# Candidate Waveforms for ARoF in Beyond 5G
## 1 Introduction  
The increasing number of mobile devices demanding internet applications has motivated the exploration of diverse possibilities and methods for achieving a higher capacity of exchanging information with enhanced coverage potential [1]. Based on ITU-T FG-IMT-2020 [2], fifth-generation (5G) networks should provide 1000 times more wireless capacity than currently available, supporting internet connectivity with exceptionally low latency (\(<\)1 ms) to over 7 trillion wireless devices among 7 billion people. Considering the capacity associated with the anticipated small cells, it is expected that data rate requirements range between 100 Mbit/s and 1000 Mbit/s and beyond, with peaks up to 10 Gbit/s. Accordingly, 5G millimeter-wave (mmWave) wireless channel bandwidths will be more than ten times greater than current 4G Long-Term Evolution (LTE) cellular channels [3, 4] to deliver an unprecedented level of service to the end user. Since wavelength shrinks by an order of magnitude at mmWave when compared to today's 4G microwave frequencies, they will be affected by a severe free space path loss (FSPL) and a considerable attenuation that is caused by diffraction and material penetration, thus elevating the importance of line-of-sight (LOS) propagation, reflection, and scattering. Therefore, cell coverage areas in 5G, are approximately in the range of 10 to 200 m [5]. This fact implies an increase in the number of cells and nodes in current mobile networks. Fiber optic networks with their immense capacities are set to be the most important connection type for front- and backhaul for such wireless networks, due to their high bit rates achieved and the long distances covered. While in current centralized radio access network (C-RAN) deployments fronthaul data is transported in digitized form, i.e., as in phase and quadrature (IQ) samples of the RF waveform, this is highly inefficient with regards to optical spectrum usage and the required data rates quickly become prohibitive as radio bandwidth and carrier frequencies grow. In this respect, analog radio over fiber (ARoF) technology, where the required RF waveform is transported in the optical network as an analog signal, paves the way as an efficient solution in terms of spectral efficiency compared to digital RoF (DRoF) [4; 6]. Thus, ARoF efficiently leverages on the advantages of fiber optics, such as low attenuation and high bandwidths. To support the integration of diversified data traffic types and integration of mobile front- and backhaul with other services in a shared network, emerging flexible, robust and high capacity passive optical networks (PONs) are considered for ARoF architectures [7]. In addition, wavelength division multiplexing (WDM) is already commonly used in aggregation and metro networks because it provides a graceful upgrade path to accessing the available optical spectrum, as well as having advantages in terms of scalability and network management. As the number of required channels growth, especially with dense 5G deployments, the number of available wavelengths must be increased to allow provisioning of sufficient capacity while minimizing waste of spectrum. In this respect, the separation among wavelengths must be reduced, resulting in ultra-dense WDM (UDWDM). In this regard, the EU-H2020 project-ITN 5G STEP FWD [8] proposes to transform the current PONs to UDWDM-PONs. The combination with ARoF, i.e., ARoF over UDWDM-PONs, is a strong candidate to be part of the 5G front- and backhaul architecture. As an illustrative case, Figure 1 shows a feasible structure of mmWave cells over UDWDM-PONs for RoF systems. However, the reduction of wavelength spacing in UDWDM-PONs and the impairments inherent to working with mmWave signals may induce system performance degradations [9]. Furthermore, the use of ARoF fronthaul directly concatenates the optical and wireless channels, creating a hybrid channel of potentially larger complexity than the pure wireless channel observed with digitized fronthaul.