The deployment of ultra-dense networks (UDNs), particularly cell-free massive MIMO (CF-mMIMO), is mainly hindered by costly and capacity-limited fronthaul links. This work proposes a two-tiered optimization framework for cost-effective hybrid fronthaul planning, comprising a Near-Optimal Fronthaul Association and Configuration (NOFAC) algorithm in the first tier and an Integer Linear Program (ILP) in the second, integrating fiber optics, millimeter-wave (mmWave), and free-space optics (FSO) technologies. The proposed framework accommodates various functional split (FS) options (7.2x and 8), decentralized processing levels, and network configurations. We introduce the hierarchical scheme (HS) as a resilient, cost-effective fronthaul solution for CF-mMIMO and compare its performance with radio-stripes (RS)-enabled CF-mMIMO, validating both across diverse dense topologies within the open radio access network (O-RAN) architecture. Results show that the proposed framework achieves better cost-efficiency and higher capacity compared to traditional benchmark schemes such as all-fiber fronthaul network. Our key findings reveal fiber dominance in highly decentralized deployments, mmWave suitability in moderately centralized scenarios, and FSO complements both by bridging deployment gaps. Additionally, FS7.2x consistently outperforms FS8, offering greater capacity at lower cost, affirming its role as the preferred O-RAN functional split. Most importantly, our study underscores the importance of hybrid fronthaul effective planning for UDNs in minimizing infrastructural redundancy, and ensuring scalability to meet current and future traffic demands.

翻译：超密集网络（UDN），特别是无小区大规模MIMO（CF-mMIMO）的部署，主要受限于成本高昂且容量受限的前传链路。本文提出一个两层优化框架，用于经济高效的混合前传规划：第一层包含近优前传关联与配置（NOFAC）算法，第二层采用整数线性规划（ILP），融合光纤、毫米波（mmWave）和自由空间光通信（FSO）技术。该框架支持多种功能拆分（FS）选项（7.2x和8）、去中心化处理层级及网络配置。我们引入分层方案（HS）作为CF-mMIMO的弹性、经济高效前传解决方案，并将其性能与无线条带（RS）使能的CF-mMIMO进行对比，在开放无线接入网络（O-RAN）架构下验证两者在不同密集拓扑中的表现。结果表明，与全光纤前传网络等传统基准方案相比，所提框架实现了更优的成本效率与更高容量。关键发现揭示了光纤在高度去中心化部署中的主导地位、毫米波在适度集中场景中的适用性，以及FSO通过桥接部署差距对两者的补充作用。此外，FS7.2x在较低成本下提供更大容量，始终优于FS8，证实其作为首选O-RAN功能拆分的角色。最重要的是，本研究强调了UDN中混合前传有效规划在减少基础设施冗余、确保可扩展性以满足当前及未来流量需求方面的重要性。