Preconfigured Uplink Resource

Description

Preconfigured Uplink Resource (PUR) is a mechanism defined in 3GPP Release 16 and later, primarily for LTE-M and NB-IoT technologies, which allows a UE to transmit uplink data on pre-allocated resources without initiating a random access channel (RACH) procedure. This is achieved by the network configuring the UE with specific time-frequency resources (e.g., periodic subframes or resource blocks) during an RRC Connected state, which persist even when the UE transitions to RRC Idle or Inactive states. The UE can use these resources to send data directly, bypassing the typical steps of random access preamble transmission, RAR reception, and scheduling request, thereby streamlining the transmission process.

Architecturally, PUR involves coordination between the UE and the eNodeB (for LTE-M/NB-IoT) or gNodeB (for NR-IoT). The configuration is established via RRC signaling, such as through an RRCConnectionSetup or RRCConnectionReconfiguration message, which includes parameters like PUR periodicity, time offset, frequency location, modulation and coding scheme (MCS), and power control settings. These resources are typically allocated in a contention-free manner, meaning they are dedicated to a specific UE, though contention-based variants may also be supported. The network maintains awareness of the PUR allocations and listens on the designated resources, enabling immediate decoding of incoming transmissions without prior scheduling grants.

How PUR works operationally: When a UE has data to send, it checks if a valid PUR configuration is active and if the current time aligns with the preconfigured resource occasion. If so, it transmits the data directly using the assigned resources, employing configured parameters for power and modulation. The network, upon successful reception, may respond with an acknowledgment or downlink data without requiring the UE to re-enter RRC Connected state fully. This reduces signaling overhead and latency, which is particularly beneficial for sporadic small data packets typical in IoT sensors. PUR configurations can be validated periodically through dedicated procedures to ensure synchronization, and they may be released or updated based on UE mobility or network conditions.

Purpose & Motivation

PUR was created to address the inefficiencies of traditional random access and scheduling request procedures for IoT devices, which often transmit small, infrequent data packets. In pre-Release 16 LTE-M and NB-IoT, each uplink transmission required a RACH procedure, involving multiple message exchanges that consumed significant energy and added latency. This was suboptimal for massive Machine-Type Communication (mMTC) use cases, such as smart meters or environmental sensors, where devices are battery-constrained and need to operate for years without recharging.

The motivation for PUR stems from the need to enhance power saving and reduce signaling overhead in IoT networks. By eliminating the RACH process for preconfigured transmissions, PUR minimizes the time the UE's radio is active, thereby extending battery life. It also reduces network congestion caused by frequent random access attempts from millions of devices, improving scalability for massive IoT deployments. This aligns with 3GPP's goals for 5G evolution, supporting ultra-lean design and efficient support for diverse IoT applications.

Historically, earlier solutions like Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) helped with energy efficiency but did not optimize the transmission phase itself. PUR complements these by streamlining uplink communication. It addresses limitations of previous approaches where latency and power consumption were trade-offs; PUR enables low-latency transmissions without sacrificing energy efficiency, making it a key enabler for critical IoT services and industrial automation within the 5G framework.