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Friday, 16 May 2014

PN (Pseudo-Noise) sequences

  PN sequence is a noise-like spreading code which is basic to the operation of spread-spectrum modulation. A PN sequence is defined as a coded sequence of 1's and 0's with certain auto-correlation properties. PN sequence is periodic i.e., a sequence of 1's and 0's repeats itself exactly with a known period.
                           The 'maximum-length sequence' represents a commonly used periodic PN sequence. Such sequences have long periods and require simple instrumentation in the form of a linear feedback shift register.
Maximum-length sequence generator for m=3 is shown in the figure below.                                                
                                                                                                                                                           
  

Thursday, 8 May 2014

Spread Spectrum Systems


Introduction:

      
  A major issue in digital communication is to provide the efficient utilization of bandwidth and power. For example, the system may be required to provide a form of secure communication in a hostile environment such that the transmitted signal is not easily detected or recognized by unwanted listeners. This requirement is catered to by a class of signalling techniques known collectively as Spread Spectrum Modulation. The primary advantage of a spread spectrum communication is its ability to reject interference whether it be the unintentional interference of another user or the intentional interference of a hostile transmitter attempting to jam the transmission. 


Definition:

  1. Spread spectrum is a means of transmission in which the data of interest occupies a bandwidth in excess of the minimum bandwidth necessary to send the data.
  2. The spectrum spreading is accomplished before transmission through the use of of a code that is independent of the data sequence. The same code is used in the receiver to de-spread the received signal to recover the original data.

Applications:

  • Military applications where resistance to jamming is of major concern.
  • Provides multipath rejection in a ground-based mobile radio environment.
  • Multiple-access communication in which a number of independent users are required to share a common channel without an external synchronizing mechanism.
  • Code-division multiple access (CDMA).