# Unlicensed Spectrum Allocation for LTE and Wi-Fi Coexistence with HAP
## 5 Performance Evaluation  
In this paper, we use MATLAB to simulate the performance of the proposed mechanism. We will compare our approach with NBS, LAA, and LTE-U. The simulation environment and the setting of parameters refer to [3; 16; 23] for NBS, LAA, and LTE-U, respectively. Table 1 lists related parameter settings in our simulations. In the LAA simulation, the four functions of LAA are fully emulated. We simply set the carrier sense adaptive transmission (CSAT) period for the LTE-U simulation to the repeating interval. The repetition interval is set according to [16]. Regardless of the value of \(\delta\), the longer the repetition interval is, the longer the contention period (CP) and the contention-free period (CFP) are. The length of the repetition interval will not have a critical impact on LTE users because the licensed band is available for the LTE users during the contention-free period. However, the Wi-Fi stations can only access the resource on the unlicensed band. Therefore, when the contention-free period (CFP) increases, the packet transmission delay will also increase accordingly for the Wi-Fi stations. On the contrary, if the repetition interval is too short, the coexistence mechanism will be performed frequently, leading to high overhead. In our simulation, the repetition interval is set to 100 ms [16]. Previously, we defined a utility function with which the weighting factor \(\alpha\) decides the relative weightings of the throughputs of Wi-Fi and LTE systems. The setting of \(\alpha\) will affect the individual throughput of Wi-Fi and LTE systems. Intuitively, the optimal value of \(\delta\) is also affected by the settings of \(\alpha\). We would like to know the impact of the setting of \(\alpha\) on the resulted \(\delta\). Therefore, we simulate the following scenario to figure out the relationship between \(\alpha\) and \(\delta\). In the scenario, there are 15 Wi-Fi stations and 50 LTE users. Generally speaking, if \(\alpha\) is larger, the communication system is friendlier to Wi-Fi stations. LTE users get more resources if \(\alpha\) is smaller. As shown in Figure 9, the relationship between \(\alpha\) and \(\delta\) is almost linear. When \(\alpha\) is small, the throughput contribution of Wi-Fi stations in the utility function is also small. Therefore, the duration of accessing the unlicensed band for LTE users will be longer such that the optimal value of \(\delta\) is smaller. In our scheme, the throughput thresholds will be set to meet the basic requirements for Wi-Fi and LTE systems. When we change the value of \(\alpha\) from 0.1 to 0.9, the value of \(\delta\) by the calculation mechanism is from 0.43 to 0.6, respectively. \(\alpha\) will be set to 0.5 in subsequent experiments. A typical scenario in Figure 10 is used to examine the feasibility and effectiveness of the proposed scheme. In the simulation scenario, there is one LTE eNB, which is a small \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Number of eNBs & 1 \\ Number of HAPs & 10 \\ Number of Wi-Fi APs & 10 \\ Radius of eNB & 120 m \\ Radius of HAP & 60 m \\ Radius of AP & 30 m \\ Transmit power on licensed band & 30 dBm \\ Transmit power on unlicensed band & 24 dBm \\ The bandwidth of licensed band & 10 MHz \\ The bandwidth of unlicensed band & 20 MHz \\ Path loss model & LTE : 140.7 + 36.7 \(\times\) log\({}_{10}\) d \\ WiFi: ITU InH [13] \\ Number of LET users & 20–60 \\ Number of Wi-Fi stations & 5–25 \\ Min. contention window & 16 \\ Max. contention window & 1024 \\ \(\alpha\) & 0.5 \\ \(P_{th,normal}\) & 0.8 \\ \(Throughput_{th,normal}^{WIFI}\) & 2 Mbps \\ Repetition Interval & 100 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. Figure 9: The \(\delta\) versus weight value \(\alpha\). base station. Moreover, multiple Wi-Fi APs are deployed, and we suppose there is a HAP located between each AP and eNB serving as a bridge. We compare our approach with other well-known schemes.