M.C. Perez

Performance comparison of different codes in an ultrasonic positioning system using DS-CDMA

In this work, several Code Division Multiple Access encoding schemes are evaluated for their application in ultrasonic sensory systems based on the determination of times-of-flight. Proper encoding improves the performance of such systems in terms of noise immunity, capability of simultaneous measurements and precision in the distance measurements. Important applications that include encoded ultrasonic signals are obstacle detection, local positioning in ubiquitous computing or non-destructive testing. Here, Kasami, Golay and Loosey Synchronous codes are experimentally tested in a privacy-oriented ultrasonic Local Positioning System based on hyperbolic trilateration.

Ultrasonic Local Positioning System with Large Covered Area

This work describes the design of a ultrasonic local positioning system (ULPS) with large covered area. The beacons have been designed with a cylindrical PVDF transducer (piezofilm transducer) to which a conical reflector has been connected to enlarge the covered area and to guarantee 3D indoor positioning. The proposed ULPS for mobile robots (MR) uses simultaneous emissions from ultrasonic beacons. In order to solve the problem of simultaneous emissions from ultrasonic beacons, the well-known technique, direct sequence code multiple division access (DS-CDMA), is used. This technique encodes the ultrasonic signal with a 255-bit Kasami code for every beacon. It implies the emitted signal by every beacon to have a wide bandwidth. PVDF-based transducers suitably guarantee this requirement. Their cylindrical or semi-cylindrical shape makes the emission pattern not suitable when using them as ultrasonic beacons, often located in the ceiling of an indoor room. To adapt the emission pattern and to increase the covered area in the ground, the design process of a conical reflector is described.

Efficient Real-Time Correlator for LS Sequences

The cross-correlation function and the side-lobes of the auto-correlation function of LS codes are zero in a certain vicinity of the zero shift. Therefore, the effects of the Inter-Symbol- Interference and Multiple-Access-Interference that appears in CDMA and multi-sensor systems are mitigated. Conventionally, the detection of these sequences has been achieved by means of straight-forward matched-filter correlators. In this paper, a new correlator which significantly reduces the total number of operations to be performed is proposed, allowing real-time operation.

Efficient Generator/Correlator of GPC Sequences for QS-CDMA

This paper presents a novel efficient generator and correlator for Generalized Pairwise Complementary (GPC) sequences. The novel generator/correlator is constructed from an efficient generator/correlator for two uncorrelated Golay pairs of length 2N · 10M · 26P. The proposed architecture permits to transmit both pairs in a quadrature modulation scheme (QAM or QPSK). The reduced number of operations needed in comparison with a straightforward correlator and the large Zero Correlation Zone (ZCZ) makes these sequences good candidates to be used in QS-CDMA (Quasi-Synchronous CDMA).

Efficient Generation and Correlation of Sequence Pairs With Three Zero-Correlation Zones

This paper presents three novel methods to efficiently generate and correlate sequence pairs with three zero correlation zones in the sum of their aperiodic correlation functions. These sequences have achieved great importance in communications systems when the maximum transmission delay is less than the length of these zones, because the multiple access interference (MAI) and the intersymbol interference (ISI) can be removed. The proposed algorithms are based on the properties of complementary sets of sequences (CSS) and they offer an efficient hardware implementation of the corresponding generators and correlators that reduces the total number of operations to be performed in comparison with straightforward implementations. That makes possible real-time operation as well as the use of very long sequences.

Ultrasonic beacon-based Local Positioning System using Loosely Synchronous codes

This work presents the development of a local positioning system (LPS), based on the transmission of ultrasonic signals, which have been previously encoded by loosely synchronous (LS) codes. The LPS consists of several ultrasonic emitters located at known positions in the environment, and of a portable receiver that computes its position by measuring the differences in times of arrival (DTOA) between a reference emitter and the others. LS codes exhibit an interference free window (IFW) in the auto-correlation and cross-correlation functions. Therefore, if the relative time-offset of the codes are within the IFW, it is possible to have simultaneous emissions without interference, as well as to reduce the multipath effect.

IR sensor array configuration and signal processing for detecting obstacles in railways

This work describes a sensor array based on optical emitters, which allows to detect obstacles in railways. The sensorial system consists of a barrier of emitters and another of receivers, placed at opposite sides of the railway. Apart from the disposition of the sensorial system, it is necessary a codification method for the emission in order to detect the reception or the non-reception of transmissions between an emitter and a receiver. The method is based on a signal codification using complementary sequence pairs, suitably adapted for their simultaneous emission through the transmission channel. A high reliability under adverse conditions is achieved with the developed system, being possible to detect the presence of obstacles, and to inform about their situation.

FPGA-based Implementation of an Ultrasonic Beacon for a Local Positioning System

Ultrasonic transducers have been often used to develop local positioning systems (LPS) in the field of intelligent spaces. In these systems, it is necessary to design an ultrasonic beacon, which is distributed in the environment to cover a determined area where mobile robots or people can be located. This work describes the design and the implementation of an ultrasonic beacon in an FPGA-based platform. The design is flexible enough to allow the real-time configuration of the ultrasonic emission, according to parameters as efficient encoding or suitable modulation schemes. Furthermore, the beacons have available a 12C link, so they are in permanent communication for exchanging configuration parameters and synchronization, not only among them, but also with a central configuration module

Efficient hardware implementation for detecting CSS-based Loosely Synchronous codes in a Local Positioning System

Loosely Synchronous (LS) codes improves the performance of Local Positioning Systems (LPS) in terms of noise immunity, capability of simultaneous measurements and precision in the location, provided that the multipath spread and the relative time offset between the codes are within the zero correlation zone (ZCZ) that they achieve in their correlation functions. Furthermore, these codes can be very efficiently generated and correlated if compared to a straightforward implementation. This work presents the hardware implementation of the correlation tasks associated to an ultrasonic LPS based on LS sequences generated from Complementary Sets of Sequences (CSS). This implementation is able to carry out all the correlation processing tasks in a time shorter than the sampling period.

Optimized Correlator for LS Codes-Based CDMA Systems

Loosely Synchronized codes, which have gained considerable interest in CDMA-based wireless communications and sensory systems, can be constructed from a set of mutually orthogonal complementary sets of sequences. In this paper a new architecture is provided for the correlation of such codes. It demands less memory and number of operations than previous ones based on efficient correlators for complementary set of sequences, and can be easily implemented in programmable devices due to its regular structure.