## 2 Concept and Proposed Architecture  
The development of architectures, technologies, interfaces and networks for 5G fronthaul has gained significant attention from both academia and industry in the last few years [16]. The migration to flexible, dynamically reconfigurable transport network segments, enabled by wavelength division multiplexing (WDM) programable optical nodes, is a necessary step for the flexible interconnection of the centralized BBU pools and multiple radio units that are distributed in the field. More specifically, the standard interconnection between each single BBU with its corresponding RU has reached a bottleneck in terms of capacity growth, the adoptability of emerging technologies and the efficient utilization of centralized resources [17,18]. The above limitations are inseparably connected to the static nature of PtP fronthaul connectivity. As such, PtMP architectures have been recently investigated throughout the literature, promising flexible bandwidth steering and the dynamic management of resource allocation among edge and radio sites, based on the traffic demand [19,20,21].  
Inspired by the legacy of WDM-PON networking [22,23], the use of optical switching and wavelength aggregation units can host the evolving heterogeneous PtMP fronthauling. Benefiting from the low-loss, protocol agnostic and SDN-compatible characteristics that they offer, WDM nodes can transparently aggregate disparate optical transmission schemes (D-/A-/SDoF) and modulation formats, enabling heterogeneous deploymentsthat comprise both fiber and converged FiWi lanes. Therefore, such architectures are capable of the co-hosting of heterogeneous fronthaul segments and radio terminals in a cooperative manner, enabling the synergy of multiple BBUs to concurrently provide enhanced capacity to selected radio sites (i.e., for hot-spot use cases) or the exploitation of advanced newly deployed RRHs that are operating at the mmWaves by varying the MNO's baseband equipment. In a few words, WDM-based transport deployment enables the adaptive transformation of baseband-to-radio hardware internetworking, based on the occurrence of a plethora of services with varying requirements [24; 25].  
Our proposed solution, which is schematically illustrated in Figure 1, aims to showcase a hybrid optical fronthaul architecture that can allow for the dynamic interconnection of central offices with RRHs (and vice versa) via both digital and analog paths. An optical node that is based on an SDN-compatible WSS was employed supporting WDM and space division multiplexing (SDM) functionalities for the dynamic distribution of the traffic to the radio sites. In detail, the proposed approach relies on the coexistence of the following fiber and FiWi fronthaul implementations:  
* Legacy CPRI connectivity in which the data transmission is carried out by fragmenting and encapsulating the radio data using well established standards such as IP or Ethernet. The resulting data stream can then be multiplexed with the other network traffic, switched in Ethernet switches and routed in IP routers (Long-Term Evolution (LTE)/4G).
* Broadband binary streams for digital optical links for B5G connectivity scenarios as in 5G-oriented RAN. Intensity modulation/direct detection (IM/DD) systems with digital modulation formats such as non-return-to-zero (NRZ) and 4-level pulse amplitude modulation (PAM-4) are promising candidates.
* B5G A-IFoF implementations which are spectrally efficient transport schemes and can support extremely high capacities.