SOFTWARE DEFINED GLOBAL POSITIONING SATELLITE RECEIVER

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1 SOFTWARE DEFINED GLOBAL POSITIONING SATELLITE RECEIVER Danel Iancu (Sandbrdge Technologe Inc., Whte Plan, NY 6 USA; dancu@andbrdgetech.com); John Gloner (Sandbrdge Technologe Inc. & Delt Unverty o Technology, Comuter Engneerng, Electrcal Engneerng Mathematc and Comuter Scence, Delt, The Netherland; jgloner@andbrdgetech.com); Vladmr Kotlyar (Sandbrdge Technologe Inc., Whte Plan, NY 6 USA; vkotlyar@andbrdgetech.com); Hua Ye (Sandbrdge Technologe Inc., Whte Plan, NY 6 USA; huaye@andbrdgetech.com); Mayan Moudgll (Sandbrdge Technologe Inc., Whte Plan, NY 6 USA; mayan@andbrdgetech.com); Erdem Hokenek (Sandbrdge Technologe Inc., Whte Plan, NY 6 USA; ehokenek@andbrdgetech.com); ABSTRACT Due to the comutatonal comlety o Global Potonng Satellte (GPS) recever, they have been tradtonally mlemented n hardware (H/W) emloyng multle arallel channel. Uually each channel reonble or trackng and demodulatng one atellte. In ome degn, one channel can be tme hared by more than one atellte. In the cae o mult rotocol communcaton ytem, a HW mlementaton become le attractve due to addtonal ch area requrement. In th aer we reent a otware (S/W) mlementaton o a GPS recever. It art o a otware communcaton baeband roceor beng degned by Sandbrdge Technologe. In our degn all uncton aocated wth the GPS recever are mlemented n S/W on the Sandbrdge Technologe Sandblater Multthreaded SB96 roceor. The devce ntended to be ued n hand held devce a moble hone and PDA.. INTRODUCTION A receved GPS gnal can be vewed a a ueroton o N DS-CDMA gnal comng rom N vble atellte. Each atellte ha a unque gnature and lghtly derent carrer requency due to the Doler Eect [] []. The GPS comote gnal, or the CA code, can be modeled a ( N = P [( n n )% N N A d [ k] g t k = n= k = ] co(π t ϕ ) η( g t n () N the number o ch n the equence (3 or the CA code), and η( the thermal noe. Wthout long generalty, the noe term n () wll not be condered urther. In order to etract the ayload normaton embedded n the comote gnal, or one atellte, we wndow ( wth the ule uncton g(t-t ) yeldng: N N N ξ, k, n g t k g t k = n= k = χ( g t m () = g t n co(π t ϕ) e jπt where j =, the carrer requency at the tranmtter (the ame or all atellte), % the mod oeraton, and ξ, k, n = A d[ k] P [( n n )% N We can vew a the Local Ocllator (LO) requency, equal to the IF requency n the recever. In equaton () the roduct o ule uncton g(t-t ) wll be non-zero and only k=k and m=n. Th mle that or a eced k=k and m=n there wll be a ngle ch elected on the tme cale. Ater ummaton and ntegraton, or all vble atellte, Equaton () become ] Where: tand or the th atellte, g(t-mt )=σ(t-mt ) σ[(m)t -], =/T the carrer requency, where or t σ ( = or t < the unt te uncton, d [k] the data n the k th, P [n] the n th ch n the k th mllecond, A the amltude, N = χ( dt = N D ( k, n n ) nϕ j = D ( k, n n ) coϕ δ ( ) N = δ ( E ) (3)

2 where E the total error caued by the wndowng wth the ule uncton due to lack o atellte-recever clock ynchronzaton and Doler ht, and D ( k, n n ) = A d [ k] P [( n n )% N. IMPLEMENTATION In equaton (3) we dere to mnmze the detecton error or a artcular atellte. Thereore we orce both condton to = and ϕ = mlyng a correcton or the Doler ht a well a or the hae ht or each atellte. Conormng to the Fourer tranorm htng roerty, the condton = can be acheved ether through requency or tme doman htng. In H/W mlementaton, the carrer tracked by advancng or retardng the LO requency and hae (requency doman h conormng to the outut o a Phae Locked Loo (PLL) crcut. For each vble atellte the ntegral dened n (3) calculated earately reultng n multle arallel rocee wth each roce eecuted by a earate dedcated H/W block called a channel. Each channel mut have t own LO, PLL and Peudo Number () generator. A otware mlementaton more ecently mlemented n the tme doman. The amled data tme doman hted by onter manulaton; the onter hted orward or retarded deendng on the drecton o the Doler ht. The LO n th cae a n-cone table and the ame or all carrer requence. The hae condton acheved by a dgtal (otware) PLL, reultng n an addtonal ht on the already hted data or each carrer earately. The equence are tored n memory. In a multthreaded envronment, all the ntegral n (3) are eecuted n arallel. The number o vrtual channel dynamcally allocated by the roceor, deendng on the avalable reource and real tme crtera. Hardware can ealy mlement the condton = wth a Voltage Controlled Ocllator (VCO) ued or LO. In otware, the requency condton cannot be eactly acheved due to a lmted amlng rate. Shtng n the tme doman wll alo nduce urou requence whch wll ncreae the detecton error. To mnmze the detecton error, overamlng requred. It can be hown that the total energy o the urou requence convergng to zero at a rate o, where N the number o overamle er cycle ( we aume n cycle/ch and k N amle/cycle then the total number o amle/ch wll be N=n k ). 3. SYSTEM DESIGN The H/W latorm ued or mlementaton hown n Fgure. The RF gnal down converted to an ntermedate requency (IF) by the Mam MAX74 GPS recever ch. The requred reerence requency rovded by a ractonal PLL clocked by a hgh recon low drt crytal ocllator. The PLL alo rovde the nut requency or the mer to urther down convert the to a 4.9 MHz ntermedate requency. The outut o the mer ltered and dgtzed at M. The Sandblater roceor alo erorm the Automatc Gan Control (AGC) uncton n otware and rovde the AGC control gnal. MAX74 GPS recever ch AGC Freq. Reerence Mer IF IF Fgure. PLL LP Flter Ocllator 3.. BASE BAND PROCESSING Data amle rom the A/D Shtng Interolaton Flterng Fgure. Co(w table Sn(w Table T T Samlng Clock A/D HW block dagram Table 3 () 3 Q data I data () Uer Layer Proceng Uer Layer Proceng DSP SB96 Search Trackng Synch Recever bae band block dagram The mled block dagram o the recever, or a ngle atellte, dected n Fgure. A ew mllecond worth o data amle, receved rom the Analog-to-Dgtal (A/D) converter are buered and roceed earately or each atellte. Frt, the data aed to the hang, nterolaton, and lterng block. The uncton erormed by th block are llutrated n Fgure 3. In order to meet the requency condton decrbed n Secton, the data hted one amle at a tme every n amle, conormng to

3 the requred Doler ht. Alo, to kee the carrer hae locked, an etra one amle ht can be erormed, t ndcated by the dgtal PLL. To ncreae the Delay Locked Loo (DLL) reoluton, the block can erorm amle nterolaton ung one o a number o well known nterolaton technque. In th ytem, we emloyed lnear nterolaton. The data then ltered ung an FIR lter and urther aed to the demodulator. The demodulaton erormed on the already hted data ung a ne/cone looku table wth eght amle er cycle. The ntegraton over eght amle rovde one I/Q amle and multaneouly erorm eght tme decmaton. The correlaton wth the equence rovde one data amle. The I/Q data ubequently roceed baed on the ntended uncton (e.g. Satellte earch, I Q, PLL, IQ, DLL, I Q,or De-readng Q) ch Fgure 3. Interolaton & htng Shang Flter From Synch & Trackng Interolaton & Flterng Block To Bae-band Proceng Interolaton, htng and lterng block. Fgure 4. Satellte earch, cro correlaton outut or, 6 atellte n vew, -3 db SNIR. Fgure 4 llutrate the earch reult or a contellaton o atellte at equal ower and -3 db gnal to noe and ntererence rato (SNIR). It how about ch beng roceed and our otware mlementaton correlatng and lockng onto the gnal. 3.. SKY SEARCH Some heavly comutatonal uncton do not requre realtme roceng. Sky earch an eamle o th tye o uncton. In the cae o the ky earch uncton, the tme tracked ung a hgh reoluton tmer, drven by the crytal ocllator, uch that the net buer o data recely laced n tme. The ky earch uncton erormed on a ew ten o mllecond o data. I the atellte contellaton known, the earch erormed only or the number o the eected vble atellte and a ew Doler requency bn around the eected Doler requence. I the contellaton not known, the earch erormed or every number and every Doler requency bn. The outut rom the demodulator, In and Quadrature hae, are correlated wth a 3 length equence at one amle er ch. It then quared and accumulated over everal mllecond. The reult comared agant a threhold. I the reult hgher than the threhold, the dereadng roce tarted wth the mamum correlaton amle oton. The deread data correlated wth an eght bt reamble reeated n every navgaton data ubrame. I the reamble ound then the earch ucceul and the data urther roceed. Otherwe the earch tart agan. Fgure DIGITAL PLL Dgtal PLL block dagram. The dgtal PLL block dagram hown n Fgure 5. The PLL nut cont o I and Q data. The outut -,, or where mean advance one amle, do not ht and - mean delay one amle. Fgure 6 and Fgure 7 how the deread data wth the PLL turned OFF and ON, reectvely. In Fgure 6 t can be een that becaue o the accumulated error n the htng roce the data cannot be roerly decoded wthout the dgtal PLL runnng. A the dgtal PLL turned ON, the decoded data equence can be ealy een n Fgure 7.

4 gnal beore decodng ( rame *4 bt): -5Hz redual doler req. w/ PLL Fgure 6. Decoded data, Doler ht 5 Hz, PLL turned OFF, SNIR -3 db. Fgure 8. Decoded data, Doler ht -5 Hz, PLL turned ON, SNIR -33 db, wth addtonal lterng. 4 3 () 3 early Data From the Demodulator 3 () 3 () _ Smoothng Flter - - late () Fgure 7. Decoded data equence {, -,, -, }; Doler ht 5 Hz, PLL turned ON, SNIR -3 db. In the cae when the receved gnal eerence lower ower level due to harh roagaton condton, addtonal lterng can dramatcally mrove the ntegrty o the data. In our S/W aroach we can dynamcally add or remove dgtal lter n the roceng chan baed uon the value o the meaured SNIR or a ecc atellte. Fgure 8 llutrate the decoded data or -33dB SNIR when addtonal otware Band Pa Flter (BPF), tuned on the IF requency wth MHz bandwdth, added. Wth a gnal ower level db le ( -33 db), t can be een that the varance on the data maller than n the -3 db SNIR cae. Fgure DIGITAL DLL Dgtal DLL block dagram. The role o the dgtal DLL to detect the ubrame boundary at le than our amle er ch reoluton n order to mrove the uer oton accuracy. The block dagram o the dgtal DLL hown n Fgure 9. It can be een that there are two dentcal branche eecutng the ame uncton, ecet or the act that the equence n one branch advanced hal a ch, whle n the other branch t delayed hal a ch. The data low a ollow: I and Q data rom the demodulator correlated wth the early and late veron o the equence or a ecc atellte. It then quared and ubtracted a decrbed n ollowng ormula and nally ltered by a moothng lter. ( I e Qe ) ( I l Ql ) where : I e, I l, Q e, Q l are the early and late I and Q.

5 GPS Perormance n SB MHz Udate Perod [econd] m, y, z oton accuracy 5 m, y, z oton accuracy 75 m, y, z, oton accuracy -.5 Fgure. GPS recever erormance / ch Fgure. Outut o the dgtal DLL. The ltered outut hown n Fgure. It can be ealy een that the curve ehbt a mnm and a mam correondng to the early and late correlaton, reectvely. The a ntercet rereent the ubrame boundary, related to the two neghborng ch. Th more accurate rame boundary rovde a more rece tme otonng or the ncomng ub rame, and ultmately rovde a better uer oton etmate. 4. RESUTS To determne the uer oton n three dmenon ( u, y u, z u ), a eudorange meaurement need to be erormed or our atellte. In our aroach we aume the uer oton aromately known o we can teratvely ue a rt order aromaton or olvng the nonlnear ytem o equaton. Ater the rt teraton, the new oton ued a ntal aromate oton and o on. Ater a ew teraton, the nal value o the oton obtaned. 5. CONCLUSIONS We have reented a otware GPS degn that rovde or dynamc trade-o n erormance veru accuracy. The degn mlemented comletely n otware and wll run n real tme on the SB96 latorm. 6. REFFERENCIES [] Ellot D. Kalan, Undertandng GPS Prncle and Alcaton, Artech Houe Inc [] Global Potonng Sytem Standard Potonng Servce Sgnal Seccaton, GPS NAVSTAR nd Edton, June, 995 [3] E. O. Brngham, The Fat Fourer Tranorm And It Alcaton, Prentce-Hall Inc [4] J. Gloner, S. Dorward, S. Jnturkar, M. Moudgll, E. Hokenek, M. Schulte, and S. Valad, Sandbrdge Sotware Tool, n Proceedng o the 3rd Internatonal Worko on Sytem, Archtecture, Modelng, and Smulaton (SAMOS 3), July -3, 3,. 4-47, Samo, Greece. [5] J. Gloner, D. Iancu, J. Lu, E. Hokenek, and M. Moudgll, A Sotware Dened Communcaton Baeband Degn, IEEE Communcaton Magazne, Vol. 4, No.,. -8, Jan., 3. We mlemented all baeband roceng or the GPS recever on the Sandbrdge Sandblater SB96 multthreaded latorm [4] [5]. The erormance or derent oton accurace hown n Fgure. We characterzed the recever erormance or varou udate erod. For hgh accuracy wth at reone (.e. 5 meter and udate reone), about MHz o roceng requred. For more tycal condton (.e. 75 meter and 5 udate reone), only 4MHz o roceng requred. Th equate to a 5% and 6% utlzaton o the SB96 latorm.