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3.3 随参信道及其传输特性  PowerPoint PPT Presentation


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3.3 随参信道及其传输特性 . 随参信道是指信道传输特性随时间随机快速变化的信道。 常见的随参信道有陆地移动信道、短波电离层反射信道、超短波流星余迹散射信道、超短波及微波对流层散射信道、超短波电离层散射以及超短波超视距绕射等信道。我们首先介绍两种典型的随参信道。 . 3.3.1 陆地移动信道 陆地移动通信工作频段主要在 VHF 和 UHF 频段,电波传播特点是以直射波为主。但是, 由于城市建筑群和其他地形地物的影响,电波在传播过程中会产生反射波、散射波以及它们的合成波,电波传输环境较为复杂,因此移动信道是典型的随参信道。 1. 自由空间传播

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3.3 随参信道及其传输特性 

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3 3

3.3


3 3

3.3.1

VHFUHF

1.

VHFUHFPT()


3 3

PR=PTGTGR

GTGRd 1(3.3 - 1)

PR=PT

Lfs=


3 3

(3.3 - 2)

Lfs=

dB

Lfs=20lg =32.44+20lgd+20lgf (dB) (3.3 - 5)

dkm fMHz(3.3 - 4)d 3 - 16


3 3

2.

3 - 17 3 - 18

R= (3.3 - 6)

:

z=


3 3

z= () (3.3 - 8)

0=-j60 (3.3 - 9)

3.

3 - 19

k=


3 3

k r0=6370km re k=


3 3

3.3.2

X60~600 km 3~30 MHz(10~100m)()


3 3

DEF1F2 3 - 20 DEF1F2 DF1EF2DEDEF2250~300 km4000 km


3 3

0

fMUF=f0 sec0 (3.3 - 11)

f00=0()

F2 F2


3 3

:

(1)

(2)

(3)

(4)

3 - 21


3 21 a b c d

3 - 21(a) (b) (c) (d)


3 3

3.3.3

(1)

(2)

(3)


3 3

1.

3 - 17

s(t)=A cosct (3.3 - 12)

n


3 3

r(t)=a1(t) cosct-1(t)+a2(t) cosct-2(t)++an(t)

cosct-n(t)

= (3.3 -13)

ai(t)ii(t)i

r(t)=

i(t)=-ci(t) (3.3 - 15)


3 3

i

(3.3 - 14)

X(t)Y(t)


3 3

n X(t)Y(t) X(t)Y(t)

x=y

(3.3 - 16)

r(t)=V(t)cosct+(t) (3.3 - 21)


3 3

V(t)=X2(t)+Y2(t) (3.3 - 22)

(t)=arctan (3.3 - 23)

2 V(t)(t)

f()=


3 3

x=y=v=

ai(t)i(t)V(t)(t)r(t)2.5r(t)r(t)r(t)fc 2 - 7 :

(1)

(2)


3 3

2.

3 - 22 k(t)

si(t)

so(t)=ksi(t)+ksit-(t)Y] (3.3 - 26)


3 3

3 22


3 3

So()=kSi()+kSi()e-j(t)

=kSi()1+e-j(t) (3.3 - 27)

H()=So() Si() =k1+e-j(t) (3.3 - 28)

1+e-j(t)


3 3

i 3 - 23(a) (3.3 - 29)


3 3

3 23


3 3

(t) 3 - 23(b)

m


3 3

1/5~1/3


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