Coverage and Handoff Enhancements using Multimedia error robustness

Description

CHEM, which stands for Coverage and Handoff Enhancements using Multimedia error robustness, is a feature specified in 3GPP Release 16 and later. It is designed to improve the user experience for multimedia services, such as voice and video calls, by enhancing coverage and handoff performance through the use of error robustness mechanisms. The feature is defined in several specifications, including 26.114 (Multimedia Telephony Services over IMS), 29.212, 29.213, 29.214 (Policy and Charging Control), and 29.512, 29.513, 29.514 (5G Policy Control). CHEM aims to address the challenges of maintaining a consistent quality of experience for multimedia sessions when users move across coverage areas or experience degraded radio conditions.

The core idea behind CHEM is to allow the network to apply different levels of error robustness to multimedia streams based on the radio conditions and handoff events. For example, during a handoff, the network may temporarily increase the robustness of the codec or apply forward error correction (FEC) to mitigate packet loss. This is achieved through coordination between the IMS application layer, the policy control framework, and the radio access network. CHEM leverages existing QoS and policy mechanisms to dynamically adjust the multimedia session parameters. The policy control function (PCF) can instruct the user equipment (UE) or the media gateway to apply specific error robustness settings when certain trigger conditions are met, such as a handoff or a drop in signal quality.

The specifications 29.212, 29.213, and 29.214 define the Policy and Charging Control (PCC) architecture for EPS, while 29.512, 29.513, and 29.514 define the corresponding 5G Policy Control framework. These specs are updated to include the necessary policy parameters and procedures to support CHEM. For instance, the PCF may receive information about the multimedia session and the current radio conditions from the Application Function (AF) and then make policy decisions to enable error robustness. The UE and the network can then negotiate the appropriate codec and error resilience settings. This dynamic adaptation ensures that the multimedia session remains intelligible even under adverse conditions, reducing the likelihood of dropped calls or degraded quality.

CHEM is particularly relevant for Voice over LTE (VoLTE) and Voice over NR (VoNR) services, as well as for video telephony. In these services, maintaining a continuous and high-quality media stream is critical. Traditional methods of improving coverage, such as adding more base stations, are expensive and not always feasible. CHEM provides a software-based approach that can be implemented in existing networks with relatively low cost. It complements other coverage enhancement techniques like MIMO, beamforming, and carrier aggregation. By focusing on error robustness at the application layer, CHEM can mitigate the effects of short-term fading, interference, and handoff interruptions.

The feature has been maintained and refined in subsequent releases (Rel-17, Rel-18, Rel-19), indicating its ongoing importance. The definition text has remained consistent across these releases, though minor editorial changes may have occurred. In summary, CHEM represents a cross-layer optimization that bridges multimedia codec adaptation with radio access and policy control, delivering a better quality of experience for mobile multimedia users.

Purpose & Motivation

CHEM was created to address the persistent challenge of maintaining high-quality multimedia services in mobile networks, particularly during handoffs and in areas with marginal coverage. Before CHEM, multimedia sessions could suffer from interruptions, packet loss, or degraded audio/video quality when users moved between cells or experienced poor radio conditions. These issues led to user dissatisfaction and increased churn for operators. Existing solutions, such as increasing transmit power or adding base stations, were not always practical or cost-effective.

CHEM solves these problems by introducing a mechanism to dynamically adjust the error robustness of multimedia streams based on network conditions. It leverages the flexibility of modern codecs and the policy control framework to adapt the media stream in real-time. This allows the network to maintain acceptable quality even when the radio conditions are challenging, without requiring changes to the underlying radio access technology. The feature is especially beneficial for operators who want to improve the coverage and reliability of their VoLTE/VoNR services without significant infrastructure investment.

From a historical perspective, CHEM emerged in Release 16 as part of the broader effort to enhance 5G services, particularly for ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB). It reflects the trend towards cross-layer optimization and application-aware network management. As multimedia services continue to evolve, CHEM provides a foundation for further enhancements in future releases.