## 4 Results and Discussion  
The proposed WDM-RoF network was positively simulated and analyzed using Optisystem 13, a commercial optical system simulator. Figure 5 shows the optical spectrum after being multiplexed by the WDM multiplexer. The spectrum was observed using an optical spectrum analyzer at the output of the WDM multiplexer.  
Single parameter optimisation (SPO) can benefit the simulation a lot. SPO assistances the simulator for optimizing all parameters to allow the access to the results of the simulation. Thus, the optimization of the length of EDFA and a maximum gain can be obtained with the software of optimization tools. Moreover, the attenuation can be measured for getting the Q-factor and BER with a minimum values from the length of fibre optimization.  
The simulation was run immediately after the connection of all components was completed and SPO was initialized so as to get the simulation result. The measurements of the proposed system started as soon as the run button was on, then a BER and Q-factor can be seen easily. An eye diagram was created to see the BER values, eye opening, in additional to the Q factor.  
The analysis of the receiver performance can be seen in the simulation run of the BER and the eye diagram diagram. The results can be seen in Figures 6-8. For receiver 1 with 5 km fibre length, the BER analyzer shows that the minimum BER for the received signal was 1.1009 \(\times\) 10\({}^{-10}\). The Q-factor for this receiver was 6.3046. The receiver results indicate a perfect performance with referring to the BER value of 10\({}^{-9}\), and more than 6 with respect to Q-factor. The noise immunity of the receiver signal affected by the eye diagram height and shown 0.00027 for receiver 1.  
For receiver 2 with 20 km fibre length, the received signal had a BER of 2.4347 \(\times\) 10\({}^{-9}\). The Q-factor of the received signal is 5.81. Thus, the BER analyzer shows that this receiver has also received a good signal quality. The eye opening of the eye diagram is 0.000157.  
Figure 5: Optical spectrum multiplexed by WDM.  
For receiver 3 with 60 km fibre length, the received signal had a BER of 1.918 \(\times\) 10\({}^{-8}\). The Q-factor of the received signal is 3.53. Here, the BER analyzer shows a bad signal quality. The eye opening of the eye diagram is 0.0000121.  
For comparison of the received signal, receiver 1 had a higher immunity to the noise than receiver 3.  
As shown in the sample above, BER was 1.10 \(\times\) 10\({}^{-10}\), and the Q-factor is 6.30. This sample result of the eye opening was 2.585 \(\times\) 10\({}^{-5}\). The sample is a good one because the BER value was less than 10\({}^{-9}\), while the Q-factor was a greater than 6. Thus, for receivers with an output just similar to the above sample, then all the proposed parameters can be achieved without difficulty.  
Figure 6: Bit error rate (BER) analyzer for receiver 1 with 5 km fibre length.  
Figure 7: BER analyzer for receiver 2 with 20 km fiber length.  
For longer distances, an optical amplifier was added, as shown in Figure 9. A specification of 20 dB gain and noise figure of 5 dB for the amplifier can be applied to the WDM-RoF system. The receiver with 20 km fibre, the BER analyzer gave a better-quality signal without distortion and noise, as shown in Figure 10a. For 60 km fibre length, the received signal had a BER of 4.56 \(\times\) 10\({}^{-10}\). The Q-factor of the received signal was 6.30, as shown in Figure 10b.  
Figure 8: BER analyzer for receiver 3 with 60 km fiber length.  
Figure 9: WDM-RoF system with amplifier.  
The performances of the receiver for these fibre lengths can be demonstrated in Table 1 without the amplifier and with the amplifier. From the table, it can be seen that the three parameters included in this simulation, BER, Q-factor, and the eye diagram, has been improved and these results show that with the amplifier, the WDM- Ref can be used for long-haul transmission system.