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DIGITAL SPREAD SPECTRUM SYSTEMS

DIGITAL SPREAD SPECTRUM SYSTEMS. ENG-737. Wright State University James P. Stephens Lecture Notes. WHAT WILL YOU LEARN ?. Learn the definition of SS Understand the reason for using SS Learn to distinguish between various modulation types

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DIGITAL SPREAD SPECTRUM SYSTEMS

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

  2. WHAT WILL YOU LEARN ? • Learn the definition of SS • Understand the reason for using SS • Learn to distinguish between various modulation types • Learn the concept of signal dimensionality and how it relates to processing gain • Understand the significance of PN sequences, their properties, how to generate them, and how to exploit them • Understand at the block diagram level, the function of each subsystem, i.e. modulation, demodulation, acquisition, tracking, etc • Learn how to determine system performance • Understand a variety of commercial SS applications • Gain exposure to new areas of digital communications

  3. SPREAD SPECTRUM SYSTEMS • Overview • Digital Communications Review • Spectrum Spreading Concepts • Pseudorandom Codes • Direct Sequence Systems • Frequency Hopping Systems • Synchronization of SS Systems • Applications of Spread Spectrum

  4. Generic Digital Communication System

  5. OVERVIEW • 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.

  6. WHY 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

  7. BASIC SPREAD SPECTRUM TECHNIQUE Interference Signal Spread Signal • The essence of interference rejection capability in SS systems can be summarized as: Recovered Data Data Signal Spreading Code Spreading Code Synchronized • 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

  8. 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

  9. DIGITAL COMMUNICATION REVIEW • Terms and definitions • Signal and system analysis • Digital modulation techniques • Detection of digital signals in noise 8

  10. INFORMATION RATE vs. SYMBOL RATE • Information in its most fundamental form is measured in “bits” (binary digits) • A signal which conveys binary information is the binary digital waveform 5 3 4 2 1 t (ms) 3T 2T 4T 5T T= 1ms Rs = Symbol (baud) rate = 1/T symbols / sec Rb = Information rate = 1/T bits / sec Example: Rs = 1000 symbols / sec Rb = 1000 bits / sec

  11. t 0 0 1 0 0 0 1 0 1 1 1 1 T = 3ms t (ms) 3T 4T T 2T INFORMATION RATE vs. SYMBOL RATE (Continued) • Binary information can be conveyed by M-ary (multilevel) digital waveforms Rs = symbol (baud) rate = 1/T symbols/sec Rb = information rate = 1/T Log2M Where M = number of levels in the M-ary waveform For the example shown: Rs = 333 symbols/sec Rb = 1000 bits/sec

  12. PERFORMANCE CRITERIADigital vs. Analog Systems • Analog communication systems reproduce waveforms (an infinite set) • A figure of merit for analog systems is a fidelity criterion (e.g. percent distortion between transmitted and recovered waveforms) • Digital communication systems transmit waveforms that represent digits (a finite set) • A figure of merit for digital systems is the quantity of incorrectly detected digits

  13. WHY DIGITAL? But let your communication be, Yea, yea; Nay, nay; for whatever is more that these, cometh of evil. - The Gospel According to St. Matthew (5:37) • Demand: Increased requirement for computer to computer communications • Signal Regeneration: Resistant to noise, interference, and distortion • Digital Signal Processing: Allows error detection and correction; permits ease of encryption and use of AJ techniques • Technology: Digital communication systems are more flexible and are better suited for future communication needs • Economics: Components are becoming increasingly more available and less expensive. Ability to rapidly troubleshoot and repair systems reduce overall maintenance costs as well as increasing system availability

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