Multi-beam antennas in a broadband wireless access system

Transcription

1 Multi-em ntenns in rodnd wireless ccess system Ulrik Engström, Mrtin Johnsson, nders Derneryd nd jörn Johnnisson ntenn Reserch Center Ericsson Reserch Ericsson SE-4 84 Mölndl Sweden E-mil: strct The pper considers high-cpcity Point-to-Multi Point rodnd wireless ccess system generlly operting ove 0 GHz. It is sed on cellulr rchitecture with fixed links etween multidirectionl rdio hu nd dispersed fixed suscrier terminls. The performed study is n up-link crrier-to-interference (C/I) performnce evlution of nrrow multi-em nd wide sector-em node ntenns in such system. It is shown tht the introduction of three-em ntenns t the rdio node improves up-link C/I-rtio with 5 d compred with wide-em sector ntenns. The replcement of wide-em node ntenns cn e mde on sector-y-sector sis. Simultions hve shown tht it is possile to get significnt interference improvement in 5 y hu network y replcing s few s 0 % of the wide-em rdio node ntenns with multi-em ntenns.. Introduction Telecommuniction opertor requirements for ccess systems s well s users incresed demnds for ndwidth re driving forces ehind the development of new rodnd ccess systems. In this context, rodnd wireless ccess is competitive nd complementry ccess technique to copper nd fier ccess. This pper considers high-cpcity Point-to-Multi Point (PMP) system, operting in the 4 to 8 GHz nd []. The PMP system follows cellulr deployment structure in which multiple squre sectors cover certin geogrphicl re. Within ech sector, there is one rdio node nd more thn 50 fixed ccess points communicting over the ir interfce. The ccess points cn e locted nywhere within the sector coverge re. The system typiclly requires line-of-sight communiction in order to void prolems with multi-pth nd dely spred. Frequencies nd polriztions re re-used in normlly regulr pttern, which cuses interference from co-chnnel sectors. One qulity mesure in cellulr systems is the crrier-to-interference rtio (C/I). In the future, demnds for incresed cpcity, etter qulity nd/or higher spectrum efficiency could e met y exploiting higher-order modultion schemes. This requires higher C/I levels, which cn e chieved y using ntenns with multiple nrrow ems t the nodes insted of the commonly used 90 sector em ntenns [].

2 . System description The investigted ccess system exploits squre cellulr deployment strtegy. hu, which contins four rdio nodes covering 60 in zimuth, serves ech cell. Typicl sector digonls re etween nd 6 km. In down-link, from the node ntenn to the ccess point, the node ntenn rdites over the whole sector. In uplink, the node ntenn receives informtion from one ccess point per time slot, exploiting Time Division Multiple ccess (TDM). The ccess point ntenns re highly directive reflector ntenns. The current system is interference limited in up-link. This is due to the fct tht the receiving rdio node hs wide-em 90 sector ntenn with three min interfering directions. Up-link trnsmission occurs from one ccess point t time in ech sector. The power control within ech sector is such tht the received power from ech ccess point is the sme regrdless of position within the sector. With frequency re-use of two nd two polriztions, the worst interference occurs when three min co-chnnel interferers re locted t the sector orders four sectors wy nd trnsmit simultneously, s illustrted with lck rrows in figure. In down-link, there is continuous rodcst from the rdio node. The receiving ccess point hs highly directive nrrow-em ntenn nd there is only one interfering direction, s illustrted y the purple rrow in figure. Figure Exmple of min up-link (lck rrows) nd down-link (purple rrow) co-chnnel interferers in 5 y hu network cell pln t frequency re-use of two. Sme upper- nd lower-cse letter is the sme frequency ut orthogonl polriztions. Rdio node hus re mrked with. t ech hu there re four 90 sector-em node ntenns. Sme colors indicte co-chnnel sectors.. Simultions The C/I-rtio hs een investigted y vrying the numer of rdio node ntenn ems from one to four in every sector in 5 y hu network. The numer ws incresed nd correspondingly the em widths were reduced to get 90 sector coverge without em overlp. frequency pln of two ws used in the simultions. In the up-link C/I-simultions, one ccess point per sector ws rndomly positioned for ech time slot ssuming uniform distriution over the sector. The glol cumultive distriution function (CDF) is the ccumulted sums of ll C/I-vlues clculted over ll rdio nodes in the cell pln nd over ll time slots. The C/I vlues were ccumulted during 000 time slots unless otherwise stted. Idel line-of-sight propgtion ws ssumed. The CDF is plotted in figure with the numer of ems s prmeter. The C/I-rtio increses monotoniclly with incresed numer of ems. With the current cell pln, the side-loe level in the ckloe region limits the performnce t higher C/I-vlues. Three-em node ntenns hve potentil increse in C/I with 5 d compred to 90 sector em ntenns t CDF of %. further increse in the numer of ems t the rdio nodes hs negligile effect on the C/I performnce.

3 00 0 em ems ems 4 ems C/I rtio (d) Figure Glol up-link CDF versus the C/I-rtio for different numer of ems of rdio node ntenns in 5 y hu cell pln. 4. Migrtion strtegy In moile cellulr systems, multi-em ntenns in the most distured sectors re considered in order to oost the cpcity nd/or lnce the network cpcity []. This pproch is most relevnt lso in fixed cellulr networks. In the previous C/I-simultions, ll node ntenns hve een of the sme type, either wide-em or multi-em ntenns. The verge interference level (I/C) in up-link for ech sector in homogeneous wide-em sector node ntenn cell pln is shown in figure. The twelve sectors experiencing the highest interference levels re esily identified (red color). Normlized Distnce Men Interference Level (d) Normlized Distnce 5 Figure verge up-link interference with respect to I/C of 5 y hu cell pln using 90 sector-em ntenns in ll sectors.

4 In the next step, three-em node ntenns replced ll wide-em node ntenns in the network. The interference level in the network is reduced compred to the cse in figure. In the twelve identified sectors, it is reduced etween 4 nd 5 d. This suggests tht sectors with the highest interference levels should e identified. The next step is to reduce the min interference directions y the introduction of multi-em node ntenns on sector-y-sector sis. s n exmple, the twelve worst sectors (the red ones in figure ) with respect to C/I performnce hve een replced with three-em node ntenns. This corresponds to 0 % of the network s ntenns in 5 y hu network. The verge interference (I/C) of the heterogeneous cell pln is shown in figure 4. It is seen tht the verge interference level is reduced in the sustituted sectors, nd tht the sectors re now more equl with respect to interference levels. Normlized Distnce Men Interference Level (d) Normlized Distnce 5 Figure 4 verge up-link interference with respect to I/C in 5 y hu cell pln with 0 % of the sectorem ntenns sustituted to three-em node ntenns. The up-link CDF per sector for the cse with only wide-em sector node ntenns in the whole network is presented in figure 5. It is clerly seen tht there re clusters with sectors tht experience similr interference environment. The twelve worst sectors re the ones previously identified in figure (red color). The CDF presented is truncted t 0.0 %, which is the reciprocl of the ccumulted numer of time slots (0 000) in the simultion. 00 Sector em, ll sectors 00 Three em, worst sectors C/I rtio (d) C/I rtio (d) ) ) Figure 5 Up-link CDF versus C/I-rtio per sector in 5 y hu network; ) wide-em rdio node ntenns in ll sectors, ) the twelve worst sectors with wide-ems ntenns replced y threeem rdio node ntenns. 4

5 The CDF per sector for the twelve sectors where three-em ntenns hve replced the wide-em ntenns is shown in figure 5. The improvement in C/I-rtio is out d t the % frction level. Simultions hve shown tht significnt improvement in interference level cn e chieved in network y replcing few wide-em ntenns with multi-em ntenns. This suggests tht the up-link interference sitution should e nlyzed in deployed network nd the interference sources nd directions identified. Simultions will then tell which wide-em rdio node ntenns should e replced y multi-em ntenns to chieve n cceptle interference level in ech sector. 5. Conclusions The potentil increse in C/I-rtio in up-link in fixed rodnd ccess system is on the order of 5 d y utilizing three-em rdio node ntenns insted of commonly used wide-em sector ntenns. The improvement in C/I cn e used to implement higher modultion schemes nd therey higher it rte or etter communiction qulity. Incresing the numer of ems of the rdio node ntenns further reduces the interference level mrginlly. Significnt improvements cn e chieved in up-link y identifying sectors with highest interference level nd replcing wide-em sector ntenns with multi-em ntenns in these sectors. In 5 y hu network study, only 0 % of the rdio node ntenns hd to e replced to improve the C/I-performnce considerly. 6. References [] K. C. Chu, K. V. Rvi,. ensou nd T. C. Pek, Wireless rodnd communictions: Some reserch ctivities in Singpore, IEEE Communictions Mgzine, pp , vol. 7, Nov [] S. ndersson et. l., dptive ntenns for GSM nd TDM systems, IEEE Personl Communictions, pp , June 999. [] F. Kronestedt nd S. ndersson, Migrtion of dptive ntenns into existing networks, Proc. VTC 98, pp , My