Adaptive Channel Estimation in WCDMA STTD Systems

Transcription

1 Aaptve Channe Estmaton n WCDMA STTD Systems J-Woong Cho an Yong-Hwan Lee Shoo of Eetra Engneerng an INMC Seou Natona Unversty Kwanak P. O. Box 3 Seou 5-7 Korea e-ma: jwh@frut.snu.a.kr yee@snu.a.kr Abstrat - The reever performane wth the use of a spae tme transmt versty (STTD) sheme s more suseptbe to the auray of hanne estmate than that wthout the use of the STTD sheme sne the espreang sgnas suffer from the effet of rosstak an the transmt power s equay ve nto mutpe transmt antennas. As a resut the effeny of hanne estmaton n the WCDMA STTD system beomes an mportant ssue more than that n the non-sttd system. In ths paper an aaptve hanne estmator () s esgne to mtgate the performane egraaton ue to naurate hanne estmaton. Numera resuts show that the performane mprovement sgnfanty nreases wth the use of the propose partuary when the hanne onton beomes worse. I. INTRODUCTION There have been propose a number of versty shemes for DS-CDMA systems []. Tme versty an be aheve wth the use of ntereavng an hanne ong. The rake reever an obtan the versty effet by ombnng eah mutpath sgna omponent. The spae versty an be reaze wth the use of mutpe antennas at the transmtter an/or the reever. Sne t may not be prata to empoy mutpe antennas n the mobe hanset the transmt antenna versty s wey empoye n the ownnk of the WCDMA system []. The transmt antenna versty sheme an be reaze usng an open-oop or ose-oop sheme. The nformaton on the hanne s not use to transmt the sgna n the open-oop versty sheme whe t s obtane from the reever to ontro the transmt gan of eah antenna n the ose-oop versty sheme []. The use of a spae tme transmt versty (STTD) sheme has been appe to the ata hanne n the WCDMA system as an open-oop transmt versty sheme []. It was shown that the performane an sgnfanty be mprove wth the use of an STTD sheme [3]. The STTD sheme empoys mutpe transmt antennas to prove both the tme an spae versty partuary when the Dopper frequeny s ow or the hanne has a sma number of mutpaths. It was reporte that the reever performane of the STTD sheme s more suseptbe to hanne estmate error than that of a non-sttd sheme sne the error an ause the rosstak between the ajaent symbos []. Moreover the hanne estmate may beome more erroneous n the STTD sheme sne the tota transmt power s spt nto mutpe transmt antennas ereasng the SIR of the esprea pot sgna. As a resut t s requre to use an effent hanne estmator to fuy obtan the effet of transmt versty. Unke n the non-sttd sheme however there have not been muh stues on hanne estmaton n the STTD sheme. The reever performane of the STTD rake reever was evauate when a brk-wa type owpass fter s use as the hanne estmaton fter (CEF) []. A few hanne estmaton shemes were propose for the STTD transever usng the orthogona property of the pot sgna transmtte from eah transmt antenna [56]. The reever performane of the STTD sheme n the WCDMA system s ompare wth that of the non-sttd sheme when a movng average (MA) FIR fter s use as the CEF [7]. It was shown that the reever performane s sgnfanty hange epenng upon the tap sze of the MA CEF an the maxmum Dopper frequeny. However there have been few stues on aaptve hanne estmaton shemes. In ths paper we propose an aaptve hanne estmator () for the WCDMA STTD system by mofyng the for non-sttd DS-CDMA system [8]. The reever performane wth the use of the propose s evauate n the STTD an non-sttd sheme. Seton Ⅱ esrbes the struture of the WCDMA STTD ownnk system. The propose s esrbe n Seton Ⅲ. The performane of the propose sheme s evauate n terms of the BER performane n Seton Ⅳ. Fnay onusons are summarze n Seton V. II. WCDMA STTD DOWNLINK SYSTEM Fg. epts the struture of the WCDMA STTD transmtter for the eate physa hanne (DPCH) []. The STTD enoe DPCH ata x (t) at antenna- an an be represente as s[ t kt x ( t) s[ t (k + ) T s t kt () x( t) s t (k + ) T where T s the symbo tme uraton. In parae wth the DPCH transmsson the orthogona pot symbo sets [A A] an [A -A] (or [-A A]) are transmtte through a ommon pot hanne (CPICH) of eah antenna urng two-symbo tme nterva. The hanne /3/$7. 3 IEEE 78

2 Data Channe Enoer TPC TFI Rate Mathng Intereaver Pot Dversty Pot M U X STTD Enoer QPSK symbos Channezaton oe an ong srambng oe C spreang ength M Ant Ant Ant x ( t ) x ( t) Ant M U X Tx. Antenna Tx. Antenna Fg.. Struture of the WCDMA STTD transmtter for DPCH transmsson. ong rate mathng an ntereavng are one as n the nonversty moe. It an be assume that the hanne between eah transmt antenna an the mobe reever suffers from nepenent fang f the transmt antennas are spae suffenty. We onser a WCDMA STTD rake reever wth L mutpath branhes. For ease of esrpton enote r an r by the vaue of the -th path reeve sgna r (t) esprea at t kt an t (k+)t respetvey as r r r (kt ) h s h s + n () r ((k + ) T ) h s + h s + n where h [ s the hanne gan of the -th path between the transmt antenna an the reever an n j s the nose n the - th path at tme t (k-+j)t j an. Here we assume that the hanne gan s not hange urng two-symbo tme nterva. After mutpyng the esprea DPCH symbo by the estmate hanne gan the esre symbo s obtane by ombnng the esprea symbos. The hanne gan for eah antenna s obtane by espreang the orthogona CPICH symbos. The esprea CPICH symbo p an p of the -th path at t kt an t (k+)t are respetvey gven by p p p (kt ) h A + βh p ((k + ) T ) h A βh A + n ' A + n ' where β s the power rato of the CPICH to DPCH sgna.e. CP β h / h. The nstantaneous hanne gan h CP of antenna- an an be obtane by orreatng the esprea CPICH symbo wth the orthogona pot symbo over two-symbo tme uraton CP h ( p [ A + p [ A ) () CP h ( p [ A p [ A ). The hanne estmate hˆ CP an be obtane by averagng the nstantaneous hanne gan h CP over N symbos N ˆCP CP h h [ k + j] an. (5) N j N (3) The esre symbo an be obtane by L L ( ˆCP ˆCP h r + h r ) ( ˆCP ˆCP h r + h r ). Assumng ea hanne estmaton the reeve sgna an be represente as [] L ( h + h ) s[ + h n + h n } L ( h + h ) s[ h n [ ] + [ ] [ ]}. k h k n k If a hanne estmaton error ours (6) an be represente as [] L L ( h ε + h jθ + h + n [ s[ h h e ( ε ε ) (8) ( h ε + h ε ) s h n ε ) s + h n jθ + h [ ] [ ] + [ ] [ ] [ ] ( [ ] [ ]) k n k s k h k h k e ε k ε k jθ where h h e jθ h h e h h ε for an an θ θ θ. It an be seen that the hanne estmate error an ause the rosstak between s ˆ an s ˆ. Moreover sne the hanne estmaton shou be one for eah transmt antenna the SIR of the CPICH pot sgna s erease by one haf egrang the auray of hanne estmate. The auray of the hanne estmator affets the reever performane n the STTD sheme muh more than that n the non-sttd sheme. III. ADAPTIVE CHANNEL ESTIMATOR We onser the use of an aaptve MA fter as the CEF sne the use of genera FIR fters oes not prove any sgnfant mprovement over the use of an MA FIR fter [8]. When a non- STTD sheme s empoye we orreate the reeve pot symbo for a gven orreator s nterva m as w ( m ) Re h h [ k m ] h (9) J } where h [ s pre-ftere output of h ˆ [ k ] to suppress the nose omponents arger than the maxmum aowabe Dopper frequeny. Wth a gven thresho η for a nteger vaues of m m satsfyng w ( m ) η an be foun n an experenng hanne [8]. The optmum tap sze of the MA CEF an be represente as a funton of m as [8] Nˆ m A ( υ + Io ζ ς I or J ) + K K os / 9 + / θ χ / 5 (6) (7) () 79

3 where A s the rato of the tota sgna to the -th path sgna power υ s the orthogonaty fator I or an I o respetvey enote the power spetra ensty of the reeve sgna from the esre base staton an from other base statons ζ s the spreang fator of the CPICH enotes the CPICH E I or ς s the normaze maxmum Dopper frequeny equa to πf m T K an θ respetvey are the Rean fator an the nent ange of the -th path sgna an χ s equa to an 5 n the ase of the ass an fat spetrum respetvey. Note that the quantty πf m T an be auate for a gven hanne onton. Sne the optmum tap sze an be represente as a funton of the symbo nterva of the orreator n () we an esgn an usng a sma number of orreators as shown n Fg. where the of the -th fnger s epte. The onssts of the hanne parameter estmator (CPE) an hanne estmaton ontroer (CEC). The CPE assfes one of the preetermne hanne ontons base on the output of the orreators. Eah orreator n the CPE generates w ( m j ) for < m < L m. Usng the property that w ( ) fast m < G m ereases as m nreases the hanne envronment an be assfe by omparng w ( m j ) wth a thresho η. If the j-th orreator output of the -th fnger beomes ess than η for the frst tme.e. w ( m j ) < η an w ( m ) η... j j G () the CPE nfers that the hanne envronment beongs to the hanne onton-j. In ths ase the tap sze of the orresponng MA fter s set to N j G. Note that N j G + orrespons to the ase when no orreator output s smaer than the thresho. Ths happens when the hanne response too sowy vares. The parameters of the CPE as m an N are etermne n the CEC onserng possbe vaues of the Rean fator path power CPICH E/Ior an Ior/Io [8]. Thus the hanne onton an be estmate n rea tme wthout exat a pror nformaton on the operatng onton. The SIR of the esprea pot symbo s reue to one haf ompare to that of the non-sttd sheme sne the transmt power s equay spt nto the two transmt antennas. However the SIR of the pot symbo beomes the same as that of the non-sttd sheme after () ue to the two-symbo averagng effet. Thus the propose for non-sttd sheme an be appe to the STTD sheme wthout muh mofaton when h CP s empoye as the nput of the. Instea sne t requres two symbos for () the tap sze of the MA CEF an the orreator s nterva of the CPE nee to be erease by one haf. For exampe assume that m an N ˆ for the n the non-sttd sheme. These vaues shou be hange to m ST an N ˆ n the ST STTD sheme. Conserng the hange of the reatonshp between m ST an N ˆ the optmum tap sze of the MA CEF n the ST STTD sheme s etermne by 3 ˆ m ( ). os / 9 / ST A υ + Io Ior + K N () ST ζ K + θ χ ς Note that the optmum tap sze s erease by a fator of / 5 / 5 when the STTD sheme s empoye. Other proeures for the esgn are the same as those n the non-sttd ase [8]. IV. PERFORMANCE EVALUATION In orer to nvestgate the effet of hanne estmaton on the reever performane n the STTD sheme the reever performane s evauate when the propose s empoye. The smuaton onton s summarze n Tabe an. Conserng the margn for e/mutpath searh s usuay set to - B [9] but the ase of -3B s aso onsere for performane omparson. Note that Ior/Io9B an Ior/Io-3B represent the ase when the mobe s near the base staton an near the e bounary respetvey. We onser the use of a CEF wth one sot averagng tme nterva n the fxe hanne estmator (FCE) for performane omparson. The propose empoys four orreators an fve CEFs.e. the CEC etermnes suh that m } an N } when K < 9 θ 9 B 5B I or I o B A an χ 5. Parameters for hanne envronment h [ k ] h [ k ] h [ Prefter Channe estmaton ontroer G m Channe parameter estmator N α Channe estmaton fter (a) Struture of the for the -th fnger G m Correator-( m) Correator-( m ) Correator- G( m ) G Correator bank ( ) w m ( ) w m w m ( G ) h ˆ [ k ] α argmn w ( m ) < η} α...g (b) Struture of the hanne parameter estmator Fg.. Propose. 75

4 Tabe. Smuaton onton. Parameter Vaue DPCH Kbps CPICH E/Ior -3B -B Channe Rayegh 3GPP moe (Case- Case-3) Power ontro Step sze: B Contro pero: sot (.667ms) Tabe. Propagaton hanne onton. 3GPP Case- 3GPP Case-3 Reatve Deay [ns] Average Power [B] Reatve Deay [ns] Average Power [B] Fg. 3 5 an 6 ept the reever performane n terms of the requre DPCH E/Ior for a BER of 3 when the propose s empoye for hanne estmaton n both the STTD an non- STTD systems. Note that the requre DPCH E/Ior for a BLER of approxmatey resuts n a BER of 3. It an be seen that the use of the propose an prove sgnfant performane mprovement over the use of the FCE n the STTD sheme ompare to that n the non-sttd sheme. The n the STTD sheme an prove performane mprovement of up to.6 B over the use of the FCE even when the CPICH power s reatvey hgh (e.g. -B). The performane mprovement nreases as the CPICH power ereases or the mobe moves towar the e bounary. Sne the MA CEF of the FCE s optmum at hgh Dopper frequeny (.e. about f 5Hz) the DPCH E/Ior gan wth the use of the nreases as f ereases. Tabe 3 summarzes the E/Ior gan wth the use of the propose over the FCE at f 6Hz. Sne the transmt power s equay ve nto the two transmt antennas n the STTD sheme the auray of hanne estmaton eterorates. Thus when s -B n the STTD sheme the SIR of the CPICH s approxmatey the same as that when s -3B n the non-sttd sheme. The reever performane of the STTD sheme epens on the auray of the hanne estmate muh more than that of the non-sttd sheme ue to the effet of rosstak. As a resut t an be seen from Tabe 3 that the DPCH E/Ior gan n the STTD sheme wth -B s arger than that n the non-sttd sheme wth - 3B uner the most of hanne ontons. Note that the transmt power of the CPICH s not ontroe n response to the hanne onton unke the transmt power of the DPCH. The performane fferene ue to the use of the an FCE n the non-sttd sheme s maxmze when the hanne has a sma number of mutpaths an ow Ior/Io as n Fg. sne the auray of the hanne estmate sgnfanty affets the reever performane. In ths ase a arge number of bt errors an our when the sgna suffers from eep fang sne the nstantaneous SIR of the CPICH pot symbo s sgnfanty erease. Ths performane egraaton ereases as the number of mutpaths nreases or Ior/Io nreases beause the mutpath versty effet s obtane or a suen SIR fa-off ours ess frequenty respetvey. When an STTD sheme s empoye there exsts at east two mutpaths even when the hanne has a snge path between eah transmt an reeve antenna makng the reever never experene fat fang n the STTD sheme. As a resut the effet of no power ontro of the CPICH n the STTD sheme s ess mportant than that n the non-sttd sheme. In ths ase the reever performane beomes ess affete by the auray of hanne estmate ompare to that n the non-sttd sheme. As a resut when the hanne has a sma number of mutpaths an ow Ior/Io the use of the n the STTD sheme oes not prove sgnfant gan over the non-sttd sheme as shown n Fg.. V. CONCLUSIONS In ths paper an has been propose for the WCDMA STTD system. The propose an sgnfanty mprove the reever performane ompare the fxe hanne estmator. The performane mprovement nreases as the transmt power of the CPICH ereases or the mobe moves towar the e bounary. In aton to performane mprovement the propose an aso be use for other purposes sne t an prove the nformaton on the spee of hanne varaton. In the next generaton mobe systems t s key to empoy mutpe antennas at the transmtter an/or the reever. In ths ase the auray of hanne estmaton beomes muh mportant ue to the effet of rosstak an spt transmt power requrng the use of an effent hanne estmator to fuy obtan the avantage of the use of mutpe antennas. RERFERENCE [] T. Ojanpera an R. Prasa Weban CDMA for thr generaton mobe ommunatons Arteh House 998. [] 3GPP 3G TS 5. Physa hannes an mappng of transport hannes onto physa hannes (FDD) Ot.. [3] S. M. Aamout A smpe transmt versty tehnque for wreess ommunatons IEEE J. Seet. Areas Commun. vo. 6 pp Ot [] A. Correa A. Hottnen an R. Whman Spae-tme transmtter versty shemes for weban CDMA Pro. VTC Sprng pp May. [5] P. Shuz-Rtth J. Batersee an G. Fok Channe estmaton for DS-CDMA wth transmt versty over 75

5 frequeny seetve fang hannes Pro. VTC Sprng pp May. [6] S. H. Won W. W. Km an S. C. Bang Channe estmaton methos aequate to spae tme transmt versty reever n the W-CDMA IMT- system Pro. VTC Fa pp Ot.. [7] Yangxn an Y. Xaohu Performane anayss of spae-tme transmt versty for weban CDMA Pro. VTC Sprng pp. 6-8a May. [8] J.-W. Cho an Y.- H. Lee An aaptve hanne estmator n pot hanne base DS-CDMA systems Pro. VTC Sprng pp May. [9] 3GPP 3G TS 5. UE Rao Transmsson an DPCH E/Ior -3 BER DPCH E/Ior -3 BER DPCH E/Ior -3 BER Reepton (FDD) Ot Iea FCE (sot) Dopper frequeny (Hz) -B Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -B -9. Fg. 3. Iea FCE (sot) DPCH E/Ior -3 BER DPCH E/Ior -3 BER Iea FCE (sot) Dopper frequeny (Hz) (b) STTD -3B Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -3B Requre E/Ior (Case- an Ior/Io9B). Dopper frequeny (Hz) DPCH E/Ior -3 BER -9. Iea FCE (sot) Dopper frequeny (Hz) DPCH E/Ior -3 BER DPCH E/Ior -3 BER DPCH E/Ior -3 BER DPCH E/Ior -3 BER Iea FCE (sot) Dopper frequeny (Hz) DPCH E/Ior -3 BER Iea FCE (sot) Dopper frequeny (Hz) -B (b) STTD -3B Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -9. Fg. 5. Iea FCE (sot) -B DPCH E/Ior -3 BER -.5 Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -3B Requre E/Ior (Case-3 an Ior/Io9B). Dopper frequeny (Hz) DPCH E/Ior -3 BER Iea FCE (sot) Dopper frequeny (Hz) -B (b) STTD -3B Dopper frequeny (Hz) Iea FCE (sot) () Non-STTD -B Fg. 6. DPCH E/Ior -3 BER Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -3B Requre E/Ior (Case-3 an Ior/Io-3B). DPCH E/Ior -3 BER B (b) STTD -3B Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -B Fg.. DPCH E/Ior -3 BER Iea FCE (sot) Dopper frequeny (Hz) () Non-STTD -3B Requre E/Ior (Case- an Ior/Io-3B). Tabe 3. Channe Case- Case-3 E/Ior gan wth the use of the over the FCE at f 6Hz. Ior/Io9B Ior/Io-3B No STTD STTD No STTD STTD (B) (B) (B) (B) (B) - < <