An Innovative Data Demodulation Technique for Galileo AltBOC Receivers

Transcription

1 Journal of Global Posiioning ysms (007) Vol.6, No.: An Innovaiv Daa Dmodulaion Tchniqu for Galilo AlBOC Rcivrs David Margaria Fabio Dovis Elcronics Dparmn, Policnico di Torino, Ialy Paolo Mulassano Navigaion Lab, Isiuo uprior Mario Bolla, Ialy Absrac. This papr dscribs an innovaiv soluion ha can b usd o rcovr h navigaion daa from Alrnaiv Binary Offs Carrir (AlBOC) modulad signals, a modulaion schm forsn for h Galilo salli navigaion sysm o ransmi four channls in h E b (64- MHz). In his papr a novl daa dmodulaion approach, calld id-b Translaor (BT), suiabl o cohrn dual b AlBOC rcivr archicurs, is inroducd validad from h analyical poin of viw. This pand approach is basd on h ida o prform a ranslaion opraion : his mans ha h wo spara in-phas componns of h AlBOC signal, conaining h navigaion daa, ar rcovrd from h rcivd signal wih a propr signal procssing, moving h informaion from h sid lobs of h AlBOC spcrum o h basb. Th innovaiv aspcs of his dmodulaion chniqu ar poind ou in h papr, highlighing h main advanags wih rspc o alrady proposd chniqus. Kywords. Daa dmodulaion, id-b Translaor, E, AlBOC, Galilo. modulaion, in ordr o xploi h widb faurs of h signals (.g. in rms of mulipah robusnss), wih an affordabl complxiy of h rcivrs archicurs as for xampl in (Dovis al. 007). In spi of h fac ha h faurs of h AlBOC modulad signals h ponial prformanc of fuur AlBOC rcivrs ar discussd in svral paprs in liraur, h rcovring of h navigaion daa (dmodulaion of h wo daa channls) is no xhausivly xamind. Only fw pans claim rcivr archicurs for rciving procssing AlBOC modulad signals: hy propos som dmodulaion sragis ha show som drawbacks, in rms of implmnaion complxiy inrfrnc vulnrabiliy. Th papr is organizd as follows: in cion a brif rviw of h AlBOC signal is providd, in cion 3 currn proposals for daa dmodulaion ar rviwd. cion 4 will inroduc h AlBOC rcivr cion will focus on h proposd id B Translaor. Th impac on h implmnaion is analyzd in cion 6, hn cion 7 will draw som conclusions. Ovrviw of h AlBOC modulaion Inroducion Th fuur Galilo sysm, a nw Global Navigaion alli ysm (GN) dvlopd by h Europan Commission h Europan pac Agncy (EA) forsn o b opraional in 03, will us h novl Alrnaiv Binary Offs Carrir (AlBOC) modulaion schm o ransmi four channls in h E b (64- MHz). vral paprs in h liraur hav addrssd h dsign of acquisiion schms racking sags for his Four channls ( Ea-I, Ea-Q, Eb-I Eb-Q ) will b ransmid in h E b by ach Galilo salli aking advanag of a novl modulaion muliplxing schm, h AlBOC modulaion. Two of four E channls ar h so-calld daa channls ( Ea-I Eb-I ), sinc hy carry navigaion daa, whras h ohr wo ( Ea-Q Eb-Q ) ar calld pilo channls ar no daa modulad. A rcivr will b abl o disinguish h four channls sinc four diffrn quasi-orhogonal Psudo-Rom Nois (PRN) cods (c Ea-I, c Ea-Q, c Eb-I c Eb-Q ) will b

2 90 Journal of Global Posiioning ysms usd for ach salli of h Galilo sysm. In his way i is possibl o rcogniz h wo daa channls ( Ea-I Eb-I ) in h rcivd signal o dmodula hir navigaion daa. I mus b noicd ha h four cods ransmid by on salli ar synchronous, wihou rlaiv bias or rlaiv chip-slip. In paricular for h daa channls h dg of ach daa symbol coincids wih h dg of a cod chip: priodic sprading cods sar coincids wih h sar of a daa symbol. A daild dscripion of h gnraion of h Galilo AlBOC modulad signal s ( E ) can b found in h Galilo Opn rvic ignal In pac Inrfac Conrol Documn (GAL O I ICD/D.0, 006). Th analyical xprssion of h s ( E ) signal is rpord hr wih h noaion usd in h Galilo O I ICD (basb complx nvlop rprsnaion): s = E [ Ea I j Ea Q [ sce j sce ( T, E [ Eb I j Eb Q [ sce j sce ( T, E [ Ea I j Ea Q [ sce P j sce P ( T, E [ Eb I j Eb Q [ sce P j sce P ( T, E In Equaion () h wo daa channls ( () E a I E b I ) ar shown wih bold yps. Thy ar dfind wih h following xprssions: [ ce a I, i dea I, [ i rct ( i TC, E a I Ea I = ) L E a I DCE a I C, E a I i= () [ ce b I, i d Eb I, [ i rct ( i TC, E b I = ) Eb I ( ) E b I DC E b I C E b I i= L, (3) whr rct is h rcangl funcion, which is qual o for 0 < < T i is qual o 0 lswhr. In Equaion () Equaion (3) h wo PRN cods cods ( ce a I ce b I ) h wo navigaion daa srams ( d E a I d E b I ) ar poind ou. Th ohr wo channls ( E a Q E b Q ), h socalld pilo channls, do no carry navigaion daa, as shown in Equaions (4) (): [ cea Q, i rct ( i TC, Ea Q Ea Q() = ) L E a Q C, E a Q i= (4) [ ceb Q, i rct ( i TC, Eb Q E b Q() = ) E b Q C E b Q i= L, () I mus also b noicd ha h AlBOC modulaion allows o us h E b as wo spara sidbs, convnionally dnod as Ea ( MHz) Eb (9.79- MHz). In his way, a singl daa channl (quivaln o a BPK signal) a pilo channl (anohr BPK signal) will b ransmid in ach sidb. Accordingly, his modulaion schm can b rad as o wo spara QPK modulaions, placd rspcivly around h Ea h Eb cnr frquncy. Th dmodulaion of h navigaion daa from h rcivd signal is hn a cumbrsom ask ha mus b carrid ou by fuur AlBOC rcivrs, sinc h wo channls Ea-I Eb-I ar ransmid in wo adjacn sidbs. 3 Exising AlBOC Dmodulaion Tchniqus A im of wriing, only fw pans (Grin, 00 D Wild al 006) claim rcivr archicurs for rciving procssing AlBOC modulad signals, considring som diffrn implmnaions of h complx corrlaion opraions ndd for h cohrn racking of h nir E b (cohrn dual b Galilo AlBOC rcivr archicur). In dail only in (Grin, 00) a possibl soluion for h daa dmodulaion is proposd. In (D Wild al 006) h rm dmodulaion is improprly usd, sinc in his documn h rcovring of h navigaion daa is no discussd, bu only som mhods dvics for racking h pilo channls ar prsnd. Th dmodulaion sragy proposd in (Grin, 00) shows som drawbacks, concrning h implmnaion complxiy inrfrnc vulnrabiliy. In his cas a no sraighforward soluion is usd o rcovr h navigaion daa. Firs, wo rplicas of h PRN cods usd in h daa channls ( ce a I ce b I, calld rspcivly c c in h pan) h corrsponding squar wav subcarrirs ar locally gnrad combind. Th obaind local signals ar corrlad wih h rcivd signal, aiming o obain h ral imaginary componns of h sum ( R R) h diffrnc ( R R ) bwn h corrlaion funcions of h wo cods. Furhr signal procssing is rquird o rcovr h navigaion daa from ( R R ) ( R R ), using a look-up abl approach. Mor dails h compl dmonsraion can b found in (Grin, 00). This dmodulaion chniqu for AlBOC signals shows h following drawbacks: cumbrsom signal procssing is rquird, sinc complx local signals mus b gnrad combind, afr h corrlaion opraions,

3 Margaria al.: Daa Dmodulaion Tchniqu Dvic uiabl o Galilo AlBOC Rcivrs 9 furhr calculaions ar rquird o dcod h navigaion daa (look-up abl); h rcivr prformanc is dgradd by corrlaion losss: his is du o h fac ha h subcarrirs locally gnrad in (Grin, 00) ar diffrn from hos usd by h Galilo sallis his implis a corrlaion loss, as sad in (ollnr Erhard, 003). In paricular in (Grin, 00) h Complx- BOC h Complx-LOC modulaions ar considrd as approximaions of h AlBOC rcivd signals. Bu h ru AlBOC modulaion ha will b usd for h Galilo E b diffrs from h Complx-LOC h Complx-BOC ssnially for h prsnc of addiional rms in h modulad signal xprssion (h so-calld produc signals) for a diffrn shap of h subcarrir wavforms (GAL O I ICD/D.0, 006); his dmodulaion chniqu is vulnrabl, sinc h wo daa channls ar joinly dmodulad, aking advanag of ( R R ) ( R R ) corrlaion rsuls. In his way an rror on on daa bi (.g. causd by an inrfring signal on a singl sidlob of h E b) can affc also h corrc dmodulaion of h ohr channl; i is no possibl o mporarily dmodula only on daa channl (.g. in a crain condiion whr h navigaion daa of h ohr channl ar no ncssary), swiching h dmodulaion scion of h ohr channl or rusing is ddicad hardwar or sofwar rsourcs. 4 Proposd Galilo AlBOC Rcivr Archicur A modifid archicur for an AlBOC rcivr, basd on h cohrn rcpion procssing of h nir Galilo E b, is dpicd in Fig.. This rcivr is similar o h ons proposd in (Grin, 00) (D Wild al 006), bu an innovaiv dsprading dmodulaion scion, ailord o h AlBOC modulaion, is usd. In Fig. a high lvl block diagram of h rcivr is prsnd: i is only inndd o simply xplain h funcioning of h rcivr. Th implmnaion dails abou h complx corrlaion discriminaion opraions h possibl opimizaions ha can b prformd in h archicur of h rcivr (.g. s Grin, 00 D Wild al 006) ar no rpord hr, du o h fac ha ar considrd background. Afr h Radio Frquncy (RF) fron nd h Inrmdia Frquncy (IF) scion, h rcivd signal is procssd by h PLL, h DLL h dmodulaion scions ha ar h mos imporan funcional blocks of h rcivr. In fac h main diffrncs bwn a convnional GP rcivr h AlBOC rcivr can b noicd in h opraions prformd by hs blocks: h Phas Lockd Loop (PLL) is usd o cohrnly rack h cnral carrir of h E b (locad a 9.79 MHz), sparaing h in-phas h quadraur componns of h rcivd signal (I Q); h Dlay Lockd Loop (DLL) is ncssary in ordr o rcovr sprading cod synchronism hn daa symbol synchronism. In fac, as prviously noicd, h four E channls of ach Galilo salli ar cohrnly ransmid, wihou rlaiv bias or rlaiv chip-slip. Th DLL funcioning is basd on h racking of h wo pilo channls ( E a Q E b Q ). This is don gnraing local rplicas of h PRN cods usd for h pilo channls ( c E a Q ce b Q ) of h subcarrir wavforms ( sce sce ( T, E / 4) ). Ths local signals ar usd o prform complx corrlaion opraions wih h I Q rcivd sampls, as discussd in (lwagn al, 004). I mus b poind ou ha h racking opraions can b prformd aking advanag of diffrn kinds of discriminaor: in Fig. h simpls on, h Early-La discriminaor, is usd for sak of simpliciy; h dmodulaion scion rcovrs h navigaion daa from h wo daa channls ( E a I E b I ), aking advanag of h synchronism rcovrd by h DLL. In paricular i is ncssary o prform h dsprading, wih local rplicas of h PRN cods usd for h daa channls ( ce a I ce b I ), h daa dcion. I mus b nod ha h dmodulaion scion in h rcivr archicur in Fig. shows rmarkabl diffrncs wih rspc o h archicur proposd in (Grin, 00). In fac a diffrn dmodulaion chniqu, basd on an innovaiv dvic calld id- B Translaor, is usd.

4 9 Journal of Global Posiioning ysms Fig. : Block diagram of a modifid cohrn dual b rcivr archicur for Galilo AlBOC signals. Th id-b Translaor (BT) Th sidb ranslaor is an innovaiv subsysm wihin h AlBOC rcivr ha can b usd o dmodula h navigaion daa includd in h widb AlBOC signal. This soluion has bn pand (Margaria, Mulassano Dovis, 007), i is basd on h ida o prform a ranslaion opraion : his mans ha h wo spara in-phas componns, conaining h navigaion daa ( E a I E b I ), ar rcovrd from h rcivd signal ( ) s E, prviously dscribd in Equaion (). To undrs h opraions prformd by h BT, i is usful o considr a simplr siuaion, as in h cas of a BOC rcivr. Wih a BOC modulaion, h signal o b ransmid is muliplid wih a rcangular subcarrir: his opraion causs a frquncy shif ha lads o h wo ypical sidlobs of h BOC spcrum (similar o h spcrum of h E AlBOC signal). To dmodula his spli-spcrum signal, onc h rcivd signal is corrcly rackd by h DLL h PLL of h BOC rcivr (h local PRN cod is synchronizd), a possibl approach is o muliply h rcivd BOC signal again wih a local rplica of h rcangular subcarrir ha can b gnrad wih h synchronism rcovrd by h DLL. This opraion

5 Margaria al.: Daa Dmodulaion Tchniqu Dvic uiabl o Galilo AlBOC Rcivrs 93 ranslas h wo sidbs of h BOC signal again o h basb: in his way, h signal bcoms again a basb signal h informaion conaind in i could b asily rcovrd wih a BPK daa dcor, afr h dsprading wih h local PRN cod. Accordingly, wih a BOC modulaion h sidb ranslaion opraion corrsponds o a simpl muliplicaion wih a local rcangular subcarrir ha r-convrs h rcivd signal in a basb signal. Howvr, wih h AlBOC modulaion his opraion is mor complx, bcaus hr ar four channls ransmid in h E b (insad of only on, as in h prvious xampl) h frquncy shifs of hs channls o h wo sidbs ar prformd aking advanag of complx xponnials. In dail h BT slcs h wo in phas daa channls Ea I Eb I movs hm from h sidbs of h AlBOC spcrum o h basb, as highlighd by h rd arrows in h schm in Fig.. modulad signal xprssion, rpord again hr for sak of clariy: s = E [ Ea I j Ea Q [ sce j sce ( T, E [ Eb I j Eb Q [ sce j sce ( T, E [ Ea I j Ea Q [ sce P j sce P ( T, E [ Eb I j Eb Q [ sce P j sce P ( T, E (6) Th dashd rms conaind in h las wo lins of Equaion (6) can b nglcd, bcaus hy corrspond o h so-calld produc signals: hs rms ar muliplid by sce P subcarrir wavform, wih smallr ampliud han sce, hy do no carry usful informaion. Th produc signals ar only ndd o obain a consan nvlop modulad signal. Mor dails can b found in (GAL O I ICD/D.0, 006), (Ris L. al, 00), (Ris L. al, 003) (ollnr Erhard, 003). Fig. 3: Thorical schm of h sidb ranslaor Fig. : Illusraion of h frquncy spcrum of h E AlBOC modulad signal h opraions prformd by h sidb ranslaor block Accordingly, h sidb ranslaor block nds o us complx xponnial muliplicaions o mov hs channls o h basb, prforming wo spara frquncy shifs, hn i mus choos h corrc channls (only h in-phas channls, conaining h navigaion daa), slcing only h ral par of h obaind signals as shown in Fig. 3. Finally h BT provids h wo rcovrd in-phas channls ( E a I E b I ), ha ar passd o subsqun dsprading BPK daa dcor blocks. In his way h navigaion daa ar rcovrd by mans of a sraighforward signal procssing opraion, simplr han h approach usd in (Grin, 00). Th opraions prformd by h sidb ranslaor block can b undrsood considring h AlBOC I is hn possibl o dcompos h modulad signal s E in is ral imaginary componns, ( ) nglcing h produc signals: s s EI EQ s = sei j seq( ) (7) E [ Ea I [ sc ( T / 4) Ea Q Eb I Eb Q sc [ Ea Q E E, E [ sc ( T / 4) Eb I Eb Q Ea I sc E E, E (8) (9) Th wo componns s E I ( ) s E Q ( ) can b considrd as h idal rcivd signals in h I Q

6 94 Journal of Global Posiioning ysms branch of h rcivr in Fig.. In fac, assuming h corrc synchronizaion of h rcivr (PLL DLL corrcly lockd) nglcing h nois, h disorions ohr propagaion ffcs, h rcivd signal s E ( ) is downconvrd o h basb is pariiond in h I Q branch of h rcivr, sparaing is ral imaginary pars. I mus b rmarkd ha aking advanag of h E AlBOC modulaion, h four channls ( ) Ea I, Ea Q, Eb I Eb Q ar ransmid in h wo sidbs of h E b. This is achivd using h subcarrir wavform sce, ha rsmbls a sampld cosin, is dlayd vrsion sce ( T, E / 4), similar o a sampld sin. Th wo subcarrir wavforms ar prsnd in dail in (GAL O I ICD/D.0, 006). In h following, for sak of simpliciy, h scond funcion is dnod as sc E. In h firs wo lins of Equaion (6) hs wo wavforms ar usd lik complx xponnials: Th firs subcarrir xponnial is obaind wih h rm [ sce j sce. I prforms a similar opraion in h frquncy domain han h complx xponnial xp( j πf sub), whr f sub is h subcarrir frquncy f sub = R, E =.34 MHz. This xponnial opras a downshif for h wo Ea channls in his way Ea I Ea Q ar shifd from h basb o h lf sidlob of h AlBOC spcrum (Ea sidb); In a similar way, h scond subcarrir xponnial [ sce j sce corrsponds o h complx xponnial xp( j πf sub) i upshifs h wo Eb channls Eb I ( ) Eb Q. Th sidb ranslaor aks advanag of his ida, prforming h opposi opraion: wih a propr us of h wo xponnials, h wo in-phas channls Ea I Eb I can b xracd from h basb rcivd signal s E ( ). To obain h Ea I channl i is ncssary o opra an upshif of h rcivd signal in h frquncy domain, muliplying i for h scond xponnial. In his way h Ea I signal bcoms cnrd o h basb i can b rcovrd slcing h in-phas (ral) componn of h rsul of h muliplicaion, as shown in h following quaions: Ea I Ea I [ sei j seq R [ sce j sce [ s EI sce seq sce R j [ sei sce seq sce (0) () sei sce seq( ) sce ( ) () Ea I imilarly o ha don for h Ea I channl, i is possibl o rcovr h Eb I signal, downshifing h rcivd signal s E ( ) wih h following opraions: Eb I Eb I [ sei j seq R [ sce j sce [ s EI sce seq sce R j [ seq sce sei sce (3) (4) sei sce seq sce ( ) () Eb I Equaions () () hn dfin h funcioning of h sidb ranslaor allows o simply rcovr h wo daa channls Ea I Eb I. 6 Implmnaion of h BT Funcional Block A possibl implmnaion of h sidb ranslaor is prsnd in Fig. 4. In his funcional block h wo opraions dscribd by Equaion () Equaion () ar dircly implmnd in h discr im domain, wih muliplicaions sums bwn h sampls of h rcivd signal h locally gnrad subcarrir wavforms. As shown in h block diagram, h rsuls of h wo quaions could b filrd, wih wo basb lowpass filrs, in ordr o rduc h inrfrnc h cross-corrlaion causd by h adjacn channls. Th shap h bwidh of h filrs mus b opimizd, bcaus a narrow b filring can rduc h prformanc of h dmodulaion scion, worsning h corrlaion propriis of h wo daa channls, bu also a filr oo wid could b an issu in prsnc of nois inrfrncs.

7 Margaria al.: Daa Dmodulaion Tchniqu Dvic uiabl o Galilo AlBOC Rcivrs 9 Fig. 4 Block diagram of h sidb ranslaor In conclusion, h sidb ranslaor funcional block provids as wo spara oupus h wo daa channls Ea I Eb I, xracd from h rcivd signal. In his way i is possibl o subsqunly rcovr h navigaion daa from h wo oupus of h BT, prforming wo spara dsprading opraions wo BPK daa dcions, as prviously rprsnd in Fig.. 7 Conclusions In his papr an innovaiv approach has bn prsnd as a valid soluion in ordr o dmodulad h navigaion daa from an AlBOC modulad signal. o Th wo daa channls Ea I Eb I of h Galilo E b ar rcovrd aking advanag of h ida o opra wo frquncy shifs on h rcivd signal; o Th frquncy shifs ar prformd using ral signals, obaind wih local rplicas of h AlBOC subcarrir wavforms sce ( ) sc E ; o Th wo signals rcovrd wih hs frquncy shifs can b sparaly filrd, in ordr o rduc inrfrncs cross-corrlaions wih adjacn channls; o Finally, h navigaion daa ar sparaly rcovrd as wo BPK signals, prforming h dsprading h dmodulaion opraions. Th proposd dmodulaion approach shows svral diffrncs wih rspc o h soluion in (Grin, 00), sinc a diffrn signal procssing is usd. This lads o h following advanags: A simplr signal procssing ha implis a saving in hardwar sofwar rsourcs. In fac h navigaion daa ar dircly rcovrd from h wo oupus of h sidbs ranslaor furhr calculaions o dcod h daa from hir sum diffrnc, as in (Grin, 00), ar no ncssary; A br rcivr prformanc, avoiding corrlaion losss in h dmodulaion scion; in fac in h proposd rcivr archicur (s Fig. ) h corrc subcarrir wavforms sce sc E ar locally gnrad usd by h sidb ranslaor o prform h frquncy shifs; An improvd robusnss of h dmodulaion scion, sinc an rror in a daa bi of on channl (.g. causd by an inrfring signal on h Ea sidb) dos no affc h corrc dmodulaion of h ohr daa channl; in fac h wo daa channls ar sparaly downconvrd dmodulad, aking advanag of h BT; A br inrfrnc rjcion, bcaus h wo low-pass filrs in h BT allow o rduc ou-ofb inrfring signals cross-corrlaions causd by PRN cods of adjacn channls; Mor flxibiliy for h funcioning of h dmodulaion scion; in fac i is possibl o mporarily dmodula only on daa channl (.g. in a crain condiion whr h navigaion daa of h ohr channl ar no ncssary), swiching h dmodulaion of h ohr channl (powr saving) or rusing is ddicad hardwar or sofwar rsourcs. Rfrncs Dovis F., Mulassano P., Margaria D. (007), Mulirsoluion Acquisiion Engin Tailord o h Galilo AlBOC ignals, in Procdings of ION GN 007, For Worh, TX (UA), p. 4-8, 007 D Wild W. al (006), A Mhod Dvic for Dmodulaing Galilo Alrna Binary Offs Carrir (AlBOC) ignals, Europan pac Agncy (Paris, FR), Inrnaional Pan (WIPO) No. WO 006/07004 A, 6 March 006. GAL O I ICD/D.0 (006), Galilo Opn rvic ignal In pac Inrfac Conrol Documn (O I ICD), Draf 0, Europan pac Agncy / Galilo Join Undraking, 3 May 006. Grin N. (00), A Hardwar Archicur for Procssing Galilo Alrna Binary Offs Carrir (AlBOC)

8 96 Journal of Global Posiioning ysms ignals, Europan pac Agncy (Paris, FR), Inrnaional Pan (WIPO) No. WO 00/0060 A, 0 January 00. Margaria D., Mulassano P., Dovis F., (007), Rcivr for AlBOC-modulad signals, mhod for dmodulaing AlBOC-modulad signals, corrsponding compur program produc, Pan Applicaion No. EP0744, July 007. Ris L. al (00), A ofwar imulaion Tool for GN BOC ignals Analysis, in Procdings of ION GP 00, Porl, Orgon, 4-7 pmbr 00. Ris L. al (003), Nw Invsigaions on Widb GN ignals, in Procdings of ENC GN 003, Graz, Ausria, April 003. lwagn J. M. al (004), Galilo AlBOC Rcivr, in Procdings of ION GN 004, Rordam, Holl, 6-9 May 004. ollnr M. Erhard Ph. (003), Comparison of AWGN Cod Tracking Accuracy for Alrnaiv-BOC, Complx-LOC Complx-BOC Modulaion Opions in Galilo E-B, in Procdings of ENC GN 003, Graz, Ausria, - April 003.