## 3 Underwater Communications  
[PERSON] and [PERSON] show the design and Implementation of an Omni-Directional Underwater Acoustic Micro-Modem Based on a Low-Power Micro-Controller Unit in [9]. The work provides the development of the cylindrically shaped micro-modem guaranteeing an omni-directional beam pattern in three dimensions. The system uses an ARM Cortex-M3 is embedded in the micro-modem to provide fast digital signal processing and to support flexible interfaces with other peripherals, and a spherical transducer for the implementation. The size of the developed cylindrical modem is only 70 mm in diameter and 35 mm in height. The authors also implemented in software and hardware the physical layer frame format and symbol structure for efficient packet-based underwater communication systems. Experiments were executed in a water tank, a pond and a river, obtaining expected results by analysis. Their experiment in the river showed that the modem was able to send data wirelessly up to 70 m with a data rate of 0.2 kbps while consuming a power of 4.5 watts. But it can be increased using a higher supply voltage or a transducer having lower resonant frequency. The modem can be used for various UWSN applications requiring acoustic communication taking profit of its features (low-power consumption, micro size, low cost, omni-directivity). The paper in [10], authored by [PERSON] _et al._, describes the physical layer of a new low-cost acoustic modem called ITACA. Its architecture combines a typical microcontroller-based core with energy-efficient mechanisms. One of the main features of the modem is the implementation of an ultra-low power asynchronous wake-up system for underwater acoustic transmission that is based on a low-cost off-the-shelf RFID peripheral integrated circuit. This allows a reduced power dissipation in stand-by mode and very low power values during reception and transmission. The modem uses coherent-FSK modulation. It also incorporates clear channel assessment (CCA) detection using both carrier frequency detection and the RSSI level to support CSMA-based medium access control (MAC) layer protocols and an efficient development of upper communication layers by supporting additional RTC (Real Time Clock) asynchronous wake-up and clear channel assessment (CCA). This asynchronous wake-up design has been tested for non-detections and false positives with excellent results. [PERSON], and [PERSON] propose in [11] the Selective Multiple Acknowledgement (SMA) method, based on Multiple Acknowledgement (MA), to efficiently reduce the amount of data transmission in an underwater environment. The transmission technique collectively transmits Acknowledgement (Ack) or Negative Acknowledgement (Nack) in order to pro information regarding the existence (or non-existence) of transmission errors. Ack and Nack are included in the Beacon frame. There is a Cluster Head (CH), which supervises scheduling and data collection, collects data via Uplink transmission within a unit cluster. Authors redesigned the transmission frame structure and took into consideration underwater transmission characteristics, which let it handle the same amount of data in a much more compact frame structure without any appreciable loss of reliability. Its method significantly reduces the number of transmissions and the frame length (which increases according to the number of participating network nodes containing Ack). They analyzed and compared in terms of performance their proposal with the conventional Automatic Repeat-reQuest (ARQ), Block Acknowledgement (BA), block response, and MA methods, obtaining better results. The proposed SMA present the smallest frame size and the lowest number of transmission with various variables. As the number of transmission targets increases, the performance of the proposed technique shows better performance, thus the efficiency of the underwater sensor network, which forms a large cluster and mostly contains uplink data is improved by the proposed method.