AltBOC for Dummies

AltBOC for Dummies or Everything YouAlways Wanted to Know About AltBOCLaurent Lestarquit, CNESGéraldine Artaud, CNESJean-Luc Issler, CNEStaking into account the limitations of the onboardOMUX, and giving it a constant-envelope in order tomaximize the amplifier efficiency. Processing thewhole signal rather than the 2 separate E5a and E5bsignals wasn’t envisioned at first.

The idea of receivingthe complete Alt-BOC arrived later when theoreticalstudies showed that the process of this wideband signalwould lead to positioning of unprecedented accuracy[1, 2, 3]. This intuition was later confirmed bymeasurements performed on the real signalbroadcasted by GIOVE-A .The understanding of Alt-BOC is challenging, so wepropose a step by step approach to apprehend it,beginning with a comprehensive insight of the 2 codeAlt-BOC, its characteristics, its properties, the possiblevariants. Then we’ll move to the 4-codes version.Indeed, the combination of four signals is possible butthe resulting signal hasn’t a constant-envelope. Theproblem was solved by using a “trick” which led to thelook-up table defined by CNES [4].

This trick wasfurther put into equation by ESA under the form of across product that led to the analytical definition of thesignal as defined in the Galileo ICD. An analyticalexpression quite difficult, but we will show how thisexpression is to be interpreted. The PSD of thisexpression is analysed and leads to surprisingconclusions.Next we will focus on some reception architecturesallowing the reception of the Alt-BOC signal.

Thereare mainly two different approaches. The simplest oneconsists in independent processing of E5a and E5b asBPSK or QPSK signals. The second consists incoherent Alt-BOC processing of the composite signalfor which many method have already been proposed.We will propose a new method for tracking the pilotcodes of the Alt-BOC, a method that is using theproperties of the 2 code Alt-BOC (which has beennoticed in [6]), and usingsub-carrier trackingtechniques as described in [7] and [8].BIOGRAPHYLaurent Lestarquit is a navigation signal expert at theCNES Transmission Technique and Signal Processingdepartment (TT). He was a member of the GalileoSignal Task Force (GSTF) and contributed to thedefinition of the Galileo signal and provided support tothe 2004 US-EU agreement of navigation system.

Heinvented the constant envelope 4-code ALT-BOCmodulation.Géraldine Artaud is a navigation engineer in theCNES Transmission Techniques department since2007. She is involved in activities related to GNSSsignal analysis, propagation, software receivers andsimulations.Jean-Luc Issler is head of the TransmissionTechniques and signal processing department ofCNES, whose main tasks are signal processing, airinterfaces and equipments in Radionavigation, TT&C,propagation and spectrum survey. He is involved in thedevelopment of several spaceborne receivers, as wellas in studies on GALILEO, including EGNOS and therelated evolutions.

With DRAST, he represents Francein the GALILEO Signal Task Force of the EuropeanCommission. He received in 2004 the AstronauticPrize of the AAAF ( French aeronautical and spaceassociation ) and in 2008 the EADS Prize of the frenchAcadémie des Sciences for his technical work onGALILEO signals and spaceborne GNSS equipments.ABSTRACTThe constant envelope Alt-BOC signal is one of themost exotic GNSS signal ever imagined: it is acomplex signal composed of 4 codes multiplexed so asto have constant envelope signal.

The main lobes ofthis signal spans over 50 MHz, meaning receiverdesiring to receive the complete signal shall have abandwidth about thirty times larger than the currentbasic GPS receivers and implement complexalgorithms, quite a challenging task for receiverdesigners! In fact, Alt-BOC was at first imagined tofulfil a need: be able to generate andmultiplex 2 or 4 navigation signal components (eachwith its own PRN code) on two close frequency bandsION GNSS 21st. International Technical Meeting of theSatellite Division, 16-19, September 2008, Savannah, GA.INTRODUCTIONAlt-BOC is a modulation whose understanding requirestrong efforts. The mathematical formulation and theequivalent look-up table provided in the GALILEOSIS-ICD are not strait-forward. Written by its inventor,961the purpose of this paper, is to help navigationengineers in understanding Alt-BOC or we should say,understanding the numerous Alt-BOC variants,explaining how the constant envelope Alt-BOC wasinvented, and highlighting its properties.An accurate definition was given in [1] : “ AnAlternative-BOC signal is a BOC-like signal havingdifferent PN codes in the lower and the upper mainsplit lobes.

Alternative-BOCs allow one signal serviceper lobe.”51.15 MHz30.69 MHz10.23 MHzE5aE5b1176.45 MHz1207.14 MHzFigure 2 : Galileo frequency plan at E5a and E5bGALILEO HISTORY & NEED FOR ALT-BOCAt that time there were 2 proposed architectures totransmit the E5a and E5b signal (figure 3). But in factthe 2 bands were so close that using a single HPA forthe 2 frequencies was necessary. If the E5a and E5bwere recombined in an OMUX after amplification, theOMUX filtering would have been so close to thedesired bands and so sharp that it would generate highpropagation delays inside the desired bands, potentiallyleading to signal distortion and propagation timeinstability. (Figure 4). Alt-BOC was needed for E5a &E5b.In addition Alt-BOC allows to transmit many sidelobesfor use by narrow correlation receivers.Let step back in time and into Galileo signal definitionhistory.

Alt-BOC appeared for the first time in the year2000. At that time, the Galileo signal plan was all butsettled, the Galileo Signal Task Force (GSTF) grouphad many issues to cope with. In the year 2000 theGalileo signal plan in what is now the L1 band wasquite different from today : there were signal carriers atthe E1 and E2 bands, which were bands 4MHz wideand 14 MHz apart from the L1 band for which Galileohad anteriority. Each of these band was to host anavigation signal.

This is when a proposal was made touse an alt-BOC signal to transmit 2 independent signalin these two bands using an unique High PowerAmplifier (HPA) working at the L1 frequency.Signal E5a( QPSK)GalileoE2OMUXGalileoGPS L1E1Signal E5b(QPSK)1575,42 MHzFigure 1 : early Galileo signal plan at L1Signal E5a &Signal E5b(ALT BOC)Yet, since the signal would be composed of a pilot anddata channel on each band, it would have been a 4code Alt-BOC that would have been needed. Indeed, a4 code Alt-LOC was studied at the time but wasquickly dropped because its envelope wasn’t constant.A constant envelope was researched, so as to allow touse the HPA at saturation, that is with an highefficiency. Furthermore, the Galileo signal plan waschanged.In the mean time, in year 2001, CNES discovered that4-code constant Alt-BOC signal is possible and this isthe Alt-BOC that came back with force to be proposedand accepted for the E5a and E5b bands, bands forwhich the need for an Alt-BOC was even higher due tothe low interval between the bands (only 10 MHzbetween main lobes !)FilterFigure 3 : 2 possible transmission architectures forE5a and E5b bands.OMUX FilterE5bGroup delayE5aFigure 4 : Graph showing the rise of the group delaysinside the desired band for the 1st proposedarchitecture.THE 2-CODES ALT-BOCThis is the easiest form of Alt-BOC, yet its propertiesare worth to be looked at.ION GNSS 21st.

International Technical Meeting of theSatellite Division, 16-19, September 2008, Savannah, GA.962First, let’s recall the expression of the BOC sub-carrierwith cosine or sine phasing. Actually, this distinctionbetween cosine and sine phasing in a BOC was onlyrealized many years after the BOC was firstintroduced, the US inventor apparently assumed a sinephasing from the start.SC4,SSBBOC subcarrier with cosine phasing :SC B ,cos (t ) = sign(cos 2πf s t )(1)SC4,SSB*BOC subcarrier with sine phasing :SC B ,sin (t ) = sign(sin 2πf s t )Figure 6 : Values taken by the SSB sub-carrierfunction on a Fresnel plot. 4 stands for the number ofvalues taken.(2)Notice also another possible expression for the SSBsubcarrier that can be directly derived from fig.6 :Time plot⎧ SC4, SSB (t ) = e j ( 4 +i⋅ 2 )⎨( i −1)Ts i⋅Ts⎩t mod Ts ∈ 4 , 4πTs/2Ts3Ts/2π[2Ts]E5aFigure 5 : BOC sub-carrier function (In this notation,SC stands for sub-carrier, B for binary)(5)E5bSC4,SSBSC4,SSB*Next, let’s built the Single Side Band (SSB) subcarriers (SCSSB) and its conjugate(SCSSB*) :SC 4, SSB (t ) =SC 4, SSB * (t ) =1212(SC(SCB ,cosB ,cos(t ) +j SC B ,sin (t ))(t ) − j SC B ,sin (t ))(3)-9Fs-7Fs-5Fs-3Fs-Fs+Fs3Fs(4)These 2 functions can take 4 values that are shown onthe Fresnel plot in Figure 6.A first remarkable properties of these functions is theirPSD (Power Spectral Density).

They have a singlefundamental harmonic located for SC4,SSB at +Fs, andfor its complex conjugate at –Fs (See figure 7). Theseare indeed SSB sub-carriers. The power of each of itsharmonics are given in table 1. Notice that for theclassical BOC sub-carrier the power are split in half inthe positive and negative harmonics PSD7FsFreq-5fs-3fs-fs+fs+3fs+5fsBOC1,64,540,5340,534,51,6SC4, SSB09,0081,0503,2SC4, SSB *3,2081,0509,009FsTable 1 : PSD table, for each harmonics of the subcarrier, expressed in % of total power.The 2 code Alt-BOC is defined as :s(t ) = c A (t ) ⋅ SCB ,SSB * (t ) + cB (t ) ⋅ SCB , SSB (t )E5a(6) can be rewrittenexpression :+ j[c B (t ) − c A (t )]SC B ,sin (t )963E5bwith the BOC subcarriers (t ) = [c A (t ) + c B (t )]SC B ,cos (t )ION GNSS 21st.