## 2 Related Work  
Due to the characteristics of the underwater acoustic channel, e.g., high propagation delay, high bit error rate, multi-path effect and Doppler effect, there is a larger difference between the UWSN and TWSN. In UWSN, we must take more care about the complex underwater environment, especially for the contaminated water. It needs an extra design for real scenarios and computational complexity. Consequently, the commonly traditional topology control in the TWSN cannot be directly applied to the UWSN. We need to redesign a series of algorithms or protocols to meet the requirements of UWSN according to the underwater environment conditions. Energy-efficiency is one of the most important targets to design a topology control mechanism in UWSN. To address this aim, clustering technology is the most common and available scheme for reliable communication and energy conservation. LEACH [7], which is a typical cluster-based topology control algorithm, periodically selects cluster-heads and uniformly drains energy by role rotation with data fusion strategy. Nevertheless, it is more suitable for ideal network model in the homogeneous TWSN, and there are some problems in its own clustering mechanism, such as uneven distribution of cluster-heads and poor energy balance. EDCS [8] is another cluster-based scheme which is proposed for heterogeneous network scenarios. Compared with the LEACH, EDCS achieves the better energy-saving effects for more general heterogeneous network because it introduces more accurate average network estimation and gravitational strategies. However, this does not apply to UWSN, which is a more complex and general heterogeneous network. Recently, many topology control algorithms have been proposed for UWSN applications [9]. As one of the most important factors, energy-efficient topology control is firstly focused on by scholars under UWSN. [PERSON] et al. [10] proposed two mechanisms, namely, the centralized topology control and the distributed topology control, to organize the network by some nodes depth adjustment. Combined with the corresponding geographic forwarding protocol, the data packets delivery ratio can be achieved into a higher level even in such hard scenarios (e.g., very sparse or dense network), while the energy consumption can also be reduced. Nevertheless, it must have the precise node position information which is provided by the extra localization system; simultaneously, the network cannot guarantee connectivity because it may have isolated nodes. [PERSON] et al. [11] proposed a new kind of greedy forwarding (NGF) strategy for the geographic-based topology control in UWSN using the splitting mechanism with Chinese remainder theorem. In NGF, it is effective for more than two nodes to participate in the forwarding of one packet instead of selecting only one node as the next-hop, because the source node can reduce the number of bits transmission by splitting mechanism. NGF can increase the lifetime and other network performance, but it needs an extra strategy to solve isolated and void nodes problem appearing in the topology formation result. A scale-free network model for calculating the edge probability is used to randomly generate the initial topology in [12]. Subsequently, a complex network theory-based topology control strategy is put forward to build a dual-clustering structure with two kinds of cluster-heads to ensure the connectivity and coverage, as well as optimize network energy consumption and propagation delay. It indicates the scale-free model can be applied in UWSN hierarchical topology but it may not satisfy the demand of specific applications. Specially, they do not mention the tradeoff between cluster-heads vulnerability and the cost from role rotations when using clustering technology. Further, to meet the requirement of diverse coverage in UWSN, [PERSON] et al. [13] proposed the traversal algorithm for diverse coverage (TADC) and the radius increment algorithm for diverse coverage (RIADC), respectively.