# Mobility-Enhanced Reliable Geographical Forwarding in Cognitive Radio Sensor Networks
## 2 Motivation  
This section discuses the motivation behind this work, which is fundamentally centered around high route instability, channel availability estimation and protection of the PU signal. High route instability: Unpredictable and fast movements of secondary users (SUs) make guaranteeing QoS requirements and minimizing interference to PUs more challenging. This is because the higher mobility of SUs can reduce channel access time and, hence, increase the number of channel switches [5; 6; 10]. This in turn incurs a higher cost in terms of energy and delay because of the frequent initiation of route maintenance or rerouting activities. Most routing protocols have bypassed this issue by either not mentioning the considered speed of the SUs [11] or pegging the movement of the nodes to very low limits [12], which have no significant difference from static scenarios because of the high tendency of spectrum correlation at such a speed [13]. In other cases, researchers take the choice of always creating routes that bypass the coverage area of the primary user (PU) [11; 14]. However, this may not always be the ideal case, because network traffic in some cases has to go through the coverage area of the PU, especially in the case of sensor networks. Since a target area can be located anywhere. At this point, it is important to mention that the fact that reactive routing generally suits mobile scenarios [15] does not make them altogether suitable for CRSNs. This is because in addition to mobility, the dynamic spectrum access component of CRSNs, which incorporates spectrum sensing and channel switching, further amplifies the issue of route instability. Thus, this still remains an open area that needs to be addressed in CRSNs. Estimation of channel availability: With respect to this metric, the majority of routing protocols in CRAHNs can be generally classified into two categories. First are protocols that derive the spectrum-availability model based solely on PUs' temporal traffic statistics [12] without considering the benefits of spatial-temporal diversity of PU signals that comes with mobility. This greatly limits the potential of the network, especially when bursty applications are considered. Second are protocols that have assumed the perfect knowledge of PU activity for channel estimation [11]. In this case, the protocol does not practically consider the dynamics of this metric in the routing decision. Both works [11; 12] also assume that cooperative sensing is used, while, in reality, implementing meaningful cooperative sensing in mobile scenarios is very challenging and adds more overhead, which are not trivial, especially in the realms of CRSNs. With energy detection, it has been shown that spatial-temporaldiversity in the received PU signals for a single mobile CR user increases with mobility [16]. In addition, it was also shown that, without cooperative detection, the mobility of the CR node improves PU detection performance, because as speed increases, the observations are less correlated. Consequently, this property reduces the frequency of periodic sensing, which means more time slots for active data transfer by the CR user. However, it is important to mention that mobility can also reduce the average received signal strength. This calls for the need to compensate the degradation in the sensitivity of energy detection by the spatial-temporal diversity [16]. Primary user protection: In order to ensure protection of the PU communication, routes are usually built to bypass the PU coverage area. Thus, the considered metrics are typically the PU's ON and OFF times and the location of the node with respect to the PU coverage area. However, this is not usually the case when CRSNs are considered. This is because an event area that is supposed to be monitored can lie within the PU coverage area. Thus, building routes for mobile CRSNs must jointly consider the PU's activity, PU signal protection and the random spatio-temporal spectrum opportunities that are mobility induced.