## 5 Hotspot Optical Fronthaul Using OFDM Multi-IF Band Transmission
### Error Vector Magnitude (EVM) Performance of the Simulated Fronthaul Link
Figure 8b-e show the EVM values per SC of all IF bands for the downlink WDM channels at \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\), respectively. Figure 8b reveals that the signal carried by \(\lambda_{1}\) and dropped by ROADM 1 exhibited SC EVM values between 3.15-3.7%. Figure 8c,d present the EVM values for \(\lambda_{2}\) and \(\lambda_{3}\), exhibiting EVM values between 3.3-4.1% and 4.6-5.7%, respectively. Finally, Figure 8e shows that the signal at \(\lambda_{4}\) exiting the last ROADM exhibited the worst performance with the EVM values of the different SCs ranging between 5-7.1%. By comparing the performance of the different WDM channels, it is observed that as the available optical power of the dropped signals was decreasing due to the ROADM losses, the EVM values were increasing. A maximum EVM difference of 3.5% was found between the worst performing SC of the different wavelength channels. By comparing the EVM measurements obtained for this multi-origin downlink capacity problem with the respective results for the single-origin capacity problem presented in Figure 7c,i, similar performance was shown for the fourth WDM channel. The performance of the other three channels (WDM1-WDM3) was found to be better for the multi-origin capacity case. Figure 8f-i present the respective EVM values per SC of all IF bands for the uplink WDM channels at \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\), respectively. Figure 8f reveals that the signal carried by \(\lambda_{1}\) and added by ROADM 1 exhibited SC EVM values between 3.1-3.7%. Figure 8g,h shows the EVM values for \(\lambda_{2}\) and \(\lambda_{3}\) added by ROADM 2 and 3, exhibiting EVM values between 3.3-4.1% and 4.6-5.7%, respectively, Figure 8i presents the EVM values for the signal carried by \(\lambda_{4}\) exiting the last ROADM. This channel exhibited the worst performance when compared with the other uplink channels with the EVM values of the different SCs ranging between 5.1-7.1%. When comparing the performance of the downlink and uplink fronthaul link, similar performance was observed. Figure 9 presents the IF spectra diagrams for three different cases: (a) the initial aggregated 4-IF band signal inserted into the EML, (b) the signal exiting the photo-receiver (PD + TIA) of the reference link without any ROADM, and (c) the respective signal transmitted through the WDM hotspot link and exiting the last ROADM. Figure 9a presents the normalized spectrum of the aggregated multi-IF band signal that was electrically generated at the Tx side and entered the EML to modulate the wavelength of 1536.9 nm (\(\lambda_{1}\)). As it can be seen, the spectral power density was constant for all IF bands of the generated signal. Almost constant power spectral density was also shown in Figure 9b for the o/e converted signal exiting the photo-receiver (PD + TIA) of the single-wavelength link where no ROADMs were used, implying a similar performance for all IF channels, as it has already been confirmed in Figure 7a,b. A spectral behavior with almost constant power spectral density was also revealed for the o/e converted multi-IF band signal at the output of the TIA connected with the drop port of the fourth ROADM, as shown in Figure 9c. In this case, the signal was transmitted through the WDM hotspot link shown in Figure 6b. This is an evidence that the EML, the fiber length of 500 m, as well as the ROADM device have affected uniformly the performance of the different IF bands, justifying the results showed in Figure 7c-j. Focusing on the spectral content of each IF band of the WDM channel shown in Figure 9c, it can be also observed that the individual SCs exhibited different power levels with a non-uniformity of \(\sim\)2.5 dB. The SCs of the respective IF band during single wavelength transmission (i.e., no ROADMs) exhibit better power uniformity with a value equal to 1 dB as shown in Figure 9b.