phy: add warning when choosing higher 3/4 sampling

common/utils/nr/nr_common.c

@@ -874,6 +874,7 @@ void get_samplerate_and_bw(int mu,
     switch(n_rb) {
     case 270:
       if (threequarter_fs) {
+        warn_higher_threequarter_fs(n_rb, mu);
         *sample_rate=92.16e6;
         *samples_per_frame = 921600;
         *tx_bw = 50e6;
@@ -973,6 +974,7 @@ void get_samplerate_and_bw(int mu,
 
     case 273:
       if (threequarter_fs) {
+        warn_higher_threequarter_fs(n_rb, mu);
         *sample_rate=184.32e6;
         *samples_per_frame = 1843200;
         *tx_bw = 100e6;

common/utils/nr/nr_common.h

@@ -37,6 +37,7 @@
 #include <stdlib.h>
 #include "assertions.h"
 #include "common/utils/utils.h"
+#include "common/utils/LOG/log.h"
 
 #define MAX_SI_GROUPS 3
 #define NR_MAX_PDSCH_TBS 3824
@@ -272,6 +273,19 @@ static __attribute__((always_inline)) inline int count_bits64_with_mask(uint64_t
   return count_bits64(v & mask);
 }
 
+static inline void warn_higher_threequarter_fs(const int n_rb, const int mu)
+{
+  LOG_W(PHY,
+        "3/4 sampling is not possible for current PRB size: %d and numerology: %d.\n "
+        "So 6/4 sampling is chosen to support x3xx type USRPs.\n "
+        "Note that this sampling rate increases fronthaul traffic, FFT buffer size and processing time by a factor of two compared "
+        "to 3/4 sampling rate.\n "
+        "Some PRACH configuration might not be supported with 6/4 FFT size.\n "
+        "Consider reducing the PRB size that would fit within the FFT size of 3/4 sampling\n",
+        n_rb,
+        mu);
+}
+
 uint64_t reverse_bits(uint64_t in, int n_bits);
 void reverse_bits_u8(uint8_t const* in, size_t sz, uint8_t* out);
 

openair1/PHY/INIT/nr_parms.c

@@ -199,14 +199,26 @@ void set_scs_parameters (NR_DL_FRAME_PARMS *fp, int mu, int N_RB_DL)
       AssertFatal(1==0,"Invalid numerology index %d", mu);
   }
 
-  if(fp->threequarter_fs)
-    fp->ofdm_symbol_size = 3 * 128;
-  else
-    fp->ofdm_symbol_size = 4 * 128;
+  // Start with FFT size 512
+  fp->ofdm_symbol_size = 512;
 
-  while(fp->ofdm_symbol_size < N_RB_DL * 12)
+  // Choose the right FFT size for the BW
+  while(fp->ofdm_symbol_size < N_RB_DL * NR_NB_SC_PER_RB)
     fp->ofdm_symbol_size <<= 1;
 
+  // Do 3/4 sampling
+  if (fp->threequarter_fs) {
+    const uint16_t threeq_fft_size = fp->ofdm_symbol_size * 3 / 4;
+    if (threeq_fft_size < (N_RB_DL * NR_NB_SC_PER_RB)) {
+      // 3/4 sampling FFT size not enough
+      warn_higher_threequarter_fs(N_RB_DL, fp->numerology_index);
+      // Choose 2 times 3/4 sampling
+      fp->ofdm_symbol_size = threeq_fft_size << 1;
+    } else {
+      fp->ofdm_symbol_size = threeq_fft_size;
+    }
+  }
+
   fp->first_carrier_offset = fp->ofdm_symbol_size - (N_RB_DL * 12 / 2);
   fp->nb_prefix_samples    = fp->ofdm_symbol_size / 128 * 9;
   fp->nb_prefix_samples0   = fp->ofdm_symbol_size / 128 * (9 + (1 << mu));