# A 100 Gbps OFDM-Based 28 GHz Millimeter-Wave Radio over Fiber Fronthaul System for 5G
## 6 Results  
The simulation results in this paper are presented for three modulation formats; 16-PSK, 16-QAM, and 64-QAM and comparing their performance each when either coherent detection or direct detection is implemented in the receiver system. In this paper, we first consider a case without DSP compensation and another case where DSP compensation is implemented. However, the simulation is investigated for an SMF of varying lengths from 5 km up to 35 km.  
The performance of this proposed OFDM-based RoF fronthaul system is evaluated using BER and QF. Table 2 shows the simulation parameters for the proposed system. Figure 9 illustrates the OFDM signal spectrums for 16-PSK, 16-QAM, and 64-QAM modulation formats at 28 GHz. As shown, the spectrums show peak OFDM power of approximately \(-51\) dBm, \(-52\) dBm, and \(-5\) dBm respectively.  
Figure 8: Block diagram of the Digital Signal Processing unit in Optisystem.  
However, we observe a change in the spectrum transmission bandwidth when the modulation type is changed by increasing the number of bits per symbol. The generated OFDM signals when modulated by the LiNbO\({}_{3}\) MZM modulator will result in a poor modulating optical signal with high harmonic powers levels at the sidebands of the modulating frequency.  
However, the optical modulating signals of the three modulation schemes are enhanced through filtering and amplification by using optical filters and amplifiers to improve the signal quality. The improved spectrums and spectrums after the modulating optical signal are transmitted over the optical fiber are shown in Figure 10. The spectrums after the modulating optical signal are transported over the optical fiber are again shown in Figure 10 (i.e., spectra of the optical fiber link for 16-PSK, 16-QAM, and 64-QAM).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Data Rate & 100 Gbits/s \\ Sequence Length & 32,768 bits \\ Number of bits/s & 4 bits/s and 16 bits/s \\ Number of Samples & 131,0/2 \\ Samples per bit & 4 \\ Number of OFDM subcarriers & 512 \\ Modulation Bandwidth & 2 GHz \\ Radio Frequency & 28 GHz mm-wave \\ Modulation Bandwidth & 2 GHz \\ SMF Attenuation Coefficient & 0.2 dB/Km \\ Chromatic Dispersion Coefficient & 17 ps (nm-km) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Simulation parameters for the proposed System.  
Figure 10: Spectrums showing improved optical signal quality through filtering and amplification.  
Figure 9: (a–c) are the up-converted modulated OFDM signal spectrums for 16-PSK, 16-QAM, and 64-QAM respectively at 28 GHz.  
In this instance, the length of the fiber is 5 km while the system data rate remains 100 Gbits/s. However, we observed that the spectrums show noise power levels of \(-37\) dBm for each of the modulation formats with variations in the signal power for each of the signal lobes. Figure 10 also illustrates the optical spectrum of the CW laser at a center frequency of 193.1 THz. The down-converted signal after detection by the coherent or direct detection receiver in frequency and time domain are shown in Figure 11 for the three modulation formats.  
Figure 11: (**a,c,e**) illustrates the down-converted OFDM signal at 28 GHz for 16-PSK, 16-QAM, and 64-respectively, (**b,d,f**) illustrates the signal in time domain respectively for 16-PSK, 16-QAM, and 64-QAM.  
In Figure 11a, the amplitude of the signal power is observed to be approximately 8 dBm while the noise power of the system is approximately \(-10\) dBm for 16-PSK format. In Figure 11c, the spectrum shows a received signal power of approximately \(-3\) dBm and noise power of approximately \(-18\) dBm when the 16-QAM format is used. In Figure 11e, the received signal power is approximately \(-6\) dBm and noise power of approximately \(-29\) dBm is observed when the 64-QAM format is used. However, from this observation the 64-QAM modulation format shows a higher signal to noise ratio compared to 16-PSK and 16-QAM formats.