4. EMC??? ? ???? ?

1
??????????-??LAN(???)-

• ?????????????
• ??????? ?????????????
• ?? ?

2
????

• ??LAN??????
• ??????????
• 802.11b?????????
• 802.11a?????????
• OFDM

3
????????
?????
???????
??
??
4
????? (Inter-Symbol Interference ISI)
?????
???????
??
??
????
????
??
??

• ??????????????????????.

5
802.11 DS/SS

• 2.5GHz
• ISM-band (Industry Scientific and Medical)
• ??????????????
• ??????????
• ?????????
• ?????
• 1Mbps/ 2Mbps
• DSSS (Direct Sequence Spread Spectrum)

6
ISM band
24712497MHz???1??????
ch.14
ch.113 ???????????????????3?????????????
2400MHz2483.5MHz???13??????
2400
2420
2440
2460
2480
2500MHz
7
802.11 DS/SS
1Msps
1Mbps 2Mbps
????????
DQPSK DBPSK
??
11Mcps
???
????
1Msps
1Mbps 2Mbps
???
DPSK??
?????????
11Mcps
???
????
(??????1,-1, 1, 1, -1, 1, 1, 1, -1, -1, -1)
8
802.11b CCK

• CCK Complementary Code Keying

6
11Mcps
????
???
S/P
11Mbps
?????? ??
DQPSK
??
2
18
1Msps
6
?? ??
P/S
11Mbps
??????? ??
DQPSK
2
???
18
9
CCK??
(d2k-1, d2k) jk (in deg)
00 0
01 90
10 180
11 270
10
CCK??
?????? j2 j3 j4
1 ? ? ?
2 ? ?
3 ? ?
4 ?
5 ? ?
6 ?
7 ?
8
11
Block Diagram of CCK Demodulator
C2
C3
C4
0
0
0
0
0
p/2
0
0
p
0
0
-p/2
0
p/2
0
x0
0
p/2
p/2
x1
0
p
p/2
x2
0
-p/2
p/2
x3
p
x4
0
0
p
x5
0
p/2
x6
0
p
p
x7
0
-p/2
p
-p/2
0
0
-p/2
0
p/2
-p/2
0
p
-p/2
0
-p/2
16-tap
32-tap
64-tap
12
Butterfly Operation
13
??????????
192ms
???PLCP?????? 144???
PLCP??? 48???
PSDU
1Mbps DSSS
1Mbps / 2Mbps DSSS 5.5Mbps/ 11Mbps CCK
96ms
????PLCP?????? 72???
PLCP??? 48???
PSDU
1Mbps DSSS
2Mbps DSSS 5.5Mbps/ 11Mbps CCK
2Mbps DSSS
14
IEEE 802.11a/g

• 5GHz?
• ??5.155.25MHz (4.9GHz?, 5.0GHz????)
• ??5.155.35MHz, 5.725MHz5.825MHz
• 6, 9, 12, 18, 24, 36, 48, and 54Mbps
• 6, 12, 24?mandatory
• IEEE802.11g
• 2.4GHz?

15
5GHz??
2005?5???
16
IEEE 802.11a ??
17
Data Rate
Rate Modulation Coding Rate Signal Bits
6Mbps BPSK R1/2 1101
9Mbps BPSK R3/4 1111
12Mbps QPSK R1/2 0101
18Mbps QPSK R3/4 0111
24Mbps 16QAM R1/2 1001
36Mbps 16QAM R3/4 1011
48Mbps 64QAM R2/3 0001
54Mbps 64QAM R3/4 0011
optional
18
802.11a??????????
19
802.11a????????????
??????? 20ms
20
OFDM??

• IEEE802.11a/g???????OFDM?????

21
OFDM

• ?????????
• ????
• ????????????????????
• ??????????????????
• ??????????

22
?????????
?????
???????
??
??
????
???
???
??
??

• ??????????

23
?????????

• 1957 ????????????
• 1966 ??????????????????
• 1971 DFT?????????????(OFDM)
• 1981 DFT/Roll-off Filtered MCM
• 1983 MCM for Microwave Digital Transmission (NTT)
• 199x xDSL/ DAB/ DVB-T/ W-LAN (IEEE802.11a/g),
  802.16a
• 2003 ISDB-T (???????????)
• PAN (IEEE 802.15.3a), 802.11n, 802.16, 802.20
• 3GPP/ 4G Mobile

24
OFDM

• Orthogonal Frequency Division Multiplex
• ?????????
• ?????????
• ???????????????????
• ??????????????????

25
OFDM????
26
OFDM????
01
???? ?????
?? ?? ??
01
???? ?????
0111101
01
???? ?????
???? ??????
?? ?? ??
???? ??????
???? ??????
27
???????????
28
??????(2)
29
??????????????(1)
????????
???????N
freq
k?????????? ?????
30
??????????????(2)
31
??????????????(3)
32
????????

• Fourier, Baron Jean Baptiste Joseph.
• 1768-1830.
• French mathematician and physicist who formulated
  a method for analyzing periodic functions and
  studied the conduction of heat.

Cited http//dictionary.com/
Cited http//www-groups.dcs.st-and.ac.uk/history
/Mathematicians/Fourier.html
33
????????

• ??????????????????????.

????????
??????
34
OFDM?????????
?????????
???????????
35
OFDM????
T
0
1
2
-1
-2
time
36
OFDM
T
time
n0
n1
n2
37
???????

• ????????????????

???(?????)
??????
????????
?????
38
???????????????
?????
???????
??
??
????
???
???
??
??
???????

• ????????

39
???????????????
T
???
time
?????
time
40
Cyclic Extension???ICI??

• ????????????

D
F
T

W
i
n
d
o
w
D
F
T

O
u
t
P
u
t
G
.
I
.
C
k
f
F
r
e
q
k
41
OFDM?????????
TsTTG
freq
freq
42
????????

• DFT (Descrete Fourier Transform)
• ??????????????????
• ????????
• ????? -N/2tsN/2ts
• ?????? 1/ts

43
DFT
44
????????

• FFT (Fast Fourier Transform)
• Cooky / Tukey (1965)
• DFT????N2
• N????????????
• ??2FFT????Nlog2N
• 8k???
• DFT 81922?67M
• FFT 819213?100k (?1/600)

45
??2?FFT???
46
?????
47
?????
48
DFT
???
?
?
49
?????
50
8 Point DFT
51
DFT????(1)
52
DFT????(2)
53
DFT????(3)
54
?????
55
????????????
Matched Filter (training Seq)
Peak Detector
56
??????????
????? ?????????????
????? ????????????
57
????????
??
????? ???
freq
freq
58
802.11a??????????????
FFT???,?????
59
???

• ??LAN??????
• 802.11b
• SS/ CCK
• 802.11a/g
• OFDM