Cristina Diego

Ultrasonic Array for Obstacle Detection Based on CDMA with Kasami Codes

This paper raises the design of an ultrasonic array for obstacle detection based on Phased Array (PA) techniques, which steers the acoustic beam through the environment by electronics rather than mechanical means. The transmission of every element in the array has been encoded, according to Code Division for Multiple Access (CDMA), which allows multiple beams to be transmitted simultaneously. All these features together enable a parallel scanning system which does not only improve the image rate but also achieves longer inspection distances in comparison with conventional PA techniques.

Modeling the Behavior of an Underwater Acoustic Relative Positioning System Based on Complementary Set of Sequences

The great variability usually found in underwater media makes modeling a challenging task, but helpful for better understanding or predicting the performance of future deployed systems. In this work, an underwater acoustic propagation model is presented. This model obtains the multipath structure by means of the ray tracing technique. Using this model, the behavior of a relative positioning system is presented. One of the main advantages of relative positioning systems is that only the distances between all the buoys are needed to obtain their positions. In order to obtain the distances, the propagation times of acoustic signals coded by Complementary Set of Sequences (CSS) are used. In this case, the arrival instants are obtained by means of correlation processes. The distances are then used to obtain the position of the buoys by means of the Multidimensional Scaling Technique (MDS). As an early example of an application using this relative positioning system, a tracking of the position of the buoys at different times is performed. With this tracking, the surface current of a particular region could be studied. The performance of the system is evaluated in terms of the distance from the real position to the estimated one.

Improved Ultrasonic Phased Array Based on Encoded Transmissions for Obstacle Detection

In this paper, a new signal processing algorithm for ultrasonic sensory systems in airborne transmission is presented. The proposal is based on a phased array (PA) where the excitation signals are loosely synchronous (LS) sequences, often used in code division multiple access communication systems. Hence, by setting a different LS sequence for each steering angle, it is possible to simultaneously acquire all the information of the scanned area with a single fire, in order to detect possible reflectors in front of the sensory array. The use of these LS sequences combined with PA techniques allows to simultaneously steer the main beam in different directions with low interference. Simulation results reveal that this approach does not only provide longer distance inspection, but also noise robustness. Furthermore, this algorithm provides higher image rate than conventional PA techniques even for high-quality images, since the image rate does not depend on the number of steering angles to be explored.

SoC-Based Architecture for an Ultrasonic Phased Array With Encoded Transmissions

This work presents a SOC-based (system-on-chip) architecture for ultrasonic imaging systems in airborne transmission which uses macro-sequences derived from complementary sets of sequences (CSS) to achieve simultaneous beam steering in several directions. By means of CSS concatenation and zero padding, a new macro-sequence can be obtained which achieves zero auto- and cross-correlation zones (ZCZ). These zones can be adjusted to provide interference-free CDMA (code division multiple access) within the scanning area. The correlation results with the proposed macro-sequences provide the A-scan lines for a final B-scan image. In contrast to other encoding schemes with similar sizes of ZCZ, the one proposed here allows an architecture design that further minimizes the computational load to make real-time processing feasible. The implementation of the proposed architecture in a FPGA (field-programmable gate array) device, including all the stages from the emission and reception modules of the imaging ultrasonic system, is presented. The ultrasonic phased array has 8 elements and permits the simultaneous analysis of 32 different angular sectors, for a maximum range of 1.5 m and an angular aperture of ±64°.

Multilevel LS sequences with flexible ZCZ length and their application to local positioning systems

In this paper multilevel complementary pairs of sequences without limitation in length and the generation of multilevel Loosely Synchronized (LS) sequences with flexible Zero Correlation Zones (ZCZ) length are proposed. The existence of efficient correlators and the fewer restrictions on the length of the multilevel LS sequences compared to binary LS ones, makes them worthwhile for their application to Local Positioning Systems (LPS).

Effect of CDMA techniques with Kasami codes on ultrasound-image quality parameters

This works presents an analysis of the benefits of CDMA (Code Division for Multiple Access) excitation techniques on the quality parameters of ultrasonic phased array images such as gratinglobes, sidelobes, dynamic range, lateral resolution and range resolution. In order to consolidate these benefits, a comparison between phased array excitation and CDMA techniques in airborne transmission is performed. Furthermore it is shown how not only image rate, but also range resolution and SNR (Signal to Noise Ratio) are improved with CDMA excitation techniques. A disadvantage of the proposal is a decreased dynamic range in the B-Scan image, therefore a different emission schema is proposed.

Local maximum detection for active sensory systems based on encoding and correlation techniques

The detection of local maxima is particularly important in sensory transmission systems that use correlation techniques. In these systems, reception blocks try to recover the information by correlating the received signal with the emitted pattern. In this paper a system capable of discriminating the auto-correlation maximum, even in high noise environments, is proposed. The local maximum detector developed is based on measuring the amplitude and on computing the first derivative.

Bearing estimation algorithm based on spectral analysis of the ultrasonic received echoes

A method to locate reflectors, based on the spectral analysis of ultrasonic received echoes, is provided in this study. The main contribution of this work lies in the spectral analysis for bearing estimation. A single ultrasonic transducer, acting as emitter and receiver, is used. The transmitted signal is a 1023-bits Kasami code BPSK modulated by a carrier that ranges from 30 kHz to 55 kHz, implying a 6-component transmitted pulse. Since the ultrasonic transducer works as an angle-dependent filter, it is possible to estimate the bearing angle of the reflector based on the received spectrum. It is shown how the amplitude of each frequency contains information about the environment, particularly about the bearing angle of the reflector. Finally, a set of experimental results is included to validate the sensory system, showing how it can deal with realistic reflectors.

Comparative analysis of an underwater acoustic relative positioning system using coded signals

Positioning systems in underwater environments constitute a relatively new field of research that is gaining increasing interest in recent days. These systems are useful to locate buoys that are monitoring the environment, as well as Remotely Operated Vehicles (ROVs) or Autonomous Underwater Vehicles (AUVs). An acoustic relative positioning system is presented in this work, based solely on pseudorandom coded acoustic signals emitted underwater and detected by matched filtering processes. One of the buoys acts as the master node, emitting its code through a hydrophone, which will be received by the other buoys. These buoys will after reply with their own code, obtaining the round-trip times of flight (RTOF) for each pair of buoys. Once the RTOFs are obtained, the distances between them can be computed knowing the sound speed value, and from them the positions are calculated by means of the Multidimensional Scaling technique (MDS). Different codes are studied, being the performance criteria the average erro...

Swell effect in shallow underwater acoustic communications

The need for monitoring underwater sensors, communications between submarines or sonar, makes underwater acoustic communications an important field of research. Electromagnetic waves are quickly attenuated in this medium and thus acoustic waves are the best option for communications. When two buoys are present, one acting as the emitter and the other as the receiver, it is crucial for the communication system to know the response of the channel. This paper presents a study of the underwater channel and proposes a model developed in Matlab where physical phenomena like attenuation, absorption and the swell effect are considered. This latter effect causes a Doppler spread in the signal, and a time-varying impulse response.