Abstract
In this article, a time-of-flight detection technique in the frequency domain is described for an ultrasonic Local Positioning System (LPS) based on encoded beacons. Beacon transmissions have been synchronized and become simultaneous by means of the DS-CDMA (Direct-Sequence Code Division Multiple Access) technique. Every beacon has been associated to a 255-bit Kasami code. The detection of signal arrival instant at the receiver, from which the distance to each beacon can be obtained, is based on the application of the Generalized Cross-Correlation (GCC), by using the cross-spectral density between the received signal and the sequence to be detected. Prior filtering to enhance the frequency components around the carrier frequency (40 kHz) has improved estimations when obtaining the correlation function maximum, which implies an improvement in distance measurement precision. Positioning has been achieved by using hyperbolic trilateration, based on the Time Differences of Arrival (TDOA) between a reference beacon and the others.
Highlights
The term LPS (Local Positioning System) refers to a system used for location and positioning in indoor environments where the use of positioning techniques based on GPS is very limited due to the weakness of the GPS signals
This paper presents an ultrasonic LPS, such as that shown in Figure 1, based on a group of transmitter beacons located at known positions, which transmit simultaneously and periodically by using DS-code division for multiple access (CDMA) (Direct-Sequence Code Division Multiple Access) technique [11]
In order to compare the sequence detection method based on cross-correlation (CC) between the received signal and the sequence assigned to each beacon with the method based on generalised cross-correlation (GCC) and on applying phase transform (PHAT) filtering, a LPS with two beacons have been considered
Summary
The term LPS (Local Positioning System) refers to a system used for location and positioning in indoor (or at least reduced) environments where the use of positioning techniques based on GPS is very limited due to the weakness of the GPS signals. LPSs have been developed using different technologies such as infrared [6], RF [7], or ultrasound [8], when ultrasonic signals are used there are several beacons emitting in the covered area and different mobile devices whose positions are going to be calculated In this process, the system measures the time of arrival (TOA) or the time-differences of arrival (TDOA) to estimate the distance or distance difference between each mobile node and the different beacons. This paper presents an ultrasonic LPS, such as that shown, based on a group of transmitter beacons located at known positions, which transmit simultaneously and periodically by using DS-CDMA (Direct-Sequence Code Division Multiple Access) technique [11]. An important feature of this LPS is the absence of synchronization between the beacons and the receiver This means that it becomes necessary to use a positioning algorithm based on hyperbolic trilateration, by using as distance measurement the time differences of arrival (TDOA) between a reference beacon.
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