Hybrid CJT-NCJT Serving Mode Empowered Energy-Efficient Cell-Free Massive MIMO Systems With Limited Fronthauls

Existing studies show that the non-coherent joint transmission (NCJT) serving mode can outperform the coherent joint transmission (CJT) serving mode in terms of energy efficiency (EE) in cell-free massive multi-input multi-output (CF-mMIMO) systems with limited fronthauls, although the CJT can achieve prominent coherent cooperative gain. However, relying solely on NCJT serving mode results in the underutilization of available fronthaul resources, failing to harvest the potential coherent gain of the system. Thus, to further improve the EE, we investigate energy-efficient resource allocation in CF-mMIMO systems under a hybrid CJT-NCJT serving framework, where users’ equipment (UEs) are categorized to operate in either CJT serving mode or NCJT serving mode. The emphasis of maximizing EE in the CJT-NCJT hybrid serving mode enabled CF-mMIMO system lies in fully utilizing fronthaul resources through UE serving mode allocation. To address this issue, we propose a matching-based serving mode allocation algorithm, embedded with a Dinkelbach-based power allocation algorithm for CF-mMIMO systems with limited fronthauls.