## 3 Results and Discussions  
In order to evaluate the data system performance in the presence of sensing system power, the EVM is analyzed at fixed sensing powers of 14, 16 and 18 dBm. In our experiments, the EVM values are estimated using Agilent VSA software, which is connected to the electrical spectrum analyzer. Firstly, the EVM performance is analyzed based on QPSK modulation using a bandwidth of 10 MHz at 2.6 GHz carrier frequency. Figure 2 shows the measured EVM values against the different data system powers. The EVM values at 1 dBm data power are \(\sim\)24.5%, \(\sim\)23.5% and \(\sim\)22.1% obtained with the sensing powers of 18 dBm, 16 dBm and 14 dBm, respectively. The optimum sensing power is 18 dBm for a 25 km sensing fiber length [13]. At 10 dBm data system power, the measured EVM values are approximately the same at \(\sim\)3.5% for the three different sensing system powers. The interference of the sensor signal and communication signal degrades the communication system performance, therefore, employing a BPF before the photodetector at the communication system receiver end is essential. Unfortunately, due to limited narrow band-pass filter availability, the wavelength spacing has been chosen in the experiments accordingly. The wavelength spacing between the distributed sensor and communication system can be closer to investigate the cross-impact on both the systems. This paper presents a proof-of-concept and feasibility of such emerging systems integration sharing a single fiber cable. In order to demonstrate the need for the optical BPF and mitigate the cross-interference in the proposed system, the EVM is analyzed with and without BPF at a fixed sensing system power \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fiber Communication System** & \multicolumn{2}{c}{**BOTDR Sensor System**} \\ \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline optical wavelength & 1550 nm & optical wavelength & 1546.12 nm \\ optical power & 0 to 10 dBm & input pump power & 14, 16 and 18 dBm \\ modulation scheme & QPSK, 16-QAM and 64-QAM & DD-MZM bandwidth & 12 GHz \\ bit rate (at channel & QPSK = 16 Mbps & & \\ bandwidth: 10 MHz) & 16-QAM= 33 Mbps & pulse width, period & 50 ns, 255\(\mathrm{\SIUnitSymbolMicro s}\) \\  & 64-QAM = 50 Mbps & & \\ baseband multiplexing & OFDM & pulse repetition rate & 4.2 kHz \\ carrier frequency & 2.6 GHz & pulse amplitude & 4 Vpp \\ channel bandwidth & 10 MHz & EDFA 1 gain, noise figure & 30 dB, 6 dB \\ RF power & 0 dBm & EDFA 2 gain, noise figure & 28 dB, 5.5 dB \\ MZM bandwidth & 20 GHz & ASE filter passband & 1545-1555 nm \\ EDFA gain, noise Figure & 28 dB, 4.8 dB & PMF length & 5 km \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Experimental system parameters. Figure 2: Measured error vector magnitude (EVM) of quadrature phase-shift keying (QPSK) for different data system powers at a fixed sensing power. of 18 dBm with QPSK modulation and the measured EVM values are shown in Figure 3. At 5 dBm data power the measured EVM values with and without BPF are 9.87% and 37.1%, respectively. Whereas, at 10 dBm data power the measured EVM values with and without BPF are 3.5% and 14.5%, respectively as shown in Figure 3. Without BPF, the sensing system power significantly influences the data signal, degrading the EVM performance. Thereafter, the EVM values are measured and analyzed for 16-QAM and 64-QAM modulation formats independently at fixed sensing powers, as illustrated in Figure 4a,b, respectively. In both modulation formats, at a 10 dBm data system power, the EVM values are approximately the same for various sensing powers. The obtained EVM values for 16-QAM and 64-QAM at 10 dBm data system power are \(\sim\)3.4% and \(\sim\)3.6%, respectively. The temperature effects have been measured in both data system and sensing system. The whole 25 km fiber was kept in a temperature-controlled oven and various temperatures were applied. The data power and sensing system powers are fixed at 10 dBm and 18 dBm, respectively.