Browsing by Subject "Underwater acoustics."

Now showing 1 - 7 of 7

Open Access
Analysis of using OFDM for short-range, multı-user, underwater acoustic communication
(2006) Öktem, Kemalettin Kerem
Acoustic waves are being used in several underwater applications, such as SONARs, underwater communication systems. Most of already developed and deployed underwater communication systems use narrow band communication and lacks layered communication approach. In this thesis, we propose a spread spectrum, layered architecture for underwater communication system, such as for SCUBA divers. The communication device shall be designed such that divers can communicate with each other in shallow water, short range in a multi-user fashion and provide not only voice communication but also data transmission as well. The device shall use Orthogonal Frequency Division Multiplexing (OFDM) as a spread spectrum technique. The OFDM technique is selected from other spread spectrum techniques due to it’s inherent ability to combat the channel impairments and flexibility of implementing the communication system using software defined radio (SDR). The spread spectrum system shall operate in 100 kHz to 300 kHz frequency band using wideband acoustic transducers. In this work, we studied a layered architecture for the communication device. We mainly studied the application layer, data link layer and physical layer in order to analyze the achievable data rate and performance. In this work, we tried to find the optimal communication parameters to achieve guaranteed communication performance for possible scenarios. The communication parameters are set in order to achieve best performance for the worst condition. Using the optimal parameters, the system shall occupy 5 users voice and data communication at the same time using the entire frequency band at the same time, however with certain Grade of Service (GOS) the capacity shall be increased. The capacity of the system shall further be increased if the system uses adaptive communication parameters that are adapted to changing channel and user conditions. The system using adaptive communication parameters shall provide at most 16 users’ voice and data communication using the entire frequency band at the same time.
Open Access
Barrel-stave flextensional transducer design
(2009) Şahin, Aykut
This thesis describes the design of low frequency, high power capability class-I flextensional, otherwise known as the barrel-stave, flextensional transducer. Piezoelectric ceramic rings are inserted inside the shell. Under an electric drive, ceramic rings vibrate in the thickness mode in the longitudinal axis. The longitudinal vibration of the rings is transmitted to the shell and converted into a flexural motion. Low amplitude displacements on its axis create high total displacement on the shell, acting as a mechanical transformer. Equivalent circuit analysis of transducer is performed in MATLAB and the effects of structural variables on the resonance frequency are investigated. Critical analysis of the transducer is performed using finite element modeling (FEM). Three dimensional transducer structure is modeled in ANSYS, and underwater acoustical performance is investigated. Acoustical analysis is performed by applying a voltage on piezoelectric material both in vacuum and in water for the convex shape barrel-stave transducer. Effects of transducer structural variables, such as transducer dimensions, shell thickness, shell curvature and shell material, on the electrical input impedance, electroacoustical transfer function, resonance frequency and quality factor are investigated. Thermal analysis of designed transducer is performed in finite element analysis. Measured results of the transducer are compared with the theoretical results.
Open Access
Design of a wideband and bi-directional transducer for underwater communications
(2007) Elmaslı, Işıl Ceren
A two ceramic layer stacked transducer structure for short range underwater communications at high frequencies is studied in this work. The structure has a wide bandwidth of one octave and operates at 350 kHz center frequency. Transducer structure inherently has two electrical and two acoustic ports. Ceramic layers are matched to water load through quarter wavelength thick matching layers on each radiating face. Using electrical ports separately to compensate for the large acoustic length of the structure in water is also investigated. It is shown that the wide bandwidth operation can be maintained. The beamwidth of the structure is narrow due to end - fire effect of two back – to – back radiating elements.
Open Access
The design of a wideband and widebeam piston transducer in a finite closed circular baffle
(2008) Şahin, Zekeriyya
The design of a high power piezoelectric underwater transducer operating at frequency range 42 kHz-78 kHz with acoustic power capability in excess of 250W is described. The transducer consists of two back-to-back elements. Each element is formed by stacked PZT-4 ceramic rings, a matching and a steel backing layer, and placed in a finite rigid circular baffle. We investigate the dependence of bandwidth and beamwidth to the combination of piston and baffle radii, a and b, respectively. With ka of 2.45 (k is the wave number) at resonance and a b/a ratio of 2, the transducer resonates at 60kHz with 60% bandwidth and has a beamwidth of 60º at each half space. We show that when two transducers are placed at right angles spatially and driven in parallel, we can obtain an omnidirectional beam pattern in the lower frequency band. The beam pattern exhibits two dips in each quadrant at the higher end of the frequency band, which are within 8 dB. We also investigate power handling capability of the transducer from thermal point of view using finite element analysis. The input impedance measurements agree well with the numerical results within the pass band.
Open Access
Implementation of an OFDM based underwater acoustic modem
(2011) Demirörs, Emrecan
In this thesis we designed and implemented an underwater acoustic (UWA) communication system employing multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM). UWA communication became more popular as there has been a growing interest in transmitting real-time data, such as video and sonar images. There are many applications where these transmissions are used. These applications are underwater wireless sensor networks(UWSN) and unmanned underwater vehicles (UUVs) for military and scientific purposes. Therefore, building an efficient UWA communication system which has a high data rate can improve these applications’ performance signifi- cantly. Currently, many underwater communication systems use single carrier modulation which have limited data rate due to complexity of their receivers, as frequency selectivity of the channel increases when the symbol rate increases, so we preferred to use multicarrier modulation in UWA communication in order to increase data rate of our system. In this thesis, we considered a system that uses zero-padded (ZP) OFDM modulation. Based on ZP-OFDM, we used a receiver model that performs pilot-tone based channel estimation, carrier frequency offset compensation based on least squares (LS) fitting error or null subcarriers if they occur and data demodulation for each OFDM block individually. We used MATLAB environment for implementing our system. The MATLAB scripts generate a data burst that contains OFDM blocks, and then it is transmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. At the other side of the system, the receiver laptop gets the data by using a DAQ Card. As the data is received, MATLAB scripts are used for demodulating it. As we built our system, we performed underwater experiments at Bilkent Lake Facility to investigate its performance in a real UWA channel. In our test, a data rate of 13.92 kbps has been achieved with quadrature phase shift keying (QPSK) modulation while the bit-error-rate (BER) was less then 9x10−2 without using any coding.
Open Access
Low power range estimation with DSSS technique in underwater acoustics
(2013) Güleryüz, Mustafa Ozan
Performance of direct sequence spread spectrum modulation (DSSS) in underwater acoustical range finding is investigated in this thesis. Range estimation using low power DSSS codes is both analyzed theoretically and implemented at 400 kHz for experimental assessment, where the chip duration is 20 µs and sequence type is maximal. The effect of sequence length on the required transmit power is measured for sequence lengths from 7 chips to 127 chips. The performance of this method is compared to the widely used tone burst pulse range estimation technique. It is found that at a sequence of 127 chips and a pulse sequence length of 2,54 ms, range is estimated with 1,5 cm resolution using source level of 132,8 dB re 1 µP arms @ 1 m source level , while it is 190,5 cm for the same length and magnitude tone burst modulation, at a reference test range of 4.5 m. Moreover, spectral height of received DSSS signal is well below the ambient noise level so that signal to noise ratio (SNR) for received DSSS signal is -14,8 dB, while it is 12,2 dB for received tone burst pulse
Open Access
Underwater acoustic modem using OFDM
(2012) Yüksel, Mine Merve
This thesis is about design, simulation and testing of an underwater acoustic modem using OFDM. The thesis work combines a theoretical part, whose objective is to understand the appropriate techniques to deal with the characteristics of the targeted channel, simulations and a practical part regarding the system deployment and experimental tests. There has been a great growing interest in transmitting real-time data and video. Unmanned underwater vehicles (UUVs) for military and scientific applications have become important. Building distributed and scalable underwater wireless sensor networks (UWSN) that will bring significant advantages and benefits to underwater applications, such as ocean observation for scientific exploration, commercial exploitation, coastline protection and target detection in military events has been in the scope of researchers. Based on these, designing a concrete system with high data rate will benefit many underwater acoustic (UWA) applications. The existing systems in literature use single carrier transmission and rely on linear or non-linear equalization techniques to suppress inter-symbol interference (ISI), however this requires complex equalizers and results in low data rates. Therefore we concentrate on multicarrier modulation. In this thesis ZP-OFDM (Zero Padded-Orthogonal Frequency Division Multiplexing) receiver is built, where CFO (Carrier Frequency Offset) compensation, pilot-tone based channel estimation, and data demodulation are carried out on the basis of each OFDM block. The implemented OFDM system has been developed in MATLAB. MATLAB scripts generate a data burst that contained OFDM blocks, and then they are transmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. At the other side of the system, the receiver laptop gets the data by using a DAQ Card. As the data is received, MATLAB scripts are demodulated and data is detected. Simulations aim to provide correct implementation of all the algorithms by coupling the generated OFDM signal to a channel using Bellhop underwater channel model and noise addition algorithm, that artificially introduces some of the real channel effects into the signal. The method is tested in a shallow-water experiment at Bilkent Lake. Over a bandwidth of 12 kHz, the data rate is 13.92 kb/s with QPSK modulation, when the number of subcarriers was 1024. Bit-error-rate (BER) is less than 9x10−2 without any coding.