# Non-Orthogonal eMBB-URLLC Radio Access for Cloud Radio Access Networks with Analog Fronthauling
## 2 System Model
### RAN Model  
We consider the same Wyner-type radio access model of [32], which is described in this subsection. The Wyner model is an abstraction of cellular systems that captures one of the main aspects of such settings, namely the locality of inter-cell interference. The advantage of employing such a simple model is the possibility to obtain analytical insights, which is a first mandatory step for the performance assessment under more realistic operating conditions [35]. As illustrated in Figure 3, the direct channel gain from the eMBB UE and the EN belonging to the same cell is set to one, while the inter-cell eMBB channel gain is equal to \(\alpha\in[0,1]\). Furthermore, the URLLC UEs have a channel gain equal to \(\beta>0\). The URLLC user is assumed to be in the proximity of the EN, and thus it does not interfere with the neighboring cells. The eMBB user, instead, is assumed to be located at the edges of the cell in order to consider worst-case performance guarantees. As a result, each eMBB user interferes with both left and right neighboring cells, following the standard Wyner model [35]. All channel gains are assumed to be constant over the considered radio resources shown Figure 3, and known to all UEs and ENs. Figure 3: Time-frequency resource allocation: (**a**) Orthogonal Multiple Access (OMA) and (**b**) Non-Orthogonal Multiple Access (NOMA). Downwards arrows denote arrival of URLLC packets. Figure 2: Model of the uplink of C-RAN system based on Analog Radio-over-Copper (A-RoC) fronthauling. As illustrated in Figure 3, we assume that the time-frequency plane is divided in \(n_{T}\) minislots, indexed as \(t\in[1,n_{T}]\), where each minislot is composed of \(n_{F}\) frequency channels, indexed as \(f\in[1,n_{F}]\), for a total of \(n_{F}n_{T}\) time-frequency radio resources. Each radio resource accommodates the transmission of a single symbol, although generalizations are straightforward. The eMBB UEs transmit over the entire \(n_{F}\times n_{T}\) time-frequency frame. In contrast, due to the latency constraints of the URLLC traffic, each URLLC transmission is limited to the \(n_{F}\) frequency channels of a single minislot, and URLLC packets are generally small compared to the eMBB frame, which requires the condition \(n_{T}\gg 1\). As illustrated in Figure 3, each URLLC UE generates an independent packet in each minislot with probability \(q\). This packet is transmitted at the next available transmission opportunity in a grant-free manner. In the case of OMA, one minislot is exclusively allocated for URLLC transmission every \(L_{U}\) minislots. Parameter \(L_{U}\) is considered here as the worst-case access latency. Accordingly, if more than one packet is generated within the \(L_{U}\) minislots between two transmission opportunities, only one of those packets is randomly selected for transmission and all the remaining are discarded. The signal \(Y_{k}^{f}(t)\) received at the \(k\)-th EN at the \(f\)-th frequency under OMA is  
\[Y_{k}^{f}(t)= \begin{cases}\beta A_{k}(t)U_{k}^{f}(t)+Z_{k}^{f}(t),&\text{if }t=L_{U},\;2L_{U},\;...\\ X_{k}^{f}(t)+\alpha X_{k-1}^{f}(t)+\alpha X_{k+1}^{f}(t)+Z_{k}^{f}(t),&\text{ otherwise}\end{cases}  \tag{1}\]  
where \(X_{k}^{f}(t)\) and \(U_{k}^{f}(t)\) are the signals transmitted at time \(t\) and subcarrier \(f\) by the \(k\)-th eMBB UE and URLLC UE, respectively; \(Z_{k}^{f}(t)\sim\mathcal{CN}(0,1)\) is the unit-power zero-mean additive white Gaussian noise; and \(A_{k}(t)\in\{0,1\}\) is a binary variable indicating whether or not the URLLC UE is transmitting at time \(t\). In case of NOMA, the URLLC UE transmits its packet in the same slot where it is generated by the application layer, so that the access latency is always minimal, i.e., \(L_{U}=1\) minislot.