SPREAD SPECTRUM
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SPREAD SPECTRUM

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SPREAD SPECTRUM

SPREAD SPECTRUM

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    SPREAD SPECTRUM SPREAD SPECTRUM Presentation Transcript

    • SPREAD SPECTRUM UNIT 3 PART 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
    • 6. 3-2 SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy.
      • Frequency Hopping Spread Spectrum (FHSS)
      • Direct Sequence Spread Spectrum (DSSS)
      Topics discussed in this section:
    • Spread Spectrum
      • A signal that occupies a bandwidth of B,
      • is spread out to occupy a bandwidth of B ss
      • All signals are spread to occupy the same bandwidth B ss
      • Signals are spread with different codes so that they can be separated at the receivers.
      • Signals can be spread in the frequency domain or in the time domain .
      6.
    • 6. Figure 3.21 Spread spectrum
    • 6. Figure 3.22 Frequency hopping spread spectrum (FHSS)
    • 6. Figure 3.23 Frequency selection in FHSS
    • 6. Figure 3.24 FHSS cycles
    • 6. Figure 3.25 Bandwidth sharing
    • 6. Figure 3.26 DSSS
    • DSSS MODULATION 6.
    • DSSS DEMODULATION 6.
    • 6. Figure 3.27 DSSS example
    • DS-SS Vs FH-SS PERFORMANCE ISSUES 6.
      • FHSS vs DSSS
      • DSSS is more susceptible to narrow band noise.
        • DSSS channel is 22 Mhz wide whereas
        • FHSS is 79 Mhz wide.
      • The FCC regulated that DSSS use a maximum of 1 watt of transmitter power in Pt-to-Multipoint system .
      • DSSS costs less then FHSS
      • FHSS can have more systems co-located than DSSS.
        • DSSS systems have the advantage in throughput
      • The Wi-Fi alliance tests for DSSS compatibility
        • No such testing alliance exists for FHSS.
      • FHSS vs DSSS contd
      • DSSS generally has a throughput of 5-6 Mbps while FHSS is generally between 1-2 Mbps.
      • Both FHSS and DHSS are equally insecure.
      • DSSS has gained much wider acceptance due to its low cost, high speed and interoperability.
        • This market acceptance is expected to accelerate.
        • FHSS advancement includes HomeRF and 802.15 (WPAN) (Bluetooth), however, it is expected to not advance into the enterprise.
    • Copyright 2005 All Rights Reserved February 2005 Co-location Comparison Date Rate in Mbps
    • UNIT - 3 THE END 6.