Transmission using Preconfigured Uplink Resource

Description

Transmission using Preconfigured Uplink Resource (PUR) is a feature introduced in 3GPP Release 16 for LTE-M and NB-IoT to enable efficient uplink data transmission from devices in RRC_IDLE mode. It allows a device to transmit data using a preconfigured uplink grant without the need to transition to RRC_CONNECTED, thereby reducing signaling overhead, power consumption, and latency. This is particularly beneficial for IoT devices that transmit small, infrequent data packets, such as smart meters or sensors.

The PUR feature is specified in TS 36.302, TS 36.321, and TS 36.331. It defines two types of transmission: Control Plane (CP) transmission using PUR and User Plane (UP) transmission using PUR. In CP transmission, data is sent via the control plane using NAS signaling, while in UP transmission, data is sent via the user plane using the data radio bearer. Both types use preconfigured resources that are allocated by the network while the device is in RRC_CONNECTED, and then used when the device returns to RRC_IDLE.

The preconfigured uplink resource includes time/frequency resources, modulation and coding scheme, and possibly a dedicated preamble for timing advance. The device must maintain timing advance validity to use PUR; if the timing advance timer expires, the device must perform a random access procedure to re-establish timing. The network can configure multiple PUR occasions, and the device selects one based on its needs. The transmission using PUR is a one-shot transmission; if it fails, the device may fall back to legacy random access.

PUR significantly improves battery life and reduces network signaling load. For a device that transmits a small packet every hour, the legacy procedure would involve random access, RRC connection setup, data transmission, and connection release, consuming significant energy and signaling resources. With PUR, the device can transmit directly from idle, saving energy and reducing latency. This is aligned with the goals of 5G for massive machine-type communications (mMTC).

The feature has evolved since Rel-16. In Rel-17, enhancements were made to support larger payloads and improved reliability. Rel-18 and Rel-19 continue to refine the feature, possibly adding support for new use cases or improving coexistence with other technologies. The definition of Transmission using PUR has remained consistent across releases, emphasizing its role as a mechanism for one uplink transmission from idle using preconfigured resources.

Purpose & Motivation

Transmission using PUR was created to address the inefficiencies of traditional uplink transmission for IoT devices. In legacy LTE and NB-IoT, a device in RRC_IDLE must perform a random access procedure and establish an RRC connection before sending data, which involves multiple message exchanges. This process consumes significant device power and network resources, especially for devices that send small amounts of data infrequently. PUR eliminates these steps by allowing data transmission directly from idle using preconfigured resources.

The motivation for PUR stems from the requirements of massive IoT, where millions of devices need to be connected with low cost, low power, and high efficiency. The 3GPP identified that signaling overhead for small data transmissions was a bottleneck, and PUR was introduced as a solution in Release 16. It builds on earlier work on early data transmission (EDT) in Release 15, which allowed data transmission during the random access procedure. PUR goes further by preconfiguring resources, thus avoiding even the random access procedure.

By reducing signaling, PUR extends battery life, which is crucial for devices deployed in remote locations where battery replacement is impractical. It also reduces network congestion by minimizing the number of signaling messages, allowing the network to support more devices. PUR is a key enabler for 5G mMTC and is expected to be widely used in applications such as smart metering, environmental monitoring, and asset tracking.