Re-implementing Muller and Mueller clock recovery with control_loop - signal-processing

Question

I'm currently implementing symbol time recovery blocks. The idea is to be able to choose different TEDs (Gardner, Zero-crossing, Early-Late, Maximum-likelihood etc). In blocks like M&M recovery, the gain parameters of the loop are expressed explicitly (gain_omega and gain_mu) which can be difficult to get right. The contro_loop class is, however, more convenient (loop characteristics can be specified by "loop bandwidth" and "damping factor"(zeta)). So my first test started with the re-implementation of the MM Clock Recovery with a control loop. The work function of this block is shown below (Comments are mine)
clock_recovery_mm_ff_impl::general_work(int noutput_items,
gr_vector_int &ninput_items,
gr_vector_const_void_star &input_items,
gr_vector_void_star &output_items)
{
const float *in = (const float *)input_items[0];
float *out = (float *)output_items[0];
int ii = 0; // input index
int oo = 0; // output index
int ni = ninput_items[0] - d_interp->ntaps(); // don't use more input than this
float mm_val;
while(oo < noutput_items && ii < ni ) {
// produce output sample
out[oo] = d_interp->interpolate(&in[ii], d_mu); //Interpolation
mm_val = slice(d_last_sample) * out[oo] - slice(out[oo]) * d_last_sample; // Error calculation
d_last_sample = out[oo];
//Loop filtering
d_omega = d_omega + d_gain_omega * mm_val; //Frequency
d_omega = d_omega_mid + gr::branchless_clip(d_omega-d_omega_mid, d_omega_lim); //Bound the frequency
d_mu = d_mu + d_omega + d_gain_mu * mm_val; //Phase
ii += (int)floor(d_mu); // Basepoint index
d_mu = d_mu - floor(d_mu); // Fractional interval
oo++;
}
consume_each(ii);
return oo;
}
Here is my code. First, the control loop is initialized the constructor
loop(new gr::blocks::control_loop(0.02,(1 + d_omega_relative_limit)*omega,
(1 - d_omega_relative_limit)*omega))
First of all I would like to eliminate a couple of doubts that I have regarding pll (the control_loop above) in symbol timing recovery particularly phase and frequency ranges (that are in turn used for wrapping). Taking an analogy from Costas loop : carrier phase is wrapped between -2pi and +2pi and the frequency offset is tracked between -1 and +1. It is quite straightforward to see why. Unfortunately I can't get my head around phase and frequency tracking in symbol recovery. From the m&m block, frequency is tracked between (1+omega_relative_limit) and (1 - omega_relative_limit)*omega where omega is simply the number of samples per symbol. Phase is tracked between 0 and omega. I dont understand why this is so and why the m&m block doesn't wrap it. Any ideas here will be appreciated.
And here is my work function
debug_time_recovery_pam_test_1_impl::general_work (int noutput_items,
gr_vector_int &ninput_items,
gr_vector_const_void_star &input_items,
gr_vector_void_star &output_items)
{
// Tell runtime system how many output items we produced.
const float *in = (const float *)input_items[0];
float *out = (float *)output_items[0];
int ii = 0; // input index
int oo = 0; // output index
int ni = ninput_items[0] - d_interp->ntaps(); // don't use more input than this
float mm_val;
while(oo < noutput_items && ii < ni ) {
// produce output sample
out[oo] = d_interp->interpolate(&in[ii], d_mu);
//Calculating error
mm_val = slice(d_last_sample) * out[oo] - slice(out[oo]) * d_last_sample;
d_last_sample = out[oo];
//Loop filtering
loop->advance_loop(mm_val); // Filter the error
loop->frequency_limit(); //Stop frequency from wandering too far
//Loop phase and frequency
d_omega = loop->get_frequency();
d_mu = loop->get_phase();
//d_omega = d_omega + d_gain_omega * mm_val;
//d_omega = d_omega_mid + gr::branchless_clip(d_omega-d_omega_mid, d_omega_lim);
//d_mu = d_mu + d_omega + d_gain_mu * mm_val;
ii += (int)floor(d_mu); // Basepoint index
d_mu = d_mu - floor(d_mu);//Fractional interval
oo++;
}
consume_each(ii);
return oo;
}
I have tried to use the block in a GFSK demodulator and I got this error
python: /build/gnuradio-bJXzXK/gnuradio-3.7.9.1/gnuradio-runtime/include/gnuradio/buffer.h:177: unsigned int gr::buffer::index_add(unsigned int, unsigned int): Assertion `s < d_bufsize' failed.
The first google search regarding this error suggests that im somehow "abusing" the scheduler since this error comes somewhere below the API. I think my calculation of d_omega and d_mu from the control loop is a bit naive but unfortunately I don't know any other way of doing so. Another alternative will be to use a modulo-1 counter (incrementing or decrementing) but I want to explore this option first.