Underwater RF system design architecture (a) Buoyant RF communication system (b) Direct RF communication System.

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... in the order of 100 W. However, it requires sophisticated antennas design and high transmission power. For RF system design which involves a communication link between underwater and terrestrial transceiver, any frequency range from MHz to GHz works effectively. Such communication systems are called buoyant RF communication system as shown in Fig. 1 (a) and they are not truly underwater communication. Other design configuration involves direct RF communication link between two transceivers submerged underwater or one set inside the water and other set in the air. This type of system design is called direct RF communication system as shown in Fig. 1 (b) and it makes use of ELF or ...

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... buoyant RF communication system as shown in Fig. 1 (a) and they are not truly underwater communication. Other design configuration involves direct RF communication link between two transceivers submerged underwater or one set inside the water and other set in the air. This type of system design is called direct RF communication system as shown in Fig. 1 (b) and it makes use of ELF or low frequency (LF) for ...

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... (i) direct LOS links, (ii) NLOS links and (iii) retro-reflector links. continuous wave (CW) laser. It was observed that even in turbid water, 5 Mbps can be achieved using high power laser. Cox et al. [132] presented back-scatter suppression in LOS link using polarization shift keying along with channel coding. LOS link configuration is shown in Fig. ...

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... optic signal at the ocean-air interface. These types of links are also called reflective links. Other way to implement NLOS links is to spread or diffuse the optical light from the LEDs or lasers in order to increase the FOV of the receiver. Such type of links are also called diffuse links. Both reflective and diffuse scenarios are presented in Fig. 11. Compared to LOS links, these links do not require stringent pointing and tracking requirements especially in a turbid environment that causes spatial dispersion of collimated light. For clear lake or ocean water, the divergence of laser beam has to be increased by making use of array of LEDs or lasers to form a cone of light defined ...

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... and tracking requirements especially in a turbid environment that causes spatial dispersion of collimated light. For clear lake or ocean water, the divergence of laser beam has to be increased by making use of array of LEDs or lasers to form a cone of light defined by inner and outer angles θ min and θ max in the upward direction as shown in Fig. 11 (a). When the angle of transmitted light is greater than the critical angle, the transmitted light strikes the ocean-air interface and is reflected back into the water due to total internal reflection (TIR). When the transmitter is at depth h, the illuminated annular surface with equal power density at depth x will be given by [15], [133] ...

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... role in link range and capacity. This is a critical problem for applications related to underwater laser imaging. Here, the increased back-scatter component leads to reduction of photons and thereby, decreases the contrast of the image. The back-scatter component can be significantly reduced by using polarization discrimination [139], [140]. Fig. 12 shows retro-reflector configuration. power, detector's sensitivity as well as extinction through the water. The models of aquatic optical attenuation can be classified into two categories: LOS configuration and NLOS configuration. Light propagation in both the configurations experience the same attenuation effects, however, in case of ...

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... system design for UOWC is shown in Fig. 13. It consists of a source that generates the information to be transmitted which is then modulated on the optical carrier to be transmitted to longer distances with a high data rate. The transmitter is equipped with projection optics and beam steering elements in order to focus and steer the optical beam towards the position of the ...

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... multiple-access (OCDMA) network based on optical orthogonal codes (OOC) for different water types. Their work is extended in [216] using cooperative diversity in underwater environments where each source node uploads its own data via relays to a remotely located optical base transceiver station (OBTS) using OCC-OCDMA technique as shown in Fig. 14. BER performance of relay-assisted OCDMA was investigated and it was observed that even a dual-hop transmission in 90 m point-to-point clear ocean link gave a performance advantage of 32 dB over direct transmission. changes very rapidly with time. In such cases, multi-hop transmission in conjunction with geographic routing protocol are ...

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... temperature fluctuations that give rise to volume expansion and contraction [222]. The volume fluctuations in turn generate a propagating pressure wave with the acoustic signal characteristics of the laser modulation signal [16]. The hybrid acoustic optic system has significant advantages in terms of throughput and energy efficiency as shown in Fig. 15. It is seen from Fig. 15 (a) that the throughput of acoustic channel saturates as the offered load increases. The throughput increases drastically with only optical as well as hybrid channel models though the hybrid model outperforms the other two. Fig. 15 (b) clearly indicates the increase in power consumption of acoustic signals with ...

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... that give rise to volume expansion and contraction [222]. The volume fluctuations in turn generate a propagating pressure wave with the acoustic signal characteristics of the laser modulation signal [16]. The hybrid acoustic optic system has significant advantages in terms of throughput and energy efficiency as shown in Fig. 15. It is seen from Fig. 15 (a) that the throughput of acoustic channel saturates as the offered load increases. The throughput increases drastically with only optical as well as hybrid channel models though the hybrid model outperforms the other two. Fig. 15 (b) clearly indicates the increase in power consumption of acoustic signals with data rate. In this case, ...

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... system has significant advantages in terms of throughput and energy efficiency as shown in Fig. 15. It is seen from Fig. 15 (a) that the throughput of acoustic channel saturates as the offered load increases. The throughput increases drastically with only optical as well as hybrid channel models though the hybrid model outperforms the other two. Fig. 15 (b) clearly indicates the increase in power consumption of acoustic signals with data rate. In this case, both hybrid and optical models yield comparable results. Therefore, the hybrid acousto-optic system provides some degree of freedom to select the optimal transmission method within least transmission time depending upon the load and ...