Sectorization with Beam Pattern Design Using 3D Beamforming Techniques

Transcription

1 Sectorizatio with Beam Patter Desig Usig 3D Beamformig Techiques Chag-She Lee, Mig-Chu Lee, Chug-Jug uag, ad Ta-Sug Lee Departmet of Electrical Egieerig, Natioal Chiao Tug Uiversity, Taiwa, ROC Tel: Abstract This paper presets a framework for a threedimesioal (3D) beam patter desig with a load-balaced cell sectorizatio strategy. First, characteristics of the 3D beam patter were observed, ad covex optimizatio was used to provide a solutio based o the criteria that describe the observatios. I additio, a user equipmet (UE) determiatio problem that arose because of cell sectorizatio was also addressed. By icorporatig a beam patter desig with a proposed UE determiatio scheme, a framework of a backward compatible system was completed. The etwork performace regardig throughput for cell sectorizatio with a dedicated beam patter desig was evaluated ad the simulatio results show that the proposed system is superior compared to the covetioal system ad other proposed sectorizatio schemes. Idex Terms: 3D beamformig, beam patter desig, cell sectorizatio, massive MIMO I. INTRODUCTION A dowlik throughput ca be potetially icreased by providig degrees of freedom (DOF) o spatial domai. Multiuser (MU) multiple-iput multiple-output (MIMO) techiques ca provide the DOF usig multiple ateas, which icrease the throughput compared to a sigle atea trasmissio. I 3rd Geeratio Partership Project (3GPP) log-term evolutio-advaced (LTE-A), evolved NodeB (enodeb) has bee equipped with multiple ateas to icrease the throughput. I log-term evolutio (LTE) Release 12, the 3GPP tries to utilize the vertical degrees of freedom. Therefore, a massive MIMO, which ivolves equippig more tha oe hudred atea elemets usig idepedet weight at enodeb becomes a potetial solutio to further icrease the throughput. owever, treatig each atea as a idepedet atea port etails icreasig the overhead of the feedback of the chael state iformatio (CSI). owever, the size of the massive MIMO system makes feedback impossible. Alteratively, because of the mature developmet of three-dimesioal (3D) beamformig techiques, groupig may elemets ito several atea ports ad usig 3D beamformig i each atea port eables the efficiet use of spatial DOF with limited CSI feedback, makig this a promisig ad feasible applicatio of the massive MIMO techiques i LTE-A. I earlier works, [1]-[3] have cofirmed that vertical sectorizatio ad horizotal sectorizatio ca icrease system This work was i part fuded by the Aimig for the Top Uiversity ad Elite Research Ceter Developmet Pla, ad the MediaTek research ceter at Natioal Chiao Tug Uiversity. throughput. A study o beam patter desig usig covex optimizatio [4] shows that beam patter with arbitrary mai lobe shape ad sidelobe level is achievable usig a sufficiet umber of ateas through covex optimizatio uder coditios of symmetry costraits. owever, previous studies did ot icorporate the beam patter desig method with sectorizatio; moreover, because the desired beam patter is usually ot symmetric i practice, system performace ca be further improved whe the symmetry costrait i [4] is removed. I additio, the traffic load is a importat factor i practice; however, i [1] the traffic load is ubalaced amog the sectors. Furthermore, previous studies did ot otice the user equipmet (UE) positio determiatio problem because the enodeb must kow the sector i which each UE is located, which is ecessary whe UE tries to access the etwork. I this paper, to efficietly use the spatial DOF, a loadbalaced cell sectorizatio scheme ad the correspodig beam patter desig usig 3D beamformig techiques are proposed. Moreover, the UE positio determiatio problem arisig from cell sectorizatio is addressed, ad a potetial solutio is proposed. Therefore, a framework for system desig that is backward compatible with the curret systems is give, ad the performace improvemet is show i simulatios. The rest of paper is orgaized as follows: Sectio II describes the proposed system model. The sectorizatio ad beam patter desig schemes ad the correspodig UE positio determiatio scheme are discussed i Sectio III. Simulatio results are show i Sectio IV. Coclusios are discussed i Sectio V. II. GENERAL INSTRUCTIONS A rectagular plaar array is composed of M ad N idetical atea elemets i y ad z directios, The correspodig iter-elemet spacig is deoted as d y ad d z i y ad z directios. By defiig u y =sisiad u z =cos, the beam patter ca be expressed as M N 2 2 j md * yuy j dzuz Gu (, u) A(, ) w e, (1) y z m, m1 1 where ad are the azimuth ad elevatio agles i spherical coordiates, I additio, G(u y,u z ) ca be represeted as a vector product A(,)w s, where MN w is composed of w m, for 1,..., N ad m 1,..., M, MN ad s is the correspodig steerig vector.

2 Accordig to [5], the azimuth ad the elevatio radiatio patter for a sigle elemet are defied as (2) ad (3), 2 A( ) mi12, A m, Am 25 db, (2) 3dB 2 A( ) mi12, SLA v, SLAv 20 db, (3) 3dB where the frot-to-back atteuatio ad sidelobe atteuatio are deoted as A m ad SLA v, Accordig to [5], the 3D sythesized beam patter ca be formulated as follows: A(, ) mimi[ A( ) AE( )], Am. (4) To execute a performace evaluatio, the sigal-toiterferece-plus-oise ratio (SINR) is measured at the groud level ad is give by M P i,(, ) SINRi,(, ), (5) N L 2 P j1, ji j,(, ) M where P is the power of the correspodig mai beam for i,(, ) L sector i ad directio (, ), P is the leakage power from i,(, ) other sectors or cells, ad 2 is the oise power. III. PROBLEM FORMULATION AND BEAM PATTERN DESIGN I this sectio, a framework of cell sectorizatio strategy ad correspodig beam patter desig is itroduced. Subsequetly, a proposed solutio for the arisig UE positio determiatio problem after sectorizatio is proposed. A. Cell Sectorizatio ad the Correspodig Beam Patter Desig The aim of the approach is to efficietly utilize both vertical ad horizotal spatial DOFs. The adopted approach ivolves partitioig the cell ito smaller sectors ad usig 3D beamformig techiques to form correspodig beams i each sector. I additio, the traffic load balace issue is emphasized as a critical factor that affects the efficiecy of frequecy reuse. Therefore, the cell should be partitioed accordig to the traffic load distributio. Based o this observatio, the remaiig task is to costruct beam patters that target the correspodig sectors without causig iterferece with other sectors ad cells. Geerally, the cell could be partitioed vertically or horizotally ito arbitrary umber of sectors, but the feasibility of the correspodig beam patters deped o the umber of ateas used. If a cell is partitioed ito too may sectors but the umber of ateas is relatively small, the beam patters satisfyig all the costraits (mai beam width, sidelobe level, etc) may be ifeasible aki to the filter desig. The detail relatio betwee performace ad umber of sectors i a cell for a fixed atea array is complicated ad eed further study. I this work, the 3 3 cell is proposed as a framework of higher order sectorizatio usig 3D beamformig. The cells with differet sectorizatio schemes are show i Fig. 1. I the proposed 3 3 cell, all sectors have a equal area because the uiform distributio of user traffic is assumed. The boudary betwee sectors is defied by a pair of rages of elevatio agle () ad azimuth agle (). The boudaries are listed as follows: Sector 1: , Sector 2: , 60 0 (6) Sector 3: , 0 60, To make a appropriate beam patter, several key poits must be oticed. First, ulls always exist whe beamformig is used. The spatial respose w s(,) was set to oe for the desired area to prevet the desired sector from a low sigal power problem caused by ulls toward the desired sector. Secod, because the formatio of sidelobes is ievitable whe usig beamformig. For other sectors or cells i which the same frequecy bad is used as a desired sector, the correspodig spatial respose w s(,) should be less tha l i to miimize the iterferece to those sectors or cells. Coversely, for sidelobes that face the sky or other sectors or cells i which usig a differet frequecy bad does ot cause iterferece, the correspodig spatial respose w s(,) should be less tha l j to miimize wastig trasmissio power. The costraits for sidelobes causig power waste are looser tha those causig iterferece i this study. owever, these costraits are ecessary otherwise most of the trasmit power would target the sky or other udesired area causig severely degradatio of the performace. Therefore, to efficietly use the limited DOF, which is proportioal to the umber of ateas used, l i for sidelobes causig iterferece is set smaller tha l j for sidelobes causig power waste. For weight vectors w that satisfy the aforemetioed costraits, the vector with the miimal orm w is preferred, because it represets the vector with miimal trasmissio power. The metioed costraits ca be formulated as a covex optimizatio problem as follows: miimize w w subject to w s(, ) 1,, U0 w s(, ) li,, Ui, i 1,2,, m w s(, ) l,, U, j 1,2,,. j The set U 0 the costraits o the mai beam ad U i represet the costraits o the sidelobes. The problem above has similar form to the oe i [4]; evertheless, the optimal beam patter is ot symmetric i our problem, thus the symmetry costrait o the beam patter i [4] must be Fig. 1 Illustratio of cell sectorizatio schemes for 3 1 [5], 3 2 [1], ad proposed 3 3 cells, j (9)

3 removed, ad the optimal weight vector becomes a complex vector. Thaks to the mature developmet of covex optimizatio tools [6], the optimal weight vector w ca be obtaied usig a SeDuMi MATLAB toolbox. The beam patters of proposed 3 3 cell are show i Fig. 2 ad Fig. 3. Although the beam patter is desiged to meet the statistical traffic load based o geometry, havig a equal load o each sector i practice is difficult. Because the optimal weight vector w does ot guaratee that every poit iside the target sector has high received power for a give beam, areas i which the sigal power is relative low persist; thus users i those areas should be assiged to other beams because they ca receive stroger power from them. By cosiderig a traffic load ad beam patter desig with practical observatios ad costraits, the proposed framework provides a fair SINR o every poit iside the cell. B. Proposed UE Positio Determiatio Approach After applyig sectorizatio to existig LTE-A systems, a ew problem arises: the enodeb must kow the sectors i which each user is located. To solve this problem, a simple but efficiet ad reliable solutio is proposed by explorig the existig precoder matrix idicator (PMI) ad chael quality idicator (CQI) feedback mechaism i LTE-A. This solutio simply modifies enodeb behavior without affectig UE, ad is backward compatible ad ca be applied to curret systems. I the LTE-A systems, the overall dowlik badwidth is divided ito several sub-bads with equal size. UE reports its ow CQI aperiodically o each sub-bad accordig to the quality of the correspodig received pilot sigal [7]. Therefore, the power disparity of the dowlik referece sigal o each sub-bad ca be itetioally desiged i enodeb ed (e.g. UEs i differet sector receive differet power levels of the dowlik referece sigal i each subbad). Because of the power disparity, the CQI report is correlated to the power disparity produced by the enodeb. Thus, from the CQI report, the enodeb locate the UE. For example, the power levels of referece sigals for sector 1, 2, ad 3 ca be [P 1, P, P, P], [P, P 2, P, P], [P, P, P 3, P], respectively, where P 1, P 2, P 3 should be sigificatly differet from P. If the locatio of the user is Sector 2, the CQI report of this user suggests that the CQI of sub-bad 2 is cosiderably differet compared with the other sub-bads. Thus, the enodeb ca determie the locatio of each user. The cocept of this solutio is similar to pulse amplitude modulatio (PAM) because iformatio is cotaied i the power level. Through this solutio, the positio of UE ca be determied without additioal UE feedback iformatio or updatig the UE device; therefore, the proposed solutio is potetially backward compatible for the LTE-A UEs. I additio, the proposed solutio determies the logical UE positio, which must be kow to the enodeb, rather tha the physical locatio. For example, if UE is physically located i Sector 1, but the strogest received pilot sigal is from the beam projected toward Sector 2 because of evirometal effects, the UE should be assiged to the beam projectig toward Sector 2, which is its logical positio because the UE ca experiece a superior chael. Therefore, the error betwee the measured UE positio ad the exact physical positio of UE is ot importat because enodeb requires its logical positio. Fially, the solutio is summarized as follows: Step 1: Desig a appropriate power disparity of the subbad for the correspodig sectors. Step 2: The UE reports the CQI based o the chael coditio. Step 3: The enodeb determies the positio accordig to the CQI report. IV. SIMULATION PARAMETERS AND RESULTS This sectio discusses the simulatios for SINR ad the throughput performace of the proposed framework. I the followig simulatios, the comparisos amog the proposed 3 3 cell, 3 2 cell [1], ad 3 1 cell [5] are show. A. Simulatio Parameters Simulatios are performed by modelig ateas i three dimesios. I the proposed 3 3 cell, a 16 5 uiform plaar array (UPA) is used to costruct the desired beam patter, i which both vertical ad horizotal atea spacig are half the wavelegth. The parameters used for 3 1 ad 3 2 cells are adopted from [5] ad [1], The detailed simulatio parameters are listed i Table I. B. Simulatio Results The simulated cell throughput results for the 3 1, 3 2, ad proposed 3 3 cells are show i Fig. 4. The overall throughput of the 3 3 cell is the best highest amog the three cells because of the icreased spectral efficiecy through frequecy reuse. Similar results ca also be foud i Fig. 5, which shows that the 3 3 cell has a 101.6% ad a 221.9% improvemet o both the average ad the worst 5% cell throughput compared with the 3 1 cell. Compared with the 3 2 cell, the 3 3 cell shows a 21.9% ad a 39.9% improvemet o the average ad the worst 5% cell throughput, I a 3 3 cell, the available resource is three times that of the 3 1 cell ad oe ad half times that of the 3 2 cell. Coversely, the iterferece i the 3 3 cell is three times that of the 3 1 cell ad oe ad half times that of the 3 2 cell if the same atea is used i the three cells. owever, Fig. 2 Beam patters for sector 1, 2, ad 3 of the proposed 3 3 cell

4 Fig. 3 Vertical beam patter for sector 3. TABLE I SIMULATION PARAMETERS Parameter 3 1 cell 3GPP case cell 3 3 cell Network layout 19 enodebs System frequecy 2000 Mz System badwidth 10 Mz Frequecy reuse factor 1 Iter-site distace 500 m BTS height 32 m UE height 1.5 m Shadowig STD 8 db Propagatio loss model L = log 10 (R), R i kilometers TX power (per site) 46 dbm Atea techiques SISO SISO MISO orizotal PBW 3dB =65 o 3dB =65 o 3dB =70 o Vertical PBW 3dB =6 o 3dB =4.4 o 3dB =10 o TX atea gai Calculated based o atea characteristics Thermal oise per z dbm Traffic distributio Uiform Traffic model Full buffer Schedulig Roud-Robi usig a appropriate beam patter desig, the iterferece i the 3 3 cell ca be greatly alleviated. owever, the cell throughput of the 3 3 cell is still larger tha that of the 3 2 ad the 3 1 cells because of efficiet resource reuse. The evaluated traffic load of the three cells is listed i TABLE II. Which shows that the 3 3 cell has a more balaced traffic load compared with the 3 2 cell. Because the 3 2 cell uses a mechaical beam tilt istead of a electrical beam patter desig, the geometry of the beam patter is fixed; however, the geometry of the beam patter does ot match the geometry of the sector. To maximize the overall throughput, the cell is partitioed ito six sectors with uequal size; however, i the 3 3 cell, the beam patter is desiged to fit the geometry of each pre-determied equalsized sector. As a result, the traffic load of the 3 3 cell is more balaced tha the 3 2 cell. TABLE II TRAFFIC LOAD FOR DIFFERENT CELL SECTORIZATION SCEMES, WERE TE NUMBER IS ORDERED IN [1, 2, 3] FOR TE 3 1 CELL, [1, 2, 3, 4, 5, 6] FOR TE 3 2 CELL, AND [1, 2, 3, 4, 5, 6, 7, 8, 9] FOR TE 3 3 CELL, RESPECTIVELY. Traffic load for each sector (%) 3 1 cell cell cell Fig. 4 CDF of cell throughput for 3 1, 3 2, ad 3 3 cells, Fig. 5 Cell average ad worst 5% throughput for 3 1, 3 2, ad 3 3 cells, V. CONCLUSIONS I this paper, a framework of cell sectorizatio to icrease the throughput icludig a 3D beam patter desig ad a sectorizatio strategy is proposed for a massive MIMO. The strategies used to partitio the cell ito several sectors have accouted for the traffic load. The beam patter desig based o covex optimizatio ca provide a beam correspodig to the sectorizatio strategy. Moreover, problems that arise from further cell sectorizatio were addressed. The simulatio results show that the proposed method ca provide a fair SINR result ad, thus, the spectral efficiecy ca be icreased. REFERENCES [1] O. N. C. Yilmaz, S. ämäläie, ad J. ämäläie, System level aalysis of vertical sectorizatio for 3GPP LTE, i Proc. IEEE Cof. ISWCS, Sep. 2009, pp [2] M. Caretti, M. Crozzoli, G. M. Dell Aera, ad A. Orlado, Cell splittig based o active ateas: performace assessmet for LTE system, i Proc. IEEE Cof. WAMICOM, Apr. 2012, pp [3]. uag, O. Alrabadi, J. Daly, D. Samardzija, C. Tra, ad R. Valezuela, Icreasig throughput i cellular etworks with higher-order sectorizatio, i Proc. Asilomar Cof. Sigal, Systems, ad Computers, Nov. 2010, pp [4] S. E. Nai, W. Ser, Z. L. Yu, ad. Che, Beampatter sythesis for liear ad plaar arrays with atea selectio by covex optimizatio, IEEE Tras. Ateas Propag., vol. 58, o. 12, pp , Dec [5] 3GPP TR V9.0.0, Evolved Uiversal Terrestrial Radio Access (E-UTRA); Further advacemets for E-UTRA Physical layer aspects.

5 [6] S. Boyd ad L. Vaderberghe, Covex Optimizatio, Cambridge Uiversity Press, [7] 3GPP TR V11.2.0, Evolved Uiversal Terrestrial Radio Access (E-UTRA); Physical layer procedures.