AltBOC for Dummies or Everything You Always Wanted To Know About AltBOC

Laurent Lestarquit, Géraldine Artaud, Jean-Luc Issler

Abstract: The constant envelope Alt-BOC signal is one of the most exotic GNSS signal ever imagined: it is a complex signal composed of 4 codes multiplexed so as to have constant envelope signal. The main lobes of this signal spans over 50 MHz, meaning receiver desiring to receive the complete signal shall have a bandwidth about thirty times larger than the current basic GPS receivers and implement complex algorithms, quite a challenging task for receiver designers! In fact, Alt-BOC was at first imagined to fulfil a need: be able to generate and multiplex 2 or 4 navigation signal components (each with its own PRN code) on two close frequency bands taking into account the limitations of the onboard OMUX, and giving it a constant-envelope in order to maximize the amplifier efficiency. Processing the whole signal rather than the 2 separate E5a and E5b signals wasn’t envisioned at first. The idea of receiving the complete Alt-BOC arrived later when theoretical studies showed that the process of this wideband signal would lead to positioning of unprecedented accuracy [1, 2, 3]. This intuition was later confirmed by measurements performed on the real signal broadcasted by GIOVE-A . The understanding of Alt-BOC is challenging, so we propose a step by step approach to apprehend it, beginning with a comprehensive insight of the 2 code Alt-BOC, its characteristics, its properties, the possible variants. Then we’ll move to the 4-codes version. Indeed, the combination of four signals is possible but the resulting signal hasn’t a constant-envelope. The problem was solved by using a “trick” which led to the look-up table defined by CNES [4]. This trick was further put into equation by ESA under the form of a cross product that led to the analytical definition of the signal as defined in the Galileo ICD. An analytical expression quite difficult, but we will show how this expression is to be interpreted. The PSD of this expression is analysed and leads to surprising conclusions. Next we will focus on some reception architectures allowing the reception of the Alt-BOC signal. There are mainly two different approaches. The simplest one consists in independent processing of E5a and E5b as BPSK or QPSK signals. The second consists in coherent Alt-BOC processing of the composite signal for which many method have already been proposed. We will propose a new method for tracking the pilot codes of the Alt-BOC, a method that is using the properties of the 2 code Alt-BOC (which has been noticed in [6]), and using sub-carrier tracking techniques as described in [7] and [8].
Published in: Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008)
September 16 - 19, 2008
Savannah International Convention Center
Savannah, GA
Pages: 961 - 970
Cite this article: Lestarquit, Laurent, Artaud, Géraldine, Issler, Jean-Luc, "AltBOC for Dummies or Everything You Always Wanted To Know About AltBOC," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 961-970.
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