Universal-Filtered Multi-Carrier Technique for Wireless Systems Beyond LTE

Transcription

1 Globecom 203 Workshop - Broadband Wreless Access Unversal-Fltered Mult-Carrer Technque or Wreless Systems Beyond LTE Vda Vaklan, Thorsten Wld, Frank Schach, Stephan ten Brnk, Jean-Franços Frgon École Polytechnque de Montréal, Dept. o Electrcal Engneerng, Montréal, QC, 3T J4, Canada {vda.vaklan, j-.rgon@polymtl.ca Alcatel-Lucent Bell Labs, Lorenzstr. 0, 70435, Stuttgart, Germany {thorsten.wld, rank.schach, stephan.tenbrnk@alcatel-lucent.com Abstract In ths paper, we propose a mult-carrer transmsson scheme to overcome the problem o ntercarrer ntererence (ICI) n orthogonal requency dvson multplexng (OFDM) systems. In the proposed scheme, called unversal-ltered mult-carrer (UFMC), a lterng operaton s appled to a group o consecutve subcarrers (e.g. a gven allocaton o a sngle user) n order to reduce out-o-band sdelobe levels and subsequently mnmze the potental ICI between adjacent users n case o asynchronous transmssons. We consder a coordnated mult-pont (CoMP) recepton technque, a number o base statons (BSs) send the receved sgnals rom user equpments (UEs) to a CoMP central unt (CCU) or jont detecton and processng. We examne the mpact o carrer requency oset (CFO) on the perormance o the proposed scheme and compare the results wth the perormance o cyclc prex based orthogonal requency dvson multplexng (CP-OFDM) systems. We use computer experments to llustrate the ecency o the proposed mult-carrer scheme. The results ndcate that the UFMC scheme outperorms the OFDM or both perect and non-perect requency synchronzaton between the UEs and BSs. Index Terms Coordnated multpont (CoMP), orthogonal requency dvson multplexng (OFDM), carrer requency oset (CFO), ntercarrer ntererence (ICI), lter bank multcarrer (FBMC). I. INTRODUCTION Coordnated mult-pont (CoMP) communcaton technques have been proposed or OFDM systems (e.g. n 3GPP LTE-Advanced standard) n order to mtgate ntercell ntererence and mprove system perormance especally or cell-edge users 5. In mult-cell wreless networks, CoMP acheves these objectves through cooperaton o multple geographcally separated base Ths work was carred out whle V. Vaklan was an ntern at Bell Labs, Alcatel-Lucent, Stuttgart, Germany (PhD@Bell Labs nternshp program). statons (BSs), the cooperaton can be consdered n transmsson o data n downlnk or recepton o users sgnals n uplnk. Downlnk COMP manly uses jont transmsson (JT) or coordnated schedulng/coordnated beamormng (CS/CB) approaches to eectvely cancel the ntererence. Uplnk CoMP perorms jont recepton (JR) multple BSs smultaneously process the receved sgnals rom user equpments (UEs) to mprove the qualty o detected sgnals 6. One o the crtcal ssues n CoMP-OFDM systems s ther senstvty to multple carrer requency osets (CFOs) between termnals and base statons. The requency oset can be caused by ether Doppler sht resultng rom termnals moblty or by oscllator requency msmatch between a transmtter and a recever. Multple CFOs n CoMP-OFDM systems destroy the orthogonalty between OFDM subcarrers and causes ntercarrer ntererence (ICI) at the recever whch leads to sgncant system perormance degradaton 7 9. A strong ICI happens due to hgh sdelobe levels o subcarrer spectrum that extend over a wde requency band. To overcome CFO ramcatons, lter-bank based multcarrer (FBMC) technque was proposed n whch prototype lters ensure a much lower sde-lobe level compared to OFDM systems 0 2. FBMC only becomes ecent wth oset QAM (OQAM), the real part o QAM symbols are mapped to one hal o the mult-carrer symbols and the magnary part are mapped to an nterlaced hal o the mult-carrer symbols. Whle ths works well wth sngle-cell, sngle user transmsson, n the JR case, we obtan addtonal ntererence paths between the nterlaced OQAM symbols. Furthermore, certan types o MIMO transmsson (e.g. Alamout 3) are not supported by FBMC/OQAM. A paramount eature requred n uture wreless communcaton systems, supportng the Internet o Thngs (IoT) and Massve Ma /3/$ IEEE 223

2 Globecom 203 Workshop - Broadband Wreless Access chne Communcaton (MMC), s to ecently support transmsson o small data packet 4. A physcal layer enablng ths target demands ecent support o short transmsson bursts. ere, FBMC/OQAM wth ts long lter lengths by desgn loses ecency. In ths paper, we propose an alternatve modulaton scheme to FBMC, a lterng operaton s appled to a group o consecutve subcarrers nstead o per subcarrer lterng used n FBMC. By usng the proposed technque called unversal-ltered mult-carrer (UFMC), the eect o sdelobe ntererence on the mmedate adjacent subchannels can be sgncantly reduced. Ths oers better ICI robustness and better sutablty or ragmented spectrum operaton. Moreover, UFMC technque uses shorter lter lengths compared to OFDM cyclc prex lengths whch makes t applcable or short bursts communcaton. The proposed technque can be consdered as a potental canddate or uture wreless systems whch have to support a plethora o low-cost devces to ntegrate the upcomng IoT and MMC. In order to serve those devces, a relaxaton o the strct LTE oscllator requrements s helpul that can be acheved usng our proposed scheme. The rest o the paper s organzed as ollows. The system model or CoMP-OFDM s descrbed n Secton II. The proposed multcarrer technque s ntroduced n Secton III. Secton IV demonstrates the smulaton results or UFMC and OFDM systems n the presence o CFO or two cases, perect and mperect CFO compensaton. Conclusons are drawn n Secton V. Notaton: Throughout ths paper, we use lowercase letters to denote the tme doman quanttes and uppercase letters to represent sgnals n the requency doman. Superscrpts ( ) T, and ( ) represent transpose, and conjugate transpose, respectvely. C represents the set o complex-valued numbers and R represents the set o real-valued numbers. Operator dag{a,a 2,,a n represents a dagonal n n matrx whose dagonal entres are a,a 2,,a n. I N represents the N N dentty matrx. Operator col{ stacks up the matrces on top o each other. II. COMP-OFDM ARCITECTURE Consder an uplnk CoMP-OFDM system wth K actve users that smultaneously transmt ther data on the same set o subcarrers to M separated base statons (see Fg. ). Each base staton s equpped wth a sngle antenna and experences multple CFOs rom K derent users. In order to avod nter-symbol ntererence (ISI), a cyclc prex (CP) o N CP samples are added at the begnnng o each OFDM symbol. Thereore, each OFDM symbol block contans N B = N + N CP samples, N s the ast ourer transorm (FFT) sze. The wreless channels between users and base staton are assumed to be requency-selectve wth L paths whch are constant over one OFDM symbol perod. The dscrete-tme doman receved sgnal at the m-th BS durng -th OFDM symbol can be expressed as K ym = Ω r h k mγ(ε k m)ω t V k + z m, () k= T k = k (0), k (),, k ( ), (2) Ω t = 0 I NCP I N R NB N, (3) Γ(ε k m)=dag {, e j2πεk m j2π(n )ε N,... e k m N, (4) Ω r = 0 I N R N N B. (5) In ths representaton, k s the transmtted sgnal rom k-th user n the -th OFDM symbol, V s the Fourer matrx, and Γ(ε k m) C N B N B s the CFO matrx. Varable ε k m = m k s the normalzed CFO, m k = m BS k UE represents the derence o the carrer requency between m-th BS and k-th UE and s subcarrer requency spacng. In (), Ω t and Ω r are the permutaton matrces to add and remove cyclc prex. Varable h k m denotes the Toepltz channel matrx whose rst column s h k m(0), h k m(),, h k m(l ), 0,, 0 T, each h k m(l) s a zero mean complex Gaussan random varable wth varance σl 2. The powers o all paths are normalzed such that L l=0 σ2 l =. In (), z m s the addtve whte Gaussan nose wth entres that are ndependent dentcally dstrbuted (..d.) complex Gaussan random varables wth zero mean and varance σn. 2 The receved sgnal at CoMP central unt, n requency-doman, can be expressed as (we drop the superscrpt or smplcty) Y = Q + Z, (6) Y =Y T, Y2 T,, YM T T, (7) Y m =Y m (0), Y m (),,Y m ( ) T, (8) = dag {, 2,, M, (9) 224

3 Globecom 203 Workshop - Broadband Wreless Access S/P N N NB Addng IFFT CP P/S UE UE BS M BS BS S/P CP Removal FFT P/S Y M Y Jont Detecton & CFO Comp. UE Fg.. System model or CoMP-OFDM n the uplnk. m, 2 m,, K m, (0) m = gure, the data stream o the k-th user k s dvded k m = dag{m(0), k m(), k,m(n k nto N ), sub-streams denoted by k,p CN, or p {, 2,,N (). Then, we use a pulse shapng T lter wth smooth edges on each resource block n tme = T, T 2,, K T C K, (2) doman that leads to substantal reducton n out-o-band leakage n requency doman. In ths way, we mnmze and s the channel requency response, and the entres the harmul ntererence rom adjacent subchannels o o are dened n (2). In equaton (6), Q s dened as the neghborng resource block. In Fg. 3, we llustrate Q = col{q, Q 2,...Q M, (3) the power spectrum o UFMC and OFDM sgnals n requency doman. In ths experment, we use a nte mpulse response (FIR) chebyshev lter. Fg. 3 (a) shows { Q m = dag Q m Q 2 m... Q K m, (4) the UFMC spectrum or N = 6 each contans 2 subcarrers. In Fg. 3 (b), we compare Q k m = VΓ(ε k m)v, (5) the power spectrum o one o UFMC wth that o the OFDM. As t can be observed n (b), OFDM qm(0) k qm() k qm(n k ) spectrum has hgh sdelobe levels resulted rom the qm( ) k qm(0) k qm(n k 2) rectangular pulse n tme doman. Ths causes severe Q k m = , ICI and perormance degradaton, the orthogonalty qm( N k +) qm( N k +2) qm(0) k (6) qm(n) k = N ( j2πq(ε k exp m + n) ). (7) q=0 Ths s a geometrc seres that can be urther smpled as 5 q k m(n) = sn π(ε k m + n) sn π (ε k m + n) ejπ( III. COMP-UFMC ARCITECTURE )(εk m+n) (8) In ths secton, we present the proposed UFMC transmsson scheme. Ths scheme s developed based on the prncple o requency dvson multplexng (FDM) n whch we dvde the nput data stream nto several lower rate sub-streams. The block dagram o a system employng UFMC s depcted n Fg. 2. As shown n ths between subcarrers collapses (e.g. by CFO). As opposed to OFDM, UFMC oers a requency well-localzed pulse shapng. In addton, the proposed UFMC oers a hgher spectral ecency n comparson to OFDM, snce t does not requre cyclc prex. The tme doman receved sgnal n CoMP-UFMC system can be expressed as y m = K h k mγ(ε k m) F Ṽ k + z m, (9) k= T k = T k,, T k,2,, T k,n C, (20) T k,p = k,p (0), k,p (),, k,p (N ), (2) Ṽ = dag {V, V 2,, V N, (22) F = F, F 2,, F N. (23) In the LTE systems, each N consecutve subcarrers n requency doman s called a resource block ()

4 Globecom 203 Workshop - Broadband Wreless Access S/P N IFFT Tx Block Flterng UE UE BS M BS BS Rx Block Flterng FFT P/S Y M Y Jont Detecton & CFO Comp. UE, V F F V,2 V 2 F 2 N p p p, p F V y F2 V 2, N V N F N F N V N Fg. 2. System model or CoMP-UFMC n the uplnk. Rel. power db Rel. power db Frequency spacng n subcarrer steps (a) Frequency spacng n subcarrer steps (b) Fg. 3. (a) Supermposed spectrum o 6 derent UFMC resource blocks wth 2 subcarrers each, carryng random QPSK data symbols. (b) Comparson o one OFDM resource block wth a UFMC resource block. and k,p s the p-th sub-stream transmtted rom the k-th user n the -th OFDM symbol, and Ṽ s the Fourer matrx. In (22), V p C N conssts o N columns o Fourer matrx V, startng rom column (p )N +. Varable F s the lter matrx, ts entry F p s the Toepltz matrx whose rst column s b p (0), b p (),, b p (L FIR ), 0,, 0 T. The coecents b p (j) or j {0,,,L FIR are the p-th resource block FIR lter coecents. Moreover, per-block lter coecents are chosen such that LFIR j=0 b p (j) 2 =. The requency doman receved sgnal at all BSs can be wrtten as (or smplcty, the superscrpt s dropped n the ollowng expressons) matrx Q can be expressed as Y = Q + Z, (24) Q = col{ Q, Q2,... QM, (25) { Q m = dag Q m Q2 m... QK m, (26) Q k m = Ṽ FΓ(ε k m) F Ṽ. (27) At the CoMP central unt, the zero-orcng (ZF) lnear equalzer s used to detect the transmtted sgnals, perormng mult-antenna combnng across the subcarrers, whch suppresses ICI and mult-user ntererence. To do so, the receved sgnal vector s multpled wth a lter matrx that can be computed as G ZF =( e e ) e, (28) e = Q or UFMC and e = Q or OFDM systems. IV. SIMULATION RESULTS In ths secton, we examne the perormance o the proposed CoMP-UFMC system and compare the results wth that o the CoMP-OFDM system. We also nvestgate the eect o requency oset between BSs and UEs on the perormance o both systems. Moreover, we study the mpact o CFO estmaton error on the symbol error rate (SER) perormance o the systems. Monte- Carlo smulatons are used to evaluate the SER o the proposed system. We carry out the smulatons or L =6, M =2, K = 2, = 72, N = 6, N = 2, and FFT 226

5 Globecom 203 Workshop - Broadband Wreless Access 0 0 Symbol Error Rate (SER) Proposed Scheme, No CFO OFDM, No CFO SNR db Symbol Error Rate (SER) 0 Proposed Scheme, ε = 0% 0 2 Proposed Scheme, ε = 20% Proposed Scheme, ε = 50% OFDM, ε =0% OFDM, ε =20% OFDM, ε =50% SNR db Fg. 4. SER perormances o CoMP-UFMC and CoMP-OFDM systems wth no CFO. Fg. 6. SER perormances o CoMP-UFMC and CoMP-OFDM systems or derent CFO estmaton errors. Symbol Error Rate Proposed Scheme, SNR = 30 db Proposed Scheme, SNR = 5 db OFDM, SNR = 30 db OFDM, SNR = 5 db ε Fg. 5. SER perormances o CoMP-UFMC and CoMP-OFDM systems or derent CFO values. sze o N = 28. We assume that the symbols are chosen rom a QPSK constellaton wth average symbol power o one. To recover the nputs sgnals at the recever, lnear zero-orcng (ZF) equalzer s appled. To make a ar comparson between UFMC and CP-OFDM systems, the lter length (L FIR ) s set to the CP length (L FIR = N CP =6). For the UFMC scheme, we use a Chebyshev wndow wth 20 db stop-band attenuaton ts center requency s set at the mddle o the resource block. Fg. 4 shows the symbol error rate (SER) o UFMC and OFDM systems versus SNR n no CFO scenaro (BSs and UEs are all synchronzed). As shown n ths gure, the proposed scheme outperorms the OFDM sys- tems. UFMC scheme provdes hgher spectral ecency compare to OFDM due to the absence o cyclc prex samples whch has to be dscarded at the recever. For the results presented n Fg. 5 and Fg. 6, we presume that each UE s synchronzed wth ts own servng BS (ε k m =0 m = k). Fg. 5 depcts the eect o CFO on the perormance o the UFMC scheme n comparson wth that o the OFDM system. In ths experment, the results are obtaned or two derent SNR values wthout CFO compensaton. For both technques, the SER perormance degrades as the CFO ncreases. Ths happens due to havng stronger ICI. Fg. 6 shows the SER perormance o the proposed scheme and the OFDM systems n the presence o the CFO estmaton error. In ths gure, ε denotes the CFO estmaton error value that s a percentage o the CFO. In ths experment, the CFO s chosen to be 0.. From ths results, we observe that the CFO estmaton error severely degrades the SER perormance o the system. For nstance, as shown n the gure, by ncreasng the CFO estmaton error rom 0% to 50%, the SER ncreases rom 0 3 to 0 2 at SNR = 25 db. The proposed scheme wth ε = 20% CFO estmaton error works better than the OFDM system wth ε = 0% CFO estmaton error. As t s evdent rom the results, the perormance gan ncreases n hgher SNR (> 5 db) as n ths range the ICI becomes more notceable. Note that n a CoMP system, the nter-cell ntererence s explctly handled by the coordnaton, so an SNR > 5 db s no uncom- 227

6 Globecom 203 Workshop - Broadband Wreless Access mon settng n an ntererence-lmted deployment, as happenng n typcal urban deployment scenaros. Thus, the SNR ranges UFMC shows stronger gans over OFDM, accordng to the smulaton results, are very relevant or CoMP. V. CONCLUSIONS In ths paper, we proposed a mult-carrer transmsson scheme n order to overcome the ICI problem and mprove the system perormance. In the proposed scheme, called UFMC, a lterng operaton s appled to a group o consecutve subcarrers (e.g. a gven allocaton o a sngle user) n order to reduce out-o-band sdelobe levels that results n a better ICI robustness and better sutablty or ragmented spectrum operaton compared to OFDM. We showed that the UFMC outperorms the CP-OFDM or both perect and non-perect requency synchronzaton between the UEs and BSs. We also studed the eect o mperect CFO compensaton on the SER perormance o the UFMC system and compare t wth that o the CP-OFDM system. The results ndcate that the proposed UFMC-scheme can be a promsng canddate or uture 5G wreless systems. Future work wll deal wth optmzng the UFMC lters. ACKNOWLEDGEMENT A part o the research leadng to these results has receved undng rom the European Communty s Seventh Framework Program FP7/ under the grant agreement n reerred as 5GNOW. REFERENCES M. Sawahash, Y. Kshyama, A. Mormoto, D. Nshkawa, and M. Tanno, Coordnated multpont transmsson/recepton technques or LTE-advanced, IEEE Wreless Commun., vol. 7, pp , March P. Baer, M. Meurer, T. Weber, and. Troger, Jont transmsson (JT), an alternatve ratonale or the downlnk o Tme Dvson CDMA usng mult-element transmt antennas, n Proc. IEEE 6th Int. Symp. Spread-Spectrum Technology and Applcatons (ISSSTA), vol., NJIT, Newark, NJ, Sept. 2000, pp S. Venkatesan, Coordnatng base statons or greater uplnk spectral ecency n a cellular network, n Proc. IEEE Int. Symp. on Personal,Indoor and Moble Rado Commun. (PIMRC), Athens, Greece, Sept. 2007, pp Q. Wang, D. Jang, G. Lu, and Z. Yan, Coordnated multple ponts transmsson or LTE-advanced systems, n Proc. IEEE on Wreless Communcatons, Networkng and Moble Computng (WCom), Bejng, Chna, Sept. 2009, pp D. Jang, Q. Wang, J. Lu, G. Lu, and C. Cu, Uplnk coordnated mult-pont recepton or LTE-advanced systems, n Proc. IEEE on Wreless Communcatons, Networkng and Moble Computng (WCom), Bejng, Chna, Sept. 2009, pp GPP TR 36.84, 3rd Generaton Partnershp Project; Techncal Speccaton Group RAN; Further Advancements or E- UTRA Physcal Layer Aspects, 3GPP, Tech. Rep. 7. Zhang, N. B. Mehta, A. F. Molsch, J. Zhang, and. Da, Asynchronous ntererence mtgaton n cooperatve base staton systems, IEEE Trans. on Wreless Commun., vol. 7, pp , Jan V. Kotzsch and G. Fettwes, Intererence analyss n tme and requency asynchronous network MIMO OFDM systems, n Proc. IEEE Wreless Commun. Networkng Con. (WCNC), Sydney, Australa, Aprl 200, pp P. Marsch and G. P. Fettwes, Coordnated Mult-Pont n Moble Communcatons: From Theory to Practce. Cambrdge Unversty Press, M. G. Bellanger, Speccaton and desgn o a prototype lter or lter bank based multcarrer transmsson, n Proc. IEEE Int. Con. on Acoust., Speech, Sgnal Process. (ICASSP), vol. 4, Salt Lake Cty, UT, May 200, pp P. Sohan, C. Sclet, and N. Lacalle, Analyss and desgn o OFDM/OQAM systems based on lterbank theory, IEEE Trans. Sgnal Process., vol. 50, pp , May B. Farhang-Boroujeny, OFDM versus lter bank multcarrer, IEEE Sgnal Process. Mag., vol. 28, pp. 92 2, May S. M. Alamout, A smple transmt dversty technque or wreless communcatons, IEEE J. Sel. Areas Commun., vol. 6, pp , Aug G. Wunder, M. Kasparck, S. ten Brnk, F. Schach, T. Wld, I. Gaspar, E. Ohlmer, S. Krone, N. Mchalow, A. Navarro, G. Fettwes, D. Ktenas, V. Berg, M. Dryjansk, S. Petrzyk, and B. Eged, 5GNOW: Challengng the LTE Desgn Paradgms o Orthogonalty and Synchroncty, n Moble and Wreless Communcaton Systems or 2020 and beyond 77th Veh. Technol. Con.: VTC203-Sprng), Dresden, Germany, June P. Moose, A technque or orthogonal requency dvson multplexng requency oset correcton, IEEE Trans. Commun., vol. 42, pp , Oct E. Dahlman, S. Parkvall, and J. Skold, 4G: LTE/LTE-Advanced or Moble Broadband: LTE/LTE-Advanced or Moble Broadband. Academc Press,