Experimental Validation of Hybrid WDM/SDM Signal Delivery for Mobile Fronthaul over PONs using SDN-Enabled Sliceable Bitrate Variable Transceivers Josep M. Fabrega 1 , Laia Nadal 1 , Ramon Casellas 1 , Michela Svaluto Moreolo 1 , F. Javier Vílchez 1 , Ricard Vilalta 1 , Ricardo Martínez 1 , Raul Muñoz 1 , Salvador Sales 2 , Ivana Gasulla 2 , José Capmany 2 1 Centre Tecnològic de Telecomunicacions de Catalunya (CTTC/CERCA), Castelldefels, Spain; e-mail: jmfabrega@cttc.es 2 ITEAM Research Institute, Universitat Politècnica de València, Valencia, 46022, Spain ABSTRACT We demonstrate combined WDM/SDM delivery of mobile fronthaul in PONs, employing SDN-enabled S-BVTs based on adaptive multicarrier modulation. Experiments show successful BBU-RRU connectivity on several deploying strategies while coping with capacities beyond 50Gb/s per flow. Keywords : Passive optical networks, space division multiplexing, wavelength division multiplexing, optical OFDM, optical fronthaul. 1. INTRODUCTION 5G demands a large increase in capacity while meeting a dynamic network management. Furthermore, a large number of antenna elements over small/femto cells are envisioned for 5G deployments. This will require a large transmission bandwidth in the fronthaul, allowing to increase data rate per end user and application demand [1]. Therefore, the current and future network paradigm is expected to be based on a hybrid configuration where the segment closest to the end user will be supported by 5G wireless communications, while optical fiber links will support radio access networks (RANs) demanding very high capacity. In order to cope with that, wavelength division multiplexing (WDM) can be employed, reusing the already deployed fiber infrastructure in the residential access, while providing flexible connectivity and high capacity [2]. Nevertheless, this solution is bounded to certain spectral regions and is not able to s cale when capacity requirements increase, as expected in highly dense urban areas [1]. To overcome this limitation, space division multiplexing (SDM) can be employed to provide spatial diversity, which, in combination with WDM, will enable a fronthaul infrastructure with unique 2-dimensional (2D) properties. A first approach for SDM can be based on bundles of standard single-mode fibers (SSMFs). A longer-term solution can rely on multicore fibers (MCFs) [3], providing a compact parallel transmission medium. In this paper, we propose and experimentally demonstrate a hybrid WDM/SDM delivery of mobile fronthaul traffic in passive optical networks (PONs), following the software defined networking (SDN) paradigm. This combination enables setting up of specific spectral/spatial channels according to the requirements of the services to deliver, enabling the configuration of virtual RANs over a passive optical infrastructure, seeing them as private network slices where flows can be allocated in a 2D space (WDM+SDM). This is enabled in the data plane level mainly by the adoption of a hybrid WDM/SDM infrastructure and SDN-enabled programmable sliceable bitrate variable transceivers (S-BVTs). The S-BVTs are able to transmit data flows with variable rate according to the network and path conditions. Specifically, we propose to use cost-effective S-BVTs bas ed on orthogonal frequency division multiplexing (OFDM) and direct detection (DD). 2. CONCEPT The network and signal delivery scheme is depicted in Fig. 1a. There, programmable S-BVTs are present at the central office in order to concurrently serve different cell sites. At the other end of the network, each cell site has a programmable BVT. Therefore space and spectrum resources can be managed by a centralized SDN controller in order to provision the different services, possibly including residential access and/or some other that could require a direct interface with the metro/aggregation segment. All the devices/systems belonging to the central office and, especially, the (S-)BVTs can be programmed by means of the corresponding SDN agents, allowing an automated channel establishment between the central office and the cell sites in a 2D space (WDM+SDM). We propose to employ the same wavelength for upstream and downstream in order to maximize the utilization of the network resources and simplify the network management. Several deployment strategies can be approached for the external plant in order to enable SDM. A firs t s tep can be using SSMF bundles, since cables deployed in the field typically have a loose-tube design containing several fibers [4]. This would enable to reuse legacy infrastructure. Nevertheless, approaching this brown field migration scenario would entail different challenges, such as the relative delay between signals propagating through different fibers and the impact of environmental conditions such as temperature variations and unequal mechanical stress. In fact, this delay is tied to the cable structure and its helix factor, leading to differences of up