# Congestion Analysis of Transport Layer in a Multicell 5G DL Communication System
## 3 Experiments  
This section presents the setup parameters for the simulations, as well as the environment scenarios selected. As presented in Table 1, we focused on downlink simulations for the two following environments: a single-cell environment and a multicell environment consisting of three cells. The channel model adopted was an urban macrocell environment without buildings, where the gNodeB (gNB) had 64 transmitter antennas and 64 receiver antennas, respectively. We considered 10 to 200 UE devices, depending on the scenario simulated, which moved at a walking speed of 3 km/h (see Figure 3 for a comparison of both environments tested). The configuration implemented consisted of a 5G standalone (SA) architecture, option two, with the core and radio components of the architecture using 5G interfaces and features. The deployment type selected was time-division duplexing (TDD) with a carrier frequency of 3.5 GHz, as this is a typical range for frequencies allocated for 5G networks, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Environment one** & Single-cell & One cell \\ \hline
**Environment two** & Multicell & Three cells \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Environments simulated in this work. Figure 3: Graphical representation of environments one and two, respectively. Figure 2: FTP traffic model two. and a bandwidth of 100 MHz, allowing for higher data rates and increased network capacity. The modulation adopted was 256 quadrature amplitude modulation (QAM) and the MIMO mode was 4 \(\times\) 4 MIMO with grid-of-beams (GoB) digital beamforming implemented. This allowed the exploitation of spatial diversity, increased data rates, and optimization of coverage by dynamically adapting beam patterns. The test environment was formed by an urban macrocell (UMa) environment without buildings, where the users moved at a walking speed of 3 km/h. To perform the simulations, we used Nokia's internal 5G simulator 5 GMax alongside the MATLAB program. See Table 2 for the detailed list of parameters adopted. Our intention was to detect and investigate the congestion that occurs in a 5G network on the DL side. The traffic model used for all simulations was FTP model two with the TCP slow-start algorithm; furthermore, 10 extended files of 5 Mbytes each, a duration of 5 s, and geometric distribution were employed in order to create enough PRB utilization and achieve divergent loading in the downlink. The slow-start algorithm was hence activated to investigate its impact on 5G network congestion and delays. This means that a single 1500-byte packet was sent into the buffer; however, several smaller packets were continuously increased until an optimal value was found. The metrics studied for this work were as follows: PRB utilization, for which all slots were included, regardless of whether there was traffic or not; data volume, defining the average volume of data waiting in the buffer; cell throughput, representing the average throughput of the cell; cell edge user throughput, consisting of the 5% tile of the user throughput distribution; user throughput; and delay, for which all slots were included, regardless of whether there was traffic or not. The user throughput was calculated as:  
\[\overline{TP_{user}}=\frac{packet\_size}{time\ of\ transmission\ +\ time\ waiting\ in\ buffer} \tag{2}\]  
Table 3 presents the scenarios and metrics simulated for each environment.