Sunday 30 August 2015

DSP Tech Brief : Signal Scrambling And Linear Feedback Shift Registers (LFSRs)

Modulated signals are often scrambled when they are modulated. There are many reasons for scrambling the signal, including : spreading the spectrum and reducing the correlation between separate channels. Most scramblers utilise a Maximum Length Pseudo-Random Binary Sequence (MLPRBS or MLS), this uses a shift register an exclusive-or gates to randomise the data, as shown in the following diagrams. There are two general arrangements for implementing a scrambler, the non-synchronising and the self-synchronising types, each has its own benefits and draw backs, the non-synchronising type requires that the transmitter and receiver are synchronised before data transmission starts however once synchronised an input bit error to the receive scrambler will cause a single bit error in the output. The self-synchronising scrambler(shown below), as the name suggests will automatically synchronise to the received bit stream but a single receive bit error will cause N+1 bit errors in the output stream, where N is the length of the shift register.

This algorithm is included in the SigLib DSP Library.

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