Fundamentals of Scrambling Techniques

Abstract

Scrambling is a binary bit-level processing applied to the transmission rate signal in order to make the resulting binary sequence appear more random. The scrambler performing this scrambling function can be implemented simply using a few shift registers and exclusive-OR gates; and the descramblers reconstructing the original bitstream out of the scrambled data stream has the same structure but with the reversed data flow. For a proper reconstruction of the original bitstream the shift registers in the descrambler should get synchronized to their counterparts in the scrambler. Depending on the synchronization method used, scrambling techniques are classified into three categories, namely the frame-synchronous scrambling(FSS), the distributed sample scrambling(DSS), and the self-synchronous scrambling(SSS).¹ In the FSS, the states of the scrambler and the descrambler shift registers get synchronized by being simultaneously reset to the prespecified states at the start of each frame; in the DSS, samples taken from the scrambler shift registers are transmitted to the descrambler in a distributed manner for use in synchronizing the descrambler shift registers; and in the SSS, the states of the scrambler and descrambler shift registers are automatically synchronized without any additional synchronization processes.