## 3 Underwater Communications
[PERSON] and [PERSON] propose in [12] a novel multiple query result merging scheme for underwater sensor networks. It is based on a relational database model and is adjusted to the practical restrictions affecting underwater communication environments. It is an energy-efficient technique for providing data on-demand in an underwater sensor network to multiple-users simultaneously. They also proposed a message payload format capable of maintaining robustness in the face of unexpected communication failures. Network simulations prove that the query management scheme can efficiently reduce redundant message transmissions and the entailed energy waste, and enhance network lifetime. It becomes more efficient with a greater number of generated queries and a relatively smaller period range. In [13], [PERSON] and [PERSON] have studied how acoustic propagation models impact the performance of high level communication protocols. First, they analyzed the evolution of underwater acoustic prediction models. Second, they analyzed the performance behavior of different MACprotocols and also checked how the scenario environmental changes impact on their network performance in terms of throughput, delay and collisions. They tested different high layer network protocols (MAC and routing protocols) from a simple to more detailed and accurate models in order to determine the influence of environmental parameters on the obtained results. They concluded that higher-level protocols are sensitive to both, physical layer parameters related to the network scenario (like ocean surface activity, scenario location, bathymetry or floor sediment composition) and the acoustic propagation model (in terms of accuracy and low-complexity). In the paper, they show the importance of defining an accurate and low complexity propagation model, and the sensibility of higher layer protocols to the time-varying scenario environmental conditions. They found that accurate and low-complexity propagation models are required for network simulation in order to obtain reliable results attained to the specific scenario and environmental parameters. This information should be taken into account when designing UWSN architectures for real network scenarios. [PERSON] _et al._ show in [14] how to remove intersymbol interference (ISI), and evaluate the performance of the implementation of three different receiver structures using real data collected in a high-frequency (10-14 kHz) field experiment. The three structures were multichannel decision feedback equalizer (McDFE), passive time reversal receiver (passive-phase conjugation (PPC) with a single channel DFE), also called PPC-DCE, and the joint PPC with multichannel DFE (PPC-McDFE). In order to test the scenario, the information of the channel characteristics for PPC processing was obtained by using the two shaping pulses: a linear frequency modulation pulse (LFM) chirp as a channel probe, and a root-raised-cosine pulse (RRC). They observed that the difference between LFM and RRC is minimal. The three structures were evaluated by off-line data processing. In the assessment, they found that it is advantageous to obtain spatial gain using the adaptive multichannel combining scheme and the matching pursuit (MP) algorithm improves the performance of communications using PPC processing. They observed that PPC-DFE achieved the worst performance in the assessment, and the performance of PPC-McDFE approximated that of McDFE. Time-variant reverberations resulted in unpredictable spatial coherence, which may impact on the performance of PPC-DFE. Therefore, it is preferable to use PPC-McDFE instead of PPC-DFE in a channel of long time spread. Moreover, they observed that PPC-McDFE is less sensitive to the channel variations than PPC-DFE.