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Chng I: GII THIU CHUNG V MNG GSM H thng thng tin di ng ton cu (ting Php: Groupe Spcial Mobile ting Anh: Global System for Mobile Communications; vit tt GSM) l mt cng ngh dng cho mng thng tin di ng. Dch v GSM c s dng bi hn 2 t ngi trn 212 quc gia v vng lnh th. Cc mng thng tin di ng GSM cho php c th roaming vi nhau do nhng my in thoi di ng GSM ca cc mng GSM khc nhau c th s dng c nhiu ni trn th gii. GSM l chun ph bin nht cho in thoi di ng (TD) trn th gii. Kh nng ph sng rng khp ni ca chun GSM lm cho n tr nn ph bin trn th gii, cho php ngi s dng c th s dng TD ca h nhiu vng trn th gii. GSM khc vi cc chun tin thn ca n v c tn hiu v tc , cht lng cuc gi. N c xem nh l mt h thng TD th h th hai (second generation, 2G). GSM l mt chun m, hin ti n c pht trin bi 3rd Generation Partnership Project (3GPP). ng v pha quan im khch hng, li th chnh ca GSM l cht lng cuc gi tt hn, gi thnh thp v dch v tin nhn. Thun li i vi nh iu hnh mng l kh nng trin khai thit b t nhiu ngi cung ng. GSM cho php nh iu hnh mng c th kt hp chuyn vng vi nhau do vy m ngi s dng c th s dng in thoi ca h khp ni trn th gii. 1.1 Lch s pht trin mng GSM Nhng nm u 1980, h thng vin thng t bo trn th gii ang pht trin mnh m c bit l Chu u m khng c chun ha v cc ch tiu k thut. iu ny thc gic Lin minh Chu u v Bu chnh vin thng CEPT (Conference of European Posts and Telecommunications) thnh lp nhm c trch v di ng GSM (Groupe Spcial Mobile) vi nhim v pht trin mt chun thng nht cho h thng thng tin di ng c th s dng trn ton Chu u.

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Ngy 27 thng 3 nm 1991, cuc gi u tin s dng cng ngh GSM c thc hin bi mng Radiolinja Phn Lan (mng di ng GSM u tin trn th gii). Nm 1989, Vin tiu chun vin thng Chu u ETSI (European Telecommunications Standards Institute) quy nh chun GSM l mt tiu chun chung cho mng thng tin di ng ton Chu u, v nm 1990 ch tiu k thut GSM phase I (giai on I) c cng b. Nm 1992, Telstra Australia l mng u tin ngoi Chu u k vo bin bn ghi nh GSM MoU (Memorandum of Understanding). Cng trong nm ny, tha thun chuyn vng quc t u tin c k kt gia hai mng Finland Telecom ca Phn Lan v Vodafone ca Anh. Tin nhn SMS u tin cng c gi i trong nm 1992. Nhng nm sau , h thng thng tin di ng ton cu GSM pht trin mt cch mnh m, cng vi s gia tng nhanh chng ca cc nh iu hnh, cc mng di ng mi, th s lng cc thu bao cng gia tng mt cch chng mt. Nm 1996, s thnh vin GSM MoU ln ti 200 nh iu hnh t gn 100 quc gia. 167 mng hot ng trn 94 quc gia vi s thu bao t 50 triu. Nm 2000, GPRS c ng dng. Nm 2001, mng 3GSM (UMTS) c i vo hot ng, s thu bao GSM vt qu 500 triu. Nm 2003, mng EDGE i vo hot ng. Cho n nm 2008 s thu bao di ng GSM ln ti con s 2 t vi trn 700 nh iu hnh, chim gn 80% th phn thng tin di ng trn th gii. Theo d on ca GSM Association, nm 2010 s thu bao GSM s t 2,5 t. (Ngun: www.gsmworld.com; www.wikipedia.org )

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Hnh 0-1 Th phn thng tin di ng trn th gii nm 2008 1.2 Cu trc a l ca mng Mi mng in thoi cn mt cu trc nht nh nh tuyn cc cuc gi n tng i cn thit v cui cng n thu bao b gi. mt mng di ng, cu trc ny rt qun trng do tnh lu thng ca cc thu bao trong mng. Trong h thng GSM, mng c phn chia thnh cc phn vng sau (hnh 1.2):

Hnh 0-2 Phn cp cu trc a l mng GSM

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Hnh 0-3 Phn vng v chia 1.2.1 Vng phc v PLMN (Public Land Mobile Network) Vng phc v GSM l ton b vng phc v do s kt hp ca cc quc gia thnh vin nn nhng my in thoi di ng GSM ca cc mng GSM khc nhau c th s dng c nhiu ni trn th gii. Phn cp tip theo l vng phc v PLMN, c th l mt hay nhiu vng trong mt quc gia ty theo kch thc ca vng phc v. Kt ni cc ng truyn gia mng di ng GSM/PLMN v cc mng khc (c nh hay di ng) u mc tng i trung k quc gia hay quc t. Tt c cc cuc gi vo hay ra mng GSM/PLMN u c nh tuyn thng qua tng i v tuyn cng G-MSC (Gateway - Mobile Service Switching Center). G-MSC lm vic nh mt tng i trung k vo cho GSM/PLMN. 1.2.2 Vng phc v MSC MSC (Trung tm chuyn mch cc nghip v di ng, gi tt l tng i di ng). Vng MSC l mt b phn ca mng c mt MSC qun l. nh tuyn mt cuc gi n mt thu bao di ng. Mi thng tin nh tuyn cuc gi
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ti thu bao di ng hin ang trong vng phc v ca MSC c lu gi trong b ghi nh v tm tr VLR. Mt vng mng GSM/PLMN c chia thnh mt hay nhiu vng phc v MSC/VLR. 1.2.3 Vng nh v (LA - Location Area) Mi vng phc v MSC/VLR c chia thnh mt s vng nh v LA. Vng nh v l mt phn ca vng phc v MSC/VLR, m mt trm di ng c th chuyn ng t do m khng cn cp nht thng tin v v tr cho tng i MSC/VLR iu khin vng nh v ny. Vng nh v ny l mt vng m thng bo tm gi s c pht qung b tm mt thu bao di ng b gi. Vng nh v LA c h thng s dng tm mt thu bao ang trng thi hot ng. H thng c th nhn dng vng nh v bng cch s dng nhn dng vng nh v LAI (Location Area Identity): LAI = MCC + MNC + LAC MCC (Mobile Country Code): m quc gia MNC (Mobile Network Code): m mng di ng LAC (Location Area Code) : m vng nh v (16 bit) Cell (T bo hay ) Vng nh v c chia thnh mt s m khi MS di chuyn trong th khng cn cp nht thng tin v v tr vi mng. Cell l n v c s ca mng, l mt vng ph sng v tuyn c nhn dng bng nhn ng ton cu (CGI). Mi c qun l bi mt trm v tuyn gc BTS. CGI = MCC + MNC + LAC + CI CI (Cell Identity): Nhn dng xc nh v tr trong vng nh v. Trm di ng MS t nhn dng mt bng cch s dng m nhn dng trm gc BSIC (Base Station Identification Code).

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Chng II : H THNG THNG TIN DI NG GSM 2.1 M hnh h thng thng tin di ng GSM

Hnh 0-4 M hnh h thng thng tin di ng GSM Cc k hiu: OSS AUC HLR MSC BSS BSC OMC SS VLR EIR : Phn h khai thc v h tr : Trung tm nhn thc : B ghi nh v thng tr : Tng i di ng : Phn h trm gc : B iu khin trm gc : Trung tm khai thc v bo dng : Phn h chuyn mch : B ghi nh v tm tr : Thanh ghi nhn dng thit b BTS : Trm v tuyn gc MS : Trm di ng ISDN : Mng s lin kt a dch v PSTN (Public Switched Telephone Network): Mng chuyn mch in thoi cng cng PSPDN : Mng chuyn mch gi cng cng CSPDN (Circuit Switched Public Data Network): Mng s liu chuyn mch knh cng cng PLMN : Mng di ng mt t cng cng

2.2 Cc thnh phn chc nng trong h thng Mng thng tin di ng cng cng mt t PLMN (Public Land Mobile Network) theo chun GSM c chia thnh 4 phn h chnh sau: Trm di ng MS (Mobile Station)

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Phn h trm gc BSS (Base Station Subsystem) Phn h chuyn mch SS (Switching Subsystem) Phn h khai thc v h tr (Operation and Support Subsystem) 2.2.1 Trm di ng (MS - Mobile Station) Trm di ng (MS) bao gm thit b trm di ng ME (Mobile Equipment) v mt khi nh gi l moun nhn dng thu bao (SIM-Subscriber Identity Module). l mt khi vt l tch ring, chng hn l mt IC Card hoc cn gi l card thng minh. SIM cng vi thit b trm (ME-Mobile Equipment) hp thnh trm di ng MS. SIM cung cp kh nng di ng c nhn, v th ngi s dng c th lp SIM vo bt c my in thoi di ng GSM no truy nhp vo dch v ng k. Mi in thoi di ng c phn bit bi mt s nhn dng in thoi di ng IMEI (International Mobile Equipment Identity). Card SIM cha mt s nhn dng thu bao di ng IMSI (International Subcriber Identity) h thng nhn dng thu bao, mt mt m xc thc v cc thng tin khc. IMEI v IMSI hon ton c lp vi nhau m bo tnh di ng c nhn. Card SIM c th chng vic s dng tri php bng mt khu hoc s nhn dng c nhn (PIN). Trm di ng GSM thc hin hai chc nng: Thit b vt l giao tip gia thu bao di ng vi mng qua ng v tuyn. ng k thu bao, chc nng th hai ny mi thu bao phi c mt th gi l SIM card. Tr mt s trng hp c bit nh gi cp cu thu bao ch c th truy nhp vo h thng khi cm th ny vo my. 2.2.2 Phn h trm gc (BSS - Base Station Subsystem) BSS giao din trc tip vi cc trm di ng MS bng thit b BTS thng qua giao din v tuyn. Mt khc BSS thc hin giao din vi cc tng i phn h chuyn mch SS. Tm li, BSS thc hin u ni cc MS vi tng i v nh vy u ni nhng ngi s dng cc trm di ng vi nhng ngi s dng vin thng khc. BSS cng phi c iu khin, do n c u ni vi phn h vn hnh v bo dng OSS. Phn h trm gc BSS bao gm: TRAU (Transcoding and Rate Adapter Unit): B chuyn i m v phi hp tc .
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BSC (Base Station Controler): B iu khin trm gc. BTS (Base Transceiver Station): Trm thu pht gc. 2.2.2.1 Khi BTS (Base Tranceiver Station): Mt BTS bao gm cc thit b thu /pht tn hiu sng v tuyn, anten v b phn m ha v gii m giao tip vi BSC. BTS l thit b trung gian gia mng GSM v thit b thu bao MS, trao i thng tin vi MS qua giao din v tuyn. Mi BTS to ra mt hay mt s khu vc vng ph sng nht nh gi l t bo (cell). 2.2.2.2 Khi TRAU (Transcode/Rate Adapter Unit): Khi thch ng v chuyn i m thc hin chuyn i m thng tin t cc knh v tuyn (16 Kb/s) theo tiu chun GSM thnh cc knh thoi chun (64 Kb/s) trc khi chuyn n tng i. TRAU l thit b m qu trnh m ho v gii m ting c th ring cho GSM c tin hnh, ti y cng thc hin thch ng tc trong trng hp truyn s liu. TRAU l mt b phn ca BTS, nhng cng c th c t cch xa BTS v thm ch cn t trong BSC v MSC 2.2.2.3 Khi BSC (Base Station Controller): BSC c nhim v qun l tt c giao din v tuyn thng qua cc lnh iu khin t xa. Cc lnh ny ch yu l lnh n nh, gii phng knh v tuyn v chuyn giao. Mt pha BSC c ni vi BTS, cn pha kia ni vi MSC ca phn h chuyn mch SS. Giao din gia BSC v MSC l giao din A, cn giao din gia BTS v BSC l giao din A.bis. Phn h chuyn mch (SS - Switching Subsystem) Phn h chuyn mch bao gm cc khi chc nng sau: Trung tm chuyn mch nghip v di ng MSC Thanh ghi nh v thng tr HLR Thanh ghi nh v tm tr VLR Trung tm nhn thc AuC Thanh ghi nhn dng thit b EIR Phn h chuyn mch (SS) bao gm cc chc nng chuyn mch chnh ca mng GSM cng nh cc c s d liu cn thit cho s liu thu bao v qun l di

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ng ca thu bao. Chc nng chnh ca SS l qun l thng tin gia nhng ngi s dng mng GSM vi nhau v vi mng khc. 2.3 Trung tm chuyn mch di ng MSC: Tng i di ng MSC (Mobile services Switching Center) thng l mt tng i ln iu khin v qun l mt s cc b iu khin trm gc BSC. MSC thc hin cc chc nng chuyn mch chnh, nhim v chnh ca MSC l to kt ni v x l cuc gi n nhng thu bao ca GSM, mt mt MSC giao tip vi phn h BSS v mt khc giao tip vi mng ngoi qua tng i cng GMSC (Gateway MSC). Chc nng chnh ca tng i MSC: X l cuc gi (Call Processing) iu khin chuyn giao (Handover Control) Qun l di ng (Mobility Management) Tng tc mng IWF(Interworking Function): qua GMSC

Hnh 0-5 Chc nng x l cuc gi ca MSC (1): Khi ch gi quay s thu bao di ng b gi, s mng dch v s lin kt ca thu bao di ng, s c hai trng hp xy ra : (1.a) Nu cuc gi khi u t mng c nh PSTN th tng i sau khi phn tch s thoi s bit y l cuc gi cho mt thu bao di ng. Cuc gi s c nh tuyn n tng i cng GMSC gn nht.

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(1.b) Nu cuc gi khi u t trm di ng, MSC ph trch m trm di ng trc thuc s nhn c bn tin thit lp cuc gi t MS thng qua BTS c cha s thoi ca thu bao di ng b gi. (2): MSC (hay GMSC) s phn tch s MSISDN (The Mobile Station ISDN) ca thu bao b gi tm ra HLR ni MS ng k. (3): MSC (hay GMSC) s hi HLR thng tin c th nh tuyn n MSC/VLR qun l MS. (4): HLR s tr li, khi MSC (hay GMSC) ny c th nh tuyn li cuc gi n MSC cn thit. Khi cuc gi n MSC ny, VLR s bit chi tit hn v v tr ca MS. Nh vy c th ni thng mt cuc gi mng GSM, l chc nng x l cuc gi ca MSC. Phn h khai thc v bo dng (OSS) OSS (Operation and Support System) thc hin 3 chc nng chnh: 1) 2) 3) Khai thc v bo dng mng. Qun l thu bao v tnh cc. Qun l thit b di ng.

2.4 Khai thc v bo dng mng: Khai thc: L hot ng cho php nh khai thc mng theo di hnh vi ca mng nh ti ca h thng, mc chn, s lng chuyn giao gia hai cell.v.v.. Nh vy nh khai thc c th gim st c ton b cht lng dch v m h cung cp cho khch hng v kp thi nng cp. Khai thc cn bao gm vic thay i cu hnh gim nhng vn xut hin thi im hin thi, chun b tng lu lng trong tng lai v m rng vng ph sng. h thng vin thng hin i, khai thc c thc hin bng my tnh v c tp trung mt trm. Bo dng: C nhim v pht hin, nh v v sa cha cc s c v hng hc, n c mt s quan h vi khai thc. Cc thit b h thng vin thng hin i c kh nng t pht hin mt s cc s c hay d bo s c thng qua kim tra. Bo dng

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bao gm cc hot ng ti hin trng nhm thay th cc thit b c s c, cng nh vic s dng cc phn mm iu khin t xa. H thng khai thc v bo dng c th c xy dng trn nguyn l ca TMN (Telecommunication Management Network - Mng qun l vin thng). Lc ny, mt mt h thng khai thc v bo dng c ni n cc phn t ca mng vin thng (MSC, HLR, VLR, BSC, v cc phn t mng khc tr BTS). Mt khc h thng khai thc v bo dng c ni ti my tnh ch ng vai tr giao tip ngi - my. Theo tiu chun GSM h thng ny c gi l trung tm vn hnh v bo dng (OMC - Operation and Maintenance Center). 2.5 Qun l thu bao: Bao gm cc hot ng qun l ng k thu bao. Nhim v u tin l nhp v xo thu bao khi mng. ng k thu bao cng c th rt phc tp, bao gm nhiu dch v v cc tnh nng b sung. Nh khai thc c th thm nhp c cc thng s ni trn. Mt nhim v quan trng khc ca khai thc l tnh cc cc cuc gi ri gi n thu bao. Khi HLR, SIM-Card ng vai tr nh mt b phn qun l thu bao. 2.6 Qun l thit b di ng: Qun l thit b di ng c b ng k nhn dng thit b EIR thc hin. EIR lu tr ton b d liu lin quan n trm di ng MS. EIR c ni n MSC qua ng bo hiu kim tra tnh hp l ca thit b. Trong h thng GSM th EIR c coi l thuc phn h chuyn mch NSS.

2.7 Giao din v tuyn s Cc knh ca giao din v tuyn bao gm cc knh vt l v cc knh logic. 2.7.1 Knh vt l Knh vt l t chc theo quan nim truyn dn. i vi TDMA GSM, knh vt l l mt khe thi gian mt tn s sng mang v tuyn c ch nh.

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GSM 900 nguyn thy Di tn s: 890 915 MHz cho ng ln uplink (t MS n BTS). 935 960 MHz cho ng xung downlink (t BTS n MS). Di thng tn ca mt knh vt l l 200KHz. Di tn bo v bin cng rng 200KHz. Ful (n) = 890,0 MHz + (0,2 MHz) * n Fdl (n) = Ful (n) + 45 MHz Vi 1 n 124 Cc knh t 1 124 c gi l cc knh tn s v tuyn tuyt i ARFCN (Absolute Radio Frequency Channel Number). Knh 0 l di phng v. Vy GSM 900 c 124 tn s bt u t 890,2MHz. Mi di thng tn l mt khung TDMA c 8 khe thi gian. Nh vy, s knh vt l GSM 900 l s 992 knh. EGSM (GSM m rng E : extended) H thng GSM nguyn thy c m rng mi bng tn thm 10 MHz (tng ng 50 knh tn s) th c gi l EGSM: Di tn s: 880 915 MHz uplink. 925 960 MHz downlink. Ful (n) = 880 MHz +(0,2 MHz)*n Fdl (n) = Ful (n) + 45 MHz. Vi n=ARFCN , 1 n 174 . Knh 0 l di phng v. DCS 1800: DCS 1800 c s knh tn s tng gp 3 ln so vi GSM 900 Di tn s: 1710 1785 MHz uplink. 1805 1880 MHz downlink. Ful (n) = 1710MHz + (0,2 MHz)*(n - 511) Fdl (n) = Ful (n) + 95 MHz Vi 512 n 885. 2.7.2 Knh logic

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Knh logic c t chc theo quan im ni dung tin tc, cc knh ny c t vo cc knh vt l. Cc knh logic c c trng bi thng tin truyn gia BTS v MS.

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CHNG III : TI U HA MNG GSM 3.1 Tnh ton mng di ng GSM 3.1.1 L thuyt dung lng v cp dch v Trong qu trnh pht trin mng, tng cng dung lng ca mng l mt nhu cu cp thit. Tuy nhin, cng cn xc nh dung lng cn tng l bao nhiu ph hp vi tng giai on pht trin ca mng v ph hp vi yu cu v mt k thut v kinh t hin ti. 3.1.2 Lu lng v knh v tuyn ng trc Trong lnh vc giao thng vn ti, ng trc cho nhiu xe c i n mi ni. Hiu qu s dng ca ng trc ln hn nhiu so vi ng ct (ch ni vi mt x vng su chng hn). Nu lin lc v tuyn bng knh v tuyn dnh ring PRM (Private Mobile Radio), th phn ln thi gian knh v tuyn khng c s dng. Ti nguyn knh v tuyn l rt hn ch, nn phi qun l n trn phm vi quc gia v quc t. T , xu hng l knh v tuyn ng trc dng chung. H thng thng tin di ng cellular p dng knh v tuyn ng trc: Mi BTS c mt s knh v tuyn dng chung cho nhiu ngi. T l ngi dng trn s knh dng chung cng cao th hiu qu s dng ng trc cng cao. Hiu sut s dng ph tn s li cng cao khi cng mt tn s m c dng li nhiu ln cc cell cch xa nhau. Lu lng: Trong h thng vin thng, lu lng l tin tc c truyn dn qua cc knh thng tin. Lu lng ca mt thu bao c tnh theo cng thc: A= Trong : C : s cuc gi trung bnh trong mt gi ca mt thu bao. t : thi gian trung bnh cho mt cuc gi. A : lu lng thng tin trn mt thu bao (tnh bng Erlang). Theo s liu thng k in hnh th: C=1 : trung bnh mt ngi c mt cuc gi trong mt gi.
C *t 3600

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t = 120s : thi gian trung bnh cho mt cuc gi l 2 pht. A =

Nh vy, phc v cho 1000 thu bao ta cn mt lu lng l 33 Erlang. 3.1.3 Cp dch v - GoS (Grade of Service) Nu mt knh b chim ton b thi gian, th knh t c dung lng cc i 1 Erl. V ngi s dng truy cp knh v tuyn theo kiu ngu nhn, nn khng th trnh khi nhng khong thi gian trng knh v tuyn , do vy knh v tuyn khng t c dung lng l tng (1 Erl). Khi s ngi dng tng ln, s cuc gi i qua knh cng tng, nn thng lng tng ln.C th xy ra tnh hung nhiu ngi dng ng thi truy cp mt knh v tuyn, khi ch c mt ngi c dng knh, nhng ngi khc b tc nghn.

Hnh 0-6-1 Lu lng: Mun truyn, c truyn, nghn Lu lng mun truyn = Lu lng c truyn + Lu lng nghn. Offered Traffic = Carried Traffic + Blocked Traffic

Cp phc v (GoS = Grade of Service): mt knh ng trc c cht lng phc v cao th xc sut nghn phi thp. Vy nn s ngi dng c th phi b gii hn, tc l lu lng mun truyn phi gi trong dung lng knh. Nu chp nhn mt cp phc v thp hn, tc l xc sut nghn ln hn, th tng ng tng c dung lng mun truyn (tng s ngi dng). GoS cng mt ngha vi xc sut nghn: Lu lng mun truyn: A (lu lng mun truyn) Lu lng b nghn : A*GoS (lu lng mt i)

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Lu lng c truyn : A*(1 - GoS) (lu lng pht ra) Theo thng k cho thy th cc thu bao c nhn s khng nhn ra c s tc nghn h thng mc di 10%. Tuy nhin mng hot ng vi hiu sut cao th mng cellular thng c GoS = 2 % ngha l ti a 2% lu lng b nghn, ti thiu 98% lu lng c truyn. M hnh ERLANG B: y l m hnh h thng thng tin hot ng theo kiu tiu hao. Thu bao khng h gi li khi cuc gi khng thnh. ng thi gi thit rng: Xc sut cuc gi phn b theo lut ngu nhin Poisson, s ngi dng rt ln so vi s knh dng chung, khng c knh d tr dng ring, cuc gi b nghn khng c gi li ngay.

Hnh 0-1-7 Xc sut nghn GoS M hnh Erlang B l m hnh thch hp hn c cho mng GSM. T cc cng thc ton hc, ngi ta lp ra bng Erlang B cho tin dng (phn Ph lc). V d: S knh dng chung l 10, GoS l 2%. Tra bng Erlang B ta c lu lng mun truyn l A = 5,084 Erl. Vy lu lng c truyn l: A*(1 - GoS) = 5,084*(1 0,02) = 4,9823 Erl. 3.1.4 Hiu sut s dng trung k (ng trc) Hiu sut s dng trung k l t s gia lu lng c truyn vi s knh ca ng trc. v d trn, ta ang xt trung k c s knh dng chung n = 10, GoS = 2 %, nn lu lng c truyn s l 4,9823 Erl. Ta c: Hiu sut s dng trung k =
4,9823 * 100 % = 49,823 % 10

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Hiu sut c v thp ny tng ng vi GoS tt (Xc sut nghn thp). Chng hn, nu GoS = 10 % (ti hn) th lu lng mun truyn l 7,511 Erl, tng ng lu lng c truyn l: 7,511*(1 0,1) = 6,7599 Erl. Khi , hiu sut s dng trung k ln n
6,7599 * 100 % = 67,599 %. 10

GoS cng tt th hiu sut s dng trung k cng thp, cn phi c nhiu knh v tuyn cho lu lng mun truyn cho. GoS cng km th vi mt lu lng cho th ch cn s knh v tuyn l t hn. Vi cng mt cp phc v, trung k cng ln (s knh dng chung ln) th hiu qu s dng trung k cng cao. S knh TCH 6 10 15 25 40 Lu lng c truyn (GoS = 2%) 2,2305 Erlang 4,9823 Erlang 8,8300 Erlang 17,155 Erlang 30,377 Erlang Hiu sut s dng trung k 37 % 49,82 % 58,86 % 68,62 % 75,94 %

3.1. 5 Cc yu t nh hng ti cht lng ph sng 3.1.5.1 Tn hao ng truyn sng v tuyn H thng GSM c thit k vi mc ch l mt mng t ong dy c v bao trm mt vng ph sng rng ln. Cc nh khai thc v thit k mng ca mnh cui cng t c mt vng ph lin tc bao tt c cc vng dn c ca t nc. Vng ph sng c chia thnh cc vng nh hn l cc cell. Mi cell c ph sng bi mt trm pht v tuyn gc BTS. Kch thc cc i ca mt cell thng thng c th t ti bn knh R = 35 km. V vy, suy hao ng truyn l khng th trnh khi. Vi mt anten cho trc v mt cng sut pht bit, suy hao ng truyn t l vi bnh phng (d.f), trong d l khong cch t trm thu n trm pht gc BTS. Trong mi trng thnh ph, vi nhiu nh cao tng, suy hao c th t l vi lu tha 4 hoc cao hn na.

- 18 -

D on tn hao ng truyn trong thng tin di ng GSM bao gm mt lot cc vn kh khn, m l do chnh bi v trm di ng lun lun di ng v anten thu thp. Nhng l do thc t ny dn n s thay i lin tc ca a hnh truyn sng, v vy trm di ng s phi vo nhng v tr tt nht thu c cc tia phn x. 3.1.5.2 Tnh ton l thuyt Cch c bn m n gin ta coi khng gian truyn sng l khng gian t do. Gi thit rng khng c tia phn x v sng v tuyn c truyn trong khng gian t do. Vi anten v hng, ta c cng thc suy hao ng truyn trong khng gian t do: Lf = 20log(4 d / ) [dB] Cng thc ny c th c vit li nh sau: Lf = 32,5 + 20logd + 20logf [dB] Trong : d = khong cch t anten pht n anten thu [km]. f = tn s lm vic [MHz]. Nhng cng thc l thuyt n gin v trn vn trn khng cn ph hp trong mi trng di ng na, ni m truyn sng do nhiu ng l ch yu. Nhng sng ny cng b tn x, nhiu x, suy gim do nhiu trng thi khc nhau ca c vt th c nh v vt th chuyn ng. Hn na, s khc x tng i lu lm ng truyn sng b un cong. M hnh mt t bng phng: M hnh mt t c trnh by trong hnh 3.3 cho thy tng tn hiu n trong my thu bao gm thnh phn n trc tip cng vi thnh phn phn x t mt t (thnh phn ny c th c coi nh l tn hiu gc t mt anten o trong lng t). Hai sng ny cng nhau to thnh sng khng gian (Space Wave).

- 19 -

Hnh 0-1-8 Truyn sng trong trng hp coi mt t l bng phng Ta c cng thc sau tnh suy hao ng truyn: L = 20.log(d2 /h1.h2 ) Nhng trong thc t, khong khng gian gia my thu v my pht thng c cc vt chn (hnh 3.4). Theo l thuyt v truyn sng v tuyn, mt chng ngi vt s lm suy gim cng ca tn hiu truyn thng. S suy gim ny ph thuc vo vt chn trong tm nhn thng ca vt chn.

Hnh 0-1-9 Vt chn trong tm nhn thng Cng thc sau dng tnh ton s suy gim do vt chn gy ra: V=
h

2( d 1 + d 2 ) d1d 2

- 20 -

Trn thc t cc loi a hnh truyn sng rt phc tp, khng mt cng thc no c th cp c ht cc loi a hnh ny. V vy, xut hin nhng m hnh truyn sng nh nhng o c thc t ca cc nh khoa hc. Nhng kt qu t nhng php o c chuyn thnh nhng th ch ra mi quan h gia cng trng v khong cch vi mt s bin nh: chiu cao anten, loi a hnh... Phng php o cng trng: Nm 1968, Y. Okumura l mt k s ngi Nht Bn a ra rt nhiu s liu v vic o cng trng tham kho. ng chia a hnh thnh 5 loi chnh 1. Vng hu nh bng phng 2. Vng nhiu i 3. Vng c chm ni c lp 4. Vng c a hnh dc 5. Vng ranh gii gia t v nc (b sng, b bin...)

Hnh 0-1-10 Biu cng trng ca OKUMURA

- 21 -

Ta thy rng s o lng ca Okumura ch cho thy s suy gim ca cng tn hiu theo khong cch, nhng n gim nhanh hn nhiu so vi nhng g ta bit trong khng gian t do. 3.1.5.3 Vn Fading Fading chun Loga: trm di ng thng hot ng cc mi trng c nhiu chng ngi vt (cc qu i, to nh...). iu ny dn n hiu ng che khut (Shaddowing) lm gim cng tn hiu thu, khi thu bao di chuyn cng thu s thay i. Fading Rayleigh: Khi mi trng c nhiu chng ngi vt, tn hiu thu c t nhiu phng khc nhau. iu ny ngha l tn hiu thu l tng ca nhiu tn hiu ging nhau nhng khc pha v bin . gim phn no tc hi do Fading gy ra, ngi ta thng tng cng sut pht ln to ra mt lng d tr Fading, s dng mt s bin php nh: phn tp anten, nhy tn ... 3.1.5.4 nh hng nhiu C/I v C/A Mt c im ca cell l cc knh ang s dng c th c s dng cc cell khc. Nhng gia cc cell ny phi c mt khong cch nht nh. iu ny c ngha l cell s b nhiu ng knh do vic cc cell khc s dng cng tn s. Cui cng vng ph sng ca trm gc s b gii hn bi l do ny hn l do tp m thng thng. V vy, ta c th ni rng mt h thng t ong hon thin l gii hn c nhiu m c qui chun, loi tr c nhiu h thng. Mt vn trong thit k h t ong l iu khin cc loi nhiu ny mc chp nhn c. iu ny c thc hin mt phn bi vic vic iu khin khong cch s dng li tn s. Khong cch ny cng ln th nhiu cng b. cht lng thoi lun c m bo th mc thu ca sng mang mong mun C (Carrier) phi ln hn tng mc nhiu ng knh I (Interference) v mc nhiu knh ln cn A (Adjacent).

- 22 -

3.1.5.5 Nhiu ng knh C/I: Nhiu ng knh xy ra khi c hai my pht pht trn cng mt tn s hoc trn cng mt knh. My thu iu chnh knh ny s thu c c hai tn hiu vi cng ph thuc vo v tr ca my thu so vi hai my pht. T s sng mang trn nhiu c nh ngha l cng tn hiu mong mun trn cng tn hiu nhiu. C/I = 10log(Pc/Pi) . Trong : Pc = cng sut tn hiu thu mong mun Pi = cng sut nhiu thu c.

Hnh 0-1-11 T s nhiu ng knh C/I Hnh 3.6 trn ch ra trng hp m my di ng (cellphone) t trong xe ang thu mt sng mang mong mun t mt trm gc phc v (Serving BS) v ng thi cng ang chu mt nhiu ng knh do nhiu pht sinh ca mt trm gc khc (Interference BS).

- 23 -

Gi s rng c hai trm u pht vi mt cng sut nh nhau cc ng truyn sng cng tng ng (hu nh cng khng khc nhau trong thc t) v im gia, my di ng c C/I bng 0 dB, c ngha l c hai tn hiu c cng bng nhau. Nu my di ng i gn v pha trm gc ang phc v n th C/I > 0 dB. Nu my di ng chuyn ng v pha trm gy ra nhiu th C/I < 0 dB. Theo khuyn ngh ca GSM gi tr C/I b nht m my di ng vn c th lm vic tt l 9 dB. Trong thc t, ngi ta nhn thy rng gi tr ny cn thit phi ln n 12 dB ngoi tr nu s dng nhy tn th mi c th lm vic mc C/I l 9dB. mc C/I thp hn th t l li bit BER (Bit Error Rate) s cao khng chp nhn c v m ho knh cng khng th sa li mt cch chnh xc c. T s C/I c dng cho cc my di ng ph thuc rt ln vo vic quy hoch tn s v mu ti s dng tn s. Ni chung vic s dng li tn s lm dung lng tng ng k tuy nhin ng thi cng lm cho t s C/I gim i. Do vic quy hoch tn s cn quan tm n nhiu ng knh C/I. 3.1.5.6 Nhiu knh ln cn C/A: Nhiu knh ln cn xy ra khi sng v tuyn c iu chnh v thu ring knh C song li chu nhiu t knh ln cn C-1 hoc C+1. Mc d thc t sng v tuyn khng c chnh thu knh ln cn , nhng n vn ngh mt s p ng nh l cho php knh ln cn gy nhiu ti knh m my thu ang iu chnh. T s sng mang trn knh ln cn c nh ngha l cng ca sng mang mong mun trn cng ca sng mang knh ln cn. C/A = 10.log(Pc/Pa) Trong : Pc = cng sut thu tn hiu mong mun Pa = cng sut thu tn hiu ca knh ln cn Gi tr C/A thp lm cho mc BER cao. Mc d m ho knh GSM bao gm vic pht hin li v sa li, nhng vic thnh cng th cng c gii hn i vi nhiu. Theo khuyn ngh ca GSM, cho vic quy hoch tn s c tt th gi tr C/A nh nht nn ln hn - 9 dB.

- 24 -

Khong cch gia ngun to ra tn hiu mong mun vi ngun ca knh ln cn ln s tt hn cho C/A. iu ny c ngha l cc cell ln cn khng nn c n nh cc sng mang ca cc knh cnh nhau nu C/A c c ngh trong mt gii hn nht nh. C hai t s C/I v C/A u c th c tng ln bng vic s dng quy hoch cu trc tn s. 3.1.5.7 Mt s bin php khc phc Vn can nhiu knh chung l mt thch thc ln vi h thng thng tin di ng t bo. C cc phng php gim can nhiu knh chung nh: 1. Tng c ly s dng li tn s (D) 2. H thp cao anten trm gc 3. S dng Anten nh hng BTS (Sector ha) Vi phng php th nht: vic tng c ly s dng li tn s D s lm gim can nhiu knh chung, tuy nhin khi s cell trong mi mng mu s tng, tng ng vi s knh tn s dnh cho mi cell s gim v nh vy th dung lng phc v s gim xung. Phng php th hai vic h thp anten trm gc lm cho nh hng gia cc cell dng chung tn s s c gim bt v nh vy can nhiu knh chung cng c gim bt. Tuy nhin, vic h thp anten s lm nh hng ca cc vt cn (nh cao tng) ti cht lng ca h thng tr nn nghim trng hn. Phng php th 3 c hai ch li: Mt l bin php lm gim can nhiu knh chung trong khi c ly s dng li tn s khng i, hai l tng dung lng h thng. Phng php ny s c trnh by trong phn sau. Ngoi ra, cc k thut khc nh: iu khin cng sut pht sng kiu ng Truyn pht gin on Nhy tn cng lm ci thin thm ng k t s C/ I ca h thng

- 25 -

Mt s k thut tng cht lng h thng: Nhy tn: Thc cht ca vic nhy tn l thc hin tri cc cm (burst) d liu trn cc knh tn s khc nhau mt cch ngu nhin, nhm gim nhiu trong ton b h thng. iu ny c ngha rt ln i vi cc mng ln m vic s dng li tn s l cc k kh khn. nhy tn cn ch trong trng hp t hp nhy tn, s tn s ny c th nhiu hn s trm thu/pht TRX ca cell. Khi chn cc tn s nhy tn khc nhau s lm cho cc cm d liu nhy tn theo cc cch khc nhau v lm gim kh nng trng tn s gia cc cm s liu trn 2 cell. Truyn pht gin on _ Discontinuous Transmission (DTX): Thc cht ca phng php DTX l BTS hay MS ch pht khi nhn c tn hiu u vo nh c tn hiu thoi v khi kt thc tn hiu n s ngng pht. Vic pht hay khng c thc hin trn c s tng khe thi gian. Mc ch ca phng php ny l tit kim nng lng v gim nhiu trn knh ln cn mt cch ti a. Khi s dng phng php truyn dn gin on ta cn thm cc thit b ph tr khc nh VAD (Voice Active Detector) pht hin tn hiu vo v to ra ting n gi khi mt pha no ngng cung cp tn hiu. iu khin cng sut thu pht ca MS v BTS: Vic iu khin tng gim cng sut thu pht ca MS v BTS cng lm ci thin ng k t s C/ I. 3.1.5.8 Phn tn thi gian Phn tn thi gian xy ra l do c nhiu ng truyn sng t my pht n my thu. Hin tng phn tn thi gian gy ra mt s vn cho mng thng tin di ng s. Vic s dng truyn dn s cng gy ra mt s vn khc nh: phn tn thi gian do cc tn hiu phn x (Reflection) gy ra. S phn tn thi gian s gy ra hin tng giao thoa gia cc k t. Gi thit chng ta pht i mt chui bit 1 v 0. Nu tn hiu phn x i chm hn tn hiu i thng ng 1 bit th my thu pht hin bit 1 t sng phn x ng thi cng pht hin bit 0 t sng i thng. Ca s thi gian c nh ngha l khong thi gian 15 ms sau khi my thu nhn c tn hiu trc tip t my pht. Gi s cc tia phn x n my thu bn

- 26 -

ngoi ca s thi gian, tc l sau 15 ms, s gy phin phc cho h thng ging nh l nhiu.Ta bit gi tr ti thiu ca C/I trong h thng GSM l 9 dB. Chng ta c th coi gi tr ny l gi tr cc i ca phn tn thi gian. Ngha l tt c cc tn hiu phn x m n tr hn 15 ms, bn ngoi ca s thi gian, phi c gi tr tng nh hn 9 dB. T s ny chnh l C/R. Cc trng hp phn tn thi gian Nhng mi trng nguy him: (l nhng mi trng c th gy nn vn v phn tn thi gian). Nhng vng ni H su hoc nhiu nh cao tng Nhng to nh cao c kt cu kim loi , ... Trong tt c nhng trng hp nh vy phn tn thi gian ch c th xy ra khi hiu qung ng gia tn hiu trc tip v tn hiu phn x t nhng chng ngi vt k trn ln hn ca s cn bng (4,5 km). Ni chung, s nguy him ca phn tn thi gian s tng cng vi khong cch gia BTS v MS. Khi mt MS gn BTS c th nhn c tn hiu phn x mnh vi hiu qung ng ln nhng vn khng nh hng g do tn hiu trc tip mnh m bo t s C/R trn ngng ti hn. Khi MS chuyn ng ra xa BTS th nguy c t s C/R thp s tng ln do tn hiu trc tip yu i. Tuy nhin, mt iu cn ch l tia phn x cng l mt phn ca sng mang cho nn vic quy hoch mt h thng cn phi ch ra c cc trng hp c th c th xy ra hin tng giao thoa k t. Phn tn thi gian vi cc trng hp khc nhau Trng hp 1:

- 27 -

Trng hp ny: Tuy hiu s qung ng = DR D0 ln (DR = D1 + D2), nhng tn hiu trc tip mnh, tn hiu phn x yu. Do vy t s C/R trn ngng.

Trng hp 2:

Trng hp ny: Hiu s qung ng vn cn kh ln nn cc tn hiu phn x nm ngoi ca s thi gian. Trong khi tn hiu n trc tip yu i, tn hiu phn x mnh hn. T s C/R gn hoc thp hn ngng. y l trng hp nguy him nht, hin tng phn tn thi gian biu hin r rng nht. Trng hp 3:

- 28 -

Trng hp ny: Tn hiu phn x mnh gn nh tn hiu trc tip, t s C/R gn hoc di ngng. Nhng do hiu qung ng nh nm trong ca s cn bng, hay cc tn hiu phn x nm trong ca s thi gian, nn trng hp ny khng b nh hng bi phn tn thi gian.

3.2 Mt s gii php khc phc Nhng gii php kh thi trnh tc hi ca phn tn thi gian l: 3.2.1 . Chn v tr t BTS: Di chuyn BTS n cng gn vt gy phn x cng tt. iu ny s m bo cho hiu khong cch lun nh nm trong phm vi ca s cn bng.

Hnh 3-2-1 t BTS gn chng ngi vt trnh phn tn thi gian Chuyn hng anten ca BTS ra khi pha vt chng ngi gy phn x nu BTS c t xa n. Anten nn chn c t s tng ch trc trn sau cao. 3.2.2 . Thay i anten v gc nghing anten: Nu vt phn x khng b chiu vo th s khng c hin tng phn x. Nh vy, ta phi c gng gim phn nng lng bc x t vt phn x m c th gy ra hin tng phn x c hi. S dng anten down tilt l mt cch c th p dng c. Anten down tilt vi rng bp sng vo khong 100, c s dng trnh chiu vo nhng vng ni v trong trng hp cn ph sng cho mt trc ng quc l. Vn chnh

- 29 -

khi s dng anten ny l chng phi c lp t tht chnh xc, sai s khng c vt qu 10. 3.2.3 . iu chnh tham s cell: Mt cch khc chng phn tn thi gian l thay i tham s ca cell. Nu mt vng no trong mt cell c cng tn hiu thp so vi vng cn li trong cell th cc tham s iu khin chuyn giao nn c thit lp tin hnh cc cuc chuyn giao ra ngay khi cell ny trc khi my di ng MS i vo vng nguy him . Cc tham s ca cc cell bn cnh cng nn c thit lp sao cho cc cuc chuyn giao khng b chuyn vo nhng vng c xy ra tn sc thi gian nm trong cell .

3.2.4 o lng: Bin php o lng c a ra trong nhng mi trng khc nhau m nhng chng ngi vt gy nn phn tn ch nm ngoi vng ellipe c to nn bi v tr gia BTS v MS v phm vi ca s cn bng (hnh 3.8). Nng lng sng phn x t l vi R-4 ca khong cch. C ngha l n s gim rt nhanh khi ra xa chng ngi vt. V li, nu BTS v MS nhn thy c nhau th tn hiu trc tip s mnh hn rt nhiu so vi tn hiu phn x v tc hi lm cho cht lng cuc kt ni khng c n nh trong thi gian phn tn thi gian s rt nh.

- 30 -

Hnh 3-2-2 Phm vi vng Elip

- 31 -

3.3 THIT K H THNG 3.3.1 H thng thng tin di ng t bo H thng thng tin di ng t bo s dng mt s lng ln cc my pht v tuyn cng sut thp to nn cc cell hay cn gi l t bo (n v a l c bn ca h thng thng tin v tuyn). Thay i cng sut my pht nhm thay i kch thc cell theo phn b mt thu bao, nhu cu thu bao theo tng vng c th. Khi thu bao di ng di chuyn t cell ny sang cell khc, cuc m thoi ca h s c gi nguyn lin tc, khng gin on. Tn s s dng cell ny c th c s dng li cell khc vi khong cch xc nh gia hai cell. Cu trc h thng thoi di ng trc y Dch v thoi di ng truyn thng c cu trc ging nh h thng truyn hnh pht thanh qung b: Mt trm pht sng cng sut mnh t ti mt cao im c th pht tn hiu trong vng bn knh n 50km.

Hnh 3-3-12 Cu trc h thng thng tin di ng trc y

H thng thng tin di ng t bo

- 32 -

Khi nim mng t ong cu trc li h thng thng tin di ng theo cch khc. Thay v s dng mt trm cng sut ln, ngi ta s dng nhiu trm cng sut nh trong vng ph sng c n nh trc. Ly v d, bng cch phn chia mt vng trung tm thnh 100 vng nh hn (cc t bo), mi cell s dng mt my pht cng sut thp vi kh nng cung cp 12 knh thoi cho mi my. Khi nng lc ca h thng v l thuyt c th tng t 12 knh thoi s dng mt my pht cng sut ln ln n 1200 knh thoi bng cch s dng 100 my pht cng sut thp. Nh vy l dung lng h thng tng ln rt nhiu. Bng cch gim bn knh ca vng ph sng i 50% (din tch vng ph sng gim 4 ln), nh cung cp dch v c th tng kh nng phc v ln 4 ln. H thng c trin khai trn vng c bn knh 1 Km c th cung cp s knh ln hn gp 100 ln so vi h thng trin khai trn vng c bn knh 10 Km. T thc t rt ra kt lun rng, bng cch gim bn knh vng i vi trm mt th nh cung cp c th phc v thm vi triu cuc gi.

Hnh 3-3-2 H thng thng tin di ng s dng cu trc t bo Khi nim cell (t bo) c s dng vi cc mc cng sut thp khc nhau, n cho php cc cell (cc t bo) c th thay i vng ph sng tu theo mt , nhu cu ca thu bao trong mt vng nht nh. Cc cell c th c thm vo tng vng tu theo s pht trin ca thu bao trong vng . Tn s cell ny c th

- 33 -

c ti s dng cell khc, cc cuc in thoi vn c duy tr lin tc khi thu bao di chuyn t cell ny sang cell khc. 3.3.2 Quy hoch Cell 3.3.2.1 Khi nim t bo (Cell) Cell (t bo hay ): l n v c s ca mng, ti trm di ng MS tin hnh trao i thng tin vi mng qua trm thu pht gc BTS. BTS trao i thng tin qua sng v tuyn vi tt c cc trm di ng MS c mt trong Cell.

Hnh 3-3-13 Khi nim Cell Hnh dng l thuyt ca Cell l mt t ong hnh lc gic:

Hnh 3-3-14 Khi nim v bin gii ca mt Cell

- 34 -

Trn thc t, hnh dng ca cell l khng xc nh. Vic quy hoch vng ph sng cn quan tm n cc yu t a hnh v mt thu bao, t xc nh s lng trm gc BTS, kch thc cell v phng thc ph sng thch hp. 3.3.2.2 Kch thc Cell v phng thc ph sng Kch thc Cell Cell ln: Bn knh ph sng khong: n km n*10 km (GSM: 35 km) V tr thit k cc Cell ln: Sng v tuyn t b che khut (vng nng thn, ven bin ) Mt thu bao thp. Yu cu cng sut pht ln. Cell nh: Bn knh ph sng khong: n*100 m. (GSM: 1 km) V tr thit k cc Cell nh: Sng v tuyn b che khut (vng th ln). Mt thu bao cao. Yu cu cng sut pht nh. Phng thc ph sng Hnh dng ca cell trong mi mt s chun ph thuc vo kiu anten v cng sut ra ca mi mt BTS. C hai loi anten thng c s dng: anten v hng (omni) l anten pht ng hng, v anten c hng l anten bc x nng lng tp trung trong mt r qut (sector). Pht sng v hng Omni directional Cell (3600) Anten v hng hay 3600 bc x nng lng u theo mi hng.

Hnh 3-3-5 Omni (3600) Cell site

- 35 -

Khi nim Site: Site c nh ngha l v tr t trm BTS. Vi Anten v hng: 1 Site = 1 Cell 3600 Pht sng nh hng Sectorization: Li ch ca sectorization (sector ha): Ci thin cht lng tn hiu (Gim can nhiu knh chung). Tng dung lng thu bao.

Hnh 3-3-6 Sector ha 1200 Vi Anten nh hng 1200: 1 Site = 3 Cell 1200

3.3.2.3 Chia Cell (Cells Splitting)

Hnh 3-3-15 Phn chia Cell ng trn quan im kinh t, vic hoch nh cell phi bo m lu lng h thng khi s thu bao tng ln, ng thi chi ph phi l thp nht. Thc hin

- 36 -

c iu ny th yu cu phi tn dng c c s h tng ca i trm c. p ng c yu cu ny, ngi ta s dng phng php gim kch thc cell gi l tch cell (cells splitting).Theo phng php ny vic hoch nh c chia thnh cc giai on sau: 1. Giai on 0 (phase 0): Khi mng li mi c thit lp, lu lng cn thp, s lng i trm cn t, mng thng s dng cc omni cell vi cc anten v hng, phm vi ph sng rng.

Hnh 3-3-16 Cc Omni (3600) Cells ban u Khi mng c m rng, dung lng s tng ln, p ng c iu ny phi dng nhiu sng mang hn hoc s dng li nhng sng mang c mt cch thng xuyn hn. Tuy nhin, mi s thay i trong quy hoch cu trc tn s phi gn lin vi vic quan tm ti t s C/I. Cc tn s khng th c n nh mt cch ngu nhin cho cc cell. thc hin c iu ny, phng php ph bin l chia cell theo th t. 2. Giai on 1 (Phase 1): Sector ha Thay anten v hng (omni) bng 3 anten ring bit nh hng di qut 1200 l mt gii php tch chia mt Cell thnh 3 Cells. l gii php di qut ha (sectorization sector ha). Cch lm ny khng i hi thm mt bng cho cc

- 37 -

Cell mi. Tuy cc Cell mi phn bit nhau theo chc nng mng nhng chng vn ti mt bng c. Khi , ti mi v tr c (Site) by gi c th phc v c 3 cell mi, nhng cell ny nh hn v c 3 anten nh hng c t v tr ny, gc gia cc anten ny l 1200.

Hnh 3-3-17 Giai on 1 :Sector ha 3. Giai on 2: Tch chia nh hn na v sau Tch chia Cell 1:3 thm ln na Hnh 4.10 trnh by vic tch chia 3 thm ln na. Ln tch ny s dng li mt bng c v thm mi gp i mt bng mi cho cc BTS mi. mt bng c, anten cn quay i 300 ngc chiu kim ng h. Nh vy tng s mt bng gp 3 ln mt bng c tr gi cho s tng dung lng mng ln gp 3 ln.

- 38 -

Hnh 3-3-18 Tch chia 1:3 thm ln na Tch chia 1:4 (sau ln u chia 3)

Hnh 3-3-19 Tch chia 1:4 (sau ln u chia 3)

- 39 -

S tch chia ny khng i hi xoay hng anten tt c cc BTS c mt bng c. V tr BTS mt bng mi c biu th trn hnh v 4.11. S ln s dng li tn s, dung lng h thng v s lng mt bng BTS u tng 4 ln so vi trc khi chia tch. Ty theo yu cu v dung lng h thng, vic chia cell c th c thc hin tip tc. Tuy nhin, mi s thay i trong quy hoch cu trc tn s phi gn lin vi vic quan tm ti t s nhiu C/I. By gi ta hy xt mt v d thy c s tng dung lng khi thu hp kch thc cell. Gi thit rng h thng c 24 tn s v chng ta bt u t mt cm 7 cell c bn knh cc i 14 km. Sau chng ta thc hin cc giai on 1 tch 3 v 1 tch 4. Cng gi thit rng mt thu bao c lu lng 0,02 Erlang vi mc phc v GoS = 5%. Vi 24 tn s, ngha l s knh logic ca h thng s l: 24 x 8 = 192 knh Trong giai on th nht, khi 1 cm (s nhm tn s) l N = 7, th s knh lu lng TCH cho mi cell l: (192 - 2 x7 )/7 = 178/7 = 25 TCH Trong giai on tip theo, khi mt cm c N = 21. S knh lu lng cho mi cell l: (192 - 21)/21 = 171/21 = 8 TCH. Cn c bng Erlang ta s c bng thng k v mt lu lng qua cc bc tch cell nh sau: Giai on 0 1 2 3 Bn knh 14 km 8 km 4 km 2 km N 7 21 21 21 TCH Phm vi S thu S thu Hiu qu trung k 76% 54% 54% 54% mi bao/ 1 bao/km2 25 499,2km2 999 2,0 2 8 166,4km 227 1,4 2 8 41,6 km 227 5,5 2 8 10,4 km 227 21,8

T bng ta thy, trong ln tch th nht, dung lng b gim (s thu bao trn 1 km2 gim t 2 xung cn 1,4) l do hiu sut trung k b gim khi s knh trn mt cell t i. Tuy nhin, y l mt bc khng th thiu c thc hin

- 40 -

cc bc tip theo. i vi cc bc tip theo l qui trnh 1 tch 4, bn knh cell gim 2 ln, nhng dung lng tng 4 ln. Nh vy, ta thy rng bin php cell split lm gim kch thc ca cell. Nhng cng lm tng dung lng h thng. Bin php ny phi c p dng theo tng giai on pht trin ca mng. Tuy nhin, bin php ny cng c mt s hn ch bi kch thc cell cng c gii hn (gii hn trn l do cng sut bc x ca BTS v MS c hn, gii hn di l do vn nhiu). ng thi vic lp t cc v tr trm mi i hi kinh ph ln, vic kho st chn c nhng v tr thch hp cng gp nhiu kh khn (nh trm t thit b, xy dng ct anten, mng in li thun tin...) gii quyt vn dung lng nhng khu vc c mt rt cao m cc bin php trn khng gii quyt c, th vic s dng cc minicell v cc microcell s tr nn ph bin vi phm vi ph sng nh, cng sut bc x ca BTS (thng l cc trm Repeater) thp. 3.3.4 Quy hoch tn s Ngy nay cc nh cung cp dch v di ng GSM s dng hai di tn s, l GSM 900 v GSM 1800. Mt s quc gia Chu M th s dng bng 850 Mhz v 1900 Mhz do bng 900 Mhz v 1800 Mhz y c s dng trc . Di tn s dng cho GSM 900 l 890 960 MHz, gm 124 tn s sng mang vi mi hng: Uplink: 890 ~ 915 MHz v Downlink: 935~960 MHz. Di tn s dng cho GSM 1800 l 1710 1880 MHz, gm 374 tn s sng mang vi mi hng: Uplink: 1710~1785 MHz v Downlink: 1805~1880 MHz. Hin nay, ti Vit Nam ang c 3 nh cung cp dch v di ng GSM l Vinaphone, Mobiphone, Viettel, cng ng thi hot ng, nn di tn s hn hp phi chia s u cho c 3 mng. Vi mng di ng VMS-Mobifone di tn c n nh cho mng nh sau:

- 41 -

GSM 900: Di tn s dng trong VMS l 41 tn s t knh 84 n 124 tng ng vi: Uplink: 906,6 MHz 914,8 MHz. Downlink: 951,6 MHz 959,8 MHz. GSM 1800: Di tn s dng trong VMS l t knh 579 n 644 tng ng vi: Uplink: 1723,6 MHz 1736,6 MHz. Downlink: 1818,6 MHz 1831,6 MHz. Ti nguyn tn s c hn trong khi s lng thu bao th ngy cng tng ln, nn vic s dng li tn s l iu tt yu. Tuy nhin, khi s dng li tn s th vn nhiu ng knh xut hin. Do cn c s hoch nh tn s tt ti thiu ha nh hng ca nhiu ti cht lng ca h thng. 3.3.4.1 Ti s dng li tn s Mt h thng t ong l da trn vic s dng li tn s. Nguyn l c bn khi thit k h thng t ong l cc mu s dng li tn s. Theo nh ngha s dng li tn s l vic s dng cc knh v tuyn cng mt tn s mang ph sng cho cc vng a l khc nhau. Cc vng ny phi cch nhau mt c ly ln mi nhiu giao thoa ng knh (c th xy ra) chp nhn c. T s sng mang trn nhiu C/I ph thuc vo v tr tc thi ca thu bao di ng do a hnh khng ng nht, s lng v kiu tn x. Mng mu (Cluster) Cluster l mt nhm cc cell. Cc knh khng c ti s dng tn s trong mt cluster. Nh khai thc mng c giy php s dng mt s c hn cc tn s v tuyn. Vic quy hoch tn s, ta phi sp xp thch hp cc tn s v tuyn vo mt mng mu sao cho cc mng mu s dng li tn s m khng b nhiu qu mc. Hnh 4.12 m t cch ph sng bng mng mu gm 7 cell n gin.

- 42 -

Hnh 3-3-20 Mng mu gm 7 cells C ly dng li tn s D dng thy rng, vi mt kch thc cell nht nh, khong cch s dng li tn s ph thuc vo s nhm tn s N. Nu N cng ln, khong cch s dng li tn s cng ln v ngc li. Ta c cng thc tnh khong cch s dng li tn s: D = R*
3* N

(trong : R l bn knh cell)

Hnh 3-3-21 Khong cch ti s dng tn s Tnh ton C/I ng thi ta c cng thc tnh t s C/I nh sau:

- 43 -

Hnh 3-3-22 S tnh C/I P l v tr ca MS thuc cell A, chu nh hng nhiu knh chung t cell B l ln nht. Ti v tr P (v tr my di ng MS) c: C. .R = I . .(D-R)
x x

C ( D R) x D I = = 1 = ( x R R
x

3. N

-1)x

Trong : x l h s truyn sng, ph bin nm trong khong t 3 n 4 i vi hu ht cc mi trng. I (dB ) = 10*lg( S cell (N) Kch thc mng 3 4 7 9 12 21 3,0 9,0 11,7 16,6 18,7 21,0 25,2
C
3. N

-1)x

T s C/I (dB) x 3,5 10,5 13,7 19,4 21,8 24,5 29,4 4,0 12,0 15,6 22,2 24,9 28,0 33,6

Bng quan h N & C/I xc nh v tr ca cc cell ng knh ta s dng cng thc: N = i2 + i.j + j2. (i; j nguyn) Theo cng thc ny: di chuyn t cell th nht i i cell theo mt hng, sau quay i 600 v di chuyn i j cell theo hng ny. Hai cell u v cui ca qu trnh di chuyn ny la hai cell ng knh.

- 44 -

Phn b t s C/I cn thit h thng c th xc nh s nhm tn s N m ta c th s dng. Nu ton b s knh quy nh c chia thnh N nhm th mi nhm s cha ( /N) knh. V tng s knh l c nh nn s nhm tn s N nh hn s dn n nhiu knh hn mt nhm v mt i trm. V vy, vic gim s lng cc nhm tn s s cho php mi i trm tng lu lng nh s gim s lng cc i trm cn thit cho ti lu lng nh trc. 3.3.4.2 Thay i quy hoch tn s theo phn b lu lng Thay i quy hoch tn s S phn b lu lng S thay i lu lng v hiu ng im nng (hotspot) hnh thnh nhu cu tng thm knh tn s mt cell no . Khi ngi ta ngh ngay n kh nng ly knh tn s cell no c lu lng rt nh thm vo cho cell no c lu lng qu ln. Tuy nhin, vic lm ny ph hng quy hoch tn s v mang li can nhiu qu mc cho php nu nh vic thc thi khng ng khoa hc.

Hnh 3-3-15 Thay i quy hoch tn s

- 45 -

Hnh 4.18 biu th mt tnh hung nh vy: y l mu ti s dng tn s 4/12. Ti mng mu X, cell D1 cn 3 knh tn s m bo lu lng, trong khi cell C3 ch cn 1 knh tn s p ng lu lng ti thi im ang xt.

Ti cell C3, c hai knh tn s 94 v 106, nh vy nn chn ti tn 94 hay 106 chuyn sang D1 ? nh hng ti
C A

Cell D1 v cell D3 l hai cell lin k. M ti tn 94, 106 ca cell C3 lin k vi ti tn 95, 107 ca cell D3. Chnh v vy, chn ti tn no d l 94 hay 106 a sang D1 th u lm tng can nhiu knh k, i vi MS bin gii D1 v D3 th t s
C ca chng gn bng 0 dB. A C I

nh hng ti

Nu chn ti tn 94 (hay 106) t cell C3 a sang D1, th c ly s dng li tn s 94 (hay 106) by gi l t cell D1 ca mng mu X n cell C3 ca mng mu Y, tc l gim i mt na so vi ban u. Ngha l nhiu knh chung tng ln nghim trng, t s C/I gim ng k. V bn knh cell R vn gi nguyn, m c ly s dng li tn s ca ti tn chuyn sang gim ch cn mt na, ngha l D/ R cn li mt na so vi quy hoch trc. V l thuyt, iu lm gim t s C/ I i chng 6 8 dB. Mun phn tch chnh xc C/ I, phi k n yu t a hnh thc t v cc nhn t mng mu. iu ny cn n cng c phn mm c bit x l vn bng my tnh. Mt trong nhng gii php cho vn ny l cu trc ng tm ca cell c tng cng thm ti tn ly t cell khc. Khi , cc ti tn sn c ban u ca

- 46 -

cell vn c dng nh vn c, cn ti tn tng cng c pht cng sut b hn mc microcell. Cc nhn t khc Cng c phn mm quy hoch v tuyn s tnh n nhiu yu t sau y khi chuyn knh tn s: S khc nhau v cng sut pht v tuyn c cc BTS. S khc nhau v anten c dng cc c s mt bng. a hnh thay i Mng mu thay i. .v.v.. V GSM l h thng b gii hn bi can nhiu, nn phi xt mu s dng li tn s no c mc can nhiu chp nhn c. Quy hoch ph sng khng lin tc Bi ton quy hoch ny phi x l c bit. Tuy nhin, c s gii bi ton ny vn l quy hoch tn s sao cho cc t s C/ I v C/ A t mc quy nh cht lng. Nhng mu thun pht sinh c th c dung ha ty hon cnh. V d: trong lng x ven quc l c th chu C/ I nh.

Hnh 3-3-16 Ph sng khng lin tc

- 47 -

3.3.4.3 Thit Patterns)

k tn s theo phng php MRP (Multiple Reuse

Thit k h thng c dung lng ln vi chi ph cho h tng l ti thiu ang ngy cng tr nn quan trng trong cuc chy ua gia cc nh iu hnh di ng. Phn ny trnh by v vic p dng k thut nhy tn kt hp vi mt phng php thit k tn s tin tin, Multiple Reuse Patterns (MRP)_a mu s dng li. Nhy tn _ Frequency Hopping Vic tng dung lng mng bng cch gim c ly ti s dng li tn s s ko theo nhng vn v nhiu tn s tr nn trm trng hn, iu ny gy kh khn cho vic thit k tn s vi cht lng tt. Mt s k thut c s dng nhm gim bt nh hng ca nhiu nh: nhy tn, iu khin cng sut, truyn pht gin on DTX (Discontinuous Transmission). Trong phn ny ta quan tm n k thut nhy tn _ Frequency Hopping. K thut nhy tn a ra hai khi nim phn tn tn s v phn tn nhiu. Phn tn tn s: Tn s c phn chia nhm cn bng cht lng tn hiu gia cc thu bao cho d thu bao ang di chuyn nhanh hay chm. iu ny c ngha l d tr cho Fainh nhanh (Rayleigh Fading) l khng cn thit. Chnh nh hiu qu ca phn tn tn s m vng ph sng c tng ln do gim c d tr cho Fainh nhanh. Ngy nay, quy hoch cell tiu biu dng 3 dB cho d tr Fainh nhanh. Phn tn nhiu: Cng nhiu c chia s u cho cc thu bao quy v mc nhiu trung bnh. Ni chung, vi mt mng li s dng k thut nhy tn th ta c th gim c ly ti s dng tn s do c th ci thin c dung lng ca h thng so vi mng khng s dng k thut nhy tn. Hiu qu ca k thut nhy tn Phn tn nhiu trong k thut nhy tn c th c nhn nhn nh s gim tng quan ca tn hiu nhiu tri qua nhng cm (burst) lin tip. Hnh 4.20 m t s suy gim tng quan tn hiu trong ba trng hp, khi ng ln uplink ca mt

- 48 -

kt ni trong cell A b gy nhiu bi cc trm di ng trong cc cell ng knh. Cell A c n nh tn s 1 v 10 trong c ba trng hp.

Hnh 3-3-17 Mt v d v hiu qu ca k thut nhy tn trn phn tp nhiu ca mt mng li. Kch thc ca mi tn phn nh nhiu tng quan gia cc cell ng knh. Trng hp th nht, mng khng s dng k thut nhy tn. MS kt ni trn knh tn s 1 trong cell A. Sau nhiu I xut hin t mt thu bao cell B ng thi hot ng trn cng knh tn s 1. Tng quan ca tn hiu nhiu trn cc cm lin tip do l rt cao. Nh vy cht lng ca kt ni l xu. Tnh hnh ch c th ci thin nu cell ng knh ngng pht tn hiu trn knh tn s ny hoc kt ni cell A c thc hin chuyn giao Handover (bi Intra-cell Handover, hay Inter-cell Handover). Trong trng hp th hai l trng hp nhy tn trong quy hoch tn s truyn thng, khi cc nhm tn s n nh cho tng cell. Kt ni trong cell A nhy trn hai knh tn s (1 v 10), cell B cng vy. Do , ngun nhiu c th thay i gia hai thu bao trong cell B, gy ra hai tn hiu nhiu I1 v I2. Bi v cng hai tn hiu nhiu ny c s khc nhau kh r rt, tng quan tn hiu nhiu c th thp hn cho cc cm lin tip. Ni cch khc, s phn tn nhiu tng ln so vi trng hp khng dng k thut nhy tn. Trng hp cui cng, mt thit k tn s bt quy tc kt hp vi k thut nhy tn. im c bit trong trng hp ny l khng c s n nh tn s s dng

- 49 -

trong mt cell v cc cell ng knh ca n. Do , cell B ch l mt cell ng knh b phn ca cell A, bi chng ch c mt tn s dng chung. Mt khc, s sp xp ny to ra s cell ng knh b phn l ln hn, trong v d trn l cell C. Trong trng hp ny, nhng cm khc nhau ca mt kt ni ti cell A s b nhiu bi cc thu bao nhng cell khc nhau. Do , cc cm lin tip s tri qua cc tn hiu nhiu I1 v I2 , thng thng l khng tng quan. Chnh v vy, trng hp ny phn tn nhiu l cao hn so vi thit k tn s theo truyn thng. M thut ng gi l Phn tn nhiu ti a _ Maximizing Interference Diversity. V d trn y trnh by cch thc c th t c phn tn nhiu ti a, mt thit k tn s khng s dng cc nhm tn s c nh l thch hp hn c. Tuy nhin, cch thit k tn s ny biu hin nhng hn ch, bao gm c vic thit k li trn phm vi rng cn thit cho mt h thng tin trin v m rng khng ngng. p dng k thut a mu s dng li_MRP c th t c phn tn nhiu ti a m vn duy tr cu trc thit k tn s. Phng php a mu s dng li MRP _ Multiple Reuse Patterns Phng php MRP l phng php tng qut t c dung lng cao bng cch s dng li tn s kt hp vi k thut nhy tn. Phng php MRP khai thc li th ca k thut nhy tn nhm tng dung lng. C s ca phng php MRP l phn chia cc tn s thnh cc mu lp bng tn s khc bit vi cc mc s dng li khc nhau v dng k thut nhy tn kt hp chng li mt mc s dng li trung bnh. Vi mc ch l trin khai c cng nhiu cng tt cc b thu pht TRX cc cell hin ti ti thiu chi ph cho lp t trm mi. Phn ny ta ch xt ti MRP s dng nhy tn bng c bn. 3.3.4.4 Phn chia bng tn: Bc u tin ca phng php MRP l phn chia ph tn sn c thnh cc bng tn khc nhau. Mt bng tn l bng tn BCCH, v mt hay nhiu bng tn TCH theo ngha rng mt tn s c dng lm tn s BCCH mt cell th s khng c s dng lm tn s TCH mt cell khc v ngc li. Bng tn BCCH dng thit k cho knh iu khin qung b BCCH. L do dng cc tn s BCCH duy nht l:

- 50 -

Lu lng khng ph thuc vo c tnh gii m BSIC: Khi MS c gng gii m BSIC (Base Station Identity Code_M nhn dng trm gc) trn knh ng b SCH (Synchronisation Channel), c tnh ny khng b nh hng bi ti lu lng. L do l lu lng c n nh vo cc tn s TCH s khng lm nhiu lon bt k tn s BCCH m knh ng b SCH nh x vo. Gii m nhn dng trm gc BSIC l rt quan trng i vi hiu sut chuyn giao (Handover). Hiu sut handover khng tt s lm tng s lng cc cuc gi b rt. n gin ha vic khai bo danh sch cell ln cn: Vi mt bng tn BCCH ring bit, s lng cc tn s cell ln cn s c gim bt. Vic thit k s n gin khi m tt c cc tn s ngoi tr tn s BCCH ca chnh cell v trong danh sch cell ln cn u c th c s dng. Nu s dng tt c cc tn s sn c nh l cc tn s BCCH s dn ti kt qu l danh sch cell ln cn di hn nh hng xu ti hiu sut handover. Vic thit k li tn s TCH khng nh hng g ti thit k tn s BCCH: Nu nhng TRX b sung c thm vo cc cell c sn, vic thit k tn s BCCH s khng b nh hng g. Hn ch duy nht cn tnh n l nhiu tn s k bn. Chnh v vy, s l hp l khi gi cng thit k tn s cho d TRX b sung c thm vo h thng. Nh iu hnh mng do bit rng nu thit k tn s BCCH tt th n vn gi nguyn c tnh trng tt, khng ph thuc vo nhng tn s TCH. Li ch ca vic iu khin cng sut v pht gin on DTX: Ch c cc tn s TCH c th s dng pht gin on v iu khin cng sut trn hng xung downlink. Vi mt bng tn BCCH ring bit, li ch y t vic iu khin cng sut v pht gin on DTX l t c trn hng xung downlink. Bc tip theo trong phng php MRP, nhng tn s cn li (TCH) c phn chia thnh nhng bng tn khc nhau. Nh vy s tn ti mt bng tn BCCH v vi bng tn TCH. tng chnh l mt vi bng tn TCH c p dng nhng mu s dng li khc nhau trn nhng b thu pht khc nhau. B thu pht TCH th

- 51 -

nht trong tt c cc cell s s dng cc tn s ca bng tn TCH th nht, bng tn TCH th hai cho b thu pht th hai, v.v L do cho vic phn chia nhng tn s TCH thnh cc bng khc nhau l: Kch c s dng li tn s trung bnh ph thuc vo phn b cc TRX ca mng li: S phn b TRX quyt nh h s s dng li tn s trung bnh m c th p dng trong mng. H s s dng li tn s trung bnh c iu chnh theo s TRX ti a cn thit cho mi cell v s lng cell cn s TRX nh vy. Theo cch ny th cht lng h thng c th kim sot tt hn nh iu chnh trong x l thit k tn s. Khi m rng thm TRX, nh hng ti thit k tn s hin ti s nh hn: Vic phn chia bng tn TCH s gii hn s lng cc yu cu ca cng tc thit k tn s khi c thm nhng TRX c b sung. Ch nhng cell c cng s TRX hoc nhiu hn mi b nh hng nu c thm nhng TRX b sung. V d, thm TRX th t vo mt cell c ba TRX s ch c nh hng ti nhng cell c bn hoc c nhiu hn s TRX. Mt bin php cu trc cho thit k tn s: Vi vic phn chia bng tn TCH thnh cc bng khc nhau, cu trc s tr nn hp l khi thit k quy hoch tn s cho b thu pht TCH th nht m khng lm thay i quy hoch BCCH hay nhng quy hoch cho nhng b thu pht TCH khc. Cu trc ny gip n gin hn trong vic a ra thit k tn s mi v trong vic pht hin ra thit k tn s khng tt. 3.3.4.5 n nh tn s Vic n nh tn s c minh ha trong hnh 4.21, mt biu ch ra cch nhng tn s khc nhau c th n nh cho mt cu hnh MRP vi ti a bn TRX mi cell. V d ny xt thit k 12/10/8/6. iu ny ngha l c 12 tn s BCCH (tn s 1, 3, 5, , 23), 10 tn s TCH cho nhm 1 (tn s 2, 4, 6, , 20), 8 tn s TCH nhm 2 (22, 24, 26, , 36) v 6 tn s TCH cho nhm 3 (25, 27, , 35). Hnh v cng ch ra s n nh tn s cho hai cell A v B vi s b thu pht theo th t l hai v bn.

- 52 -

Hnh 3-3-18 V d v thit k tn s vi phng php MRP Cell A c n nh tn s BCCH th 1 v tn s TCH th 6. Do cell A s s dng nhy tn bng c bn trn hai tn s. Trong khi cell B c n nh tn s BCCH th 23 v cc tn s TCH th 20, 26, 35. Do , cell B s dng nhy tn bng c bn trn bn tn s. Ch rng, nhng tn s BCCH khng cn xc nh r v tr, do bt k tn s no trong di tn c sn u c th chn lm tn s BCCH min sao s chia tch BCCH/ TCH c tha mn. Khng cn phi lc no cng tun th cht ch vic n nh tn s theo phng php MRP. Nu mt cell tn ti nhng vn v cht lng th c th gii quyt vn ny bng thay i mt tn s trong cell sang mt tn s tri lut, tn s m ban u c s dng trong nhm b thu pht khc. Tuy nhin, theo khuyn ngh th vic tun th cu trc MRP nn thc hin mt cch cht ch nht c th. 3.3.4.6 Thit k tn s Phng php MRP c pht trin nhm x l c trng tiu biu ca mng li khi s phn phi TRX l khng ng u. iu ny rt quan trng khi mng t bo c s khc nhau v nhng c tnh mng nh kch c cell, s ph tn sn c v a hnh. C ngha l trong mng li, mt s cell c nhiu TRX trong khi c nhng cell vi s TRX t hn.

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tm hiu cc trng thi s dng li tn s khc nhau ca nhng cell khc nhau vi s TRX l khc nhau, ta xem xt v d sau: Cu hnh MRP 12/8/6/4 c chn cho tng s 30 tn s sn c. Trong , 12 tn s BCCH, ba nhm tn s TCH ln lt gm 8, 6, 4 tn s. Trong v d ny ta gi thit rng t l cc cell c 2, 3, 4 TRX ln lt l 20%, 30%, 50%. H s s dng li tn s trung bnh ca mt cell = Tng s tn s trong nhm n nh cho cell / S TRX ca cell Do , cc cell khc nhau s c h s s dng li tn s khc nhau: h s bng 10 vi cell c 2 TRX, bng 8,7 vi cell c 3 TRX, v bng 7,5 vi cell c 4 TRX. S TRX /cell T l cell (%) MRP groups H s s dng li tn s TB S dng li tn s TB thc t (Gii hn trn) phn tn 2 20% 12 / 8
12 + 8 =10 2

3 30% 12 / 8 / 6
12 + 8 + 6 = 8,7 3

4 50% 12 / 8 / 6 / 4
12 + 8 + 6 + 4 = 7,5 4

10 Nh

9,0 Ln

8,5 Rt ln

H s s dng li tn s trung bnh thc t c hiu theo ngha ri rc, v khng phi tt c cc cell u trang b y thit b. V d, TRX th 3 c s dng trn 80% tng s cell, do vy m h s s dng li thc t ca TRX ny ri rc s l 6/ 0,8 = 7 (lm trn t 7,5), ty thuc vo phn b a l ca nhng cell vi TRX th 3. Do , gii hn trn ca h s s dng li tn s thc t ca cell c 3 TRX s l: (12+8+7)/3 = 9,0. Li ch ca nhy tn s tng cng vi s lng nhng tn s trong chui nhy tn. Nhng cell c nhiu TRX hn tng ng vi hiu qu s dng li cao hn, cng ng ngha vi mc nhiu l cao hn, nhng vi phng php MRP iu ny c cn bng vi mt phn tn nhiu l ln hn. V d trn minh ha MRP c th iu chnh thit k tn s theo phn b TRX trong h thng. Tuy nhin, cng phi ch rng MRP khng cn thit phi

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thc hin trn ton b h thng, m ch cn p dng cho nhng vng c dung lng cao. Cng c th s dng cc cu hnh MRP khc nhau cho nhng vng a l khc nhau trong mng. Mu MRP ti H Ni nm 2007 ca VMS_Center1 l cu hnh 15/ 12/ 9 /3: Group BCCH TCH1 TCH2 TCH3 Patch 84 11 3 95 10 3 11 93 1 86 11 4 99 Cell A 88 11 5 10 7 90 92 12 0 94 11 7 10 5 91 96 11 8 10 9 Cell B 10 98 11 9 87 0 12 4 10 2 10 4 12 1 85 10 1 Cell C 10 10 11 6 12 2 89 8 12 3 97 0 11 6

3.3.5 Antenna Anten l thit b thc hin vic chuyn i nng lng gia sng c dn hng (v d trong cp ng trc) v sng trong mi trng khng gian t do, hoc ngc li. Anten c th c s dng pht hoc thu tn hiu v tuyn. Trong thng tin di ng, vic s dng anten thch hp s c vai tr rt quan trng, quyt nh ti cht lng h thng. 3.3.5.1 Kiu loi anten: Trong thng tin di ng ngi ta thng dng hai loi anten chnh l: Anten v hng (omni anten): Pht x tn hiu theo mi hng (3600). Anten nh hng (sector anten): Ch pht x theo mt hng nht nh. S dng Anten nh hng c hiu qu chng nhiu ng knh cao hn so vi Anten v hng. Gim nhiu ng knh s dng anten nh hng (Sector ha) Ta bit vn nhiu giao thoa ng knh thng lin quan n vic s dng li tn s v mt trong nhng dng ca loi nhiu ny l t cc thu bao ang

- 55 -

hot ng nhng v tr cao (cc qu i, trn cc to nh cao tng...) gy nhiu ti cc cell c cng tn s lm vic. Khi dng Omni Antenna:

Hnh 3-3-19 Anten v hng (Omni antenna) Ta gi thit hai cell E1 v E2 s dng chung mt tn s v E1 c a th cao hn so vi E2. Mt thu bao MS ang di chuyn t E1 ti E2. Khi thu bao di chuyn cng gn E2, kh nng gy nhiu ca n ti E2 cng ln.

Khi dng Sector Antenna:

Hnh 3-3-20 c Sector ha By gi ta cng vn dng E1 v E2. Nhng c sector ho thnh: EA1, EB1, EC v EA2, EB2, EC2. MS di chuyn v pha E2, xut pht t EA1 (c khong cch ln nht ti E2). Khi MS vt qua v tr trm EA1, n c chuyn giao ti EB1 v khong cch t MS ti E2 gn hn. EB1 cng tn s vi EB2 nhng nh a hnh ta thy, cc nhiu n to ra u nm pha sau anten ca EB2 (v l anten nh hng nn c

- 56 -

t s nng lng hng trc trn hng sau = 6 15 dB). iu ny c ngha l kh nng chng nhiu ca h thng tng t 6 15 dB. Tng t nh vy khi MS i ti EA2 n ch to nhiu cho EA1 t pha sau ca anten EA1. Tm li dng sector anten l mt bin php lm tng t s C/ I ca h thng.

3.3.5.2 tng ch anten (Gain of an Antenna) tng ch ca mt anten l t s, thng tnh bng dB, gia cng sut cn thit ti u vo ca mt anten chun khng suy hao vi cng sut cung cp u vo ca anten sao cho mt hng cho trc to ra cng trng hay mt thng lng cng sut nh nhau ti cng mt c ly. Nu khng c ghi ch g thm, th tng ch anten c tnh i vi hng pht x ln nht. Ty thuc vo s la chn vo anten chun, c cc loi tng ch anten sau: 1. Tng ch tuyt i hay tng ch ng hng (Gi) khi anten chun l mt anten ng hng bit lp trong khng gian. 2. tng ch ng vi mt dipol na bc sng (Gd) khi anten chun l mt dipol na bc sng bit lp trong khng gian v mt phng vung gc ca n cha hng pht x. 3. tng ch ng vi mt anten thng ng ngn (Gv) khi anten chun l mt dy dn thng ngn hn nhiu so vi mt phn t bc sng, vung gc vi mt phng dn in l tng cha hng pht x. 3.3.5.3 Cng sut bc x ng hng tng ng - EIRP Cng sut bc x ng hng tng ng EIRP (Equivalent Isotropic Radiated Power) l tch s ca cng sut sinh ra cung cp cho mt anten vi tng ch ca anten hng nht nh ng vi anten ng hng ( tng ch ng hng hay tng ch tuyt i) EIRP c xc nh bi cng thc: PEIRP (W) = Pt (W)* 10(G - L)/10 Hay
PEIRP ( dB ) = Pt ( dB ) L + G

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Trong :
- PEIRP (dBm): cng sut bc x ng hng tng ng; - Pt (dBm): tng cng sut ca cc my pht; - L (dB): tng suy hao t cc my pht n anten (v d do combiner,

feeder);
- G (dBi): tng ch cc i ca anten tng ng vi anten ng hng.

3.3.5.4 cao v gc nghing (down tilt) ca anten: Khi anten t thng ng, hng bp sng chnh s nm trn mt ng thng nm ngang.

Hnh 3-3-21 Anten v hng c gc nghing bng 0 nhng khu vc th trn nh hay nng thn, lu lng ca h thng thp nn vic ti s dng tn s l khng cn thit. Do vy, ta nn s dng cc v tr cao hay t anten cao ti a ho vng ph sng. Tuy nhin nhng khu vc th ln, lu lng h thng cao, kch thc cell hp th c l thch hp nht l gim cao anten c lm gim can nhiu knh chung. Tuy nhin, nu t qu thp, cc vt cn (nh cao tng...) s c nh hng ln ti cht lng h thng. Do vy, hin nay cao anten cc th thng l 30 50 m. gii quyt phm vi vng ph sng hp, mt k thut c a ra l lm nghing hng bp sng chnh ca anten (down tilt). thy r hiu qu ca downtilt i vi cht lng h thng ta xt minh ha sau:

- 58 -

Chng ta bit rng cng sut bc x ca anten cng gim khi cng ri xa bp sng chnh. th thc nghim sau y (c xy sng t c tnh bc x ca anten trong mt phng ng) ch r quan h . th ny s dng cho 3 loi anten c rng bp sng trong mt phng ng ln lt l 70 , 140 , 280 . Trong : Trc X biu din gc l gc gia hng ta ang xt v hng bc x chnh trn mt phng ng (Vertical Angle Degree 0C). Trc Y biu din s suy hao cng trng (Gain Reduction - dB).

Hnh 3-3-22 th quan h gia gc thng ng v suy hao cng trng Gi thit c hai cell A v B s dng cng tn s. Bn knh mi cell r = 500m, khong cch hai cell l d = 4 km, cao anten l h = 30 m, rng bp sng l 70 .

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Hnh 3-3-23 V d v hiu qu ca downtilt S dng th thc nghim ta tnh c: Suy hao tn hiu nhiu ca cell A gy ra ti cell B:
= arctg ( ) = arctg (
h d 30 ) = 0,43 0 4000

T th thc nghim ta c Gain Reduction = 0,2 (dB). Suy hao tn hiu ca anten A ti bin ca cell A:
= arctg ( ) = arctg (
h r 30 ) = 3,44 0 500

Nn Gain Reduction = 4 (dB). By gi ta nghing gc ca anten A i mt gc 4,930, khi hng bp sng chnh lch i gc 4,930 . Lc ny: Suy hao tn hiu nhiu ca cell A gy ra ti cell B: = dt - = 4,930 0,430 = 4,50 Nn Gain Reduction = 6,2 (dB). Suy hao tn hiu ca anten A ti bin ca cell A: = dt = 4,930 3,440 = 1,490 Nn Gain Reduction = 0,5 (dB). Nh vy ta thy, tn hiu nhiu do cell A gy ra cho cell B lc ny b suy hao ng k (suy hao thm 6 dB), ng thi suy hao tn hiu trong cell A gim ng k ngha l cht lng ph sng cell A c ci thin.

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Qua th d trn ta thy, vi vic nghing gc ca anten th cht lng ph sng ca c hai cell A v B u c ci thin. Va lm cht lng thu cell A tng ln, va lm gim nhiu do cell A gy ra cho cell B. Vic nghing gc anten c th dng gii quyt vn ph sng. Tuy nhin vic p dng nghing gc anten cn c s phn tch k cng nhng yu t lin quan c th xy ra trong vng ph sng. 3.3.6 Mt tr ngi trong vic pht trin mng thng tin di ng t bo l vn pht sinh khi mt thu bao di ng di chuyn t cell ny sang cell khc. Cc khu vc k nhau trong h thng t bo s dng cc knh v tuyn c tn s khc nhau, khi thu bao di ng di chuyn t cell ny sang cell khc th cuc gi hoc b rt hoc t ng chuyn t knh v tuyn ny sang mt knh khc thuc cell khc. Thay v cuc gi b rt, qu trnh Handover (ting M: Handoff) gip cho cuc gi c lin tc. Qu trnh Handover xy ra khi h thng thng tin di ng t ng chuyn cuc gi t knh v tuyn ny sang knh v tuyn khc khi thu bao di ng di chuyn t cell ny sang cell khc lin k vi n. Trong qu trnh m thoi, hai thu bao cng chim mt knh thoi. Khi mt thu bao di ng chuyn ng ra khi vng ph sng ca cell cho trc, tn hiu u thu ca cell ny s gim. Khi , cell ang s dng s yu cu mt Handover (chuyn giao) n h thng. H thng s chuyn mch cuc gi n mt cell c tn s vi cng tn hiu thu mnh hn m khng lm gin on cuc gi hay gi cnh bo n ngi s dng. Cuc gi s c tip tc m ngi s dng khng nhn thy qu trnh Handover din ra. 3.3.6.1 Phn loi Handover H thng phn loi cc qu trnh chuyn giao cuc gi thnh nhng loi sau: Intra-cell Hand Over. Inter-cell Hand Over. Intra-MSC Hand Over. Inter-MSC Hand Over.

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Intra-cell Hand Over (Chuyn giao trong ni b t bo): Th tc chuyn giao thc hin gia hai knh vt l ca cell ang phc v. Intra-cell Hand Over khng c s dng khi thu bao di chuyn sang cell khc, ngoi tr trng hp nu mc nhiu trn knh ring l cao th mt s chuyn giao sang mt knh vt l khc phi c thc hin.

Hnh 3-3-24 Intra-cell Handover Inter-cell Hand Over (Chuyn giao lin t bo): MS c chuyn mch sang mt knh v tuyn mi ca mt cell khc nhng c iu khin ca cng mt b iu khin trm gc BSC.

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Hnh 3-3-25 Inter-cell Handover Intra-MSC Hand Over (Chuyn giao trong ni b MSC): Chuyn mch knh v tuyn gia hai BSC ca cng mt tng i di ng MSC.

Hnh 3-3-26 Intra-MSC Handover Inter-MSC Hand Over (Chuyn giao lin MSC): Chuyn mch knh v tuyn gia hai tng i di ng MSC.

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Hnh 3-3-27 Inter-MSC Handover Trong trng hp ny, MSC ban u gi ton quyn iu khin cuc gi v sp t truy nhp mng cho n khi kt thc cuc gi. Cuc gi c nh tuyn vt l li t MSC ban u trc tip n MSC ch.

3.3.6.2 Khi to th tc Handover Th tc Handover s c khi to vo bt k lc no mi khi xut hin nhu cu chuyn i ng v tuyn gia BSS v MS sang mt knh mi, c th l cng BSS hay mt BSS khc. BSS s nhn thy yu cu thit lp th tc Handover duy tr s lin tc ca cuc gi trong khi MS di chuyn. Nu cell ch u tin nm trong cng BSC th th tc Handover s c thc hin bi BSC. Nu khng, BSC s gi n MSC mt danh sch u tin cc cell ch. Trong danh sch ny, MSC ch cn nhc n ba cell u tin. Gii thut MSC (vi v d l c ba cell c cn nhc) l bc th nht s th vi cell u tin, nu khng c th s chuyn sang th vi cell th hai ri th ba, m khng c vic tnh n nhng kha cnh lu lng. Nu nh trong khi th tc handover ny ang din ra m c thm mt yu cu Handover khc nhn c t BSC, danh sch s c cp nht tnh ton n cc cell mi. Vi mi ln th, MSC yu cu mt ti nguyn tn s thch hp ti phn h trm gc BSS ch m

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theo mc nh l khng cho php vic xp hng i, hoc l cho php hng i nu cho php nu nh c ch th tng ng trong yu cu handover nhn c t BSS ngun. Nu nh MS khng hon thnh vic t c mt knh mi t cell ch th n s gi mt bn tin thng bo chuyn giao tht bi thng qua cell trc ti mng li v MSC s hi phc li kt ni trc ca MS hoc nu MSC nhn thy MS mt kt ni th cuc gi s c gii phng. 3.3.6.3 Quy trnh chuyn giao cuc gi V c bn, th tc Handover bao gm 4 giai on: Giai on 1: BSC quyt nh thc hin th tc handover Giai on 2: Mt kt ni mi c thit lp, song song vi kt Giai on 3: MSC chuyn cuc gi sang kt ni mi. Giai on 4: Kt ni gc c gii phng. m bo kt ni ca cuc gi. ni gc.

Giai on 1:

Hnh 3-3-28 G 1: Trong lc kt ni, MS vn tip tc o c mc thu v cht lng truyn dn ca cell phc v v nhng cell xung quanh.

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Nhng kt qu o c c gi ti BSC v l cn c yu cu mt th tc Handover sang mt cell khc c mt cht lng truyn dn tt hn.

Hnh 3-3-29 Quyt nh chuyn giao_Handover Decision Handover Margin: Thc cht ca Handover Margin l s chnh lch gia mc thu ca cell ang phc v v cc cell ln cn. Khi mc thu ca mt cell ln cn no vt qu mc thu ca cell ang phc v mt khong ln hn gi tr Handover Margin nh sn th mt Handover Alarm s c gi v h thng nhm a n quyt nh chuyn giao. Thng thng th th tc Handover s c thc hin ngay sau . Nu vic t gi tr Handover Margin qu thp s dn ti vic Handover qu nhiu, nhng ngc li khi gi tr ny t qu ln c th lm cho cht lng cuc gi b gim xung. V vy, ty vo tnh cht ph sng ca tng vng cng nh mc nhiu ca tng cell phc v, Handover Margin cn c iu chnh thch hp t c cht lng tt nht. V d nh vng b nhiu nhiu c th t gi tr Handover Margin thp MS c th nhanh chng chuyn giao sang cell khc c cht lng tt hn.

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Gi tr khuyn ngh l 4 dB, trn mng VMS_Mobifone hin ang s dng Handover Margin l 5 dB. BSC s thng bo cho MSC v s cn thit thc hin th tc Handover, v khai bo thng tin vi MSC1.

Hnh 3-3-30 G 1: BSC khai bo thng tin vi MSC Giai on 2: MSC1 yu cu mt Handover Number (HON) t MSC2 v thng bo vi MSC2 thng tin v cell B.

Hnh 3-3-31 G 2: MSC1 yu cu MSC2 cp Handover Number

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M HON (Handover Number) ch quan trng trong trng hp Inter-MSC Handover. N c dng MSC1 thit lp kt ni knh lu lng vi MSC2. Cu trc ca m HON cng ging nh m MSRN v cng c cung cp bi VLR mi. MSC2 yu cu VLR cung cp mt m HON, ng thi yu cu BSC cung cp knh v tuyn. Sau , knh v tuyn v m HON c gi li cho MSC1.

Hnh 3-3-32 G 2: Cp m HON v knh v tuyn cho MSC1 Giai on 3:

Hnh 3-3-33 G 3: MSC1 chuyn mch kt ni cho MS trn knh lu lng thit lp vi MSC2

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Vi m HON, MSC1 c th thit lp knh lu lng kt ni vi MSC2. MSC1 thng bo cho MS v knh v tuyn m n phi chuyn mch ti.

Giai on 4: Gii phng kt ni vi BTS c.

Hnh 3-3-34 Kt ni vi BTS c c gii phng

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Chng IV :CC CH TIU CHT LNG H THNG 4.1 Cc ch tiu cht lng

4.1.1 Khi nim v cht lng dch v QOS QOS (Quality of Service) c th xem nh l nhng ch tiu nh gi mng li m bt c mt h thng thng tin di ng no u phi c. Ch tiu cht lng mng li y phi l nhng tiu ch thc s cht lng chng hn nh ting ni trong tro, t rt cuc gi v khng b nghn mch. nh gi c cht lng mng chng ta phi xc nh nhng i lng c trng (key indicators), qua cho php nhng ci nhn chnh xc v s hot ng ca mng li cng nh cht lng ca mng. 4.1.2 Cc i lng c trng 4.1.2.1 T l thit lp cuc gi thnh cng CSSR (Call Setup Successful Rate) C th nh ngha CSSR nh l t l m ngi s dng (thu bao) thnh cng trong vic bt u thc hin cuc gi xt trn c hai chiu gi i v gi n (lu l nhng cuc gi c ni nhng b rt trong trng hp ny vn c coi l thnh cng). Thnh cng y ta c th tm coi l khi ngi s dng quay s v bm YES, cuc gi chc chn c ni (trng hp gi i). Trong trng hp gi n, s khng thnh cng c th hiu n gin l mt ai thc s gi n thu bao nhng thu bao vn khng nhn c mt tn hiu bo gi no mc d anh ta vn bt my v nm trong vng ph sng. CSSR c th c tnh nh sau: CSSR = Tng s ln thc hin (nhn) thnh cng cuc gi / Tng s ln thc hin (nhn) cuc gi Theo khuyn ngh Alcatel v ch tiu cht lng h thng th t l thit lp cuc gi thnh cng CSSR cn t l 92%. 4.1.2.2 T l rt cuc gi trung bnh (Average Drop Call Rate - AVDR) AVDR l t l s cuc gi b rt mch trn tng s cuc gi thnh cng. AVDR c th c tnh nh sau:

- 70 -

AVDR = Tng s ln rt mch / Tng s ln chim mch TCH thnh cng ngoi tr trng hp Handover (AVDR = Total drops/ Total TCH seizures excluding TCH seizures due to HO) i lng ny nn s dng nh gi cht lng ton mng, ch khng nn p dng cho tng cell ring l v rng mi cell khng ch mang nhng cuc gi c bt u t n (trn c hai ngha gi i v gi n) m n cn phi chu trch nhim ti nhng cuc gi c handover t nhng cell khc sang - iu c ngha l n b chim mch nhiu hn rt nhiu ln. Hn na i vi mi cell, vic mang mt cuc gi do handover hay bnh thng l c cng mt bn cht. 4.1.2.3 T l rt mch trn TCH (TCH Drop Rate - TCDR) TCDR c th tm nh ngha l t l rt mch tnh trn cc knh TCH ca tng cell ring bit. TCDR= Tng s ln rt mch/ Tng s ln chim mch thnh cng (TCDR= Total TCH Drops/ Total TCH Seizures) Tng s ln chim mch y c th xut pht t bt c nguyn nhn no, k c Handover. C rt nhiu nguyn nhn gy nn rt mch, loi tr nguyn nhn do my di ng gy ra ta c th a ra nhng nguyn nhn chnh sau y: Do b nhiu qu nhiu hoc do cht lng knh truyn qu thp Do tn hiu qu yu Do li ca h thng chng hn nh phn cng trc trc Do s dng cc gi tr khng chun ca cc tham s BSS Do khng Handover c (thiu neighbour cell chng hn) Nhm d dng hn cho cng tc k thut, TCDR c phn ra lm hai i lng mi: Rt mch do li h thng: TCDR-S (Drop due to System): tham s ny bao gm tt c cc li do h thng chng hn nh software, transcoder ..c tnh theo t l phn trm trn tng s ln rt mch. Vi mt h thng tt, t l ny l rt nh (thng vo khong 2-5 % tng s ln rt mch).

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Rt mch do li tn s v tuyn RF : TCDR-R (Drop due to RF): tham s ny bao gm tt c cc li nh mc tn hiu km, cht lng qu km, qu nhiu, Handover km... cng c tnh theo t l phn trm trn tng s ln rt mch. TCDR-R + TCDR-S = 100% 4.1.2.4 T l nghn mch TCH (TCH Blocking Rate - TCBR) TCBR c nh ngha nh t l chim mch khng thnh cng do nghn knh thoi (khng c knh TCH ri) trn tng s ln h thng yu cu cung cp knh thoi. TCBR = Tng s ln b nghn / Tng s ln yu cu ng thng (TCBR = Total blocks / Total TCH attempts) T s ny phn nh mc nghn mch trn tng cell ring l hay trn ton h thng. Khi t s ny mt cell (hay khu vc) no tr nn qu cao iu c ngha l rt kh thc hin c cuc gi trong cell (hay khu vc) . Tuy nhin tham s ny khng phn nh mt cch chnh xc yu cu v lu lng trn mng v rng khi mt ngi no mun thc hin mt cuc gi trong vng mt pht chng hn, ngi ta s c nhiu ln c th ni c mt knh thoi v nh vy s th c th l rt nhiu ln (c th l hng chc) c th ch thc hin mt cuc gi duy nht ko di mt pht. iu ny lm tng t l nghn mch ln rt nhanh, vt qu c bn cht thc t ca vn . V vy nh gi mt cch chnh xc hn, ngi ta s dng mt i lng khc l cp phc v GoS (Grade of Service). i khi ta khng hiu ti sao m t l TCBR li rt cao mt s cell, trong trng hp ny cch tt nht l tham kho thm cc i lng Maxbusy v Congestion time cho cell Maxbusy: S knh ln nht b chim ti cng mt thi im Congestion time: Tng s thi gian m ton b s knh b chim ht (Tng s thi gian nghn).

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Lu lng v Grade of Service (GOS): Lu lng mang bi h thng trong khong thi gian t c nh ngha nh sau: C = n*T/t Trong T l thi gian m thoi trung bnh n s cuc gi trong khong thi gian t. n v ca lu lng c tnh bng Erlang (E), nu nh thay t=3600, ta c Eh (Erlang gi). Mt cch hon ton n gin, ta c th tnh lu lng nh sau: C = Tng thi gian chim mch/ Thi gian o Lu lng ca h thng cng phn no y cho thy s hot ng ca mng. Nu nh lu lng ca mt cell no y gim i mt cch bt bnh thng, iu c ngha l hoc vng ph sng ca cell b thu hp li (do tt cng sut hay anten hng) hoc mt nhm thu pht no y ca cell khng hot ng. Lu lng ca h thng c mt tng quan tng i i vi t l nghn TCH (TCBR) trnh by trn, khi lu lng tng vt mt gi tr no (tu thuc vo dung lng ca cell) th t l TCBR cng tng ln rt nhanh theo n. Tuy nhin trong mt s trng hp, ngay c khi c lu lng rt thp, t l TCBR vn rt cao. Khi khng c mt cch l gii no tt hn l mt s khe thi gian timeslot trn cell khng hot ng. Gi bn ca h thng BH (busy hour) c tnh nh l gi m lu lung i qua h thng l ln nht. V do khi thit k mt h thng no , nhm tho mn yu cu v lu lng mt cch tt nht ngi ta thng s dng cc s liu thng k cho gi bn. Trong mt h thng vi mt s hu hn knh thoi v mi thu bao chim mch ht mt thi gian trung bnh T no y, ta thy ngay rng khi s thu bao tng ln hay ni cch khc khi m lu lng tng ln th xc sut b nghn mch cng tng ln v khi lu lng tng ln n mt mc no th tnh trng nghn mch khng th chp nhn c na. Vy lm sao c th nh gi mc nghn mch

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ny mt cch chnh xc? Ngi ta s dng mt i lng l cp phc v GOS thc hin iu . GOS c th c nh ngha nh l xc sut b nghn mch cho mt thu bao khi thc hin cuc gi trong mt khu vc c mt lu lng yu cu (offerred traffic) xc nh no . Vn ny sinh ra l lu lng yu cu y l g? N c th c coi nh l lu lng m h thng c th mang c trong gi bn trong trng hp khng c nghn mch hay ni cch khc i l khi s knh thoi ca h thng tng ln. Ngi ta c th tnh GOS cho mt h thng vi t - knh v A - lu lng yu cu nh sau: GOS (t,A) = (GOS (t-1,A)) GOS (0,A) = 1. Tuy nhin Lu lng yu cu l mt ci g c v khng thc, khng th cn o ong m c v ngi ta ch c th o c lu lng thc mang bi cc knh thoi m thi. V vy ngi ta tnh lu lng yu cu A nh sau: A = C*(1+GOS) Trong C - lu lng o c trn h thng Nhng vn li l lm sao tnh c GOS. tnh GOS u tin ngi ta gi s A= C, da vo cng thc (*) ta c th tnh c GOS1 no y, v khi : C1=A/(1+GOS1) Nu nh C1 va tnh c li nh hn C thc, ngi ta li tng A ln mt cht chng hn A= C + 0,00001, li tnh theo cch trn v c nh th cho ti khi Cn tnh c gn vi C thc nht. Khi gi tr tnh c GOSn chnh l gi tr ca GOS cn tm. Khi tnh c lu lng yu cu A, ta c th d dng xc nh s knh cn thit bng cch tra bng. 4.1.2.5 T l rt mch trn SDCCH (SDCCH Drop Rate - CCDR) CCDR c nh ngha nh l t l gia tng s ln rt mch trn knh SDCCH v tng s ln chim SDCCH thnh cng. CCDR = Tng s ln rt trn SDCCH/ Tng s ln chim SDCCH thnh cng (CCDR = SDCCH drops / SDCCH seizures)
(*)

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CCDR cng rt quan trng, n mt phn nh gi t l thnh cng ca cuc gi ni chung. Ni chung trong thng tin di ng GSM v v mt kha cnh no , t nht l trn mt tn s v tuyn RF, CCDR v TCDR c cng bn cht, nu nh CCDR cao th t l TCDR cng cao v ngc li. V rng thi gian chim mch trn SDCCH l rt ngn (trung bnh khong 3s) so vi thi gian chim mch trn TCH (trung bnh khong 65 s) nn CCDR cng nh hn TCDR rt nhiu. Tuy nhin, khi CCDR tr nn ln mt cch khng bnh thng so snh vi TCDR, iu c ngha l c mt ci g khng n hoc l do cc tham s ca phn BSS hoc l do knh tn s c cha SDCCH qu nhiu. 4.1.2.6 Mt s i lng c trng khc Nhng i lng c trng di y tuy khng phn nh mt cch trc tip cht lng ca h thng nhng rt cn thit cho cng tc nh gi cht lng h thng. S knh hot ng (Available Channels) y l mt ch tiu rt quan trng cho nhng ngi theo di hot ng ca mng li. Thng thng i vi mi cell trong mt h thng GSM, s knh ny (trong trng hp bnh thng) s l 6, 7, 14, 15, 22, 23, 30 tu thuc vo cu hnh ca cell. Tuy nhin khi theo di cc bo co v mng, i khi ta thy s knh ny l mt s khc nhng con s trn thm ch l mt s vi du phy km theo (v d 13,2) - iu ny c ngha l trong sut thi gian m ta quan st c mt lc no y mt s timeslots trn cell khng hot ng hoc l c ton b cell b s c. Vic mt s timeslot khng hot ng nh hng trc tip n t l handover thnh cng s cp n phn sau. T l thnh cng handover n (Incoming HO Successful Rate - IHOSR) IHOSR c nh ngha nh l t l gia s ln nhn handover thnh cng v tng s ln c yu cu chp nhn handover. IHOSR = Tng handover vo thnh cng / Tng handover vo (IHOSR = Incoming HO Success / Total Incoming HO request by BSS)

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IHOSR ca mt cell rt quan trng, n nh hng trc tip n cht lng ca khu vc c cha cell . Nu IHOSR l thp, n s lm tng t l rt mch nhng cell xung quanh n v thm ch lm nh hng n cht lng thoi ca cuc gi bi v nu mt ln handover khng thnh cng th cuc gi hoc s b rt hoc h thng s phi thc hin mt ln handover khc v mt ln nh th lung tn hiu thoi s b ct v lm cho ngi nghe cm gic bi t on trong m thoi. IHOSR cn phn nh c cht lng phn cng ca cell, chng hn sleeping TRXs trn cell. T l thnh cng handover ra (Outgoing HO Successful Rate - OHOSR) OHOSR c nh ngha nh l t l gia s ln handover ra thnh cng v tng s ln c yu cu handover. OHOSR = Tng handover thnh cng / Tng s ln quyt nh handover (OHOSR = HO Success / Total HO request by BSS ) EMPD EMPD c nh ngha nh l t s gia traffic tnh theo pht v tng s ln rt mch. EMPD = 60* traffic / Tng s cuc rt EMPD biu th s tng quan gia traffic v s rt mch, n phn nh mt cch r rng cht lng ca h thng v c th dng lm thc o chung cho cc h thng s dng cc thit b khc nhau v hot ng nhng khu vc c c th kinh t khc nhau.

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PH LC

Minh ha cng tc cng tc ti u ha ti VMS- Mobifone o kim tra Handover gia hai trm Cng vi cc cng c khc, my TEMS c s dng thng xuyn trong vic o v kim tra cht lng h thng. Di y gii thiu vic s dng my TEMS T68i ca Ericsson o kim tra handover t trm Trng nh sang trm i La.

Hnh 0-1 o kim tra Handover t trm Trng nh sang trm i La

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Vi my TEMS ta c th o c mc thu ca cell phc v v cc cell ln cn, cc thng s ca knh hin ti. Nh trn y ta thy cc thng s o c sau khi handover nh sau:

Mc thu ca cell phc v (lc ny l cell i La) v cc cell Cell Name i La Trng nh ARFCN BSIC RxLevel (dBm) 84 1-2 -53 103 98 85 102 104 2-3 1-5 -57 -73 -78 -79 -80

ln cn:

Cc thng s ca knh hin ti: CGI (MCC, MNC, LAC, CI) Bng tn BCCH ARFCN BSIC Timeslot 452 01 111 10991 900 84 1-2 6

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Hnh 0-2 Kt qu o Handover gia hai trm l tt T biu ph tn hiu trn ta thy trc thi im handover mc thu ca cell phc v gim xung thp hn so vi mc thu ca cell ln cn, ng thi t s tn hiu trn nhiu C/ I cng gim xung ch cn 12 dB, MS yu cu thit lp th tc Handover. Sau khi Handover: mc thu cell phc v l -53 dBm, t s C/ I c ci thin l 22dB. Kt lun: Kt qu Handover gia hai trm l tt.

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KT LUN *** n vin thng trnh by nhng nt c bn nht v mng thng tin di ng GSM, cng vi mt s cng tc ti u ha h thng c thc hin ti mng VMS_MobiFone. Ti u ho l mt cng vic kh khn v i hi ngi thc hin phi nm vng h thng, ngoi ra cng cn phi c nhng kinh nghim thc t v s tr gip ca nhiu phng tin hin i c th gim st v kim tra ri t mi a ra cc cng vic thc hin ti u ho. Em rt mong c c nhng kin nh gi, gp ca cc thy v cc bn n thm hon thin. H Ni, thng 02 nm 2012

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Ti liu tham kho [1] PTS.Nguyn Phm Anh Dng, Thng tin di ng GSM, Nh xut bn bu in, H Ni 1999. [2] V c Th, Tnh ton mng thng tin di ng s CELLULAR, Nh xut bn gio dc, H Ni 1999. [3] V c Th , Tnh ton mng di ng s cellular [4] http://www.wikipedia.org [5] http://www.tapchibcvt.gov.vn [6] http://www.google.com.vn