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 Each channel 1MHz wide (Hence the term Narrowband)
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FHSS - Frequency Hopping Spread Spectrum - Frequency...

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