Signal Processing From MS to BTS

7/29/2019 Signal Processing From MS to BTS

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Qu trnh xl tn hiu tMS ti BTS v ng c li.

I. Qu trnh xl tn hiu t MS n BTS

1. A/D- Ly mu : Tn hiu thoi c ly mu 8khz (1s ly 8000 mu).- L ng tha: GSM dng 8192 mc l ng t biu din, tc l mi mu s ng v i

13bits.Vy Tc bt lc ny l 8000*13=104.000 bits/s = 104kbps. Tc ny l qu l n sov i knh truyn v tuyn.

2. Phn on- Phn mnh dliu x l. Trong GSM ng i ta schia dliu thnh cc block bits

xl. mi block ny ko di trong 20ms. Vy b c ny mi block sc 20ms*104kbps= 2080 bits/1 block 20ms.

3. M ha ting- Mc ch l nn dliu v lm gim d thng tin to ra lung stc thp ph

h p v i knh truyn tip kim ti nguyn , m vn m bo cht l ng.- Tng block s c nn li t2080bits --> nn--> 260bits.

Trong truyn dn stn ti 3 loi m ho :

+)M ho dng sng+) M ho dng thoi (Vocoder)+) M ho lai ghp .


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M ho dng sng :

- Lung s c truyn i sau khi loi bbt d thng tin , bng vic sdng cc b nn dn thch h p

-Dng m ho ny c dng trong thoi c nh (iu chxung m PCM ) tn hiu c truyn i v chloi bmt vi thng tin d tha , nn bn thu skhi phc ctn hiu ging nh bn pht cht l ng thoi rt tt , nhng tc lung sli cao

M ho dng thoi ( Vocoder):

- Thay v truyn thng tin nh m ho dng sng , th m ho dng thoi ch truyn i ccthng scn thit ca ting ni

- Bn thu s cn cvo cc thng s d on v m tli tn hiu bn pht

- Chnh v vy m m ho dng thoi cho tc bt rt thp nhng li cho cht l ngthoi ko cao

M ho lai ghp :

- p dng nhng u im ca 2 loi m ho sng v thoi , v kt h p 2 loi m ho ny ta c m ho dng lai ghp

- Thu c lung stc thp nhng li m bo cht l ng thoi cao. Nhng phctp ca my mc th kh cao

- Loi m ho ny c p dng trong hthng GSM

Blc phn tch LPC (Linear Prediction Coding) : Blc m ha d on tuyn tnh, l blco ca blc tuyn tnh bc 8. Do cc on lin tip ca ting ni kh ging nn n ch gi i


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skhc nhau gia cc on lin tip ny m thi. Sau PLC chui c chia thnh cc khi 5ms/40mu.

Blc LTP ( Long Term Prediction) : Blc d on di hn, blc ny thc hin trtn hiuhin th i vi on c trNr mu v c nhn v i hsbr. TrNr vo khong 40 120 mu (5-

15ms) c th tng ng v i tn s c bn ca ting ni.Tn hiu d sau LTP c a qua blc thng thp v c ly mu u v i tn sly mu8/3kHz.

Thut ton nn bt l LPC (cho ra 36bits), LTP (cho ra 36bits) v RPE (cho ra 188bits).Cng 3 ng ra ny li vi nhau th c 260bits. Vy tc bt lc ny gim xung cn:260bits/20ms = 13kbps.

4. M ha knh

M ha knh c sdng pht hin v hiu chnh li trong lung bit thu gim tlbitli BER.

Sdng 2 dng m ho knh :

+) M ho khi tuyn tnh : pht hin li

+) M ho xon : dng sa li

chng li tt th ta phn cp thng tin :

+)Cp Ia : 50 bt : cp quan trng nht .

+)Cp Ib : 132 bt : cp quan trng va .


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+) Cp II : 78 bt : cp ko quan trng

Ca s 160bit/20ms c phn cp :

Ch c Ia v Ib mi c m ho sa li . Cn II ko c chng li

Bt u m ho knh :

B1 : m ho khi sa li cho 50 bt loi 1a c thm 3 bt kim tra CRC

B2 : 132 bit loi 1b tip theo cng 53 bit trc c sp xp li v ni thm 4 bit zero vocui thnh 189 bit.

B3: Thc hin m ho xon 189 bt v i t lm r=1/2, rng buc k = 5 u ra ta c2.189 = 378 bt.

B4 : Cui cng 378 bt sau m ho xon c ghp v i 78 bt loi 2 c 456 bt /20mstng ng v i tc 456 bt/20ms =22,8 kbit/s

lung bt sau m ho knh 22.8Kbit/s c a vo ghp xen

5. Ghp xen

*) Do m ho xon ch sa c li bt n , nn trnh li bt lin tip n mc mxon ko sa c li th sdng Ghp Xen

*) Ghp xen c tc dng phn tn cc bt thng tin lin k bn thu , sau gii ghp xenth nhm bt b li sthuc nhng nhm bt khc nhau v ko to ra li bt bi.


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an xen mc 1 : Cc bit sau m ha c tchc li v c ghp theo 8 bn cm, mibn cm c 57 bit.

an xenmc 2 : Cc bn cm li c an xen to ln mc 2

Vi cch an xen ny nu ta mt mt bn cm th chmt 12,5% thng tin ca mt khi bntin v sau khi sp xp li cc bt scch nhau 8bit.

6. Mt m ha- bo vtn hiu khi scan thip ca ng i th3.


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- Mt m ha thc hin bng cch XOR gia mt chui ngu nhin vi 114 bit c mtm ha.

- Chui ngu nhin c to ra tskhung v m Kc theo thut ton A5. Kha Kc gingnhau gia thu v pht, s khung thay i tcm ny n cm khc. Nh vy, 22 bit s khung kt h p v i 64 bit Kc theo thut ton A5 to ra chui ngu nhin 144 bit.

7. Lp khun cm- V i thng tin sau khi mt m ho th c a vo lp khun cm a vo iu ch v pht i ,V Cn c nhng bt dn ng ; hay iu khin c chn vo gip my

thu thu ti u - 2 bn cm 57 bt sau mt m ho c t vo cm NB cng v i cc bt dn ng + bt

ui+ phng v.- Ta c tng sbt sau lp khun cm l

NB = 57. 2 +26+ 2.3 + 2 + 8,25 = 156,25 bittc l : 156,25 bt/20ms =270,8 kbit/s

8. iu ch


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iu ch theo s GMSK 0.3 (0.3 m t rng bng 3dB ca blc dng xung gauss lin h v i tc bit BT = 0.3). GMSK l loi iu chFM s c bit.Bit 1 v bit zero c biu din bng dch tn sng mang RF mt l ng +67.708KHz. Tc knh ca GSM l 270.833kbps bng ng 4 ln sdch tn sng mang, iu ny lm gim rng bng ca ph iu chv ci tin c dung l ng knh. Tn hiu sau iu ch GMSK iqua blc Gauss lm trn sdch chuyn tn t ngt (trnh tri phsang cc knh ln cn).II. TRuyn dn tBTS t i MSC

1. T BTS n TRAU

Lu : TRAU c th t cc v tr khc nhau.

- Nu TRAU t BSC th sc thm bo hiu bsung vo ting truyn cc thng tiniu khin TRAU tb iu khin chuyn i m txa RTH. Sbsung thm 60 bitvo 260 bit ting trong 20ms, nng tng sbit nn 320bit v tc truyn dn t16kbps.

- Nu TRAU t BTS th ng truyn lc ny l 64kbps.2. T TRAU n MSC- Nu TRAU t BSC, hoc BTSth ng truyn dn tc 64kbps theo lut A, tiu

chun G.711.- Nu TRAU t ti MSC th ng truyn dn l 16kbps.


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Xl tn hiu svo MS.

1. Bgii iu Ch Gii iu ch: Tn hiu n ng i dng c th c xc nh b i TS vARFCN. D liu trn mt khe tng ng c gii iu chkhi bsung d liu ng b c cung cp theo to dng cm. Sau khi gii iu ch, ccthng tin nh phn c gii bo mt, gii ghp xen, gii m knh v gii mha ting ni.

2. Bcn bng Viterbitruyn dn v tuyn donh h ng phn xtnhiu vt khc nhau dn ngiao thoa gia cc k hiu ISI gy ra li bit. gii quyt vn ny ng ita sdng nguyn l ca my thu ti u, GSM c thc hin b cnbng Viterbi. bcn bng c thxy dng c m hnh knh cc th iim khc nhau, chui bit h ng dn (S = 26) c pht i gia cm, chuiny sphn nh tng i ng tnh trng truyn sng cho my thu, do my

thu bit tr c chui ny nn da trn s sai lch ca chui ny my thuc thxy dng c m hnh knh thi im ang xt. Sau my thu s cho cc th p bit khc nhau c thc qua m hnh knh v chn th p nocho u ra ging th p thu nht, vic la chn t h p no nhanh nhtng i ta sdng thut ton viterbi. Bcn bng Viterbi GSM cho php x


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l tn hiu phn xtrt i 15us. B ny cng a thng tin mm t i bgiim knh to iu kin cho bny hiu chnh li tt hn.

3. Gii mt m

4. Gii php xem

5. Gii m Viterbi

6. Gii m tingGii m ting bao gm cc giai on:

To li cc mu 8 kHz bng cch bsung 27 mu khng vo 13 muting trong cc khi 5 ms. Lc LTP bao gm cc mu ca khi 5ms hin th i v ba khi tr c

. Lc LPC theo cc thng s c truyn. Khnhn mnh (De-emphasis).

Lc LTP Lc LPC Kh nhn mnhChui kch thch( b sung27 bit vo 13 bit/ mu)

Ting

7. D/A

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Signal Processing From MS to BTS