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mobile computing unit 1

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MOBILE COMPUTING ❖ Spread Spectrum ▪ Direct Sequence Spread Spectrum ▪ Frequency Hopping Spread Spectrum  Problem with radio transmission: Frequency dependent, fading can wipe out narrow band signals for duration of interference.  Solution: Spreads the narrow band signal into a broad band signal using special code.  Spread spectrum technology involves spreading the bandwidth needed to transmit data.  Advantage: Resistance to narrowband interference.  Disadvantages: ❖ Increased complexity of receivers that have to despread a signal ❖ Large frequency band is needed due to the spreading of the signal. ➢ Alternatives: Direct Sequence, Frequency Hopping  In the above figure: i) Shows an idealized narrowband signal from a sender of user data (here power density dP/df versus frequency f). ii) The sender now spreads the signal, i.e., converts the narrowband signal into a broadband signal. The energy needed to transmit the signal is the same, but it is now spread over a larger frequency range. The power level of the spread signal can be much lower than that of the original narrowband signal without losing data. Depending on the generation and reception of the spread signal, the power level of the user signal can even be as low as the background noise. This makes it difficult to distinguish the user signal from the background noise and thus hard to detect. iii) The sum of interference and user signal is received. The narrowband and the broadband in ...
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I was struggling with this subject, and this helped me a ton!

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