## 1 Introduction  
Broadband mobile connectivity is rapidly evolving towards the fifth generation (5G) era, defining a new set of requirements beyond the capabilities of the legacy long-term evolution (LTE) based fourth generation (4G) networks [1]. The widespread use of emerging applications, such as 4K high definition streaming, augmented/virtual reality applications (AR/VR), immersive video conference, etc., has created the need for ubiquitous high bandwidth connectivity and enhanced mobile broadband(eMBB) services [2; 3]. Stimulated by this completely new landscape, expert alliances, such as the International Telecommunication Union (ITU) [4] and Next Generation Mobile Networks (NGMN) [5], have defined certain key performance indicators (KPIs) for 5G broadband access networks. According to these indicators, user data rates of 1 Gb/s and peak rates of 20 Gb/s with latency less than 10 ms are targeted, resulting in high area connection densities up to 10\({}^{6}\) devices per km\({}^{2}\) and capacities up to 10 Mb/s/m\({}^{2}\)[4; 5]. Towards satisfying these ambitious set of requirements, 5G technology focuses on the deployment of New Radio (NR) systems and exploitation of millimeter-wave (mm-wave) radio in the access networks [6] through Multiple-Input Multiple-Output (MIMO) systems [7], targeting to migrate from the congested 6 GHz radio to larger contiguous spectral bands. The above came into realization also with 3 GPP Release 15 for 5G NR that specifies frequencies up to 52.6 GHz and channel bandwidths up to 400 MHz [8], while the future 3 rd Generation Partnership Project (3 GPP) Release 16 will include higher spectrum at the 57 GHz-71 GHz band. Within this framework, the first 5G trials and deployment roadmaps are expected to be installed initially on overcrowded privately-owned and small geographical scale hotspot areas [9; 10], e.g., sport stadiums, airports, etc., where the presence of many users can lead to economically viable network installations at reduced costs [11]. The employment of higher spectral bands necessitates the densification of the access points (APs), due to their inherently high propagation and attenuation losses [12]. However, when applying the traditional macro-cell distributed radio access network (RAN) architectures at ultra-dense mm-wave environments, it comes at the expense of increased acquisition, deployment and operational costs, which in itself is detrimental to the mobile network operators' (MNOs) business model [13]. Moreover, when using backhauling in ultra-dense mm-wave networks, a tremendous burden is placed to the management layer that must now coordinate and cancel interference from an order of magnitude more sources that are densely placed in the complex and ever-changing mm-wave environment [14]. For these reasons, operators are currently shifting to centralized solutions, such as the centralized-RAN (C-RAN) [15; 16], where all demanding processing operations that were part of the Base Station (BS), are now moved into the baseband unit (BBU), placed in a centralized location. In parallel, the operationally simple remote radio heads (RRHs) are dispersed across the APs and connected to the BBU through what is termed as fronthaul. Thus, it becomes apparent that centralization reduces greatly the capital and operational expenditures of deploying dense networks [17], while at the same time the formation of the single-point origin of the downlink signals enables improved radio coordination and increased energy efficiency through optimized capacity sharing and statistical multiplexing. Despite the cost-reduction and energy efficiency improvement, centralization results in a tremendous increment in time-sensitive traffic in the fronthaul part of the network [18; 19], which, in the dense mm-wave AP era will be required to include the transportation of several ultra-broadband >400 MHz NR mm-wave channels at a small latency and energy envelope.