## 1 Introduction  
With the rise of Smart Building, Smart Cities and Industry 4.0, future mobile networks will encounter the challenge of delivering enormous data rates for the interconnection of a massive number of machines and user devices. Gbit per second scale user connectivity, along with low latency reliability, dynamicity and fast service deployment times, are some of the critical requirements that drive the evolution towards the Beyond-5G (B5G) era [1,2]. In order to deliver a plethora of advanced services with much variation of the delivery of key performance indicators (KPIs), fronthaul networks should evolve to provide flexible, high-bandwidth and low-latency connectivity to the remote radio heads (RRHs), adoptingarchitectural transformations that will lead to enhanced scalability and interoperability with Wireless Fidelity (Wi-Fi), fixed and cloud networks [3]. Within the already mature, digitized fronthauling environment, it is inevitable that Common Public Radio Interface (CPRI)-based links will remain the most common interface for the interconnection between the baseband units (BBUs) and the Remote Radio Heads (RRHs), despite the inherently limited bandwidth efficiency of the CPRI protocol and the flexibility limitations that have been widely discussed during the past years [4]. Nonetheless, a series of brand-new technological enablers have arisen, promising large bandwidth availability, flexibility and easy deployment, hence increased scalability. More specifically, the adoptions of large unlicensed bands at high radio frequencies (i.e., V-band, D-band) have combined to advanced radio techniques (such as cooperative beamforming and massive multiple input-multiple output (MIMO) transmission) and the embracement of photonic processing/networking solutions seems to be a promising path towards surpassing the capacity and scalability bottleneck of the current deployments. On top of that, innovative beyond-CPRI fronthaul alternatives, such as the analog radio-over-fiber (A-RoF) and sigma-delta-over-fiber (SDoF) schemes have been widely visited as possible candidates to host these technological blocks, showcasing the Gbps scale connectivity over fiber and converged wired-wireless topologies [5; 6]. As such, it is essential to migrate to a network infrastructure that can efficiently integrate the various heterogenous technologies and enable the internetworking of the existing small cells and future deployment extensions. For the implementation of such densified and versatile network deployments, cloud radio access networks (C-RAN) are highly attractive, as they offer greater network scalability, efficient transport and increased network function virtualization (NFV) [4]. The cloud-based virtual BBUs (vBBUs) can support the dynamic allocation of baseband processor platform resources based on the traffic demand. Within this centralized, reconfigurable universe the vBBUs can concurrently and interoperably accommodate heterogeneous, remotely located radio units that are equipped with different interfaces, reached via the already existing optical paths or those that are beyond-legacy, currently emerging optical transport schemes. The adaptation of the optical transport architectures, interconnecting central units (CUs), distributed units (DUs) and RUs in this everchanging, multi-technology ecosystem has been widely discussed during the past years [7]. Specifically, the transition to hybrid topologies co-hosting inhomogeneous transceivers and protocols in a transparent manner while interconnecting BBUs and RUs in a versatile and reconfigurable way is necessary [7; 8]. For this purpose, the current static point-to-point (PtP) optical interconnection of a single BBU with an RRH should evolve and be replaced with scalable, Point-to-Multi-Point (PtMP) layouts offering wavelength aggregation and the reconfigurable routing of variable types of waveforms and protocols.