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synchronous scrambler vs self synchronizing scrambler | Difference between synchronous scrambler and self synchronizing scrambler

This page compares Synchronous scrambler vs Self synchronizing scrambler and mentions difference between synchronous scrambler and self synchronizing scrambler used in data communication including their advantages (benefits) and disadvantages (drawbacks).

Introduction: In wireless communication, scrambler is used to remove long sequence of ones and zeros by randomization of data. It is used before FEC encoder or modulator or line encoder. Scrambler is used at the transmitter side. Descambler is used at the receiver side to recover original bit pattern from randomized data bits. The same is shown in the physical layer of wimax as per IEEE 802.16 standard.

Scrambler in wimax physical layer

There are two main functions of scrambler in the system.
• It provides more transitions in the data by removing long string of similar data bits i.e. 1s and 0s. This helps in accuate timing recovery at the receiver.
• It disperses energy on carrier signal and hence reduces ICI (Inter Carrier Interference).
• The scrambling concept is used in R8ZS and HDB3 coding techniques to maintain DC balance.

Synchronous scrambler | Additive scrambler

It uses LFSR (Linear Feedback Shift Register) to generate PRBS (Pseudo Random Binary Sequence) sequence or pre-stored PRBS is used. Synchronous scrambler transforms data stream by using modulo-2 sum of input data and PRBS sequence as shown in the figure. Hence synchronous scrambler is also known as additive scrambler. The circuit is as per PRBS generator 1 + X14 + X15 polynomial.

Synchronous scrambler and descrambler
Figure-1 : Synchronous scrambler and descrambler circuit

The same SYNC word is used for scrambler and descrambler at the transmit and receive end to enable synchronous operation of both.

Advantages of Synchronous scrambler

Following are the benefits or advantages of Synchronous scrambler.
➨It offers error detection capability.
➨It removes long string of zeros and ones and provide more transitions in the data pattern to make synchronization easier without the need of separate clock signal.
➨It does not have any DC components.

Disadvantages of Synchronous scrambler

Following are the drawbacks or disadvantages of Synchronous scrambler.
➨It fails to generate random sequences in worst case conditions.
➨It should be reset by SYNC word otherwise massive error propagation occurs.

Self synchronizing scrambler | Multiplicative scrambler

The scrambler and descrambler circuit for self synchronizing scrambler and descrambler is shown in the figure. It does not require frame SYNC word for synchronization unlike synchronous scrambler. Hence it is called self-synchronizing scrambler.

This scrambler performs multiplication of input by transfer function in Z-space. Hence self synchronizing is also known as Multiplicative scrambler.

Self Synchronizing Scrambler and Descrambler
Figure-2 : Self synchronizing scrambler and descrambler circuit

They are defined by polynomial 1 + X14 + X15. Multiplicative scrambler is recursive where as descrambler is non-recursive.

Advantages of Self synchronizing scrambler

Following are the benefits or advantages of Self synchronizing scrambler.
➨It does not require SYNC word called "SEED" for its operation unlike synchronous scrambler.
➨The other advantages are similar to the one listed in synchronous scrambler section above.

Disadvantages of Self synchronizing scrambler

Following are the drawbacks or disadvantages of Self synchronizing scrambler.
➨It fails to generate random sequences in worst case conditions.
➨Single bit error at the input of descrambler leads to error multiplication at its ouput. An error will result into "W" errors where "W" is number of taps used in scrambler.



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