In this paper, we propose flexible functional split schemes for a multi-tier C-RAN architecture based on the concept of virtual remote radio heads (V-RRHs), aiming at optimizing the resource utilization while reducing the overall cost of BBU pool. A V-RRH consists of a digital unit (DU) that hosts a few baseband (BB) processing functionalities and a radio unit (RU) with RF functionality. Specifically, three types of V-RRHs differing in the number of BB processing function slices are considered in three functional split options. We aim at finding the optimal functional split option for each task and thus allocate the appropriate function slice block (FSB) of BBU pool for it. The issue of optimally allocating FSBs to tasks is normalized as a zero–one (0–1) packing problem which is NP-complete. To solve the 0–1 packing problem, we divide it into three stages and solve them by a pure integer non-linear programming (PINLP) model. Numerical results show that the proposed flexible functional split strategies comprehensively outperform the existing C-RAN and D-RAN schemes in terms of the overall cost.

在本文中，我们基于虚拟远程无线电头（V-RRH）的概念，为多层C-RAN架构提出了灵活的功能拆分方案，旨在优化资源利用率，同时降低BBU池的总体成本。V-RRH由承载一些基带（BB）处理功能的数字单元（DU）和具有RF功能的无线电单元（RU）组成。具体地，在三个功能划分选项中考虑了BB处理功能片的数量不同的三种类型的V-RRH。我们旨在为每个任务找到最佳的功能分割选项，从而为其分配适当的BBU池功能切片块（FSB）。将FSB最佳分配给任务的问题被归类为NP完全的零一（0-1）打包问题。为了解决0-1包装问题，我们将其分为三个阶段，并通过纯整数非线性规划（PINLP）模型进行求解。数值结果表明，在总体成本方面，所提出的灵活的功能拆分策略在总体上优于现有的C-RAN和D-RAN方案。