Open radio access network (ORAN) provides an open architecture to implement radio access network (RAN) of the fifth generation (5G) and beyond mobile communications. As a key technology for the evolution to the sixth generation (6G) systems, cell-free massive multiple-input multiple-output (CF-mMIMO) can effectively improve the spectrum efficiency, peak rate and reliability of wireless communication systems. Starting from scalable implementation of CF-mMIMO, we study a cell-free RAN (CF-RAN) under the ORAN architecture. Through theoretical analysis and numerical simulation, we investigate the uplink and downlink spectral efficiencies of CF-mMIMO with the new architecture. We then discuss the implementation issues of CF-RAN under ORAN architecture, including time-frequency synchronization and over-the-air reciprocity calibration, low layer splitting, deployment of ORAN radio units (O-RU), artificial intelligent based user associations. Finally, we present some representative experimental results for the uplink distributed reception and downlink coherent joint transmission of CF-RAN with commercial off-the-shelf O-RUs.

翻译：开放无线接入网（ORAN）提供了实现第五代（5G）及未来移动通信无线接入网（RAN）的开放架构。作为向第六代（6G）系统演进的关键技术，无小区大规模多输入多输出（CF-mMIMO）能够有效提升无线通信系统的频谱效率、峰值速率和可靠性。从CF-mMIMO的可扩展实现出发，我们研究了ORAN架构下的无小区RAN（CF-RAN）。通过理论分析与数值仿真，我们探究了新架构下CF-mMIMO的上行和下行频谱效率。随后，我们讨论了ORAN架构下CF-RAN的实现问题，包括时频同步与空中互易性校准、低层分割、ORAN无线单元（O-RU）部署以及基于人工智能的用户关联。最后，我们展示了使用商用现成O-RU的CF-RAN上行分布式接收与下行相干联合传输的代表性实验结果。