## 3 Research of the Key Elements
### Injection of eNB Deviation by Fiber Optical Repeater  
In the uplink, a high level of interfering signal power at the eNB receiver can pose a hindrance to communication services for cellular phones. The transmission power in a cellular phone is set to ensure the required Eb/No. Notably, the fiber optic repeater, being an active component, will powerfully amplify the transmission interfering signal energy level, which includes thermal noise power. Consequently, the output noise power of the fiber optic repeater, \(P_{rep}\), can be expressed as follows:  
\[P_{rep}=10\ log[KTB]+P_{eNB}+NF_{rep}+G_{rep} \tag{10}\]  
Here, \(NF_{rep}\) is the noise figure of the fiber optical repeater and \(G_{rep}\) is the gain of the fiber repeater. The propagation model of the path loss from eNB to fiber optical repeater is \(L_{p}\). Therefore, the noise power signal injected by the fiber optical repeater in the eNB receiver, \(P^{\prime}_{rep}\)[17], can be written as Equation (11):  
\[ \begin{split} P^{\prime}{}_{rep}&=P_{rep}-L_{p}\\ &=10\ log[KTB]+P_{\text{th,eNB}}+NF_{rep}+G_{rep}-L_{p}\\ &=-119\text{dBm}+P_{\text{th,eNB}}+NF_{rep}+G_{rep}-L_{p}\\ \end{split}  \tag{11}\]  
When the fiber optical repeater is injected in the eNB receiver, the total noise power, \(P_{eNB,total}\)[18], can be expressed as Equation (12):  
\[ \begin{split} P_{eNB,total}&=P_{eNB}-P^{\prime}{}_{rep }\\ &=10\ log[10P_{eNB}+10P^{\prime}{}_{rep}]\\ &=P_{eNB}+10\ log\bigg{[}1+10^{\frac{NF_{rep}+NF_{eNB}+-L_{p}}{ 10}}\bigg{]}\\ &=P_{eNB}+\Delta N_{eNB}\end{split}  \tag{12}\]where  
\[\Delta N_{eNB}+10\,log\bigg{[}1+10^{\frac{N\!F_{rep}+N\!F_{eNB}+-L_{p}}{10}}\bigg{]} \tag{13}\]  
The above analysis demonstrates that an injected fiber optical repeater increases the noise power in the eNB receiver compared to one without a fiber optical repeater. Furthermore, there are many factors that may affect the noise floor of the eNB receiver. Parameters related to the above include the noise figure of eNB, \(N\!F_{eNB}\) the noise figure of the fiber optical repeater, \(N\!F_{rep}\) the gain of the fiber optical repeater, \(G_{rep}\) and \(L_{p}\), the path lossfrom eNB to fiber optical repeater. The equation can be presented as Equation (13):  
* Case 1: When \(N\!F_{rep}-N\!F_{eNB}+G_{rep}-L_{p}=0\), by \(\Delta N_{eNB}\), the noise floor has increased to 3 dB, in the receiver terminal of \(eNB\). * Case 2: When \(N\!F_{rep}-N\!F_{eNB}+G_{rep}-L_{p}=-6\), by \(\Delta N_{eNB}\), the noise floor has increased to 0.97 dB, in the receiver terminal of \(eNB\). * Therefore, in this case, the injected fiber optical repeater does not have an influence on the eNB receiver. Usually, the noise figure of eNB, \(N\!F_{eNB}\) = 2 dB. According to Equation (10), the eNB noise power injected by the fiber optical repeater is \(P_{rep}=-125\) dBm. That is why, in engineering practice, the noise figures of the eNB and fiber optical repeater are constant, the noise floor increase in eNB receives the port, and \(\Delta N_{eNB}\) influences the path loss of the eNB transmitter to fiber optical repeater and fiber optical repeater \(G_{rep}\). The noise figure of eNB, \(N\!F_{eNB}\) = 2 dB. The noise figure of fiber optical repeater, \(N\!F_{rep}\) = 5 dB as usual. The gain of the fiber optical repeater, Grep, should be lower than the 8 dB path loss. By limiting Grep, the \(\Delta N_{eNB}\) would be controlled and kept to under 1 dB. Since, in the network design, the hypothetical range of the objective coverage plane is relatively high, there is a need to connect parallel several fiber optical repeaters. In this case, the noise floor of the eNB receiver, \(P_{eNB,total}\) is the noise figure and the thermal noise power signal injected by the fiber optical repeater to the eNB receiver port, \(P_{rep}\), is shown to be \(P_{rep,total}=10 log\left[10P_{eNB}+10(P_{rep}\eta^{\prime})\right]\). The quantity of fiber optical repeaters is given by n. For the purpose of controlling the Prep,total, the \(\Delta N_{eNB}\) must be under 1 dB.