DIGITAL SPREAD SPECTRUM SYSTEMS

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DIGITAL SPREAD SPECTRUM SYSTEMS
ENG-737

• Wright State University
• James P. Stephens

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FOURIER BASED TRANSFORM DOMAIN SYSTEM

• Describe a methods for implementing an
  interference-avoiding digital communications
  system
• Waveform will exhibit Low Probability of
  Intercept (LPI) characteristics
• Specify the waveform in the Transform Domain
  (Fourier) where non-desirable signal components
  can be ignored
• Resulting signal avoids interference by
  adapting itself away from interference and
  jamming

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INTRODUCTION

• Potentially most significant development in RF
  communication technology
• Historically, waveforms designed in time-domain
  and frequency domain characteristics accepted as
  consequence
• Future waveforms will be designed in transform
  domain (TD) (i.e. frequency, time-scale, etc.)
• Emphasis of this paper/briefing is on
  interference avoidance considerations (others are
  possible)

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CONVENTIONAL COMMUNICATION SYSTEM
Information Source
Voice Data Video
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FOURIER BASED TRANSFORM DOMAIN SYSTEM
TDCS Transmitter
Estimate Spectrum
Establish Magnitudes
TX
X
Memory
Modulate
FFT-1
Basis Function
Data
Random Phase
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SPECTRUM OF SAMPLED ENVIRONMENT
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ESTIMATED SPECTRUM AND THRESHOLD
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THEORETICAL WAVEFORM SPECTRUM
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RANDOM PHASE MAPPING

• øi Î 0, 2p/2r,, 4p/2r,, . . . , 2p(2r-1)/2r
• i 1, 2, . . . , N

Ree jøi
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TIME DOMAIN SIGNAL (FUNDAMENTAL MODULATION
WAVEFORM)
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MODULATION TECHNIQUES
Others are possible
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SYNCHRONIZATION

• Must align receivers locally generated basis
  function with transmitted basis function
• Match filter or correlator structures are
  possible
• In CSK, the waveform must be synchronized to
  the symbol period before demodulation can be
  accomplished
• In an m-ary CSK, a correlation peak will appear
  on one of the m time slots in the periodic
  correlation
• Initial acquisition would be aided by coded bit
  patterns that aid receiver in determining
  placement of symbol slots.

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FOURIER BASED TRANSFORM DOMAIN SYSTEM
TDCS Receiver
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PERFORMANCE ISSUES

• TDCS transmitter/receiver must view same
  spectral environment, i.e. must yield identical
  basis function
• Operational Scenario - Group of aircraft
  flying deep interdiction mission
• Smoothing of the TD coefficients effects
  environment estimate
• CDMA can be implemented by using orthogonal
  coding
• Adaptability is an important feature in the
  design
• Quantization effects must be considered or
  signal artifacts are generated

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8-BIT QUANTIZED SIGNAL SPECTRUM
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ADAPTIVE SYSTEM TIMING DIAGRAM

• Made adaptive by periodically resampling the
  environment and generating a new basis function
• Rate of resampling depends on how fast the
  environment is changing
• Resampling impacts the time that the system is
  available for transmitting

Ts time to transmit data symbol TT time to
transmit n symbols To time to sample
environment Tp time to process the samples TF
frame time, period at which system revisits the
environment
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DISCRETE COSINE DOMAIN

• An alternative for generating basis functions
• DCT replaces the complex exponential of the DFT
  with a cosine function that is not complex
• DCT produces only spectrum amplitudes and does
  not contain phase information
• Therefore, amplitudes are the only parameter
  which may be manipulated to produce basis
  function
• Once sampled environment is compressed by
  DCT, frequencies are eliminated and remaining
  frequencies are used to generate the basis
  function

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WAVELET DOMAIN

• Wavelets and wavelet packets provide a
  fundamentally different basis for analyzing
  signals compared to Fourier
• Generating a wavelet basis function is
  accomplished by examining the time-scale
  coefficients and using only those which do not
  contain the interference
• Like DCT, wavelet domain yields only
  amplitudes, thus the PN code must manipulate
  amplitudes not phase
• Difficulty is selecting appropriate wavelet
  family and scale level for characterizing the
  interference
• May not be able to localize interference to a
  consistent set of time-scale coefficients

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COIF4 WAVELET ESTIMATE OF ENVIRONMENT
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INTERFERENCE AVOIDANCE TRANSFORM DOMAIN SYSTEM
System Concept
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SUMMARY

• Transform domain techniques are shown to
  implement an interference avoidance waveform
• May be accomplished by the use of Fourier,
  discrete cosine, or wavelet basis function
• May be implemented using a variety of
  modulation and synchronization techniques
• Performance is dependent on accuracy of the
  estimate of the environment and quantization
  effects resulting from sampling and D/A
  conversions
• To achieve adequate data throughput, the
  complex DSP operations require multiple
  processors with high-speed data exchanges that
  are expensive and currently not well suited for
  integration into portable radios.