hack: do not use BCH RBs for DL scheduling in subframe 0

The problem is that when using those RBs with MCS 28 we may exceed the
code rate 0.93 and according to 36.213 7.1.7 the UE may skip decoding
PDSCH entirely in this case.

This is a hack. The real solution is to check that the code rate is below
0.93 for each scheduling decision and, I don't know, reduce the MCS if the
code rate is above, so that in the end it is below. That means that we need
a proper resource grid for the configuration of the eNB and this is not an
easy thing (at least from my point of view) given all the possible
configurations for the eNB, so I prefer not to do it rather than do something
incorrect, thus this hack.

The problem of this hack is that we won't use all the available RBs for
scheduling, potentially reducing the maximum throughput achievable.

To be fixed properly at some time, by someone who understands fully the
resource grid and all the possible combinations (fdd/tdd, number of
antennas, whatever else).

openair2/LAYER2/MAC/eNB_scheduler.c

@@ -586,6 +586,23 @@ eNB_dlsch_ulsch_scheduler(module_id_t module_idP,
     memset(cc[CC_id].vrb_map_UL, 0, 100);
     cc[CC_id].mcch_active = 0;
     clear_nfapi_information(RC.mac[module_idP], CC_id, frameP, subframeP);
+
+    /* hack: skip BCH RBs in subframe 0 for DL scheduling,
+     *       because with high MCS we may exceed code rate 0.93
+     *       when using those RBs (36.213 7.1.7 says the UE may
+     *       skip decoding if the code rate is higher than 0.93)
+     * TODO: remove this hack, deal with physical bits properly
+     *       i.e. reduce MCS in the scheduler if code rate > 0.93
+     */
+    if (subframeP == 0) {
+      int i;
+      int bw = cc[CC_id].mib->message.dl_Bandwidth;
+      /* start and count defined for RBs: 6, 15, 25, 50, 75, 100 */
+      int start[6] = { 0, 4, 9, 22, 34, 47 };
+      int count[6] = { 6, 7, 7,  6,  7,  6 };
+      for (i = 0; i < count[bw]; i++)
+        cc[CC_id].vrb_map[start[bw] + i] = 1;
+    }
   }
 
   /* Refresh UE list based on UEs dropped by PHY in previous subframe */