DIGITAL SPREAD SPECTRUM SYSTEMS

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DIGITAL SPREAD SPECTRUM SYSTEMS
ENG-737 Lecture 2

• Wright State University
• James P. Stephens

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DIGITAL MODULATION TECHNIQUES

• Information signals (baseband signals) must be
  processed before transmission to be compatible
  with the channel
• A baseband signal modulates a high frequency
  carrier by varying the carriers 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)

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BINARY PHASE SHIFT KEYING (BPSK)
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BPSK CIRCUIT IMPLEMENTATION
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BINARY AMPLITUDE-SHIFT KEYING (BASK)
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SPECTRUM OF BASK

• ASK differs from PSK in that ASK has carrier
  component due to DC value of m(t)

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BINARY FREQUENCY-SHIFT KEYING (BFSK)
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BFSK POWER SPECTRUM
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SPREAD SPECTRUM

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

1. The transmitted signal occupies a bandwidth which
   is much greater than the minimum necessary to
   send the information.
2. Spreading is accomplished by means of a spreading
   signal called a code signal, which is
   independent of the data.
3. At the receiver, despreading is done by
   correlating the received SS signal with a
   synchronized replica of the spreading signal.

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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

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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

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BASIC SPREAD SPECTRUM TECHNIQUE
Demod

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

1. Multiplication once by the spreading code spreads
   the signal bandwidth
2. Multiplication twice by the spreading code
   followed by filtering, recovers the original data
   signal
3. The desired signal gets multiplied twice, but the
   interference gets multiplied only once

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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

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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

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CONCEPT OF DIMENSIONALITY
Low SNR
½ PT
c
Jammer ½ J
- f
f
c
c
At the receivers antenna
Low SNR
½ PT
Signal ½ PT
c
c
Jammer
- f
f
c
c
After despreading by receiver
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CONCEPT OF DIMENSIONALITY (Cont)
High SNR
Signal ½ PT
c
Jammer
- f
f
c
c
Output of filter
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PROCESSING GAIN PG
Processing gain is the improvement seen by a
spread spectrum system in SNR, within the
systems information bandwidth, over the SNR in
the transmission channel.
PG BS / Ri PG(dB) 10 log (Bs / Ri)
Typical PG 20 to 60 dB
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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
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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
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AFIT BUILT DSSS THEORY OF OPERATION
TRANSMITTER
So(t)
FM Exciter
Audio
DBM
FM Osc
BPF
X
Amp
X3
X3
X4
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
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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
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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)/x2
• Spectral line spacing 1/NTc
• Null-to-Null BW 2 1/Tc
• Number of spectral lines N

0 to 1st Null
40
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DIRECT SEQUENCE BPSK
Direct sequence, suppressed carrier, biphase,
code modulated
Direct sequence, unsuppressed carrier, biphase,
code modulated
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POWER DISTRIBUTION OF SINC2 SPECTRUM
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EFFECT OF HELICAL BANDPASS FILTER
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SPECTRAL CHARACTERISTICS
Rc 2.7875 MHz Tc 0.3587 ?s N 255 BW 5.575
MHz 1/NTc 10.93 kHz

• Sin (x)/x2
• Spectral lines 1/NTc
• Null-to-Null BW 2 1/Tc
• Number of spectral lines N

40
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EFFECT OF NARROWBAND FM MODULATION ON THE PSD OF
DSSS
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BASIC FREQUENCY HOPPER ARCHITECTURE
Typically FSK
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BASIC INDIRECT FREQUENCY SYNTHESIZER
Phase-Locked Loop
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BASIC FREQUENCY HOPPER SYSTEM WITH WAVEFORMS
Mixer
IF BPF
To demodulator
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3
PN Generator
Frequency Synthesizer
1 PN Code
3 Code Reference
4 FSK Reference
2 FH Carrier
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FREQUENCY HOPPING CORDLESS TELEPHONE
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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

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CORDLESS PHONE WAVEFORMS
Base and Handset Unit
Handset Unit
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CORDLESS PHONE ACQUISITION FREQUENCIES
Spectrum Plot
Time-Frequency Plot
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CORDLESS PHONE COMMUNICATION FREQUENCIES
f1
f1
f2
f2
Spectrum Plot
Time-Frequency Plot
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TIME-FREQUENCY DISTRIBUTION OF FH RADIO
(a)
Dwell
4-ary
(b)
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FREQUENCY HOPPING EXAMPLE USING 8-ARY FSK
Data
Tone 1
Tone 3
Tone 6
0 0 1
1 1 0
0 1 1
Frequency Hopping Band
Time
Symbol Interval (20 ms)
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FREQUENCY HOPPING EXAMPLE WITH DIVERSITY (N 4)
Data
Tone 1
Tone 3
Tone 6
0 0 1
1 1 0
0 1 1
Frequency Hopping Band
5 ms / chip
Each Symbol Repeated 4 times
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FAST HOPPING VS. SLOW HOPPING
(a) Fast Hopping Example 4 hops / bit
(b) Slow Hopping Example 3 bits / hop