Preamble-Assisted Channel Estimation in OFDM-based Wireless Systems

Transcription

1 reamble-asssed Channel Esmaon n OFDM-based reless Sysems Cheong-Hwan Km, Dae-Seung Ban Yong-Hwan Lee School of Elecrcal Engneerng INMC Seoul Naonal Unversy Kwanak. O. Box 34, Seoul, Korea e-mal: {kkns, qks83}@l.snu.ac.kr, ylee@snu.ac.kr Absrac Accurae channel sae nformaon (CSI) s ndsensable for he use of channel-adave ransmsson echnques, bu may no easly be achevable near he cell boundary n cellular envronmens. Ths aer consders he mrovemen of CSI esmaon accuracy n OFDM-based wreless acke ransmsson sysems by exlong he reamble sgnal as well as he lo sgnal. The CSIs resecvely esmaed from he receved reamble lo sgnal are combned accordng o he channel correlaon beween he reamble he lo sgnalng. The combnng wegh s analycally deermned o mnmze he mean squared error. hen mulle anennas are emloyed for nerference cancellaon, he CSI of nerference channels can also be esmaed n a smlar manner. The roosed scheme s aled o he moble-max sysem o verfy he effecveness. Smulaon resuls show ha he roosed scheme s que effecve near he cell boundary. I. INTRODUCTION In recen years, orhogonal frequency dvson mullexng (OFDM) has been recognzed as one of key ransmsson echnques for nex generaon wreless communcaon sysems []. I can rovde hgh secral effcency mgae ner-symbol nerference n frequency selecve fadng envronmens. Accurae channel sae nformaon (CSI) s ndsensable for he emloymen of adave ransmsson echnques, bu may no be easly achevable near he cell boundary n mul-cell envronmens []. As a consequence, he erformance of OFDM ranscever may sgnfcanly deerorae near he cell boundary. In racce, he CSI can be esmaed from lo sgnal usng a convenonal esmaon echnque such as he leas square (LS) mehod [3]. Snce he LS esmaon yelds a mean-squared error (MSE) nversely rooronal o he sgnal-o-nerference lus nose rao (SINR), may no work roerly near he cell-boundary due o serous nerference effec. An erave nerference canceller n he me doman has recenly been roosed [4]. However, can be aled only o a block ye arrangemen of lo sgnals, where all subcarrers are reserved for lo sgnalng a a secfc erod. acke-based wreless ransmsson sysems (e.g., moble-max) emloy a reamble sgnal for he urose of synchronzaon, whch s ofen orhogonal o reamble sgnals ransmed from oher cells [5]. As a consequence, he reamble sgnal s less nerfered han he lo sgnal whch s common o all cells. The naure of wreless channel causes he ransmsson of reamble lo sgnal o exerence correlaon n he me frequency doman. The accuracy of he CSI esmaon can be mroved by exlong he channel correlaon beween he reamble lo sgnal. The CSI esmaed from he receved lo sgnal can be mroved wh he ad of he CSI esmaed from he receved reamble sgnal. The wo CSI can be combned o mnmzed he MSE. hen he recever has mulle receve anennas, can cancel ou oher cell nerference (OCI) by emloyng adave anenna echnques such as he mnmum MSE (MMSE) nullng scheme whch requres he CSI of nerferng channels as well as he arge CSI [6]. The roosed scheme can also be aled o he esmaon of nerferng CSI. The res of he aer s organzed as follows. Secon II nroduces he downlnk model of an OFDM-based wreless sysem n consderaon. Secon III descrbes he roosed esmaon scheme, where he arge nerferng CSIs are esmaed by combng he CSI esmaed from he receved reamble lo sgnal. Secon IV verfes he erformance by comuer smulaon when he roosed scheme s aled o he moble-max sysem. Fnally, conclusons are gven n Secon V. II. SYSTEM MODEL Consder a mul-cell mul-secor srucure as llusraed n Fg., where he TBS denoes he arge base saon (BS) servng he arge moble saon (MS) he IBS denoes a BS causng nerference o he MS. Defne he arge channel he -h nerference channel by he channel beween he TBS he MS, he -h IBS

2 f f f Fg.. Mul-cell mul-secor srucure. where H(, f, n ) (, f, n ) are resecvely he frequency resonse of he arge channel lo sgnal ransmed from he TBS, H(, f, n ) (, f, n ) are resecvely he frequency resonse of he -h nerference channel he lo sgnal ransmed from he -h IBS, (, f, n ) s he frequency resonse of he background nose oher mnor nerferng sgnals. The addve nose erm can be modeled as a comlex zero-mean Gaussan rom varable wh varance. I can be assumed ha he lo sgnal s ransmed wh he same average ransm ower,.e., E{ (, f, n) } = E{ (, f, n) } =. For smlcy of descron, he receve anenna ndex n wll be omed snce he esmaon rocedure for each receve anenna s dencal. Consder reambles near he lo sgnal Y (, f ) n he frequency doman (e.g., he reambles a he ( f ) - h, f -h ( f ) -h subcarrers). For ease of descron, assume ha he reamble a he ( f ) -h, f -h ( f ) -h subcarrers s ransmed from secor α, β γ, resecvely. Then, he receved reamble ransmed from he TBS he -h IBS can be reresened resecvely as Y(, f H (, f ) S (, f ) (, f ) () Fg.. reamble lo sgnal srucure. he MS, resecvely. Assume ha each cell s dvded no hree secors (say, α, β γ secor), he BS has a sngle ransm anenna he MS has N receve anennas, ha he arge MS s locaed n he β secor of he TBS. Assume also ha he BS ransms reamble lo sgnals for he urose of synchronzaon channel esmaon, resecvely, as llusraed n Fg., where all he secors ransm he same lo sgnal a he same me hrough he same frequency b, bu hey ransm reamble sgnals a he same me hrough dfferen frequency bs. Consder he ransmsson of lo sgnal allocaed o he -h OFDM symbol he f -h subcarrer. Assumng ha he major nerference comes from a mos wo adjacen secors, he receved lo sgnal hrough he n - h anenna can be reresened as Y(, f, n H (, f, n) (, f, n) H(, f, n) (, f, n) () = (, f, n) Y (, f ( ) H(, f ( ) ) S(, f ( ) ) (3) (, f ( ) ) where S(, f ) S (, ( ) f ) denoe he reamble ransmed from he TBS he -h IBS, resecvely. I can also be assumed ha he he reamble sgnal s ransmed wh he same average ransm ower,.e., { } { } E S (, ) (, ( ) f = E S f S. Le ρ be he correlaon coeffcen beween he channel of he lo sgnal he reamble ransmed from he TBS, defned by where { } ( ) H(, f) H(, f) ρ E (4) H(, f) E denoes he execaon he suerscr denoes comlex conjugae. Smlarly, le ρ be he correlaon coeffcen beween he channel of he lo sgnal he reamble ransmed from he -h IBS, defned by ( ) H(, f) H(, f ( ) ) ρ E. (5) H(, f)

3 III. CONVENTILANL CHANNEL ESTIMATION The arge he -h nerference channel can be esmaed from he receved lo sgnal usng a convenonal LS esmaon mehod as [3] Y (, ) f H (, f (, f) (, f) = H(, f) H(, f) (6) = (, f) (, f) (, f) Y (, ) f H (, f (, f) (, f) = H(, f) H(, f) (, f) (, f ) (, f ) (, ). (, ) (, ) j H j f j= f f j I can be shown ha he corresondng MSEs of he arge CSI nerference CSI esmaon are resecvely { (, ) (, ) } conv, E H f H f = = (7) (8) { (, ) (, ) } E H f H f conv, = j j= j where are he gan of he arge he -h nerference channel, resecvely. I can be conjecured from (8) (9) ha he LS esmaon may no rovde good erformance n he resence of large nerference (e.g., near he cell-boundary). Furhermore, hs esmaon mehod does no exlo eher he channel correlaon beween he OFDM symbols nor ha beween he subcarrers. IV. ROOSED CHANNEL ESTIMATION e consder he mrovemen of he CSI esmaon accuracy n he resence of nerference by exlong he channel correlaon roeres n he me frequency (9) doman. The arge CSI s resecvely esmaed from he receved reamble lo sgnal usng a convenonal LS mehod. Then, hese wo CSIs are combned for beer CSI esmaon, whch s also used for he esmaon of nerference channel. A. reamble Channel Esmaon The CSI can be esmaed from he reambles ransmed from he TBS IBSs usng he LS mehod as Y (, f) (, f) H (, f = H (, f ) (0) (, ) (, ) S f S f Y (, f ( ) ) (, ( ) f S(, f ( ) ) = H(, f ( ) ) (, f ( ) ) S(, f ( ) ) H () Unlke he CSI esmaon from he lo sgnal, hs CSI esmaon from he reamble sgnal s only affeced by addve nose. B. Targe Channel Esmaon The arge CSI can be esmaed from (6) (0) as H (, f H () where H (, ) = H f H(, f) s a wegh vecor for arge CSI esmaon. e analyze he wegh vecor mnmzng he MSE of esmaon by alyng ener equaon [7]. Le R be he auocovarance marx cross-covarance vecor of he arge channel resecvely defned by ρ = R E{ HH} = (3) ρ S { H ( (, )) } E H f =. (4) ρ The omum wegh vecor deermned by w for he arge CSI can be = R. (5)

4 ro, ( Re { ()}) ρ ( ()) = R E{ VV } = (9) ρ ( () ) S The corresondng MSE of he arge CSI esmaon can be reresened as [7] { (, ) (, ) } E H f H f ro, = R H where he suerscr H denoes conjugae ransose. (6) C. Inerference Channel Esmaon Snce he sgnal from he arge channel behaves as an addve nose n he esmaon of nerference CSI, he nerference CSI can be esmaed as Y (, ) (, ) (, ) f H f f H (, f. (7) (, f) The nerference channel can furher be re-esmaed as H (, f V (8) where V (, ) (, ( ) = H f H f ) s a wegh vecor for he -h nerferng CSI esmaon. The auo-covarance marx cross-covarance vecor of he -h nerference channel can resecvely be reresened as (9) { V ( (, )) } ( ()) E H f = (0) ρ where () denoes he frs elemen of. Smlarly, he omum wegh vecor for he -h nerferng CSI s deermned by = R. () The corresondng MSE of he -h nerference CSI esmaon s reresened as { (, ) (, ) } E H f H f ro, = R H V. ERFORMANCE EVALUATION. () The erformance of he roosed scheme s verfed by comuer smulaon when aled o he moble-max sysem, where he MS has wo receve anennas o cancel ou oher cell nerference hrough a MMSE nullng scheme [6]. The smulaon arameers are summarzed n Table I [5]. I s assumed ha he gan of wo nerferng channels s he same (.e., = ) he symbol dsance n he me doman beween he reamble lo sgnalng s d =. Fg. 3 decs he correlaon beween he reamble lo sgnal accordng o he user mobly symbol dsance n he me doman. I can be seen ha he channel correlaon beween he reamble lo sgnalng decreases as he mobly ncreases /or he symbol dsance beween he wo sgnals ncreases. Fg. 4 decs he normalzed MSE of he lo CSI esmaon scheme when he MS has a mobly of 30 km / h d = 0. I can be seen ha he LS scheme accuracy s sgnfcanly affeced by he SINR, bu he roosed scheme s no. Ths s manly due o he use of adave wegh mnmzng he MSE. I can also be seen ha he roosed scheme noceably ouerforms he LS esmaon scheme. Fg. 5 decs he MSE of he lo CSI esmaon normalzed wh resec o he channel gan accordng o he user mobly he symbol dsance n he me doman. I can be seen ha he LS scheme rovdes erformance almos ndeenden of he user mobly snce does no exlo he channel correlaon, bu he roosed scheme s susceble o he mobly because exlos he channel correlaon beween he reamble he lo sgnalng, whch s affeced by he mobly he symbol dsance. Neverheless, he roosed scheme ouerforms he LS esmaon by combnng he wo CSIs. VI. CONCLUSION In hs aer, we have roosed a channel esmaon scheme ha ulzes he reamble as well as he lo sgnal. The roosed scheme esmaes he channel by combnng he CSI esmaed from he receved reamble lo sgnal. The combnng wegh s analycally deermned o mnmze he MSE of he channel esmaon. hen he roosed scheme s aled o he moble-max sysem, he smulaon resuls show ha he roosed scheme s que effecve n he cell boundary, esecally hghly correlaed channel envronmens.

5 ACKNOLEDGEMENT Ths work was n ar suored by he IT R&D rogram of MKE/IITA (008-F-007-0, Inellgen reless Communcaon Sysems n 3 Dmensonal Envronmen). REFERENCES [] S. Hara R. rasad, Mulcarrer Technques for 4G Moble Communcaons, Arech House, frs edon, 003. [] T. Tang R.. Heah, Sace-Tme Inerference Cancellaon n MIMO-OFDM Sysems, IEEE Trans. Vehc., vol. 54, no. 5, , Se [3]. G. Jeon, K. H. ak, Y. S. Cho, An Effcen Channel Esmaon Technque for OFDM Sysems wh Transmer Dversy, n roc. IEEE IMRC 000, , Se [4] K. Hayash H. Saka, Downlnk Channel Esmaon for Mul-cell Block Transmsson Sysems wh Cyclc refx, n roc. IEEE VTC 007 srng, , Ar [5] IEEE 80.6e, Draf IEEE sard for local meroolan area neworks, Se [6] D. Tse. Vswanah, Fundamenals of wreless communcaon, Cambrdge Unversy ress, 005. [7] S. Haykn, Adave Fler Theory, rence Hall, fourh edon, 00. TABLE I SIMULATION ARAMETERS ARAMETERS Values Carrer frequency.3 GHz Bwdh 0 MHz OFDM symbol duraon 5. us Number of subcarrers 04 Channel Raylegh fadng ower delay rofle edesran A Doler secrum Jakes model Normalzed MSE Normalzed MSE LS scheme (arge channel) LS scheme (nerference channel) roosed scheme (arge channel) roosed scheme (nerference channel) SINR [db] d : 0 Mobly: 30km/h Fg. 4. MSE accordng o he SINR Mobly [km/h] d : 0 SINR : -4dB LS scheme (arge channel) LS scheme (nerference channel) roosed scheme (arge channel) roosed scheme (nerference channel) (a) MSE accordng o he mobly..0 Correlaon Symbol dsance (d ): 5 Symbol dsance (d ): 0 Symbol dsance (d ): 5 Symbol dsance (d ): 0 Normalzed MSE 0. SINR : -4dB Mobly : 60km/h LS scheme (arge channel) LS scheme (nerference channel) roosed scheme (arge channel) roosed scheme (nerference channel) Dsance Mobly (km/h) Fg. 3. Correlaon beween reamble lo sgnal. (b) MSE accordng o he symbol dsance Fg. 5. MSE erformance accordng o he channel correlaon.