to make link quality estimations and detect route disconnections. MoMoRo was evaluated both in indoor and outdoor testbeds. The triggering of the handoff is initiated by the link packet losses, and the selection of the new attachment point is based on the fuzzy logic estimator.