How to Make a New SDU in the RLC SRB TX Buffer Get Scheduled Immediately

[hdtuanss]

Hi everyone,

I'm currently modifying the DU-high component of srsgNB to push an SDU into the RLC SRB TX buffer and send it to the UE. My goal is for the new SDU to be scheduled immediately after being pushed during the Registration Procedure.

To achieve this, I saved the SRB0 TX buffer pointer during its creation and called the handle_sdu() function during Registration Procedure between UE and gNB, as shown below:

//In file: lib/du/du_high/du_manager/procedures/ue_creation_procedure.cpp
void ue_creation_procedure::create_rlc_srbs()
{
  // Create SRB0 RLC entity.
  du_ue_srb& srb0 = ue_ctx->bearers.srbs()[srb_id_t::srb0];
  srb0.rlc_bearer = create_rlc_entity(make_rlc_entity_creation_message(du_params.ran.gnb_du_id,
                                                                       ue_ctx->ue_index,
                                                                       ue_ctx->pcell_index,
                                                                       srb0,
                                                                       make_default_srb0_rlc_config(),
                                                                       du_params.services,
                                                                       ue_ctx->get_rlc_rlf_notifier(),
                                                                       du_params.rlc.pcap_writer));

  // Create SRB1 RLC entity.
  if (ue_ctx->bearers.srbs().contains(srb_id_t::srb1)) {
    du_ue_srb& srb1 = ue_ctx->bearers.srbs()[srb_id_t::srb1];
    srb1.rlc_bearer =
        create_rlc_entity(make_rlc_entity_creation_message(du_params.ran.gnb_du_id,
                                                           ue_ctx->ue_index,
                                                           ue_ctx->pcell_index,
                                                           srb1,
                                                           ue_ctx->resources->srbs[srb_id_t::srb1].rlc_cfg,
                                                           du_params.services,
                                                           ue_ctx->get_rlc_rlf_notifier(),
                                                           du_params.rlc.pcap_writer));

  }

   //save the latest rlc_tx_upper_layer_data_interface to use later
    rlc_tx_upper_layer_data_interface* rlc_tx = srb0.rlc_bearer->get_tx_upper_layer_data_interface();
    test_framework->update_ue_rlc_srb0_tx_pointer(rlc_tx);
  }
}

Later, when I want to send a new SDU during Registration Procedure, I call:

rlc_tx->handle_sdu(std::move(new_pdu), false);

However, I'm observing a delay in the scheduling. For example, if I push the SDU at (SFN, slot) = (100, 1), it is scheduled for transmission at (102, 1). Also, when I push two SDUs in consecutive slots, they are combined into a single MAC PDU.

My question is: Why is there a delay in SDU scheduling for RLC SRB0, and how can I make the SDU be scheduled immediately in the next available downlink transmission?

Thanks in advance!