# Mobility-Enhanced Reliable Geographical Forwarding in Cognitive Radio Sensor Networks
## 5 Mror: Mobile Reliable Opportunistic Routing for CRSN
### VMH Zoning System  
In order to ensure the stability of the built routes, MROR introduces the concept of the virtual mobile handover (VMH) zoning system. VMH defines an area in the just created virtual clusters beyond which an LMR node has to hand over its role to the most deserving node in the virtual cluster. Once the LMR nodes have been identified, the VHM initiator function is automatically activated. This function helps the LMR node when it is appropriate for it to initiate the handover process to the next most deserving node in its virtual cluster. The VHM initiator function is formulated as:  
\[\frac{v_{n_{i}}}{0.6\left(d_{sk}^{lmr_{h-\alpha}}-d_{sk}^{lmr_{h-\alpha+1}} \right)-\left(d_{sk}^{n_{i}}-d_{sk}^{lmr_{h-\alpha+1}}\right)}\leq t_{ho} \tag{29}\]  
where \(t_{ho}\) is the expected time it takes for a successful handover operation. Due to the speed of the node, this function was designed to be primarily triggered when an LMR node has covered 60% of its VC. This value was chosen because, since the protocol was designed to always select routes with  
Figure 5: VCG organization after the VCG formation stage. VCG, virtual contention group; PU, primary user. the lowest channel switch, at 60% of the previous VC, the system gives more chances to the handing over node to still participate in forwarding packets before it enters into the next VMH zone. Thus, \(v_{n_{i}}\) represents the speed of the node \(i\), \(d_{sk}^{lmr_{h-a}}\) is the distance-to-sink of the handing over node at the time the VCG formation ended. Likewise, \(d_{sk}^{lmr_{h-a+1}}\) is the distance-to-sink of the preceding LMR node at the time the VCG formation ended, and \(d_{sk}^{il}\) is the present distance-to-sink of node \(i\). \(a\) is used to represent the VC id in which the handover is being initiated. On the other hand,  
\[t_{ho}=\frac{2t_{s,hp}+t_{w,rp}+t_{t/o}}{p_{s}^{2}} \tag{30}\]  
where \(t_{s,hp}\) is the expected time it takes the handing over LMR node to successfully announce the handover bidding process. This value is multiplied by two, because a handover exercise is only successful after the LMR node receives a broadcast reply from an eligible node, which marks the end of the handover process. The expected time the node will wait for a reply is represented by \(t_{w,rp}\), and \(t_{t/o}\) is the expected time the node's timer runs out (_i.e._, deciding that a neighbor does not exist). This subsequently triggers the route maintenance module, as discussed in Section 5.8. As illustrated in Figure 6, once an LMR node successfully broadcasts its handover request packet (hrp), the region between \(d_{sk}^{n_{i}}\) and \(d_{sk}^{lmr_{h-a}}\), which is referred to as the handover region, is partitioned into \(G\) priority regions. Every priority region \(pr_{i}\) is made to match a back-off window size \(CW_{i}\). Thus, based on a node's location in the handover region, it backs-off for \(\sum_{j=1}^{i-1}CW_{j}+cw_{i}\), where \(cw_{i}\) is randomly selected, such that \(cw_{i}\in[0,CW_{\max}]\) where \(CW_{\max}CW_{i}-CW_{i-1}\), \(\forall i\). Hence, based on how close a node is to \(d_{sk}^{lmr_{h-a}}\), this backoff scheme prioritizes them. When a node receives an hrp packet, it backs-off for \(\sum_{j=1}^{i-1}CW_{j}+cw_{i}\), as corresponds to its priority area. In this way, a node in any priority region will wait in its priority slot for an average of \(CW_{\max}/2\) in addition to the waiting period of all of the previous priority slots. During this period, if it overhears a handover reply from another node, it assumes the LMR position as occupied and refrains from sending. At this stage, the handover process is declared completed. Figure 6: MROR virtual mobile handover zoning system.