DIGITAL SPREAD SPECTRUM SYSTEMS

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DIGITAL SPREAD SPECTRUM SYSTEMS
ENG-737

• Wright State University
• James P. Stephens

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FREQUENCY HOPPING

• Data is sent during the dwell time of a frequency
  hopping radio
• Modulation is typically Binary FSK
• The frequency shift is small compared to the
  frequency hop center frequency channels
• If the data is voice as in a tactical military
  radio or cordless telephone, it is digitized
  according to some digital voice standard
  (vocoder)
• Various vocoders have been adopted, but a common
  speech vocoder is known as CVSD (continuously
  variable, slope, delta) modulation
• Often, forward error correction (FEC) is
  employed, however, speech can tolerate
  considerable disruption before speech becomes
  unintelligible
• Speech data must be compressed to allow
  continuous transmission during time transmitter
  is transitioning to a new frequency

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FREQUENCY HOPPINGExample

• CVSD speech ASICs often use 16 kbps, typically,
  for high quality speech
• If we wish to use employ frequency hopping, how
  much compression must we use?
• Assume the channel bandwidth (demodulator) can
  only support 20 kbps
• Then 16K/20K 0.80 ? 80 duty cycle
• If we need to send 100 bits per dwell, what is
  our hop rate?
• 100 bits (1/20K) 5 ms (Dwell time)
• 5 ms / 0.8 6.25 ms (Hop time) ? 160 hps

6.25 ms
100 data bits
5 ms
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FREQUENCY HOPPINGClarifying Processing Gain

• A FH transmitter dwells for a period t1(time per
  hop) at each center frequency
• Hopping takes place over M frequencies
• PG Td BWss number of frequencies (M) ( for
  FH)
• Example
• Assume contiguous coverage, BWss 20 MHz
• N 1000 frequencies
• N 10 log 1000 30 dB
• If 20 MHz / 1000 20 kHz channel bandwidth
  (contiguous)
• PG 20 MHz / 20 KHz 1000 30 dB
• But not so if channels overlap or are
  non-contiguous

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FREQUENCY HOPPER RECEIVER
st(t)
ht(t)
Sync is usually based on time-of-day and
correlation
1 . . . . .k
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FREQUENCY HOPPER RECEIVER

• The frequency synthesizer output is a sequence of
  tones of duration Tc, therefore,
• ?
• ht(t) S 2p(t nTc) cos(?nt ?n )
• n - ?
• where p(t) is a unit amplitude pulse of duration
  Tc starting at time t 0
• ?nt and ?n are the radian frequency and phase
  during the nth frequency hop interval
• The frequency ?n is taken from a set of 2k
  frequencies

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FREQUENCY HOPPER RECEIVER

• The transmitted signal is the data modulated
  carrier up-converted to a new frequency ( ?0 ?n
  ) for each FH chip
• ?
• st(t) sd(t) S 2p(t nTc) cos(?nt ?n )
• n - ?
• The transmitted power spectrum is the frequency
  convolution of Sd (f) and Ht (f)

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FREQUENCY HOPPER RECEIVER

• Example
• FH, 250 hps, 2 ms dwell time, 48 bits per dwell
• Hop time 1 /250 4 ms
• ds 48 / 2 ms 24 kbps (signaling rate during a
  dwell)
• dr 48 / 4 ms 12 kbps (channel rate
  throughput)
• Minimum spacing for FSK tones are
• 1 / T 24 kHz (non-coherent FSK)
• 1 / 2T 48 kHz (coherent FSK)

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FREQUENCY SYNTHESIZERS

• There are two fundamental techniques for
  implementing frequency synthesis
• Direct
• Indirect
• In the direct implementation, a number of
  frequencies are mixed together in various
  combinations to give all of the sum and
  difference frequencies
• Example
• cos(2??1) cos(2??2) 1/2 cos(2? (?1- ?2))
  1/2 cos(2? (?1 ?2))
• The selection is made based upon a digital
  control word as to which filters pass the
  selected tone
• The direct implementation becomes very difficult
  when a large number of frequencies must be used
• Size and weight of the filters are major factors
  in the choice to use this technique

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SIMPLE DIRECT FREQUENCY SYNTHESIZER
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BASIC ADD-AND-DIVIDE FREQUENCY SYNTHESIZER
A control word selects the gate on f2 fm which
are mixed with a reference frequency which
usually specifies the frequency separation or
spacing
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INDIRECT SYNTHESIZERS

• Any synthesizer that employs a phase-locked loop
  is called an indirect synthesizer
• The output of the phase detector is filtered and
  drives a variable controlled oscillator (VCO)
• The phase detector drives the oscillator in the
  direction necessary to make ?? 0
• Any change causes the VCO to change in the
  opposite direction, thereby keeping the device
  locked to the input
• Frequency synthesis is accomplished by adding a
  divide-by-n block in the feedback path
• The VCO will lock to a multiple of the reference
  selected by n

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BASIC INDIRECT FREQUENCY SYNTHESIZER
The divide-by-n is changed digitally by the code
generator to select another output frequency
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NUMERICALLY CONTROLLED OSCILLATORS (NCO)

• More recent technique of frequency synthesizers
  are NCOs, also called direct digital
  synthesizers (DDS)
• DDSs are available as ASICs, see appendix 9 in
  text
• NCOs are available as FPGA cores, i.e. drop-in
  modules
• These devices simply have a sinusoid stored into
  memory that is outputted when selected.
• One such device uses a 32-bit tuning word to
  provide 0.0291 Hz tuning resolution and can
  change frequencies 23 million times per second,
  i.e.43 ns switching time
• These devices can control the phase, often with
  5-bits, in increments of 180, 90, 45, 22.5, 11.25
  degrees or combinations there of

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BASIC NUMERICALLY CONTROLLED OSCILLATOR
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DIRECT DIGITAL SYNTHESIZER
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MULTIPLE CORRELATORS FOR FREQUENCY HOPPING
ACQUISITION
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MULTIPLE CORRELATORS FOR FREQUENCY HOPPING
ACQUISITION
Time Delay
3 2 1 0
f1 f2 f3 f4 4
f4 f1 f2 f3 0
f3 f4 f1 f2 0
f2 f3 f4 f1 0
f1 f2 f3 f4 4
Delay
f1 f2 f3 f4
Let f1 101 MHz f2 107 MHz f3
105 MHz f4 103 MHz
Outcomes
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REVISITING PROCESSING GAIN

• What is processing gain?
• From Peterson / Ziemer / Borth
• The amount of performance improvement that is
  achieved through the use of spread spectrum is
  defined as processing gain
• That effectively means that processing gain is
  the difference between a system using spread
  spectrum and system performance when not using
  spread spectrum. . .all else equal
• An approximation is
• Gp BWss / ri
• Some authors use other definitions
• Some system marketers use improper definitions
  just to make their system sound superior to
  competitors
• The particular definition chosen is of little
  consequence as long as it is understood that real
  system performance is the primary concern

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REVISITING PROCESSING GAIN (Cont.)

• We could define processing gain as
• Gp td / tc
• Where td is the data bit time and tc is the chip
  time
• In the case of frequency hopping, a jammer or
  interferer can place all of his energy on a
  single narrowband signal, therefore, if the
  signal hops over M frequencies, the jammer must
  distribute power over all M frequencies with 1/M
  watts on each frequency
• Therefore, Gp M BWss / BWd (frequency
  hopping)
• however, we must assume contiguous,
  non-overlapping frequencies
• If overlapping occurs, Gp is reduced because the
  jammer can affect performance in adjacent
  channels. Thus Gp must be reduced by the amount
  of the overlap
• If non-contiguous, Gp gt M if jammer does not know
  system channelization since power will be wasted
  in regions where hopper never transmits

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REVISITING PROCESSING GAIN (Cont.)

• Sklar defines processing gain as
• How much protection spreading can provide
  against interfering signal with finite power
• Spread spectrum distributes a relatively
  low-dimensional signal into a large-dimensional
  signal space
• The signal is thereby hidden so to speak in the
  signal space since the jammer does not know how
  to find it
• Dixon, however is not very consistent
• Page 6 A signal-to-noise advantage gained by
  modulation and demodulation process is called
  process gain
• Page 10 What is really meant by Gp in spread
  spectrum is actually jamming margin
• Gp BWss / BWinf (which assumes BWinf Rinf
  (1 Hz/bit))

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REVISITING PROCESSING GAIN (Cont.)

• Note if
• Gp BWss / BWinf BWss / Rinf
• where Rinf 1 / Td
• Then Gp TdBWss (time-bandwidth product)

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REVISITING PROCESSING GAIN (Cont.)

• Example
• Assume contiguous coverage for a frequency
  hopping radio
• BWss 20 MHz, N 1000 frequencies
• Gp N 10 log 1000 30 dB
• If
• 20x106 / 1000 20 kHz channelization
• Gp 20x106 / 20x103 1000 30 dB
• But not equivalent if channels overlap or are
  non-contiguous

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COUNTERMEASURES
Electronic Attack (EA)

• To interfere with the enemys effective use of
  the electromagnetic spectrum
• Communications jamming involves the disruption of
  information, i.e. voice, video, digital
  command/control signals
• Rule One Jam receiver, not the transmitter

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JAMMING MARGIN

• In general, the major factors which influence
  communicating in a jamming environment are
• Processing Gain
• Antenna gain (Tx, Rx, and jammer)
• Power (Tx and jammer)
• Receiver sensitivity and performance
• Geometrical channel
• Item 5 deals with issues such as directivity and
  line-of-sight features. Adaptive array
  processing and null steering are used to gain
  directivity advantages over a jammer or group of
  jammers

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SIGNAL-TO-JAMMING RATIO

• Assume the jammer power dominates thermal noise
  (AWGN)
• The free-space propagation equation is
• (S/J)R PTGTGRdJ2 / PJGJdT2
• GR is the ratio of gain in the direction of the
  communication transmitter to gain in the jammer
  direction

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SIGNAL-TO-JAMMING RATIO (Cont.)

• Since,
• (Eb/Jo) (S/J)R PG
• Where,
• (S/J)R the received signal energy-to-noise
  power spectral density ratio
• Then,
• (Eb/Jo) min required to achieve an acceptable
  PE performance must satisfy
• (Eb/Jo) min ? PTGTGR PG dJ2 / PJGJdT2
• Therefore, to improve performance we can increase
  PT, GT, GR, PG, or dJ
• Or decrease PJ, GJ, or dT

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JAMMING STRATEGIES

• Noise
• Barrage
• Partial Band
• Narrowband
• Tone
• Single
• Multiple
• Swept
• Pulsed
• Smart
• Synchronized (coherent repeater)
• Non-synchronized (spectral matching)
• Knowledge based

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PROBABILITY OF BER VERSUS SNR
Digital signals are highly susceptible to gradual
degradation
BER
SNR (Eb/N0)
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KNOWLEDGE POWER RELATIONSHIP IN JAMMING
Brute Force Jamming
Power Required to Jam Victim
Smart / Responsive Jamming
Knowledge Required About Victim
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JAMMING TECHNIQUES
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JAMMING TECHNIQUES (Cont)
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JAMMING TECHNIQUES (Cont)
G3
G2
G1
WSS
STEPPED TONES
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DSSS IMMUNITY TO WIDEBAND NOISE
Noise jammer rejected by receiver

• Least power efficient technique but more covert
  than CW
• Requires no knowledge of signal
• High collateral damage (fratricide)
• Jamming power may be adjusted for gradual
  degradation

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DSSS IMMUNITY TO CW
CW Interferer rejected by receiver

• Requires high power to overcome DSSS processing
  gain
• More power efficient than wideband noise
• Non-covert, target may employ filter to remove
  jammer

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SPECTROGRAM AS VIEWED AT TARGET RECEIVER

• Receive Time 1.7 ms
• Jam Time 2.0 ms
• Link SNR 20 dB
• Jammer BW increased to make jammed regions visible

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ENEMY LINK Pe VERSUS Eb/N0
Enemy Link (J/S6 dB)
Enemy Link (J/S3 dB)
Enemy Link (J/S0 dB)
Enemy Link (no jamming)
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JAMMING STRATEGIES AGAINST DSSS

• Most effective (non-adaptive) technique is
  provided by single-tone jammer at or near the
  carrier frequency
• This stresses the carrier suppression of balanced
  demodulators
• CCM
• Use an adaptive notch filter to delete the tone
• Detect the tone by a PLL and then subtract it
  from the signal or spatially null the jammer
• Decipher the PN code, replicate it as a jamming
  signal which will not be eliminated by the
  processing gain
• Most effective if jammer can become synchronized
  to the receiver
• CCM
• Make the PN code generators programmable so that
  the code can be readily changed or use complex,
  adaptive, codes

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JAMMING STRATEGIES AGAINST DSSS (Cont.)

• Determine the carrier frequency and chip rate,
  then jam with a PN signal having these parameters
  (spectral matching)
• Less effective than 1) or 2), but more difficult
  to counter
• CCM - Use an adaptive code rates (ditter)
• Attack the acquisition process using a
  combination of 1) or 3)
• CCM Use short code for quick acquisition, then
  switch to longer code
• Pulse jamming and swept jamming at the carrier
  frequency
• Generally less effective than other methods
• Can be vary effective against AGC and tracking
  loops of target receiver if knowledge of receiver
  design is known
• CCM Use interleaving and error corrective
  coding

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JAMMING STRATEGIES AGAINST FH

• Repeater jamming which involves intercepting
  signal, determining the center frequency, and
  transmitting a tone at that carrier frequency
• Very effective against slower FH systems
• CCM
• Increase hop rate
• Partial band or multitone
• Jammer places a series of tones across bandwidth
  where the received power per jamming tone exceeds
  the systems received power per hop
• CCM
• Use error corrective coding with interleaving
• Swept frequency
• Increases the BER, but is less effective than 1)
  or 2)
• CCM
• Use error corrective coding with interleaving
• Note Generally speaking, FH systems are less
  susceptible to attacks on acquisition than are
  DSSS

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THE TACTICAL SCENARIO
Hopper Link
Jamming Link
Monitor Link
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GEOMETRY FOR FREQUENCY HOP REPEAT JAMMER

• Th is the hopping period and ? is the fraction
  of hopping period within which the jammer must
  operate to be effective (Typically 50 of the
  dwell time)

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GEOMETRY FOR FREQUENCY HOP REPEAT JAMMER

• For jamming to be effective we must have
• d2 d3 d1

Propagation time for Jammer
Where, tp jammer processing time c speed
of light (3 x 108 m/sec) (1 - ?) fraction of
dwell to be jammed
Source Modern Communications Jamming Principles
and Techniques - Poisel
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HOP RATE VERSUS STAND-OFF DISTANCE