# Mobility-Enhanced Reliable Geographical Forwarding in Cognitive Radio Sensor Networks
## 3 Related Work  
One of the most relevant works in this regard is the spectrum-aware routing protocol for cognitive _ad hoc_ networks (SEARCH) [12]. SEARCH is a geographic forwarding-based scheme, which jointly undertakes path and channel selection to avoid regions of PU activity during route formation. During the route operation, it can adapt to the newly-discovered and lost spectrum opportunity while considering node mobility in a distributed environment. The key functionality of the SEARCH approach is its evaluation to decide on when the coverage region of the PU should be circumvented and when changing the channel is a preferred option. Although SEARCH is also a geographical forwarding scheme as MROR, MROR implements a lossy link-aware geographic forwarding for CRSNs, which is a more realistic consideration. The MROR approach to routing is more energy efficient and more scalable than the SEARCH approach in the following ways: (i) in SEARCH, route search is broadcast via all channels in order to identify all possible routes to the sink. Thus, the authors did not consider energy, which is a fundamental routing metric in resource-constrained CRSNs [3]. Contrarily, MOR uses the concept of the common control channel (CCC). The design of a simple CCC acquisition scheme for CRSNs has also been proposed by the same authors [19]; (ii) In MORR, the data-forwarding phase is more stateless and scalable, as the next hop is not restricted to a single node as in SEARCH; rather, the next hop is instantaneously decided upon based on the best node that meets the required QoS criteria; (iii) Fundamentally, SEARCH restricts the movement of the nodes to low movement, which has no significant difference from static scenarios because of the high tendency of spectrum correlation at the considered speed. Furthermore, its routes are built to always bypass PU coverage with the aid of anchor locations, which are out of PU coverage. This is the main reason why speed was not considered as a routing metric. As already mentioned, MCRSNs have to consider all possibilities of having routes both within and outside the PU coverage area. Thus, MORR considers a mobility channel availability model as a fundamental routing metric that protects the PU signal within the PU coverage area by considering the PU's activity, but also increases the routes throughput by seizing the spatio-temporal spectrum opportunities that are mobility induced. Similar to SEARCH, the geographic routing protocol for cognitive radio mobile _ad hoc_ networks (TIGHT) [14], is also built to completely bypass PU activity areas. However, unlike in SEARCH, TIGHT proposes three routing modes. The greedy mode in TIGHT utilizes greedy geographic forwarding to route a packet until it encounters a PU region, after which it routes the packet around the PU region to where greedy forwarding can take over. The other two modes that work best when PUs are active most of the time include the optimal mode, which routes a packet along an optimal trajectory to the sink, and the suboptimal mode, which routes it along a suboptimal trajectory. However, unlike SEARCH and MORR, TIGHT considers only a single PU channel scenario, which is a simpler case to handle. This is because the multi-channel issue, which is a major characteristic of CR networks, is easily bypassed by making all nodes tune to the only PU channel available by default. In addition, just as in SEARCH, by always avoiding PU active areas, it bypasses the mobility-induced spectrum opportunity fluctuation. Furthermore, it does not address the issue of route stability with respect to the speed of the mobile node. In [11], a distributed cognitive radio routing protocol for _ad hoc_ networks (CRP) was proposed. The main difference CRP has in contrast to SEARCH is, in addition to bypassing the PU activity area, CRP also supports the on-demand creation of multiple classes of routes based on service differentiation in CR networks.