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Advantages of Scrambling | Disadvantages of scrambler in data communication

This page covers advantages and disadvantages of Scrambling performed by digital scrambler. It mentions benefits or advantages of Scrambling and drawbacks or disadvantages of Scrambling used in data communication.

What is Scrambling?

Introduction:
The process of randomization of binary data is known as scrambling. The logic circuit which performs randomization is known as scrambler. As shown in the figure, it is a generic linear feedback shift register with EX-OR gate.

The function of the scrambling is to remove long string of ones (1s) and zeros (0s) from the digital binary data. Scrambler is used in physical layer transmitter where as descrambler is used at receiver.

scrambler circuit used for scrambling

The scrambling does not increase or decrease the bit rate i.e. the input and output bits are same in size. It helps in synchronization as long string of 0s and 1s are removed from the digital data which creates more level transitions in the data pattern.

There are two types of scrambler.
1. Additive or synchronous scrambler (as shown in figure), It uses modulo-two addition.
2. Multiplicative or self-synchronizing scrambler, it performs multiplication of input signal by scrambler's transfer function in z-domain.

wimax physical layer Block diagram for transmitter part

It is used besides other modules such as FEC encoder (Convolutional, Reed solomon, CTC), interleaver and data modulation as shown in physical layer >>. It is also used along with block coding techniques such as 4B/5B or 8B/10B to achieve better performance results.

Benefits or advantages of Scrambling

Following are the benefits or advantages of Scrambling:
➨It does not increase data rate unlike block coding technique.
➨It eliminates long string of 0s to provide more transitions in the data. This helps receiver for synchronization to recover the original bit pattern.
➨It does not have any DC components as it creates balance between positive voltage levels and negative levels during encoding process in line coding techniques such as R8ZS and HDB3.
➨It offers error detection capability.

Drawbacks or disadvantages of Scrambling

Following are the drawbacks or disadvantages of Scrambling:
➨Multiplicative descrambler produces error multiplication. A single bit error at the input of such descrambler produces error in decrambled data. Hence it is used with other FEC techniques.
➨Additive scrambler should be reset by "frame sync" initially during descrambling otherwise massive error propagation occurs.
➨Both scrambler types fail to produce random sequences in worst case conditions.



MATLAB code of scrambling

Following is the MATLAB code of scrambling circuit shown above. This is as per IEEE 802.16 WiMAX OFDM physical layer specifications. For more information visit MATLAB Source code >>.

Scrambler_input=[80 255 16 9 48 255 80 0 25 0 145]

s=20255; %Initialization of scrambler circuit

rand_data=zeros(size(Scrambler_input));

for j=1:size(Scrambler_input,2);

for i=1:8

msb=bitxor(bitget(s,1),bitget(s,2));

s=bitshift(s,-1);

s=bitset(s,15,msb);

t=bitxor(bitget(Scrambler_input(j),9-i),msb);

rand_data(j)=bitset(rand_data(j),9-i,t);

end

end

scrambler_out=rand_data


Line coding techniques

Difference between Unipolar Polar and Bipolar coding    RZ vs NRZ vs Manchester coding    RZ vs NRZ pulse shapes    Advantages and disadvantages of NRZ encoding    RZ encoding    2B1Q coding    8B6T coding    4D PAM5 coding    MLT-3 coding    4B/5B encoding    8B/10B encoding    R8ZS scrambling    HDB3 scrambling   


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