DIGITAL SPREAD SPECTRUM SYSTEMS

1. DIGITAL SPREAD SPECTRUM SYSTEMS ENG-737 Wright State University James P. Stephens

2. 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

3. 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)

4. Modulator Amplifier Multipliers RF Amp Mixer Detector Amplifier Output CONVENTIONAL COMMUNICATION SYSTEM Information Source Voice Data Video

5. FOURIER BASED TRANSFORM DOMAIN SYSTEM TDCS Transmitter Estimate Spectrum Establish Magnitudes TX X Memory Modulate FFT-1 Basis Function Data Random Phase

6. SPECTRUM OF SAMPLED ENVIRONMENT

7. ESTIMATED SPECTRUM AND THRESHOLD

8. THEORETICAL WAVEFORM SPECTRUM

9. n Stage LFSR 1 0 0 1 0 0 1 0 1 . . . r Phase mapper taps {e jøi } Im[e jøi] 2r points RANDOM PHASE MAPPING • øi Î [ 0, 2p/2r,, 4p/2r,, . . . , 2p(2r-1)/2r] • i = 1, 2, . . . , N Re[e jøi]

10. TIME DOMAIN SIGNAL (FUNDAMENTAL MODULATION WAVEFORM)

11. ANTIPODAL • s1(t) = bf • s2(t) = -bf • Error performance in the presence of AWGN is identical to other antipodal signaling schemes • Benefits are when interferers are present • BINARY CSK • s1(t) = bf • s2(t) = s1((t - T/2))T • Cyclically time shifted version of the basis function • 3-dB performance loss compared to antipodal • m-ary signaling schemes are possible MODULATION TECHNIQUES Others are possible

12. SYNCHRONIZATION • Must align receiver’s 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.

13. T ó õ X ( )dt Data Decision Rule FFT-1 Conjugate Spectrum Magnitude X Random Phase Local BF reference FOURIER BASED TRANSFORM DOMAIN SYSTEM TDCS Receiver

14. 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

15. 8-BIT QUANTIZED SIGNAL SPECTRUM

16. 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

17. 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

18. 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

19. COIF4 WAVELET ESTIMATE OF ENVIRONMENT

20. INTERFERENCE AVOIDANCE TRANSFORM DOMAIN SYSTEM System Concept

21. 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.