# A 100 Gbps OFDM-Based 28 GHz Millimeter-Wave Radio over Fiber Fronthaul System for 5G
## 4 Remote Radio Head (RRH) System  
The RRH system is made up of the OTR down-conversion system, DSP compensation unit and an OFDM receiver system. The OTR is an optical detection system consisting of photodetectors. In this paper, OCD or DD is used to implement the OTR system. Figures 6 and 7 illustrates the coherent and direct detection systems, respectively. In the OCD system, the incoming optical signal from the fiber coupled with a coherent signal generated from a narrow-linewidth laser with a launch power of \(-2\) dBm is combined using fiber couplers and the signal is detected by two pairs of photodetectors. The polarization controller (90\({}^{\mathrm{o}}\) phase shift) is used to perform mixing of the signals to match the polarization states of the two photodetectors. The two optical signals in the complex form are expressed as;  
\[\mathrm{E_{ROS}}=\mathrm{A_{ROS}}\exp[-\mathrm{i}(\omega_{\mathrm{o}}t+\varphi _{\mathrm{ROS}})], \tag{11}\]  
where \(\mathrm{A_{ROS}}\) is the amplitude, \(\omega_{\mathrm{o}}\) is the carrier frequency and \(\varphi_{\mathrm{ROS}}\) is the phase. However, the optical field associated with the LO is also expressed as;  
\[\mathrm{E_{LO}}=\mathrm{A_{LO}}\exp[-\mathrm{i}(\omega_{\mathrm{LO}}t+\varphi _{\mathrm{LO}})], \tag{12}\] \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Fiber Attenuation Coefficient, \(\alpha\) & 0.2 dB/km \\ Reference Wavelength & 1550 nm \\ Dispersion, D & 16.75 ps/nm/km \\ Differential Group Delay & 0.2 ps/km \\ Fiber Length & 5–35 km \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Single Mode Fiber (SMF) simulation parameters. Figure 5: RoF transport link system configuration. where \(\rm{A_{LO}}\) is the amplitude of the local oscillator, \(\rm{\omega_{LO}}\) is the frequency of the local oscillator and \(\rm{\varphi_{LO}}\) is the phase of the local oscillator. Assuming that the two optical fields, \(\rm{E_{ROS}}\) and \(\rm{E_{LO}}\) are identically polarized, the total optical power incident at the photodetector is given as \(\rm{P=}{\left|{E_{ROS}+E_{LO}}\right|^{2}}\). However, the two equations result in the equation below;  
\[\rm{P(t)=P_{ROS}+P_{LO}+2\sqrt{P_{ROS}P_{LO}}\cos(\omega_{IF}t+\varphi_{ROS}- \varphi_{LO})} \tag{13}\]  
where \(\rm{P_{ROS}=A_{ROS}}^{2}\), \(\rm{P_{LO}=A_{LO}}^{2}\), and \(\rm{\omega_{IF}=\omega_{O}-\omega_{LO}}\). This output signal is demodulated by a quadrature demodulated. The filtered I and Q components are demodulated according to the following expressions respectively;  
\[\rm{V_{I}(t)=}[\rm{GV_{in}(t)}\ \cos\ (2\pi f_{c}t+\phi)]*h_{low}(t) \tag{14}\]  
\[\rm{V_{Q}(t)=}[-\rm{GV_{in}(t)}\sin(2\pi f_{c}t+f_{c})]*h_{low}(t) \tag{15}\]  
where, \(\rm{V_{in}}\) is the electrical signal at the input, G is that gain, \(\rm{f_{c}}\) is the carrier frequency, \(\rm{\phi_{c}}\) is the carrier phase and \(\rm{h_{low}}\) is the response time of the internal low pass filter (LPF). In-Phase, Quadrature Imbalance and Non-Linear Dispersion Compensation Based Digital Signal Processing  
In this paper, a DSP compensation scheme is implemented in the receiver system to improve the system performance. The DSP scheme is used to compensate for the in-phase and quadrature imbalances which may occur at several points along the transmission path. These imbalances may occur from inappropriate bias voltage settings of the MZM, misalignment of the polarization controller and the photodetector responsivity mismatch. Figure 6: Illustration diagram of the implemented coherent detection system in Optisystem. Figure 7: Illustration diagram of the implemented direct detection system in Optisystem. Figure 8 illustrates the block diagram of the DSP unit in Optisystem. Also, the DSP scheme is used to compensate for chromatic dispersion and non-linearity.