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??????? ??????? ?????? ??? (Spread Spectrum) Chapter 2a

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A Basis for a Jamming Game (cont. ... Communicator also collects jamming energy. What is the Pr that exactly N communicator receivers will be jammed? ... – PowerPoint PPT presentation

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Title: ??????? ??????? ?????? ??? (Spread Spectrum) Chapter 2a


1
??????? ??????? ?????? ???(Spread Spectrum)
Chapter 2a
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2
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??? 1 ???? ?????? ???????? Spread Spectrum ????? ??????? ??? ??? ????- ????? ???? ???????? ?? ?????? ?????? ??????? ????? ??????? ??????? ??????, ?????? ???????? ?? ???????, ?????? ????? ????? ???????, ?????? ????????.
??? 2 ???? ??????? ??????? ??? (Spread Spectrum) - ?????? ??????? ??????? ??????? ??? ????? ????? ??? ???????? ?? ????? ????? DS)) ??????? ???? TH) ) ??????? ???? (FH)
??? 3 ????? ??? ??????? ??????? ??? - LFSR, Gold Sequence, Walsh
??? 4 ??????? ?? ?????? ?? ????? ??? ????? (DS) ??????? ?? ?????? ?? ?????? ??? (FH) ????? ?????, ????? ??????? ?? ????? Spread Spectrum
??? 5 ????? ?????? ??????, ??????? ?? ?????? Spread Spectrum ?? ????? ?????? ??????, ???????? Viterbi
??? 6 ??????? CDMA ??????? ????
??? 7 ??????? ?????????? ?? ?????? Spread Spectrum
8 ??"?
8 ??"?
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3
1. Introduction
  1. Secure communications
  2. EW Electronic Warfare
  3. ECCM
  4. Communication System Requirements
  5. Global System Approach to ECCM

4
1.1 Secure Communication ????? ??????? ?????
?????
  • Electronic Warfare (EW) ????? ?????????
  • The enemy uses Electronic Counter Measures (ECM)
  • Secure Communication
  • The communicators use Electronic Counter-Counter
    Measures (ECCM).

ECCCM.
5
1.2 Electronic Warfare (EW)
  • Interception ????? ????? ?????? ("?????")
  • LPI Low Probability of Interception
  • Data Extraction ????? ?????? ????? ???????
  • COMMINT, ELINT, HUMINT
  • Jamming ????? ????? ???????
  • Deception
    ?????

6
1.3 ECCM ????? ???-??? ????? ?????????
  • Encryption
  • Power Control
  • Adaptive Antenna Array
  • Adaptive Time and Frequency Filtering
  • Spread Spectrum
  • Direct Sequence (DS)
  • Frequency Hopping (FH)
  • Time Hopping (TH)
  • Error Correcting Codes
  • Alternative Routing

7
1.4 communication System Requirements
  • Functional Requirements
  • Data Flow
  • Operation and Special Features
  • EW Threat
  • Interception
  • Data Extraction
  • Jamming
  • Deception
  • Collocation Requirements
  • Users Number
  • Relative Location
  • Mode of Channel Use
  • Resources
  • Budget
  • Weight and Volume
  • Frequency Range

8
1.5 Global System Approach to ECCM
  • System Control
  • Time and Frequency Management
  • Alternative Routing
  • Communication Protocols ??? ??????
    ?????????? ?? ????????? ??"?
  • Communication Controller
  • Error Correcting Codes and Interleaving
  • Cryptography
  • Modem and Spread Spectrum
  • Modulation
  • Direct Sequence
  • Frequency Hopping
  • IF/RF and Antennas
  • Power Control
  • FDMA or TDMA
  • Directional Antennas
  • Adaptive Antennas Arrays

9
2. Introduction to Spread Spectrum Systems
  • Definition
  • Spread Spectrum System Concept
  • PN Waveforms
  • Time-Frequency waveforms
  • Spread Spectrum Process

10
2.1 Spread Spectrum System Definition
  • A System is considered to be a spread spectrum
    system if
  • a. The effective bandwidth WSS is much larger
    then the data rate Rb.
  • b. the receiver is able to select the channel
    (or the waveform) where the signal is present and
    disregard the energy of the unused channels (or
    the waveforms).
  • c. The waveforms are random or pseudo random,
    known to the communication but unknown to the
    enemy.

11
2.2 Spread Spectrum System Concept
12
A Basis for a Jamming Game
  • Assume Communication space is divided
    between K Transmitters.
  • Wss Hz total available BW
  • Ts is transmitted signal period

Complex envelope of k-th Tx signal
Scenario of Orthogonal communication complex of
multiplicity K
13
A Basis for a Jamming Game (cont.)
  1. Observed signal at i-th receiver
  1. Strategy for i-th Rx project the signal onto
    the set of basis functions for the i-th Tx
    signal space

14
A Basis for a Jamming Game (cont.)
  • In the absence of J(t) and ni(t) - the i-th Rx
    can correctly discover the data symbols used
    by i-th Tx
  1. Both J(t) and ni(t) can be expanded in terms of
    orthonormal basis

15
A Basis for a Jamming Game (cont.)
  1. In general

No useful jamming signal
Total energy in the Jamming signal
  • Jammer has full knowledge of timing 0, Ts
  • Jammer has full knowledge of the set
  • The EJ can be partitioned into K parts, the i-th
    part representing the energy to jam the i-th Rx

16
A Basis for a Jamming Game (cont.)
Thus,
Additive partitions is direct result of the
orthogonality requirement. Similarly, total Tx
energy is the sum of K signals each related
with the i-th Tx
17
(1) Energy Allocation Strategies
  • Within the orthogonal communication system
    complex of multiplicity K, consider strategies to
    allocate
  • Es communicator energy
  • EJ Jammer energy
  • to the K links.

Communicator strategy Randomly select Each
receiving units. The remaining links are not
used. KS diversity factor. Strategy fully known
to communicator Rx.
Jammer strategy Randomly select receivers
out of K to jam. Each receiving units. The
remaining links are not used.
18
(2) Energy Allocation Strategies
  • Communicator Rx collects all Es units of
    transmitted energy in Ks receivers (and further
    uses diversity combiner).
  • Communicator also collects jamming energy. What
    is the Pr that exactly N communicator receivers
    will be jammed?!
  • - the multiplicity of orthogonal system
    complex, should be as large as possible.
  • Communicator can select Ks 1 to minimize .
  • Using only 1 out of K available links is -
  • pure spread-spectrum strategy

19
(3) Energy Allocation Strategies
  • The strategy to use any of K orthogonal links
    increases Es/EJ at receiving antenna to K
    (Es/EJ)
  • Energy gain, EG, against jamming

That is, EG is the ratio of the signal space
dimension (what the Jammer needs to jam) to the
total dimensions actually used by communicator
over KS links.
  • For pure spread Spectrum strategy

20
2.3 Spread Spectrum Pseudo Random Waveforms
  • a. Frequency Hopping
  • where
  • is a pseudo random sequence of
    frequencies
  • is a random sequence of phases
  • is the time duration of one hop and
  • b. Direct Sequence
  • where
  • is the time duration of one chip and
  • is a complex discrete pseudo random
    sequence

21
2.3 Spread Spectrum Pseudo Random Waveforms
(cont.)
  • c. Time Hopping
  • where
  • is the time duration of one
    time hop and
  • is the start time of one time
    hop

d. Hybrid methods DS-FH, TH-DS, TH-FH, .. e.
Pure random Spread Spectrum Transmitted
Reference (TR) Spread Spectrum Stored Reference
(SR) f. Matched filter (MF) generating a
wideband Tx signal by pulsing a filter having a
long, wideband PN controlled impulse response
22
2.4 Time and Frequency Occupancy of Spectrum
Waveforms
Frequency Hopping when TsgtTh fast FH










WSS
Frequency
TIME
Fast FH
23
2.5 Complex Spectrum Spreading Process
  • a. Multiplication
  • where
  • are complex
    discrete pseudo random process waveforms and
    is complex data waveform
  • For example is a direct sequence and
    is a frequency hopping sequence.

Data modulator
Data Source
Spreading function generator
24
2.5 Spectrum Spreading Process (cont.)
  • b. Switching

The previous method is susceptible to repeater
Jamming transmitting a replica with different
modulation. This reduces multiplicity K factor of
a SS system to unity. Here, data signal
is quantized to M-levels
Per each level a distinct oneoutof M
possible code sequences is transmitted.
25
c. Compress and Shift (delay modulation)
  • Reorganization of modulated data into the
    transmission intervals of a time hopping signal.

26
2.6 Receiver and demodulation aspects
  • Basically, Rx computes the inner product (matched
    filter, correlation receiver)
  • One or both of the complex signals appears as
    real IF or RF signal

SS receiver needs sync. circuits to determine
the inner product
27
Overview of Spread Spectrum Sync.
  • Baseband processing
  • baseband correlation
  • Baseband matched filters
  • Bandpass processing
  • IF multiplication, bb integration
  • IF multiplication and integration (bp
    correlator)
  • Recovery of
  • requires 3 levels of sync.

28
Levels of Spread Spectrum Sync.
  • Correlation interval sync
  • Correlator requires pulse START and STOP
    correlation. In bp implementation this provides
    timing for the sampling and initialization of
    bp filters at the beginning of correlation..
  • In DS correlation sync related with the symbol
    clock period.
  • In FH (fast hopping) each symbol lasts several
    hops. Interval sync pulse indicates Th . No
    meaning to correlate over range with random phase
    transitions.
  • SpSp generator sync
  • Timing signal to control the local SpSp generator
    with the incoming SpSp signal. In DS this
    requires sync to the chip rate 1/Tc. In FH - to
    1/Th
  • Carrier sync
  • For ideal bp to bb operation the frequency
    and phase of local oscillator should be in sync.
    to received signal.
  • In DS both carrier and phase sync is available.
    In FH only freq is attained

29
Bibliography
  1. R. C. Dixon, Spread Spectrum Systems. New Work
    John Wiley, 1976
  2. M. K Simon et. Al., Spread Spectrum
    Communication, Vol. I,II,III, Rockville Maryland
    Computer Science Press,1985.
  3. R. Ziemer, Peterson, Introduction to Spread
    Spectrum Communications, Prentice Hall,1995 2Ed
  4. J. K. Holmes, Coherent Spread Spectrum System,
    New Work John Wiley Sons, 1982 ISBN
    0-471-03301-4.
  5. R. Skaug, J. F. Hjelmstad, Spread Spectrum in
    Communication, London Peter, Peregrinus Ltd.,
    1985, ISBN 0-86941-034-0.
  6. D. J. Torrieri, Principles of Military
    Communication System, Dedham, Massachusetts
    Artech House Inc., 1981 ISBN 0-89006-102-5.

30
3. Anti-jamming Spread Spectrum Systems
  1. Processing Gain
  2. Common Jammers Classification
  3. Anti-jamming System Configuration
  4. Interleaving

31
3.1 Processing Gain (PG)
  • total spread-spectrum signal bandwidth
    available
  • data rate in bits per second

(3,2)
TRANSMITTER
RECEIVER
JAMMER
32
Basic parameters (independent of SpSp type,
coding type etc)
  • Regardless of signal and jammer waveforms we can
    define

bit-energy-to-jammer noise
  • In dB the bit-energy-to-jammer noise

33
Basic parameters- remarks
  • We assume that jammer power is much higher than
    thermal noise. Thus Eb/NJ dictates error
    performance in AWGN channels.
  • PG is defined as the ratio

Independent of modulation, coding etc
PG definition may not agree with
34
3.2 Common Jammers Classification
  • Wide Band White Noise Jammer
  • Partial Band White Noise Jammer

35
CW Jammer
  1. Multitone Jammer

36
Common Jammers Classification (contd)
  • Pulse Jammer
  • Frequency Follower Jammer
  • Echo Jammer

37
3.3 Anti-jamming System Configuration
Encoder
Interleaver
Modulator
Spreader
Data
Channel
JSSI
Data
Demodulator
Deinterleaver
Decoder
Despreader
Coding Channel
Figure 1 The antijamming system model.
38
3.4 Interleaving
  • The interleaving process transforms a memory
    channel to a memoryless channel.
  • The interleaver is scrambling the order of the
    symbols in the transmitter
  • The deinterleaver is reordering the symbols in
    the receiver

I6
x51 x21 x11 x1
x52 x22 x12 x2
x53 x23 x13 x3
x54 x24 x14 x4
x55 x25 x15 x5
x56 x26 x16 x6
x57 x27 x17 x7
x58 x28 x18 x8
x59 x29 x19 x9
x60 x30 x20 x10
Block Interleaver Write symbols
column-by-column Read row-by-row
N10
N x I matrix
39
4. Direct Sequence (DS) Spread Spectrum
  1. Overview of DS spread spectrum system
  2. Parameters of a DS System
  3. Spectra in the DS Spread Spectrum System
  4. Performance Comparison in the Presence of a
    Narrow-Band Interference
  5. Uncoded DS/BPSK Spectrum System With Pulse Jammer
  6. Performance of the Uncoded DS/BPSK System.Worst
    Case Pulse Jammer Vs. Constant Power Jammer

40
4.1 Overview of DS spread spectrum system
1
2
3
4
  • Multiplication by in the transmitter is
    spreading the signal spectrum.
  • The sequence is despreading the signal
    spectrum.
  • Usually and the despread signal is
    equal to the original unspread
  • signal. For the special popular case of a BPSK
    system the transmitter can be implemented in an
    equivalent way.

BPSK MODULATOR
PN GENERATOR
PN GENERATOR
CHANNEL
TRANSMITTER
RECEIVER
41
(No Transcript)
42
Plot of DS time signal
43
4.2 Parameters of a Direct Sequence System
  • - bit duration
  • - chip duration
  • - unspread signal bandwidth
  • - spread spectrum signal bandwidth
  • - processing gain
  • - number of chips in one bit
  • - spectrum spreading ratio
  • - signal power
  • - signal bit energy
  • - jammer power

44
4.3 Spectra in the Direct Sequence Spread
Spectrum System
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