Chapter8 Digital Satellite Communications

Chng 8: Gi thng tin v tinh

(Satellite Packet Communications)GVHD : PGS.TS. Phan Hng LinHc vin : Hunh Thanh KhoaMSHV : 9091401401/2010

**Mc lcTng quan gi thng tin v tinhThit b s bTruyn thng tin bng FDMA: kiu M/G/1Truyn thng tin bng TDMAAloha thun ty: Chuyn mch gi v tinh (pure Aloha)Aloha phn khe (plotted Aloha)S d tr/bo lu giThut ton chia cy (tree algorithm)

**Tng quanThit k cho vic truyn thoi v d liu vi phn phi c nh v phn phi yu cu s dng giao thc a truy cp nh FDMA hay TDMA.Hot ng nh h thng chuyn mch. (mng li in thoi)Truyn d liu a dng hn so vi truyn thoi.Truyn d liu c x l ng dng phc tp cao trong yu cu truyn. di ca thng tin t mt k t n n hng nghn bytes.Mt thng tin c sn sng lp tc bi tn hiu iu khin v phi c truyn t ngun trong thi gian rng buc cho php.C li ch ch kinh t ln trn mch chuyn mch v tinh thng thng, nht l khi c s lng ln ngi dng trn cng phn b a l.Cho php thc thi giao thc a truy cp cho cp pht ng ca dung lng v tinh n tt c ngi dng t c thng k trung bnh ca ti trm.Hiu sut ca giao thc a truy cp l lu lng knh v tinh theo c tnh tr gi trung bnh. Lu lng knh v tinh l lng gi thng tin truyn thnh cng.

Digital Satellite Communications

8.1. Thit b s b tm hiu truyn thng tin gi, ta gi nh:Mi ngi dng to thng tin theo xc sut Poisson vi s lng trung bnh tin trn mi n v thi gian.Tin bao gm mt hay nhiu gi, c di trung bnh 1/ n v thi gian. Mi gi mang mt a ch ch, nn khi truyn trn knh v tinh khng c giao thoa vi ngi dng khc, n s c nhn ti a ch chnh xc.Phn b Poisson:

Mt trm: t s ca lu lng trung bnh v 1/ thi gian dch v

Xc sut ca chnh xc k tin trong di t theo ham Poisson

H thng ch c s dng ph bin (tin n, i trong b nh m cho dch v, v truyn i). N x l tin ti (hm mt xc sut thi gian ti, hm mt di tin), quy tc dch v (h thng u tin), s lng dch v (chuyn mch ra), v kch thc b m (hu hn, v hn).

8.1. Thit b s b (tt)K hiu : A/B/CA: thi gian tin tiB: xc sut di tin hay xc sut thi gian dch vC: s lng dch vA hay B c th l 3 loi sau:M (Markov): s dng cho tn hiu n Poisson hay hm mt theo hm m.D (deterministic tt nh): s dng cho mt thi gian dch v l hng sG (general Khi qut): s dng cho cc hm xc sut ty V d:M/M/1: c tn hiu n phn b theo hm Poisson, mt dch v theo hm m v 1 dch vM/G/1: c tn hiu n phn b theo hm Poisson, mt dch v khi qut v 1 dch v


Truyn thng tin bng FDMA: M/G/1 queueMc tiu: phn tch tr tin trung bnh theo knh v tinh thng qua lu lng dng phn phi c nh FDMA nh mt giao thc a truy cp.Gi s h thng trng thi cn bng, n tin tn ti trong b m ti t0 v n tin c x l sau thi gian t tnh t t0. k tin n trong thi gian t th s tin tn ti trong b m ti cui thi gian t ln=max(n-1,0)+k=n-1+n+k vi n=(n=0)?1:0K vng ca k trong thi gian t vi g(t) hm mt xc sut bt k ca thi gian dch v vi trung bnh 1/ v phng sai 2.hay


Truyn thng tin bng FDMA (tt)K vng ca bnh phng gi tr k tr tin trung bnh: l khong thi gian gia lc tin ti ca tin ti b m v bt u ca tin hon tt vi g(t) hm mt xc sut bt k ca thi gian dch v vi trung bnh 1/ v phng sai 2.hayThi gian dch v trung bnh: thi gian trung bnh trong qu trnh ch n khi c x l hay thi gian ch ca tin.


Truyn thng tin bng FDMA (tt)Tc thng tinV d: FDMA c N=200 ngi, R=12.8 Mbps, b=104 bits, =3 (thng tin/giy), phn phi theo PoisonDung lng knh ca mi ngi dng:

Tc trung bnh mi knh FDMA:

tr trung bnh mi ngi dng vi TR=270 ms: Chiu di thng tin phn b theo hm m:Chiu di thng tin phn b hng s: tr trung bnh cua FDMA


Truyn thng tin bng TDMAMc tiu: phn tch tr tin trung bnh ca h thng TDMAThi gian mt khe ca khung

Tng tr ca khung tr trung bnh cua TDMA


Pure Aloha Aloha thun tyGiao thc Aloha l mt cch thc truy cp ngu nhin Thng lng knh trung bnhAloha: R=250 kbps, b=1000 bit, =b/R=4 ms, TR=250 ms, N= FDMA and TDMA: N = 1500 S l hm c gi tr kp tr pure Aloha


Slotted Aloha Aloha chia kheGiao thc Aloha l mt cch thc truy cp ngu nhin Thng lng knh trung bnh tr pure Aloha


Slotted Aloha Aloha chia khe (tt)Xt N trm mt t c th l mt trong hai trng thi: ang x l hoc b chn. tr trung bnh trc khi truyn li Xc sut chuyn tip pu m cc knh trng thi i s chuyn n trng thi j trong khe k tip tr Slotted Aloha


Slotted Aloha Aloha chia khe (tt)TR=0.250ms, =4ms (TR/=62,5 khe), N=500, v =7,36x10-4, K=100, S=(N-n)/. Ta c:


Slotted Aloha Aloha chia khe (tt)Trng thi n nh ca knh slotted AlohaCn bng GlobalCn bng kpKhng cn bngQu tiGiao thc slotted Aloha c iu khin


S hn ch gi d liuTrng thi n nh ca knh slotted AlohaGiao thc slotted Aloha c iu khin


TREE ALGORITHM: Thut ton cyCy nh phnCn trn v cn di ca tr trung bnh theo dung lng cho thut ton cyCp khe trong thut ton chia cy


Thank you


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Chapter8 Digital Satellite Communications