# Planning of Optical Connections in 5G Packet-Optical xHaul Access Network
## 3 Main Assumptions
### Transmission Model  
In this article, we assume the properties of WDM transmission system \(x\)Wave 400G [16] applied in the xHau 5G optical-packet access network. As discussed in Section 2, the system is characterized by a narrow inter-channel spacing. The benefit of a small inter-channel spacing is the reduction of signal distortion caused by dispersion, while the drawback is the need to use lasers with low chirp, temperature drift in the transmitter, and greater nonlinear distortions. In this case, the limitation of the transmission distance is caused by the power budget. Therefore, we present a detailed model for estimating this budget in the following section. Table 1 shows the system parameters required to calculate the transmission distance \(L\) expressed by Equation (1), determined on the basis of the power and attenuation budget. The number of OADMs introduced into the optical path is denoted by \(N\). \[L=\min\left(\frac{P_{\text{TXoma}}-P_{\text{RXoma}}-4 NA_{\text{C}}-2 NA_{\text {MUX}}-A_{\text{MM}}}{\alpha},L_{\text{max}}\right)\,\text{[km]}. \tag{1}\]  
Minimal transmitter outer optical modulation amplitude (\(P_{\text{TXoma}}\)) is defined as the minimal difference between two outer (maximal and minimal) optical power levels in PAM-4 signal at transmitter output. Receiver sensitivity (\(P_{\text{RXoma}}\)) is defined as the difference between two outer (maximal and minimal) optical power levels in PAM-4 signal at receiver input sufficient to obtain bit error rate less than \(2.4\times 10^{-4}\) before FEC correction. Power budget is the difference between \(P_{\text{TXoma}}\) and \(P_{\text{RXoma}}\) (see Reference [25], Figure 2-1). The G.652D fiber loss coefficient value of 0.40 dB/km given in Reference [23] is increased here by 0.10 dB/km taking into account additional splices losses and environmental margin. To calculate the influence of connectors on attenuation budget, we exploit a statistical approach due to a large number of connectors utilized (\(4N\)), and, as a consequence, mean value represents connector loss. The maximum transmission distance (\(L_{\text{max}}\)) of the flexiHaul xWave 400G system is limited to 20 km [16]. MUX and DEMUX modules always operate in pairs, so, if they have the same order of filters, the total attenuation we estimated is at approximately 2/3 of combined maximum MUX/DEMUX attenuations (e.g., 3.6 dB instead of 5.4 dB) because the channel added in the MUX farthest from the output is the most attenuated, while, in the DMUX, it is dropped first and the least attenuated. This applies to both the MUX/DEMUX pair which is part of OADM (see Figure 1) and to the first/last MUX/DEMUX pair in each optical path. Attenuation of cut-through OADM optical path is increased by patchord attenuation equal to \(2\times A_{\text{C}}\). Figure 2 shows the system configuration with indicated parameters necessary to estimate the transmission distance from Equation (1). A minimum of one wavelength is \begin{table}
\begin{tabular}{c c} \hline \hline Fiber loss coefficient (\(\alpha\)) [dB/km] & 0.5 \\ Connector loss (\(A_{\text{C}}\)) [dB] & 0.25 \\ Maximum number of carrier wavelengths (\(W\)) & 4 \\ MUX4 loss (\(A_{\text{MLX}}\)) [dB] & \(1.8\times\{1.0,0.9,0.8\}\) \\ Maintenance margin (\(A_{\text{MM}}\)) [dB] & 1.0 \\ Minimal transmitter outer optical modulation amplitude & 3.0 \\ (\(P_{\text{TXoma}}\)) [dBm] & \(-14.0\) \\ Maximum transmission distance (\(L_{\text{max}}\)) [km] & 20.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Transmission system parameters. dedicated to a single remote site, so the maximum remote sites number equals \(W\). In the information available to us about the parameters of the flexiHaul xWave 400G system, there is no exact data for the attenuation of MUX and OADM modules. Therefore, the budget calculations are based on the parameters of similar, off-the-shelf components, made in the same TFF technology, available from various manufacturers [27,28]; hence, we use several variants for values of \(A_{\text{MUX}}\) adopted in the analysis.