You should have received a copy of the GNU Lesser General Public License
along with this program; if not, see <http://www.gnu.org/licenses/>.
- */
+*/
#include <stdio.h>
#include <stdlib.h>
static void qpsk_demod(complex float, int *);
static void ofdm_txframe(struct OFDM *, complex float [OFDM_SAMPLESPERFRAME], complex float *);
static int coarse_sync(struct OFDM *, complex float *, int);
+static void freq_shift(struct OFDM *, complex float [], complex float [], int);
/* Defines */
* 270.0 - 359.9 = 10
*/
static const complex float constellation[] = {
- 1.0f + 0.0f * I,
- 0.0f + 1.0f * I,
- 0.0f - 1.0f * I,
+ 1.0f + 0.0f * I,
+ 0.0f + 1.0f * I,
+ 0.0f - 1.0f * I,
-1.0f + 0.0f * I
};
*/
static const char pilotvalues[] = {
-1, -1, 1, 1, -1, -1, -1, 1, -1,
- 1, -1, 1, 1, 1, 1, 1, 1, 1
+ 1, -1, 1, 1, 1, 1, 1, 1, 1
};
/* Functions */
ofdm->foff_est_gain = 0.01f;
ofdm->foff_est_hz = 0.0f;
+ ofdm->rx_foff_hz = 0.0f;
ofdm->sample_point = 0;
ofdm->timing_est = 0;
ofdm->nin = OFDM_SAMPLESPERFRAME;
ofdm->foff_est_hz = val;
}
+void ofdm_rx_shift_freq(struct OFDM *ofdm, float val) {
+ if (val > -500.0f && val < 500.0) {
+ ofdm->rx_foff_hz = val;
+ }
+}
+
/*
* --------------------------------------
* ofdm_mod - modulates one frame of bits
}
}
+/*
+ * -----------------------------
+ * Frequency shift modem signal
+ * -----------------------------
+ *
+ * The use of complex input and output allows single sided
+ * frequency shifting (no images).
+ */
+
+static void freq_shift(struct OFDM *ofdm, complex float rxbuf_fcorr[], complex float rx[], int nin) {
+ int i;
+
+ float woff = TAU * ofdm->rx_foff_hz / OFDM_FS;
+
+ for (i = 0; i < nin; i++) {
+ rxbuf_fcorr[i] = rx[i] * cexpf(I * woff * i);
+ }
+}
+
/*
* ------------------------------------------
* ofdm_demod - Demodulates one frame of bits
*
* D P DDDDDDD P DDDDDDD P DDDDDDD P D
* ^
+ * Obviously this introduces some latency.
*/
void ofdm_demod(struct OFDM *ofdm, int *rx_bits, COMP *rxbuf_in) {
}
/*
- * get latest freq offset estimate
- *
- * foff_est_hz will be 0.0 unless
- * foff_est_en is enabled
+ * get user and calculated freq offset
*/
float woff_est = TAU * ofdm->foff_est_hz / OFDM_FS;
+ freq_shift(ofdm, ofdm->rxbuf_fcorr, ofdm->rxbuf, ofdm->nin);
+
/* update timing estimate -------------------------------------------------- */
if (ofdm->timing_en == true) {
complex float work[(en - st)];
+ /*
+ * Adjust for the frequency error by shifting the phase
+ * using a conjugate multiply
+ */
+
for (i = st, j = 0; i < en; i++, j++) {
- work[j] = ofdm->rxbuf[i] * cexpf(-I * woff_est * i);
+ work[j] = ofdm->rxbuf_fcorr[i] * cexpf(-I * woff_est * i);
}
ft_est = coarse_sync(ofdm, work, (en - st));
/* down-convert at current timing instant---------------------------------- */
for (j = st, k = 0; j < en; j++, k++) {
- work[k] = ofdm->rxbuf[j] * cexpf(-I * woff_est * j);
+ work[k] = ofdm->rxbuf_fcorr[j] * cexpf(-I * woff_est * j);
}
matrix_vector_conjugate_multiply(ofdm, ofdm->rx_sym[0], work); /* sym[0] = previous pilot */
* So we will now be starting at this pilot symbol, and continuing past the next pilot symbol
* rr = 0 = this pilot, rr = 8 = next pilot
*
- * Each symbol is of course OFDM_M samples long.
+ * Each symbol is of course (OFDM_M + OFDM_NCP) samples long.
*
* In this routine we process the data symbols. These are the ones we want right now.
*/
/* down-convert at current timing instant---------------------------------- */
for (j = st, k = 0; j < en; j++, k++) {
- work[k] = ofdm->rxbuf[j] * cexpf(-I * woff_est * j);
+ work[k] = ofdm->rxbuf_fcorr[j] * cexpf(-I * woff_est * j);
}
matrix_vector_conjugate_multiply(ofdm, ofdm->rx_sym[rr + 1], work); /* sym[1..9] = this pilot + Data + next pilot */
/* down-convert at current timing instant---------------------------------- */
for (j = st, k = 0; j < en; j++, k++) {
- work[k] = ofdm->rxbuf[j] * cexpf(-I * woff_est * j);
+ work[k] = ofdm->rxbuf_fcorr[j] * cexpf(-I * woff_est * j);
}
matrix_vector_conjugate_multiply(ofdm, ofdm->rx_sym[OFDM_NS + 2], work); /* sym[10] = last pilot */
/*
* We are finished now with the DFT and down conversion
- * From here on down we are in the frequency domain
+ * From here on down we are in baseband frequency domain
*/
/* est freq err based on all carriers ------------------------------------ */
if (ofdm->foff_est_en == true) {
/*
- * Subtract the two complex pilot frequency values
+ * Conjugate multiply the two complex pilot frequency values
* sym[1] is the 18 (this) pilot carriers, sym[9] is (next) 18 pilot carriers.
*
- * So we sum the carriers all together. Since we hope the two pilots are identical,
- * then by conjugating one of them, we should get a real only value, and the
+ * So we sum the carriers all together. Since we hope the two pilots have identical
+ * phases, then by conjugating one of them, we should get a real only value, and the
* imaginary part should be zero.
*
* If we find the angle using atan2, then it would be atan2(im, re).
* but if the im is zero, then we basically at 0 degrees radian at some real x.
*
- * If the two pilots are different, then we will get a positive or negative angle
- * in radians, and we can use this to correct the frequency.
+ * If the two pilot phases are different, then we will get a positive or negative angle
+ * offset by some angle in radians, and we can use this to correct the frequency.
*/
complex float freq_err_rect = conjf(vector_sum(ofdm->rx_sym[1],
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