Link to System mapping

Description

Link to System (L2S) mapping is a critical methodology defined in 3GPP for system-level simulations of mobile networks. It serves as a bridge between detailed, computationally intensive link-level simulations (which model the physical layer transmission and reception of individual radio frames over a specific channel) and higher-level system simulations (which model network topology, user mobility, traffic patterns, and resource management). The core challenge L2S addresses is the vast difference in simulation timescales and complexity; link-level simulations operate on symbol or bit times, while system-level simulations operate on call or packet times.

The methodology works by abstracting the physical layer's behavior into a set of lookup tables or mathematical models. These models, often called 'interface tables' or 'effective SINR mapping' (ESM) techniques, are generated offline from extensive link-level simulations. They map a set of instantaneous channel quality indicators (like SINR per subcarrier in OFDM systems) observed by a receiver to a single, effective quality metric. This effective metric is then used to determine the probability of correctly decoding a transport block based on the chosen modulation and coding scheme (MCS). This abstraction allows system simulators to predict block error rates (BLER) and throughput without running real-time link simulations.

Key components of the L2S framework include the channel model definition, the link-level simulator used to generate the performance tables, and the specific mapping algorithm (e.g., Exponential ESM, Mutual Information ESM). Its role is foundational in the standardization process, allowing different companies and researchers to compare the performance of proposed radio technologies (like new MCS tables, scheduling algorithms, or antenna configurations) under consistent and comparable assumptions. It is extensively used for evaluating coverage, capacity, and mobility performance for technologies from GSM to 5G NR.

Purpose & Motivation

L2S mapping exists to make the performance evaluation of complex mobile radio systems feasible and standardized. Before its formalization, comparing different network proposals or technologies was difficult due to inconsistent simulation assumptions and methodologies. Each vendor or research group might use proprietary, non-comparable link models, making objective assessment of gains from new features nearly impossible.

The creation of L2S, formalized in 3GPP TR 45.914 for GSM/EDGE and later applied to UMTS, LTE, and NR, was motivated by the need for a common, agreed-upon evaluation framework. It solves the problem of computational intractability; directly coupling a full link simulator into a system simulator covering hundreds of cells and thousands of users is prohibitively slow. By providing a simplified yet accurate abstraction, L2S enables large-scale, statistically significant network performance studies within reasonable computational time.

Historically, it addressed the limitations of overly simplistic system models that used fixed BLER curves or ignored fast fading, which led to inaccurate capacity predictions. L2S introduced a standardized way to incorporate the impact of realistic, time-varying radio channel conditions on system-level key performance indicators (KPIs), ensuring that performance claims during 3GPP standardization are based on a consistent and transparent methodology.

Key Features

• Abstraction of physical layer performance into pre-computed lookup tables
• Enables computationally efficient large-scale system-level simulations
• Standardized methodology ensuring comparable results across different evaluators
• Supports multiple Effective SINR Mapping (ESM) algorithms for different technologies
• Applicable across multiple 3GPP generations (GSM, UMTS, LTE, NR)
• Integrates detailed channel models (e.g., tapped delay lines, spatial correlation) into network performance analysis

Evolution Across Releases

Defining Specifications