Change the calculation of UE group A random access preamble selection to choose a random number from [0 nof_groupA_preambles) rather than incrementing on the preambleTransmissionCounter. (#200)

srsue/src/mac/proc_ra.cc

@@ -237,7 +237,8 @@ void ra_proc::step_resource_selection() {
     }
     if (sel_group == RA_GROUP_A) {
       if (nof_groupA_preambles) {
-        sel_preamble = preambleTransmissionCounter%nof_groupA_preambles;
+        // randomly choose preamble from [0 nof_groupA_preambles)
+        sel_preamble = rand() % nof_groupA_preambles;
       } else {
         rError("Selected group preamble A but nof_groupA_preambles=0\n");
         state = RA_PROBLEM;
@@ -245,6 +246,7 @@ void ra_proc::step_resource_selection() {
       }
     } else {
       if (nof_groupB_preambles) {
+        // randomly choose preamble from [nof_groupA_preambles nof_groupB_preambles)
         sel_preamble = nof_groupA_preambles + rand() % nof_groupB_preambles;
       } else {
         rError("Selected group preamble B but nof_groupA_preambles=0\n");