CODEWITHSUNDEEP
gnuradiognuradio-companion
Moses Browne Mwakyanjala
Moses Browne Mwakyanjala

Reputation: 550

Using GNU Radio scrambler and descrambler

I'm trying to use the GNU Radio descrambling blocks. I have a block written by a third party that takes of descrambling. The polynomial used is x17 + x12 + 1.

The code is given below

descrambler_cc_impl::descrambler_cc_impl()
  : gr::sync_block("descrambler_cc",
          gr::io_signature::make(1, 1, sizeof(unsigned char)),
          gr::io_signature::make(1, 1, sizeof(unsigned char)))
{
    lsr = 0;
}

/*
 * Our virtual destructor.
 */
descrambler_cc_impl::~descrambler_cc_impl()
{
}

int
descrambler_cc_impl::work(int noutput_items,
    gr_vector_const_void_star &input_items,
    gr_vector_void_star &output_items)
{
  const unsigned char *in = (const unsigned char *) input_items[0];
  unsigned char *out = (unsigned char *) output_items[0];
  int i;
  for (i = 0; i < noutput_items; i++) {
out[i] = (lsr & 1) ^ ((lsr >> 12) & 1) ^ ((lsr >> 17) & 1);
lsr = lsr << 1;
lsr = lsr | (in[i] & 1);
  }

  // Tell runtime system how many output items we produced.
  return i;
}

Now I want to use the GNU Radio descrambler block. From
this link, I calculated the descrambling parameters as follows : Mask - 0x0210001 ; seed - 0x00; length - 24.

Unfortunately, it is not working as its counterpart in the code shown above. Could someone provide guidance as to why this is not working?

Upvotes: 3

Views: 3056

Answers (1)

Moses Browne Mwakyanjala
Moses Browne Mwakyanjala

Reputation: 550

Sorry for a late update on the answer. The explanation below will clear everything up

The GNU Radio block Descrambler implements a multiplicative descrambler of a given mask, seed and length. The mask can be calculated from the scrambling polynomial. In GNU Radio, the polynomial has to be written in little-endian bit order before the mask is calculated. For the polynomial above, p(x) = x^17 + x^12 + 1 , the mask is calculated by arranging the coefficients of lower powers first i.e. coef(x^1), coef(X^2) ... coef(x^17) for p(x) above. This is shown below:

mask = 0000 0000 0010 0001 = 0x0021.

From the source code of this block, it can be deduced that length in this context is the number of bits a shift register needs to shift when a new bit is being inserted. Therefore, length can be calculated as

length = deg (p (x)) − 1

which is, for our case, 17 - 1 = 16.

Upvotes: 5

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