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DIGITAL SPREAD SPECTRUM SYSTEMS. ENG-737 Lecture 2. Wright State University James P. Stephens. DIGITAL MODULATION TECHNIQUES. Information signals (baseband signals) must be processed before transmission to be compatible with the channel

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digital spread spectrum systems

DIGITAL SPREAD SPECTRUM SYSTEMS

ENG-737

Lecture 2

Wright State University

James P. Stephens

digital modulation techniques
DIGITAL MODULATION TECHNIQUES
  • Information signals (baseband signals) must be processed before transmission to be compatible with the channel
  • A baseband signal modulatesa high frequency carrier by varying the carrier’s phase, amplitude, or frequency
  • In digital systems there are 3 basic modulation schemes:
    • Phase shift keying (PSK)
    • Amplitude shift keying (ASK)
    • Frequency shift keying (FSK)
  • In general modulated signal is of the form:

S(t) = A(t) cos [2pfot + f(t)]

spectrum of bask
SPECTRUM OF BASK
  • ASK differs from PSK in that ASK has carrier component due to DC value of m(t)
spread spectrum
SPREAD SPECTRUM

DEFINITION:

Spread spectrum (SS) is a means of signal transmission in which:

  • The transmitted signal occupies a bandwidth which is much greater than the minimum necessary to send the information.
  • Spreading is accomplished by means of a spreading signal called a ‘code’ signal, which is independent of the data.
  • At the receiver, despreading is done by correlating the received SS signal with a synchronized replica of the spreading signal.
why use spread spectrum
WHY USE SPREAD SPECTRUM ?
  • Interference Suppression
    • Antijam capability
    • Natural interference rejection
    • Self-interference (multipath protection)
  • Energy Density Reduction
    • Low probability of intercept (LPI)
    • Low probability of exploitation (LPE)
    • National allocation regulations
  • High-Resolution Ranging
  • Multiple Access
    • Communications resource sharing
    • Communications privacy
spread spectrum techniques
SPREAD SPECTRUM TECHNIQUES
  • Direct Sequence (DS) - A carrier is modulated by a digital code sequence in which bit rate is much higher than the information signal bandwidth.
  • Frequency Hopping (FH) - A carrier frequency is shifted in discrete increments in a pattern dictated by a code sequence.
  • Time Hopping (TH) - Bursts of the carrier signal are initiated at times dictated by a code sequence.
  • Hybrid Systems - Use of combination of the above.
  • Others - Carrier-less based, transform domain systems
basic spread spectrum technique

Interference Signal

Spread Signal

Recovered Data

Data Signal

Spreading Code

Spreading Code

Synchronized

BASIC SPREAD SPECTRUM TECHNIQUE

The essence of interference rejection capability in SS systems can be summarized as:

Demod

  • Multiplication once by the spreading code spreads the signal bandwidth
  • Multiplication twice by the spreading code followed by filtering, recovers the original data signal
  • The desired signal gets multiplied twice, but the interference gets multiplied only once
signal dimensionality
SIGNAL DIMENSIONALITY
  • An arbitrary M-ary signal set:
  • Can be completely specified by a linear combination of orthonormal basis functions:
  • The signal set is said to be D-dimensional when D is the minimum number of orthonormal basis functions necessary to span the signal set
  • It can be shown that :

D  2 BD T

Where,

T = signaling interval

BD = bandwidth of the D-dimensional signal set

  • AWGN has infinite power and constant energy in all dimensions, therefore increasing D yields no performance against AWGN
  • A jammer or interference source has a fixed finite power, increasing the dimensionality of our signal space in a manner unknown to the jammer will provide increased performance
  • The jammer is forced to spread his finite power over all the coordinates that we might use
dimensionality example

Jammer

J(t)

Xmitter

Receiver

s(t)

DIMENSIONALITY EXAMPLE
  • r(t) = s(t) + J(t), neglecting noise
  • r(t) = m(t) b(t) + J(t)

Where,

m(t) is the message data (1)

b(t) is the spreading code (1)

  • Note that b(t) b(t) = b2(t) = 1 , for all t
  • At the receiver r(t) b(t) = m(t) b(t) b(t) + J(t) b(t)

Where,

b(t) is an embedded reference code at the receiver

r(t) b(t) = m(t) + J(t) b(t)

  • The jammer has been spread and the message m(t) has been despread
concept of dimensionality
CONCEPT OF DIMENSIONALITY

Low SNR

½ PT

Jammer = ½ J

c

- f

f

c

c

At the receiver’s antenna

Low SNR

½ PT

Signal = ½ PT

c

c

Jammer

- f

f

c

c

After despreading by receiver

concept of dimensionality cont
CONCEPT OF DIMENSIONALITY (Cont)

High SNR

Signal = ½ PT

c

Jammer

- f

f

c

c

Output of filter

slide17

PROCESSING GAIN = PG

Processing gain is the improvement seen by a spread spectrum system in SNR, within the system’s information bandwidth, over the SNR in the transmission channel.

PG = BS / Ri

PG(dB) = 10 log (Bs / Ri)

Typical PG = 20 to 60 dB

slide18

JAMMING MARGIN = MG

Jamming margin takes into account the requirement for a useful system output SNR and allow for internal losses.

MG = GP – [Lsys + (S/N)out ] , dB

Where,

Lsys = system implementation losses

(S/N)out = SNR at information, despread, output

basic spread spectrum system configuration
BASIC SPREAD SPECTRUM SYSTEM CONFIGURATION

Transmitter

Receiver

DBM

DBM

FM Receiver

FM Exciter

RF Preamp

X

Amp/Filter

X

Audio

Audio

PN Code Generator

Synchronous

Oscillator

PN Code Generator

Embedded Reference

slide20

AFIT BUILT DSSS

THEORY OF OPERATION

TRANSMITTER

So(t)

FM Exciter

Audio

DBM

FM Osc

BPF

X

Amp

X4

X3

X3

ST1(t)

ST2(t)

12.388 MHz

111.5 MHz

PN Code Gen

Divide

by

40

bT(t)

2.7875 MHz

Detailed DSSS System Block Diagram

slide21

AFIT BUILT DSSS

THEORY OF OPERATION

RECEIVER

SI(t)

DBM

SR1(t)

SR2(t)

X

FM Receiver

Audio

Preamp

446.0 MHz

bR(t)

PN Code Gen

Divide

by

40

Synchronous

Oscillator

2.7875 MHz

111.5 MHz

Detailed DSSS System Block Diagram

slide22

DIRECT SQUENCE BPSK SPECTRAL CHARACTERISTICS

Rc = 2.7875 MHz

Tc = 0.3587 s

N = 255

BW = 5.575 MHz

1/NTc = 10.93 kHz

  • PSD = {Sin (x)/x}2
  • Spectral line spacing = 1/NTc
  • Null-to-Null BW = 2 * 1/Tc
  • Number of spectral lines = N

0 to 1st Null

40

slide23

DIRECT SEQUENCE BPSK

Direct sequence, suppressed carrier, biphase, code modulated

Direct sequence, unsuppressed carrier, biphase, code modulated

slide26

SPECTRAL CHARACTERISTICS

Rc = 2.7875 MHz

Tc = 0.3587 s

N = 255

BW = 5.575 MHz

1/NTc = 10.93 kHz

  • {Sin (x)/x}2
  • Spectral lines – 1/NTc
  • Null-to-Null BW – 2 * 1/Tc
  • Number of spectral lines = N

40

slide28

NRZ data

d(t)

Sd(t)

x

High Pass

Filter

Data

Modulator

St(t)

ht(t)

√2P cos(ω0t)

Frequency

Synthesizer

1 2 3 . . . . k

Code

Generator

FH code clock

TRANSMITTER

BASIC FREQUENCY HOPPER ARCHITECTURE

Typically

FSK

basic indirect frequency synthesizer

Reference Oscillator

Phase Detector

Filter

VCO

÷ n

Digital word from frequency map

BASIC INDIRECT FREQUENCY SYNTHESIZER

Phase-Locked Loop

slide30

2

1

Frequency

Synthesizer

PN

Generator

f3 + IF

f4 + IF

f1

f2

f3

f4

f2 + IF

f1 + IF

BASIC FREQUENCY HOPPER SYSTEM WITH WAVEFORMS

Mixer

IF BPF

To demodulator

4

3

PN

Generator

Frequency

Synthesizer

1 – PN Code

3 – Code

Reference

4 – FSK

Reference

2 – FH Carrier

dwell time and hop rate

Hop time

Dwell time

t

f1

f2

DWELL TIME AND HOP RATE
  • Hop time is the period of the hop cycle
  • Hop rate = 1 / Hop time
  • Dwell time is the time when radio is transmitting
  • Duty cycle is the % time the radio is transmitting versus the hop time:
  • % Duty Cycle = (Dwell time / Hop time) x 100
slide33

CORDLESS PHONE WAVEFORMS

Base and Handset Unit

Handset Unit

slide34

CORDLESS PHONE ACQUISITION FREQUENCIES

Spectrum Plot

Time-Frequency Plot

slide35

CORDLESS PHONE COMMUNICATION FREQUENCIES

f1

f1

f2

f2

Spectrum Plot

Time-Frequency Plot

frequency hopping example using 8 ary fsk

Legend

Tone # Tone Data Symbol

f0 + 175 Hz

f0 + 125 Hz

f0 + 75 Hz

f0 + 25 Hz

f0

f0 - 25 Hz

f0 - 75 Hz

f0 - 125 Hz

f0 - 175 Hz

000

001

010

011

100

101

110

111

0

1

2

3

4

5

6

7

FREQUENCY HOPPING EXAMPLE USING 8-ARY FSK

Data

Tone 1

Tone 6

Tone 3

0 0 1

1 1 0

0 1 1

Frequency Hopping Band

Time

Symbol Interval (20 ms)

frequency hopping example with diversity n 4

Legend

Tone # Tone Data Symbol

f0 + 700 Hz

f0 + 500 Hz

f0 + 300 Hz

f0 + 100 Hz

f0

f0 - 100 Hz

f0 - 300 Hz

f0 - 500 Hz

f0 - 700 Hz

000

001

010

011

100

101

110

111

0

1

2

3

4

5

6

7

Symbol Interval (20 ms)

FREQUENCY HOPPING EXAMPLE WITH DIVERSITY (N = 4)

Data

Tone 1

Tone 6

Tone 3

0 0 1

1 1 0

0 1 1

Frequency Hopping Band

5 ms / chip

Each Symbol

Repeated 4 times

fast hopping vs slow hopping

1

0

Bits

Frequency

Time

Chip Duration

Bits

101

100

101

110

011

000

011

110

Frequency

Time

Chip Duration

FAST HOPPING VS. SLOW HOPPING

(a) Fast Hopping Example: 4 hops / bit

(b) Slow Hopping Example: 3 bits / hop