O-RAN for Energy-Efficient Serving Cluster Formulation in User-Centric Cell-Free MMIMO

TL;DR

The paper tackles energy efficiency in 6G-like User-Centric Cell-Free MMIMO by formulating Serving Clusters (SCF) that coordinate multiple APs. It proposes an O-RAN-based rApp–xApp framework where the Non-RT RIC rApp generates SCF policies (notably the Serving Cluster Size, $SCS$) and the Near-RT RIC xApp enforces them using KPI data and RSRP measurements via A1/O1/E2, validated with an advanced OFDMA-based UCCF MMIMO simulator incorporating 3D Ray Tracing. A key contribution is the demonstration of up to ~37% energy efficiency gain over Network-Centric designs at an optimal $SCS$ (approximately $SCS=3$), with larger $SCS$ sometimes degrading EE for many users due to distant O-RUs. The work highlights the importance of state-aware policy selection for energy efficiency and points to future directions such as Digital Twin integration and micro-sleep control of O-RUs to maximize gains in practical deployments.

Abstract

The 6G Massive Multiple-Input Multiple-Output (MMIMO) networks can follow the so-called User-Centric Cell-Free (UCCF) architecture, where a single user is served by multiple Access Points (APs) coordinated by the Central Processing Unit (CPU). In this paper, we propose how O-RAN functionalities, i.e., rApp-xApp pair, can be used for energy-efficient Serving Cluster Formulation (SCF). Simulation studies show up to 37\% gain in Energy Efficiency (EE) of the proposed solution over the state-of-the-art Network-Centric (NC) designs.

Figures (3)

• Figure 1: The concept of rApp-xApp cooperation for EE-oriented SCF in O-RAN UCCF MMIMO network.
• Figure 2: Distribution of per-user EE for varying Serving Cluster Size.
• Figure 3: RAN EE for varying serving cluster size.