## 5 Hotspot Optical Fronthaul Using OFDM Multi-IF Band Transmission
### Error Vector Magnitude (EVM) Performance of the Simulated Fronthaul Link
(**e**) EVM per SC for all IF bands for WDM3 (\(\uplambda_{3}\) = 1538.46 nm) at a minimum ROP of \(-\)13 dBm, (**f**) constellation diagrams of the worst performing SC for the first and last IF band at \(\uplambda_{3}\). In order to investigate the scenario where the available downlink capacity resources is needed to be uniform distributed on the hotspot area (denoted as the multi-origin capacity problem), we carried out EVM measurements for the case that the WDM channels were dropped one by one in the peripheral of the stadium by the respective ROADMs. The multi-origin capacity scenario was investigated also for the uplink fronthaul link by performing EVM measurements for the WDM channels that were consecutively added by the respective ROADMs of the uplink bus and received by the array of PDs at the centralized BBU side. Figure 8a presents the employed setup for the simultaneous downlink and uplink transmission of four WDM channels, each one carrying four IF band signals with the format parameters presented in Table 1. Focusing on the downlink direction, the BBU's Tx has transmitted four WDM channels as shown in Figure 6b, with each of them chosen to exhibit a TOP equal to 1.1 dBm. The WDM signal was transmitted through a 500 m length SMF to the hotspot, where the WDM channels at \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\) were consecutively dropped from ROADMs 1-4, respectively. The signal at the output of each ROADM was received by a photo-receiver (PD + TIA) before entering the evaluation unit shown in Figure 6b. Taking into account the power budget of the downlink and a TOP equal to 1.1 dBm, the ROP at the output of the fourth ROADM was found to be equal to \(-13\) dBm. Considering that the total loss of the ROADM module was equal to 3.1 dB, the ROP values at the output of the ROADMs 3, 2, 1 have been calculated equal to \(-9.9\) dBm, \(-6.8\) dBm, and \(-3.7\) dBm, respectively. Figure 8: (**a**) Simulation setup of the WDM downlink and uplink optical bus where downlink \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\) data signals are subsequently dropped by ROADMs 1–4 and uplink \(\lambda_{4}\), \(\lambda_{3}\), \(\lambda_{2}\), \(\lambda_{1}\) data signals are subsequently added by ROADMs 4,3,2,1, respectively. EVM per SC for all IF bands for the (**b**) downlink WDM1 (\(\lambda_{1}=1536.9\) nm) at a ROP of \(-3.7\) dBm, (**c**) downlink WDM2 (\(\lambda_{2}=1537.67\) nm) at a ROP of \(-6.8\) dBm, (**d**) downlink WDM3 (\(\lambda_{3}=1538.46\) nm) at a ROP of \(-9.9\) dBm and (**e**) downlink WDM4 (\(\lambda_{4}=1539.25\) nm) at a ROP of \(-13\) dBm, (**f**) uplink WDM1 (\(\lambda_{1}=1536.9\) nm) at a ROP of \(-3.7\) dBm, (**g**) uplink WDM2 (\(\lambda_{2}=1537.67\) nm) at a ROP of \(-6.8\) dBm, (**h**) uplink WDM3 (\(\lambda_{3}=1538.46\) nm) at a ROP of \(-9.9\) dBm and (**i**) uplink WDM4 (\(\lambda_{4}=1539.25\) nm) at a ROP of \(-13\) dBm. Focusing on the uplink direction, each RRH added a different WDM channel into the uplink bus through the respective ROADM. Particularly, WDM channels at \(\lambda_{4}\), \(\lambda_{3}\), \(\lambda_{2}\), \(\lambda_{1}\) were consecutively added into the link from ROADMs 4, 3, 2 and 1, respectively, each one with a TOP equal to 1.1 dBm. The uplink 4-wavelength WDM signal at the exit of the hotspot was then transmitted through a SMF with a length of 500 m, demultiplexed by the DEMUX of the BBU and received by its array of photo-receivers as presented in Figure 1b. The ROP of the channels carried by \(\lambda_{1}\), \(\lambda_{2}\), \(\lambda_{3}\), \(\lambda_{4}\) was calculated equal to \(-\)3.7 dBm, \(-\)6.8 dBm, \(-\)9.9 dBm and \(-\)13 dBm, respectively, using the same procedure as in case of the downlink. Finally, the electrical converted signals at the output of the photo-receivers sent to the evaluation unit of the BBU, comprising the same circuit shown in Figure 6b for the end user.