From 3763d04578da2d763961c59f4de186182644fe8e Mon Sep 17 00:00:00 2001
From: Xavier Arteaga <xavier.arteaga@softwareradiosystems.com>
Date: Tue, 28 Sep 2021 16:18:43 +0200
Subject: [PATCH] implement helper function to calculate FFT size for a given
 sampling rate

---
 lib/include/srsran/phy/common/phy_common_nr.h |  8 +++++
 lib/src/phy/common/phy_common_nr.c            | 31 +++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/lib/include/srsran/phy/common/phy_common_nr.h b/lib/include/srsran/phy/common/phy_common_nr.h
index 931be19b4..f33d2adfa 100644
--- a/lib/include/srsran/phy/common/phy_common_nr.h
+++ b/lib/include/srsran/phy/common/phy_common_nr.h
@@ -596,6 +596,14 @@ SRSRAN_API srsran_mcs_table_t srsran_mcs_table_from_str(const char* str);
  */
 SRSRAN_API uint32_t srsran_min_symbol_sz_rb(uint32_t nof_prb);
 
+/**
+ * @brief Computes the minimum valid symbol size for a given amount of PRB
+ * @attention The valid FFT sizes are radix 2 and radix 3 between 128 to 4096 points.
+ * @param nof_prb Number of PRB
+ * @return The minimum valid FFT size if the number of PRB is in range, 0 otherwise
+ */
+SRSRAN_API int srsran_symbol_sz_from_srate(double srate_hz, srsran_subcarrier_spacing_t scs);
+
 /**
  * @brief Computes the time in seconds between the beginning of the slot and the given symbol
  * @remark All symbol size reference and values are taken from TS 38.211 section 5.3 OFDM baseband signal generation
diff --git a/lib/src/phy/common/phy_common_nr.c b/lib/src/phy/common/phy_common_nr.c
index 8a5d410df..9f514ccbf 100644
--- a/lib/src/phy/common/phy_common_nr.c
+++ b/lib/src/phy/common/phy_common_nr.c
@@ -244,6 +244,37 @@ uint32_t srsran_min_symbol_sz_rb(uint32_t nof_prb)
   return 0;
 }
 
+int srsran_symbol_sz_from_srate(double srate_hz, srsran_subcarrier_spacing_t scs)
+{
+  // Make sure srate is valid
+  if (!isnormal(srate_hz) || srate_hz < 0.0) {
+    return SRSRAN_ERROR;
+  }
+
+  // Convert srate to integer and Hz
+  uint32_t srate_int_hz = (uint32_t)srate_hz;
+
+  // Get subcarrier spacing in Hz
+  uint32_t scs_int_hz = SRSRAN_SUBC_SPACING_NR(scs);
+
+  // Check the symbol size if a integer
+  if (srate_int_hz % scs_int_hz != 0) {
+    ERROR("Invalid sampling rate %.2f MHz with subcarrrier spacing %d kHz", srate_hz / 1e6, scs_int_hz / 1000);
+    return SRSRAN_ERROR;
+  }
+
+  // Calculate symbol size in samples
+  uint32_t symbol_sz = srate_int_hz / scs_int_hz;
+
+  // Verify the symbol size can have an integer cyclic prefix size
+  if ((symbol_sz * 144U) % 2048 != 0 && (symbol_sz * (16U << (uint32_t)scs)) % 2048 != 0) {
+    ERROR("The sampling rate %.2f MHz with subcarrrier spacing %d kHz", srate_hz / 1e6, scs_int_hz / 1000);
+    return SRSRAN_ERROR;
+  }
+
+  return (int)symbol_sz;
+}
+
 float srsran_symbol_offset_s(uint32_t l, srsran_subcarrier_spacing_t scs)
 {
   // Compute at what symbol there is a longer CP