BO COBI TP LN THNG TIN DI NG TI: Tm hiu v truyn lan sng v tuyn trong khng gian t do H Ni, thng 5/2015

MC LC

DANH MC HNH V2LI NI U3A.Gii thiu chung4I.Gii thiu nhm4II. ti4III.Lp k hoch v phn cng cng vic.43.1Lp k hoch43.2 Phn chia cng vic4B. Ni dung6I.C bn v sng v tuyn (tham kho ti trang 3-15 trong ti liu [1])61.Sng in t - Electromagnetic Wave62.Sng Radio7II.S truyn lan sng v tuyn trong khng gian t gio ( Tham kho trong [2]).91.M hnh truyn lan sng v tuyn trong khng gian t do92.Cc phng thc c bn trong truyn lan v tuyn113.Nhiu x134.Link Budget Analysis205.M hnh truyn sng ngoi tri216.M hnh truyn sng trong nh24III.Kt lun25TI LIU THAM KHO26

DANH MC HNH V

Hnh 1.1 H thng to sng in t6

Hnh 1.2 Phn cc sng in t7

Hnh 1.3 c tnh sng in t8

Hnh 1.4 c tnh v pha ca sng in t8

Hnh 2.5 M hnh truyn sng trong khng gian9

Hnh 2.6 Hin tng phn x11

Hnh 2.7 Hin tng nhiu x12

Hnh 2.8 Hin tng tn x13

Hnh 2.9 Nhiu x hnh li dao13

Hnh 2.10. S tnh ton nhiu x hnh li dao14

Hnh 2.11. Vng Fresnel15

Hnh 2.12 lch pha tn hiu vng Fresnel16

Hnh 2.13 Vng Fresnel Line - of - Sight18

Hnh 2.14 Nhiu nhiu x hnh li dao trong mt vng20

Hnh 2.15 Suy hao ng truyn theo bn knh vi m hnh Hata.23

LI NI U

S pht trin nhanh chng ca cng ngh thng tin di ng trong nhng nm qua v d bo s bng pht ca cng ngh ny trong nhng nm ti s dn ti s thiu ht ngun nhn lc c trnh v kinh nghim cao trong lnh vc ny. Cc trng i hc trn th gii, trong c Vit Nam v ang nghin cu nhiu chng trnh v bin php c th o to chuyn gia v k s v tuyn c trnh cao. Mn hc Thng Tin Di ng l mt trong nhng mn hc lin quan n lnh vc v v tuyn, nhm cung cp thm cho sinh vin kin thc nn tng v thng tin v tinh, thng tin di ng.

Thng qua mn hc, c s ng ca ging vin hng dn, nhm chng em chn ti cho bi tp ln mn Thng tin Di ng l: Tm hiu v truyn lan sng v tuyn trong khng gian t do.

Ngoi phn m u, gii thiu chung v kt lun, ni dung bi tp ln gm:

I. Gii thiu chung v lan truyn v tuyn

II. Lan truyn v tuyn trong khng gian t do

Chng em xin gi li cm n chn thnh nht ti Thy T.S Trn Quang Vinh tn tnh hng dn nhm hon thnh bi tp ln ca mnh. D rt c gng nhng bi tp ln khng th trnh khi thiu st, nhm rt mong nhn c s ng gp kin ca Thy cng cc bn bo co ny c hon thin hn.

A. Gii thiu chung

I. Gii thiu nhm

Tn nhm: Nhm 3

Lp: Ging ng TC 212 Th 3 6h45 9h15. M lp: 79011

Thnh vin

STT

H v tn

MSSV

Ghi ch

1

H Xun Linh

20112617

Trng nhm

2

Nguyn Hng Thi

20112674

3

inh Trng Chuyn

4

Cao Nguyn Duy

II. ti

Tm hiu v Truyn lan sng v tuyn trong khng gian t do da trn ti liu Free Space Radio Wave Propagation

III. Lp k hoch v phn cng cng vic.

3.1 Lp k hoch

Bi tp ln thc hin trong vng 5 tun (bt u t tun 8 v ktthc vo tun 13)

Tun

Cng vic

Ghi ch

Tun 6 - 7

Nhn ti, tm hiu chung v thng tin di ng

Tun 8-9

Phn cng dch v c hiu bi bo

Tun 10

Ghp ni, chnh sa phn dch

Tun 11-13

Chnh sa bo co, hp nhm chun b bo v

3.2 Phn chia cng vic

STT

Thnh vin

Cng vic phn cng

Ghi ch

1

H Xun Linh

Tm kim ti liu lin quan, dch bi, tng hp phn dch, lm bo co

2

Nguyn Hng Thi

Dch bi, lm slide

3

nh Trng Chuyn

Dch bi

4

Cao Nguyn Duy

Dch bi

B. Ni dungI. C bn v sng v tuyn (tham kho ti trang 3-15 trong ti liu [1])1. Sng in t - Electromagnetic Wave

Khi nim

Sng in t l sng c th di chuyn qua chn khng trong khng gian. Sng c hc (mechanical wave) cn s tn ti ca vt cht truyn dn truyn ti nng lng ca n t im ny n im khc.

To ra sng in t

Nguyn tc c bn: lc cc ht in tch.

truyn thng tin, s dao ng cc ht in tch phi c kim sot, ngha l cc ht in tch nn dao ng ln v xung mt cch nhp nhng. Qu trnh ny to ra cc bc x c th lp li chnh n. lm iu ny, ngi ta s dng mt mch in c cun cm v t to ra dng in dao ng. Sau truyn dng in ny vo mt anten.

Hnh 1.1 m t h thng to sng in t. Cun dy (tuning coil) v cc bnh Leyden ng vai tr l cc t in v cun cm. Khi nhn telegraph, ngun pin s to ra chnh lch in th b nh la (spark gap), lm pht ra mt tia in gia hai u ca b nh la. Vic ny lm ng mch v cung cp dng in n mch c to ra bi cun dy v bnh Leyden. Dng in s di chuyn vi tn s ca mch ny, sau c a n anten v truyn i.

(Hnh 1.1 H thng to sng in t)

Mt s tnh cht ca sng in t

Sng c to ra c c t tnh v in tnh, c kh nng lan truyn qua khng gian.

Bc x (radiation) in t to ra t trng (Magnetic Field) v in trng (Electric Field).

Hai trng ny vung gc vi nhau v hng di chuyn cng vung gc vi c hai trng.

Tc dao ng ca electron quyt nh tn s sng.

Mt phng ca in trng s xc nh s phn cc (polarization) ca sng

Hnh 1.2 Phn cc sng in t

2. Sng Radioa. Khi nim

Sng Radio cng l mt dng sng v tuyn, l mt dng sng bc x in t c bc sng trong ph in t di hn tia hng ngoi. Sng v tuyn c tn s 300 GHz n 3KHz, tng ng vi bc sng 1 millimeter (0.039 in) n 100 kilometer (62 mi). Ging nh cc loi sng in t khc, sng radio c tc bng tc nh sng 3.108m/s.

b. S lan truyn sng radio

Theo nguyn l maxwell:

Do thnh phn in v t lun tng tc, sn sinh ra nhau nn sng lun c mt in t trng

Cc ht mang in v t ny tng tc vi cc ht ln cn (trong trng hp in t ca n) trong mi trng ng cht nn cc ht ny cng nhim in t v tip tc lm nhim in t cc ht ln cn

C th, sng s lan truyn i xa n khi ht cng sut pht hoc b di li (do phn x hay khc x).

Trong mi trng khc cht, sng s b phn x hay khc x.

c. Cc c tnh ca sng in t

Sng in t c bn c tnh c bn, l bc sng (wavelength), tn s (Frequency), bin (amplitude), v pha (phase).

Hnh 1.3 c tnh sng in t

Pha:

s khc bit gia cc sng hnh sin chng ln nhau.

o t 0-360o.

0o cng pha

90o lch pha /4

180o ngc pha, hy tn hiu ban u

360o cng pha.

(Hnh 1.4 c tnh v pha ca sng in t)Tc , bc sng v tn s

Tc nh sng = bc sng () x tn s (f) = 3.108m/s

Sng

Tn s

Bc sng

Dng AC

60Hz

5000Km

FM radio

100MHz

3m

Cellcular

800MHz

37.5cm

Ka band Satellite

20GHz

15mm

Ultraviolet light

1015Hz

10-7m

II. S truyn lan sng v tuyn trong khng gian t do ( Tham kho trong [2]).1. M hnh truyn lan sng v tuyn trong khng gian t do

Hnh 2.5 M hnh truyn sng trong khng gian

Mc d cc tn hiu EM khi n thng qua cc knh khng dy yu dn i do nhiu yu t khc nhau, nhng trong mt s trng hp, s truyn pht l ng hng, khng c vt cn ging nh thng tin v tinh. Mc nng lng nhn c bi anten pha nhn c tnh theo cng thc Friss nh sau:

Pr = Pt

Trong :

Pt l nng lng tn hiu ti anten pht

Pr l nng lng tn hiu ti anten thu

l bc sng mang (m)

Gt l mc khuch i (gain) ca anten pht

Gr l mc khuch i (gain) ca anten thu

d l khong cc gia cc anten (T-R) (m)

L l tham s suy hao ca h thng do suy hao trn ng dy (ni b, trong thit b, phn cng), suy hao do cc b lc, suy hao ca anten (L 1). Nu L = 1 ngha l khng c suy hao do phn cng.

khuch i ca cc anten c lin quan n hiu qu ca chng v ph thuc trc tip vo bn knh ca cc anten, c th hin theo cng thc sau:

G = 4Ae/

Path Loss (suy hao trn ng truyn) l mc suy hao tin hiu c o bng dB v l 1 i lng > 0, c nh ngha l s khc nhau gia mc nng lng hiu dng pht ra v mc nng lng nhn c ti cc anten, th hin theo cng thc sau:

PL(dB) = 10logPt/Pr

Cc trng ca mt anten c th c phn loi theo 2 vng, trng xa v trng gn. Trong trng xa, cc hot ng truyn dn sng nh sng my bay v c nng lng t l nghch vi khong cch. Vng trng xa cng c gi l vng Fraunhofer v cng thc Friis vn ng trong trng ny. Do vy, phng trnh Friis ch c s dng vi trng khong cch xa (df), ph thuc vo bn knh ca anten pht vi cng thc sau:

df = 2/

Ta cng c th thy phng trnh Friis khng c nh ngha dnh cho trng hp d = 0. V l do ny, chng ta s dng mt khong cch gn hn (do) nh mt im tham chiu. Nng lng nhn c, Pr(d) c tnh theo cng thc sau:

Pr(d) = Pr(do)

2. Cc phng thc c bn trong truyn lan v tuyn

Lan truyn v tuyn radio propagation l mt hnh vi ca sng radio khi chng c pht t mt im n mt dim khc, hoc khi chng di chuyn gia nhng thnh phn khc nhau ca bu kh quyn.

V cng l sng in t, sng radio cng b nh hng bi cc hin tng: reflection (phn x), diffraction (nhiu x), v scattering (tn x). Sau y chng ta s tm hiu k hn v nh hng ca cc hin tng trn i vi lan truyn v tuyn.

a. Reflection phn x

Phn x xy ra khi bc x in t gp mt vt cn ln hn nhiu so vi cc bc sng (chng hn: b mt tri t, nh cao tng,). Khi mt sng radio gp phi cc vt cn c cc tnh cht in khc nhau, mt phn sng s lan truyn vo bn trong vt cn, trong khi nng lng b phn x tr li.

Nu mi trng ti ca cc sng in t l cht in mi, th mt phn nng lng c phn x tr li v 1 phn c truyn i tip. Nu mi trng l vt dn hon ton, tt c nng lng s c phn x tr li mi trng ban u. Lng nng lng phn x tr li ph thuc vo s phn cc ca cc sng in t.

Hnh 2.6 Hin tng phn x

b. Diffraction nhiu x

ng i gia transmitter v reciver b cn bi mt vt c cnh trn. Sng lan truyn s vng qua vt cn nh b mt tri t, cc ta cao c, ngay c khi tm nhn thng (Line of Sight LoS) khng tn ti.

Hin tng ny c th c gii thch bi quy tc Huygen, theo , nguyn l ny cho rng mi im nm trn u sng l cc ngun cho cc sng th cp mi; v s lan truyn ca ton b l tng ca cc sng th cp n t mi im trong mi trng m sng i qua. Cch tip cn ny cho php gi thch nhiu hienj tng quang hc v hin tng sng ni chung, nh hin tng nhiu x.

Hnh 2.7 Hin tng nhiu x

c. Scattering tn x.

Khi gp vt th nh hn so vi bc sng ca sng ang lan truyn (chng hn: bin bo, ct n). Tn hiu b phn tn thnh nhiu ng tn hiu c cng yu hn.

Thng th nng lng nhn c bn thu c cng mnh hn so vi cc hin tng phn x v nhiu x. l v cc cy ln, ta nh, ct n c th tn x nng lng v tt c mi hng. iu ny gip cung cp thm nng lng bn thu. nhm b mt c th nghim da theo tiu chun Rayleigh, l nh ngha mt mc nng lng quan trng hc ng vi mt gc ti nht nh i, c tnh theo cng thc:

hc =

Hnh 2.8 Hin tng tn x

3. Nhiu x

Nhiu x l hin tng c gii thch bng s lch hng ca cc sng in t t cc ng thng do nh hng ca cc vt cn. V d nh: m thanh c th nghe c trong phng, ni ngun m mt phng khc m khng c bt k vt cn no. Tng t nh hin tng nhiu x xy ra vi nh snh nhng cng nh sng nhiu x l khng ng k. iu ny l do vt cn hay cc khe h cn truyn ln lt cc bc sng ca sng in t c th nhn c tn hiu tt nht bn thu. Ngun bc x c th l mt cht im c th pht ra theo mi hng.

Hnh 2.9 Nhiu x hnh li dao

a. Nhiu x hnh li dao Knife Edge Diffration Geometry

Nh hnh 2.10 cc gi thit a ra l chiu cao vt cn = h, c t 1 khong cch d1 so vi ngun pht sng, v cch d2 so vi my thu. Chnh lch khong cch gia ng trc tip v ng tnh theo nh hnh v l:

=

Hnh 2.10. S tnh ton nhiu x hnh li dao

Hay ta c th tnh theo cng thc sau:

= h2(d1 + d2)/(2d1d2)

lch pha:

= 2/ = 2h2(d1 + d2)/(2d1d2.)

t = + , gc b nn xem ~ tan, ta c th vit li nh sau:

tan = tan + tan = h/d1 + h/d2 = h(d1 + d2)/d1d2

n gin ha, chng ta thng s dng cng thc nhiu x Fresnel Kirchoff sau:

v = h =

Do lch pha c biu din li nh sau:

= v2/2

b. Vng Fresnel - khi nim v s tn hao do nhiu x

C mt vi hin tng m chng ta cn bit n, u tin l nguyn tc truyn sng Huygens trong ni rng sng s bt u to cc sng trn mi bt u ti mi im ca mt u sng. S nhiu x y xy ra l khi sng truyn i chm mt i tng trong khng gian, sng trn s b nhiu x ra pha ngoi v hng y ln pha trc trong hnh dng mt na vng trn nh hnh di y:

Hnh 2.11. Vng Fresnel

Th hai, tt c cc chm tn hiu RF s m rng khi chng lan truyn qua mi trng khng kh, gc bp sng m chng m rng c th b gii hn bi cc anten nh hng. Cui cng, cc tn hiu c th giao thoa vi nhau, c bit l ti cng tn s, do cc sng lch pha vi nhau. dch pha ny c tnh nm t 0 n360ovi s cng pha hon ton xy ra ti 0 v360ov lch pha hon ton ti 180o. Cc sng chng ln nhau c gi l cng pha, tri qua s giao thoa vi bin tn hiu ca chng c cng thm vi nhau to ra mt tn hiu mnh, nhng cc tn hiu lch pha tri qua s giao thoa trit gim ln nhau lm gim s khuch i ca tn hiu, c th ln n im trit tiu hon ton nu chng lch pha nhau 180o. iu ny c th d dng hn hiu trc quan nh th hin bi cc s di y:

Hnh 2.12 lch pha tn hiu vng Fresnel

Nh cp trc , cc i tng vng b khut ngun sng th s b tn hao nhiu x nhiu hn. H qu ca tn hao nhiu x c gii thch bi vng Fresnel nh mt hm v s khc bit ng sng.

L thuyt ca Vng Fresnel ly mt ng lin kt v tuyn trc tip gia hai thit b mng v tuyn ang hot ng vi cc im A v B mi u v khng gian ba chiu xung quanh ng lin kt nh hng n tn hiu ng gp ti im nhn cui. Mt s cc sng tn hiu truyn i trc tip dc theo tuyn ng t A n B trong khi cc sng khc truyn i lch khi ng lin kt thng trn nhng tuyn ng lch trc, dn n tuyn truyn dn ca chng c mt khong cch di hn gy ra s dch pha gia cc chm sng khc nhau. Bng cch a ra cc khi nim c gii thch trn v p dng mt s tnh ton cho pht hin rng, c nhng vng hnh vng elip nm xung quanh ng lin kt AB trc tip m c nh hng n cng tn hiu c truyn n trm nhn.

C mt s lng v hn cc vng Fresnel c th tnh c, nhng vng Fresnel nh hng nhiu nht n hiu sut ca mng v tuyn l vng Fresnel th nht (1st Fresnel Zone). Nu c bt k vt cn tr, chng hn nh cc ta nh, cy ci hoc nhng ngn i, nm trong vng Fresnel 1 ny th tn hiu s b nh hng bi cc vt cn ny v do n s b suy yu ti trm tip nhn. Theo quy lut ngn tay ci, khi quy hoch nhng lin kt v tuyn, vng Fresnel 1 nn lun phi c thng thong, tuy nhin iu ny c th l khng thc t. V vy thc t cho rng khng nn qu 40% vng Fresnel 1 b che khut tc l 60% phi thng thong, tuy nhin khuyn ngh cho hiu sut ti u phi l khng qu 20% b che khut hoc b chn t hn. Lu rng nhng vt cn tr trong vng Fresnel 1 s to ra tn hiu lch pha t 0-90o, lch pha 90-270os thuc vng Fresnel th hai, lch pha 270-450 thuc vng th ba.

Hnh 2.13 Vng Fresnel Line - of - Sight

tnh ton Vng Fresnel cho mng v tuyn ca bn, u tin thit lp cc ng RF LoS (Line-of-Sight), l cc ng lin kt theo phng din sng v tuyn trc tip, theo tm nhn thng gia anten trm pht v anten trm thu (hoc cc trm thu pht). Phng trnh tng qut tnh bn knh vng Fresnel ti bt k im P gia cc im u cui ca lin kt v tuyn c th c ch ra bng phng trnh sau y:

Fn =

Trong :

Bn knh vng Fresnel th n (m)

Khong cch ca im P n im A (m)

Khong cch ca im P n im B (m)

Bc sng ca tn hiu (m)

Bn knh ca vng Fresnel l ti gi tr cao nht (khong cch ln nht) ngay ti im trung tm RF LoS v i vi cc ng dng thc t ca vic quy hoch mng v tuyn, n th hu ch bit bn knh ti a ca vng Fresnel 1.Do , cc khong cch gia im A v B ti im P s ging ht nhau v bng cch chuyn i cc gi tr bc sng sang tn s tn hiu chng ta c nh sau:

=

d1 = d2 do d1 + d2 = D

Trong

Tc nh sng (3x108m/s)

Khong cch tng cng (m)

Tn s tn hiu (Hz)

Do , nu cc gi tr tn s v khong cch tng cng c chuyn i tng ng thnh GHz v km, phng trnh cho bn knh ti a ca vng Fresnel 1 c th c tnh theo cch sau y:

r = 8.657

c. M hnh nhiu x hnh li dao

M hnh nhiu x hnh li dao l mt trong nhng m hnh nhiu x n gin nht tnh ton, c lng tn hao nhiu x. N xem cc i tng nh nh cao c, cc nh ni ging nh l cc li giao. Lc ny, cng in trng Ed ca mt sng nhiu x li giao c tnh bi cng thc:

Ed/Eo = F() = (1+j)/2

Mc khuch i nhiu x do cc i tng li dao c tnh bng cng thc:

Gd(db) = 20log|F()|

Khi c nhiu hn mt vt cn, cc m hnh tng ng c th tm tng t da trn 1 m hnh nhiu x hnh li dao ging nh hnh di:

Hnh 2.14 Nhiu nhiu x hnh li dao trong mt vng

4. Link Budget Analysis4.1 M hnh path-loss Log-distance

Theo nh m hnh nng lng nhn c khong cch d c ra bi cng thc:

PL(d)( => = + 10nlog(

Gi tr ca n ty thuc vo cc mi trng truyn dn. n = 2 l dnh cho khng gian t do. 4 n 6 ng vi cc mi trng c vt cn l cc ton nh, 3 n 5 ng vi mi trng th. L yu t quan trng la chn khong cch quy chiu chnh xc do. i vi cell trung bnh c bn kn ph sng 1Km, khi cc h thng micro-cell?

4.2 Log Normal Shadowing

Phng trnh cho Log normal shadowing c a ra bi cng thc sau:

PL(dB) = (dB) + X = ) + 10nlog( + X

y X l phn b ngu nhin tun theo phn b Gauss (n v l dB) vi lch chun cng c n v dB. Trong thc t, gi tr ca n v c o t c s d liu c sn.

Nng lng trung bnh nhn c:

Hm Q c a ra theo cng thc sau:

Q(z) = 0.5(1-erf())

V

Q(z) = 1 Q(-z)

V vy xc sut cc mc tn hiu nhn c (dB) s vt qu mt gi tr nht nh l

P(Pd > ) = Q()

5. M hnh truyn sng ngoi tri

C rt nhiu m hnh lan truyn v tuyn ngoi tri, v d nh m hnh Longley-Rice, m hnh Durkins, m hnh Okumura, m hnh Hata M hnh Longley Rice l mt trong nhng m hnh c s dng ph bin nht di tn t 40MHz-100GHz trn cc a hnh khc nhau. Mt s thay i c bn so vi m hnh s khai l thm vo cc yu t th (UF). Di y chng ta s bn v m hnh Okumura Hata ni ting.

M hnh Okumura - Hata

M hnh Okumura l mt trong nhng m hnh c s dng rng ri nht d on tn hiu trong khu vc th. M hnh ny c p dng cho cc tn s trong khong 150-1920MHz (mc d n thng c ngoi suy ln n 3000Mhz) v khong cch thu pht t 1km cho n 100km. N c th c s dng cho cc trm gc khc nhau vi chiu cao ht nm trong khong t 30m n 1000m.

Trong m hnh ny, ban u suy hao ng truyn c tnh bng cch tnh h s hiu chnh anten cho cc vng th l hm ca khong cch gia trm gc, trm di ng v tn s. Kt qu c iu chnh bng cc h s cho cao anten trm gc v trm di ng .

PLd(db) = L(fc,d) + Amu(fc,d) G(ht) G(hr) Garea

Lp= 69,55+26,16.lgfc 13,28.lghb a(hm) + (44,9-6,55.lghb).lgR (dB)

Trong :

fc : Tn s hot ng (MHz);

Lp : Tn hao cho php.

hb : cao anten trm gc (m);

R : Bn knh (km).

a(hm) : H s hiu chnh cho cao anten di ng (dB)

Di thng s s dng cho m hnh Hata l:

+H s hiu chnh (hm) c tnh nh sau [6]:

- i vi thnh ph ln:

a(hm) =8,29.(lg1,54hm)2 - 1,1 (dB) vi fc 200MHz (4.13)

a(hm) =3,2.(lg11,75hm)2 - 4,97 (dB) vi fc 400MHz (4.14)

- i vi thnh ph nh v trung bnh :

a(hm) = (1,11.lgfc 0,7).hm (1,56.lgfc 0,8) (dB)(4.15)

Nh vy bn knh c tnh :

(4.16)

- Vng ngoi : Vi vng ngoi h s hiu chnh suy hao so vi vng thnh ph l [6]:

Lno = Lp - 2 (dB)(4.17)

- Vng nng thn: Vi vng nng thn h s hiu chnh suy hao so vi vng thnh ph l[6]: Lnt = Lp 4,78.(lgfc)2 +18,33(lgfc) - 40,49 (dB) (4.18)

Kho st phng trnh (4.16) vi cc s liu sau: tn s fc=880 (MHz), cao anten trm gc hb=30 (m), cao trm di ng hm=1,5 (m). Hnh 4.5 biu din ng suy hao theo bn knh c kho st theo m hnh truyn sng Hata-Okumura.

(ng cong suy hao theo bn knh)

Hnh 2.15 Suy hao ng truyn theo bn knh vi m hnh Hata.

Hnh 2.15 biu din bn ng cong suy hao ca bn vng: vng nng thn, vng ngoi , vng thnh ph ln v vng thnh ph trung bnh v nh. Vng nng thn c suy hao thp nht trong cc vng, ng vi bn knh 5 km th vng nng thn suy hao 123 dB, vng thnh ph 151 dB. Suy hao ca mi vng ph thuc a hnh mi trng truyn sng ca vng .

6. M hnh truyn sng trong nh

Knh pht thanh trong nh khc vi cc knh v tuyn di ng truyn thng im l khong cch, bn knh bao ph c nh hn nhiu, v s thay i ca mi trng cng lm cho khong cch gia Tx Rx nh hn. Vic cc yu t ngoi cnh nh cc loi nh, cc vt liu xy dng, hay cch b tr cc ton nh, cng lm nh hng mnh n s lan truyn sng bn trong chng. Truyn sng v tuyn trong nh cng b chi phi bi cc yu t nh ngoi tri, chng hn tnh cht phn x, nhiu x v tn x vi cc iu kin bin i. Chung quy li th cc knh trong nh c th c phn lp tng ng ging nh tm nhn thng hoc l b che khut.

7.1 S phn vng tn hao bn trong mt tng (intra floor)

Cu trc bn trong v bn ngoi ca mt ta nh c nhng s khc nhau c trng, chnh v th n to ra nhng phn vng khc nhau. Phn vng c hnh thnh theo cu trc xy dng ca ta nh c gi l phn vng cng, v cc phn vng c th c di chuyn m khng ko di ln n trn nh c gi l phn vng mm. Cc phn vng khc nhau da trn cc t tnh vt l, in ca chng, lm cho chng tr nn kh khn hn trong vic p dng m hnh c th lp t trong nh.

7.2 S phn vng tn hao gia cc tng vi nhau (inter floor)

S tn hao gia cc tng ca mt ta nh c xc nh bi kch thc bn ngoi v vt liu xy dng ca ta nh. Ngay c s lng ca s trong mt ta nh, hay mu sn ca chng cng c nh hng n s tn hao gia tng tng.

7.3 M hnh tn hao ng truyn Log-distance

Tn hao ng truyn trong nh c tnh da trn cng thc sau:

PL(db) = PL(do) + 10nlog10(d/do) + X

Trong n ph thuc vo ton nh v khu vc xung quanh n, v X l mt thng s i din cho n v o dB vi lch chun l dB.

III. Kt lun

Trong bi tp ln ny, chng ta cp ch yu n ba m hnh lan truyn sng in t chnh, l: lan truyn trong khng gian t do, khc x, nhiu x. y l nhng m hnh ph bin c p dng v n cng gii thch cho s tn hao ng truyn. V phn path-loss (suy hao ng truyn), mt yu t ph bin c cp n l mi hnh suy hao ng truyn log-distance (truy cp t xa). Ngoi ra, cn cp v nhng vn lin quan n s lan truyn sng v tuyn ang c p dng trong i sng thc t ca chng ta.

TI LIU THAM KHO

[1] ng Thanh Bnh Chng 2 S lan truyn v tuyn .

[2] Chapter 4 - Free Space Radio Wave Propagation

[3] T. S. Rappaport, Wireless Communications: Principles and Practice, 2nd ed. Singapore: Pearson Education, Inc., 2002.

[4] S. Haykin and M. Moher, Modern Wireless Communications. Singapore: Pear-son Education, Inc., 2002.

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