ch_awgn: Improve API

The API of functions srsran_ch_awgn_X was causing confusion between
noise variance and standard deviation. Also, in the complex-valued case,
the noise variance did not take into account the fact that the signal is
two-dimensional.

All uses of these functions are modified accordingly.

lib/examples/npdsch_enodeb.c

@@ -683,10 +683,9 @@ int main(int argc, char** argv)
         // find the noise spectral density
         float snr_lin = srsran_convert_dB_to_power(file_snr);
         float n0      = abs_avg / snr_lin;
-        float nstd    = sqrtf(n0 / 2);
 
         // add some noise to the signal
-        srsran_ch_awgn_c(output_buffer, output_buffer, nstd, sf_n_samples);
+        srsran_ch_awgn_c(output_buffer, output_buffer, n0, sf_n_samples);
       }
 
       /* send to file or usrp */

lib/examples/pdsch_enodeb.c

@@ -992,7 +992,7 @@ int main(int argc, char** argv)
         if (!null_file_sink) {
           /* Apply AWGN */
           if (output_file_snr != +INFINITY) {
-            float var = srsran_convert_dB_to_amplitude(-(output_file_snr + 3.0f));
+            float var = srsran_convert_dB_to_power(-output_file_snr);
             for (int k = 0; k < cell.nof_ports; k++) {
               srsran_ch_awgn_c(output_buffer[k], output_buffer[k], var, sf_n_samples);
             }

lib/include/srsran/phy/channel/ch_awgn.h

@@ -1,23 +1,15 @@
 /**
+ *  \file ch_awgn.h
+ *  \brief Additive white Gaussian noise channel object
  *
- * \section COPYRIGHT
+ * \copyright Copyright 2013-2021 Software Radio Systems Limited
  *
- * Copyright 2013-2021 Software Radio Systems Limited
+ * \copyright By using this file, you agree to the terms and conditions set
- *
- * By using this file, you agree to the terms and conditions set
  * forth in the LICENSE file which can be found at the top level of
  * the distribution.
  *
  */
 
-/**********************************************************************************************
- *  File:         ch_awgn.h
- *
- *  Description:  Additive white gaussian noise channel object
- *
- *  Reference:
- *********************************************************************************************/
-
 #include "srsran/config.h"
 #include <stdint.h>
 
@@ -29,7 +21,7 @@ extern "C" {
 #endif
 
 /**
- * The srsRAN channel AWGN implements an efficient Box-Muller Method accelerated with SIMD.
+ * \brief srsRAN channel AWGN implements an efficient Box-Muller Method accelerated with SIMD.
  */
 typedef struct {
   float*   table_cos;
@@ -39,7 +31,7 @@ typedef struct {
 } srsran_channel_awgn_t;
 
 /**
- * Initialization function of the channel AWGN object
+ * \brief function of the channel AWGN object
  *
  * @param q AWGN channel object
  * @param seed random generator seed
@@ -47,7 +39,7 @@ typedef struct {
 SRSRAN_API int srsran_channel_awgn_init(srsran_channel_awgn_t* q, uint32_t seed);
 
 /**
- * Sets the noise level N0 in decibels full scale (dBfs)
+ * \brief the noise level N0 in decibels full scale (dBfs)
  *
  * @param q AWGN channel object
  * @param n0_dBfs noise level
@@ -55,7 +47,7 @@ SRSRAN_API int srsran_channel_awgn_init(srsran_channel_awgn_t* q, uint32_t seed)
 SRSRAN_API int srsran_channel_awgn_set_n0(srsran_channel_awgn_t* q, float n0_dBfs);
 
 /**
- * Runs the complex AWGN channel
+ * \brief the complex AWGN channel
  *
  * @param q AWGN channel object
  * @param in complex input array
@@ -65,7 +57,7 @@ SRSRAN_API int srsran_channel_awgn_set_n0(srsran_channel_awgn_t* q, float n0_dBf
 SRSRAN_API void srsran_channel_awgn_run_c(srsran_channel_awgn_t* q, const cf_t* in, cf_t* out, uint32_t length);
 
 /**
- * Runs the real AWGN channel
+ * \brief the real AWGN channel
  *
  * @param q AWGN channel object
  * @param in real input array
@@ -75,15 +67,31 @@ SRSRAN_API void srsran_channel_awgn_run_c(srsran_channel_awgn_t* q, const cf_t*
 SRSRAN_API void srsran_channel_awgn_run_f(srsran_channel_awgn_t* q, const float* in, float* out, uint32_t length);
 
 /**
- * Free AWGN channel generator data
+ * \brief AWGN channel generator data
  *
  * @param q AWGN channel object
  */
 SRSRAN_API void srsran_channel_awgn_free(srsran_channel_awgn_t* q);
 
+/**
+ * \brief signal \p input with AWGN to obtain signal \p output (complex case).
+ *
+ * @param[in] input Input signal
+ * @param[out] output Output signal
+ * @param[in] variance Noise variance
+ * @param[in] len Number of samples
+ */
 SRSRAN_API void srsran_ch_awgn_c(const cf_t* input, cf_t* output, float variance, uint32_t len);
 
-SRSRAN_API void srsran_ch_awgn_f(const float* x, float* y, float variance, uint32_t len);
+/**
+ * \brief Perturb signal \p input with AWGN to obtain signal \p output (real case).
+ *
+ * @param[in] input Input signal
+ * @param[out] output Output signal
+ * @param[in] variance Noise variance
+ * @param[in] len Number of samples
+ */
+SRSRAN_API void srsran_ch_awgn_f(const float* input, float* output, float variance, uint32_t len);
 
 SRSRAN_API float srsran_ch_awgn_get_variance(float ebno_db, float rate);
 

lib/src/phy/ch_estimation/test/chest_nbiot_test_dl.c

@@ -169,7 +169,7 @@ int main(int argc, char** argv)
 
       if (have_channel) {
         // Add noise
-        float std_dev = srsran_convert_dB_to_amplitude(-(snr_db + 3.0f)) * 0.1f;
+        float std_dev = srsran_convert_dB_to_power(-(snr_db + 20.0f));
         srsran_ch_awgn_c(est.pilot_recv_signal, est.pilot_recv_signal, std_dev, SRSRAN_REFSIGNAL_MAX_NUM_SF(1));
       }
 

lib/src/phy/channel/ch_awgn.c

@@ -119,7 +119,6 @@ static inline void channel_awgn_run(srsran_channel_awgn_t* q, const float* in, f
 
 #if SRSRAN_SIMD_F_SIZE
   for (; i < (int)size - SRSRAN_SIMD_F_SIZE + 1; i += SRSRAN_SIMD_F_SIZE) {
-
     if (i % AWGN_TABLE_READ_BURST == 0) {
       idx1 = channel_awgn_rand(q);
       idx2 = channel_awgn_rand(q);
@@ -145,7 +144,6 @@ static inline void channel_awgn_run(srsran_channel_awgn_t* q, const float* in, f
 #endif /* SRSRAN_SIMD_F_SIZE */
 
   for (; i < size; i++) {
-
     if (i % AWGN_TABLE_READ_BURST == 0) {
       idx1 = channel_awgn_rand(q);
       idx2 = channel_awgn_rand(q);
@@ -197,19 +195,21 @@ void srsran_ch_awgn_c(const cf_t* x, cf_t* y, float variance, uint32_t len)
 {
   cf_t     tmp;
   uint32_t i;
+  float    stddev = sqrtf(variance);
 
   for (i = 0; i < len; i++) {
     __real__ tmp = rand_gauss();
     __imag__ tmp = rand_gauss();
-    tmp *= variance;
+    tmp *= stddev * (float)M_SQRT1_2;
     y[i] = tmp + x[i];
   }
 }
 void srsran_ch_awgn_f(const float* x, float* y, float variance, uint32_t len)
 {
   uint32_t i;
+  float    stddev = sqrtf(variance);
 
   for (i = 0; i < len; i++) {
-    y[i] = x[i] + variance * rand_gauss();
+    y[i] = x[i] + stddev * rand_gauss();
   }
 }

lib/src/phy/fec/convolutional/test/viterbi_test.c

@@ -184,13 +184,13 @@ int main(int argc, char** argv)
     for (uint32_t i = 0; i < snr_points; i++) {
       ebno_db   = SNR_MIN + i * ebno_inc;
       esno_db   = ebno_db + srsran_convert_power_to_dB(1.0f / 3.0f);
-      var[i]    = srsran_convert_dB_to_amplitude(esno_db);
+      var[i]    = srsran_convert_dB_to_power(-esno_db);
-      varunc[i] = srsran_convert_dB_to_amplitude(ebno_db);
+      varunc[i] = srsran_convert_dB_to_power(-ebno_db);
     }
   } else {
     esno_db    = ebno_db + srsran_convert_power_to_dB(1.0f / 3.0f);
-    var[0]     = srsran_convert_dB_to_amplitude(esno_db);
+    var[0]     = srsran_convert_dB_to_power(-esno_db);
-    varunc[0]  = srsran_convert_dB_to_amplitude(ebno_db);
+    varunc[0]  = srsran_convert_dB_to_power(-ebno_db);
     snr_points = 1;
   }
 

lib/src/phy/fec/ldpc/test/ldpc_chain_test.c

@@ -314,6 +314,7 @@ int main(int argc, char** argv)
   int    n_error_words_avx512_flood    = 0;
 #endif // lV_HAVE_AVX512
 
+  float noise_var     = srsran_convert_dB_to_power(-snr);
   float noise_std_dev = srsran_convert_dB_to_amplitude(-snr);
 
   int16_t inf15  = (1U << 14U) - 1;
@@ -371,12 +372,12 @@ int main(int argc, char** argv)
     }
 
     // Apply AWGN
-    srsran_ch_awgn_f(symbols_rm, symbols_rm, noise_std_dev, batch_size * (rm_length + F));
+    srsran_ch_awgn_f(symbols_rm, symbols_rm, noise_var, batch_size * (rm_length + F));
 
     // Convert symbols into LLRs
     for (i = 0; i < batch_size; i++) {
       for (j = 0; j < rm_length + F; j++) { //+F because we have already considered fillerbits when modulating.
-        symbols[i * finalN + j] = symbols_rm[i * (rm_length + F) + j] * 2 / (noise_std_dev * noise_std_dev);
+        symbols[i * finalN + j] = symbols_rm[i * (rm_length + F) + j] * 2 / noise_var;
       }
       // the rest of symbols are undetermined, set LLR to 0
       for (; j < finalN; j++) {

lib/src/phy/fec/ldpc/test/ldpc_rm_chain_test.c

@@ -409,6 +409,7 @@ int main(int argc, char** argv)
   int    n_error_words_avx512_flood    = 0;
 #endif // LV_HAVE_AVX512
 
+  float noise_var     = srsran_convert_dB_to_power(-snr);
   float noise_std_dev = srsran_convert_dB_to_amplitude(-snr);
 
   int16_t inf15  = (1U << 14U) - 1;
@@ -478,12 +479,12 @@ int main(int argc, char** argv)
     }
 
     // Apply AWGN
-    srsran_ch_awgn_f(rm_symbols, rm_symbols, noise_std_dev, batch_size * rm_length);
+    srsran_ch_awgn_f(rm_symbols, rm_symbols, noise_var, batch_size * rm_length);
 
     // Convert symbols into LLRs
     for (i = 0; i < batch_size; i++) {
       for (j = 0; j < rm_length; j++) {
-        rm_symbols[i * rm_length + j] = rm_symbols[i * rm_length + j] * 2 / (noise_std_dev * noise_std_dev);
+        rm_symbols[i * rm_length + j] = rm_symbols[i * rm_length + j] * 2 / noise_var;
       }
     }
 

lib/src/phy/fec/polar/test/polar_chain_test.c

@@ -319,13 +319,13 @@ int main(int argc, char** argv)
     for (int i = 0; i < snr_points; i++) {
       snr_db        = SNR_MIN + i * snr_inc;
       snr_db_vec[i] = snr_db;
-      var[i]        = srsran_convert_dB_to_amplitude(-snr_db);
+      var[i]        = srsran_convert_dB_to_power(-snr_db);
     }
     snr_db_vec[snr_points] = 101; // include the no noise case
     snr_points++;
   } else {
     snr_db_vec[0] = snr_db;
-    var[0]        = srsran_convert_dB_to_amplitude(-snr_db);
+    var[0]        = srsran_convert_dB_to_power(-snr_db);
     snr_points    = 1;
   }
 

lib/src/phy/fec/turbo/test/turbodecoder_test.c

@@ -206,11 +206,11 @@ int main(int argc, char** argv)
     for (uint32_t i = 0; i < snr_points; i++) {
       ebno_db = SNR_MIN + i * ebno_inc;
       esno_db = ebno_db + srsran_convert_power_to_dB(1.0f / 3.0f);
-      var[i]  = srsran_convert_dB_to_amplitude(-esno_db);
+      var[i]  = srsran_convert_dB_to_power(-esno_db);
     }
   } else {
     esno_db    = ebno_db + srsran_convert_power_to_dB(1.0f / 3.0f);
-    var[0]     = srsran_convert_dB_to_amplitude(-esno_db);
+    var[0]     = srsran_convert_dB_to_power(-esno_db);
     snr_points = 1;
   }
   for (uint32_t i = 0; i < snr_points; i++) {

lib/src/phy/mimo/test/precoder_test.c

@@ -154,10 +154,10 @@ void populate_channel(srsran_tx_scheme_t type, cf_t* h[SRSRAN_MAX_PORTS][SRSRAN_
 static void awgn(cf_t* y[SRSRAN_MAX_PORTS], uint32_t n, float snr)
 {
   int   i;
-  float std_dev = srsran_convert_dB_to_amplitude(-(snr + 3.0f)) * scaling;
+  float var = srsran_convert_dB_to_power(-snr) * scaling * scaling;
 
   for (i = 0; i < nof_rx_ports; i++) {
-    srsran_ch_awgn_c(y[i], y[i], std_dev, n);
+    srsran_ch_awgn_c(y[i], y[i], var, n);
   }
 }
 

lib/src/phy/phch/test/npdsch_npdcch_file_test.c

@@ -213,8 +213,8 @@ int main(int argc, char** argv)
 
       // add some noise to the signal
       if (snr != -1.0) {
-        float nstd = powf(10.0f, -snr / 20.0f);
+        float var = powf(10.0f, -snr / 10.0f);
-        srsran_ch_awgn_c(buff_ptrs[0], buff_ptrs[0], nstd, nread);
+        srsran_ch_awgn_c(buff_ptrs[0], buff_ptrs[0], var, nread);
       }
 
       // try to decode

lib/src/phy/sync/test/sync_nbiot_test.c

@@ -175,8 +175,8 @@ int main(int argc, char** argv)
   if (snr != -1.0) {
     snr -= 10.0;
     printf("Adding AWGN with target SNR: %.2fdB\n", snr);
-    float nstd = srsran_convert_dB_to_amplitude(-snr);
+    float var = srsran_convert_dB_to_power(-snr);
-    srsran_ch_awgn_c(fft_buffer, fft_buffer, nstd, SFLEN);
+    srsran_ch_awgn_c(fft_buffer, fft_buffer, var, SFLEN);
   }
 
   // look for NPSS signal

lib/src/phy/sync/test/sync_sl_test.c

@@ -166,8 +166,8 @@ int main(int argc, char** argv)
 
     // ADD CHANNEL NOISE
     if (snr < 50) {
-      float std_dev = powf(10.0f, -(snr + 3.0f) / 20.0f);
+      float var = powf(10.0f, -snr / 10.0f);
-      srsran_ch_awgn_c(output_buffer, output_buffer, std_dev, output_buffer_len);
+      srsran_ch_awgn_c(output_buffer, output_buffer, var, output_buffer_len);
     }
 
     // ADD FREQUENCY OFFSET