1. טכניקות בתקשורת מרחיבת סרט (Spread Spectrum) Chapter 2a
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טכניקות בתקשורת מרחיבת סרט (Spread Spectrum) Chapter 2a
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Dr. Moshe Ran- Spread Spectrum

2. Dr. Moshe Ran / Spread Spectrum
נושאי לימוד
פרק 1
מבוא הסטורי לטכניקות Spread Spectrum הרחבת ספקטרום – לשם מה? חזרה- מושגי יסוד ועקרונות של מערכות תקשורת ספרתיות; רעשים והפרעות במערכות תקשורת, דרישות מערכתיות על התקשורת, השוואת שיטות אפנון ספרתיות, יעילות ספקטרלית.
פרק 2
מבוא למערכות מרחיבות סרט (Spread Spectrum) - : קונספט ומודלים למערכות מרחיבות סרט; שיטות הרחבת סרט המבוססות על הרחבה ישירה DS)) דילוגים בזמן TH) ) דילוגים בתדר (FH)
פרק 3
סדרות קוד למערכות מרחיבות סרט - LFSR, Gold Sequence, Walsh
פרק 4
ביצועים של מערכות עם הרחבת סרט ישירה (DS) ביצועים של מערכות עם דילוגי תדר (FH) ; שיטות גילוי, עקיבה וסנכרון של אותות Spread Spectrum
פרק 5
קודים לתיקון שגיאות, ביצועים של מערכות Spread Spectrum עם קודים לתיקון שגיאות, אלגוריתם Viterbi
פרק 6
עקרונות CDMA בתקשורת תאית
פרק 7
שימושים ואפליקציות של מערכות Spread Spectrum
8 שו"ת
8 שו"ת
8 שו"ת
8 שו"ת
8 שו"ת
8 שו"ת
4 שו"ת
Dr. Moshe Ran / Spread Spectrum

3. Dr. Moshe Ran / Spread Spectrum
1. Introduction
Secure communications
EW – Electronic Warfare
ECCM
Communication System Requirements
Global System Approach to ECCM
Dr. Moshe Ran / Spread Spectrum

4. 1.1 Secure Communication שיטות לתקשורת צבאית חסינה
Electronic Warfare (EW) לוחמה אלקטרונית
The enemy uses Electronic Counter Measures (ECM)
Secure Communication
The communicators use Electronic Counter-Counter Measures (ECCM).
ECC…CM….
Dr. Moshe Ran / Spread Spectrum

5. 1.2 Electronic Warfare (EW)
Interception יירוט אותות תקשורת ("האזנה")
LPI – Low Probability of Interception
Data Extraction פענוח ופיצוח אותות התקשורת
COMMINT, ELINT, HUMINT
Jamming חסימת אותות התקשורת
Deception הונאה
Dr. Moshe Ran / Spread Spectrum

6. 1.3 ECCM אמצעי נגד-נגד לוחמה אלקטרונית
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
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

8. 1.5 Global System Approach to ECCM
System Control
Time and Frequency Management
Alternative Routing
Communication Protocols כאן מחפשים פרוטוקולים לא סטנדרטיים בד"כ
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
Dr. Moshe Ran / Spread Spectrum

9. 2. Introduction to Spread Spectrum Systems
Definition
Spread Spectrum System Concept
PN Waveforms
Time-Frequency waveforms
Spread Spectrum Process
Dr. Moshe Ran / Spread Spectrum

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.
Dr. Moshe Ran / Spread Spectrum

11. 2.2 Spread Spectrum System Concept
DATA
SOURCE
JAMMER
LINK
SELECTOR TR #1 #2 #K
COMMON CLOCKS
AND KEYS
RCV #1
RCV #2
RCV #K
DIVERSITY
COMBINER
USER
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

13. A Basis for a Jamming Game (cont.)
Observed signal at i-th receiver
Strategy for i-th Rx: project the signal onto the set of basis functions for the i-th Tx signal space
Dr. Moshe Ran / Spread Spectrum

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
Both J(t) and ni(t) can be expanded in terms of orthonormal basis
Dr. Moshe Ran / Spread Spectrum

15. A Basis for a Jamming Game (cont.)
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
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

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.
Dr. Moshe Ran / Spread Spectrum

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”
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

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
is the time duration of one chip and
is a complex discrete pseudo random sequence
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

22. 2.4 Time and Frequency Occupancy of Spectrum Waveforms
Frequency Hopping: when Ts>Th “fast FH”
WSS
Frequency
Fast FH
TIME
Dr. Moshe Ran / Spread Spectrum

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 Source
Data
modulator
Spreading function generator
Dr. Moshe Ran / Spread Spectrum

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 one–out–of M possible code sequences is transmitted.
Dr. Moshe Ran / Spread Spectrum

25. c. Compress and Shift (delay modulation)
Reorganization of modulated data into the transmission intervals of a time hopping signal.
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

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.
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

29. Dr. Moshe Ran / Spread Spectrum
Bibliography
R. C. Dixon, Spread Spectrum Systems. New Work: John Wiley, 1976
M. K Simon et. Al., Spread Spectrum Communication, Vol. I,II,III, Rockville Maryland: Computer Science Press,1985.
R. Ziemer, Peterson, Introduction to Spread Spectrum Communications, Prentice Hall,1995 2Ed
J. K. Holmes, Coherent Spread Spectrum System, New Work: John Wiley & Sons, 1982 ISBN
R. Skaug, J. F. Hjelmstad, Spread Spectrum in Communication, London: Peter, Peregrinus Ltd., 1985, ISBN
D. J. Torrieri, Principles of Military Communication System, Dedham, Massachusetts: Artech House Inc., 1981 ISBN
Dr. Moshe Ran / Spread Spectrum

30. 3. Anti-jamming Spread Spectrum Systems
Processing Gain
Common Jammers Classification
Anti-jamming System Configuration
Interleaving
Dr. Moshe Ran / Spread Spectrum

31. Dr. Moshe Ran / Spread Spectrum
3.1 Processing Gain (PG)
= total spread-spectrum signal bandwidth available
= data rate in bits per second
(3,2)
TRANSMITTER
RECEIVER
JAMMER
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

34. 3.2 Common Jammers Classification
Wide Band White Noise Jammer
Partial Band White Noise Jammer
Dr. Moshe Ran / Spread Spectrum

35. Dr. Moshe Ran / Spread Spectrum
CW Jammer
Multitone Jammer
Dr. Moshe Ran / Spread Spectrum

36. Common Jammers Classification (cont’d)
Pulse Jammer
Frequency Follower Jammer
Echo Jammer
Dr. Moshe Ran / Spread Spectrum

37. 3.3 Anti-jamming System Configuration
Data
Encoder
Interleaver
Modulator
Spreader
JSSI
Channel
Data
Decoder
Deinterleaver
Demodulator
Despreader
Coding Channel
Figure 1: The antijamming system model.
Dr. Moshe Ran / Spread Spectrum

38. Dr. Moshe Ran / Spread Spectrum
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
I=6
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
N=10
N x I matrix
Dr. Moshe Ran / Spread Spectrum

39. 4. Direct Sequence (DS) Spread Spectrum
Overview of DS spread spectrum system
Parameters of a DS System
Spectra in the DS Spread Spectrum System
Performance Comparison in the Presence of a Narrow-Band Interference
Uncoded DS/BPSK Spectrum System With Pulse Jammer
Performance of the Uncoded DS/BPSK System. Worst Case Pulse Jammer Vs. Constant Power Jammer
Dr. Moshe Ran / Spread Spectrum

40. 4.1 Overview of DS spread spectrum system2 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
Dr. Moshe Ran / Spread Spectrum

41. Dr. Moshe Ran / Spread SpectrumPN
GENERATOR
BPSK
MODULATOR PN
GENERATOR
BPSK
MODULATOR
Dr. Moshe Ran / Spread Spectrum

42. Dr. Moshe Ran / Spread Spectrum
Plot of DS time signal
Dr. Moshe Ran / Spread Spectrum

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
Dr. Moshe Ran / Spread Spectrum

44. 4.3 Spectra in the Direct Sequence Spread Spectrum System
Dr. Moshe Ran / Spread Spectrum