OFDMA Trong LTE Tung

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ofdma trong lte

Text of OFDMA Trong LTE Tung

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B GIO DC V O TOTRNG I HC GIAO THNG VN TIKHOA IN - IN T------------BO CO MN HCCNG NGH OFDMA V NG DNG TRONG H THNG LTEGio vin hng dn: TS. Nguyn Cnh MinhHc vin: Nguyn Thanh Tng ng Hu ThunKha: K 22.1H Ni-2015

Hc vin : Nguyn Thanh Tng GVHD: TS.Nguyn Cnh Minh ng Vn ThunMC LCMC LC1DANH MC BNG BIU1DANH MC HNH V21.a truy nhp phn chia theo tn s trc giao (OFDMA)21.1. Nguyn l21.2. u nhc - im ca OFDMA42. K thut a truy nhp phn chia theo tn s trc giao OFDMA53. S truyn dn ng xung OFDMA73.1. Ti nguyn vt l ng xung73.2. Cc tn hiu tham kho ng xung123.3. Cc tn hiu v knh vt l ng xung.143.4. Kin trc my pht tn hiu vt l.153.5. Ghp s liu ng xung.163.6 . Xo trn.203.7. S iu ch203.8. Thng tin lp lch ng xung203.9. M ha knh.213.10. To tn hiu OFDM.21KT LUN23

DANH MC BNG BIUBng 3.1. Cc tham s khi ti nguyn18Bng 3.2. rng bng tn khi ti nguyn vt l v s khi ti nguyn vt l ph thuc vo rng bng tn .18Bng 3.3 Cc tham s OFDM22DANH MC HNH VHnh 1.1. Khc bit ca OFDM v OFDMA3Hnh 2.1. Cp pht sng mang con OFDM v OFDMA7Hnh 3.1. Ti nguyn vt l ng xung ca LTE8Hnh 3.2. Cu trc min thi gian tn s ng xung ca LTE10Hnh 3.3. Cu trc khung con v khe ng xung10Hnh 3.4. Mt khi ti nguyn12Hnh 3.5. Cu trc tn hiu tham kho ng xung trong trng hp CP bnh thng (7 k hiu OFDM trong mt khe).14Hnh 3.6. S khi to tn hiu bng gc ng xung.16Hnh 3.7. Li ti nguyn thi gian-tn s ng xung17

1.a truy nhp phn chia theo tn s trc giao (OFDMA)1.1. Nguyn la truy nhp phn chia theo tn s trc giao (OFDMA : Orthogonal frequency division multiple access) c xy dng trn c s nguyn l ghp knh phn chia theo tn s trc giao (OFDM: orthogonal frequency division multiplex). Trong OFDMA, mt s cc sng mang con, khng nht thit phi nm k nhau, c gp li thnh mt knh con (sub-channel) v cc user khi truy cp vo ti nguyn s c cp cho mt hay nhiu knh con truyn nhn ty theo nhu cu lu lung c th.OFDMA s dng ging vi k thut OFDM, nhng thm vo chc nng chia tng s sng mang bng cch s dng tn hiu OFDM gp thnh cc nhm ca cc sng mang khng k nhau, m nhng user khc nhau c ch nh cc sng mang khc nhau. iu ny l cn thit vi vic chia tng s sng mang OFDM cho nhiu hn mt ngi s dng mt thi im.OFDMA cho php nhiu ngi dng cng truy cp vo mt knh truyn bng cch phn chia mt nhm cc sng mang con (subcarrier) cho mt ngi dng ti mt thi im. cc thi im khc nhau, nhm sng mang con cho mt ngi dng cng khc nhau.

Hnh 1.1. Khc bit ca OFDM v OFDMAT hnh 1.5 trn ta thy im khc nhau l OFDMA chia cc subcarrier thnh tng nhm gi l subchannel (mu sc khc nhau). V mi mt subchannel s dnh ring cho 1 ngi dng user.Trong OFDMA, mi ngi c cp pht mt tn s sng mang con (knh tn s) trong tng s sng mang con kh dng ca h thng. V mt ny ta thy OFDMA ging nh FDMA, tuy nhin nh s dng cc sng mang con trc giao vi nhau nn mt ph cng sut ca cc knh song mang con ny c th chng ln ln nhau m khng gy nhiu cho nhau. Chnh v vy m ta khng cn c cc on bng bo v gia cc knh (hay ni chnh xc hn l ch cn cc on bng bo v kh hp) v nh tng c dung lng h thng OFDMA so vi FDMA.1.2. u nhc - im ca OFDMA OFMDA l phng php a truy nhp da trn nn OFDM nn n mang nhng c tnh ca OFDM l: OFDM tng hiu sut s dng bng cch cho php chng lp nhng sng mang con. Bng cch chia knh thng tin ra thnh nhiu knh con fading phng bng hp, cc h thng OFDM chu ng fading la chn tn s tt hn nhng h thng sng mang n. OFDM loi tr nhiu symbol (ISI) v xuyn nhiu gia cc sng mang (ICI) bng cch chn thm vo mt khong thi gian bo v trc mi symbol. S dng vic chn knh v m knh thch hp, h thng OFDM c th khi phc li c cc symbol b mt do hin tng la chn tn s ca cc knh. K thut cn bng knh tr nn n gin hn k thut cn bng knh thch ng c s dng trong nhng h thng n sng mang. OFDM t b nh hng vi khong thi gian ly mu (sample timing offsets) hn so vi h thng n sng mang. OFDM chu ng tt nhiu xung vi v nhiu xuyn knh kt hp. Ngoi nhng u im trn th OFDM cng c nhng hn ch. Symbol OFDM b nhiu bin vi mt khong ng ln. V tt c cc h thng thng tin thc t u b gii hn cng sut, t s PARR cao l mt bt li nghim trng ca OFDM nu dng b khuch i cng sut hot ng min bo ha u khuch i tn hiu OFDM. Nu tn hiu OFDM t s PARR ln hn th s gy nn nhiu xuyn iu ch. iu ny cng s tng phc tp ca cc b bin i t analog sang digital v t digital sang analog. Vic rt ngn (clipping) tn hiu cng s lm xut hin c mo nhiu (distortion) trong bng ln bc x ngoi bng. OFDM nhy vi tn s offset v s trt ca sng mang hn cc h thng n sng mang. Vn ng b tn s trong h thng OFDM phc tp hn h thng n sng mang. Tn s offset ca sng mang gy nhiu cho cc sng mang con trc giao v gy nn nhiu lin knh lm gim hot ng ca cc b gii iu ch mt cch trm trng. V vy, ng b tn s l mt trong nhng nhim v thit yu cn phi t trong b thu OFDM . Nhng c im vt tri hn ca OFDMA Bng thng rng v linh hot hn (t 1.25MHz 20MHz) Tc truyn dn cao hn Nh c tnh trc giao m tit kim bng thng hn rt nhiu nh hng ca truyn tin hiu a 2. K thut a truy nhp phn chia theo tn s trc giao OFDMAOFDMA l m rng v mt logic ca OFDM trong u cui xa s dng tp hp con ca cc sng mang ph mang d liu. Ton b sng mang ph c thit lp thng qua OFDM, nhng tp hp con ca cc sng mang ph c xc lp vi mi s dng c th phn b ngu nhin trong s ton b cc sng mang ph. iu ny tng t nh FDMA nu mt d liu s dng c phn chia nh hn trong s nhiu sng mang ph, tuy nhin do quan h trc giao ca cc sng mang trong OFDM, nn n d dng lc ra sng mang ph, v vy bng tn phng v v b lc khng yu cu kht khe nh FDMA. Vi OFDMA, nhiu tn s c thit lp nh iu ch OFDM bng chui nhy tn s dng phn bit tn hiu ca cc u cui xa khc nhau. Khi nim a truy nhp c th m rng hn phn chia nh hn ph tn s v mt thi gian. Cc u cui xa ring bit c n nh TS nh trong TDMA trong khong thi gian cho php truy nhp cc sng mang ph truyn d liu thng qua mt chui nhy m duy nht. Cng nh h thng nhy tn bt k, cc mu nhy tn c th trc giao vi nhau, cho N tn s trong h thng, c th xy dng c N mu nhy tn trc giao. Bi v tt c cc sng mang ph s dng trn mt trm trung tm lin lc vi cc u cui xa khc nhau l trc giao vi nhau, cho nn nhiu gia cc mng c nh gi v vn mo pha/tn s t a ng . my pht, mt s sng mang con OFDM c cp cho mt ngi s dng theo cch lin tc hoc ghp xen. Vi cch OFDMA khi, cch ny n gin nhng thng lng gim do pha inh knh. Vi cch OFDMA ghp xen, cch ny thu c phn tp knh nhng cn ng b nghim ngt. S liu ngi s dng c iu ch vi s iu ch bng gc. Cc k hiu iu ch bng gc c cp cc sng mang con nh s dng bn cp pht c cp pht bi s nh v sng mang con. Sau , k hiu OFDMA c truyn i. pha thu, s liu ca ngi s dng th ui thu c bng s nhn bit cp pht sng mang con. S khi c th ca h thng OFDMA s c trnh by trong phn v SC-FDMAVi OFDM chun, truyn dn UE c th rt hp c th chu pha nh bng hp v can nhiu. l do ti sao 3GPP chn OFDMA cho ng xung, c th hp tc vi cc thnh phn ca TDMA. OFDMA cho php mt tp sng mang con c cp ng gia nhng ngi s dng khc nhau trn knh. Kt qu l h thng tr nn mnh m hn v dung lng tng ln.

Hnh 2.1 Cp pht sng mang con OFDM v OFDMA

hiu r hn v k thut a truy nhp OFDMA c ng dng nh th no trong ng xung LTE th chng ta s tm hiu s truyn dn ng xung. 3. S truyn dn ng xung OFDMA 3.1. Ti nguyn vt l ng xungCng ngh a truy nhp ng xung trong LTE da trn OFDM. Ti nguyn vt l ng xung ca LTE c th xem nh l li tn s - thi gian (hnh 3.7) trong mi phn t ti nguyn tng ng vi mt sng mang con OFDM trong khong thi gian mt k hiu.

Hnh 3.1. Ti nguyn vt l ng xung ca LTEi vi ng xung LTE, khong cch gia cc sng mang c chn vi f = 15kHz. Vi thc hin my pht/my thu da trn FFT, tc ly mu tng ng s l fs = 15000.N, trong N l kch thc FFT. V th n v thi gian Ts c nh ngha trong mc trc c th coi l thi gian ly mu ca thc hin my pht, my thu da trn FFT vi N = 2048. cn nhn mnh rng mc d n v thi gian Ts c a vo chun truy nhp v tuyn ch lm cng c cho nh ngha cc khong thi gian khc nhau v khng t ra bt c quy nh no i vi thc hin my pht v (hoc) my thu, ngha l tc ly mu. Trong thc t, mt thc hin my pht/my thu vi N = 2048 v tc ly mu tng ng fs = 30,72MHz s thch hp cho cc bng thng LTE rng hn c gi tr 15MHz v cao hn. Tuy nhin i vi cc bng thng truyn dn nh hn, kch thc FFT nh hn v tc ly mu tng ng thp hn cng c th s dng rt thch hp. V d, i vi truyn dn bng thng 5MHz, kch thc FFT N = 512 v tc ly mu tng ng fs = 7.68MHz c th l .L do tip nhn khong cch gia cc sng mang con bng 15kHz cho LTE l n gin ha thc hin cc u cui a ch LTE. S dng kch thc FFT ly tha hai v khong cch gia cc sng mang con f = 15kHz, tc ly mu fs = f.N s l bi hoc c s ca tc chip WCDMA/HSPA 1^= 3.84Mchip/s.Ngoi khong cch gia cc sng mang con 15MHz, khong cch gia cc sng mang con rt ngn flh = 7,5kHz cng c nh ngha cho LTE. Mc ch s dng khong cch gia cc sng mang con rt ngn l thc hin cc truyn dn a phng/qung b da trn MBSFN Trong mc cn li ca mc ny v cc chng sau ta s coi rng khong cch gia cc sng mang con l 15kHz nu khng c gii thch g thm.T minh ha trn hnh 3.8, trong min tn s cc sng mang con c nhm thnh cc khi ti nguyn tng ng vi bng thng khi ti nguyn chun 180kHz. Ngoi ra sng mang con DC (mt chiu) ti tm ca ph ng xung s khng c s dng. S d khng s dng sng mang con DC l v n c th trng vi tn s ca b dao ng ni ti my pht trm gc v (hoc) my thu u cui di ng. Hu qu l c th b nhiu cao do r b dao ng ni.Tng s cc sng mang con trn mt sng mang ng xung, k c sng mang con DC, v th s bng Nsc= 12.NRB+1, trong NRB l s lng cc khi ti nguyn. c t lp vt l LTE cho php ng xung c th c s lng khi ti nguyn bt k trong di t 6 khi ti nguyn cho n hn 100 khi ti nguyn. iu ny tng ng vi bng thng truyn dn trong di t 1MHz n 20MHz vi tnh ht mn. iu ny m bo tnh linh hot bng thng/ph ca LTE rt cao, t nht l t gc c t lp vt l. Tuy nhin cc yu cu tn s v tuyn ca LTE ch c c t cho tp c hn cc bng thng tng ng vi tp c hn s lng cc khi ti nguyn NRB. Hnh 3.8 minh ha cu trc min thi gian - tn s ng xung LTE.

Hnh 3.2. Cu trc min thi gian tn s ng xung ca LTE

TCP = 160.Ts (k hiu OFDM th nht), 144.Ts (cc k hiu OFDM cn li).TCP = 512.Ts .Hnh 3.3. Cu trc khung con v khe ng xung

Hnh 3.3 cho thy cu trc min thi gian cho truyn dn ng xung ca LTE. Mi khung con 1ms gm hai khe ng kch thc c di Tslot = 0,5ms (15360.Ts). mi khe gm mt s k hiu OFDM. Mt khung bao gm hai khe ng kch thc. Mi khe gm 7 hoc 6 k hiu OFDM trong trng hp CP bnh thng v trong trng hp CP m rng.Theo