In this paper, we propose an improved energy optimization clustering algorithm (EOCA) for the multi-hop underwater acoustic cooperative sensor networks (UWA-CSN) due to the limited energy supply of the underwater sensor nodes. The proposed EOCA scheme considers multiple factors, such as the number of neighbor nodes, the residual energy of each node, the motion of the sensor nodes caused by the ocean currents, and the distance between the sink node and each underwater sensor node. All these factors are considered within the framework of the multi-hop cooperative communication. Moreover, in the proposed energy optimization scheme, we define a residual energy based maximum effective communication range (reE-MECR) according to the residual energy of each underwater sensor node, which can adaptively control the energy consumption of data transmission for each hop. Compared with the existing clustering scheme, the experimental results demonstrate that the proposed EOCA scheme can not only prolong the life time of the multi-hop UWA-CSN, but also keep a good communication and networking performance, including the package delivery ratio, the energy consumption efficiency, and the network coverage area. The proposed scheme can obtain both the clustering benefit and the cooperative communication benefit for the multi-hop UWA-SN in terms of energy saving in underwater environments.

Multipath due to reflections of the sea surface, seabed, and obstacles, as well as inhomogeneity within the ocean, is an important characteristic of underwater acoustic channels. Mutual interference among multiple paths causes severe amplitude fading and frequency selective fading. The guard interval is an effective anti-multipath method, but an excessively long guard interval will reduce the data rate of multi-carrier underwater acoustic communication. In this paper, we propose a new antimultipath multi-carrier communication method based on orthogonal chirp division multiplexing (OCDM) that uses chirp signals for carrier modulation. As OCDM exploits the multipath components for diversity gain, the system robustness is improved. The new method also adds a data pick-based rake receiver for maintaining good communication performance, even at short guard intervals. We detail the implementation and parameter selection of the anti-multipath OCDM system and compare its performance with the traditional orthogonal frequency division multiplexing (OFDM) scheme using simulations; under a severe multipath simulation condition (the delay spread is longer than the guard interval), the anti-multipath OCDM achieves a bit error rate (BER) of 10-6, while the OFDM has a BER floor of 10-3. The simulation results verify the feasibility of the proposed method and the superiority of its anti-multipath performance.

Dynamic coded cooperation (DCC) allows relay diversity without altering the transmission procedure from the source to the destination. In this paper, we propose a practical orthogonal frequency-division multiplexing (OFDM)-modulated DCC scheme for underwater relay networks, where OFDM modulation accommodates multipath fading channels with large delay spread. Two cooperation strategies are studied, where the relay transmits either identical or different OFDM blocks as the source during the cooperation phase. The block-level synchronization between the OFDM blocks from the source and the relay is achieved by a delay control mechanism at the relay, by knowing the distances among the source, relay, and destination. Two OFDM-DCC design examples are presented, one based on nonbinary rate-compatible low-density parity-check (LDPC) codes applied across multiple OFDM blocks, and the other using layered interblock erasure correction and intrablock error-correction coding. In addition to simulation studies, one particular design has been implemented on practical OFDM modems, and tested in a swimming pool and in a recent sea experiment. The proposed OFDM-DCC scheme is particularly appealing to underwater acoustic (UWA) networks where a relay node with abundant resources (e.g., a surface buoy) can enhance communications among underwater nodes without changing their transmission procedure.

The dynamic topology, narrow transmission bandwidth, and limited energy of sensor nodes in mobile underwater acoustic sensor networks (UASNs) pose challenges to design an efficient and robust network for underwater communications. In this paper, we propose a novel machine learning-based clustering and routing scheme, named energy-efficient clustering and cooperative routing based on improved K-means and Q-learning (ECRKQ), to reduce and balance energy consumption among sensor nodes in a mobile UASN and improve the bandwidth utilization. In the cluster head (CH) selection stage, ECRKQ modifies the K-means algorithm to dynamically select a CH based on the residual energy of the node and the distance from the node to the centroid in a cluster. In the clustering stage, ECRKQ adopts the Q-learning algorithm by incorporating the residual energy of the CH, the energy consumption of data transmission from the node to the CH, and the energy consumption of the data transmission from the CH to the base station into the Q-value function. In the data transmission stage, ECRKQ applies the dynamic coded cooperation (DCC) transmission to improve the bandwidth utilization and the robustness of the underwater communications. In the DCC transmission, cooperative nodes are also dynamically selected based on the residual energy and the energy consumption of transmitting a packet to their destinations. In the simulation, we apply the ocean current drifting model to emulate the position variation of nodes caused by ocean currents in a mobile UASN. The simulation results show that the proposed ECRKQ scheme can achieve more balanced energy consumption among sensor nodes in a mobile UASN than that of the existing scheme.

With the advancement and implementation of marine strategies in various countries, it is of great significance to collect marine data by using underwater acoustic sensor networks (UWA-SNs). Due to the difficulties in charging or replacing batteries, how to prolong the lifetime of underwater sensor nodes is one of the key problems to be solved. In this paper, we propose a new energy consumption balanced protocol, named as dynamic clustering K-means (DC-K-means) based simplified balanced energy adaptive routing (S-BEAR), to prolong the lifetime of a UWA-SN by balancing the energy consumption of underwater sensor nodes and avoiding the energy hole. First, by considering the residual energy of the underwater sensor nodes, we propose the DC-K-means algorithm to cluster the underwater sensor nodes and optimize the topology of data transmission among underwater sensor nodes so that the overall energy consumption of the system can be reduced and the problem of energy consumption balance of underwater sensor nodes can be solved. Then, the S-BEAR protocol is designed for multi-hop transmissions, where the data collected by each cluster head is transmitted to the surface receiving terminal equipment, and thus further balances and saves the energy consumption of the system. The simulation results demonstrate that the proposed DC-K-means based S-BEAR scheme has the advantages of more balanced energy consumption among sensor nodes and higher overall residual energy as compared to the state-of-the-art schemes.

In this paper, we consider a practical underwater data collection network, where one destination needs to collect data from multiple underwater nodes. With the conventional automatic-repeat-request (ARQ) protocol, the destination requests retransmission from each node individually without any node cooperation. We propose two protocols, selective relay cooperation and dynamic network coded cooperation, utilizing the fact that underwater nodes can overhear the transmission of the others. In the selective relay cooperation, one node can be selected as a relay to transmit the data from another undecoded node in the retransmission phase. In the network coded cooperation, the selected relay nodes transmit network coded packets to the destination. The relay nodes participating the cooperation are selected by the destination based on the channel quality, as measured by the effective signal-to-noise ratio. In addition to simulation results, we have carried out several lake tests based on a full protocol implementation. Simulation and field testing results demonstrate that the proposed schemes can gain significant performance improvement compared with the conventional ARQ scheme.