Statistical Approach to Mitigating 3G Interference to GPS in 3G Handset

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1 Int. J. Communications, Ntwork and Systm Scincs, 1, 3, doi:1.436/ijcns Publishd Onlin Sptmbr 1 ( Statistical Approach to Mitigating 3G Intrfrnc to GPS in 3G Handst Abstract Tahr AlSharabati, Yinchao Chn Univrsity of South Carolina Columbia, South Carolina, USA sharabat@mail.sc.du, CHENYIN@cc.sc.du Rcivd Jun 1, 1; rvisd July 18, 1; accptd August, 1 In this papr, w show how statistical dcision thory can b usd to solv ral lif product problms. Global Positioning Systm (GPS) prformanc in a mobil handst dgrads whnvr it is simultanously usd with a 3G data or voic srvic. This dgradation is du to th 3G transmittr intrfrnc. Mitigation mthods to intrfrnc in GPS hav bn proposd. Howvr, most of ths mthods dpnd on hardwar and signal procssing or just hardwar solutions. In som cass, it mayb difficult to implmnt ths hardwar mthods, spcially in mobil handsts du to its small siz, printd circuit board (PCB) layout issus and th addd cost. In this papr, a novl signal procssing statistical algorithm approach is proposd to mitigat th 3G intrfrnc to GPS. This statistical approach utilizs th knowldg of th statistical charactristics and distributions of both th GPS signal and nois. Thn th mthod utilizs th probabilitis to mak a statistical dcision to rmov th ffct of nois. This mthod dos not rquir room on th PCB of th mobil handst and thrfor no layout challngs aris. In addition, cost is minimizd and th product turn in cycl is shortnd. This papr offrs thortical as wll as practical insight to th GPS opration during 3G call insid th mobil phon. Kywords: Dtction and Estimation, Global Positioning Systm (GPS), Satllit Acquisition, Wid Band Cod Division Multipl Accss (WCDMA) 1. Introduction Global Positioning Systm (GPS) plays a major rol in our daily livs. Its applications xtnd from naval, arospac, first tim mrgncy rspondrs and location basd-finding and dirctions in cll phons to nam a fw. Th xistnc of GPS in cll phons prsnts its own challngs sinc it co-xists with othr srvics such as 3G WCDMA and GSM carrirs [1]. Th spatial distanc or proximity of ths srvics and subsystms and thir componnts insid th cll phon is vry small. Espcially sinc th mobil handst is bing packd with as many srvics as possibl with th smallst form factor. This distanc is small nough to th point whr it dos not prsnt nough attnuation to th 3G transmittd Radio Frquncy (RF) signals toward othr rcivrs insid th handst. This, in turn, will prsnt itslf as intrfrnc to th rcivrs of th srvics that xist in th handst spcially th GPS rcivr. Th ffct of th distanc on th attnuation or path loss of th transmittd signal strngth is govrnd by th quation: A 3.4 1log( rf ) (1) whr A is th attnuation in db, f is th frquncy of th transmittd signal and r is th distanc btwn th transmittr antnna and th GPS rcivr. Th spctrum sparation or proximity btwn th frquncy bands of th mobil handst wirlss srvics is anothr sourc of intrfrnc. Th wirlss srvics ar allocatd frquncy bands for thir transmittrs and thir rcivrs. Th closr th frquncy of a transmittr to th frquncy of a rcivr, th mor of intrfrnc th frquncy will prsnt. This is du to th non idal charactristics of th band pass filtr of th transmittr []. As a mattr of fact, som filtr dsigns xhibit fly back, which is wors insrtion loss charactristics spcially towards th band dgs of th filtr. Ths band dgs could b th oprating frquncis of anothr srvic lik th GPS. For xampl, th GPS rciv frquncy (calld L1 band) is 17.4 MHz, whil th DCS band lowst transmit frquncy (or channl) in GSM is Copyright 1 SciRs.

2 MHz and that of WCDMA band IV is MHz. Th frquncy sparation is lss than 13 MHz. It should b mntiond that architctur and systm nginrs do a thorough intrfrnc linup analysis and calculations to assss th ffcts of diffrnt scnarios of th ffcts of intrfrnc of ach band to itslf and othrs with th givn componnts of th subsystms bfor or on th onst of any transcivr chipst dploymnt. Th ffcts of intrfrnc, if not mitigatd, on rcivrs and th GPS rcivr in particular ar svr. Ths ffcts could rang from dgradation in usr position accuracy or loss of satllit acquisition. This situation, whn it happns to a rcivr in a product lik mobil phons, is calld d sns i.. th rcivr is d snsd. In [1], th GPS rcivr was d snsd by. db. Shown in Figur 1 is a bar graph of th GPS satllits whr ight of thm hav bn acquird basd on an acquisition mtric of., whr th horizontal axis is th satllit numbr and th vrtical axis is th mtric valu, (Prmission from [3]) whn thr is no 3G intrfrnc addd to th data collctd in [3]. Any satllit with an acquisition mtric of. and abov is considrd acquird.. Analysis In th following w prsnt an analysis and xampls for calculating two of th important paramtrs of th GPS rcivr, namly; th avrag signal strngth and th root man squard (RMS) cod tracking rror..1. Calculating th Avrag Signal Strngth Th avrag carrir strngth ( C av ) (or C s ) of th acquird satllits can b calculatd basd on Equation (9) in [4]. Equation (9) is rpatd hr for clarity purposs for th cas1 Db and assuming that.33 ( is th normalizd GPS cod tracking rror varianc in units of sc [], D is th two sidd GPS corrlator spacing and b r Tc () b is th normalizd bandwidth, assuming r 6MHz, T c is th chip priod ): From this quation, th carrir to nois powr ratio Cs N is: o Cs db 1.67dB. No Assuming systm nois figur (NF) of db and thrmal nois powr (N o ) of -174dB/Hz. From: C No NF Cs No av (3) C av ~ = 1 dbm. This is th avrag signal strngth or carrir powr of on of th acquird satllits. Figur shows a plot of th avrag carrir powr of th acquird satllits of Figur 1. Th horizontal axis is th satllit numbr, whil th vrtical axis is th avrag carrir signal strngth in dbm. Not how th valus corrspond to th mtric valus in Figur 1... Calculating th RMS Cod Error On of GPS masurmnt accuracy paramtrs is th root man squard (RMS) cod tracking rror in units of mtrs. Lt s dnot th RMS cod tracking rror as thn [], *c (4) whr σ is th standard dviation in units of chips [6] and c is th spd of light. Each chip priod is approximatly1 s. For xampl, for th abov cas whr.33, 4.66 m. This is th cod tracking rror. Figur 3 displays th RMS cod tracking rror (in mtrs) of th acquird satllits. It is vidnt from this plot and that of Figur how th avrag signal powr affcts this paramtr. Th strongr th powr is; th lss th rror is and thrfor th accurat th usr position is. 3. Procdur B (1. B T) 1 b 1 D c T ( D) N o N o 3.1. GPS Data and 3G Nois Gnration Bing insid th phon, th GPS rcivr suffrs th most intrfrnc from th 3G transmittrs insid th Usr Equipmnt (UE) whil thy ar transmitting. From this point on, th 3G transmittd signals will b rfrrd to as 3G nois bcaus thy prsnt thmslvs as nois to th GPS rcivr. Th GPS data usd in this papr ar data collctd in [3], which is data output of th ADC in that platform. Th 3G nois will b simulatd on th basband lvl by using th WCDMA Quadratur Phas Shift Kying (QPSK) modulation schm which is: L L ( ) ( ) 1, 1 Db (9) in [4] T C s b 1 b Cs Copyright 1 SciRs.

3 73 Acquisition Mtric Not acquird signals Acquird signals Acquisition Rsults Acquird Satllit's Numbr Figur 1. Numbr of acquird satllits whn 3G nois is not prsnt. Avrag Carrir Powr Cav (dbm) Avrag Carrir Powr of Acquird Satllits Acquird Satllits' Numbrs Figur. Plot of th avrag powr of th acquird satllits. Cod Tracking Error E (m) RMS Cod Tracking Error of Acquird Satllits Acquird Satllits' Numbrs Figur 3. Plot of RMS cod tracking rror of th acquird satllits. Vrms V3 G() t ( di()cos( t wift ) dq()sin( t wift )) 4 4 di(), t dq() t 1 () whr di(), t dq() t ar th in phas and quadratur phas basband componnts of th data strams (which ar random and normally distributd with zro man and varianc of on) and w IF is th intrmdiat frquncy of th down convrtd signal. Th 3G nois is sampld at th rat of th sampling frquncy of th ADC in [3]. Thn th 3G and GPS data ar addd togthr to form th GPS plus intrfrnc signal. 3.. Dtrmining 3G Nois Lvl This task will tak into considration th total systm architctur componnts of both th 3G transmit (3G TX) sction and GPS rciv sction. Figur 4 shows a block diagram of a typical GPS-3G coxistnc scnario [1]. Th GPS sction could b th hardwar platform in [3]. Going from right to lft; th hart of th GPS (GPS RX) rcivr is th GPS Radio Frquncy Intgratd Chip (RFIC). Most of GPS RFICs hav on board cross functional sctions that span from RF to basband. Th SiG 41L [8] is on of SiG s GPS lin of products. It has a low nois amplifir (LNA) on board and it outputs srial IQ data. So, its functionality span from RF to bits. A bandpass filtr is usd to filtr out any signal outsid th dsignd bandwidth of th GPS RX. At th forfront is th GPS antnna and antnna matching circuitry to bring th antnna impdanc to a -ohm systm. On th intrfring 3G TX sid, th TX-RX 3G antnna accommodats both th WCDMA and GSM carrir bands. Thn, th WCDMA/GSM Powr Amplifir (PA) along with th duplxr. Basd on typical valus of 3GPP WCDMA transmittr nois into GPS, valus of 3G nois into GPS [9] ( 14 dbm/hz), duplxr insrtion loss and spatial isolation, a valu of V rms (3G nois) in Equation () was calculatd. This valu cam to b ~.8 mv. In ths calculations (for illustration purposs only), th spatial isolation is assumd to b 1 db, th total rciv lin up gain is ~1 db in ohm systm. First th powr (P db ) of th intrfrr was calculatd and thn it was convrtd to voltag basd on a ohm systm: PdB nois into GPS + Linup GaindB + (6) IsolationdB = db P dbw pdbw pdb 3 (7) (8) 1 log( Vrms / ) V.8 mv rms Copyright 1 SciRs.

4 WCDMA /GSM PA Duplxr 3G TX WCDMA/ GSM Antnna TX Isolation (I) 3G Antnna GPS Antnna Antnna Match BPF GPS RX Intrnal LNA Figur 4. Typical co-xistnc 3G-GPS scnario. GPS RFIC Approach to th Algorithm Prvious publishd work has focusd on suggsting ithr a combination of signal procssing and hardwar or just hardwar solutions to mitigating th intrfrnc. In [7], antnna null string was suggstd to minimiz Wid Band (WB) RF intrfrnc (RFI). Whil this mthod works in products othr than cll phons, it is difficult to implmnt in th cll phon bcaus it rquirs complx implmntation of antnna dsign, RF front nd lctronics, fdback and signal procssing. This mthod can not b implmntd in th cll phon du to its complxity and immaturity for mobil phon nvironmnt. Othr mthods ar proposd to prvnt pulsd RFI [7]. Whil th mthods just mntiond aimd at prvnting intrfrnc from occurring, thy do not mitigat th ffcts of intrfrnc aftr it happns. Ths mthods do not offst th ffcts of rrors gnratd onc th intrfrnc is mixd along th lin up and down convrtd to basband with th dsird GPS signal. Instad, a statistical mthod is proposd to mitigat ths rrors. Th GPS data output from th ADC is Gaussian distributd with zro man and thrfor thir probability distribution function is normal and is in th form of: x 1 px ( ) (9) Figur shows that distribution and wights in bar graph of th valus and thrfor it givs th probability of ach valu output from th Analog to Digital Convrtr (ADC). Th rd trac is th Normal distribution fit of th data. Th horizontal axis ar th valus output from th ADC, th vrtical axis ar thir wights. This figur dos not litrally giv th probability valus, but it shows th wights of ths valus.. Algorithm and Rsults.1. Mitigation Algorithm Dvlopmnt Th rcivr tris to mak a dcision or bst guss about a symbol s i givn that r was rcivd. In othr words, th rcivr tris to dcid th probability of s givn that r was rcivd; p(s r) basd on th tst statistic: T p( r s ) p( s ) (1) i i Sinc th data follow th Gaussian distribution, thn for N = [1], whr N is th numbr of dimnsions of th signal spac: ( rs ) i pr ( si ) (11) and (1) bcoms; ( rs ) i pr ( si) ps ( i) ps ( i) (1) ps ( i ) is th probability of th nois fr quantizd output GPS signal from th ADC and is th standard dviation of th 3G nois. Th novlty of this approach is as follows: sinc th Coars Acquisition (C/A) cod of th satllits is comprisd of 1 s and 1 s ( 1 for ), th prototyp mssags can b rducd to two lvls, namly; 1 and 1 and thrfor calculating th probabilitis in Figur rducs to calculating th probabilitis of th ngativ valus of th signal and th probabilitis of th positiv valus of th signal. W just nd to rcovr th right phas of th signal. Th dcision rducs down to dciding whthr a 1 or a 1 is bing snt. In this cas (1) gnrats two valus: and ( rs ) i p(1 si) ps ( i) ps ( i ) (13) ( r s ) i p( 1 si) p( si) p( si ) (14) Th dcision gos to th on with th highr statistic... Rsults To dtrmin usr position, th GPS rcivr has to acquir at last four satllits. Figur 6 shows th rsults of acquisition of GPS data plus 3G nois whn no dtction algorithm is usd [], namly PRN#: 1, 18, 1. From th rsults, only thr satllits wr acquird, not nough to dtrmin usr position. Whil aftr using th dtction algorithm, fiv satllits (Figur 7) ar acquird, namly PRN#: 6, 1, 18, 1, and. This will Copyright 1 SciRs.

5 734 hlp th GPS rcivr in calculating th usr position vn if th usr is using th mobil handst in ithr a data call or a voic call. Basd on Equations (), (3) and (1) in [4], valus of th avrag carrir powr ( C av ) wr approximatd and a comparison plot for thr cass was obtaind. Th first cas corrsponds to th acquird satllits in Figur 1 whr th rsults ar for th raw data whr no 3G nois was addd to th data and thrfor no dtction algorithm was usd. Th scond cas corrsponds to th acquird satllits in Figur 6 whr 3G nois was addd to th data and no dtction and stimation algorithm was usd. Th third cas corrsponds to th acquird satllits in Figur 7 whr 3G nois was addd to th data and th dtction and stimation algorithm was usd. Figur 8 shows such a comparison. It is vidnt from this plot how th proposd dtction algorithm and stimation not only rcovrs th satllits lost du to nois, but it improvs th avrag dtctd powr of thos satllits. By using (4), a comparison was mad for th cod 3 Probability Dnsity Distribution of GPS Data GPS data GPS data GPS data Fit GPS Data Fit Dnsity Discrtizd GPS Data Output of ADC Figur. Distribution of GPS data from th ADC in [3]. 4. Acquisition rsults Not acquird signals Acquird signals 4 3. Acquisition Mtric Satllit numbr Figur 6. Rsults of acquisition whn no dtction usd. Copyright 1 SciRs.

6 Acquisition Rsults wtih Nw Dtction Algorithm Not acquird signals Acquird signals 6 Acquisition Mtric Satllits' numbr Figur 7. Rsults of acquisition whn using dtction algorithm. Avrag Carrir Powr Cav (dbm) Comparison of Avrag Carrir Powr of Acquird Satllits 3G 3G Nois Addd-No Dtction No No 3G 3G Nois-No Dtction 3G 3G Nois Addd-Dtction Usd Usd Acquird Satllits' Numbrs Figur 8. Comparison of avrag carrir powr of acquird satllits whn (1) No 3G nois addd. () 3G nois addd-no dtction algorithm usd. (3) 3G nois addd- dtction algorithm usd. Copyright 1 SciRs.

7 736 Comparison of RMS Cod Tracking Error of Acquird Satllits 6 Cod Tracking Error E (m) G Nois Addd-No Dtction 3G Nois Addd-No Dtction No 3G Nois-No Dtction 3G Nois Addd-Dtction Usd Usd Acquird Satllits' Numbrs Figur 9. Comparison of RMS cod tracking rror of acquird satllits whn (1) No 3G nois addd. () 3G nois addd-no dtction algorithm usd. (3) 3G nois addd- dtction algorithm usd. tracking rror for th sam thr cass as shown in Figur 9. Th standard dviation was calculatd for ach trackd satllit. It is sn how th proposd algorithm rducs th rror by showing lss numbrs in mtrs. This mans that th usr position is mor accurat for th satllits acquird with nois and dtction usd than undr nois without dtction algorithm. It should b mntiond that th situation in Figur 6 is vry optimistic. In many cass, th GPS rcivr loss track of all satllits and hnc no GPS fix can b obtaind. Th suggstd algorithm guarants to rcovr most if not all th satllits that ar trackd as if th 3G nois is not prsnt. 6. Conclusions A cross functional ffort has bn displayd in this papr. Th ffort spannd from prsnting architctural analysis and calculations for a 3G-GPS RF TX-RX systm focusing on th intrfrnc rlatd aspcts and sourcs, to stimating th avrag powr of th rcivd signal, to laying out th algorithm analysis and drivation of th mitigation schm. W showd how th mitigation schm can rcovr th acquisition of th satllits that wr lost du to th intrfrnc. Also, th avrag carrir powr and RMS cod tracking rror of th rcovrd satllits ar improvd and thrfor th usr position accuracy is improvd vn though th mobil handst is in a 3G data or voic srvic whn using th GPS srvic. This algorithm could b implmntd at th phasing stag of a mobil handst dvlopmnt and manufacturing. 7. Rfrncs [1] T. Sharabati and Y. Chn, Mitigating 3G Carrir Intrfrnc to GPS Du to Co-Existnc in 3G Handst, IEEE NAECON, 9, pp [] T. Sharabati, Proposal for Mitigating 3G Intrfrrnc to GPS, Univrsity of South Carolina, South Carolina, 9. [3] K. Borr, D. M. Akos, N. Brtlsn, P. Rindr and S. H. Jnsn, A Softwar Dfind GPS and Galilo Rcivr Birkhausr, Birkhausr, Boston, 7. [4] W. B. John and R. K. Kvin, Extndd Thory of Early-Lat Cod Tracking for A Band limitd GPS Rcivr, Navigation, Journal of th Institut of Navigation, Vol. 47, No. 3,, pp [] W. B. John, Effct of Narrowband Intrfrnc on GPS Cod Tracking Accuracy, ION NTM, 6-8 January, pp [6] B. Michal and A. J. V. Dirndonck, GPS Rcivr Architctur and Masurmnts, Procdins of th IEEE, Vol. 87, No. 1, 1999, pp [7] P. Ward, Intrfrnc Hads Up, GPS World, Vol. 19, No. 6, Jun 8, pp [8] S. Smiconductor, SE41L GNSS Rcivr IC, Jun 8. [9] A. Tchnologis, UTMS17/1(171-17MHz) and UMTS17 (17-178MHz), Avago Tchnologis, AV- 197EN, 9. [1] W. Couch Lon II, Digital and Analog Communication Systms, 3rd dition, Collir Macmillan, Nw York, 199. Copyright 1 SciRs.