A Carrier Estimation Method for MF-TDMA Signal Monitoring

Transcription

1 117 JOURNAL OF NETWORKS, VOL. 7, NO. 8, AUGUST 1 A Carrer Etmaton Method for MF-TDMA Sgnal Montorng X Lu School of Electronc and Informaton Engneerng, Behang Unverty, Bejng, Chna Emal: Autn_lu@139.com Wenquan Feng, Chunheng L, Chao Ma School of Electronc and Informaton Engneerng, Behang Unverty, Bejng, Chna Emal: fengwenquan@buaa.edu.cn, lchunheng@buaa.edu.cn, cnnfhvj@163.com Abtract Baed on the need of MF-TDMA gnal montorng, th paper nvetgate the etmaton method for frequency offet and carrer phae. The procedure for tmng recovery ntroduced, and then an effectve algorthm deduced for etmatng carrer frequency and phae. The propoed open-loop algorthm demand no preamble and ha a wde capture range. In addton, the overall block dagram of a montorng recever, whch we degned wth the propoed etmaton algorthm, preented. to etablh a montorng ytem at NCC, o a to fnd out anomale and make adjutment n tme. Index Term Carrer phae etmaton; Frequency offet etmaton; MF-TDMA; VSAT; I. INTRODUCTION MF-TDMA a two-dmenonal multple acce type whch combne frequency dvon wth tme dvon. By frequency hoppng, varable rate and vrtual crcut technology, flexble MF-TDMA network can be acheved between dfferent termnal taton [1]. Becaue of t flexblty and extenblty, MF-TDMA atellte communcaton ytem ha been appled more and more. Key pont n MF-TDMA atellte communcaton ytem nvolve tmng recovery, capture and ynchronzaton, control of power and frequency, etc. Baed on th, th paper dcue on the carrer etmaton method for MF-TDMA gnal n VSAT atellte network montorng. A hown n Fg 1, VSAT atellte communcaton ytem compred of the network control center (NCC), the atellte tranponder and VSAT taton []. There are three typcal tranmon pattern n VSAT atellte network: CDMA, SCPC and TDMA. If multple carrer coext n a common tranponder band, TDMA tranmon turn to be MF-TDMA, whch can provde more changeful ervce and more flexble. MF-TDMA network more effcent whle the mplementaton technque are complex: olar outage, ranfall, or mproper operaton on VSAT taton wll mpact the qualty of communcaton. Conequently, t neceary Fgure 1. VSAT ytem confguraton In general, VSAT network power lmted, rather than frequency lmted ytem, n whch modulaton cheme that ha the hghet power effcency hould be adopted. Therefore, coherent QPSK or BPSK elected n mot cae. For the dgtal communcaton ytem wth PSK modulaton, there are two type of carrer ynchronzaton method [3]: one ung a feedback cloed-loop to track the carrer frequency and phae of the receved gnal, uch a quadratc loop and Cota loop; and the other one the drect etmaton of receved carrer ung open loop tructure. In MF-TDMA ytem, carrer parameter may vary a the burt lot come from dfferent VSAT taton, whch have caued conderable dffculty for cloed-loop carrer recovery. For th reaon, the open loop maxmum lkelhood etmaton of carrer parameter motly tuded at preent. The open loop etmaton method can be dvded nto two group accordng to weather the tranng equence preamble or not: the DA (data-aded) algorthm and the NDA (non-data-aded) algorthm [4]. For the applcaton feld of MF-TDMA gnal montorng, a t dffcult to be nformed of preamble equence, therefore the NDA algorthm are uually adopted. Baed on the premed demand, th paper do:1.434/jnw

2 JOURNAL OF NETWORKS, VOL. 7, NO. 8, AUGUST propoe an open loop carrer etmaton method and the correpondng archtecture of montorng recever [5-6]. II. ARCHITECTURE OF THE MONITORING RECEIVER In th paper, we take QPSK for example and expre the receved MF-TDMA gnal a: We conder performng the montor at ntermedate frequency, thu the archtecture of the amplng recever can be hown a n Fg 5. ( ) () () () exp ( π ϕ k k k () θk) r t = A t j f t + t + + n t (1) n whch Ak () t the ampltude of receved gnal, f k the carrer frequency, θ k the carrer phae devaton, ϕ t the modulated ymbol phae takng value n k () π π π π }, ( ) { 14,34,54,74 nt the addtve whte Gauan noe, and the ubcrpt k repreent the ordnal number of the multple carrer. Fg repreent the tme-frequency two-dmenonal expanon of th MF-TDMA gnal a n (1). Spectrogram of th gnal and the tme-doman envelope of a ngle channel are hown n Fg 3 and Fg 4. Fgure 5. Sytem dagram of montorng recever Frt, the recever quantze the IF mult-carrer gnal by ung band-pa amplng, and the local NCO et to the center frequency of one of the MF-TDMA carrer. After the quadrature down-converon, the amplng rate reduced to nteger multple of the ymbol rate to facltate further proceng. Then, after the matched flter, we get the complex ample of the baeband TDMA gnal. For MF-TDMA gnal montorng ytem, the top prorty to obtan meaurement ndce (uch a lot power, frequency offet, EVM, etc.) of each channel. If there any anomalou value, the montorng ytem hould gve an alarm tmely. In th paper, we focu on the carrer etmaton method of the baeband ample. PSD(W/Hz) Power Envelope x 1-14 Fgure. Sketch map of tme-frequency expanon Frequency(MHz) Fgure 3. Spectrogram of MF-TDMA gnal III. ALGORITHM DESCRIPTION In tradtonal recever, feedback loop are commonly ued for carrer ynchronzaton, n whch the hang-up effect may lmt the capture range and lowdown the proce of carrer acquton. Accordngly th artcle propoe ung open-loop etmaton method n montorng recever, and put forward correpondng algorthm utable for MF-TDMA burt communcaton. A. Tmng recovery Montorng ytem receve burt gnal from dfferent VSAT taton, o eparate etmaton for each burt lot needed. The purpoe of tmng recovery to fgure out the optmal amplng tme, o a to gan the maxmum recevng gnal-to-noe rato (SNR) durng demodulaton. In th paper, Gardner tmng-error detecton algorthm appled for optmal amplng-pont verdct. Pvotal dea of Gardner algorthm are lted a follow: take ample pont, of whch any adjacent two are half a ymbol cycle apart, nto account, and ue ther ampltude and polarty change to calculate the tmng error [11]. The pecfc algorthm Symbol ur ( ) = Ir ( 1/)[ Ir ( ) Ir ( 1)] + Qr ( 1/)[ Qr ( ) yr ( 1)] () Fgure 4. Tme-doman envelope of a ngle channel where I and Q repreent the nphae and quadrature ample, and the ndex r ued to degnate the number of ymbol: r repreent the current ample pont, r-1 repreent the ample pont of the lat ymbol, and ue r-

3 117 JOURNAL OF NETWORKS, VOL. 7, NO. 8, AUGUST 1 1/ to denote the ample lyng mdway between the rth and the (r-1)th ample. Therefore, the error calculaton only requre to ample the baeband gnal at the tme of the ymbol rate. Gardner algorthm ndependent from carrer etmaton, a a reult, the tmng recovery can be performed before or after the carrer ynchronzaton. In the montorng recever a we decrbed n Fg 5, modfed open-loop tmng recovery method adopted for the convenence of proceng. We frt reduce the amplng frequency, after the quadrature down-converter, to Q tme of the ymbol rate (Q an even nteger greater than ). After the matched flter, ue () to calculate tmng error of the Q column ampled data. And fnally, we pck the column whch ha the mnmum varance a the optmal ample. A neceary conderaton that, durng one frame perod, there are tme lot when no data burt but only ext noe, and t meanngle to calculate tmng error at uch dle lot. Alo, tmng error of burt data lot may dffer, a they came from dfferent VSAT taton. In order to mprove the accuracy of the tmng recovery, we ue a module called Slot Extracton to extract data lot from the energy envelop of modulated carrer. Then, tmng error calculated for each data lot eparately. B. Carrer etmaton For non data-aded (NDA) carrer etmaton, the Vterb & Vterb algorthm, a n [8], a clac openloop nonlnear phae etmaton algorthm, and ha been wdely ued. Generally, the proce of V&V etmaton can be dvded nto three tep: frt, the modulaton removed by a nonlnear tranformaton; then, average the phae n the etmaton nterval; at the fnal, equvocaton of etmated carrer phae hould be elmnated. Conder that the MF-TDMA gnal beng montored n VSAT network relayed tranparently by the geoynchronou atellte, the dynamc range of carrer frequency mall enough that the Doppler hft can be neglected. Therefore, domnated carrer frequency offet come from the devaton of frequency tandard on dfferent VSAT equpment. Due to the need of average proce on etmated phae, the V&V algorthm requre the normalzed frequency offet to meet: ΔfT.5, n whch Δ f the carrer frequency offet, and T the perod of one ymbol. When the frequency offet Δ f exceed a certan lmt, the performance of V&V etmaton algorthm wll decend obvouly. For th reaon, we conder to evaluate the frequency offet and try to make correpondng compenaton before the phae etmaton [9-1]. The tructural model of the carrer etmaton method propoed n th paper hown a n Fg 6. f Δ Fgure 6. Structural model of the carrer etmaton For the faclty of analyzng, we aume that the tmng of ample ha been accurately recovered after the matched flter, and the complex baeband ample can be expreed a n (3). r = x + j y ( ( π ϕ ( π ) θ )) = exp j Δf T + M + + n (3) n whch ( M ) ϕ π repreent the M-ary PSK modulated u data, θ the ntal phae, and n repreent the complex addtve Gauan noe whoe two-de power pectral denty N. To nvetgate the complex ample n the etmaton nterval N N, we frt remove the modulaton by a Mth-power nonlnear tranformaton, namely, M j( πδf T+ ϕ( π M) + θ ) M j( M arg( r )) ( e ) e z = + n = r (4) Such a tranformaton wll change the carrer phae π, π π M, π M. When the nterval from [ ] nto [ ] SNR of r hgh, we can take a mple approxmaton a follow [7] ( ) ( π ϕ ( π ) θ ) arg r = Δf T + M + + u (5) n whch the equvalent phae noe to n, who alo Gauan dtrbuted wth zero mean, and yeld σu = σn. The reult of th nonlnear tranformaton can be wrtten a ( z ) M arg( r ) ( ) α = arg = = M πδf T + θ + u + π (6) Wthout the nfluence of modulaton, carrer frequency α. Note offet can be evaluated form ample ere { } that ample { α } are lnearly proportonal to the frequency offet Δ f. In partcular, the maxmum

4 JOURNAL OF NETWORKS, VOL. 7, NO. 8, AUGUST lkelhood (ML) etmaton equvalent to an unbaed leat mean-quare (LMS) etmaton, wth the followng reult: N N Δ f = α 8πT = N N N = 3 α πtl L 1 = N ( ( )) n whch L the total number of ample n the confdence nterval, and L=N+1. The varance of th etmaton read: wth Subttutng 3 ( 16 T L( L 1 α )) f (7) σ = σ π (8) Δ ( E N ) σ = σ = σ = 4 (9) α u n b o σ nto (8) yeld α 3 ( 4 ( 1 ) b o) σ = πt L L E N (1) Δ f A we can ee, t much eaer to avod π phae ambgute f the phae dfference { β } are ue, ntead of the abolute phae{ α }: wth * ( zz ) ( ) β = α α = arg 1 1 = M πδ ft + M u u 1 Then (7) can be rewrtten n term of { β }: Δ f = w β = w arg z z (11) N N * () () ( 1) (1) = N = N () 3( 1 )( ) 4π ( 1 ) w = N + + N T L L (13) From the about dervaton of etmaton method we can ee, the algorthm performance cloely related to two factor, namely, the magntude of carrer offet and SNR. Frtly, the carrer frequency offet cannot exceed the algorthm capablty, whch derved when calculatng argument{ β } n (11). To avod phae alang, we make ( ) ( π π) M πδ ft + M u u, (14) 1 thu M πδ ft < π, and o Δ ft < 1M. Secondly, SNR of the receved gnal wll alo have gnfcant mpact on the performance of etmaton. Note that n (11), the noe tem rae when SNR decreae; meanwhle, (5) wll no longer be workable when SNR too low. The lower SNR become, the harder t wll be to elmnate the π phae ambgute, and thu algorthm performance wll degrade everely. In order to verfy the performance of the algorthm, we conducted a ere of mulaton. Mean value and varance of the etmated frequency offet are hown n Fg 7 and Fg 8. Thee curve are calculated over dfferent value of the carrer phaeθ. Smulaton reult how that the preented frequency etmaton algorthm unbaed, and the etmaton range cloe to theoretcal upper lmt (1/M) n cae of hgh SNR. Mean of Etmated Frequency offet: f*mt Eb/No=5dB Eb/No=1dB Eb/No=15dB Eb/No=dB Eb/No=5dB fd-et v fd Frequency offet: f*mt Fgure 7. Mean value v normalze frequency offet For the pecfc mplementaton, we can contruct the ML etmator a a FIR flter, a hown n Fg 6, and tore { w() } a flter coeffcent n advance. Snce the frequency offet Δ f ha been corrected and the redual error mall enough, the devaton of carrer phae can be etmated, for ntance, by ung V&V algorthm a n Fg 6. IV. SIMULATION RESULTS AND PERFORMANCE ANALYSES

5 1174 JOURNAL OF NETWORKS, VOL. 7, NO. 8, AUGUST 1 Var of Etmated Frequency offet Eb/No=5dB Eb/No=1dB Eb/No=15dB Eb/No=dB Eb/No=5dB var of fd-et v fd Frequency offet: f*mt Fgure 8. Varance v normalzed frequency offet The varance of etmated frequency at dfferent Eb/No (db) n Fg 9 ndcate that, at hgh SNR, performance of the algorthm propoed approachng MCBR [1], namely, 3 ( 16 T L( L 1 α )) σ = σ π (15) Δ f Fg 1 mage the change of contellaton n dfferent ector n the montorng recever. Smulaton are performed a Eb/No vare, and the reult how hat, wth the algorthm we propoed n th paper, the frequency offet and the phae devaton of carrer can be effcently etmated and corrected. Var of Etmated Frequency offet f*mt=. f*mt=. f*mt=.4 f*mt=.6 f*mt=.8 MCRB Eb/No(dB) Fgure 9. Varance v Eb/No Fgure 1. Change n contellaton V. CONCLUSION In th paper, we preent a carrer etmaton method for the need of MF-TDMA gnal montorng: the algorthm demand no ad of pror data, and ha a low computatonal complexty. The propoed method cont of the etmaton of carrer phae and frequency offet. Smulaton reult have proved that, the preented frequency etmaton algorthm unbaed, and the etmaton performance cloe to theoretcal upper lmt. In concluon, the recever archtecture we decrbed wth th algorthm ha a wde capture range and very utable for the montorng of burt communcaton gnal. REFERENCES [1] Jun He and Hua Deng, MF-TDMA Sgnal Detecton and Parameter Etmaton Sytem, Communcaton Technology, vol. 9, pp. 3-33, 8. [] X. T. Vuong, F. S. Zmmermann and T. M. Shmabukuro, Performance analy of Ku-band VSAT network, IEEE Communcaton Magazne, vol. 6, pp. 5-33, [3] Xupng Jang, A Non-Data-aded Frequency Offet Etmaton Algorthm for Burt Tranmon, Electronc Scence and Technology, vol. 3, pp. 1-1, 1. [4] Zhenyan Qn and Jyu Zheng,"A None-Data-Aded Carrer Frequency Offet Etmaton Algorthm Baed on Maxmum Lkelhood", Journal of Gu ln Unverty of El ectronc Technology, Vol. 9, No. 3, pp. 6-8, 9. [5] Lue, M. and R. Reggannn, Carrer frequency recovery n alldgtal modem for burt-mode tranmon. Communcaton, IEEE Tranacton on, vol. 43(34): pp , [6] B. E. Paden, Matched nonlnearty for phae etmaton of a pkmodulated carrer, IEEE Tranacton on Informaton Theory, vol. IT-3, pp , 1986.

6 JOURNAL OF NETWORKS, VOL. 7, NO. 8, AUGUST [7] F. Gardner, A BPSK/QPSK Tmng-Error Detector for Sampled Recever, IEEE Tranacton on Communcaton, vol. 34, pp , [8] A. J. Vterb and A. M. Vterb, Nonlnear etmaton of PSKmodulated carrer phae wth applcaton to burt dgtal tranmon, IEEE Tranacton on Informaton Theory, vol. 9, pp , [9] Kay, S., A fat and accurate ngle frequency etmator. Acoutc, IEEE Tranacton on Speech and Sgnal Proceng, vol. 37(1): pp , [1] Xao, Y.C., et al., Fat and accurate ngle frequency etmator. Electronc Letter,. Vol. 4(14): pp , 4. [11] S. Belln, C. Molnar and G. Tartara, Dgtal frequency etmaton n burt mode QPSK tranmon, IEEE Tranacton on Communcaton, vol. 38, pp , 199. [1] A.N. D'Andrea, U. Mengal, R. Reggannn, The modfed cramer- rao bound and t applcaton to ynchronzaton problem, IEEE Tran. Commun, vol. 4, pp , X Lu, wa born n Laonng provnce of Chna, n He receved h B.S. degree n electronc and nformaton engneerng and n appled mathematc n 1. He currently a PhD tudent n School of Electrc and Informaton Engneerng n Behang Unverty (BHU). H reearch nteret n atellte telemetry trackng and command (TT&C), and nter-atellte communcaton technology. Wenquan Feng, wa born n Schuan provnce of Chna, n 197. He receved h PhD degree n Behang Unverty (BHU) and currently a profeor of School of Electrc and Informaton Engneerng n BHU. In recent year, he ha preded over more than 1 centfc program, publhed over paper, ncludng over 1 EI-ndexed paper, edted and publhed two monograph. H reearch nteret n modern dgtal communcaton ytem, telemetry and telecontrol technology, aeropacecraft tet and mulaton, etc. Chunheng L, profeor of School of Electrc and Informaton Engneerng n Behang Unverty (BHU). He receved h PhD degre n communcaton and electronc ytem, n BHU, In recent year, he ha preded over more than centfc program, guded graduated tudent, edted and publhed 4 monograph, and publhed over 8 centfc paper. H reearch nteret nclude paceborne SAR ytem and mulaton technology, nformaton fuon on mutource remote enng mage, gnal gatherng and proceng,etc. Chao Ma, wa born n Nemenggu provnce of Chna, n He receved h B.S. degree n Behang Unverty (BHU) n 1, and currently a graduated tudent n School of Electrc and Informaton Engneerng n BHU. H reearch nteret n atellte telemetry trackng and command techmology.