# Non-Orthogonal eMBB-URLLC Radio Access for Cloud Radio Access Networks with Analog Fronthauling
## 1 Introduction
### C-RAN Based on Analog Radio-over-X Fronthauling  
The advent of 5G is introducing advanced physical layer technologies and network deployment strategies such as massive MIMO, mmWave, small-cell densification, mobile edge computing, etc. (see [8; 9] for an overview). C-RAN is an enabling technology that is based on the colocation of the Base Band Unit (BBU) of Edge Nodes (ENs) that are densely distributed in a given indoor or outdoor area. This solution has the advantages of allowing for centralized BBU signal processing, providing network scalability, increasing spectral efficiency, and reducing costs.  
The most typical C-RAN architecture relies on digital optical fronthaul links to connect ENs to BBUs. This solution, known as Digital Radio-over-Fiber (D-RoF), is adopted in current 4G mobile networks, and is based on the transmission of in-phase and quadrature baseband signals, upon digitization and packetization according to the CPRI protocol [10].  
Over the last years, several alternative C-RAN architectures have been proposed that redistribute the RAN functionalities between BBU and ENs, obtaining different trade-offs in terms of bandwidth and latency requirements, advanced Cooperative Multi-Point processing capabilities, and EN cost and complexity [11]. For scenarios with stringent cost and latency constraints, a promising solution is to use analog fronthauling.  
With analog fronthauling, focusing on the uplink, the ENs directly relay the radio signals to the BBUs after frequency translation and, possibly, signal amplification. This has the advantages of avoiding any bandwidth expansion due to digitization; guaranteeing ENs synchronization; minimizing latency; reducing hardware cost; and improving energy efficiency [12; 13; 14]. A C-RAN architecture based on analog fronthauling is also known in the literature as Analog Radio-over-X (A-RoX), where X depends on the technology employed for the fronthaul, which can be either Fiber (A-RoF [12]), Radio (A-RoR [15]), or Copper (A-RoC [13]), as depicted in Figure 1.  
In particular, A-RoF provides an effective example of analog fronthauling, due to its capability to support the transport of large bandwidths [12; 16]. However, A-RoF requires the deployment of a fiber optic infrastructure whose installation is not always feasible, e.g., in dense urban areas. In such scenarios, a possible solution is to rely on the A-RoR concept, thus employing point-to-point wireless links, mainly based on mmWave or THz bands, with several advantages in terms of flexibility, resiliency, hardware complexity and cost [15; 17]. Another application scenario where the installation of fiber links may be too expensive to provide satisfactory business cases is indoor coverage. For indoor deployments, A-RoC [18; 19; 20] has been recently proved to be an attractive solution, especially from the deployment costs perspective [21], since it leverages the pre-existing Local Area Network (LAN) caching infrastructure of building and enterprises. Moreover, LAN cables are equipped with four twisted-pairs with a transport capability up to 500 MHz each, or 2 GHz overall, for radio signals, thus providing enough bandwidth for analog fronthaul applications [22]. Over the last years, A-RoC based on LAN cables has become a standard solution for in-building commercial C-RAN deployments, allowing to extend the indoor coverage over distances longer than 100 m [23].  
In this paper we study the coexistence of URLLC and eMBB services under both OMA and NOMA assuming a C-RAN multi-cell architecture based on analog fronthauling (see Figure 1) by using information theoretical tools.