## 5 Hotspot Optical Fronthaul Using OFDM Multi-IF Band Transmission
### Error Vector Magnitude (EVM) Performance of the Simulated Fronthaul Link  
Initially, the performance of the reference single-wavelength link without any ROADM, using four aggregated OFDM IF channels modulated with 64-QAM format, was examined. The SMF length interconnecting the EML with the VOA of the Rx was considered equal to 500 m, based on the typical stadium periphery specifications. The total received optical power (ROP) before the PD was adjusted to be equal to \(-13\) dBm for all IF bands for comparison purposes against the WDM hotspot link. Figure 7a shows the EVM values per SC for each IF channel, revealing similar performance for all IF OFDM signals with an average EVM of 4.6%. Figure 7b depicts two indicative constellation diagrams for the worst-performing SCs of the outermost IF bands of 4.4 GHz (IF1) and 6.2 GHz (IF4), exhibiting EVM values of 5.1% and 5.2%, respectively. It should be noted that the performance of all SCs was within the acceptable threshold of 8% as it is defined by 3 GPP for the base station radio transmission and reception of NR carriers with 64-QAM format [68; 69]. After the evaluation of the single-wavelength link, we investigated the additional degradation added by the 4-\(\lambda\) WDM transmission and the ROADM modules for the longest reach RRH of the hotspot fronthaul architecture, i.e., when the WDM data signals crossed through the first three ROADMs and being dropped at the last ROADM. This study corresponds to the single origin capacity problem that requires all the resources to be allocated in a specific area of the hotspot. The SMF length interconnecting the BBU with the first ROADM was considered equal to 500 m and the ROP corresponding to each wavelength was equal to \(-13\) dBm, resulted by taking into account the maximum losses of the individual devices employed in the hotspot link and a moderate transmitted optical power (TOP) per wavelength equal to 1.1 dBm. This value is lower than the maximum available TOP of 2.8 dBm that each EML can output for modulated signals [55] and it was chosen in order to present the EVM measurements for a non-optimal ROP. Figure 7c presents the EVM values per SC for all IF channels of the first WDM wavelength of 1536.9 nm. As it is shown, all IF bands at the center frequencies of 4.4 GHz, 5 GHz, 5.6 GHz, and 6.2 GHz exhibited similar performance with an average EVM value of 6.3%. The constellation diagrams for the worst performing SC of the first IF the last IF band are depicted in Figure 7d, exhibiting EVMs of 7% and 6.9%, respectively. Similar performance was obtained for all the other WDM channels with EVM values within the threshold of 8%. Figure 7e shows the EVM measurements versus the SC index for all four IF bands carried by the second WDM wavelength of 1537.67 nm. As it can be seen, all IF bands exhibited again similar performance with the EVM values of the SCs ranging between 5.2% and 7.1%. Despite these slight deviations, all SCs achieved EVM values within the 3 GPP threshold limit of 8%. Figure 7f presents the constellation diagrams for the worst performing SCs of the first and last IF, exhibiting an EVM of 7.1% and 7.05%, respectively. Figure 7g,h depict the respective EVM measurements and constellation diagrams for the third WDM wavelength of 1538.46 nm, respectively, while Figure 7i,j present the respective set of results for the fourth WDM channel at 1539.25 nm, exhibiting similar performance within the 8% EVM limit. Figure 7: Link w/o ROADM and single \(\uplambda\) transmission of OFDM 64-QAM: (**a**) EVM per SC for all IF bands for \(\uplambda_{1}\) at a ROP of \(-\)13 dBm, (**b**) constellation diagrams of the worst performing SC for the first and last IF band. Hotspot WDM Link with ROADM: (**c**) EVM per SC for all IF bands for WDM1 (\(\uplambda_{1}\) = 1536.9 nm) at a minimum ROP of \(-\)13 dBm, (**d**) constellation diagrams of the worst performing SC for the first and last IF band at \(\uplambda_{1}\).