## 4 Discussion
### Performance Indicators  
Throughput is a key measure of a user's data transmission efficiency in a wireless network. It represents the amount of actual data successfully transmitted per unit of time. The throughput calculation in this paper takes into account the user's SNR and the type of network connected (VLC or Wi-Fi), and the throughput calculation is expressed as:  
\[r_{j}=\begin{cases}\frac{B_{\text{VLC}}}{2}\cdot\log_{2}\Big{(}1+\frac{ \epsilon}{2\pi}\xi_{\text{VLC}}^{i,j}\Big{)},&\text{for VLC AP}\\ B_{\text{WIFI}}\cdot\log_{2}\Big{(}1+\xi_{\text{WIFI}}^{j}\Big{)},&\text{for WiFi AP}\end{cases} \tag{22}\]  
where \(\xi_{\text{VLC}}^{i,j}\) and \(\xi_{\text{WIFI}}^{j}\) are the SNR between the user and the AP, and \(B_{\text{VLC}}\) and \(B_{\text{WIFI}}\) are the bandwidths of VLC and Wi-Fi, respectively.  
The handover rate is an important measure of the frequency of the handover in a network. It is often used to evaluate the efficiency and stability of network handover algorithms. The calculation of the handover rate is expressed as follows:  
\[\text{Handover Rate}=\frac{\text{N}_{\text{HO}}}{\text{T}} \tag{23}\]  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Room size (length by width by height) & 15 m \(\times\) 15 m \(\times\) 3 m \\ The cost of VHO, \(C_{V}\) & 500 ms \\ The cost of HHO, \(C_{H}\) & 200 ms \\ The physical area of the PD, \(A_{\text{eff}}\) & 1 cm\({}^{2}\) \\ Field of view semi-angle of the PD, \(\psi_{\text{c}}\) & 90\({}^{\circ}\) \\ Detector responsivity, \(\gamma\) & 0.53 A/W \\ Half-intensity radiation angle, \(\phi_{1/2}\) & 60\({}^{\circ}\) \\ System bandwidth per VLC, \(B_{\text{VLC}}\) & 20 MHz \\ Transmitted optical power per VLC, \(P_{\text{VLC}}^{\text{VLC}}\) & 3.5 W \\ Noise power spectral density in VLC, \(N_{\text{VLC}}\) & \(10^{-21}\) A\({}^{2}\)/Hz \\ System bandwidth per Wi-Fi, \(B_{\text{WIFI}}\) & 20 MHz \\ Transmitted power per Wi-Fi, \(P_{\text{WIFI}}\) & 15 dBm \\ Noise power spectral density in Wi-Fi, \(N_{\text{WIFI}}\) & \(-\)174 dBm/Hz \\ \hline \hline \end{tabular}
\end{table}
Table 1: Simulation parameters.  
where NHO is the number of handovers occurring during the observation time T. Frequent handover not only consumes additional network resources but also interrupts data transmission, significantly affecting the user experience and the overall stability of the network.  
(3) Fairness  
Jain's fairness index is an important index to measure the fairness of resource allocation in network handover, and the calculation of fairness is expressed as:  
\[J=\frac{\left(\sum_{i=1}^{N}x_{i}\right)^{2}}{N\sum_{i=1}^{N}\left(x_{i} \right)^{2}} \tag{24}\]  
where \(N\) denotes the number of users and \(x_{i}\) denotes the amount of resources allocated to the user (e.g., bandwidth, throughput), which in this paper refers to the user's throughput. The range of values is [0, 1]. This index can help evaluate whether users have fair access to network resources when the handover is between different access technologies. A high fairness index implies that the difference in throughput between users is slight, and the allocation of network resources is more balanced.