Radio Resource LCS Protocol

Description

The Radio Resource LCS Protocol (RRLP) is a Layer 3 application protocol defined for the Location Services (LCS) feature in 2G GSM and 3G UMTS networks. It operates as a peer-to-peer protocol between a target Mobile Station (MS) and a Serving Mobile Location Centre (SMLC) or a Location Measurement Unit (LMU). The protocol messages are transported over the existing radio interface signaling connections, typically using the DTAP (Direct Transfer Application Part) mechanism to carry them within higher-layer messages between the MS and the core network, which then relays them to the SMLC.

RRLP defines a set of request and response messages to facilitate various positioning methods. Key procedures include the provision of assistance data from the network to the MS and the delivery of location measurements or a computed position from the MS to the network. For example, in Assisted-GPS (A-GPS) positioning, the SMLC sends an RRLP Assistance Data message containing GPS satellite ephemeris, almanac, and timing information to the MS. The MS uses this data to quickly acquire GPS signals and then responds with an RRLP Measure Position Response message containing pseudorange measurements or the calculated latitude/longitude.

The protocol supports multiple positioning methods: Cell-ID (with optional timing advance), Observed Time Difference of Arrival (OTDOA) for UMTS, and Enhanced Observed Time Difference (E-OTD) for GSM. For OTDOA, the network provides assistance data about neighboring cells' timing, and the MS measures the observed time differences of received pilot signals. RRLP carries these measurements back to the SMLC, which calculates the position. The protocol is designed to be efficient over the constrained bandwidth of the radio interface, using compact information elements (IEs) to encode assistance data and measurement results. Its operation is controlled by the LCS client request, which specifies the required quality of service (QoS) for the location fix, such as accuracy and response time.

Purpose & Motivation

RRLP was created to standardize the signaling needed for network-based and mobile-based positioning in GSM and UMTS networks, fulfilling regulatory (e.g., E911) and commercial location-based service requirements. Before standardized protocols like RRLP, proprietary solutions existed, hindering interoperability between network equipment and handsets from different vendors. RRLP provided a universal language for location-related communication over the air interface.

It solved the problem of enabling accurate positioning in handsets that may not have full, autonomous GPS capabilities. By providing network assistance (A-GPS), RRLP dramatically reduces the Time To First Fix (TTFF), improves sensitivity for indoor operation, and reduces the handset's power consumption. For network-based methods like OTDOA, it provided a mechanism for the network to request specific radio measurements from the handset to compute its location, even without GPS. The creation of RRLP was a critical step in making reliable, standardized location services a ubiquitous feature of cellular networks, enabling emergency services, navigation, fleet tracking, and a host of other applications.

Key Features

• Supports multiple positioning methods (Cell-ID, OTDOA/E-OTD, A-GPS)
• Defines procedures for transfer of positioning assistance data from network to MS
• Carries location measurements (e.g., pseudoranges, OTDOA) or computed position from MS to network
• Operates over standard radio signaling channels (using DTAP transport)
• Includes QoS parameters for location requests (accuracy, response time)
• Protocol defined in ASN.1, ensuring unambiguous encoding/decoding

Evolution Across Releases

Defining Specifications