## 5 Hotspot Optical Fronthaul Using OFDM Multi-IF Band Transmission
### Error Vector Magnitude (EVM) Performance of the Simulated Fronthaul Link
This additional signal distortions within the IF bands of the WDM channel is also consistent with the EVM analysis presentedin Figure 7. Particularly, as shown in Figure 7c,e,g,i, the EVM values during WDM transmission are ranging between 5.1-7.1%, while the respective EVM values the single-wavelength transmission were lower, ranging between 4-5.2% (Figure 7a). Figure 10 shows the received EVM for the worst performing SC of the multi-IF transmission versus the ROP, for each of the WDM channels \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\) transmitted through the first three ROADMs and dropped by the fourth ROADM. A fiber length of 500 m was employed in order to interconnect the BBU with the first ROADM of the optical bus. The respective EVM curves for the single-\(\lambda\) transmission of \(\lambda_{1},\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\) without the use of ROADMs are also presented in Figure 10.  
Figure 10: EVM versus ROP (dBm) for the worst performing SC of each WDM channel. Respective EVM curves for the single-\(\lambda\) transmission at \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\). Figure 9: IF Spectra of (**a**) the OFDM 64-QAM signal inserted the EML, (**b**) o/e converted and amplified signal exiting the TIA for the reference link without any ROADM, and (**c**) the signal exiting the TIA for the hotspot link with the first three ROADMs in through mode and the ROADM4 in drop mode. As it is shown, all WDM channels exhibit similar performance, achieving EVMs within the 3 GPP threshold of 8% when the ROP is higher than \(-\)14.3 dBm. It should be also noted that a relatively low power penalty of 1.2 dB was obtained with respect to the single-\(\lambda\) transmission at the EVM of 8%. A power budget analysis of the proposed hotspot fronthaul architecture was carried out to calculate the actual ROP of the signal reaching PD of the fourth RRH. The purpose of this analysis is to compare the ROP calculated by the power budget with the required ROP of \(-\)14.3 dBm, dictated by the EVM measurements (Figure 10). Table 2 summarizes the power budget parameters of the main building blocks incorporated in the WDM hotspot link. By considering an EML modulated average TOP of 2.8 dBm [55] and subtracting the losses of the MUX, SMF [59] and the four ROADMs (Section 4.3), an actual ROP of \(-\)11.3 dBm was calculated before the PD of the fourth ROADM for a SMF length of 500 m. The fact that the ROP of the power budget analysis is higher than the ROP of \(-\)14.3 dBm required in order to achieve an EVM of 8%, showing actually a power margin of 3 dB (3 dB = \(-\)11.3 dBm \(-\) (\(-\)14.3 dBm)), indicates that all WDM multi-IF band signals can be successfully transmitted over the proposed hotspot fronthaul link with sufficient power budget. The EVM values of the worst performing SC for the ROP of \(-\)11.3 dBm (maxTOP = 2.8 dBm) was found to range between 5.6%-6.3%. The minimum and maximum EVM values correspond to the \(\lambda_{1}\) and \(\lambda_{4}\), respectively. The respective EVM value for the moderate TOP of 1.1 dBm, used for the EVM versus SC index measurements, was found to be 7-7.1%.