chuyên Đề wireless bài 6 các khái niệm cơ bản về rf

C bn v sng v tuyn

hiu ti sao v lm th no cc thit b wireless hot ng c th vic nm r cc kin thc c bn v trng in t, anten v mt s cc thut ng lin quan l rt cn thit. Nu khng c nhng kin thc c bn ny, c th bn s khng lp t c chnh xc cc thit b wireless v kh x l s c (troubleshoot).

Trong mt phin truyn thng, v tn cng bn cht ca d liu l bao gm cc bit 0 v 1, bn pht d liu cn c mt cch thc gi cc bit 0 v 1 gi cho bn nhn. Mt tn hiu xoay chiu hay mt chiu t n s khng thc hin tc v ny. Tuy nhin, nu mt tn hiu c thay i v dao ng, d ch mt t, s thay i ny s gip phn bit bit 0 v bit 1. Lc , d liu cn truyn s c th gi v nhn thnh cng da vo chnh s thay i ca tn hiu. Dng tn hiu iu ch ny cn c gi l sng mang (carrier signal).

C ba thnh phn ca dng sng c th thay i to ra sng mang, l bin , tn s v pha. Tt c cc dng truyn thng dng sng v tuyn u dng vi dng iu ch truyn d liu. m ha d liu vo trong mt tn hiu gi qua sng AM/FM, in thoi di ng, truyn hnh v tinh, ta phi thc hin mt vo kiu iu ch trong sng v tuyn ang truyn.

Mt ngi dng bnh thng th khng quan tm n vic tn hiu c iu ch nh th no. H ch cn bit l thit b hot ng nh mong i. Tuy nhin, tr thnh mt ngi qun tr mng khng dy gii, vic c thm kin thc v iu g ang thc s din ra khi hai trm khng dy giao tip vi nhau cng rt cn thit. Phn cn li ca bi ny s trnh by v sng v truyn sng.

Bin v bc sng

Truyn thng v tuyn bt u khi cc sng v tuyn c to ra t mt my pht v gi n my nhn mt v tr khc. Sng v tuyn tng t nh cc cn sng m bn hay gp bin hay h. Sng c hai thnh phn chnh: bin v bc sng.

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Bin l chiu cao, mnh hoc cng sut ca sng. Nu bn ang ng trc bin khi cc cn sng i vo b, bn c th cm nhn sc mnh ca nhng con sng ln so vi nhng cn sng nh. Thit b ng-ten cng thc hin mt chc nng tng t nhng vi sng v tuyn. Cc sng ln thng to ra nhiu tn hiu in trong mt ng-ten, gip cho tn hiu d nhn dng nhn ra hn.

Bc sng l khong cch gia hai im tng t trn hai nh sng lin tip. Bin v tn s c hai u l cc thuc tnh ca sng.

Bc x in t

u tin ta xt n sng in t. Bc x in t bao gm sng radio, vi ba, hng ngoi, nh sng kh kin, tia cc tm, tia X, v tia gamma. Tt c chng u truyn i vi vn tc nh sng l c = 3x108 m/s v to ra ph in t. S khc nhau gia cc loi sng in t ny ph thuc vo bc sng ca mi th v chnh ci gi l bc sng ny lin quan trc tip n nng lng ca sng (bc sng cng nh th nng lng cng cao).

Khi chng ta cung cp mt dng in xoay chiu c tn s cao vo mt dy dn th dy dn s to ra sng in t (radio wave) v truyn ra khng gian theo phng thng v mi hng. V d d hnh dung nht l s pht nh sng ca mt tri.

Nu ly mt vin th xung h nc, ta s to ra nhng gn sng nhp nh ln xung v di chuyn dn ra xa, sng in t cng bc x ra ngoi vi hnh dng y ht nh th v pht nng lng ra mi trng xung quanh.

Pha (Phase)

Pha l mt thut ng mang tnh tng i. N ch ra mi quan h gia hai sng c cng tn s. xc nh pha, bc sng c chia thnh 360 phn, c gi l . Nu bn ngh thng s ny ta nh thi gian bt u th nu c mt sng bt u t im 0 v mt sng khc bt u lc 90 , hai sng ny c xem l lch pha nhau 90 .

Trong mt mi trng l tng, sng c to ra v truyn t mt my ny v nhn mt cch hon ho bn my kia. Tuy nhin, truyn thng v tuyn khng xy ra trong mi trng l tng. C nhiu ngun gy nhiu v nhiu vt c nh hng sng khi n ang di chuyn. Hnh v di y v hai sng ang lch pha nhau 90 .

Thi gian v pha

Gi s bn c hai ng h ang ng yn v c hai cng chnh v 12 gi. Lc 12 gi, bn khi ng ng h u tin v sau mt gi, bn khi ng ng h th hai. ng h th hai c xem l i chm hn ng h th nht mt gi. Khi thi gian tri i, ng h th hai cng vn i sau ng h th nht 1 gi. C hai ng h cng duy tr mt ngy 24 gi nhng c hai khng ng b vi nhau. Cc sng lch pha nhau th cng l hai sng xut pht nhng thi gian khc nhau. C hai sng s hon thnh chu k 360 nhng n s lch pha vi nhau.

Cc phng thc iu ch

d liu c th c truyn, tn hiu phi c x l sao cho bn my nhn c cch phn bit bit 0 v 1. Phng php x l tn hiu sao cho n tng trng cho nhiu mu d liu c gi l iu ch. Phng thc ny s bin tn hiu vo trong sng mang. Phng thc ny m ha d liu sao cho n c th truyn. C ba kiu iu ch: iu bin (Amplitude Shift Keying - ASK), iu tn (Frequency Shift Keying - FSK), v iu pha (Phase Shift Keying - PSK).

tng cho phn iu ch tn hiu trong WLAN l cha cng nhiu d liu cng tt vo trong tn hiu v gim thiu lng d liu c th b mt do nhiu. Khi d liu b mt, n phi c truyn li, v v vy lm tn ti nguyn ca mng khng dy.

C hai k thut khc nhau c dng m t d liu:

Trng thi hin hnh (current state): vi k thut ny, gi tr hin hnh ca tn hiu c dng phn bit gi tr 0 v 1. K thut ny s gn mt gi tr c th ch ra gi tr nh phn l 0 hay l 1. mt thi im c th, gi tr ca tn hiu s xc nh gi tr nh phn. V d, bn c th m t gi tr 0 v 1 bng mt cnh ca bnh thng. Mi mt pht, bn kim tra xem ca l ng hay m. Nu ca l ang m, n tng trng cho gi tr 0. Nu ca ang ng, n tng trng cho gi tr 1. Tnh trng hin thi ca cnh ca, ng hay m, s xc nh gi tr 0 hay 1.

Chuyn trng thi (state transition): vi k thut ny, s thay i hay chuyn tn mc tn hiu s c dng phn bit 0 v 1. K thut ny c th m t gi tr 0 bng cch thay i pha ca sng mt thi im c th, trong khi gi tr 1 s c trng bng vic gi nguyn pha. mt thi im c th, yu t c s thay i hay khng c s thay i trong pha ca tn hiu s c dng xc nh gi tr nh phn. V d cnh ca bn trn c th c dng li mt ln na minh ha. C mi mt pht, nu cnh ca ang di chuyn (d m hay ng), n tng trng cho gi tr 0. Nu cnh ca ng yn (d ang m hay ng), n tng trng gi tr 1. Trong v d ny, trng thi chuyn i (di chuyn hay khng di chuyn) s xc nh gi tr 0 hay 1.

iu bin

iu bin thay i bin / cao ca tn hiu m t d liu nh phn. iu bin dng k thut trng thi hin hng, trong mt mc bin c dng tng trng mc 0 v mt mc c dng tng trng mc 1. Hnh bn di m t lm th no mt dng sng c th iu ch mt m ASCII, k t K dng phng php iu bin. Bin ln tng trng cho bit 1, trong khi bin nh tng trng cho mc 0.

Chnh bin ca sng s xc nh d liu ang c truyn. Bn my nhn, u tin my nhn s chia tn hiu nhn c ra thnh nhng khon thi gian c gi l thi gian ly mu. My nhn sau s kim tra sng tm ra bin . Ty thuc vo gi tr bin ca sng, my nhn s xc nh gi tr nh phn ang c truyn.

Nh bn cng c th bit, cc tn hiu khng dy th c th kh d on v cng c th b nhiu t nhiu ngun. Khi nhiu xy ra, n thng nh hng n bin ca tn hiu. V khi c mt s thay i trong bin c th lm cho my nhn din dch sai gi tr d liu, k thut ny phi c dng mt cch cn thn.

iu tn

iu tn thay i tn s tn hiu m t d liu nh phn. iu bin dng k thut trng thi hin hnh, trong mt mc tn s c th tng trng cho bit 0 v mt tn s khc tng trng cho bit 1. S thay i tn s s xc nh d liu ang c truyn. Bn my nhn ly mu ca tn hiu, n s xc nh tn s ca sng, v tu thuc vo gi tr tn s, my nhn s xc nh gi tr nh phn.

Hnh di y s m t lm th no mt dng sng c th m t k t K ca bng m ASCII dng k thut iu tn. Mc tn s nhanh hn c din dch nh mc 1 v mc tn s thp hn c din t nh mc 0.

Trong cc chun 802.11 ban u, k thut iu tn FSK c dng. Khi yu cu truyn thng nhanh hn, k thut FSK s i hi nhiu k thut t hn h tr tc nhanh hn. iu ny lm cho n khng cn thc t.

iu pha

K thut ny s thay i pha ca tn hiu m t d liu nh phn. iu pha dng k thut thay i trng thi, trong mt pha dng m t bit 0 v mt pha khc dng m t mc 1. S thay i trng thi ca pha s xc nh d liu ang c truyn. Khi my nhn ly mu tn hiu, n s xc nh pha v trng thi ca bit.

Hnh di y m t lm th no mt dng sng c th m t k t K dng k thut iu pha. Pha thay i u chu k dng m t gi tr nh phn l 1. Nu pha khng thay i u chu k th m t gi tr 0. iu pha dng nhiu trong cc chun 802.11. Mt cch tiu biu, bn my nhn s ly mu tn hiu v so snh pha ca mu hin hnh vi pha trc v xc nh s khc nhau. S khc nhau trong pha (lch pha) s c dng xc nh gi tr bit.

Cc phin bn cao cp ca iu pha c th m ha nhiu bit. Thay v dng hai pha xc nh gi tr nh phn, bn pha c th c dng. Mi pha trong b bn ny c th dng m t hai gi tr nh phn (00,01,10,11) thay v ch 1 (0 hoc 1), nh vy s gip ngn thi gian truyn. Hnh di y m t k t K trong bng m ASCII c th c m ha v truyn dng k thut ny. Bn ch l c t chu k ly mu hn hnh v trc.