# A Survey on Underwater Acoustic Sensor Network Routing Protocols
## 5 The Cross-Layer Design Method
### The Optimization-Based Method
#### 5.1.2 Location Information-Based Routing Protocols  
With the development of underwater GPS and underwater location algorithms, it is attractive that the routing protocols utilize the location information to reduce the blindness in routing discovery, which can save energy and reduce propagation delay remarkably. As we know, if the source nodes know the location of other nodes (such as the sink node and the relay nodes), the process of routing discovery will be more effective than that of the location information-free routing protocols. Even though location information-based routing protocols are effective, considering the expensive underwater GPS devices needed and the complexity of the underwater location algorithm, the performance of the location-based routing protocols will decrease. The challenge is to find a more simple approach to get the location information of nodes.  
The routing protocol proposed in [69] is based on HH-VBF [43], but by taking the power level, the distance to the destination node, and the transmission range into account, the performance of adaptive hop-by-hop vector-based forwarding (AHH-VBF) [69] is much better than that of HH-VBF in terms of data delivery ratio, energy consumption, and end-to-end latency. Considering that greedy flooding is energy intensive, in the focused beam routing (FBR) protocol [70], the authors try to use the location information of source nodes and destination nodes to build the routing path. To improve the routing robustness, the nodes can dynamically change their transmission power according to the network conditions. FBR can achieve better energy consumption performance, but it is not flexible and appropriate for sparse networks. The principle of FBR can be found in Figure 5. In [71], the protocol relies on the degree of information of neighbor nodes to make routing decisions. The nodes that have different node degree belong to different layers. The node ID, the coordinate information, and the residual energy level are used to decide the node degree. The information can only be transmitted from low level degree to high level degree nodes. The authors in [72] investigate the function between the corresponding node distance and packet size. Based on this function, two geographical routing algorithms are proposed. The algorithms take the transmission power, the channel quality, and the application requirements into account to choose the next hop relay nodes. Realizing the fact that an underwater sensor network is an intermittently connected network and different applications have different requirements, in [73], the authors propose a routing protocol in which the routing is performed adaptively based on the message types and application requirements. The algorithm allows packets with higher priority to forward more copies. The routing decisions are made based on the packet priority afterwards. As a result, the algorithm can not only satisfy different application requirements, but also achieve a good trade-off between delivery ratio, average end-to-end delay, and energy consumption.  
In this section, routing protocols that introduce the need for location information to improve the routing performance were discussed. The location-based routing protocols have better performance than the location information-free routing protocols, so the performance in [70; 71; 72; 73] is much better than that of the routing protocols that were introduced in Section 5.1.1. The reason is that once the nodes  
Figure 5: The principle of FBR.  
know their own location of that of the destination node, the routing discovery and data transmission will be less blind and more effective than in the location information-free routing protocols. However, the disadvantage is that the location information of underwater nodes is hard to get, and sometimes it is impossible in an underwater environment.