Scrambler and Descrambler MATLAB Code

This section provides MATLAB source code for a scrambler (or randomizer) and a corresponding de-scrambler. The de-scrambler code is included to validate the operation of the scrambler.

A scrambler, often called a randomizer, is primarily used to remove long sequences of 0s and 1s from the data stream. This is crucial in wireless communication systems, as it prevents the receiver from losing synchronization due to repetitive patterns. It’s typically implemented within the physical layer (Layer 1).

The circuit depicted above is commonly employed as a scrambler in various IEEE wireless standards, including WiMAX and WLAN. Below, we’ll explore the MATLAB code implementation of this circuit.

Scrambler MATLAB 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

Explanation:

• Scrambler_input: This array represents the input data to be scrambled.
• s = 20255: This initializes the scrambler’s internal state register. The initial value is crucial for both scrambling and de-scrambling.
• The nested loops iterate through each byte of the input data and each bit within the byte.
• msb=bitxor(bitget(s,1),bitget(s,2)): This line calculates the Most Significant Bit (MSB) by XORing the first and second bits of the state register s. This implements the feedback polynomial of the Linear Feedback Shift Register (LFSR).
• s=bitshift(s,-1): Shifts the state register s one bit to the right.
• s=bitset(s,15,msb): Sets the 15th bit of s to the calculated MSB, completing the feedback mechanism.
• t=bitxor(bitget(Scrambler_input(j),9-i),msb): XORs the current input bit with the calculated MSB to generate the scrambled bit.
• rand_data(j)=bitset(rand_data(j),9-i,t): Sets the corresponding bit in the rand_data array with the scrambled bit.
• scrambler_out: Contains the scrambled output data.

Descrambler MATLAB Code

s=20255; %Initialization of de-scrambler circuit
descrambler_in=zeros(size(scrambler_out));

for j=1:size(scrambler_out,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_out(j),9-i),msb);
        descrambler_in(j)=bitset(descrambler_in(j),9-i,t);
    end
end

descrambler_out=descrambler_in

Explanation:

The de-scrambler code mirrors the scrambler code, ensuring that the original data can be recovered.

• s = 20255: The de-scrambler must be initialized with the same seed value as the scrambler.
• The logic within the loops is identical to the scrambler, effectively reversing the scrambling process.
• descrambler_out: Contains the de-scrambled data, which should match the original Scrambler_input.

Output in the MATLAB Window

The output from the MATLAB code should demonstrate that the de-scrambled data perfectly matches the original input data, confirming the correct implementation of both the scrambler and de-scrambler.