Bit scrambling, specifically for radio, is a method of 'forcing' more bit changes in transmitted data without increasing the length of transmission, such as Manchester Encoding.

Many methods can be chosen to complete this, however, a common method is using the G3RUH scrambler, which takes the XOR of the 12th and 17th positions in a bit register and depending on whether scrambling or descrambling will be manipulated by a bit stream input.

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Scrambler

Above is the algorithm to scramble the data. The input data is parsed, bit by bit, through the shift register (which would normally be filled with the preamble data from a packet) to scramble the data.

The XOR of bits is actually an addition of the bits in binary.

To do this in C++, the shift register values are assigned to a variable:

// 12th bit
bit11 = (bit_register >> 11) & 0b1;
// 17th bit
bit16 = (bit_register >> 16) & 0b1;

Then the algorithm is applied to the data:

// XOR the 12th and 17th positions
bit_xor = bit11 ^ bit16;


bit0 = bit_input ^ bit_xor;

// Shift register and add new bit0
bit_register = (bit_register << 1) + bit0;


// For this algorithm output is bit0
output = (output << 1) + bit0;

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Descrambler

Above is the algorithm to descramble the data. This is reversing the operation that was applied during scrambling to recover the data.

// XOR the 12th and 17th positions
bit_xor = bit11 ^ bit16;

bit_register = (bit_register << 1) + bit0;

// For this algorithm, output is XOR of the input and 12th + 17th bits
output = (output << 1) + (bit_input^bit_xor);