# A Survey on Underwater Acoustic Sensor Network Routing Protocols
## 5 The Cross-Layer Design Method
### The Optimization-Based Method
#### 5.1.1 Location Information-Free Routing Protocols  
The location information free routing protocols do not need node location information. Due to the absence of location information, these routing protocols use the flooding model, the deep information, or the hierarchy model to send data packets from the source node to the sink node. Due to the fact that getting location information in the underwater environment is much more difficult than that in the terrestrial environment, in underwater environments, the routing protocols always assume that the location information is unknown. However, without the help of the location information, the routing discovery process is always blind, which increases the energy consumption and propagation delay. The challenge of location information free routing protocols is how to reduce this blindness and improve the algorithm efficiency. There are many papers devoted to the design of cross-layer design routing protocols without utilizing node location information, such as [60; 61; 62; 63; 64; 65; 66; 67; 68]. The routing protocol in [60] uses the history of successful transmission to the neighbor nodes, the link quality, the residual energy, and the buffer space to select next hop relay nodes. For this reason, the protocol has good next hop node selection and energy consumption performance. In [62], the nodes are divided into different levels based on their residual energy, only the nodes in the same energy level can be chosen as a next hop relay node. The work in [63] is also an energy efficient routing protocol. In [63], each node can decide a specific forwarding group rather than all the neighbor nodes to reduce unnecessary broadcasts and transmissions. The relay nodes are chosen based on the distance to the sink node and their residual energy, which can successfully prolong the network lifetime. Like [62], the work in [65] divides the nodes into different levels based on their power level. The nodes can adapt their transmission power based on the network conditions. The power level and the residual energy of nodes are used to choose the next hop relay nodes. The proposal in [66] is similar to the protocol introduced in [62; 65]. In [66], the depth information and the residual energy of nodes are chosen to decide the next hop relay nodes. The nodes whose depth is smaller than the source node and whose residual energy is higher will be chosen. The result is that the network lifetime can be prolonged when using [66]. A similar routing protocol can be found in [64], which proposes a set of routing protocols and scheduling to reduce the effect of interference and energy consumption. Some papers not only consider the distance and residual energy, but also combine the network constraints with other methods to improve the network performance. In [61], the authors introduce MIMO-OFDM (Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing) into underwater acoustic sensor network routing protocol design, which successfully achieves excellent energy consumption and propagation delay performance. The work in [67] is a tier-based UASN routing protocol. In this routing protocol, the network topology is partitioned into tiers, and the next hop relay nodes towards the surface sink are limited to the nodes that belong to the upper tier. The protocol in [68] is an adaptive tree reconfiguring protocol where the data aggregation tree is reconfigured by using a dynamic pruning and grafting function. The function can change the routing path based on the information of the aggregation count and minimum residual energy of nodes. In this section, cross-layer design location information-free routing protocols are reviewed. In cross-layer design methods, more than one network constraint can be considered in routing protocol design. The most common factors are depth information [63; 66; 67] and the residual energy [60; 62; 65; 68].