# Evaluating the Impact of Pre-Configured Uplink Resources in Narrowband IoT
## 2 NB-IoT
### Operational States
#### 2.1.2 Preconfigured Uplink Resources  
The objective of employing PURs is to further enhance the efficiency of signaling, a goal distinct from the approach of EDT [24]. The initial phase comprises Msg 1 and Msg 2, which initialize contact with the eNodeB and equip the UE with a valid Timing Advance (TA) for UL synchronization. It is important to note that the initial phase only needs to be completed once, until the UE moves into the proximity of a different eNodeB or the timer for the resources assigned to the UE expires. Subsequently, Msg 3 facilitates the first scheduled UL transmission, housing both an RRC message and a UL data payload. Ultimately, Msg 4 functions to acknowledge UL data transmission, address contention, potentially convey DL data payloads, and orchestrate UE transitions to the CONNECTED state, as shown in Figure 5. It is important to note that PURs achieve their full potential when the User Equipment remains stationary. However, if the UE moves from the proximity of one eNodeB to another, it must acquire new resources each time it needs to transmit a packet. This process is no different from the mechanism used by EDT. The strategy behind pre-configuring the UE with UL radio resources, under the assumption of a valid TA, is to eliminate the necessity for Msg 1, Msg 2, and conventional connection establishment procedures. The activation of PURs involves dedicated RRC signaling while the UE is in the CONNECTED state, encompassing the acquisition of UE-specific radio resources and a Radio Network Temporary Identifier (PUR C-RNTI), among other parameters. A comprehensive PUR configuration incorporates periodic time-frequency resources, a Modulation and Coding Scheme (MCS), a Transport Block Size (TBS), PUSCH (Physical Uplink Shared Channel) repetitions, Demodulation Reference Signal (DMRS) configurations, and power control parameters. Triggers for UE configuration with PURs include transmitting a PUR Configuration Request message by the UE to an eNB while in an RRC_CONNECTED state or network-driven activation based on subscription data or identified periodic traffic patterns. Before executing a PUR transmission, the UE evaluates the validity of the TA through an individual or combined-attribute evaluation, which includes whether the serving cell has changed, the expiration of the PUR TA timer, and variations in received signal power (RSRP). The successful receipt of UL PUR transmissions by the eNB can lead to the conclusion of the PUR procedure in only two messages, UL and DL, recognized through layer-1 signaling in DL Control Information (DCI) or layer-2/3 signaling in an RRC message. The adaptable periodicity of PURs reflects their suitability for periodic traffic scenarios. However, the configuration flexibility allows the UE to skip transmitting for several consecutive PUR occasions before automatic configuration release. Unlike EDT, the successful reception of UL PUR transmissions can sometimes be adequately acknowledged through layer-1 signaling if there are no pending DL data. Such acknowledgments might include adjustments to TA, the number of PUSCH repetitions, or even an indication to employ EDT or the ordinary connection establishment procedure. In alternative scenarios, responses to UL PUR transmissions might be conveyed via layer-2/3 signaling (RRC messages) not only for acknowledgments but also for downstream user data transmission, PUR configuration adjustments, and potential UE state transitions. Two distinct schemes exist for PUR transmissions: dedicated PURs and shared PURs. In the former, UL time-frequency resources are exclusively allocated to a single UE at any given time, particularly suitable for UEs transmitting periodically in the UL. Shared PURs allow the concurrent use of the same UL resources by up to two UEs, distinguished through orthogonal demodulation reference signal sequences. This approach accommodates both periodic and pseudo-varying traffic patterns.