Positioning Radio Resource Management

Description

Positioning Radio Resource Management (PRRM) is a critical functional entity within the Radio Access Network (RAN) architecture, defined to manage the radio resources required for user equipment (UE) positioning. It operates as part of the Radio Resource Control (RRC) layer, interacting with core network elements like the Serving Mobile Location Centre (SMLC) or Location Management Function (LMF). The primary role of PRRM is to handle the setup, maintenance, and release of dedicated radio resources for positioning purposes, which includes the configuration of measurement gaps, the scheduling of positioning reference signals (PRS), and the coordination of UE measurement reporting. This management is essential because positioning activities, such as Observed Time Difference of Arrival (OTDOA) or Assisted Global Navigation Satellite System (A-GNSS), consume uplink and downlink bandwidth and require precise timing synchronization; PRRM ensures these resources are allocated efficiently without causing excessive interference or degrading the quality of service for other communication channels.

Architecturally, PRRM functions are distributed between the network and the UE. On the network side, the RAN Node (e.g., Node B, eNodeB, or gNB) executes PRRM procedures based on requests from the core network's positioning server. These procedures involve deciding which physical resource blocks, time slots, and frequency bands to allocate for positioning signals. For downlink-based methods like OTDOA, PRRM configures the transmission of PRS from multiple cells, ensuring they are transmitted with sufficient power and at specific periodic intervals to be detectable by the UE. For uplink-based methods like Uplink Time Difference of Arrival (UTDOA), PRRM schedules the UE's transmission of sounding reference signals (SRS) dedicated for positioning. The PRRM entity also manages the associated signaling, such as RRC messages that convey positioning assistance data (e.g., PRS configuration info) to the UE and that command the UE to perform specific measurements.

Key components of PRRM include the measurement control and reporting mechanisms, the resource scheduling algorithms, and the interfaces for positioning coordination. The measurement control involves the RRC protocol sending Measurement Control messages to the UE, which specify the type of positioning measurement (e.g., Reference Signal Time Difference (RSTD) for OTDOA), the cells to measure, and the reporting criteria. PRRM works in tandem with other RAN functions like Admission Control and Packet Scheduling to ensure that positioning resource allocations are admitted only if sufficient radio resources are available, thereby maintaining overall network stability. Its role is particularly vital in crowded urban environments or during emergency scenarios where accurate location information is paramount; by dynamically managing resources, PRRM helps balance positioning accuracy with the network's primary objective of providing reliable voice and data services.

Purpose & Motivation

PRRM was introduced to address the growing need for network-assisted positioning services in cellular networks, mandated by regulatory requirements like Enhanced 911 (E911) in the United States and similar emergency caller location services worldwide. Prior to its standardization, early mobile networks lacked dedicated mechanisms to manage radio resources for positioning, leading to ad-hoc implementations that could severely impact network capacity and performance. For instance, without coordinated resource management, positioning measurements could cause excessive interference, drain UE battery life due to continuous searching, or fail in dense urban canyons where signal reflections are prevalent. The creation of PRRM provided a standardized, efficient framework within 3GPP to integrate location services directly into the RAN's resource management paradigm.

The technology solves the problem of how to perform accurate UE positioning without degrading the quality of the primary communication services. It enables the network to dynamically allocate and de-allocate the necessary time-frequency resources for positioning signals and measurements on-demand. This is crucial for techniques like OTDOA, which require multiple base stations to transmit specially designed reference signals (PRS) at known times. PRRM ensures these transmissions are scheduled in a coordinated manner across cells, minimizing intra-cell and inter-cell interference. Furthermore, it manages the trade-off between positioning accuracy (which benefits from more resources and frequent measurements) and network efficiency. By having a dedicated management entity, the network can support a variety of positioning methods—including those based on LTE, NR, and hybrid approaches—in a scalable and future-proof manner.

Historically, PRRM's introduction in 3GPP Release 99 laid the groundwork for all subsequent enhancements in cellular positioning. It represented a shift from standalone GPS solutions, which were unreliable indoors, to network-assisted hybrid methods. The initial architecture defined the basic procedures for resource negotiation between the RAN and the core network location server. Over the releases, as positioning accuracy requirements became more stringent (e.g., for commercial location-based services and IoT asset tracking), PRRM evolved to support new signals, higher bandwidths, and carrier aggregation, but its core purpose of efficient radio resource stewardship for positioning has remained constant.

Key Features

• Manages allocation of physical resource blocks (PRBs) for positioning reference signal transmission
• Coordinates measurement gaps and scheduling to enable UE positioning measurements without disrupting data services
• Supports multiple positioning methods including OTDOA, A-GNSS, and E-CID through standardized RRC procedures
• Interfaces with core network location servers (e.g., SMLC, LMF) to execute positioning requests
• Controls UE measurement reporting configurations and criteria for positioning
• Balances positioning accuracy requirements with overall network capacity and interference levels

Evolution Across Releases

Defining Specifications