The need for underwater wireless communications exists in applications such as remote control in off-shore oil industry, pollution monitoring in environmental systems, collection of scientific data recorded at ocean-bottom stations, speech transmission between divers, and mapping of the ocean floor for detection of objects, as well as for the discovery of new resources. Wireless underwater communications can be established by transmission of acoustic waves. Underwater communications,...
More DescriptionThe need for underwater wireless communications exists in applications such as remote control in off-shore oil industry, pollution monitoring in environmental systems, collection of scientific data recorded at ocean-bottom stations, speech transmission between divers, and mapping of the ocean floor for detection of objects, as well as for the discovery of new resources. Wireless underwater communications can be established by transmission of acoustic waves. Underwater communications, which once were exclusively military, are extending into commercial fields. The possibility to maintain signal transmission, but eliminate physical connection of tethers, enables gathering of data from submerged instruments without human intervention, and unobstructed operation of unmanned or autonomous underwater vehicles (UUVs , AUVs).Underwater communications in general mainly gets affected due to* Channel VariationsChannel variations are variations in: - Temperature - Salinity of water - pH of water - Depth of water column or pressure and - Surface/bottom roughness.* Multipath Propagation The channel can be considered as a wave guide and due to the reflections at surface and bottom we have the consequence of multipath propagation of the signal.* Attenuation Acoustic energy is partly transformed into heat and lost due to sound scattering by inhomogeneities.* Doppler Shift - Due to the movement of the water surface, the ray getting reflected from surface can be seen as a ray actually getting transmitted from a moving transmitter, and thereby, having Doppler shift in the received. When the receiver and transmitter are moving with respect to each other, the emitted signal will either be compressed or expanded at the receiver. Thereby, Doppler effect is observed.Channel variations and multipath propagation keep a lot of hurdles for the achievement of high data rates and robust communication links. Moreover, the increasing absorption towards higher frequencies limits the usable bandwidth typically to only a few kHz at large distances.The channel has been modeled by considering multipath propagation, surface and bottom reflection coefficients. In order to achieve high data rates it is natural to employ bandwidth efficient modulation. In our case Quadrature Phase-Shift Keying (QPSK, which is equivalent to 4-QAM) modulation techniques have been used for transmitter and receiver.A random bit generator is employed as the bit source. The transmitter converts the bits into QPSK symbols and the output from transmitter is fed into "Underwater Acoustic Channel". The receiver block takes the output from the channel, estimates timing and phase offset, and demodulates the received QPSK symbols into information bits.The QPSK modulation technique is extensively being used in several applications like CDMA (Code Division Multiple Access) cellular service, wireless local loop, Iridium (a voice/data satellite system) and DVB-S (Digital Video Broadcasting-Satellite). In our case the idea of receiver design has been taken from these applications.We have considered in depth the channel variations and multipath propagation as our investigation. Thus we present a reliable simulation environment for underwater acoustic communication applications (reducing the need of sea trails) that models the sound channel by incorporating multipath propagation, surface and bottom reflection coefficients, attenuation, spreading and scattering losses aswell as the transmitter/receiver device employing Quadrature Phase-Shift Keying (QPSK) modulation techniques.