Design of new codes for reducing interference between GNSS-Like signals transmitted indoors
Résumé
In order to extend the GNSS coverage to indoor environments, several systems have been proposed for indoor positioning. Among them, those using local GNSS-like signal transmitters are of specific interest since they could allow a single device, the GNSS receiver, to achieve the continuity of the positioning service from outdoors to indoors. Since these local antennas broadcast GNSS signals similar to those broadcasted by the satellites, the indoor signals may interfere with the outdoor ones. Thus, it increases the noise level at the outdoor receiver's end and the error on the computed positions. In this paper, we propose new codes that aim to reduce this interference for both the GPS and the GLONASS signals (considering one could evenly consider the two constellations). Then the new proposed codes and modulations are described and the interference levels estimated in a simulated environment (typical of a museum or a conference centre). The basic underlying idea is that if the interference level is significantly decreased, then the maximum allowed transmitted power level (due to US and European regulations for instance) could potentially be increased. Then, the number or required transmitters could be decreased for the same coverage. The performance of the new proposed codes is tested in the case of the repealite based indoor positioning system, already described in the literature. In comparison with existing codes (PRN 33 to 36 for GPS Gold codes and specific m-sequence for GLONASS), the new so-called IMBOC codes (IMBOC stands for Indoor Modified Binary Offset Carrier) allow a reduction of the interference levels of typically 15dB outdoors. Spectrum considerations are also detailed, as it should be