Link Selector Parameter

Description

The Link Selector Parameter (LSP) is a fundamental control mechanism within the GSM/EDGE Radio Access Network (GERAN) architecture, specifically designed for the optimization of data transmission over the radio interface. It operates within the context of the radio resource management (RRM) layer, where the network continuously monitors channel quality indicators such as the received signal strength (RSSI) and the bit error rate (BER). Based on these measurements, the network calculates an appropriate LSP value. This value is then signaled to the mobile station (MS) and is used to determine the most suitable Modulation and Coding Scheme (MCS) for the ongoing data transfer. For instance, in good radio conditions, a higher LSP might correspond to a more efficient, higher-order modulation scheme like 8-PSK with less robust channel coding, maximizing throughput. Conversely, in poor conditions, the LSP would trigger a fallback to a more robust scheme like GMSK with stronger error correction, ensuring data integrity at the expense of raw speed.

Architecturally, the LSP is managed by the Base Station Controller (BSC) in the GERAN. The BSC collects measurement reports from both the mobile station and the Base Transceiver Station (BTS). Using algorithms defined in the relevant 3GPP specifications, the BSC computes the LSP. This parameter is then transmitted to the MS via Layer 3 signaling messages on the Slow Associated Control Channel (SACCH) during a dedicated connection. The mobile station's physical layer uses the received LSP to configure its transmitter and receiver accordingly, selecting the pre-defined combination of modulation type, code rate, and, implicitly, the required transmission power level. This closed-loop control is continuous, allowing dynamic adaptation to fading, interference, and user mobility.

The role of the LSP is critical for achieving the promised data rates of Enhanced Data rates for GSM Evolution (EDGE). It is a key enabler for incremental redundancy (a type of Hybrid ARQ) where the LSP influences the selection of initial coding for a data block. By finely tuning the link adaptation process, the LSP directly impacts key performance indicators such as user throughput, block error rate (BLER), and overall network capacity. Its proper functioning ensures that radio resources are used efficiently, providing the best possible data service within the constraints of the instantaneous radio environment, which is a cornerstone principle of modern cellular systems.

Purpose & Motivation

The Link Selector Parameter was introduced to address the challenge of efficiently transmitting packet-switched data over the inherently variable and noisy GSM radio channel. Early GSM primarily handled circuit-switched voice, which used a constant, robust modulation (GMSK). With the introduction of high-speed circuit-switched data (HSCSD) and later GPRS and EDGE for packet data, a more dynamic approach was needed. Static link configurations would either waste capacity in good conditions or suffer high error rates in bad conditions. The LSP was created as a standardized control parameter to enable dynamic link adaptation.

Its creation was motivated by the need to maximize spectral efficiency—the amount of data transmitted per unit of bandwidth—which is a scarce and expensive resource for operators. By allowing the network to command the mobile station to switch between multiple modulation and coding schemes (MCS-1 through MCS-9 in EDGE) in real-time, the LSP ensures that the highest possible data rate is used for the current channel quality. This solves the problem of either over-provisioning (using too robust a scheme, wasting capacity) or under-provisioning (using too aggressive a scheme, causing retransmissions and delays).

Historically, it provided the necessary granular control to realize the performance gains of EDGE over basic GPRS. Without such a parameter, EDGE's advanced 8-PSK modulation would be unusable in marginal coverage areas. The LSP, therefore, was a key technological component that allowed GSM networks to evolve into competitive mobile data platforms before the widespread deployment of 3G UMTS.

Key Features

• Dynamic selection of Modulation and Coding Scheme (MCS) based on real-time radio measurements.
• Closed-loop control managed by the Base Station Controller (BSC) and executed by the Mobile Station (MS).
• Enables efficient use of EDGE modulation schemes (GMSK and 8-PSK).
• Integral to power control algorithms for data channels.
• Supports Incremental Redundancy (IR) by defining the initial coding for a data block.
• Signaled over the Slow Associated Control Channel (SACCH) during dedicated connections.

Evolution Across Releases

Defining Specifications