Serving and Neighbour cell Pathloss

Description

Serving and Neighbour cell Pathloss (SNPL) is a fundamental radio measurement defined in 3GPP specifications, primarily for UMTS and HSPA networks. Path loss represents the attenuation of signal strength as a radio wave propagates from a transmitter (a Node B/base station) to a receiver (a User Equipment/UE). SNPL specifically refers to the UE's ability to measure and report the path loss for both its currently connected serving cell and for detected neighboring cells. This measurement is derived from the received signal power of known reference signals, such as the Common Pilot Channel (CPICH) in UMTS, and knowledge of the transmitted power of these signals, which is broadcast by the cell. The UE calculates path loss as the difference (in dB) between the transmitted reference signal power and the received power.

The architecture for SNPL involves the UE's physical layer measurement capabilities, the Radio Resource Control (RRC) protocol for reporting, and network algorithms in the Radio Network Controller (RNC). The UE continuously monitors the CPICH Received Signal Code Power (RSCP) for the serving and listed neighbor cells. Using the broadcast CPICH transmission power parameter, it computes the path loss. These measurements are then reported to the network via RRC measurement reports, either periodically or when triggered by specific events (e.g., when a neighbor cell's path loss becomes better than the serving cell's by a certain threshold).

In the network, the RNC utilizes SNPL reports for several critical Radio Resource Management (RRM) functions. It is a key input for handover (HO) decisions, helping determine when a UE should be handed over to a cell with a more favorable propagation path. It also aids in cell reselection for idle mode UEs. Furthermore, SNPL is indirectly used in power control algorithms; knowledge of path loss helps the network estimate the required uplink transmit power for a UE to achieve a target signal quality at the base station, thus optimizing system capacity and reducing interference. Its role is foundational for maintaining call quality, ensuring coverage continuity, and managing network load.

Purpose & Motivation

SNPL was introduced to provide the network with a direct, propagation-based metric for mobility and resource management, which is more fundamental than simple received power measurements. Prior to or alongside SNPL, measurements like Received Signal Strength Indicator (RSSI) or RSCP were used. However, these received power measurements alone do not account for variations in base station transmit power. A cell with high transmit power might have a strong RSCP even if the path loss is poor, which could mislead handover algorithms. SNPL solves this by normalizing the received power with the known transmitted power, giving a true measure of the radio channel's attenuation.

The primary problem SNPL addresses is the need for accurate and consistent cell quality comparison for mobility decisions. For efficient handovers, the network needs to know which cell provides the best radio link, not just which one is being received most powerfully at a given moment. By using path loss, the network can predict the uplink performance more accurately and make decisions that balance uplink and downlink performance. This leads to more stable connections, reduced dropped calls, and better overall utilization of radio resources. Its creation was motivated by the evolution towards more autonomous and intelligent RRM in 3GPP networks, moving beyond simple thresholds to algorithms based on channel conditions.

Key Features

• Provides a normalized measure of radio channel attenuation for serving and neighbor cells
• Derived from CPICH RSCP and broadcast CPICH transmission power in UMTS
• Used as a key input for handover decision algorithms in the RNC
• Supports idle mode cell reselection procedures
• Aids in uplink power control estimation
• Reported by the UE to the network via RRC measurement control procedures

Evolution Across Releases

Defining Specifications