PHY: make sf_extension a parameter

This parameter is meaningful in TDD, to decide
when to start DL at eNB side. Since there is a
need for the PA to be operational, we need to
transmit a bit before the DL subframe coming
after an UL subframe. (We transmit zeros.)

We used to use N_TA_offset which may be too much.
Default value is now N_TA_offset/2 and can be
changed in the configuration file, in the RUs
section, like:

RUs = (
    {
       local_rf       = "yes"
         nb_tx          = 1
         nb_rx          = 1
         att_tx         = 0
         att_rx         = 0;
         bands          = [7];
         max_pdschReferenceSignalPower = -27;
         max_rxgain                    = 105;
         eNB_instances  = [0];
         sf_extension = 312;
    }
);

N_TA_offset is 624 (for 30.72MHz). In the example
above, we set sf_extension = 312, which is also
the default.

The value to put in the configuration file is for
30.72MHz. The value is scaled accordingly at runtime
(thus only one value to set for every RB configuration,
25, 50 or 100, leading to less problems when adapting
configuration files).

This option is for experts and should not be changed
randomly.

openair1/PHY/defs_eNB.h

@@ -312,6 +312,8 @@ typedef struct RU_t_s{
   LTE_DL_FRAME_PARMS frame_parms;
   ///timing offset used in TDD
   int              N_TA_offset; 
+  /// SF extension used in TDD (unit: number of samples at 30.72MHz) (this is an expert option)
+  int              sf_extension;
   /// RF device descriptor
   openair0_device rfdevice;
   /// HW configuration

openair2/ENB_APP/enb_paramdef.h

@@ -101,6 +101,7 @@ typedef enum {
 #define CONFIG_STRING_RU_NBIOTRRC_LIST            "NbIoT_RRC_instances"
 #define CONFIG_STRING_RU_SDR_ADDRS                "sdr_addrs"
 #define CONFIG_STRING_RU_SDR_CLK_SRC              "clock_src"
+#define CONFIG_STRING_RU_SF_EXTENSION             "sf_extension"
 
 #define RU_LOCAL_IF_NAME_IDX          0
 #define RU_LOCAL_ADDRESS_IDX          1
@@ -122,6 +123,7 @@ typedef enum {
 #define RU_NBIOTRRC_LIST_IDX          17
 #define RU_SDR_ADDRS                  18
 #define RU_SDR_CLK_SRC                19
+#define RU_SF_EXTENSION_IDX           20
 
 
 
@@ -149,7 +151,8 @@ typedef enum {
 {CONFIG_STRING_RU_ATT_RX,                   	 NULL,       0,       uptr:NULL,       defintval:0,		TYPE_UINT,	  0}, \
 {CONFIG_STRING_RU_NBIOTRRC_LIST,                 NULL,       0,       uptr:NULL,       defintarrayval:DEFENBS,  TYPE_INTARRAY,    1}, \
 {CONFIG_STRING_RU_SDR_ADDRS,                 	 NULL,       0,       strptr:NULL,     defstrval:"type=b200",   TYPE_STRING,      0}, \
-{CONFIG_STRING_RU_SDR_CLK_SRC,               	 NULL,       0,       strptr:NULL,     defstrval:"internal",   TYPE_STRING,      0}, \
+{CONFIG_STRING_RU_SDR_CLK_SRC,               	 NULL,       0,       strptr:NULL,     defstrval:"internal",    TYPE_STRING,      0}, \
+{CONFIG_STRING_RU_SF_EXTENSION,                  NULL,       0,       uptr:NULL,       defuintval:312,          TYPE_UINT,        0}, \
 }
 
 /*---------------------------------------------------------------------------------------------------------------------------------------*/

targets/RT/USER/lte-ru.c

@@ -835,7 +835,7 @@ void tx_rf(RU_t *ru) {
         (prevSF_type == SF_UL) &&
         (nextSF_type == SF_DL)) {
       flags = 2; // start of burst
-      sf_extension = ru->N_TA_offset;
+      sf_extension = ru->sf_extension;
     }
 
     if ((fp->frame_type == TDD) &&
@@ -843,7 +843,7 @@ void tx_rf(RU_t *ru) {
         (prevSF_type == SF_UL) &&
         (nextSF_type == SF_UL)) {
       flags = 4; // start of burst and end of burst (only one DL SF between two UL)
-      sf_extension = ru->N_TA_offset;
+      sf_extension = ru->sf_extension;
     }
 
 #if defined(__x86_64) || defined(__i386__)
@@ -1386,6 +1386,14 @@ int setup_RU_buffers(RU_t *ru) {
        * TODO: find a proper cleaner solution
        */
       ru->N_TA_offset = 0;
+
+    if      (frame_parms->N_RB_DL == 100) /* no scaling to do */;
+    else if (frame_parms->N_RB_DL == 50)  ru->sf_extension /= 2;
+    else if (frame_parms->N_RB_DL == 25)  ru->sf_extension /= 4;
+    else { printf("not handled, todo\n"); exit(1); }
+  } else {
+    ru->N_TA_offset = 0;
+    ru->sf_extension = 0;
   }
 
   if (ru->openair0_cfg.mmapped_dma == 1) {
@@ -2883,6 +2891,8 @@ void RCconfig_RU(void) {
         RC.ru[j]->max_pdschReferenceSignalPower     = *(RUParamList.paramarray[j][RU_MAX_RS_EPRE_IDX].uptr);;
         RC.ru[j]->max_rxgain                        = *(RUParamList.paramarray[j][RU_MAX_RXGAIN_IDX].uptr);
         RC.ru[j]->num_bands                         = RUParamList.paramarray[j][RU_BAND_LIST_IDX].numelt;
+        /* sf_extension is in unit of samples for 30.72MHz here, has to be scaled later */
+        RC.ru[j]->sf_extension                      = *(RUParamList.paramarray[j][RU_SF_EXTENSION_IDX].uptr);
 
         for (i=0; i<RC.ru[j]->num_bands; i++) RC.ru[j]->band[i] = RUParamList.paramarray[j][RU_BAND_LIST_IDX].iptr[i];
       } //strcmp(local_rf, "yes") == 0