A MODIFIED DIRECTIONAL FREQUENCY REUSE PLAN BASED ON CHANNEL ALTERNATION AND ROTATION

Transcription

1 A MODIFIED DIRECTIONAL FREQUENCY REUSE PLAN BASED ON CHANNEL ALTERNATION AND ROTATION Vncent A. Nguyen Peng-Jun Wan Ophr Freder Computer Scence Department Illnos Insttute of Technology Chcago, Illnos and ABSTRACT In prevous work, we presented a novel Channel Alternaton and Rotaton (CAR) scheme that coordnates channel assgnment wth antenna drectvtes. In CAR, each cell type s allocated an extra channel set that provdes network desgner the flexblty to alternate and rotate channels accordng to nearest front lobe nterference avodant strategy to enhance co-channel nterference rato (C/I). CAR relaxes the constrants assumed n conventonal reuse plans to allow deployment of smaller, unconventonal reuse cluster szes based on C/I requrements, thus ncreases frequency reuse effcency. In ths paper, we present a new reuse plan n whch 2 extra channel sets are allocated to each cell type and assgned accordng to CAR strategy. Ths reuse plan, referred to as 2x(3+2), ncreases channel capacty by 20% n comparson wth conventonal 4x3 reuse plan whle stll provdes sgnals above the mnmum acceptable C/I margn. 2x(3+2) reuse plan s smple and can be mplemented wthout costs. INTRODUCTION Unlke Omn-drectonal antenna whch power radates equally n all drectons, drectonal antennas project man beams power onto front lobe regons, thus sde and back lobes nterference s reduced. To take advantages of the antenna drectvtes to enhance C/I, most cellular systems employ three to drectonal antennas at each base staton (BS) [2][3]. Fg. 1 depcts a typcal 3dB beamwdth, antenna s radaton pattern obtaned at [7]. Wth n whch, front lobe regon s generally wthn 0 ± 60 from the bore-sght (at azmuth angle 0 0 ). In a conventonal cellular network, the entre rado frequency s assgned to each reuse cluster of N adjacent cells. To provde equdstant separaton among co-channel cells, N must be a rhombc number determned by the two shft parameters and j, and expressed as, 2 2 = + j j (1) N + Thus, N s restrcted wthn a fnte set of values e.g. 3, 4, 7. Small N or short reuse dstance ncreases frequency reuse effcency, whch drectly determnes the system capacty, however, decreases C/I, whch affects the Qualty of Servce (QOS). Larger N or longer reuse dstance ncreases QOS, however, decreases system capacty. Furthermore, n tr-sectored cellular system employng fxed channel assgnment, 3 unque and dsjoned channel sets are assgned once to a tr-sectored cell n the cluster and repeated unformly n all others. Thus, tr-sectored drectonal antenna systems are generally denoted as N * 3. Wth those fxed constrants, conventonal reuse plans have not taken full advantage of antenna drectvtes to maxmze frequency reuse effcency. Fg. 2 depcts the conventonal 4x3 reuse plan and worse C/I scenaro, that 0 s, when MS s at the boundary at ± 60 of a servng sector. Several other researches have been done to enhance C/I and thus ncrease channel capacty [2][3][4][6]. Although mprovement n C/I and channel capacty s realzed, all proposed schemes requre antennas replacement and or modfcaton to the cell structures. Thus, they are not practcal for mprovng exstng drectonal cellular systems, wthout sgnfcant costs. In prevous work [1], we presented CAR reuse scheme n whch cell layout s based on two-ter cell-reuse structure and each cell type s allocated an extra channel set to gve network desgner the flexblty to assgn channels accordng to nearest front lobe nterference avodant strategy to enhance C/I. CAR relaxes the constrants requred n conventonal system to allow deployment of reuse cluster szes based on C/I requrement rather than 3,4,7 as determned by (1). In 2x(3+1), 3x(3+1), 4x(3+1), and 5x(3+1) reuse plans, we obtan reuse factor of 2.6, 4, 5.3, and 6.6 respectvely. CAR reuse plan can ncrease /01/$17.00 (c) 2001 IEEE 1439

2 Fg. 2. 4x3 reuse plan and worse nterference Fg drectonal antenna pattern channel capacty up to 31.25% whle stll mantan C/I margns comparable to targeted conventonal N*3 reuse plans. Furthermore, CAR uses exstng drectonal antenna nfrastructure, t truly does not mpose any cost. In CAR, each cell only uses 3 of the 4 allocated channel sets, thus the extra alternate channel set, f used and carefully planned, can double the capacty any one sector. However, snce the total number of channel sets n 3x(3+1) and conventonal 4x3 reuse plan are equal, wthout employng alternate channel, the system capactes of the two schemes reman the same. In ths paper, we present a new reuse plan n whch 2 extra channel sets are allocated to each cell type and assgned accordng to CAR strategy to maxmze frequency reuse effcency. Ths reuse plan, referred to as 2x(3+2), ncreases channel capacty by 20% over conventonal 4x3 system whle stll provdes sgnals at and above the mnmum acceptable C/I margn. The remander of ths paper s organzed as follows. Secton II further descrbes how drectonal antenna systems are exploted n CAR, and presents the 2x(3+2) reuse plan. In Secton III, we demonstrate the performance advantages of 2x(3+2) over conventonal 4x3 and CAR 3x(3+1) reuse plan based on system capacty and worse C/I. Fnally, Secton IV concludes ths paper. FREQUENCY REUSE PLANNING A. Conventonal 4x3 vs. CAR 3x(3+1) Reuse Plan In conventonal 4x3 reuse plan depcted n Fg. 2, four cells are grouped nto a cluster and labeled A, B, C, and D Fg. 3. 3x(3+1) reuse plan and worse nterference and assgned equdstant to all other co-channel cells. Each cell has 3 sectors and each sector s allocated a unque channel set, whch s assgned once n the cluster and repeated unformly n all others. Thus 3 dsjoned channel sets are allocated to each cell type and 12 channel sets are used system-wde, whch are typcally labeled 1 to 12 and assgned as follows: A = {1,5,9}, B ={2,6,10}, C= {3,7,11}, and D = {4,8,12}. For smplcty, only type A cells are labeled and ndexed from the top-left of the cell grd accordng to nterlockng cell structure based on rectangular cell lattce. In worse nterference scenaro depcted n Fg. 2, among the sx co-channel nterferers of channel 1 n cell A 4,2, A 2,4 and A 6,4 are from back lobes, A 0,2 and A 8,2 are from sde lobes, and A 2,0, A 6,0, are from the antennas front lobes. The other front lobe nterferer, A 10,0, s farther way and has lesser mpact on C/I n comparson to A 2,0 and A 6,0. Hence, C/I s manly the result of nterference from the 1440

3 two nearest front lobes nterferers, specfcally A 2,0 and A 6,0. Unlke conventonal tr-sectored antenna systems, n CAR, each cell type s allocated an extra channel set used for channel alternaton that results n k+1 sets per cell type and N ( k +1) sets system-wde. Thus, n 3x(3+1) reuse plan depcted n Fg. 3, a total of 12 channel sets are used and assgned as follows: A = {1,4,7,10}, B = {2,5,8,11}, and C = {3,6,9,12}. Snce each cell s assgned only 3 of the 4 4 allocated sets, there are () 3 = 4 unque patterns per cell type, e.g., A 1 = {1,4,7}, A 2 = {1,4,10}, A 3 = {1,7,10}, and A 4 = {4,7,10}. Therefore, the 3x(3+1) reuse plan conssts of repeatng patterns of 12 cells and each channel s used 3 tmes n the pattern, as llustrated n Fg 3. Consder cell A 0,0 on the top left corner, channel 1 (sector 1) ponts drectly toward ts nearest column-adjacent cochannel cell A 0,2, snce channel 1 s replaced by channel 10, front lobe nterference s avoded. Channel 10 s then alternated by channel 1 n the next column-adjacent cochannel cell A 0,4. Thus, 1 and 10 become alternatng channel pars. Furthermore, channel 4 and 7 are also rotated n each nearest column-adjacent co-channel cell, nterference s greatly reduced, as they become sde lobe nterferers nstead. Also notce that n subsequent rowadjacent co-channel cells, alternatng channel pars become rotatng channel pars and vce versa. Thus, although channel 10 from the row-adjacent co-channel cell, A 3,1, ponts drectly toward cell A 0,0, however, channel 10 s not used n A 0,0, there s no nterference. Due to channel rotaton, channel 1 becomes back lobe nterferer; therefore, nterference s neglgble. Other adjacent co-channels are assgned lkewse. Snce front lobe nterference from nearest co-channel cells s avoded, nterference s manly from antennas sde and back lobes that are ether reduced or neglgble, and from antennas front lobes from second ter co-channels that are farther away. Thus CAR schemes allow co-channels to be assgned closer n comparson wth ther counterparts. In worse nterference scenaro, also depcted n Fg. 3, when MS s at the boundary of sector 4 n cell A 6.4, among the sx co-channel cells, due to channel alternaton, A 3,3 and A 9,5 do not contans channel 4, thus there s no nterference; snce channel 4 n A 6,2 and A 6,6 are rotated, they become back lobe and sde lobe nterferers; smlarly A 3,5 and A 9,3 also become back and sde lobe nterferers. All front lobe nterferers, A 0,4, A 3,1, A 12,2, and A 12,6 are from second ter co-channel cells whch are farther away. B. CAR 2x(3+2) Reuse Plan Snce TDMA system such as IS-136 requres 14 db, to further enhance frequency reuse effcency from 4x3 reuse plan, n ths paper, we present a reuse plan n whch the number of cell type s reduced to 2 and the number of alternate channels s ncreased to 2 to a total of 5 channel sets per cell type and 10 channel sets system-wde. Ths scheme results n a novel reuse plan referred to as 2x(3+2). Based on channel separaton factor N, the channel allocatons are as follows: A = {1,3,5,7,9}, and B = {2,4,6,8,10}. C. CAR 2x(3+2) Algorthm To smplfy 2x(3+2) channel assgnment algorthm, we agan employ a rectangular cell-lattce I [ j, k ] ndexed accordng to nterlockng rows and columns startng from the top left of the grd, e.g. j =0,k=0. 1. Dvde the avalable channels nto 10 channel sets and allocate to each cell-type as llustrated n Table I. 2. Start from the left column of cell-type I; For I = A to B a) Label 5 channel sets allocated to type I cell as {C 1, C 2, C 3, C 4, C 5 } b) Assgn the frst 3 channels to each sector of the frst cell of type I counter-clockwse startng from bottom to sde and top sector, whch are denoted f210 0, f90 0, and f330 0 based on antenna s drecton shown n Fg. 4, respectvely. c) For each remanng type I cells n the same column Advance to next adjoned co-channel cell. Assgn prevous (channel prevously used n) f90 0 to current f210 0 and prevous f330 0 to current f90 0. If current f90 0 s C5 then assgn C 1 to f330 0, else assgn the next channel n sequence. d) For each subsequent column of type I cells Move to frst cell on the top of the column. Assgn prevous column adjacent f330 0 to current f210 0 If current f210 0 s C5 then assgn C 1 to f90 0, else assgn the subsequent channel to f90 0. If current f90 0 s C5 then assgn C 1 to f330 0, else assgn the next channel to f330 0 Repeat step c. Applyng the above algorthm, we obtan 2x(3+2) reuse plan depcted n Fg. 5. Ths reuse plan conssts of repeatng patterns of 10 cells whch are as follows: A 1 = {1,3,5}, A 2 = {3,5,7}, A 3 = {5,7,9}, A 4 = {7,9,1}, and A 5 = {9,1,3} and B 1 = {2,4,6}, B 2 = {4,6,8}, B 3 = {6,8,10}, B 4 = {8,10,2}, and B 5 = {10,2,4}. Table 1 summarzes the 1441

4 Cell Type Reuse Plan A B C D 4x3 1,5,9 2,6,9 3,7,11 4,8,12 3x(3+1) 1,4,7,10 2,5,8,11 3,6,9,12 Pattern1 1,4,7 2,5,8 3,6,9 Pattern2 1,4,10 2,5,11 3,6,12 Pattern3 1,7,10 2,8,11 3,9,12 Pattern4 4,7,10 5,8,11 6,9,12 2x(3+2) 1,3,5,7,9 2,4,6,8,10 Pattern1 1,3,5 2,4,6 Pattern2 1,3,9 2,4,10 Pattern3 1,7,9 2,8,10 Pattern4 3,5,7 4,6,8 Pattern5 5,7,9 6,8,10 Table 1. Channel allocaton Fg. 4. Sector Orentatons channel allocaton and assgnment of the three schemes dscussed above for comparsons. Note that n 2x(3+2), the same channel sets are repeated n adjoned co-channel cells; however, snce they all pont at dfferent drectons, a back lobe co-channel separaton equal R s obtaned and thus nterference s mnmal. Yet, to further mnmze nterference, we suggest usng drectonal antennas wth hgh front-to-back rato to maxmally suppress sde and back lobe radaton. PERFORMANCE EVALUATION In 2x(3+2), each channel s used 3 tmes n the repeatng pattern, thus the actual reuse factor for 2x(3+2) reuse plan labeled N generalzed n [1] as, car N ( k + x) N car = (2) j where j s the number of tmes the same channel set s repeated n the pattern. Thus the reuse factor for 2x(3+2) s 3.3 n comparson wth reuse factor N=4 n 4x3 and 3x(3+1). To determne ts performance, worse nterference scenaro s assumed. Thus C/I for user located at the frnge of a sector s consdered and provded n [1] as, C I = 10log n = 1 G( θ ) 0 D G( θ ) R y (3) Fg. 5. Channel assgnment and reuse cluster n 2x(3+2) where R s the radus of the cell and also the dstance from MS to the servng BS, normalzed to 1; D s the dstance th from MS to co-channel BS; n s the number of cochannel nterferers, and γ s the path loss exponent set equal to 4; G ( θ 0) and G (θ ) are antenna gans by MS th from the servng BS and from co-channel BS at angle θ wth respect to antenna bore-sght (at 0 0 ) respectvely, and expressed n decbels as, G ( θ ) db G θ = (4) ( ) In our analyss, we nclude all co-channel nterferers from adjacent co-channel cells and all second and thrd ter front lobe and sde lobe nterferers that nterference may be sgnfcant, generally when D < 7R and θ <90 0. However, we exclude and assume no shadowng effect. On the other hand, we also omt antenna down tltng factor, whch can ncrease C/I by several db. In conventonal plans where channels are assgned unformly system-wde, thus, worse nterference s theoretcally the same for all sectors and corners. However, n 2x(3+2), due to channel rotaton and channel assgnment, whch are based on antenna drectons, some corners and sectors 1442

5 Fg. 6a: Worse C/I n F210 0 (channel 5). Fg. 6b: Worse C/I n adjoned F90 0 (channel 7). D / R θ G( θ ) db ( ) G(θ ) D / R y d d d d d d C/I db Table 2. Worse C/I n 2x(3+2) reuse plan (sector f210) wthn the cell are dfferent. Therefore, we compute C/I for both corners of each sector. Assume that the user s at the cell border of sector 5 n cell A 6,4, among 8 neghborng co-channel cells, only one of the two adjoned co-channel cells, A 4,4 contans set 5, but t ponts away at angle f90 0, thus t ether becomes sde or back lobe nterferer. A 8,2 and A 8,6 are the only frst ter front lobe nterferers, whch have the strongest mpact on C/I. All others do not contan 5, hence cause no nterference, or pont to other drectons, thus have mnmal nterference. Two other front lobe nterferers are from second ter co-channels; however, due to small N, they too generate consderable nterference. Fg 6a represents the worse nterference scenaro where user s at the boundary corner n sector f210 0 (channel 5), snce t s closer to the strongest nterferer A 8,6 and A 8,2. At the same locaton, the gven user can also be served by adjoned sector f90 0 (channel 7). Front ths sector, both adjoned co-channel cells A 4,4 and A 8,4 contans 7; however, they pont at dfferent drectons, therefore nterference s mnmal. The two strongest nterferers are from A 4,6 and A 10,6. Fg 6b depcts the nterference of ths d D / R θ G( θ ) db ( ) G(θ ) D / R y d d d E-04 d d d d E-04 d E-04 d E-04 d E-04 d E-05 C/I db Table 3. Worse C/I n adjoned sector (f90) adjoned sector. In addton, at that gven corner locaton, the user also receves 4 other sgnals from neghborng cells. Startng n clockwse drecton, they are channels 10, 6, 9, and 1. Usng (3), (4), and drectonal antenna pattern depcted n Fg. 1, we obtaned C/I for each scenaro descrbed. Table 2 and 3 llustrate the calculatons of the worse nterference scenaro depcted n Fg. 6a and ts adjoned sector shown n Fg. 6b. In worse scenaro depcted n Fg. 6a, 2x(3+2) reuse plan only provdes 12.3 db, whch s below the C/I threshold for QOS n TDMA systems such as IS-136. However, unlke conventonal plan where C/I below the mnmum s unacceptable, snce all sectors provde about the same C/I protecton level, n 2x(3+2), C/I s vared from sde to sde and sector to sector. Also n practce, MS always montors the sgnals and selects the strongest avalable one to operate wthn. In whch case, the adjoned sector (channel 7) and two other overlappng sectors (channel 10 and 1) 1443

6 Sector C/I (db) Table 4. Worse C/I and sgnal avalablty n 2x(3+1) Reuse Plan Reuse Factor Capacty Per Cell Increment 4x % 3x(3+1) % 2x(3+2) % 20.00% Table 5. Channel capacty provde 14.6 and 14.7 db, whch are above the 14 db requrement. Smlarly, for example, f the user s served by sector 6 n whch worse C/I s at 12.9 db, there are adjoned sector (channel 10) and two overlappng sectors (channel 7 and 1) that provde C/I above 14 db. Thus for any corner locaton whch C/I falls below the acceptable margn, there s an adjoned sector and two other overlappng sectors that provde C/I above the 14 db whch MS can select from. The avalablty of sgnals and worse C/I are summarzed n Table 4. Due to reducton n reuse factor, some tradeoff n QOS s unavodable. Thus, antenna down tltng s necessary to reduce nterference and ncrease C/I. Also load balancng wthn each cell can be used to fully maxmze ts gan snce some sectors must assume addtonal load. Note that f the MS moves farther away from the edge or further nsde the cell, sgnal strength wll ncrease accordngly. Snce 2x(3+2) reuse plan only requres reuse factor of 3.3 or 10 channel sets system-wde, n comparson wth conventonal 4x3 and CAR 3(3+1) reuse plan whereas 12 channels are used, 2x(3+2) ncreases channel capacty by 20%. These comparsons are llustrated n Table 5. CONCLUSION Snce nterference from antenna back lobe s neglgble and nterference from the sde lobe s sgnfcantly reduced, CAR scheme was proposed to take full advantage of antenna drectvtes to enhance C/I and ncrease channel capacty. In CAR, each cell type s allocated an ex an extra channel set, whch provdes network desgner the flexblty to s used to strategcally rotate and alternate channels to mnmze the effects of and to avod front lobe nterference to and from the nearest co-channels. Snce reducng the separaton between co-channel cells hence reduces C/I protecton level, conventonal reuse plan tghter than 4x3 s not practcal for TDMA systems that requre C/I of 14 db [2][6]. In ths paper, we presented a hgh capacty drectonal reuse plan referred to as 2x(3+2) n whch 2 alternate channels are used for channel rotaton and alternaton. Performance analyss shows that 2x(3+2) reuse plan can ncrease channel capacty by 20% n comparson wth conventonal 4x3 reuse plan, due manly to smaller reuse factor of 3.3 vs. 4, respectvely. Yet, 2x(3+2) stll provdes sgnals wth acceptable C/I protecton level. Gven a scenaro where mprovement n channel capacty for system that requre 14 db s desred, 2x(3+2) s a vable canddate snce conventonal 3x3 reuse plan cannot be deployed due to low C/I. However, due to reducton n reuse factor, some tradeoff n QOS s unavodable, partcularly n the corner regons and f sgnfcant shadow fadng s present. Thus, antenna down tltng and load balancng wthn each cell may also be necessary to fully maxmze ts gans. CAR 2x(3+2) reuse plan s smple to swtch from 4x3 trsectored drectonal antenna systems. Snce modfcaton to system nfrastructure s not requred, t does not mposng any costs. REFERENCES [1] V. A. Nguyen, P. Wan, and O. Freder, Channel alternaton and rotaton for tr-sectored drectonal antenna cellular systems, IEEE Vehcular Technology Conference, Fall 2001, n press. [2] L.-C. Wang, A new cellular archtecture based on an nterleaved cluster concept, IEEE Transactons on Vehcular Technology, vol. 48, no. 6, pp , Nov [3] H. Tawfk, Frequency plannng consderatons for dgtal cellular systems, IEEE Vehcular Technology Conference, vol. 40, pp , [4] S. Faruque, Drectonal frequency reuse for cellular communcatons, IEEE Personal Wreless Communcatons, pp , [5] P. S. Rha, Frequency reuse scheme wth reduced co-channel nterference for fxed cellular systems, IEE Electroncs Letters, vol. 34, no. 3, pp , Feb. 98. [6] J. Xang, A Novel two ste frequency reuse plan, IEEE Vehcular Technology Conference, pp , [7] CSA Wreless, [8] L.-C. Wang, C. K. Chawla, and L. J. Greensten, Performance studes of narrow-beam trsector cellular systems, IEEE Vehcular Technology Conference, vol. 2, pp , [9] I. Katzela, M. Naghshneh, Channel assgnment schemes for cellular moble telecommuncaton systems: a comprehensve survey, IEEE Personal Communcatons, vol. 3, no. 3, pp , June [10] G. K. Chan, Effects of sectorzaton on the spectrum effcency of cellular rado systems, IEEE Transactons on Vehcular Technology, vol. 41, pp , Aug. 92. [11] Y. Knoshta, D. Asano, Enhanced conceptual desgn formulae for frequency channel double reuse dgtal systems usng sectored cells, IEEE Vehcular Technology Conference, vol. 1, pp , [12] L.-C. Wang, K. K. Leung, Performance enhancement n narrow-beam quad-sector cell and nterleaved channel assgnment n wreless networks, Global Telecommuncatons Conference, pp ,