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ELIMINATION OF NAWIOW-BCIND INTERFERENCE I N BPGK-IU)DU-ATED SI 6Wk RECEPTION'

Michael A. Soderstrand'Herschel H. L w m i sK. V. RangaraoE le c tri ca l and Computer E ngineering

Naval P ostgraduate S ch wlMonterey, CA 93943-5100

CIBSTRI\CTFour different classes of adaptivesig nal cancelers can be used t o

elim ina te narrow-band inter fere nce froma broadband signal: (1 ) cascadedsecond-order notch f i l te r s , ( 2 ) high-order in- lin e notch fi l te r s , (3) second-order bandpass noise cancelers, and (4)hig h- wdw bandpass noi se cancelers. Ofthe four, a structure based on second-order bandpass f i l t e r s used to cancelthe narrow-band s ignal performed be tterthan the other structures . The adaptivealgorithm fo r these f i l t e r s has beenproposed by Kwan and Martin and modifiedby P etrag lia, M itra and Szczupak. W epropose a modification of the structuret o improve the adaptation and s imp lif ythe hardware. The new s tru cture i sapplied t o narrow-band Bi-Phase S hi ft-Key (BPSK) inte rfe renc e i n broadbandBPSK signals w i t h and without backgroundnoise. The s truc ture i s able to removeinterfere nce under many di ffe re ntconditions.

1.0 IntroductionA common signa l proces sing problem i sthe reception of a re la tiv e ly weakbroad-band s igna l such as a spread-spectrum Bi-Phase S hift-K ey (BPSK)

modulated s ignal i n the presence ofnarron-band interference. These narrow-band in te rf ere rs may have le s s energythan the broad-band s ignal, but becausethe energy i s concentrated over a narrowbandwidth they mask the broad-bandsignal .'This work was supported i n pa rt by agrant from the U nited S tates A i r Force.'H.A. Soderstrand i s a Vis i t ingP rofessor a t the Naval P ostgraduateSchool from the Department o f E le c tri ca lE ngineering and Computer Science a t theU niversity of C alifornia, Davis.

I n th is paper we compare severalapproaches t o e limina te the narrow-bandinterference while preserving thedes ired broad-band signal. S ection 2describes four dif ferent structures fo radaptive interferen ce can cellation. Thestruc ture s a re compared and a ba s is i sgiven fo r the se lection o f the second-order band-pass canceler as the bestdesign. Se ction 3 gives detailedinformation on the neu algorithm fo r thesecond-order band-pass cance ler andexperimental data t o demonstrate i t sadvantages over other algorithms such asthe algorithm by Kwan and M artin C l l .Conclusions are given i n S ection 4.

2.0 Adaptive Notch F il te rsNotch fi l te r s fo r removing multiple

narrow-band inter fere nce can be put in tofour broad categories E21 il lu s tra ted i nF igure 1. The f i r s t two categories,F igures l a and lb, are cascaded secondorder notches w ith each second-ordersec tion removing one frequency. Thenext two categories, F igures ICnd Id,are higher-order notches tha t elim ina temultiple frequencies. I n a l l of thecategories, it i s possible to use FIRf i l t e r s tie : a l l zer o f i l ter s ) whichare e as il y p ipe line d and can be madetr u l y li ne a r phase. However, I I Rf i l te r s out perform FIR f i l te rs . ThusI I R pipe lining may become an importantis sue C21.

2.1 2nd-Ordw Cascaded Notch F i l te r sThe second-order notch f i l t e r i s usedi n cascade and in- line with the signalas shown i n F igure la. The trans ferfunction for the notch f i l te r i s givenby:

a - 2 r k i z - ' + az-'1 - 2 r k i z - ' + r ' z - 'H,(z) = ( l a )

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For r greater than 0 and le ss than 1 theparameter k i can be adjusted betueen -1and +l t o yi e ld a symmetric notch f i l t e rwith unity gain a t DC and the N yquistfrequency and notch frequency determinedby ki. I f r i s kept cons tant, then the3db notch width i s also kept constantand the parameter a i s also constant.Thus ki may be adapted to r m v e onenarrow-band signa l without effe ctin g r ,a, or the notch bandwidth. CI cascade ofsuch f i l ters can be used t o removemultiple narrou-band signals.

a. Cascade-in-line 2nd Order F i l te r s

u u Ub. Cascade of 2nd-order Cancelers

c. High-order-in-line Notch F il te r s

d High-ordw S ignal CancelerF igure 1. Four Notch F il te rs

2.2 2nd-Ordu Cascaded S ignal CancelerThe cascaded second-order s igna lcanceler approach shown i n F igure l b hasthe advantage tha t the des ired s igna ldoes not pass through the adaptive

filter. Instead the bmd-prs s filter i sused to detect the narra rband s ignalwhich i s then wbtractwd f ramthrdes ired s ign al C23. A conr$mt 3dbbandwidth notch can be rchievrd byselecting a band-pass fi lter w i t h thetransfer functimr

h(1 - z - ~ )H,,(z) = (2a)1 - Zk i r - ' + r 2 z - 2uhwer

6% w i t h the 2nd-order in - line notch,the signal canceler keeps r , a, h r andthus the notch bandwidth constant dilradapting k i t o c ancrl the narrau-4mdinterference. The 2nd-o rdr siqna l-cancrlwr structure i s also n ice foradaptation because it s relatively easyto generate se ns itivity functibns h i c hare related t o the gradient H,, withres pec t to the parameter ki C1,23.

I n a r ecen t paper 133 we proposed amodific ation t o the Kwan and Martinf i l ter . This Soderstrand Notch F i l te rusmi only 34-1 2nd order f i l te r s t o f w mN notches ra the r than N(N+3)/2 req uire dby Kwan and Martin. The Soderstrandalgorithm i s ne11 sui ted t o removingnarrou-band interference C23.

2.3 Higher-Ordr In-Linr Notch F i l te rThe notch f i l t e r of Figure ICas

the advantage tha t it can be implementedas an F IR f i l te r ( e g g tapped-delay-line)and thus i s more ea s ily pipelined. Sucha f i l t e r has the additional advantage oli ne ar phasp. H o u e v u , an F IR filterw i l l re qu ire many ueights t o obta in goodperfwmance.h I I R in- l ine f i l t e r w i l l allow goodperformance w i t h much femr ueights.Houever, the I I R f i l t e r i s d i f f i c u l t t omake adaptive and pipe lin ing of the I I Rfilter i s more di f f i c u l t than p ipe l in ingthe FIR f i l t e r . This filter could beadapted by having a discrete set of pre-selected notches which could be switched

i n by a detection ci rc uit that sinplylooks fo r maximums i n th e spectrum ofthe i nput s ign al C23.

2.4 Higher-Ckdu Signal C Inc rlerThe sign al canceler of F igure i d hasmost of the b en efits and disadvantagesof the higher-order in- lin e notchdiscussed above. However, since the

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canceler i s not in-l ine with the signal,the des ired s ignal does not need to passthrough the filtw. cllsp, it nay beeas ier t o dceign the adaptive po rtio n oft h i s f i l t e r . ch additional advantagecomes f roa the a bi l i ty t o adapt thef i l t e r o ff- lin e and then switch it i nonce the filter i s able to enhance theperformance of the overall system C23.

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2.5 I I R V FIR R eal izationrThe second order notch of F igure l aand the second-order band-pass cancelerof F igure l b would have to be I I Rreal izations t o achieve a s uff ic ientlynarrow bandwidth to cancel in te rfe rin gs igna ls without adverse eff ec t on thedesired signa l. The f i l te r s of F iguresI C and l d could be I I R or FIR. However,high-order FIR fi l te r s provide linearphase and easy adaptation. Higher order

I I R f i l te r s would add great complicationwithout s igni fica ntly improving f i l te rperfwmance..CI tapped-delay-line adaptive filter

ww ld be well suited t o the cascaded-in-l in e structure of F igure lc. A nin teres ting approach, however, i s theuse of the recursive DFT to rea l i ze thes tructure of Figure Id. Using a Fas tF w r i w Transfwm (FFT) chip or aRecursive DFT using Residue NumberC kithretic C21, w e could obtain a seriesof band-pass f i l te r s and the adaptivealgorithm would simply adapt the weightsof the output subtracter tha t wouldcancel th e various s ine waves C23.

I...-_ .....

38 Men Znd-Ordw S ignal CancelerThe high-order re al iz ati on s of F igurel c and I d both suffer from di f f ic u l t ie si n adapting the f i l t e r weights. Of the

second-order structures, the W e rs tra n df i l t e r of F igure lb i s best C23. Oneexample of a problem with the f i l te r ofFigure l a occurs when two narrow-bandsignals are close to one another,F igure 2 shows th is di ff i c ul ty. Thespectrum of the inpu t signa l i s shown i nF igure 2a and cm ri s ts of 3 s in e wavesa t 0 . ifs, 0 . 125fs, and 0.375fs (36O,4S0, and 1 3 5 O ) i n Gaussian noise. Eachs ine nave and the nois e have the sameenergy i n the signal. F igure 2b showsthe spectrum of the output of the th i r dstage of the f i l te r . Y ou can see theattenuation of the two low-frequencynarrowband signals, but the thi rdnarrow-band s ign al i s s t i l l present.F igure 2c shows the parameter adaptationfo r th i s example. W e had hoped thateach of the three notch f i l te r s woulde l minate one of the narrorrbandsignals . However, ins tead of completely

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elimin ating one si ne wave, the fi r s tsection of the notch f i l te r centersi ts e l f ha lf way between the two sinewaves. Then the second and th i rdsec tion each remove one of the s inesleaving the th i rd sine wave untouched.

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I T EMT I WSc. P arameter M apta tio nF igure 2 Cascade-in-line F il te r

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W e repeated the same experiment us ingthe S odwstrand algorithm C31. F igure 3shows the r es ul ts o f th i s experiment.W e used the same input as F igure 2a.F igure 3a rhows the o utput spectrum fromthe Soderstrand fi l te r . C learly the newf i l te r has eliminated a l l 3 narrow-bandf i l ters. F igure 3b s h o w s the parameteradaptation f o r the Soderstrand fi l te rc 2 3 .

m y OF OUTPUT FRQl3-NOMI SODERSIlwlD nDnPTl ut PlLTW1 4 0 . . - ' , . ' , , , ,t I

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~ e se lee lie 2i-Zse -jae & d I& seeITEWTIOIISb. Parameter AdaptationF igure 3 New Soderstrand F il te r

4.0 ConclusionsThe fi n a l te s t of the new algorithmi s to apply it to re a l data. The 3-dp lots o f F igure 4 show the res ul ts ofapplying the new Soderstrand algo rithmto a broad-band BPSK sig nal corrupted by

tw o narrow-band BPSK sig na ls and bynoise. These p lo ts are obtained us ingthe C yclic S pectral A nalysis SoftwarePackage C41. F igure 4a shows thedes ired BPSK s igna l. F igure 4b showsthe corrupted signal. F igure 4c, whichmatches very c losely w ith F igure 4a, i sthe corrupted signa l a fte r passingthrough the new f i l te r . These re s ul tsco nfirm performance of the new alg ori thmi n elimin ating narrow-band interference.

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a. Broad-band BPSK signal

a #I

b. Corrupted s igna l

anc. Output of S odwstrand F il te rF igure 4. Recovery of BPSK S ignal

R e fw MCCS1 Kwan and Martin, "Adaptive Detection

t Enhancement of M ulti pl e Sinusoids,"IEEE Trans. CAS, vol. CAS-36, no. 7,pp. 937-9459 J uly 1989.

2. W ers tra nd , Rangarao, Bernstein, andL o o m i s , Now Algwithms fo r th e-tac tion and E limination of S ineWaves and Mhw Nurow-Band S ignalsi n the Resencr o+ Broadband Noi se,Naval P wtgrad School TechnicalReport, A pr il 1991.

3. Soderstrand, Loomis, and Rangarao,"Improved Real-Time M aptive Detec-tio n, Enhancement, or E limination ofM ultip le Sinusoids," Proc. 1991Midwest Symposium cA8, Ilonterey, May1991.

4. S chell and Spooner, C yc lic S pectralAna lysis S oftuare Package UI wrManual, S tati s tic a l S ignal Proc . ,Inc, Yo untville CA , J une 38 , 1990.

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