phy: Document what the various pss_signal_* fields represent

Between the padding and conjugation applied, it's non-obvious what the contents of these fields are.
Also get rid of unused pss_signal_time_scale

Author: kotarou3

lib/include/srsran/phy/sync/pss.h

@@ -76,12 +76,15 @@ typedef struct SRSRAN_API {
   uint32_t frame_size;
   uint32_t N_id_2;
   uint32_t fft_size;
-  cf_t*    pss_signal_freq_full[3];
 
-  cf_t* pss_signal_time[3];
-  cf_t* pss_signal_time_scale[3];
+  // pss_signal_freq: One raw ZC sequence for each N_id_2, without the DC bin
+  // pss_signal_time: conj(IDFT{pss_signal_freq outer frequencies padded to fft_size}) right-padded to frame_size+fft_size
+  // pss_signal_freq_full: DFT{pss_signal_time}
+  // Note: Conjugate is applied so we can find the correlation with convolution functions
+  cf_t   pss_signal_freq[3][SRSRAN_PSS_LEN];
+  cf_t*  pss_signal_time[3];
+  cf_t*  pss_signal_freq_full[3];
 
-  cf_t   pss_signal_freq[3][SRSRAN_PSS_LEN]; // One sequence for each N_id_2
   cf_t*  tmp_input;
   cf_t*  conv_output;
   float* conv_output_abs;

lib/src/phy/sync/pss.c

@@ -437,7 +437,7 @@ float compute_peak_sidelobe(srsran_pss_t* q, uint32_t corr_peak_pos, uint32_t co
 }
 
 /** Performs time-domain PSS correlation.
- * Returns the index of the PSS correlation peak in a subframe.
+ * Returns the end index of the PSS correlation peak in a subframe.
  * The frame starts at corr_peak_pos-subframe_size/2.
  * The value of the correlation is stored in corr_peak_value.
  *
@@ -458,9 +458,10 @@ int srsran_pss_find_pss(srsran_pss_t* q, const cf_t* input, float* corr_peak_val
 
     /* Correlate input with PSS sequence
      *
-     * We do not reverse time-domain PSS signal because it's conjugate is symmetric.
      * The conjugate operation on pss_signal_time has been done in srsran_pss_init_N_id_2
-     * This is why we can use FFT-based convolution
+     * This is why we can use FFT-based convolution.
+     * We do not reverse time-domain PSS signal because its conjugate is symmetric, but
+     * it does mean the correlation peak is on the end of the signal rather than start
      */
     if (q->frame_size >= q->fft_size) {
 #ifdef CONVOLUTION_FFT