FHSS - Frequency Hopping Spread Spectrum

FHSS - Frequency Hopping Spread Spectrum - Frequency...

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Frequency Hopping Spread Spectrum What is it? o Modulation technique where signal is repeatedly switching frequencies during radio communication, minimizing probability of jamming/detection o First used during Cuba Crisis o Implemented In 802.11b (WiFi) Receiver hops between frequencies in synchronization with transmitter based on pseudo-random frequency sequence (explained later) Advantages o Eavesdroppers hear only noise when in wrong frequency gap o Attempts to jam signal on a frequency block succeed only at knocking out a few bits o Used extensively when in hostile territory Disadvantages o If message is lost due to interference, transmitter must resend data o Can use Adaptive Frequency Hopping (Smart Hopping) to avoid this situation Preview channel before sending out signal If noise encountered, put channel at end of queue, if not, transmit as normal
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FHSS Block Diagram Figure 1 Frequency Hopping (Graphical Context) Figure 2
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FHSS Performance Considerations Large Number of Frequencies Used o Uses the ISM Band (Industrial, Scientific, and Medical) from 2.402 GHz – 2.479 GHz o Has possible 79 hopping channels o
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