Added linear phase derotation in UL

This is necessary to compensate the rotation caused by the timeshifted OFDM symbol (1/8 CP)

openair1/PHY/INIT/nr_init.c

@@ -535,6 +535,7 @@ void nr_phy_config_request(NR_PHY_Config_t *phy_config) {
   RC.gNB[Mod_id]->configured     = 1;
 
   init_symbol_rotation(fp);
+  init_timeshift_rotation(fp);
 
   LOG_I(PHY,"gNB %d configured\n",Mod_id);
 }

openair1/PHY/MODULATION/nr_modulation.c

@@ -745,6 +745,75 @@ void init_symbol_rotation(NR_DL_FRAME_PARMS *fp) {
   }
 }
 
+void init_timeshift_rotation(NR_DL_FRAME_PARMS *fp)
+{
+  const double fd = 15e3*(float)(1<<fp->numerology_index);
+  const double Tc=(1/480e3/4096);
+  const double Nu=2048*64*(1/(float)(1<<fp->numerology_index));
+  const double Ncp0=16*64 + (144*64*(1/(float)(1<<fp->numerology_index)));
+  const double Ncp1=(144*64*(1/(float)(1<<fp->numerology_index)));
+  double t0;
+  double poff;
+  double exp_re;
+  double exp_im;
+  double f;
+  int16_t *timeshift_symbol_rotation;
+  int i;
+
+  for (i = 0; i < fp->ofdm_symbol_size/2; i++) {
+    timeshift_symbol_rotation = fp->timeshift_symbol_rotation[0];
+    f = fd * i;
+    t0 = Ncp0 / 8;
+    poff = 2 * M_PI * -t0 * Tc * f;
+    exp_re = cos(poff);
+    exp_im = sin(-poff);
+    timeshift_symbol_rotation[i*2] = (int16_t)floor(exp_re * 32767);
+    timeshift_symbol_rotation[i*2+1] = (int16_t)floor(exp_im * 32767);
+
+    timeshift_symbol_rotation = fp->timeshift_symbol_rotation[1];
+    t0 = Ncp1 / 8;
+    poff = 2 * M_PI * -t0 * Tc * f;
+    exp_re = cos(poff);
+    exp_im = sin(-poff);
+    timeshift_symbol_rotation[i*2] = (int16_t)floor(exp_re * 32767);
+    timeshift_symbol_rotation[i*2+1] = (int16_t)floor(exp_im * 32767);
+
+    if (i < 10)
+      LOG_I(PHY,"Timeshift symbol rotation %d => (%d,%d) %f\n",i,
+            fp->timeshift_symbol_rotation[1][i*2],
+            fp->timeshift_symbol_rotation[1][i*2+1],
+            poff);
+
+  }
+
+  i = 1;
+  for (int j = fp->ofdm_symbol_size-1; j > fp->ofdm_symbol_size/2-1; j--) {
+    timeshift_symbol_rotation = fp->timeshift_symbol_rotation[0];
+    f = -fd * i;
+    t0 = Ncp0 / 8;
+    poff = 2 * M_PI * -t0 * Tc * f;
+    exp_re = cos(poff);
+    exp_im = sin(-poff);
+    timeshift_symbol_rotation[j*2] = (int16_t)floor(exp_re * 32767);
+    timeshift_symbol_rotation[j*2+1] = (int16_t)floor(exp_im * 32767);
+
+    timeshift_symbol_rotation = fp->timeshift_symbol_rotation[1];
+    t0 = Ncp1 / 8;
+    poff = 2 * M_PI * -t0 * Tc * f;
+    exp_re = cos(poff);
+    exp_im = sin(-poff);
+    timeshift_symbol_rotation[j*2] = (int16_t)floor(exp_re * 32767);
+    timeshift_symbol_rotation[j*2+1] = (int16_t)floor(exp_im * 32767);
+
+    if (j > fp->ofdm_symbol_size-10-1)
+      LOG_I(PHY,"Timeshift symbol rotation %d => (%d,%d) %f\n",-i,
+            fp->timeshift_symbol_rotation[1][j*2],
+            fp->timeshift_symbol_rotation[1][j*2+1],
+            poff);
+    i++;
+  }
+}
+
 int nr_layer_precoder(int16_t **datatx_F_precoding, char *prec_matrix, uint8_t n_layers, int32_t re_offset)
 {
   int32_t precodatatx_F = 0;

openair1/PHY/MODULATION/slot_fep_nr.c

@@ -346,7 +346,7 @@ void apply_nr_rotation_ul(NR_DL_FRAME_PARMS *frame_parms,
 			  
   int symb_offset = (slot%frame_parms->slots_per_subframe)*frame_parms->symbols_per_slot;
 
-  for (int symbol=0;symbol<nsymb;symbol++) {
+  for (int symbol=first_symbol;symbol<nsymb;symbol++) {
     
     uint32_t rot2 = ((uint32_t*)frame_parms->symbol_rotation[1])[symbol + symb_offset];
     ((int16_t*)&rot2)[1]=-((int16_t*)&rot2)[1];
@@ -356,5 +356,18 @@ void apply_nr_rotation_ul(NR_DL_FRAME_PARMS *frame_parms,
 		      (int16_t *)&rxdataF[frame_parms->ofdm_symbol_size*symbol],
 		      length,
 		      15);
+
+    int16_t *shift_rot;
+    if (symb_offset+symbol == (7 * (1 << frame_parms->numerology_index)))
+      shift_rot = frame_parms->timeshift_symbol_rotation[0];
+    else
+      shift_rot = frame_parms->timeshift_symbol_rotation[1];
+
+    multadd_cpx_vector((int16_t *)&rxdataF[frame_parms->ofdm_symbol_size*symbol],
+          shift_rot,
+          (int16_t *)&rxdataF[frame_parms->ofdm_symbol_size*symbol],
+          1,
+          length,
+          15);
   }
 }

openair1/PHY/defs_nr_common.h

@@ -329,6 +329,9 @@ struct NR_DL_FRAME_PARMS {
   /// sequence which is computed based on carrier frequency and numerology to rotate/derotate each OFDM symbol according to Section 5.3 in 38.211
   /// First dimension is for the direction of the link (0 DL, 1 UL)
   int16_t symbol_rotation[2][224*2];
+  /// sequence used to compensate the phase rotation due to timeshifted OFDM symbols
+  /// First dimenstion is for different CP lengths
+  int16_t timeshift_symbol_rotation[2][4096*2] __attribute__ ((aligned (16)));
   /// shift of pilot position in one RB
   uint8_t nushift;
   /// SRS configuration from TS 38.331 RRC