136 research outputs found
Interferometric synthetic aperture sonar system supported by satellite
Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 200
Doppler compensation algorithms for DSP-based implementation of OFDM underwater acoustic communication systems
In recent years, orthogonal frequency division multiplexing (OFDM) has gained considerable attention in the development of underwater communication (UWC) systems for civilian and military applications. However, the wideband nature of the communication links necessitate robust algorithms to combat the consequences of severe channel conditions such as frequency selectivity, ambient noise, severe multipath and Doppler Effect due to velocity change between the transmitter and receiver. This velocity perturbation comprises two scenarios; the first induces constant time scale expansion/compression or zero acceleration during the transmitted packet time, and the second is time varying Doppler-shift. The latter is an increasingly important area in autonomous underwater vehicle (AUV) applications. The aim of this thesis is to design a low complexity OFDM-based receiver structure for underwater communication that tackles the inherent Doppler effect and is applicable for developing real-time systems on a digital signal processor (DSP). The proposed structure presents a paradigm in modem design from previous generations of single carrier receivers employing computationally expensive equalizers. The thesis demonstrates the issues related to designing a practical OFDM system, such as channel coding and peak-to-average power ratio (PAPR). In channel coding, the proposed algorithms employ convolutional bit-interleaved coded modulation with iterative decoding (BICM-ID) to obtain a higher degree of protection against power fading caused by the channel. A novel receiver structure that combines an adaptive Doppler-shift correction and BICM-ID for multi-carrier systems is presented. In addition, the selective mapping (SLM) technique has been utilized for PAPR. Due to their time varying and frequency selective channel nature, the proposed systems are investigated via both laboratory simulations and experiments conducted in the North Sea off the UK’s North East coast. The results of the study show that the proposed systems outperform block-based Doppler-shift compensation and are capable of tracking the Doppler-shift at acceleration up to 1m /s2.EThOS - Electronic Theses Online ServiceIraqi Government's Ministry of Higher Education and Scientific ResearchGBUnited Kingdo
Shallow Water '06: A Joint Acoustic Propagation/Nonlinear Internal Wave Physics Experiment
Since the end of the Cold War, the US Navy has had an increasing interest in continental shelves and slopes as
operational areas. To work in such areas
requires a good understanding of ocean
acoustics, coastal physical oceanography,
and, in the modern era, autonomous
underwater vehicle (AUV) operations.
Each area presents challenges for both
the scientist and the Navy. In physical
oceanography, a complex interplay
among winds, rivers, tides, and local
bathymetry drives a non-stationary, shelf-break
front and the nonlinear internal
wave (NLIW) field. These strongly
affect acoustic systems but are not
adequately understood. A key oceanographic
challenge is to model the fully
four-dimensional ocean from the large-scale
circulation down to fine scales,
which include NLIW packets, internal
tides, jets, and density fronts. Both Navy
acoustics systems and Navy operations
need the “local ocean weather” as
well as the “ocean climate” as part of
the routine forecast, but the former is
not yet available.Keywords: Ocean acoustics, Coastal physical oceanography, AUV operations, Autonomous underwater vehicle operations, Acoustic propagatio
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