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HỆ THỐNG DVB-T • Giới thiệu hệ thống DVB-T • Tiêu chuẩn DVB-T • Thông số hệ thống DVB-T • Định dạng tín hiệu DVB-T; cấu trúc khung DVB-T DVB-T • Điều chế tín hiệu DVB-T và Phát • Giới thiệu mô hình tính nhiễu trong DVB-T • Mạng DVB-T đơn tần. Biên soạn: GV Lê Chu Khẩn

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  • H THNG DVB-T Gii thiu h thng DVB-T Tiu chun DVB-T Thng s h thng DVB-T nh dng tn hiu DVB-T; cu trc khungDVB-T

    iu ch tn hiu DVB-T v Pht Gii thiu m hnh tnh nhiu trong DVB-T Mng DVB-T n tn.

    Bin son: GV L Chu Khn

    Gii thiu h thng DVB-T Tiu chun DVB-T Thng s h thng DVB-T nh dng tn hiu DVB-T; cu trc khungDVB-T

    iu ch tn hiu DVB-T v Pht Gii thiu m hnh tnh nhiu trong DVB-T Mng DVB-T n tn.

    Bin son: GV L Chu Khn

  • H THNG TRUYN DN TN HIU DVB-T

    S khi iu ch tn hiu DVB-T (part 1)

  • S khi iu ch tn hiu DVB-T (part 2); FEC

  • TIU CHUN DVB-T- rng bng thng 6/7/8 MHz- 2 mode: 2K (IFFT 2046 im [s sng mang 2048] & 8K(IFFT 8192 im[s sng mang 8192])- Chiu di mt symbol: 250 s (2K mode) hoc 1 ms (8Kmode).- Khong cch sng mang ph 4 kHz (2k) v 1 kHz chomode 8k- Khong bo v t 1/32 n 1/4 chiu di ca symbol-Kiu iu ch QPSK, 16QAM hoc 64QAM- Code Rate: ; ; 7/8;- DVB S cng nh DVB T c FEC. Trong DVB- T thayi tc d liu bng cch thay i t l m.

    - rng bng thng 6/7/8 MHz- 2 mode: 2K (IFFT 2046 im [s sng mang 2048] & 8K(IFFT 8192 im[s sng mang 8192])- Chiu di mt symbol: 250 s (2K mode) hoc 1 ms (8Kmode).- Khong cch sng mang ph 4 kHz (2k) v 1 kHz chomode 8k- Khong bo v t 1/32 n 1/4 chiu di ca symbol-Kiu iu ch QPSK, 16QAM hoc 64QAM- Code Rate: ; ; 7/8;- DVB S cng nh DVB T c FEC. Trong DVB- T thayi tc d liu bng cch thay i t l m.

  • -Trong DVB-T c la chn kiu m ha theo ng cp(hierarchical coding ) cho cc lung HP (high priority)- Trong m ha theo ng cp, b iu ch c hai lung dliu vo lm vic theo hai cu hnh c lp nhau, c hai khiFEC v phn bit nhau theo FEC. Nh vo kt qu FEC nym la chn kiu iu ch cho ph hp cho cc lung c u tin cao (HP). Mt lung truyn dn c tc thp, khnng sa li rt tt, c dnh truyn cho cc lung HP(thng chn t l m ). Lung truyn dn kia tc ln,kh nng sa li km hn, dnh truyn cho cc lung LP(thng chn t l m )- Trong DVB-T s dng iu ch OFDM kt hp (coherentOFDM)- Trong OFDM kt hp: cc sng mang ti (payload carrier)c nh x hon ton trong th chm sao m khng sdng kiu m ha vi sai

    -Trong DVB-T c la chn kiu m ha theo ng cp(hierarchical coding ) cho cc lung HP (high priority)- Trong m ha theo ng cp, b iu ch c hai lung dliu vo lm vic theo hai cu hnh c lp nhau, c hai khiFEC v phn bit nhau theo FEC. Nh vo kt qu FEC nym la chn kiu iu ch cho ph hp cho cc lung c u tin cao (HP). Mt lung truyn dn c tc thp, khnng sa li rt tt, c dnh truyn cho cc lung HP(thng chn t l m ). Lung truyn dn kia tc ln,kh nng sa li km hn, dnh truyn cho cc lung LP(thng chn t l m )- Trong DVB-T s dng iu ch OFDM kt hp (coherentOFDM)- Trong OFDM kt hp: cc sng mang ti (payload carrier)c nh x hon ton trong th chm sao m khng sdng kiu m ha vi sai

  • - Trong DVB-T, IFFT 2048 (2K) im hoc 8192(8K) im. Lthuyt dng 2048 (2K) hoc 8192(8K) sng mang truyn dnd liu.- Tuy nhin, ch s dng mt s cc sng mang ny truyndn d liu (ti). Mode 8K s dng 6048 payload carriers vmode 2K s dng 1512 cho payload carriers. (S sng mang payload trong mode 8K gp bn trong mode 2K)-Ngoi cc sng mang ti DVB-T cn c cc sng mang khcnh sau:- Cc sng mang th ng (t bin zero)

    Payload carriers t v tr c nh Continual pilots t v tr c nh Scattered pilots chuyn i v tr trong ph TPS carriers t v tr c nh

    CC DNG SNG MANG CA TN HIU DVB- T- Trong DVB-T, IFFT 2048 (2K) im hoc 8192(8K) im. Lthuyt dng 2048 (2K) hoc 8192(8K) sng mang truyn dnd liu.- Tuy nhin, ch s dng mt s cc sng mang ny truyndn d liu (ti). Mode 8K s dng 6048 payload carriers vmode 2K s dng 1512 cho payload carriers. (S sng mang payload trong mode 8K gp bn trong mode 2K)-Ngoi cc sng mang ti DVB-T cn c cc sng mang khcnh sau:- Cc sng mang th ng (t bin zero)

    Payload carriers t v tr c nh Continual pilots t v tr c nh Scattered pilots chuyn i v tr trong ph TPS carriers t v tr c nh

  • - Payload carrier: S dng cho truyn ti d liu- Cc sng mang pha cnh trn v pha di (trong ph)c t bin bng zero khng qua iu ch- Cc continual pilots: nh v ti cc trc thc (I), ti pha0 hoc 180 v c bin xc nh. Cc pilots ny ccng sut hn 3dB so vi cng sut trung bnh ca tn hiuc s dng so pha ti u thu v cho AFC (automaticfrequency control). VD: s dng kha tn s tn hiu tiu thu v ti u pht.- Cc scattered pilots: B tn x ton b trn ph ca knhDVB-T qua li gia cc symbol v hnh thnh mt tn hiuo gy kh khn trong c lng knh.- ng vi mt symbol, c mt scattered pilot ti sngmang th 12 . Mi scattered pilot s nhy v pha trc 3 vtr sng mang symbol tip theo

    - Payload carrier: S dng cho truyn ti d liu- Cc sng mang pha cnh trn v pha di (trong ph)c t bin bng zero khng qua iu ch- Cc continual pilots: nh v ti cc trc thc (I), ti pha0 hoc 180 v c bin xc nh. Cc pilots ny ccng sut hn 3dB so vi cng sut trung bnh ca tn hiuc s dng so pha ti u thu v cho AFC (automaticfrequency control). VD: s dng kha tn s tn hiu tiu thu v ti u pht.- Cc scattered pilots: B tn x ton b trn ph ca knhDVB-T qua li gia cc symbol v hnh thnh mt tn hiuo gy kh khn trong c lng knh.- ng vi mt symbol, c mt scattered pilot ti sngmang th 12 . Mi scattered pilot s nhy v pha trc 3 vtr sng mang symbol tip theo

  • ScatteredPilot ti Sngmang th 0

    Scattered Pilot tiSng mang th 12

    S thay i v tr giacc scattered PilotsScatteredPilot ti Sng

    mang th 0

    Scattered Pilot tiSng mang th 12

    - Nh vy, trong mi trng hp s c 2 payload carrierskhng bao gi tr thnh scattered pilot.- Cc scattered pilots lun nm trn trc I ti 0 v180 , c cng bin nh cc continual pilots.

    S thay i v tr giacc scattered Pilots

  • - Cc sng mang TPS (TPS carriers) c t ti cc tn sc nh. VD sng mang s 50 l mt sng mang.- TPS carrier c nhim v truyn dn cc thng s bo hiu.- Cc TPS carrier i in cc knh thng tin m pha phtthng bo pha thu cc thng s truyn dn hin ti. Ccthng tin ny iu ch theo DBSK (differential bi-phaseshift keying), c nh v ti trc I 0 hoc 180 vm ha theo kiu visai. VD thng bo s khc nhau gia haisymbol gn nhau.- Mt bng tin TPS hon thin, c truyn trong 68symbol gm 67 bit.

    - Cc sng mang TPS (TPS carriers) c t ti cc tn sc nh. VD sng mang s 50 l mt sng mang.- TPS carrier c nhim v truyn dn cc thng s bo hiu.- Cc TPS carrier i in cc knh thng tin m pha phtthng bo pha thu cc thng s truyn dn hin ti. Ccthng tin ny iu ch theo DBSK (differential bi-phaseshift keying), c nh v ti trc I 0 hoc 180 vm ha theo kiu visai. VD thng bo s khc nhau gia haisymbol gn nhau.- Mt bng tin TPS hon thin, c truyn trong 68symbol gm 67 bit.

  • Tm li, cc sng mang TPS thng ba cho u thucc thng tin nh sau:+ Mode (2K, 8K)+ Length of the guard interval (1/4, 1/8, 1/16, 1/32)+ Type of modulation (QPSK, 16QAM, 64QAM)+ Code rate (1/2, 2/3, 3/4, 5/6, 7/8)+ Hierarchical coding

    + Mode (2K, 8K)+ Length of the guard interval (1/4, 1/8, 1/16, 1/32)+ Type of modulation (QPSK, 16QAM, 64QAM)+ Code rate (1/2, 2/3, 3/4, 5/6, 7/8)+ Hierarchical coding

  • VD: V tr cc dng sng mang ca tn hiu DVB-T gm:payload carriers; Continual v Scattered Pilots; TPScarriers trong iu ch 64QAM

  • Bng lit k chc nng cc bit trong sng mang TPS

  • V tr cc sng mang cho continual pilot trong mode 2K v mode 8K

  • V tr cc sng mang TPS cho mode 2K v Mode 8K

  • S lng cc sng mang trong DVB-T

  • Chm sao cho tn hiu DVB-T i vi cc kiu iu ch QPSK;16QAM; 64 QAM (Gm cc sng mang Payload; Continual Pilothoc scattered Pilot; TPS carrier

  • CC THNG S H THNG DVBMt s cc thng s quan trong nh sau: Tn s ly mu IFFT rng bng thng tn hiu DVB-T rng ph cho knh DVB-T: 8/7; 6 MHz. Tc d liu Mc tn hiu cho mi sng mangThng s c bn trong DVB-T l tn s ly mu cho IFFT viknh 8 MHz c nh ngha nh sau:

    Mt s cc thng s quan trong nh sau: Tn s ly mu IFFT rng bng thng tn hiu DVB-T rng ph cho knh DVB-T: 8/7; 6 MHz. Tc d liu Mc tn hiu cho mi sng mangThng s c bn trong DVB-T l tn s ly mu cho IFFT viknh 8 MHz c nh ngha nh sau:

    - Nhng c rt nhiu sng mang khng c s dng trongmode 2K cng nh mode 8K nn bng thng tn hiu DVB-Tthc t nh hn bng thng knh truyn 8/7; 6 MHz.- R rng nht l bng thng knh 8 MHz, ch s dng 7,6 MHzcn mi bn 200 KHz cha trng trnh giao thoa gia ccknh k cn

    f sample IFFT 8MHz = 64/7 MHz = 9.142857143 MHz.

  • - Trong rng 7.6 MHz cha 6817 (8K) hoc 1705(2K)sng mang- Tm tn s ly mu IFFT trong Knh 7 MHz hoc knh 6MHz bng cch nhn t l 7/8 hoc 6/8.f sample IFFT 7 MHz= 64/7 MHz 7/8 = 8 MHz;f sample IFFT 6 MHz= 64/7 MHz 6/8 = 48/7 MHz =6.857142857 MHz;- Khong cch gia hai sng mang k nhau

    f sample IFFT 7 MHz= 64/7 MHz 7/8 = 8 MHz;f sample IFFT 6 MHz= 64/7 MHz 6/8 = 48/7 MHz =6.857142857 MHz;- Khong cch gia hai sng mang k nhau

    f = f sample IFFT /Ntotal_carriersf 2k = f sample IFFT /2048;f 8k = f sample IFFT/8192;VD: Mode 2K, bng 7 MHz. th f = 3906,25 Hz # 4 KHz

  • Khong cch cc sng mang ph k nhau mode 2K v mode 8Ktrong DVB-T

    Nh vy, chiu di mt symbol: tsymbol = 1/f;Nh vy, chiu di mt symbol: tsymbol = 1/f;

    Thi gian truyn mt symbol trong mode 2K v mode 8K

  • - rng bng thng tn hiu DVB-T c c t khong cch ccsng mang ph f v s sng mang c s dng trong mode 2K(1705) v mode 8K (6817)

    rng bng thng ca tn hiu DVB-T

  • Ph ca tn hiu DVB-T trong mode 8K cng nh trong mode2K ca knh 8/7/6 MHz

  • Tc truyn dn mt symbol:

    Tc truyn dn tng cng ca d liu:

    Tng thi gian mt symbol

  • Tc mt symbol trong mode 2K v mode 8K trong DVB-T

  • Gross_data_rate trong DVB-T

  • -Tc thc ca d liu (Data net rate): ph thuc vo tl m v loi m. VD thng dng m RS (188,204) th:net_data_rate = gross_data_rate 188/204 code_rate;- Nh vy, do gross_data_rate bng nhau trong mode 2Kv mode 8K (do h s 4 trong 2 kiu mode). Vy s tnhnet_data_rate cho 2 trng hp nh sau:

  • Net_data_rate vi iu ch khng theo ng cp trong knh DVB_T8MHz

  • Net_data_rate vi iu ch khng theo ng cp knh DVB-T 7 MHz

  • Net_data_rate vi iu ch khng theo ng cp trong knh DVB-T6 MHz

  • - i vi iu ch ng b: Gross data rate phn phi theo 2:4 cholung High Priority (HP) v Low Priority (LP) vi iu ch64QAM. Vi kiu iu ch 16QAM th t l ny l 2:2. Net datarate cng ty thuc vo t l m cho lung HP v cho LP. Ta tnhnh sau:

  • - i vi sng mang TPS, t cng cng sut sngmang ti Payload. Cn i vi cc Continuals pilot hocscattered Pilots c cng sut hn cng sut sng mangti Paload khong 2.5 dB. C th mc in p caContinual Pilot ln hn 4/3 ln so vi sng mang tiPayload. ng vi mc cng sut 16/9 so vi sng mangti. c th:20 log(4/3) = 2.5 dB; T s in p ca continual vscattered pilots so vi trung bnh tn hiu ca payloadcarrier.10 log(16/9) = 2.5 dB; T s cng sut ca continual vscattered pilots so vi trung bnh tn hiu payloadcarrier;

    - i vi sng mang TPS, t cng cng sut sngmang ti Payload. Cn i vi cc Continuals pilot hocscattered Pilots c cng sut hn cng sut sng mangti Paload khong 2.5 dB. C th mc in p caContinual Pilot ln hn 4/3 ln so vi sng mang tiPayload. ng vi mc cng sut 16/9 so vi sng mangti. c th:20 log(4/3) = 2.5 dB; T s in p ca continual vscattered pilots so vi trung bnh tn hiu ca payloadcarrier.10 log(16/9) = 2.5 dB; T s cng sut ca continual vscattered pilots so vi trung bnh tn hiu payloadcarrier;

  • - Cng sut sng mang ti:

  • th chm sao tn hiu trong qu trnh iu ch QPSK; 16QAM;64QAM nh sau:

  • IU CH DVB-T V PHT

    Cc cu hnh pht tn hiu DVB-T thc t

  • S khi u thu tn hiu DVB-T(Part 1)

  • Data sau bdemapper

    S khi thu tn hiu DVB-T (channel coding) part 2

  • S khi b x l set top box cho pha thu

  • Echos (multipath reception)InterfererNoise (AWGN)Doppler shift

    NHIU GIAO THOA TRN NG TRUYNDVB-T V CC NH HNG CA N

    Echos (multipath reception)InterfererNoise (AWGN)Doppler shift

    Crest factor limitationIntermodulationNoiseIQ errorsInterferer

    Cc loi nhiu trn ng truyn DVB-T t pht ti thu

  • - Cht lng truyn dn tn hiu DVB-T cng ph thucvo iu ch v pht tn hiu DVB-T.- V d: H s gn sng (crest factor: CF), lm nh hngcht lng ng truyn DVB-T.- CF l t s gia bin nh ln nht v gi tr RMS catn hiu DVB-T- Theo l thuyt, CF khong 35 n 41 dB. Nhng vi gitr CF ny s lm qu ti cc b khuch i cng sut.- Thc t, CF khong 12 n 13 dB trc khi tn hiu DVB-T cp cho b khuch i cng sut.

    - Cht lng truyn dn tn hiu DVB-T cng ph thucvo iu ch v pht tn hiu DVB-T.- V d: H s gn sng (crest factor: CF), lm nh hngcht lng ng truyn DVB-T.- CF l t s gia bin nh ln nht v gi tr RMS catn hiu DVB-T- Theo l thuyt, CF khong 35 n 41 dB. Nhng vi gitr CF ny s lm qu ti cc b khuch i cng sut.- Thc t, CF khong 12 n 13 dB trc khi tn hiu DVB-T cp cho b khuch i cng sut.

  • PEP: Cng sut nh =

    CF ca tn hiu COFDM tnh nh sau:CF ca tn hiu COFDM tnh nh sau:Gi tr bin in p nh casng mang thnh phn ca tn hiuCOFDMGi tr bin in p nh tn hiu

    COFDM

  • Ta c:

    Ta c:S sng mang s dng

    Vi: 2K: 1705 sng mang; 8K: 6817 sng mang

  • CC M HNH TNH TON NHIU TRONGNG TRUYN DVB-T

    Knh truynnhiu Gaussian

    Knh truynnhiu Ricean

  • Knh truyn nhiu Rayleigh

  • Knh truyn k nnh hng dchDoppler khi uthu di chuyn vntc v.

    nh hng cadch Doppler khitruyn a ng

  • v: tc di chuyn pha thuf : tn s phtc : Vn tc sng nh sng (299792458 m/s): Gc gia tia trc tip v tia a ng.VD: f= 500 MHz; vn tc 200 Km/h. dch dopplerkhong 94 Hz.VD: f= 500 MHz; vn tc 200 Km/h. dch dopplerkhong 94 Hz.

  • BER TRONG TN HIU DVB-T

    th BER trong h thng DVB-T

  • Chnh lch gia:T l cng sut sng mang/cng sutnhiu: (C/N) v Cng sut tn hiu / cng sut nhiu(S/N)

    T l cng sut sng mang/cng sut nhiu: (C/N)

    Chnh lch gia:T l cng sut sng mang/cng sutnhiu: (C/N) v Cng sut tn hiu / cng sut nhiu(S/N)

  • C/N nh nht cn t c trong iu ch khng ng cp

  • T l C/N nh nht trong trng hp iu ch ngcp cho lung u tin thp LP)

  • H THNG DVB-T N TN

    Cu trc mng n tn DVB-T

  • - OFDM & COFDM rt thch hp cho mng SFN- Cao tn (RF) kha pha vi tn hiu GPS (GlobalPositioning System).- GPS pht ra 1 PPS(Pulse Per Second) t v tinh, cp chob giao ng 10 MHz. pha thu tn hiu GPS. Xem nh yl tn hiu ng h chun cp cho ton b h thngDVB-T- Khong cch gia pht v thu cng c gii hn lmvic t cc thng s ti u (tham kho bng sau).- Khong cch gia Tx v Rx cng ph thuc vo khongbo v v vn tc truyn sng (nh sng)- Hin tng ISI c th trnh c nu tr ca ngtruyn khng ln hn chiu di khong bo v

    - OFDM & COFDM rt thch hp cho mng SFN- Cao tn (RF) kha pha vi tn hiu GPS (GlobalPositioning System).- GPS pht ra 1 PPS(Pulse Per Second) t v tinh, cp chob giao ng 10 MHz. pha thu tn hiu GPS. Xem nh yl tn hiu ng h chun cp cho ton b h thngDVB-T- Khong cch gia pht v thu cng c gii hn lmvic t cc thng s ti u (tham kho bng sau).- Khong cch gia Tx v Rx cng ph thuc vo khongbo v v vn tc truyn sng (nh sng)- Hin tng ISI c th trnh c nu tr ca ngtruyn khng ln hn chiu di khong bo v

  • Khong cch pht t Tx-Rx cho php ng vi cckhong bo v khc nhau trong mode 2K v mode 8K(knh 8MHz)

  • Khong cch pht t Tx-Rx cho php ng vi cckhong bo v khc nhau trong mode 2K v mode 8K(knh 7MHz)

  • Khong cch pht t Tx-Rx cho php ng vi cckhong bo v khc nhau trong mode 2K v mode 8K(knh 6MHz)

  • VD: Vn tc truyn sng C= 299792458 m/s. th tr khitruyn trong khong cch 1 Km l t1km = 1000/C= 3.336 s.Nh vy, trong mode 8K c khong bo v ln hn rtnhiu so vi tr trn, nn mode 8 K rt thch hp chomng SFN khi khong cch truyn ln, tr tng

  • CU TRC KHUNG DVB-T

    S gi lung truyn dn (TPS) sau MPEG cho mt SuperFrame

  • Phn phi tn hiu ng b cho mng DVB-T

  • Cu trc MegaFrameCu trc MegaFrame

    Thi gian ca MegaFrame

  • Cu trc gi ca mt Mega Frame


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