# Prediction-Based Energy Saving Mechanism in 3 GPP NB-IoT Networks
## 3 Problem Statement for Scheduling Request  
Even after the radio bearer is established and the RRC is in the connected state (RRC_Connected), the uplink packets cannot be transmitted without a scheduling request procedure. In this section, we present the scheduling request procedure and the energy consumption problem in NB-IoT networks. Similar to LTE networks, the NB-IoT radio resources for packet transmission are shared among UEs, and an evolved node B (eNB) scheduler dynamically assigns these resources to each UE based on the scheduling policy. Scheduling commands, which contain assigned time, resource, and decoding information, represent DCI transmitted via the NPDCCH. The UE decodes the NPDCCH information by using a radio network temporary identifier (RNTI) at specific times with the decoding period configured when the connection is established. This RNTI-based decoding identifies whether the radio resource for the NPUSCH is assigned to the UE. Figure 8 illustrates the uplink packet transmission procedure including the physical control channel in the case that the UE does not have any assigned radio resources for uplink. In contrast to LTE networks, the NB-IoT does not have dedicated radio resources for scheduling requests. Hence, the uplink radio resources can be only requested through a random-access procedure. Since this procedure competes with that of other devices, the contention resolution could fail and the UE retries random access after a back-off time. As shown in step (1) of Figure 8, the preamble is transmitted via a narrowband random-access channel (NPRACH). Then, the eNB sends the random-access response through the NPDSCH and NPDCCH. The NPDCCH includes the DCI to decode the NPDSCH, and the NPDSCH carries an identifier for contention resolution and a scheduling command for the next NPUSCH, as shown in steps (2) and (3) of Figure 8. The eNB scheduler has no information on the amount of data in the UE buffer. Hence, it first assigns a small radio resource to receive the UE buffer status information. Next, when the UE receives the uplink scheduling command, it sends the buffer status report through the scheduled NPUSCH, as shown in step (4) of Figure 8. Finally, the eNB acknowledges the UE buffer status and continuously assigns uplink radio resources until the data transfer corresponding to the reported size is completed, as illustrated in steps (5) though (8) in Figure 8.  
Figure 7: Packet transmission scheme: (**a**) existing long-term evolution (LTE); (**b**) NB-IoT with control-plane (CP) cellular IoT (CIoT) optimization. Thus, although the radio bearer is established, additional interactions are required due to the scheduling request procedure. Given that the random-access procedure can be used to send a RRC connection request besides the scheduling request, the response message is typically composed of a 88-bit scheduling command, a size appropriate for that type of message [15]. Therefore, even packets with few bytes are difficult to transmit in MSG3 (step (4) of Figure 8), and another uplink transmission is required to complete the packet transmission, like step (6) of Figure 8. Given the NB-IoT repeating transmission for coverage enhancement, as the number of uplink application packets increases, so does the energy consumption. Clearly, there is no effect of the scheduling request when a session completes after the transmission of one small packet, as that shown in Figure 7b. However, if a handshake between the UE and network is required, as shown in Figure 9, the battery consumption by the scheduling request increases. Figure 9a illustrates the scenario where the network server responds to the application report, and a radio link control (RLC) ACK packet corresponding to the downlink data should be transmitted with the scheduling request procedure. Figure 9b illustrates the scenario of a session triggered by the network command. In the worst case, two scheduling request procedures can occur by the RLC ACK and application response due to different processing times.