## 3 Routing Protocols for UWSN
### Protocols Featuring Channel Properties
#### 3.3.2 Location-Aware Routing Protocol (LARP)  
Even though existing routing protocols have improved UWSN drastically, they still pose many problems. [PERSON] et al. proposed a novel routing protocol which gives a better performance in delivery ratio and reduce overhead in compare to all other existing routing protocols. This is called Location-Aware Routing Protocol (LARP) [58]. It adopts a Received Signal Strength Indicator (RSSI) method to know the location of the nodes [70] and it utilizes this location information of the nodes for the packet data transmission. It is considered that there are two kinds of nodes in the network-(1) anchor nodes (2) general nodes. Anchor nodes are the reference nodes and are used to determine the location of the other nodes. Anchor nodes are equipped with GPS module which broadcast the location information to the beacon. It is considered that anchored nodes have- (i) the property to know the precise location of the anchor nodes by GPS technology or other methods [71], (ii) enough energy and storage capability, (iii) enough RF transmission range to cover the whole network scale, (iv) the property of random mobility.  
Figure 8: Packet transmission in Directional Flooding-Based Routing (DFR) [57].  
With the help from 3 or more anchor nodes, a general node usually finds its own positioning and they save their positioning information. If these general nodes come near to any anchor node, they transmit their location information to the anchor node to store. Figure 9 depicts data transmission of LARP where N1, N2 and N3 are anchor nodes. S is designated as source and D is designated as destination for the transmission. At first, the source node looks for an anchor node within its range or waits for any anchor node to come within its range. Source node requests for the destination node location to the anchor node. When anchor node broadcast the destination ID, all other anchor nodes will check that if there is destination node D within their range. If any of the anchor nodes find the destination node, the source node will get the information about the destination node.  
Now the node S will broadcast the 'destination location' request within its range. If the destination node D is within the range and receives the destination location request, it will reply to the node S. After that, node S will directly transmit to node D. Note here that, no other node will reply to the request upon receiving. When the source node gets no reply, the source node broadcasts'moving direction' requests. Moreover, Source node S collects all the information of the nodes within its range. Now the node which is moving in the direction of D, reply to node S. In the next step, the data packet is delivered to that particular node. Thus, the next best hop is selected. If it happens that, there are more than one node which is directed towards the destination D, then their speed is taken into consideration. The node with the greater speed is chosen as the best hop.  
If it happens that there is no node directed towards the destination node then no node replies back to the node S and it waits until it gets any suitable node to make it the next best hop. In Figure 7, node 3 is selected as the next hop. In this way best hop is selected recursively until the data packet is delivered to the destination node. Simulations show that LARP gives a decent packet delivery ratio, lower normalized overhead. Moreover, it gives more reliable transmission than the other routing protocols even though it possesses some drawbacks. It needs a very dense network to work perfectly, besides a source node had to wait for an indefinite time if it does not get any anchor node within the range. It may also happen that the source node may have to wait for a long period of time to get a node which is directed towards the destination node to select it as the next best hop.