In some telecommunication systems, scrambling of transmitted data is applied to provide some level of security. For example, the communication system might look like:

Data -> Scrambling -> Compression -> Error correction encoding

Could one possibly match the scrambling with the error-correcting code to obtain some benefit? Why is scrambling typically separated from the other blocks of the transmitter?

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    $\begingroup$ Are you sure that scrambling comes before compression and error correction coding? In some systems, scrambling is done after error correction coding, just before modulation, to break up patterns of long strings of zeroes or ones in the input to the modulator (Long sequences without bit transitions make life more difficult at the receiver). So distinguish between scrambling for privacy or security and scrambling for other reasons. $\endgroup$ Dec 4 '11 at 16:26
  • $\begingroup$ I agree with Dilip. Scrambling before data compression would actually be a bad thing, as the scrambling will make the datastream appear to be more random (without the long sequences of repeated symbols) and therefore be less compressible. $\endgroup$
    – Jason R
    Dec 4 '11 at 16:56
  • $\begingroup$ I know there are two types of scrambling but I mean security that is for sure before the error correction coding. Please let me know why it comes before? $\endgroup$
    – Hossein
    Dec 6 '11 at 4:55

There are two possible scramblers, one meant for security and privacy of the information, and the other for making sure that the sequence of symbols transmitted over the channel has properties that are desirable for digital transmission purposes. Either or both might be present in a system, or both could be absent too. Furthermore, the scramblers might well be non-interchangeable since the scrambling needed for privacy need not be the same as that required for transmission purposes.

The question then is, where should these scramblers be placed in the transmitter. The channel scrambler obviously should be between the error-correction encoder (whose output is to be transmitted) and the modulator that creates the actual signals that are broadcast over an antenna or transmitted down a cable or twisted pair of wires (or recorded on a magnetic medium or CD or DVD or what have you). The question of where the privacy scrambler should be placed is more complicated. @JasonR has contended in comments that putting it before the data compression unit makes the input to the compressor less compressible. This is certainly something to think about, but I am not completely sure about that it is always a major concern because the scrambler for security and privacy might be doing other things than breaking up long sequences of repeated symbols. In any case, the location of the privacy scrambler before the data compression unit may also be dictated by privacy and security concerns that may have nothing to do with efficient communication. If I were in charge of privacy and security, I would make darn sure that the chain looked like

data --> ENCRYPTION -- > scrambling -->

in my domain before the dsp and comm people even get to see the stream of symbols. Let them worry about how to transmit the stream I give them as efficiently as possible; my job is security and so I make sure that things are nailed down tight when they leave my shop!

  • $\begingroup$ Just as a note, if I was building the system and wanted to include compression, it would be directly after the "data" stage in Dilip's chain, as well-encrypted data is essentially incompressible. After that, tack on whatever encryption and/or privacy scrambling that you might want. $\endgroup$
    – Jason R
    Dec 7 '11 at 13:23
  • $\begingroup$ Thanks Jason R,you are right but there are two types of compression methods.Before and after scrambling :)). What you say is in general true. $\endgroup$
    – Hossein
    Dec 9 '11 at 15:31

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