Description
Link Quality Control (LQC) is a fundamental feature within the GSM/EDGE Radio Access Network (GERAN) architecture, specified in 3GPP TS 45.912. Its primary function is to maintain and optimize the quality of the radio link between the Mobile Station (MS) and the Base Transceiver Station (BTS) in a dynamic radio environment. LQC operates by continuously collecting measurement reports from both the MS and the BTS. These reports include key metrics such as Received Signal Level (RXLEV), Received Signal Quality (RXQUAL), and timing advance. The network, specifically the Base Station Controller (BSC), processes these measurements to make real-time decisions on transmission parameters.
The core mechanisms of LQC include Power Control and Handover Control. Power Control dynamically adjusts the transmission power of the MS and the BTS to use the minimum necessary power to maintain an acceptable link quality. This reduces interference, conserves battery life in the MS, and increases overall network capacity. Handover Control uses the quality measurements to decide when to transfer an ongoing call from one cell to another (e.g., due to deteriorating signal strength or quality) to ensure service continuity. The BSC evaluates the measurement reports against predefined thresholds and algorithms to trigger these actions.
LQC is deeply integrated into the GSM radio resource management procedures. It works in conjunction with other features like Discontinuous Transmission (DTX) and Frequency Hopping. The algorithms for LQC are implemented in the BSC software, using inputs from the Layer 1 measurement procedures defined in the radio specifications. Its role is critical for the autonomous, efficient management of the radio interface, ensuring reliable voice and data services while maximizing the utilization of the scarce radio spectrum.
Purpose & Motivation
Link Quality Control was created to address the fundamental challenges of providing reliable mobile voice services over a shared, interference-prone radio medium. Early mobile radio systems suffered from issues like co-channel and adjacent channel interference, rapid signal fading, and limited battery capacity in handsets. Without dynamic control, calls would drop frequently, quality would be poor, and network capacity would be severely limited.
The introduction of LQC in GSM, and its continued definition in later releases including GERAN evolution, solved these problems by enabling the network to adapt to changing radio conditions in real-time. It automated the optimization process that would otherwise require manual engineering. By minimizing transmission power, it directly tackled interference, which is the primary capacity limiter in cellular networks. By managing handovers based on quality, it maintained service continuity as users moved. This automated, measurement-based control was a key innovation that contributed to GSM's commercial success and the high spectral efficiency of 2G networks.
Key Features
- Dynamic Uplink and Downlink Power Control based on RXLEV/RXQUAL measurements
- Measurement-based Handover decision algorithms for service continuity
- Utilization of RXQUAL (Bit Error Rate) for precise link quality assessment
- Integration with Discontinuous Transmission (DTX) for interference reduction
- Support for both slow and fast associated control channels for signaling
- Algorithm execution in the Base Station Controller (BSC) for centralized control
Evolution Across Releases
Introduced in 3GPP Release 8 within the GERAN specifications (TS 45.912). The initial architecture defined LQC as a core part of radio resource management, utilizing measurement reports from MS and BTS to perform power control and handover decisions. It established the fundamental algorithms for maintaining link quality in GSM/EDGE networks.
Defining Specifications
| Specification | Title |
|---|---|
| TS 45.912 | 3GPP TR 45.912 |