Sync in OFDM

7/28/2019 Sync in OFDM

1/23

SEMINAR:NG B TRONG OFDM

(SYNCHRONIZATION IN OFDM)

NGI BO CO: VHNGCNG

PH TRCH MN HC: PGS-TS NGUYN HU PHNG

KHOA IN T VIN THNGCAO HC KHA 22-2012

CHUYN NGNH: K THUT IN T, HNG T-VT-MTTRNG I HC KHOA HC T NHIN TPHCM

--- TP.HCM-2013 ---


2/23

MC LC:

I- Khi nim v ng b trong H thng OFDM

II- Tng quan Gii php thng thng ng b ha trong OFDM

III- Nghin cu Trng hp: ng b ha tn s

m trong OFDM qua Tiu chun ng Cho


3/23

I-Khi nim v ng b trongH thng OFDM

1.Vn ng b (Synchronization):ng b l phn thit yu ca my thu OFDM.

Nu khng c ng b trong qu trnh gii iu chs b gim cp hiu sut vn hnh h thng do:- Sai hng Dao ng (oscilator impairments);- Sai lch ng h (Clock differences);

- Tp nhiu pha (phase noise).


4/23

1.Vn ng b:

My thu cn phi c ng b vi bin ca k hiu(symbol) trong min thi gian; ng thi phi ckha vi cc tn s sng mang con (subcarrier).


5/23

2. lch nh th (Timing Offset):Khng c can nhiu lin sng mang (ISI), tn hiu thu

c l: (1)

l ng bao ca mo dng tr (delay distorsion),d l lch thi gian ly mu (sampling time offset).Sau bin i Fourrier nhanh (FFT), ta c:

(2)

lch pha a vo tc dng n tt c cc sng mangcon mt cch tuyn tnh. Nhng s xoay pha ny c th

chnh sa trong tng c lng knh.


6/23

2. lch Tn s (Frequency Offset):OFDM nhy vi lch tn s v n to ra cannhiu lin sng mang (ICI): mi sng mang con b

can nhiu bi tt c cc sng mang con khc. Nuykl tn hiu thu c khng c lch thi gian,v fl lch tn s, th:

(3)Sau FFT ta c:

(4)Trong (F) l thnh phn suy gim(attenuation component) ng thi cng l thnhphn can nhiu (interference component) t nhngsng mang con khc.


7/23

4. Tp nhiu Pha (Phase Noise):

Tp nhiu pha sinh ra trong nhng tng trunggian ca qu trnh gii iu ch, nh l gia RFv IF.Tp nhiu pha l qu trnh ngu nhin c trungbnh zero ca lch pha dao ng. ph mt

cng sut (PSD) ca tp nhiu pha cchun ha vi cng sut ca ng ra b daodng sng sin.Tp nhiu pha tiu biu c ch trong hnh.PSD ny c lc thng cao (high-pass) vi

Vng kha pha (PLL) v b dao ng ckha cho iu ch trung gian (intermediatemodulation).


8/23

4. Tp nhiu Pha (Phase Noise) (tt):

Ng ra ca b gii iu ch my thu l:

(5)Vi (k) l tp nhiu pha cho sng mang con thk, c th xp x bng 1+j(k), do :

(6)


9/23

4. Tp nhiu Pha (Phase Noise) (tt):

Thnh phn th nh trong phng trnh l nhiulon pha ngu nhin (random phase disturbance),n xy ra trong mi k hiu. Thnh phn ny cgi l Sai li pha chung CPE (Common Phase

Error), v c th sn sng b trit tiu bng ccho lch pha ca sng mang con dn dng(pilot subcarrier) , ri tr xoay pha ny cho ttc cc sng mang con.

Thnh phn th 3 trong phng trnh chnh l cannhiu lin sng mang ICI, v c tnh k d(peculiar). Can nhiu ny c th c x l nhmt tp nhiu Gauss nu nhng sng mang con

c lp vi nhau.


10/23

II-Tng quan Gii php thng thng ng b ha trong OFDMNhng thut ton ng b ha OFDM c th

chia thnh 2 loi nh sau:

- Nhng phng php c d liu tr gip, cngi l nhng phng php DA (Data Aided),

hoc l pp ng b ha Thi gian-Tn s viTr gip Dn ng (Pilot-AssistedTime/Frequency Synchronization).- Nhng phng php khng c d liu tr

gip, cn gi l nhng phng php NDA(Non-data Aided), hoc l ng b ha Thigian-Tn s M (Blind Time-FrequencySynchronization).


11/23

1. ng b ha Thi gian-Tn s vi Tr gipDn ng (Pilot-Assisted Time/Frequency

Synchronization):- Nhng phng php t c s trn tn hiu dnng (pilot) thch hp hn vi truyn thng tc cao, bi v s ng b ha phi c thc hinvi kh nng nhanh nht.- Ban u s ng b ha frame c thc hin tch im bt u mt frame v nhng knh tiubiu c th a vo lch tn s cha bit v tr thi gian cha bit. iu ny c thc hin

bng cch ly tng quan (correlation) tn hiung vo vi mt preamble bit.


12/23

1. ng b ha Thi gian-Tn s vi Tr gip Dnng(tt):

Sa sai tn s c thc hin nh B dao ng iukhin bng in p VCO u cui tng t. Ngoi racn c th thc hin digital bng cch nhn tn hiu thuvi mt tn hiu c lng pha trc FFT.


13/23

2. ng b ha Thi gian-Tn s M (Blind Time-Frequency Synchronization):

- ng b ha m l php c lng khng c dnng v t c s trn phng php c lng khnng ln nht MLE (Maximum Likelihood Estimation).- Nh bit, k hiu OFDM m rng tun hon, l s

lp li c bn ui ca k hiu trong khong phng v(guard interval). c tnh dng tun hon(cyclostationary) ny c th gip cho vic ly trungbnh qua mt s ln k hiu OFDM.

- Kt qu l h thng c lng c nh th k hiuto, lch tn s fo, v rng k hiu T.


14/23

2. ng b ha Thi gian-Tn s M (tt):

c lng t ti u (Optimum estimator) ti a habiu thc sau y tm kt qu:

(7)

Trong y l tn hiu thu c, TG l di khong

phng v (guard interval length).


15/23

III- Nghin cu Trng hp: ng bha tn s m trong OFDM qua Tiu

chun ng Cho:Cng trnh ng b ha Tn s M trong OFDMqua Tiu chun ng Cho (Blind frequencysynchronization in OFDM via diagonality criterion)

ca Timo Roman, Samuli Visuri and Visa KoivunenSenior Member, IEEE, Phng Th nghim X l Tn hiu,Khoa K thut in, Trng i hc K thut Hensinki,Phn Lan . Cng trnh ny c ti tr mt phn biVin Hn lm Phn Lan v Trng Cao cp GETA. C th download ti: http://ieeexplore.ieee.org/xpl/RecentIssue.jsp

?punumber=78 hoc:

http://lib.tkk.fi/Diss/2006/isbn951228104X/article4.pdf
http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=78http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=78http://lib.tkk.fi/Diss/2006/isbn951228104X/article4.pdfhttp://lib.tkk.fi/Diss/2006/isbn951228104X/article4.pdfhttp://lib.tkk.fi/Diss/2006/isbn951228104X/article4.pdfhttp://lib.tkk.fi/Diss/2006/isbn951228104X/article4.pdfhttp://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=78http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=78


16/23

Gii thiu gii php:* Roman et. al. gii quyt bi ton c lng lchtn s sng mang (CFO) cho OFDM trong trng hp

cc knh chn lc tn s. CFO ph hy tnh trc giaogia cc sng mang lm cho nhng ma trn hipphng sai ca tn hiu mt tnh cho (non-diagonal)trong min tn s.

* Phng php ng b ha m c xut gip tito cu trc cho bng cch lm cc tiu nhng thnhphn cng sut khng ng cho. Nh vy c tnh

trc giao ca OFDM vi cyclic prefix c phc hi.

* Phng php c gi l m v n khng cn kinthc tin nghim ca d liu pht hay ca knh, vkhng cn bt k mt sng mang con o no.


17/23

Gii thiu gii php(tt):*Phng php dn xut c mt li gii dng kn, chora c lng CFO chnh xc vi hiu qu tnh ton

trong nhng mi trng fa-ing a ng.* Tnh vng chc ca ton t c lng c chngminh. Tc hi t ca thut ton nh l mt hm sca kch thc mu cng c phn tch. c c

nng sut vn hnh vi mu ln, cn Cramer Rao CRB(Cramer Rao bound) c dn xut cho bi ton CFOm. CRB dn xut gi thit mt m hnh Gauss tngqut cho tn hiu OFDM, c th c p dng cho cphng thc iu ch chu tuyn (circular) v khng

chu tuyn (non-circular).* Cui cng, nhng kt qu gi lp trn vic c lngCFO c bo co da trn vic s dng nhng mhnh knh thc t.


18/23

Kt qu nh gi:

Cng thc dng kn kt qu ca thut ton:

(8)CFO c lng c bng cch chn gi tr tngng vi cc tiu ca hm chi ph:

(9)


19/23

Kt qu gi lp:

[3] Estimated frequency offset and cost function over time (1realization), = 0.43 and SNR=15dB


20/23

Kt qu gi lp(tt):

[3] Covariance matrix structure without (on the left)

and with (on the right)frequency offset compensation for = 0.43, QPSK modulation

and SNR=15 dB. Dark colors correspond to high absolutevalues. Estimation has been performed over 10000 blocks. Totaloff-diagonal power gets reduced by 19.8 dB. In successful CFO

compensation, non-diagonal elements are zero and only thediagonal terms remain.


21/23

KT LUN:

ng b ha trong OFDM l mt rng buc ca k thutny. Thiu ng b thi gian hay tn s s lm cho nng

sut vn hnh ca h thng gim st.

C nhiu phng php v thut ton ng b ha choOFDM.

Nhng phng php DA cn phi c tn hiu dn ng(pilot). Nhng phng php NDA khng cn nhng trgip d liu tin nghim, nhng thut ton rt cng knh,i hi kh nng phn cng.

L thuyt X l tn hiu s v Cc phng php thng kl nhng kin thc tin quyt cho vic nghin cu vn ny./-


22/23

CM N RT NHIU

THANK YOU VERY MUCH


23/23

CM N RT NHIUTHANK YOU VERY MUCH

Sync in OFDM

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