The paper will consider the viability of exploiting the spatial diversity that exists in doubly-spread, underwater acoustic communications channels to provide enhanced capacity using an OFDM-MIMO system. The proposed method offers two forms of diversity. First, coded orthogonal frequency-division multiplexing (COFDM) provides frequency diversity by exploiting the frequency selectivity inherent in channels that suffer from multipath propagation. Second, multiple transducers are used to exploit the spatial diversity that exists in underwater acoustic channels, also due to multipath propagation. This allows the multi-transducer communication system to be viewed as a multi-input, multi-output (MIMO) system to provided potential capacity gain.
"where α (f ) represents the medium absorption coefficient, A ∈   dB is the so-called transmission anomaly, which accounts for the degradation of the acoustic intensity caused by multiple path propagation, refraction, diffraction, and scattering of sound and ρ has a unit mean Rayleigh cumulative distribution D ρ (ρ) = 1 − exp −πρ 2 /4 . ρ is a normalized random variable that represents the power gain of the fading. "
[Show abstract][Hide abstract] ABSTRACT: In this paper, we propose UW-OFDMAC, a distributed Medium Access Control (MAC) protocol tuned for Under-Water Acoustic Sensor Networks (UW-ASNs). It is a transmitter based Orthogonal Frequency Division Multiple Access (OFDMA)scheme that integrates an original power and OFDMA parameters self-assignment algorithm to set the optimal transmit power, subcarrier spacing and guard interval duration. UW-OFDMAC aims at achieving two objectives, namely, guarantee high bandwidth efficiency and low energy consumption. Simulation results show that UW-OFDMAC outperforms the basic OFDMA protocol tuned for the underwater environment.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
[Show abstract][Hide abstract] ABSTRACT: With the advancement of technology in communication field, underwater wireless communication (UWC) has been introduced for military & commercial applications but not advance as Radio wireless communication because in UWC, Bandwidth is extremely limited. The attenuation of acoustic signal increases with frequency and range. A widespread technology Multi Input Multi Output- orthogonal-Frequency Division Multiplexing (MIMO-OFDM) is used in UWC for high data rate signal processing. MIMO-OFDM takes advantage of the multipath properties of environments using base station antennas that do not have Line of Sight (LOS). A set of independent orthogonal subcarriers are used to transmit data. The total bandwidth is divided into a large number of narrowband channels each one non-interfering with each other. The use of both technologies together gives the attractive solution future underwater wireless communication. This paper explores the advancement in design of MIMO-OFDM system design for High Rate Signal Processing in Underwater Wireless Communication. These results suggest that MIMO-OFDM is an appealing solution for high data rate transmissions over underwater acoustic channels.
National Conference on Advances in Digital Signal Processing (NCADSP-2010), Chitkara University, Punjab; 10/2010
"The combination of MIMO and OFDM is an appealing low-complexity solution for highly spectrally efficient communications over the bandwidth-limited frequencyselective underwater channels. Previous work on MIMO underwater communications includes space-time coding to increase link reliability (an Alamouti design has been conceptually described and tested through simulation in ) and spatial multiplexing to increase the data rate (a 2×4 system was demonstrated using experimental data in ). "
[Show abstract][Hide abstract] ABSTRACT: Frequency and time correlation of the underwater channel are exploited to obtain an adaptive channel estimation algorithm for MIMO spatial multiplexing with low complexity and low overhead. Non-uniform Doppler compensation is performed by extending the adaptive synchronization method to multiple transmitters. Algorithm performance is demonstrated on experimental data recorded in several shallow water channels over distances on the order of 1 km. Nearly error-free performance is observed for two and four transmitters with BCH(64,10) encoded QPSK signals. We report in detail on an experiment where an information rate of 23 kbps (4 transmitters times 0.75 sps/Hz/transmitter times 2 bits/symbol times 10/64 times 24 kHz) is achieved using 2048 carriers in a 24 kHz bandwidth.
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