G3RUH Multiplicative scrambler in C++

Question

I have written and tested a G3RUH (1 + X^12 + X^17) descrambler. It seems to work quite well in an SDR project I'm working on. I'm currently working on a scrambler and I can't get it to work. Below is the test code I use. It has scrambling, descrambling, data-generating, and error testing functions. I was wondering if you could spot what I'm doing wrong.

#include <iostream>
#include <vector>
#include <cassert>
using namespace std;

void
Scramble(std::vector<uint8_t> &u8vIn)
{
    uint32_t m_nLSR = 0;
    uint8_t bit;
    std::vector<uint8_t> buffer(u8vIn.size());
    for(unsigned int i = 0; i < u8vIn.size(); i++)
    {
        bit = u8vIn[i]^((m_nLSR >> 12) & 1)^((m_nLSR >> 17) & 1);
        m_nLSR = (m_nLSR << 1) | bit;
        buffer[i] = bit;
    }
    u8vIn.clear();
    u8vIn = buffer;
}

void
Descramble(std::vector<uint8_t> &u8vIn)
{
    uint32_t m_nLSR = 0;
    uint8_t bit;
    std::vector<uint8_t> buffer(u8vIn.size());
    for(unsigned int i = 0; i < u8vIn.size(); i++)
    {
        bit = (m_nLSR & 1)^((m_nLSR >> 12) & 1)^((m_nLSR >> 17) & 1);
        m_nLSR = (m_nLSR << 1) | u8vIn[i];
        buffer[i] = bit;
    }
    u8vIn.clear();
    u8vIn = buffer;
}

void
GenRand(int nLength, std::vector<uint8_t> &u8vIn)
{
    // PRBS 7 : x^7 + x^6 + 1
    uint8_t  nPRBS7Start  = 0x7f;
    uint8_t  nPRBS7Out    = nPRBS7Start;
    for(int i = 0; i < nLength; i++)
    {
        uint8_t newbit = (((nPRBS7Out >> 6) ^ (nPRBS7Out >> 5)) & 1);
        nPRBS7Out = ((nPRBS7Out << 1) | newbit) & 0x7f;
        u8vIn.push_back(newbit);
    }
}

int
Bert(std::vector<uint8_t> u8Seq1, std::vector<uint8_t> u8Seq2)
{
    assert(u8Seq1.size() == u8Seq2.size());
    int errors = 0;
    for(unsigned int i = 0; i < u8Seq1.size(); i++)
        errors += u8Seq1[i]^u8Seq2[i] ? 1:0;
    return errors;
}


int main()
{
    // Generate random data
    std::vector<uint8_t> prbs, buff;
    GenRand(128,prbs);
    buff = prbs;

    // G3RUH Scrambling
    Scramble(prbs);

    // G3RUH Descrambling
    Descramble(prbs);

    // BERT
    int errors = Bert(buff,prbs);
    cout << "NumErrors=" << errors<< endl;
    return 0;
}

Answer 1

I believe this line

bit = (m_nLSR & 1)^((m_nLSR >> 12) & 1)^((m_nLSR >> 17) & 1);

in your Descramble method is incorrect because it doesn't use the incoming data the first time through.

I believe changing it to use the actual data:

bit = u8vIn[i]^((m_nLSR >> 12) & 1)^((m_nLSR >> 17) & 1);

makes it work correctly because if I do that, I get:

NumErrors=0

as the output.

Answer 2

Apparently, and according to my calculations which are based on the script, the scrambler code seems okay.

On the other hand, the error calculator code should always result in zero as the variables buff and prbs are equalised to each other and they are fed into an XOR gate.

As far as I understand from the script, prbs is the input to the scrambler. Actually, it is the same as the variable u8vIn which is used as the digital input signal of the scrambler. However, in place of doing prbs = u8vIn while declaring functions, those functions work for prbs = &u8vIn. After changing &u8vIn to u8vIn in the function declaration lines, the error value results in zero which is the expected thing.