# The Multiband over Spatial Division Multiplexing Sliceable Transceiver for Future Optical Networks
## 1 Introduction  
Global data traffic has experienced significant growth in recent years, and it is expected to continue increasing primarily driven by the emergence of groundbreaking 6G applications and services, such as augmented reality, holographic communications, and even more sophisticated AI applications, which will require stringent requirements in terms of data rate/bandwidth and latency having an impact in the evolution of optical networks [1; 2]. This exponential growth will imply a change in terms of volume and distribution of traffic in the network, highlighting the importance of flexible, robust, scalable, reliable, and high-capacity optical networks [3]. According to [1], in the aggregation metro segment, the flow capacities from access nodes directed towards additional nodes, particularly regional nodes, are foreseen to escalate to a maximum of 2 Tb/s in the medium term and an impressive 8 Tb/s in the long term. According to the estimations derived from traffic analysis of [1], regional and national nodes should efficiently manage the aggregated traffic, ranging from a few Tb/s in the short term, ten Tb/s in the medium term, and potentially reaching up to a hundred Tb/s in the long term. This underscores the need for robust and scalable infrastructure to accommodate the increasing demands on network resources over various time horizons from the access to the backbone network segments [4]. The landscape of 6G networks will also see significant enhancements in the wireless segment [5]. This includes more efficient wireless links that complement the wired infrastructure, providing seamless communication with final user devices enabling optical/wireless convergence [6; 7]. The wireless segment for 6G is currently receiving high attention, and novel technologies such as THz, millilitre wave and visible light communications, integrated and free-space optics or power-over-fibre deployment as well as theuse and adoption of machine learning techniques, softwarization and massive multiple-input multiple-output signalling are under active investigation to provide additional features [7; 8; 9; 10; 11; 12; 13; 14]. Notably, there is a focus on incorporating high-accuracy positioning, imaging and sensing capabilities, which are expected to play an important role for enabling services for all verticals within 6G networks. The integration/convergence of both wired and wireless technologies is essential to meet the diverse and demanding requirements of 6G applications and support the incoming traffic increase [15]. The combination of high-capacity optical networks and advanced wireless links is crucial for ensuring the delivery of low-latency, high-throughput, and reliable connectivity. Moreover, as part of the evolution towards 6G, networks are anticipated to become more energy-efficient, contributing to the reduction in carbon footprints and aligning with global sustainability goals [16]. In this context, key advanced technologies including multiband (MB) and spatial division multiplexing (SDM) have been recently investigated for evolving current optical networks towards enabling the expected network capacity scaling. MB technology enables the utilization of additional bands beyond C-band, such as E-, S-, L-, O- and U-bands, in order to increase the available bandwidth of the fibre and optical spectrum resources [17]. This not only allows for the simultaneous transmission of data across various frequency bands, but also enhances the network's resilience and flexibility. Additionally, SDM enables to further enhance overall network capacity and scalability by exploiting multiple spatial channels, which can be based on a bundle of standard single-mode fibres (SSMFs) or even speciality fibres such as multicore fibres (MCFs) and few mode fibres (FMFs) [18; 19]. By exploiting spatial diversity, SDM provides an advance in network capacity, enabling optical networks to accommodate an ever-expanding volume of data traffic.