I believe that I read, for digital modulation with multiple levels, that the AGC makes use of the fact that the symbol sequence, before noise etc., can be modeled as a uniform distribution.

$$\text{Source $\rightarrow$source-coding $\rightarrow$FEC$\rightarrow$ interleaving $\rightarrow$ bit-scrambling $\rightarrow$ modulation $\rightarrow$ channel}$$

What other advantages are there to bit scrambling (not for privacy/security).



In normal usage, scrambling is used for two reasons:

  1. It is used to remove the possibility of a long sequence of 1's and 0's in the bit sequence. The long sequence of 1's and 0's make timing synchronisation and clock synchronisation tougher at the receiver as regular transitions help in working of adaptive circuits like AGC and phase locked loop.

  2. It eliminates the dependence of signal's power spectrum on the transmitted information sequence thereby keeping it below the maximum power spectral density requirement. If scrambling is not done, power might be concentrated in a narrow frequency band thereby causing intermodulation and crossmodulation distortion to adjacent channels.

Reference: http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Scrambler.html

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