Download Unit 1 Introduction to Spread

Unit 1 Introduction to SpreadSpectrum Systems
Department of Communication Engineering, NCTU
1
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
What does it mean by spread spectrum communications
 Spread the energy of an information bit over a bandwidth
which is much larger than the information bit rate
Why spread the energy of information bit?
 Originally to provide a system which is robust to jamming
in a military environment
 Intentional jammer v.s. unintentional interference
Jammer
Interferenec
Broadband noise
Barrage jammer
Multiuser interference
Narrowband noise
Partial band jammer
Multi-system /
multiuser Interference
Pulse noise
Pulse jammer
Multiuser Interference
Multiple sinusoids
Multi-tone jammer
Harmonic/image
Department of Communication Engineering, NCTU
2
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
The effect of a typical narrow band interference
 Suppose BPSK is used, the transmitted signal of the form
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In the presence of interference plus noise, we have
r(t) = s(t) + j(t) + n(t)
Suppose the power spectral density of j is equal to Sj = J/2B
Department of Communication Engineering, NCTU
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Spread Spectrum
Lecture 1 Introduction
Sau-Hsuan Wu
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Matched-filter the received signal and threshold
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The discretized received signal is of the form
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where S = Es / T
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and
Department of Communication Engineering, NCTU
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Spread Spectrum
Lecture 1 Introduction
Sau-Hsuan Wu
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The bit error rate of detection is
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where Rs = 1/ Ts
The loss is this system relative to a system with no
interference is
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How to cut this loss?
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Increase the signal energy
Design a system to operate at a lower signal to noise ratio →
better channel CODEC
Increase B → Spread the interference energy (?)
Department of Communication Engineering, NCTU
5
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Define an effective Es / N0 as
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Suppose the target BER is 10-3 ⇒ (Es/N0)eff =6.8dB
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The J/S satisfies
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If Rs / B =1 S > 4.8J
Now, suppose we use a signaling scheme which expands
the bandwidth by a factor of 100 so that Rs / B =1 /100
⇒ S > J / 21
How to dilute the noise PSD?
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Department of Communication Engineering, NCTU
6
Spread Spectrum
Lecture 1 Introduction
Sau-Hsuan Wu
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Power spectrum densities of the signal and the
interference
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PSD after spreading
Department of Communication Engineering, NCTU
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Spread Spectrum
Lecture 1 Introduction
Sau-Hsuan Wu
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Goal is to have the PSD of the desired signal after despreading looks like below
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How to do energy spreading
 The most simpler way is direct-sequence spread spectrum
Department of Communication Engineering, NCTU
8
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Direct sequence spread spectrum (DS-SS)
 Date sequence : d(t) = 1 1 -1 1 -1
 Code sequence : c(t) = -1 -1 -1 1 1 -1 1
Department of Communication Engineering, NCTU
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Spread Spectrum
Lecture 1 Introduction
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At the transmitter, we do signal spreading
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At the receiver, we do de-spreading
Department of Communication Engineering, NCTU
Sau-Hsuan Wu
10
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Slow frequency hopping (FH)
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Data sequence: +1 -1 | -1 -1 | +1 +1 | -1 +1 | -1 -1
Code sequence: -1 -1 -1| +1 -1 +1| +1 -1 -1| -1 +1 -1| +1 +1 -1
0
1
2
3
4
5
6
7
Department of Communication Engineering, NCTU
11
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Hybrid time and frequency hopping (Ts > Tc)
Department of Communication Engineering, NCTU
12
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Time-hopping (pulse position modulation) TH-PPM
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code sequence : 0 1 0 0 0 0 0 1 0 0 -1 0 -1
Transmitted waveform
0
1
0
0
frame
0
0
0
1
0
0
Department of Communication Engineering, NCTU
-1 0
-1
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Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Important issues related to spread spectrum techniques
 How to get receiver’s spreading sequence in sync with the
transmitter’s spreading sequence?
 How to generate spreading sequence which is difficult to
predict or at least exhibit certain properties?
 How many chips per bit should be used?
 How to build a frequency synthesizer which is
programmable and capable of rapid transitions from one
frequency to the next (inexpensive would be nice too)?
Department of Communication Engineering, NCTU
14
Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Are there other reason for spread spectrum
communications
 Provide multiple access mechanism
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Traditional approaches are frequency division multiple access
and time division multiple access
Spread spectrum enables another multiple access scheme
called spread spectrum multiple access , also know as code
division multiple access (CDMA)
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DS-CDMA
Each user is assigned a different spreading sequence and all users
occupy the entire frequency band at all times
FH-CDMA
Each user follows a different hopping pattern,
TH-CDMA
Department of Communication Engineering, NCTU
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Spread Spectrum
Lecture 1 Introduction
Department of Communication Engineering, NCTU
Sau-Hsuan Wu
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Spread Spectrum
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Lecture 1 Introduction
Sau-Hsuan Wu
Advantages of CDMA over FDMA or TDMA
 Provides a means to support multiple users and
simultaneously to be robust to interference from sources
other than the system users
Problems with CDMA
 Multiple access interference
 Effective interference PSD
Department of Communication Engineering, NCTU
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