# A 100 Gbps OFDM-Based 28 GHz Millimeter-Wave Radio over Fiber Fronthaul System for 5G
## 2 Baseband Unit (BBU) System  
In this section, the BBU system consist of the OFDM transmitter and the RTO converter. The OFDM transmitter system consists of a pseudo-random binary sequence generator (PRBSG) used to generate a stream of binary digits (data) and a PSK or QAM sequence generator is used to generate a multi-bits-per-symbol sequence and to carry the number of bits per symbol. The OFDM technique is used to map the data of the baseband signal, and the OFDM signal generated is filtered by a low-pass cosine roll-off filter (LPCR-off filter) which is used to shape the modulated intermediated frequency (IF) carrier to achieve negligible inter-symbol interference. The resulting modulated OFDM signal is used to modulate the 28 GHz RF carrier generated by using a quadrature modulator (QM). The generated analog signal from the QM is then mapped unto the optical carrier generated from a continuous wave (CW) laser using external intensity modulation as shown in Figure 4.  
Figure 4: Block diagram of the Baseband Processing Unit system. Figure 3: Proposed OFDM-Based RoF Fronthaul system architecture. In this paper, 512 OFDM subcarriers with an average OFDM power of 15 dBm and 10 cyclic prefix points are set as simulation specification for the OFDM modulator. However, all the individual symbols required for one OFDM symbol period is given as:  
\[\text{OFDM Symbol Period}=\frac{\text{Individual Symbol Period}}{\text{Total Number of Sub-carriers Possible}} \tag{1}\]  
Also, the subcarrier frequencies are denoted as;  
\[\text{F}_{\text{sub}}=\frac{1}{\text{T}_{\text{symbol}}} \tag{2}\]  
where, \(\text{T}_{\text{symbol}}\) is the duration of an OFDM symbol. The implementation of the LPCR-Off Filter is based on the following transfer function given as;  
\[\text{H}(\text{f})=\left\{\sqrt{ \begin{array}{c}0.5\cdot\alpha^{2}\cdot \Big{[}1+\cos\Big{(}\frac{|\text{f}|-\text{f}_{1}}{\tau_{\text{P}}\cdot\Delta \text{f}_{\text{FWHM}}}\cdot\pi\Big{)}\Big{]}\\ 0\end{array} }\right\}, \begin{array}{c}(|\text{f}|<\text{f}_{1})\\ (\text{f}_{1}\leq|\text{f}|<\text{f}_{2}),\\ (\text{f}_{2}\leq|\text{f}|)\end{array}  \tag{3}\]  
where \(\alpha\) is the insertion loss, \(\text{f}_{\text{c}}\) is the filter cutoff frequency, and \(\text{r}_{\text{P}}\) is the roll-off factor specified as 0.2 and it is the measure of the excess bandwidth of the filter beyond the Nyquist bandwidth of \(\frac{1}{2\Gamma}\). The parameter \(\text{f}_{1}\) and \(\text{f}_{2}\) are also given as:  
\[\text{f}_{1}=1-\text{r}_{\text{P}}\text{f}_{\text{c}}\ \big{(}0\leq\text{r}_{ \text{P}}\leq 1\big{)} \tag{4}\] \[\text{f}_{2}=1+\text{r}_{\text{P}}\text{f}_{\text{c}}\ \big{(}0\leq\text{r}_{ \text{P}}\leq 1\big{)} \tag{5}\]  
If the excess bandwidth of the filter is denoted as \(\Delta\text{f}\) then the roll-off factor, \(\text{r}_{\text{P}}\) is expressed as:  
\[\text{r}_{\text{P}}=\frac{\Delta\text{f}}{\Big{(}\frac{1}{2\Gamma}\Big{)}}= \frac{\Delta\text{f}}{\frac{\text{R}_{\text{s}}}{2}}=2\Gamma\Delta\text{f}, \text{ where R}_{\text{s}}=\frac{1}{\Gamma} \tag{6}\]  
The QM as shown in Figure 5 is used to generate any desired RF signal. The local oscillator generates the RF signal such that the I and Q phase components are multiplied by a cosine and sine electrical carriers generated from an electrical wave generator and summing the IQ signal at the output. However, the total output signal is modulated according to:  
\[\text{V}_{\text{out}}(\text{t})\text{=G[I(t)\cos(2\pi\text{f}_{\text{c}}\text {t}+\text{$\phi$}_{\text{c}})-Q(t)\sin(2\pi\text{f}_{\text{c}}\text{t}+\text{$ \phi$}_{\text{c}})]+b} \tag{7}\]  
where, I and Q are the electrical signal, G is the gain, b is the bias, \(\text{f}_{\text{c}}\) is the carrier frequency, and \(\text{$\phi$}_{\text{c}}\) is the phase of the carrier. In this paper, an LiNbO\({}_{3}\) MZM as shown in Figure 4 is used to externally modulate the RF signal.