CELL SEARCH ROBUST TO INITIAL FREQUENCY OFFSET IN WCDMA SYSTEMS

Transcription

1 CELL EARCH ROBUT TO INITIAL FREQUENCY OFFET IN WCDMA YTEM June Moon and Yong-Hwan Lee chool of Electrcal Engneerng eoul Natonal Unversty an 56-, hllmdong, Kwanak-Ku, 5-74, eoul, Korea Abstract - The WCDMA system, an asynchronous CDMA scheme, uses dfferent downlnk scramblng codes for each base staton, not requrng synchronzaton between the base statons as n the synchronous CDMA systems. However, the moble staton needs to obtan ntal code synchronzaton by resolvng both the code and tmng uncertanty. The 3GPP group adopts the use of a three-step cell search scheme []. For ease of ntal cell search, the base staton transmts two synchronzaton codes through the synchronzaton channel. The moble staton searches the peak correlaton value of the prmary synchronzaton code to acqure the slot tmng. For relable detecton, the recever combnes the correlaton values from several slots. In ths paper, we propose a new combnng scheme that can reduce search tme and also provde performance robust to a wde range of ntal frequency offset. The proposed scheme utlzes a dfferental detecton method, applcable to the transmt antenna dversty mode as well as the sngle antenna mode. The proposed scheme can also estmate the ntal frequency offset for the automatc frequency control (AFC) performance, elmnatng the need of ntal frequency acquston crcut. Keywords Acquston, WCDMA, Cell search, Frequency offset estmaton I. INTRODUCTION Although synchronzaton between the base statons s not requred n the WCDMA system, the moble staton should handle ntal cell search process. In synchronous CDMA systems such as I-95 and cdma, the moble staton needs to search only the code tmng for ntal code synchronzaton. nce dfferent downlnk scramblng codes are used by each base staton n the WCDMA system, the moble should resolve both the tmng and code uncertanty for ts ntal code synchronzaton []. The use of 5 complex Gold codes as the downlnk scramblng code makes t mpractcal to exhaustvely search all the possble codes for the code tmng. Ths problem can be allevated usng a three-step cell search scheme adopted as the 3GPP specfcaton [,3]. The three-step operaton for ntal cell search n the WCDMA system can be processed usng two synchronzaton channels (CH) and common plot channel (CPICH) n thedownlnk physcal channel [3-4]. The CH comprses the prmary CH (P-CH) and secondary CH (-CH). The P-CH provdes the nformaton on the slot tmng and the -CH provdes nformaton on the code group and frame boundary. In the frst step of the three-step cell search, the recever searches for the slot tmng by correlatng the receved sgnal wth the P-CH code usng a matched flter (MF) whose tap coeffcent s matched to the P-CH code. nce the sgnal-to-nose power rato (NR) of the transmtted P- CH code s very low, t s requred to combne the correlaton values from multple slots to properly acqure the slot tmng. A number of combnng technques such as coherent combnng, non-coherent combnng and dfferental combnng have been proposed [3,5,6]. Coherent combnng can provde optmum performance under deal condton but t cannot be appled to real envronment where the channel dstorton and RF mparment exst [3]. Non-coherent combnng has wdely been used due to ts practcal aspects [5]. It was shown that dfferental coherent combnng scheme can outperform non-coherent schemes under mld Doppler condton [6]. However, t cannot be appled to the transmt dversty mode (TDM), a tme swtched transmt dversty (TTD) scheme s used n the synchronzaton channel [4], snce the dfferental product s extracted from the two consecutve slots where the phase of the receved sgnal alternately vares over the slots. Moreover, t s necessary to employ a peak detecton scheme operatng ndfferently of the TDM, snce the recever does not know whether a TDM s used or not durng the ntal cell search stage. In ths paper, we propose an nner-slot dfferental combnng scheme that can be applcable to both sngle and double antenna mode. nce the proposed scheme s based //$7. IEEE PIMRC

2 on a dfferental detecton scheme, t can provde performance robust to ntal frequency offset. The performance of the proposed scheme s compared to that of the conventonal non-coherent scheme n terms of the mean search tme. Followng ths Introducton, the proposed scheme s descrbed n ecton II. ecton III evaluates the performance of the proposed scheme n comparson to conventonal schemes. Fnally, conclusons are gven n ecton IV. Two rays from B Matched Flter (Cp) One rays from Bj lot-wse accumulaton Tslot Fnd maxmum II. THE PROPOED CHEME The cell searcher acqures the slot tmng usng the P-CH code n the frst step of the three-step cell search scheme. The base staton transmts a P-CH code of length 56 repeatedly durng the frst symbol of a slot through the P- CH. The P-CH s transmtted wth a % transmsson duty cycle. The recever can fnd the slot tmng by correlatng the receved sgnal wth the locally generated P- CH code. nce the P-CH codes are not contnuously transmtted, t can be correlated usng a passve type correlator such as the MF whose peak value can be used for acqurng the slot tmng. In the recever front-end, after down-convertng, low-pass flterng and samplng at tme t = TC, a baseband receved sample r [] can be represented as Fnd the peak Fg.. A combnng scheme appled to peak detecton experence flat Raylegh fadng. nce the P-CH code s transmtted durng the frst symbol (56-chp duraton) of the slot ( symbols), P [] can be represented as a perodc functon wth a perod of 56, P, < 56 P [ ] = (), 56 < 56 The P-CH code can be generated usng two component codes X and X of length 6, C PCH ] = ( + j) X [ ( dv)6] X [ (mod)6] (3) [ r[ ] = jθ [ ] P[ ]/ α [ ] e C [ k] + n[ ] () PCH where where P [] s the power of the P-CH code, α [] s gan varaton due to the channel fadng, θ [] denotes the phase dstorton due to the channel and RF carrer offset, C PCH [] s the P-CH code desgned to reduce the MF complexty [7], kt C s the amount of delay due to the channel and recever flterng and n [] s the nose term comprsng the ntra-cell nterference, nter-cell nterference and thermal nose. In ths paper, we assume that the sgnals r[] X [5] X [4] X [3 ] X [] X [5] X [4 ] X [3 ] X [] X [] X [8 ] X [7 ] X [4 ] X [3 ] X [] a k, 3[ n] a k, [ n] a k, [ n] a k, [ n] Fg.. tructure of the herarchcal correlator (64-chp partal correlaton) X = [,,,-,-,,-,-,,,,-,,-,,] X = [,,,,,,-,-,,-,,-,,-,-,] and (dv) 6 and (mod) 6 are the quotent and remander of / 6, respectvely. The receved sgnal s correlated wth the locally generated P-CH code usng the MF. As n the acquston problem n the D/ system, the correlaton length of the correlator should be determned consderng the amount of ntal frequency offset due to the RF carrer nstablty. When the frequency nstablty s large as much as ppm, t s known that the use of 64-chp partal correlaton s most approprate provded that the correlaton length s gven by a power of [5]. Note that effcent Golay correlators cannot be employed because they cannot perform the partal correlaton. The use of herarchcal correlators [8] can be employed for partal correlaton as shown n Fg.. Let a k, j [ ], j =,,3, be the j-th partal correlaton value correspondng to the k-th slot, where s the chp tme ndex. Then t can be represented as (4)

3 a 64( j ) [ ] = + r[ + l] x[ l (dv) 6] x[ l (mod)6] + η[ ] 64 k, j l= 64 j where η [] s the nterference term due to the correlaton of n [] and C PCH []. For relable detecton, the partal correlaton value a k, j [ ] should be combned over multple slots as shown n Fg.. It s practcal to employ a noncoherent combnng scheme that provdes the optmum performance when the phases of the combnng samples are ndependently and unformly dstrbuted as shown n Fg. 3 (a). The non-coherently combned output Z non [] over Ns slots can be represented as N 3 k = j= k, j ] (5) Z [ ] = a [ (6) non When there exsts an ntal frequency offset, the phase varaton between the two consecutve partal correlaton lot- 56Tc P-CH lot-k a, a, a, a,3 a k, a k, a k, a k,3 a Ns-, a Ns-, a Ns-, a Ns-,3 lot-(ns-) Σ Σ Σ Σ Z non [] (a) A non-coherent combnng scheme mag a k, j+ φ [ ] 64 f o Tc a k, j real [ ] (b) The phase varaton between two consecutve partal correlaton values lot- 56Tc P-CH lot-k a, a, a, a,3 a k, a k, a k, a k,3 a Ns-, a Ns-, a Ns-, a Ns-,3 lot-(ns-) * * * * * * * * * Σ [] Z dff (c) The proposed nner-slot dfferental combnng scheme Fg. 3. Combnng schemes Table mulaton parameter Item Value CPICH Ec/Ior - db PCCPCH Ec/Ior - db DPCH Ec/Ior -8 db (F = 8) CH Ec/Ior (P-CH/-CH) -3dB / -3dB OCN F=8, 6 channels mulaton rate Chp rate Propagaton channel Flat Raylegh fadng values heavly depends on the magntude of frequency offset, as shown n Fg. 3 (b). Assumng that the magntude of frequency offset does s unchanged durng 64-chp partal correlaton nterval, dfferental combnng of the nner-slot partal correlaton values can be used for peak detecton. Fg. 3 (c) depcts the proposed nner-slot dfferental combnng scheme. The dfferentally combned output Z dff [] over Ns slots can be represented as Ns k = j= * k, j[ ] ak, j+ [ ] Z [ ] = a (7) dff nce there are 56 chps n each slot, the slot tmng chp ndex can be estmated n a maxmum lkelhood (ML) manner MAX {,,,559} = argmax Z [ ] (8) The amount of phase varaton between the two consecutve partal correlaton values s related to the ntal frequency offset f O. Fg. 3 (b) llustrates a phase rotaton of φ = 64 f O T C. The ntal frequency offset can be estmated after peak detecton usng the combned value Z ] as dff { Z [ ]} dff [ MAX f ˆ o = arg (9) dff MAX 64Tc Unlke conventonal dfferental combnng scheme that extract the dfferental products from the two consecutve slots, the proposed scheme can work well regardless of the TDM because the dfferental products are extracted from the partal correlaton values wthn a slot. III. PERFORMANCE EVALUATION To evaluate the performance of the proposed scheme, t s assumed that the chp tmng s known, a coherent combnng scheme s used n the second step, the false alarm probablty of the thrd step s desgned to -4, and the

4 st step nd step 3rd step Fal! Fal! Fal! OK! Fg. 4. Tmng dagram of the three-step cell searcher Mean search tme [ms] Mean search tme [ms] 8 NO TTD 6 4 fo=khz TTD fo=khz fo=khz (a) ngle antenna mode fo=khz Doppler = Hz Noncoherent combne Dff. coherent combne (b) TTD mode Doppler = Hz Noncoherent combne Dff. coherent combne Fg. 5. Mean search tme as a functon of Ior/Ioc Mean search tme [ms] NO TTD TTD Doppler frequency [Hz] three-step operaton s processed n parallel as shown n Fg. 4. The processng tme of each step s set to a sngle frame nterval for the parallel processng, resultng that the number Ns of combnng slots s 5. The test envronments are based on the model proposed by the 3GPP to perform the user equpment (UE) conformance test [9] as summarzed n Table. The performance of the cell searcher s evaluated n Fg. 5 n terms of the mean search tme as a functon of the rato of the ntra-cell to nter-cell nterference power, Ior/Ioc. The sold lnes depct the performance of conventonal noncoherent combnng scheme wth the sngle antenna and double antennas mode when the Doppler frequency s Hz. It can be seen that the mean search tme decreases as Ior/Ioc ncreases and that the proposed scheme outperforms the conventonal scheme regardless of the use of the TDM. nce the proposed scheme employs dfferental detecton, t can provde mproved performance when the amount of ntal frequency offset s large. It can be seen n Fg. 5 (b) that the performance gan can further be obtaned n the TTD mode. Ths s manly due to that the NR gan from dfferentally coherent combnng makes the detectors work well at low NR when the dversty s employed. Although the transmt antenna dversty n the base staton s optonal n the 3GPP specfcaton, t s expected that the TTD s wdely employed to ncrease the system capacty. Fg. 6 depcts the search performance n terms of the Doppler spread. The tme dversty gan can be obtaned by combnng the correlaton values over multple slot ntervals. It can be seen that the mean search tme decreases as the Doppler frequency ncreases because of the ncreased tme dversty gan. The combnng process can obtan full dversty gan when the combnng nterval (.e., one slot duraton) becomes larger than the coherence tme of the Raylegh fadng channel,.e., 56 TC > () f Thus, the maxmum dversty gan can be obtaned when f d > 75Hz. nce the loss n coherent ntegraton ncreases as the Doppler frequency ncreases, the searcher can provde the best performance around f d = 75Hz, as can be seen n Fg. 6. It can also be seen that the better performance s obtaned under the TTD mode. d, fo=khz L=4, Ior/Ioc = -3dB Noncoherent combne Dff. coherent combne Fg. 6. Mean search tme as a functon of the Doppler frequency

5 rms frequency offset error [Hz] rms frequency offset error [Hz] 5 (.ppm) 5 (.5ppm) 5 5 (.ppm) 5 (.5ppm) 5 It should be noted that the proposed scheme can estmate the ntal frequency offset. Fg. 7 depcts root-mean-squared (rms) estmaton error, gven by ˆ ε = E{( f f ) } () f NO TTD o It can be seen that the estmaton error decreases as Ior/Ior ncreases and that the estmaton performance s ndfferent from the amount of ntal frequency offset. The rms estmaton error s less than KHz when Ior/Ioc s hgher than -6dB. Ths s equvalent to the use of a GHz oscllator wth nstablty less than ppm. Ths mples that there s no need of a frequency acquston crcut for the automatc frequency control (AFC) requred after the cell search. nce t takes ~3ms for a conventonal frequency acquston crcut to properly work [], the proposed scheme can reduce the tme for synchronzaton n addton to the mproved search performance. o Doppler = Hz (a) ngle antenna mode TTD fo = 5k fo = k fo = k (b) TTD mode Doppler = Hz fo = 5k fo = k fo = k Fg. 7 Intal frequency offset estmaton performance IV. CONCLUION The peak detector of the frst step plays the most mportant role to mprove the search performance. An nnerslot dfferental peak detecton algorthm has been proposed and ts performance s verfed by computer smulaton. Numercal results show that t outperforms conventonal non-coherent combnng schemes n both TDM modes. Although the proposed combnng scheme employs a dfferental detecton method, t can easly be appled to the TTD mode. In addton, t can estmate the ntal frequency offset wth an accuracy of KHz, correspondng to ppm oscllator n the WCDMA operatng envronment, removng the need of a frequency acquston crcut for the AFC. REFERENCE [] ARIB, Japan s Proposal for Canddate Rado Transmsson Technology on IMT-: W-CDMA, June 998. [] K. Hguch and M. awahash, F. Adach, Fast Cell earch Algorthm n Inter-Cell Asynchronous D- CDMA Moble Rado, IEICE Trans. Commun., vol. E8-B, no. 7, pp , July 998. [3] 3GPP T 5.3, spredng and modulaton (FDD), June.,. [4] 3GPP T 5., Physcal channels and mappng of transport channels onto physcal channels, ept.,. [5] Y.-P.E. Wang and T. Ottosson, Cell search n W- CDMA, IEEE J. on elected Areas n Commun., vol. 8, pp , Aug.. [6] Y. K. Jeong, K. B. Lee and O.-. hn, "Dfferentally Coherent Combnng for lot ynchronzaton n Inter- Cell Asynchronous D/ ystems," Proc. on PIMRC, pp , ept.. [7] emens, A new correlaton sequence for the Prmary ynchronsaton Code wth good correlaton propertes and low detector complexty, Tdoc MG UMT-L 47/98, Oct. 4-6, 998. [8] emenas and Texas Instruments, Generalsed Herarchcal Golay equence for PC wth low complexty correlaton usng pruned effcent Golay correlators, 3GPP TG RAN WG TGR-554/99, 999. [9] 3GPP T 5., UE Rado Transmsson and Recepton, Oct.,. [] Y.-P.E. Wang and T. Ottosson, Intal frequency acquston n W-CDMA, Proc.VTC 99, pp. 3-7, ept. 999.