Lab 9 直序列展頻通訊Direct Sequence Spread Spectrum (DSSS)

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EE4650 通訊實驗
Lab 9
直序列展頻通訊
Direct Sequence Spread Spectrum
(DSSS)
1
國立清華大學電機系 EE4650: 通訊系統實驗, Instructor: Prof. 吳仁銘, jmwu@ee.nthu.edu.tw
Lab 9 Direct Sequence Spread Spectrum (DSSS)
RF Frontend
Baseband Tx
0101
Error
Correction
coding
Digital
Modulation
D/A
BPF
PA
Fading
Channel
MSeq
010110
cos(2fct)
AWGN
Baseband Rx
0101
010110
Error
Correction
decoding
2
RF Frontend
Digital
Demodulation
A/D
BPF
cos(2fct)
LNA
實驗目的
 瞭解展頻通訊的設計原理及應用。
 了解虛擬随機碼的邏輯電路。
實驗器材
桌上型電腦
LabVIEW Full Development System
LabVIEW Modulation Toolkit 3.1 (and up)
3
Spread Spectrum
 Invented by the Hollywood actress, Hedy Lamarr in 1940.
 Spread signal over a wide frequency range higher than the
minimum necessary bandwidth.
 Advantages :
- Interference rejection
- Multipath rejection
- Multiple access communication
 Two main techniques:
- Direct sequence spread spectrum (DSSS)
- Frequency hopping spread spectrum (FHSS)
 Spread spectrum techniques is now widely
used in modern wireless communications.
(Hedy Lamarr, 1914 -2000)
Spread Spectrum, CDMA, and Multi‐User Access
Application showcases:
- GPS,
- 2G cellular system: IS-95,
- 3G cellular systems: CDMA-2000,
WCDMA
M-Sequence
 M-sequence is also called PN sequence, and is periodic
binary sequence with noise-like waveform.
 Usually generated by a feed back shift regestor with m
shift-register and period of N  2m  1
 Simple implementation with Flip-Flops and
combination logics gate.
M-Sequence Example
p(x)=1+x+x3
S0=S1+S3
xor
Ss 0
3
1
2
s2
S
1
s
S
2
1
3
S3s
Q: Is this an M-sequence?
0
cycle
S0
S1
S2
S3
0
0
1
1
1
1
1
0
1
1
2
0
1
0
1
3
0
0
1
0
4
1
0
0
1
5
1
1
0
0
6
1
1
1
0
7
0
1
1
1
A: Yes. The states S0S1S2=111, 011,101,010,001,100,110,… repeats every
7 cycles. The output sequence is 1110100.1110100.1110100…
 The output can also be found by the long division of 1/P(x)
7
M-Sequence Properties
 Balance Property:
The number of 1’s about the same number of 0’s.
Pr( x  1)  Pr( x  0) 
 Run property
1
2
Subsequence of run-length = n makes about (½)n of the overall
M-sequence.
Run-length = 1, makes
1
1
of the sequence, run-length = 2, makes of the sequence.
2
4
 Correlation property
The auto-correlation of a M-sequence is similar to [n]
The cross-correlation between two M-sequence is zero
Applications of M-Sequence
 Random Binary Data as Testing source
 Binary noise
 Pseudo-random noise (AWGN noise)
 Direct Sequence Spread Spectrum (DSSS)
 Code Division Multiple Access (CDMA)
Direct-Sequence Spread Spectrum (DSSS)
with BPSK system.
Direct Sequence Spread Spectrum (DSSS)
 Each bit in original signal is represented by multiple
bits in the transmitted signal, using a spreading code.
 Signal energy spread over a wider frequency band.
DSSS Tx / Rx with BPSK
 Transmitter
x(t )  m(t ) A cos( 2πfct   )
 b(t )c(t ) A cos( 2πfct   )
x (t )
 Receiver
BPSK
Demod
w(t )  y(t )c(t ) A cos( 2πfct   )
  x(t )  i(t )  j(t ) c(t ) A cos( 2πfct   )
1 A2
 b(t )(  cos( 4πfct   ))  i(t )  j(t ) c(t ) A cos( 2πfct   )
2 2
Example of DSSS using BPSK
13
實驗步驟
在LabView 中LFSR做出一隨機產生碼，記錄其輸出的值和輸出波形圖。
實驗步驟
 DSSS Encoding
Spread Symbols.vi
15
實驗步驟
 DSSS Decoding
- 調整Noise的大小 (0.003/0.03/0.1/0.3)，Spreading sequence length
長度 (=4/6/8/10)，message bits = 1000, 測試BER
Spread Symbols.vi
Bernoulli
Noise.vi
Despread
Symbols.vi
16
The Birth of Spread Spectrum
Communications
How "The Most Beautiful Girl In The World" and
"The Bad Boy of Music" Catalyzed A Wireless Revolution-In 1941
Hedy Lamarr (1914-2000)
Lamarr told Antheil about her idea for a Secret Communications System that
could guide torpedoes to their target without being intercepted by the enemy, by
sending messages between transmitter and receiver over multiple radio
frequencies in a random pattern.
US Patent Number 2,292,387 for the
Secret Communications System
 Two pages of drawings from Lamarr and
Antheil's patent.
 Note the player-piano-like slotted paper
on the second sheet.
Facts of Spread Spectrum
 On August 11, 1942, Lamarr and Antheil were awarded
US Patent Number 2,292,387 for the Secret
Communications System.
 The Secret Communications System was never used
during World War II.
 First developed electronic spread-spectrum system
that handled secure communications for the US is
during the Cuban Missile Crisis in 1962.
 In the mid-1980s, the US military declassified spreadspectrum technology, and the commercial sector began
to develop it for consumer electronics.
 Today, it's an increasingly important component of
mobile telephony. CDMA (Code Division Multiple
Access) technology uses spread spectrum. Spread
spectrum has proven highly useful in cellular
telephones

Lab 9 直序列展頻通訊Direct Sequence Spread Spectrum (DSSS)

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