# A Co-Design-Based Reliable Low-Latency and Energy-Efficient Transmission Protocol for UWSNs
## 2 Related Works
### Low-Latency Transmission Protocols  
In depth based routing (DBR) algorithms such as [7], EEDBR [13], the sender in these protocols will sense its current depth position relative to the surface and put this value in the packet header. When a receiver receives a packet, it will compare its current depth with the depth value in the packet to determine whether to forward the packet further or not without control packets. However, it is very likely that multiple nodes will forward the same packet and also receive the same packet multiple times. The holding time mechanism is often used in such protocols to alleviate this issue. During this period, the mechanism will discard the packet if it detects the same packet again, otherwise it will forward the packet. If the holding time mechanism cannot avoid the multiple copies, this will result in void communication regions. DVOR [14] is an opportunistic routing protocol that uses the query mechanism to set up distance vectors for all nodes, the sensor nodes having their own distance vector that is determined by the hop-count from the sink. The relay priority for each node can be calculated from the hop-count in the distance vector.  
The receiver will decide whether to discard or rebroadcast it, and when to rebroadcast it is determined according to its distance vector. DVOR avoids the problems of void regions and long tours, however, simultaneous forwarding by multiple nodes with the same hop-count may cause multi-copy problems. DQELR [15] implements a deep Q network (DQN) algorithm to route adaptively according to network conditions. To improve the performance of transmission, the authors combine unicast and broadcast communications using a hybrid approach. This takes time and adequate data to deploy in different networks. DMR [16] divides the network into four parts and deploys a sink in each part. Compared to the one-sink case in which data needs multiple hops to the sink nodes, multiple sinks will reduce latency. However, the long detour issue may occur inside each part and the new sink will die earlier than other nodes.  
ALOHA-based UWAN MAC protocols are the simplest MAC protocols, with which a node transmits a data frame anytime at its will without any control packets. Ref. [6] shows that collision probability is around 0.4 for most traffic arrival rates underwater. Obviously in most cases, this type of protocol cannot be used directly underwater. The protocol [17] consists of a TDMA scheduling scheme for interference reduction and a hierarchical routing scheme for reliable data transmission in UWSNs. The article describes a tightly coupled MAC and routing protocol, which may not work well with other protocols. The optimization of energy costs is not considered. ST-MAC [18] formulates a TDMA-based scheduling problem as an NP-complete vertex coloring problem for a conflict graph. The graph consists of links and their relationships. Through a link-scheduling algorithm it allocates transmission time to nodes. Similarly in [19], which proposes an interference-free graph according to the links between nodes, the two protocols take advantage of the spatial-temporal reuse strategy for the TDMA. Both of these will improve transmission efficiency.