Đề tài 4G

ti nghin cu cng ngh 4GSV: L Qu Quang & Nguyn Thanh Nam

ti khoa hcNi dung nghin cu: Mng 4G Nhm sinh vin nghin cu: 1. L Qu Quang2.

Nguyn Thanh Nam

Ngi hng dn:Vu Duy Thun

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Li noi uTiu chun vin thng lin tc thay i v tin b cng ngh v oi hoi chung ta cho ni dung chuyn giao cho cc thit b cm tay ca moi ngi nhanh hn l mt ng lc ng sau nhng s pht trin. Ti thi im ny, bn c th nhn thc c rng mt phn ln in thoi di ng hin i v cc dch v bng thng rng di ng cho my tnh v cc ch s hu my tnh xch tay kt ni vi mng 3G. Tn ny cp n mt thc t rng iu ny c coi l th h th 3 ca cng ngh mng vin thng di ng, v ti thi im ny, mng 3G c kh nng tc ti t 3,6 Mbps v 7,2 Mbps. Tc ny s ph thuc vo cc nh cung cp mng m bn chn, mc ph sng 3G v kh nng ca nn tng in thoi di ng bn ang s dng nhn tn hiu. Tuy nhin, c nhiu cng ty ang lm vic trn vic to ra cc cng ngh 4G hoc th h th 4 ca mng di ng. Mc d ti thi im vit 4G khng phi l mt cng ngh thng nht tiu chun ha v c nhng cng ty khc nhau lm vic trn cc sn phm 4G s khng thc hin trong cch tng t hoc cng cp. Ti thi im ny cng ngh 4G ang tm kim tng t nh trong nhiu cch cc mng khng dy m bn c th c s dng ni lm vic ca bn hoc trong nh ring ca bn, ch c trn mt quy m ln hn nhiu v tch hp vo cc thit b di ng cng nh cc gii php my tnh bn. i vi ngi s dng in thoi di ng, n s cung cp mt kt ni bng thng rng di ng cc cuc gi thoi, cc phng tin truyn thng trc tuyn v truy cp internet s c lin tc bn tay. Vy cng ngh 4G la gi?

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Mc lc:Trang Li noi u.. 21. Chng 1: Lich s phat trin mang vin thng. 4 2. Chng 2: Cng ngh 4G 7 2.1. Muc tiu va phng phap tip cn 7 2.1.1. Muc tiu gia inh.. 7 2.1.2. Phng phap tip cn. 9 2.2. iu cn bit v 4G 10 3. Chng 3: 4G Cng ngh ca tng lai. 17

3.1. Tnh u vit ca mng 4G so vi mng 3G 17 3.2. Tai sao ban nn chon 4G?............................................................... 19 3.3. Cha th t chun 4G... 214. Chng 4: Tng lai cng ngh 4G ti Vit Nam. 22

4.1. Cc nh mng ang trin khai th nghim mng 4G 22 4.2. Nm 2018 mi c th cp php mng 4G. 23

oan kt 24

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Chng 1: Lich s phat trin mang vin thngNgc dng thi gian...1G(th h u tin). 1G l mt h thng tng t, v c pht trin trong nhng nm by mi, 1G c hai ci tin ln, y l s pht minh ra b vi x l, v k thut s chuyn i ca cc iu khin lin kt gia in thoi v trang web di ng. 1G tng t h thng truyn thng di ng nhn thy hai ci tin quan trng trong nhng nm 1970: pht minh ra b vi x l v s ha ca iu khin lin kt gia cc Mobilephone v trang web di ng. Nng cao h thng in thoi di ng (AMPS) ln u tin c a ra bi M v l mt in thoi di ng h thng 1G. Cn c vo FDMA, n cho php ngi dng thc hin cuc gi bng ging ni trong 1 quc gia. Trong hn mt thp k qua, th gii chng kin s thnh cng to ln ca mng thng tin di ng th h th hai 2G. Mng 2G c th phn ra 2 loi: mng 2G da trn nn TDMA(Time division multiple access) v mng 2G da trn nn CDMA(Code division multiple access). nh du im mc bt u ca mng 2G l s ra i ca mng D-AMPS (hay IS-136) dng TDMA ph bin M. Tip theo l mng CdmaOne (hay IS-95) dng CDMA ph bin chu M v mt phn ca chu , ri mng GSM dng TDMA, ra i u tin Chu u v hin c trin khai rng khp th gii. S thnh cng ca mng 2G l do dch v v tin ch m n mng li cho ngi dng, tiu biu l cht lng thoi v kh nng di ng.

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Hnh 1: S tm lc qu trnh pht trin ca mng thng tin di ng t bo Tip ni th h th 2, mng thng tin di ng th h th ba 3G v ang c trin khai nhiu ni trn th gii. Ci tin ni bt nht ca mng 3G so vi mng 2G l kh nng cung ng truyn thng gi tc cao nhm trin khai cc dch v truyn thng a phng tin. Mng 3G bao gm mng UMTS s dng k thut WCDMA, mng CDMA2000 s dng k thut CDMA v mng TD-SCDMA c pht trin bi Trung Quc. Gn y cng ngh WiMAX cng c thu nhn vo h hng 3G bn cnh cc cng ngh ni trn. Tuy nhin, cu chuyn thnh cng ca mng 2G rt kh lp li vi mng 3G. Mt trong nhng l do chnh l dch v m 3G mang li khng c mt bc nhy r rt so vi mng 2G. Mi gn y ngi ta mi quan tm ti vic tch hp MBMS (Multimedia broadcast and multicast service) v IMS (IP multimedia subsystem) cung ng cc dch v a phng tin. 3G, th h th ba H thng 3G ha hn dch v thng tin lin lc nhanh hn, dn n ging ni, fax v chuyn giao d liu Internet kh nng, mc ch ca 3G l cung cp cc dch v ny bt c lc no, bt c ni no trn khp th gii, vi chuyn vng lin mch gia cc tiu chun. ITU IMT-2000 l mt tiu chun ton cu cho 3G v m ra cnh ca mi to iu kin cho cc dch v sng to v ng dng v d, gii tr a phng tin, v cc dch v da trn v tr, cng nh mt ton b rt nhiu. Nm 2001, Nht Bn nhn thy mng 3G u tin ra mt.Cng ngh 3G h tr khong 144 Kbps, vi tc di chuyn cao, tc l trong mt chic xe. 384 Kbps ti a phng, v ln n 2Mbps cho cc trm c nh, tc l trong mt ta nh.Va gi y chung ta ang tin ti mt mang vin thng vi tc truyn tai d liu cao,vi tc cao thi no h tr xem video cht lng cao,m thanh trung thc hn va nhiu

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hn na.o la tng lai cua mang 4G mang lai.Vy khai nim 4G bt ngun t u? C nhiu nh ngha khc nhau v 4G, c nh ngha theo hng cng ngh, c nh ngha theo hng dch v. n gin nht, 4G l th h tip theo ca mng thng tin di ng khng dy. 4G l mt gii php vt ln nhng gii hn v nhng im yu ca mng 3G. Thc t, vo gia nm 2002, 4G l mt khung nhn thc tho lun nhng yu cu ca mt mng bng rng tc siu cao trong tng lai m cho php hi t vi mng hu tuyn c nh. 4G cn l hin th ca tng, hy vng ca nhng nh nghin cu cc trng i hc, cc vin, cc cng ty nh Motorola, Qualcomm, Nokia, Ericsson, Sun, HP, NTT DoCoMo v nhiu cng ty vin thng khc vi mong mun p ng cc dch v a phng tin m mng 3G khng th p ng c. Nht, nh cung cp mng NTT DoCoMo nh ngha 4G bng khi nim a phng tin di ng (mobile multimedia) vi kh nng kt ni mi lc, mi ni, kh nng di ng ton cu v dch v c th cho tng khch hng. NTT DoCoMo xem

4G nh l mt m rng ca mng thng tin di ng t bo 3G. Quan im ny c xem nh l mt quan im tuyn tnh trong mng 4G s c cu trc t bo c ci tin cung ng tc ln trn 100Mb/s. Vi cch nhn nhn ny th 4G s chnh l mng 3G LTE , UMB hay WiMAX 802.16m. Nhn chung y cng l khuynh hng ch o c chp nhn Trung Quc v Hn Quc. Gn y trn nhiu blog cng ngh a thng tin: In-Stat ni rng ITU s cng b trong 2008/2009, 4G chnh l LTE, UMB v IEEE 802.16m WiMAX. Bn cnh , mc d 4G l th h tip theo ca 3G, nhng tng lai khng hn ch gii hn nh l mt m rng ca mng t bo. V d chu u, 4G c xem nh l kh nng m bo cung cp dch v lin tc, khng b ngt khong vi kh nng kt ni vi nhiu loi hnh mng truy nhp v tuyn khc nhau v kh nng chn la mng v tuyn thch hp nht truyn ti dch v n ngi dng mt cch ti u nht. Quan im ny c xem nh l quan im lin i. Do , khi nim ABC-Always Best Connected (lun c kt ni tt nht) lun c xem l mt c tnh hng u ca mng thng tin di ng 4G. nh ngha ny c nhiu cng ty vin thng ln v nhiu nh nghin cu, nh t vn vin thng chp nhn nht hin nay.

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D theo quan im no, tt c u k vng l mng thng tin di ng th h th t 4G s ni ln vo khong 2010-2015 nh l mt mng v tuyn bng rng tc siu cao.

Chng 2: Cng ngh 4G chng 1 chng ti khi qut v lch s pht trin ca mng vin thng cn chng 2 ny chng ti s i su vo vn mng vin thng 4G trong tng lai khng xa v c pht trin nhiu nc trn th gii.Vy mng 4G c nhng im g mi? Mc tiu ca mng 4G hng ti?

2.1. Muc tiu va phng phap tip cn.2.1.1. Muc tiu gia inh.*Trc y h thng 4G c hinh dung ban u cua C quan d an nghin cu phong v (DARPA) DARPA la chon kin truc phn phi va cui cung kt thuc gioa thc internet (IP),va tin rng giai oan u trong peer to- peer mang,trong o mi thit bi di ng se la ca hai b thu phat va mt b inh tuyn cho cac thit bi khac trong mang,loai bo nhng yu kem spoke-va-trung tm cua 2G va cac h thng di ng 3G.K t khi h thng 2.5G GPRS,h thng di ng cung cp c s ha tng kep:chuyn mach goi cac nut cho cac dich vu d liu,va chuyn mach cac nut cho cac cuc goi bng giong noi. Trong cc h thng 4G, c s h tng chuyn mch knh b b ri v ch c mt mng chuyn mch gi c cung cp, trong khi h thng 2.5G v 3G yu cu c hai chuyn mch gi v chuyn mch nt mng , tc l hai c s h tng song song. iu ny c ngha rng trong 4G, cc cuc gi thoi truyn thng c thay th bng in thoi IP. H thng di ng nh 4G cho php di ng lin mch, do chuyn tp tin khng b gin on trong trng hp mt thit b u cui di chuyn t mt t bo (mt trm c s vng ph sng) khc, nhng bn giao c thc hin. Thit b u cui cng gi cng mt a ch IP trong khi di chuyn, c ngha l mt my ch in thoi di ng c th truy cp min l n nm trong vng ph sng ca bt k my7


ch. Trong h thng 4G di ng ny c cung cp bi giao thc IP di ng , mt phn ca phin bn IP 6, trong khi trc cc th h di ng ch c cung cp bi lp vt l v giao thc lp datalink. Ngoi di ng thng sut, 4G cung cp kh nng tng tc linh hot ca cc loi mng khng dy hin c, chng hn nh v tinh, di ng khng dy, WLAN, PAN v h thng truy cp vo mng c nh khng dy. Trong khi duy tr tnh di ng lin mch, 4G s cung cp d liu t l rt cao vi k vng ca 100 dch v khng dy Mbit / s. Tng bng thng v tc d liu truyn dn cao hn s cho php ngi dng 4G kh nng s dng video nt cao v cc tnh nng hi ngh truyn hnh ca cc thit b di ng gn lin vi mt mng 4G. Cc h thng khng dy 4G c d kin s cung cp mt gii php IP ton din cc ng dng a phng tin v dch v c th c giao cho ngi s dng trn mt c s bt c lc no, bt c ni no, vi mt t l tha ng d liu cao, cht lng cao v bo mt cao. 4G c m t nh MAGIC: in thoi di ng a phng tin, bt c lc no bt c ni no, h tr tnh di ng ton cu, gii php khng dy tch hp, v cc dch v c nhn ty chnh.Mt s tnh nng chnh (ch yu l t cc im ca ngi s dng) ca cc mng di ng 4G l: Cao kh nng s dng: bt c lc no, bt c ni no, v vi bt k cng ngh H tr cho cc dch v a phng tin vi chi ph truyn dn thp C nhn Dch v tch hp

*Hin nay 4G ang c pht trin ph hp vi cht lng dch v (QoS) v cc yu cu t l do vic pht trin ca cc ng dng 3G nh truy cp bng thng rng di ng , Dch v nhn tin a phng tin (MMS), video chat , truyn hnh di ng , m cn cc dch v mi nh HDTV . 4G c th cho php roaming vi cc mng khng dy ti a phng, v c th tng tc vi video k thut s pht sng h thng.

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Trong vn hc, cc yu cu ca 4G gi nh hoc d kin s c thay i trong nhng nm trc khi IMT-Advanced c quy nh bi ITU-R. y l nhng v d v cc mc tiu c nu trong nhiu ngun khc nhau: Mt tc d liu danh ngha ca 100 Mbit / s trong khi khch hng v th cht di chuyn tc cao tng i so vi nh ga, v 1 Gbit / s trong khi khch hng v ch cc v tr tng i c nh theo quy nh ca ITU-R .Mt tc d liu ca t nht 100 Mbit / s gia hai im bt k trong th gii .Smooth bn giao qua mng khng ng nht .Kt ni lin mch v chuyn vng ton cu trn nhiu mng.Cao cht lng dch v cho th h tip theo h tr a phng tin (m thanh, thi gian thc d liu tc cao, ni dung video HDTV, truyn hnh di ng, ...) .Kh nng tng tc vi cc tiu chun khng dy.Mt IP, chuyn mch gi mng.IP da trn femtocell (nh cc nt kt ni vi c s h tng Internet bng thng rng c nh).

2.1.2. Phng phap tip cn.Tng vt l k thut chuyn dn nh sau:

MIMO: t c cc cao hiu qu quang ph bng cc phng tin ch bin khng gian bao gm nhiu ng-ten v nhiu ngi s dng MIMO Tn s-min-cn bng, v d iu ch a sng mang ( OFDM ) trong ng xung tu sn bay duy nht tn s min cn bng(SC-FDE) ng ln: Khai thc ti sn knh chn lc tn s m khng c cn bng phc tp Ghp knh thng k tn s-min, v d nh (OFDMA ) hoc (n sng mang FDMA) (SC-FDMA, hay cn gi l tuyn tnh precoded OFDMA, LP-OFDMA) trong ng ln: t l bit bin bng cch ch nh knh ph khc nhau cho ngi s dng khc nhau da trn knh iu kin Turbo nguyn tc iu chnh li m: gim thiu cc yu cu SNR pha tip nhn Ph thuc vo knh lp k hoch : s dng cc knh thi gian khc nhau Lin kt thch ng: thch ng iu ch v m sa li Qu trnh chuyn tip, bao gm c chuyn tip cc mng c nh (FRNs), v chuyn tip cc khai nim hp tc x , c gi l giao thc ch a

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2.2. iu cn bit v 4G* 3 Cng ngh tin 4G va c s cua 4GLTE (Long-Term Evolution) T chc chun ha cng ngh mng TTD t bo th h th ba 3G UMTS 3GPP (3rd Generation Partnership Project) bao gm cc t chc chun ha ca cc nc chu , chu u v Bc M bt u chun ha th h tip theo ca mng di ng 3G l LTE. LTE c xy dng trn nn cng ngh GSM (Global System for Mobile Communications), v th n d dng thay th v trin khai cho nhiu nh cung cp dch v. Nhng khc vi GSM, LTE s dng phng thc ghp knh phn chia theo tn s trc giao (OFDM) - truyn d liu tc cao bng cch phn chia thnh cc sng mang con trc giao. LTE s dng ph tn mt cch thch hp v mm do, n c th hot ng bng tn c rng t 1,25 MHz cho ti 20 MHz. Tc truyn d liu ln nht (v l thuyt) ca LTE c th t ti 250 Mb/s khi rng bng tn l 20 MHz. LTE khc vi cc cng ngh tin 4G khc nh WiMAX II ch, n ch s dng OFDM hng ln, cn hng xung n s dng a truy nhp phn chia theo tn s n sng mang nng cao hiu qu trong vic iu khin cng sut v nng cao thi gian s dng pin cho thit b u cui ca khch hng. UMB (Ultra Mobile Broadband) T chc chun ha cng ngh mng TTD th h th ba CDMA2000 3GPP2 (3rd Generation Partnership Project 2) c thnh lp v pht trin bi cc t chc vin thng ca Nht Bn, Trung Quc, Bc M v Hn Quc cng vi cc hng nh Alcatel-Lucent, Apple, Motorola, NEC v Verizon Wireless. Thnh vin ca 3GPP2, Qualcomm l hng i u trong n lc pht trin UMB, mc d hng ny cng ch tm c vo vic pht trin LTE. UMB da trn CDMA (Code Division Multiple Access) c th hot ng bng tn c rng t 1,25 MHz n 20 MHz v lm vic nhiu di tn s. UMB c xut vi tc truyn d liu ln ti 288 Mb/s cho lung xung v 75 Mb/s cho lung ln vi rng bng tn s dng l 20 MHz. Cng ngh ny s cung cp kt ni thng qua cc sng mang da trn a truy nhp phn chia theo m CDMA. IEEE 802.16m (WiMAX II)

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IEEE 802.16 l mt chui cc chun do IEEE pht trin, chng h tr c c nh (IEEE 802.16-2004) v di ng (IEEE 802.16e-2005). IEEE 802.16m (hay cn gi l WiMAX II) c pht trin t chun IEEE 802.16e, l cng ngh duy nht trong cc cng ngh tin 4G c xy dng hon ton da trn cng ngh a truy nhp phn chia theo tn s trc giao OFDMA (k thut a truy cp vo knh truyn OFDM). Cng ngh WiMAX II s h tr tc truyn d liu ln ti 100 Mb/s cho cc ng dng di ng v c th ln ti 1Gb/s cho cc ngi dng tnh. Khong cch truyn ca WiMAX II l khong 2 km mi trng thnh th v khong 10 km cho cc khu vc nng thn. Hng Intel ang dn u v xut s dng v pht trin WiMAX II cho h thng 4G, mt chin lc m cc Hng Alcatel-Lucent, AT&T, Motorola, Nokia, Samsung, Sprint Nextel v cc thnh vin khc ca WiMAX Forum cng h tr tch cc.

*Multiplexing v truy cp cc chng trnhTiu chun 4G kt hp cc yu t ca nhiu cng ngh sm v thng bn s c v cc gii php s dng M (mt cypher), tn s hoc thi gian lm c s ghp knh quang ph hiu qu hn. Trong khi Spectrum c coi l hu hn, Cooper ca Lut ch ra rng chng ti pht trin cch thc hiu qu hn bng cch s dng quang ph cng nh php lut ca Moore cho thy kh nng ca chng ti tng ch bin. Khi tiu chun khng dy pht trin, cc k thut truy cp s dng cng c biu hin gia tng nng lc, hiu qu v kh nng m rng. Cc tiu chun th h u tin khng dy s dng TDMA v FDMA . Trong cc knh khng dy, TDMA c chng minh l km hiu qu hn trong vic x l cc knh d liu tc cao, v n i hi thi gian bo v ln lm gim bt tc ng a ng. Tng t nh vy, FDMA tiu th bng thng nhiu hn cho bo v trnh s can thip nh cung cp dch v lin. V vy, trong h thng th h th hai, mt b tiu chun s dng s kt hp ca FDMA v TDMA v cc thit lp khc gii thiu mt chng trnh truy cp c gi l CDMA . Cch s dng CDMA tng nng lc h thng, nhng nh l mt nhc im l thuyt t mt gii hn mm v n hn l gii hn cng (tc l thit lp mng CDMA khng vn t chi khch hng mi khi n n gn gii hn ca n, kt qu trong mt t chi dch v cho tt c cc khch hng khi qu ti mng, mc d kt qu ny c trnh trong vic trin khai thc t bng11


cch kim sot nhp hc ca mch chuyn mch hoc cc dch v bitrate thng tin lin lc c nh). Tc d liu cng tng ln nh chng trnh ny truy cp (cung cp mng khng n c kh nng ca mnh) l hiu qu x l cc knh a ng.iu ny cho php h thng th h th ba, chng hn nh IS2000 , UMTS , HSXPA , 1xEV-DO , TD-CDMA v TD-SCDMA , s dng CDMA l chng trnh truy cp. Tuy nhin, vn vi CDMA l n b t tnh linh hot ph ngho v tnh ton chuyn su cn bng min thi gian (s lng cao ca php nhn trong mt giy) cc knh bng rng. Gn y, chng trnh truy cp mi nh FDMA trc giao (OFDMA), Single Carrier FDMA (SC-FDMA), Interleaved FDMA v CDMA a sng mang (MC-CDMA) c tm quan trng nhiu hn cho cc h thng th h tip theo.y l da trn thut ton FFT hiu qu v cn bng min tn s, dn n mt s thp hn ca php nhn trong mt giy. H cng lm cho n c th kim sot bng thng v to thnh quang ph mt cch linh hot. Tuy nhin, h yu cu phn b knh nng ng tin tin v lp k hoch thch ng giao thng. WiMax c s dng OFDMA trong ng xung v ng ln. i vi cc th h k tip UMTS , OFDMA c s dng cho ng xung.Ngc li, IFDMA ang c xem xt cho cc ng ln OFDMA ng gp nhiu hn na cc PAPR cc vn lin quan v kt qu hot ng phi tuyn ca b khuch i. IFDMA cung cp bin ng t in nng hn v do trnh cc vn khuch i. Tng t nh vy, MC-CDMA l trong ngh cho cc tiu chun IEEE 802,20 . Cc chng trnh truy cp cung cp cc hiu qu tng t nh cng ngh c nh CDMA. Ngoi ra, kh nng m rng v tc d liu cao hn c th t c. Cc li th quan trng khc ca cc k thut truy cp cp trn l h yu cu phc tp hn cho cn bng ngi nhn. y l mt li th gia tng, c bit l trong cc mi trng MIMO k t khi truyn dn ghp knh khng gian MIMO h thng vn i hi phi cn bng phc tp cao ngi nhn. Ngoi nhng ci tin trong cc h thng ghp knh, ci thin iu ch k thut ang c s dng. Trong khi , tiu chun trc phn ln c s dng giai on chuyn keying , h thng hiu qu hn nh 64 QAM ang c ngh s dng vi Tin ha di hn tiu chun 3GPP.

*H tr IPv6.

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Khng ging nh 3G, m l da trn hai c s h tng song song bao gm cc mch chuyn mch gi v chuyn sang cc nt mng tng ng, 4G s c da trn chuyn mch gi ch. iu ny s i hi phi truyn d liu c tr thp . Bi thi gian m 4G c trin khai, qu trnh cn kit a ch IPv4 c d kin s l trong giai on cui cng ca n. V vy, trong bi cnh ca 4G, h tr IPv6 l iu cn thit h tr mt s lng ln cc thit b khng dy cho php. Bng cch tng s lng a ch IP , IPv6 loi b s cn thit cho dch a ch mng (NAT), mt phng php chia s mt s a ch gii hn trong mt nhm ln hn ca cc thit b, mc d NAT vn s c yu cu giao tip vi cc thit b trn hin c IPv4 mng. Tnh n thng 6 nm 2009 , Verizon ng chi tit k thut yu cu bt k thit b 4G trn mng ca mnh h tr IPv6.

*Nng cao h thng ng-tenHiu sut ca i pht thanh truyn thng ph thuc vo mt h thng ngten, gi l ng-ten thng minh hoc thng minh . Gn y, nhiu cng ngh ngten ang ni ln t c cc mc tiu ca cc h thng 4G nh tc cao, tin cy cao, v cc thng tin lin lc tm xa. Trong u nhng nm 1990, phc v cho nhu cu tc tng trng d liu ca d liu thng tin lin lc, nhiu chng trnh truyn ti c xut.Mt cng ngh ghp knh khng gian , t c tm quan trng cho vic bo tn bng thng ca n v hiu qu nng lng. Ghp knh khng gian lin quan n vic trin khai nhiu ng-ten ti my pht v ngi nhn.Sui c lp sau c th c truyn ng thi t tt c cc ng-ten.Cng ngh ny, c gi l MIMO (nh l mt chi nhnh ca ng-ten thng minh ), nhn tc d liu c bn (nh hn) s lng cc ng-ten truyn ti hoc s nhn c ng-ten. Ngoi ra, tin cy trong truyn d liu tc cao trong cc knh fading c th c ci thin bng cch s dng ng-ten ti my pht hoc ngi nhn. iu ny c gi l truyn ti hocnhn s a dng. C hai truyn / nhn s a dng v truyn ghp knh khng gian c phn loi vo thi gian khng gian k thut m ha, m khng nht thit i hi nhng kin thc knh ti my pht. Cc th loi khc l nhiu cng ngh ng-ten vng kn, yu cu kin thc knh ti my pht.

*Software-Defined Radio (SDR)

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c tnh c k vng nht ca mng 4G l cung cp kh nng kt ni ABC, mi lc, mi ni. tha mn c iu , mng 4G s l mng hn tp (bao gm nhiu cng ngh mng khc nhau), kt ni, tch h p nhau trn nn ton IP. Thit b di ng ca 4G s l a cng ngh (multi-technology), a mt (multi-mode) c th kt ni v i nhiu loi mng truy nhp khc nhau. Mun vy, thit b di ng s s dng gii php SDR (Software Defined Radio) c th t cu hnh nhiu loi raio khc nhau thng qua mt phn cng raio duy nht. SDR l mt trong nhng hnh thc ca kin trc khng dy m (OWA). K t khi 4G l mt tp hp cc tiu chun khng dy, hnh thc cui cng ca mt thit b 4G s c coi l tiu chun khc nhau. iu ny c th c thc hin hiu qu bng cch s dng cng ngh SDR c phn loi khu vc ca hi t pht thanh.Lch s ca 4G v cng ngh tin-4G

Tnh n thng 12 nm 2011, c mng 4G thc hin y cc tiu chun ca Lin minh Vin thng quc t c th t c 1Gbit / s trong khi vn phng phm Tuy nhin, trong thng 12 nm 2010, ITU cng nhn rng cc phin bn hin ti ca LTE, WiMax v pht trin cng ngh 3G m khng thc hin y "IMTAdvanced yu cu tuy nhin c th c coi l" 4G ", min l h i din tin thn IMT-Advanced v" ng k mc ci thin hiu sut v kh nng i vi cc h thng th h th ba ban u trin khai ". Trong nm 2002, tm nhn chin lc cho 4G m ITU ch nh l IMTAdvanced c t ra. Nm 2005, cng ngh truyn dn OFDMA c chn l ng c vin cho HSOPA ng xung, sau i tn thnh 3GPP Long Term Evolution (LTE) khng kh giao din E-UTRA. Trong thng 11 nm 2005, KT chng minh dch v WiMAX di ng ti Busan, Hn Quc. Trong thng 4 nm 2006, KT bt u u tin ca dch v WiMAX di ng thng mi th gii ti Seoul, Hn Quc. [ Trong gia nm 2006, Sprint Nextel cng b s u t 5 t USD trong WiMAX buildout cng ngh trong nhng nm tip theo (5760000000 $ trong iu kin thc t). K t , Sprint phi i mt vi tht bi nhiu, dn n thit hi14


dc qu. Ngy 07 thng 5 nm 2008, Sprint, Imagine,Google ,Intel ,Comcast, Bright House , v Time Warnercng b mt tng hp ca mt trung bnh 120 MHz ca quang ph, Sprint sp nhp ca n phn chia Xohm WiMAX vi Clearwire to thnh mt cng ty s t tn "Clear". Trong thng 2 nm 2007, cc cng ty Nht Bn NTT DoCoMo th nghim mt mu th nghim 4G h thng thng tin lin lc vi 4x4MIMO gi l VSFOFCDM ti 100 Mbit / s trong khi di chuyn, v 1 Gbit/ s trong khi vn phng phm. NTT DoCoMo hon thnh mt th nghim m h t mt t l gi tin truyn ti ti a khong 5 Gbit / s trong ng xung vi 12x12 MIMO s dng mt bng thng 100 MHz tn s trong khi di chuyn 10 km / h,v ang c k hoch pht hnh u tin mng li thng mi trong nm 2010. Trong thng 9 nm 2007, NTT Docomo chng minh e-UTRA d liu tc 200 Mbit / s vi in nng tiu th thp hn 100 mW trong khi th nghim. Trong thng 1 nm 2008, y ban Truyn thng Lin bang M (FCC) bn u gi ph tn s 700 MHz truyn hnh tng t trc y bt u. Kt qu l, phn ln nht ca quang ph Verizon Wireless v ln nht bn cnh AT & T.C hai ca cc cng ty ny tuyn b nh ca h v h tr LTE. Trong thng 1 nm 2008, y vin EU Viviane Reding xut ti phn b l 500-800 ph MHz cho truyn thng khng dy, trong c WiMAX. Ngy 15 Thng Hai, 2008 - Skyworks Solutions pht hnh mt m-un front-end cho e-UTRAN. Trong nm 2008, ITU-R thnh lp cc yu cu thc hin chi tit ca IMTAdvanced, bng cch pht hnh mt Th Thng t ku gi cho cc ng c vin cng ngh truy cp v tuyn (chut) cho IMT-Advanced. Thng 4 nm 2008, ngay sau khi nhn c th lun lu, 3GPP t chc mt hi tho v IMT-Advanced ni m n c quyt nh LTE Advanced, mt s tin ha ca LTE tiu chun hin hnh, s p ng hoc thm ch vt qu yu cu IMT-Advanced sau chng trnh ngh ITU-R . Trong thng t 2008, LG v Nortel chng minh e-UTRA tc d liu 50 Mbit / s trong khi i du lch ti 110 km / h. Ngy 12 thng 11 nm 2008, HTC cng b in thoi WiMAX u tin cho php in thoi di ng, Max 4G15


Thng 12 nm 2008, Tng cng ty San Miguel , ng Nam ca tp on thc phm v nc gii kht ln nht, k kt mt bin bn ghi nh vi Qatar Telecom QSC ( Qtel ) xy dng bng thng rng khng dy v truyn thng di ng d n ti Philippines. Lin doanh c thnh lp wi-tc Vit Nam, trong cung cp 4G trong nc.Khong thi gian ny Globe Telecom tung ra dch v WiMAX u tin Vit Nam. Ngy 03 thng 3 nm 2009, ca LRTC Lithuania cng b hot ng u tin "4G" mng WiMAX di ng trong cc quc gia Baltic. Trong thng 12 nm 2009, Sprint bt u qung co dch v "4G" cc thnh ph c la chn ti Hoa K, mc d tc ti xung trung bnh ch 3-6 Mbit / s vi tc nh cao ca 10 Mbit / s (khng c sn trong tt c cc th trng). Ngy 14 Thng 12 2009, trin khai LTE thng mi u tin trong Scandinavian th Stockholm v Oslo bi cc nh iu hnh mng ca TeliaSonera Thu in, Phn Lan v Na Uy ca thng hiu Netcom (Na Uy). TeliaSonera mang nhn hiu mng "4G".Cc thit b modem trn cung cp c sn xut bi Samsung(dongle GT-B3710), v c s h tng mng c to ra bi Huawei (Oslo) v Ericsson (Stockholm). TeliaSonera c k hoch tung ra trn ton quc LTE trn khp Thy in, Na Uy v Phn Lan. TeliaSonera s dng quang ph bng thng 10 MHz, v duy nht-trong-mt, m nn cung cp lp vt l net bitrate ln n 50 Mbit / s ng xung v 25 Mbit / s trong ng ln.Kim tra gii thiu cho thy mt giao thc TCP thng qua 42,8 Mbit / s ng xung v 5,3 Mbit / s ng ln Stockholm. Ngy 25 Thng Hai 2010, E-xt-ni-a ca EMT m LTE "4G" mng li lm vic trong ch th nghim. Ngy 04 thng 6 nm 2010, Sprint Nextel pht hnh in thoi thng minh WiMAX u tin M, HTC Evo 4G. Trong thng by nm 2010, Uzbekistan 's MTS trin khai LTE Tashkent. Ngy 25 thng 8 nm 2010, Latvia ca LMT m LTE "4G" mng li lm vic trong ch kim tra 50% lnh th. Ngy 06 thng 12 2010, ti Hi tho Thng tin v tuyn th gii ITU 2010, ITU cho rng LTE, WiMax v tng t nh "pht trin cng ngh 3G" c th c coi l "4G".16


Ngy 12 Thng 12, 2010, VivaCell-MTS ra mt ti Armenia 4G/LTE mng th nghim thng mi vi mt th nghim trc tip c tin hnh ti Yerevan Ngy 28 thng 4 nm 2011, Lithuania 's Omnitel m LTE "4G" mng li lm vic ti 5 thnh ph ln nht. Trong thng 9 nm 2011, Saudi vin thng Tt c ba ngi khng l Trung tm giao dch chng khon , Mobily v Zain cng b rng h s cung cp 4G LTE cho tc cao USB cho my tnh di ng, vi s pht trin hn na cho in thoi vo nm 2013. Nm 2011, Argentina Claro ra mt mng 4G HSPA + trong nc. Nm 2011, Thi Lan 's TrueMove-H ra mt 4G HSPA + mng sn c trn ton quc. Ngy 31 Thng 1, 2012, Thi Lan ca trng Quc t M v cc cng ty con ca n UBND huyn theo hp tc vi cc CAT Telecom cho 1800 bng tn s MHz v TOT cho bng tn s 2300 MHz khi ng lnh vc LTE th nghim u tin Thi Lan bng cch y quyn t NBTC Vy inh hng trong tng lai cua cng ngh 4G la?

Chng 3: Mng 4G - Cng ngh ca tng lai3.1.Tnh u vit ca mng 4G so vi mng 3G3G v 4G phn loi cng ngh truyn thng khng dy tiu chun nht nh v im chun. Trong qu trnh tin ha ca in thoi di ng, cc tiu chun 3G v mng 4G cch mng ha kh nng th h tip theo ca thu bao in thoi di ng . C hai tiu chun nhm mc ch cung cp tc d liu cao l yu t ti quan trng cho cc ng dng khc nhau sp ti v nhu cu ca ngi s dng nh a phng tin, trc tuyn, hi ngh... *Cng ngh truyn thng khng dy 3G

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y l th h th ba ca mng di ng ang hng ti tc d liu cao cho cc ng dng nh gi in thoi video, video v m thanh, hi ngh truyn hnh v cc ng dng a phng tin . C hai hp tc tn ti c th l 3GPP v 3GPP2 sau ny l mt trong nhng tiu chun cho 3G da trn cng ngh CDMA. Theo ITU (Lin minh Vin thng quc t) cc yu cu sau y phi c p ng bi bt k mng c gi l mt mng 3G nh xut bi 3GPP. - Tc truyn d liu (xung lin kt) 144Kbps ti thiu cho vic di chuyn cc thit b cm tay v 384Kbps cho thit b c nh. - V nhu cu bng thng v truy cp internet bng thng rng 2Mbps cng theo quy nh ca 3GPP. Chnh k thut a truy nhp c s dng bi cc mng 3G CDMA bin th. i vi cc mng CDMA hin c cho GSM s tip tc s dng WCDMA (Wide band CDMA) s dng 5 MHz chiu rng bng tn knh c kh nng cung cp 2Mbps tc d liu. Ngoi ra cng ngh CDMA khc nh CDMA2000, CDMA2000 1x EV-DO c s dng nhng ni khc nhau trn ton th gii cho cc mng 3G. *Cng ngh truyn thng khng dy 4G y l th h tip theo ca cc mng di ng nh c quy nh bi ITU v ngi tin nhim ca cc mng 3G. Hin nay c hai cng ngh y ha hn ang c xem xt trong khi ni chuyn v di chuyn ln 4G do t l d liu cao nh tc 100Mbps trong mi trng di ng v 1Gbps trong mi trng c nh. WiMAX (Worldwide Interoperability for Microwave Access) v LTE (Long Term Evolution) l nhng cng ngh ang c xem xt. Thng s k thut sau y cn phi c p ng bi bt k mng no c coi l 4G: - Tc 100Mbps tc d liu trong mi trng di ng v 1Gbps trong mi trng c nh - Mng li hot ng trn cc gi tin IP (Tt c cc mng IP) - Nng ng knh phn b knh bng thng khc nhau t 5MHz n 20 MHz theo yu cu ca ng dng * S khc nhau gia cng ngh mng 3G v 4G 1. Tc ti d liu 3G trong khong 2Mbps ch tnh trong khi cc chi tit k thut 4G n phi l 1 Gbps v tc mi trng di ng tc ti 3G khong 384Kbps v 100 Mbps trong mng 4G.18


2. K thut a truy nhp c s dng 3G l CDMA v cc bin th ca n . 4G c hai cng ngh (LTE v WiMAX) s dng OFDMA (Orthogonal Frequency Division Multiple Access) trong nhn d liu. 3. Trong ng ln LTE s dng SC-FDMA (Single Carrier FDMA) v WiMAX tip tc s dng OFDMA trong khi mng 3G s dng CDMA bin th.

3.2 Tai sao ban nn chon 4G?

Thin v hng lin iMng 4G s khng phi l mt cng ngh tin tin vt bc, kh nng p ng tt c cc loi hnh dch v cho tt c cc i tng ngi dng. Nhng cng ngh nh m ni ln gn y nh WiMAX 802.16m, Wibro, UMB, 3G LTE, DVBHmc d chng p ng tc truyn ln, tuy nhin chng ch c xem l nhng cng ngh pre-4G (tin 4G). Mng 4G s l mt s hi t ca nhiu cng ngh mng hin c v ang pht trin nh 2G, 3G, WiMAX, Wi-Fi, IEEE 802.20, IEEE 802.22, pre-4G, RFID, UWB, satellite cung cp mt kt ni v tuyn ng ngha rng khp (ubiquitous), mi lc, mi ni, khng k mng thuc nh cung cp no, khng k ngi dng ang dng thit b di ng g. Ngi dng trong tng lai s thc s sng trong mt mi trng t do, c th kt ni mng bt c ni u vi tc cao, gi thnh thp, dch v cht lng cao v mang tnh c th cho tng c nhn.

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Hnh 2: M hnh mng hn tp

Khch hng l thng Hin ti khi chng ta mua mt kt ni di ng, kt ni y gn vi mt hp ng, vi cc rng buc ca nh cung cp mng. Ngi dng hu nh khng c bt c s la chn no khc ngoi dch v m nh cung cp cung ng. Mi ngi t nht cng c vi loi hp ng khc nhau s dng cc loi hnh dch v khc nhau: hp ng dng in thoi di ng, hp ng dng in thoi c nh, hp ng dng Internet, hp ng dng GPS, hp ng dng dch v TV di ng,.Mi lin lc, kt ni ca ngi dng iu chu s qun l cht ch ca nh cung cp dch v (nn cn gi l "network-centric). Thc t, ngi dng chnh l mc ch cui cng m mt sn phm hay mt cng ngh mun hng ti. Do vy, liu ch cn cung cp tc d liu cao l p ng nhu cu ca ngi dng cha hay 4G cn phi p ng cc yu cu khc na? Sau y chng ta th cng nhau xem xt nhng g ngi dng cn m cng ngh mng hin ti cha p ng c. y chnh l cha kha cho s thnh cng ca 4G! Tnh hung 1: Trc khi bn i ra khi nh n ni lm vic, bn cn bit nhng thng tin nh gi tu/but, tnh trng kt xe trn ng, cng nh d bo thi gian cn thit i n ch lm vic. Mt khi ngi dng chn mt phng tin i li, th thng tin v thi gian, thi im chuyn i phng tin tip theo,..s c cp nht lin tc vi thi gian thc. Trong lc ngi trn phng tin cng cng, bn mun c e-mail, nghe raio, xem TV, kt ni vi intranet ca cng ty chun b ti liu cho bui hp,. Tnh hung 2: Bn c th s rt thch nhn c nhng thng tin shopping, hng gim gi, thng tin vui chi gii tr hp dn khi bn ngi relax nh hay ang trong xe but. Tuy nhin s c nhiu bn li rt ght nhng thng tin kiu th ny. Do , dch v ny phi ty theo s thch, thi quen ca tng ngi dng. Cng tng t v d khi bn i du lch sang mt thnh ph hay nc no , bn s rt hi lng khi nhn c nhng thng tin hng dn nh bn , nhng a danh cn tham quan, cc mn ngon nn thng thc Mi khi n trc mt a im tham quan bn s

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nhn c thng tin c th v lch s, c im ni bn ang tham quan. c bit hn na nu cc thng tin cung cp n bn theo ng ting m ca bn. Trn y ch l hai tnh hung tiu biu m ngi dng trong tng lai ch i. lm c iu , h thng mng 4G phi t ngi dng vo v tr trung tm (user-centric), v cc dch v trong tng lai s phi tnh n s thch, yu cu, a im, tnh hung, thuc tnh ca tng ngi dng nh ngh nghip, tui tc, quc tch.

3.3. Cha th t chun 4G.Mng 4G c tnh u vit so vi mng 3G, tuy nhin chng ta vn ang trn con ng t mc chun 4G. Ti phin hp ton th ca Hi ng thng tin v tuyn (Radiocommunication Assembly - ITU-R) ngy 18/1/2012 ti Geneva, khuyn ngh ITU-R M.2012 v chun giao din IMT-Advanced nht tr ph chun. Chun giao din v tuyn IMT-Advanced, hay cn c nhc n nh l chun 4G, gm 2 loi giao din v tuyn l LTE-Advanced v WirelessMANAdvanced (c i tn t WiMAX). Trong , chun giao din v tuyn LTEAdvanced do 3GPP pht trin (phin bn LTE Release 10 v tip sau) cn chun WirelessMAN-Advanced do IEEE pht trin (chun WirelessMAN-Advanced kt hp vi chun IEEE 802.16). Vic Khuyn ngh ITU-R M. 2012 c thng qua s l mt mc quan trng thc y vic a ra thng mi cc cng ngh theo chun 4G v th trng vin thng th gii trong mt vi nm ti s c nhiu t ph. Theo nh ngha trc y th chun 3G c tc l 2Mbit/s. Nhng nh ngha chun 4G cao hn gp 500 ln chun 3G. IMT-Advanced s cung cp cc dch v bng thng rng vi cht lng cao hn, tc ln hn so vi nhng cng ngh hin c. Tc d liu ng xung cao nht l 100Mbit/s trong trng hp di chuyn nhanh v 1Gbit/s cc kt ni trong khi di chuyn chm. IMT-Advanced c so snh nh kt ni cp quang n my di ng, tc v mt l thuyt nhanh gp t nht 500 ln so vi kt ni 3G.

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Theo li ca ng Francois Rancy, G Cc V tuyn ITU-R. Hin ti, mt s nh mng nng cp s dng cng ngh HSDPA c tc d liu ng xung cao nht t 42Mbit/s (v d Truemove-H, Pelephone Israel, Deutchsche Telekom), ngoi ra nu s dng HSPA+ v anten MIMO cng nh dull-cell HSDPA th c th cung cp tc d liu ng xung ln ti 84 Mbit/s. Tuy nhin, cc cng ngh ny vn cn mt khong cch so vi nh ngha ca IMT-Advanced/4G. IMT-Advanced vt tri khng ch v tc m cn v hiu qu s dng ph tn. IMT-Advanced c th truyn d liu ln hn trn bng thng hp hn. iu ny gip cc nh mng i mt vi s gia tng nhanh ca lu lng d liu trong tng lai gn. Trn th gii cc nh mng bt u c k hoch trin khai mng s dng cng ngh IMT-Advanced trong vi nm ti. c bit, mt s mng, trong c mt vi nh khai thc ang s dng WiMAX, tuyn b s nng cp ln dng LTEAdvanced. C th, Clearwire (M) s trin khai LTE Advanced theo chun TDD-LTE thay th mng WiMAX ca mnh. Thng 10/2011, Spint Nextel tuyn b s trin khai LTE Advanced ti bng tn 800MHz vo na u nm 2013. Bn cnh , AT&T s trin khai LTE Advanced trong nm 2013. Vi s pht trin chng mt ca cng ngh v nhu cu s dng dch v cht lng cao ca con ngi, chng ta ang ch i cng ngh 4G s xut hin trong tng lai khng xa.

Chng 4: Tng lai cng ngh 4G ti Vit Nam4.1.Cc nh mng ang trin khai th nghim mng 4GTrong khun kh hi tho ti Vietnam Comm 2011, ng Bi Quc Vit, Gim c Trung tm Thng tin v Quan h cng chng ca VNPT cho rng, hin c ti 7 mng vin thng xin cp th nghim LTE - mng 4G. Th nhng, trong s ch c 3 doanh nghip chnh thc th nghim dch v ny l VNPT, Viettel v FPT. Hin ti, trong s cc doanh nghim vin thng xin th nghim 4G ch c VNPT v Viettel ang trin khai mnh nht. Ngy 10/10/2010, VNPT tuyn b22


hon thnh trm BTS theo cng ngh LTE u tin t ti ta nh Internet, l 2A, lng Quc t Thng Long, Cu Giy, H Ni vi tc truy cp Internet c th ln n 60 Mbps. Giai on 1 d n th nghim cung cp dch v v tuyn bng rng cng ngh LTE ca VNPT s c VDC trin khai vi 15 trm BTS ti H Ni, bn knh ph sng mi trm khong 1km. Viettel cng tuyn b phi hp vi Huawei tin hnh lp t, tch hp thit b LTE ti qun Tn Bnh, TP.HCM v H Ni. Ngy mai (12/5), Viettel s t chc hp bo cng v vic th nghim 4G vi tn gi 4G tri nghim cng ngh tng lai. FPT Telecom cho bit h cng tin hnh th nghim LTE sau khi c B TT&TT ng . Nh vy, ngoi VNPT, Viettel v FPT Telecom th hin cha c doanh nghip no a ra tuyn b v vic th nghim ca mnh. Nhiu kin cho rng, hin 3G Vit Nam vn ang giai on u pht trin, nn 4G vn l cu chuyn ca tng lai. Cn gii chuyn mn th ni vic th nghim LTE ti thi im ny khng thc s quan trng bi cng ngh vn ang trong qu trnh hon thin v qu t thit b u cui cho 4G. Hn na, th trng Vit Nam khng phi l con chim u n trin khai cc cng ngh mi. Nh vy, vic tin hnh th nghim, u gi tn s 4G s kh khn cho nhng doanh nghip vin thng khng c h tng mnh.

4.2.Nm 2018 mi c th cp php mng 4GD tho quy hoch pht trin vin thng quc gia n nm 2020 nu r, hin ti cc doanh nghip cn m rng vng ph sng, nng cao hiu qu s dng h thng mng 3G. Theo , n nm 2015, nhng n v ny mi c xem xt trin khai dch v vin thng di ng th h tip theo. "Ti thi im nm 2015, cn c vo s pht trin chung ca th gii v iu kin c th Vit Nam, chng ta mi cn nhc c nn u t mng 4G khng, hay vn tip tc pht trin 3G v nng cp ln 3,5G", ng L Nam Thng, Th trng B Thng tin v Truyn thng cho bit. Theo B trng Thng tin Truyn thng Nguyn Bc Son, d kin, n nm 2018, B mi xem xt cp php mng 4G cho cc doanh nghip. iu ny nhm gip doanh nghip kp thu hi s vn u t cho cng ngh 3G, ngi tiu dng cng khng lng ph vi s tin mua thit b u cui cho dch v.

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Trc , Lin minh Vin thng Quc t chp nhn LTE-Advanced v WirelessMAN-Advanced l hai cng ngh t tiu chun mng 4G vi tc truyn dn nhanh gp ti 500 ln so vi mng 3G. Theo thng k mi y ca cng ty chuyn nghin cu th trng Nielsen, hin Vit Nam c gn 12 triu thu bao 3G, chim khong 10% trn tng s hn 100 triu thu bao di ng. Dch v 3G chnh thc c cung cp ti th trng vin thng Vit Nam t thng 10/2009. Hin nay, c nc c 4 mng di ng hot ng trong lnh vc ny l Viettel, MobiFone, VinaPhone, Vietnammobile, qua 2 hnh thc: truy cp trn in thoi v USB 3G. Trc khi sp nhp vo Viettel, EVN Telecom cng nm trong danh sch.

oan kt.Co th hin nay mang 4G cha hn ph bin Vit Nam nhng mt mng 4G vi tc cao hn hn s gip cho tc truyn ti ca d liu trn cc h thng mng c ci thin ng k v a cc dch v cao cp nh s dng ng dng di ng, xem video trc tip trn mng, hi ngh truyn hnh hay chi game trc tuyn s bng n thc s. Tuy nhin, im li hi nht ca mng 4G l n c th thay th mt cch hon ho cc ng truyn Internet c nh (k c ng truyn cp quang) vi tc khng thua km, vng ph sng rng ln hn v c tnh di ng rt cao. Vi nhng tinh nng vt tri o chng ta hon ton c th tin tng vo 1 s t ph ca cng ngh 4G trong tng li.Va s tht la 4G a ang va se tr thanh 1 im nhn ln trong lich s phat trin nganh cng ngh thng tin cua nhn loai.

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