In creating a Java version of a couple of routines in fdmdv.c, I had
to do things a bit different, due to the pointers. But I did get rid
of the S[] arrays out of global memory and they now go in and out of
the stack. Attached is the essential bits. Not a big savings, but at
least you don't have 4K of memory sitting idle. I could do a lot with
4K back in the Z80 days :-)
So I converted that back to C to see if it might be useful for you.
/*
* LPF and pick peak part of freq estimate
*/
static float lpf_peak_pick(int pass, float *foffset, COMP S[], int nin) {
COMP s[MPILOTFFT];
COMP *pilot_lpf;
COMP *pilot_baseband;
int i, j, k;
if (pass == 1) {
pilot_lpf = pilot_lpf1;
pilot_baseband = pilot_baseband1;
} else {
pilot_lpf = pilot_lpf2;
pilot_baseband = pilot_baseband2;
}
// LPF cutoff 200Hz, so we can handle max +/- 200 Hz freq offset
for (i = 0; i < (NPILOTLPF - nin); i++) {
pilot_lpf[i] = pilot_lpf[nin + i];
}
for (i = (NPILOTLPF - nin), j = 0; i < NPILOTLPF; i++, j++) {
pilot_lpf[i].real = 0.0;
pilot_lpf[i].imag = 0.0;
for (k = 0; k < NPILOTCOEFF; k++) {
pilot_lpf[i] = cadd(pilot_lpf[i], fcmult(pilot_coeff[k], pilot_baseband[j + k]));
}
}
/*
* decimate to improve DFT resolution, window and DFT
* calc decimation rate given new sample rate is twice LPF freq
*/
int mpilot = (int) FS / (2 * 200); // 20
for (i = 0; i < MPILOTFFT; i++) {
s[i].real = 0.0;
s[i].imag = 0.0;
}
for (i = 0, j = 0; i < NPILOTLPF; i += mpilot, j++) {
s[j] = fcmult(hanning[i], pilot_lpf[i]);
}
kiss_fft(fft_pilot_cfg, (kiss_fft_cpx *) s, (kiss_fft_cpx *) S);
// peak pick and convert to Hz
float imax = 0.0;
float mag;
int ix = 0;
for (i = 0; i < MPILOTFFT; i++) {
mag = S[i].real * S[i].real + S[i].imag * S[i].imag;
if (mag > imax) {
imax = mag;
ix = i;
}
}
float r = (2.0 * 200.0) / (float) FFT_SIZE; // maps FFT bin to frequency in Hz
if (ix >= (FFT_SIZE / 2)) {
*foffset = (float) (ix - FFT_SIZE) * r;
} else {
*foffset = (float) ix * r;
}
return imax;
}
/*
* Estimate frequency offset of FDM signal using BPSK pilot. Note that this
* algorithm is quite sensitive to pilot tone level wrt other carriers, so
* test variations to the pilot amplitude carefully.
*/
static float rx_est_freq_offset(COMP rx_fdm[], int nin) {
COMP pilot[M + M / P];
COMP prev_pilot[M + M / P];
COMP S[MPILOTFFT];
float *foff1;
float *foff2;
int i, j;
// get pilot samples used for correlation/down conversion of rx signal
for (i = 0; i < nin; i++) {
pilot[i] = pilot_lut[pilot_lut_index++];
if (pilot_lut_index >= (M * 4)) {
pilot_lut_index = 0;
}
prev_pilot[i] = pilot_lut[prev_pilot_lut_index++];
if (prev_pilot_lut_index >= (M * 4)) {
prev_pilot_lut_index = 0;
}
}
/*
* Down convert latest M samples of pilot by multiplying by ideal
* BPSK pilot signal we have generated locally. The peak of the
* resulting signal is sensitive to the time shift between the
* received and local version of the pilot, so we do it twice at
* different time shifts and choose the maximum.
*/
for (i = 0; i < (NPILOTBASEBAND - nin); i++) { // 230 - 160 = 0..69
pilot_baseband1[i] = pilot_baseband1[i + nin];
pilot_baseband2[i] = pilot_baseband2[i + nin];
}
for (i = 0, j = (NPILOTBASEBAND - nin); i < nin; i++, j++) { // 70..159 (90)
pilot_baseband1[j] = cmult(rx_fdm[i], cconj(pilot[i]));
pilot_baseband2[j] = cmult(rx_fdm[i], cconj(prev_pilot[i]));
}
float freq1 = lpf_peak_pick(1, foff1, &S[0], nin);
float freq2 = lpf_peak_pick(2, foff2, &S[0], nin);
if (freq1 > freq2) {
return *foff1;
} else {
return *foff2;
}
}
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