basic simulator: handle discontinuities in output

In TDD mode the eNB does not output data all the time,
only in downlink and special subframes.

The basic simulator needs a continuous stream of samples.

So let's fill the blank with silence.

targets/ARCH/tcp_bridge/tcp_bridge_oai.c

@@ -44,6 +44,7 @@ typedef struct {
   int sock;
   int samples_per_subframe;
   uint64_t timestamp;
+  uint64_t next_tx_timestamp;
   int is_enb;
 } tcp_bridge_state_t;
 
@@ -141,11 +142,27 @@ int tcp_bridge_write(openair0_device *device, openair0_timestamp timestamp, void
 {
   if (cc != 1) { printf("tcp_bridge: only 1 antenna supported\n"); exit(1); }
   tcp_bridge_state_t *t = device->priv;
+  /* deal with discontinuities in output (think: eNB in TDD mode) */
+  if (t->next_tx_timestamp && timestamp != t->next_tx_timestamp) {
+    uint32_t b[4096];
+    uint64_t to_send = timestamp - t->next_tx_timestamp;
+    memset(b, 0, 4096 * sizeof(uint32_t));
+    while (to_send) {
+      int len = to_send > 4096 ? 4096 : to_send;
+      int n = fullwrite(t->sock, b, len * 4);
+      if (n != len * 4) {
+        printf("tcp_bridge: write error ret %d error %s\n", n, strerror(errno));
+        abort();
+      }
+      to_send -= len;
+    }
+  }
   int n = fullwrite(t->sock, buff[0], nsamps * 4);
   if (n != nsamps * 4) {
     printf("tcp_bridge: write error ret %d (wanted %d) error %s\n", n, nsamps*4, strerror(errno));
     abort();
   }
+  t->next_tx_timestamp = timestamp + nsamps;
   return nsamps;
 }