## 2 Converged 5G mm-wave A-RoF OFDM Digital Communication System  
OFDM, a multicarrier modulation approach, is broadly used in wireless and optical communication systems due to its high bandwidth capability and its ability to counter both inter-carrier interference (ICI) and inter-symbol interference (ISI) using a cyclic prefix (CP) [5]. OFDM, when utilized with high-order QAM, enhances the system's efficiency, permitting a high throughput with low errors. Figure 1 shows a very basic block diagram for an OFDM transceiver with a converged 5G mmWave A-RoF system.  
A Pseudo-Random Binary Sequence (PRBS) is used to produce the bit rate, according to the modulation type and the used bitrate. The digital bits are divided into data flows, and each serial flow of input data is then converted to 'n' parallel bit flows of lower bitrates. In this way, symbol mapping is applied. Subsequently, the signal is modulated on to orthogonal subcarriers in the digital domain by applying the inverse fast Fourier transform (IFFT). Thus, a complex frequency domain signal is represented as a time domain signal. A CP is added after the IFFT to mitigate the inter-channel interference. The CP is a guard time interval placed at the start or at the end of each OFDM symbol after the IFFT. After adding the cyclic prefix, the parallel sub-channels are converted to serial sub-channels, and the resulting signal is fed into a DAC transforming the digital signal into the analog domain before transmitting the signal to a channel. Subsequently, a mm-wave signal at 60 GHz is generated, and the converged mm-wave and A-RoF signals are sent to a SSMF with a dispersion 17 ps/nm/km. The signal is then sent to a photodetector to convert it to an electrical signal.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Literature Review** & \begin{tabular}{c} **Approach/Methods** \\ **Discussed** \\ \end{tabular} & **Pros** & **Cons** \\ \hline \multirow{4}{*}{[16,19]} & Use cascade least & \multirow{4}{*}{Use electronic dispersion compensation} & BPSK is used and less \\  & & \begin{tabular}{c} variable step \\ size equalizer \\ \end{tabular} & & \begin{tabular}{c} sion compensation \\ \end{tabular} & transmission distance \\ \hline \multirow{4}{*}{[22]} & Modified CMA & \multirow{4}{*}{Simple algorithm} & Considered multi \\  & is used & & path channel instead \\  & & & of SMF \\ \hline \multirow{4}{*}{[23]} & Joint equalization by & \multirow{4}{*}{Better equalization} & Complex algorithm \\  & DD-LMS and & & and considered multi \\  & MCMA algorithm & & path channel instead \\  & & & of SMF \\ \hline \multirow{4}{*}{[24]} & Used blind nonlin- & \multirow{4}{*}{\
\begin{tabular}{c} Compensates fiber \\ nonlinearity and long \\ reach system \\ \end{tabular} } & Use EDFA’s for long \\  & & & reach, expensive \\ \hline \multirow{4}{*}{[This Work]} & Converged & \multirow{4}{*}{
\begin{tabular}{c} Electronic dispersion \\ compensation in \\ Matlab, no amplifiers \\ used, cost \\ signal processing \\ \end{tabular} } & Difficult to use higher \\  & & & order modulation \\ \cline{1-1}  & & & formats in \\ \cline{1-1}  & & & such system \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._The bottom part of Figure 1 shows the structure of an OFDM decoder. The ADC is required to convert the continuous received signal into discrete samples. Serial to parallel conversion is performed, and the cyclic prefix is removed. After removing the cyclic prefix, the FFT procedure includes the subcarrier transformation in the digital domain [30], and the signal is then changed back to the frequency domain. The parallel bit streams are then fed into a parallel-to-serial converter, and the QAM demodulator converts the received symbols into bits.