## 5 Hotspot Optical Fronthaul Using OFDM Multi-IF Band Transmission
### Simulated Setup  
#### 5.1.1 Single-wavelength link without ROADM  
For the performance evaluation of the reference single wavelength optical IFoF fronthaul link when OFDM signals are employed, we simulated the setup shown in Figure 6a. This setup relies on the simulation models described in the first 2 sub-sections of Section 4. With the chirping properties having been accurately modelled for two different modulation formats, i.e., single-carrier QPSK and 16 QAM, that were the only available experimental-verified results [41], this simulation model can be reliably employed towards evaluating the performance of the underlying fronthaul even for different modulation formats like OFDM, which are considered to be more suitable for the next-generation mobile traffic [46]. A different network performance is obviously expected when a different modulation format is used, suggesting that our simulation model can be reliably employed for validating the performance of different network configurations and different modulation formats before engaging into the real experimental deployment of the system. A number of four OFDM signals were electrically generated by the respective OFDM encoder that was responsible for the distribution of the input bit sequence into 256 data streams by means of the Inverse Fast Fourier Transform (IFFT) method and the encoding of each SC with 64-QAM modulation format. The real and imaginary I and Q electrical constituents at the output of each OFDM encoder were then pulse shaped by RRC filters with a roll-off factor equal to 0.2 and up-converted to the  
Figure 6: Simulated setup of the IFoF optical fronthaul using OFDM, for (**a**) the reference link employed for the transmission of a single \(\lambda\) and (**b**) the hotspot link comprising the optical WDM Tx at the BBU and the ROADM network at the RRHs’ side. respective IF frequency by the multiplication with the cosine and sine waves of the respective LO. These signals are then summed to give the transmission signal of each IF band [66]. The up-converted OFDM signals at the different IF frequencies were SC multiplexed and fed into the EAM section of the EML in order to modulate a CW in the window of 1536.9-1539.25 nm provided by the laser section of the EML. Table 1 summarizes the format parameters of the multi- IF band OFDM signal for the employed SC modulation format of 64-QAM. Four IF bands at 4.4 GHz, 5 GHz, 5.6 GHz, and 6.2 GHz were used with each individual band occupying a bandwidth of 0.5 GHz and consisting of 256 SCs. Each of the SCs has a baud rate of 1.953 Mbd, giving a total data rate of 3 Gb/s per IF band. The OFDM waveforms were separated by a guard band of 100 MHz, while the aggregate data rate of the four channels loaded on the EML optical input was equal to 12 Gb/s, occupying bandwidth of 2.3 GHz. The voltage swing of the multi-IF band signal inserted into the EAM was set at 0.7 Vpp, while the bias voltage of the EAM was equal to -0.4 V in order to ensure operation at the linear regime. The resultant IFof data signal exiting the EML with a maximum optical power of 2.8 dBm per wavelength [55], was transmitted through a standard SMF to the PD for o/e IF conversion. After the amplification by the low noise TIA, the output aggregated multi-IF bands were sent to the evaluation unit, where they were down-converted by the multiplication with the respective LOs, pulse-shaped and decoded by the OFDM decoder, emulating the signal processing operations that would normally be performed at the wireless end-users' site. The OFDM decoder was based on a typical signal processing circuit that performs the OFDM demodulation by means of the nc-points FFT algorithm [66]. After the decoding of the OFDM-64 QAM signals, the resultant streams were fed into the oscilloscope (OSC) and the EVM tester for the evaluation of the system. A variable optical attenuator (VOA) was employed to vary the optical power entering the PD for the EVM measurements.