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Mobile comm. 1

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  • 1. Mobile Communications Course: Communication Systems Dr. Muhammad Saleem Awan
  • 2. Mobile communication is burdened with particular propagation complications, making reliable wireless communication more difficult than fixed communication between and carefully positioned antennas. • The antenna height at a mobile terminal is usually very small, typically less than a few meters. Hence, the antenna is expected to have very little 'clearance', so obstacles and reflecting surfaces in the vicinity of the antenna have a substantial influence on the characteristics of the propagation path. • The propagation characteristics change from place to place and, if the terminal moves, from time to time.
  • 3. Spread Spectrum An increasingly popular form of encoding for wireless communications is known as spread spectrum. • The use of spread spectrum makes jamming & interception more difficult and improves reception. • The basic idea of spread spectrum is to modulate the signal so as to increase significantly the bandwidth (spread the spectrum) of the signal to be transmitted.
  • 4. Spread Spectrum A comment about pseudorandom numberspseudorandom numbers: These numbers are generated by an algorithm using some initial value called the seed. The algorithm is deterministic and therefore produces sequences of numbers that are not statistically random. • The important point is that unless you know the algorithm and the seed, it is impractical to predict the sequence. Hence, only a receiver that shares this information with a transmitter will be able to successfully decode the signal.
  • 5. Frequency Hopping Spread Spectrum The signal is broadcast over a seemingly random series of radio frequencies, hopping from frequency to frequency at split-second intervals. A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message.
  • 6. Frequency Hopping Spread Spectrum
  • 7. Frequency Hopping Spread Spectrum
  • 8. Direct Sequence Spread Spectrum Each bit in the original signal is represented by multiple bits in the transmitted signal using a spreading code known as chipping code. The chipping code spreads the signal across a wider frequency band in direct proportion to the number of bits used. Therefore, a 10-bit chipping code spreads the signal across a frequency band that is 10 times greater than a 1-bit chipping code.
  • 9. Direct Sequence Spread Spectrum Note that an information bit of 1 inverts the pseudorandom bits in the combination, while an information bit of 0 causes the pseudorandom bits to be transmitted without inversion.
  • 10. Direct Sequence Spread Spectrum The amount of spreading that is achieved is a direct result of the data rate of the pseudorandom stream; the greater the data rate of the pseudorandom input, the greater the amount of spreading.
  • 11. Direct Sequence Spread Spectrum In this case, the information stream and the pseudorandom stream are both converted to analog signals and then combined, rather than performing the exclusive-or of the two streams and then modulating.