## 5 Hotspot Optical Fronthaul Using OFDM Multi-IF Band Transmission
### Simulated Setup
#### 5.1.1 Single-wavelength link without ROADM
1.2 Wavelength Division Multiplexing (WDM) Hotspot Link with Reconfigurable Optical Add-Drop Multiplexers (ROADMs)  
The proposed optical fronthaul architecture consisted of two identical optical buses extending around the stadium area, each one consisting of four ROADMs. Without loss of generality, we focused our study on one of the two identical buses in the downlink communication, as depicted in Figure 6b, yet equal optical performance is also expected for the second bus or the uplink communication. The 4-\(\lambda\) WDM Tx of the BBU comprised four EMLs modulating the respective CWs at 1536.9 nm, 1537.67 nm, 1538.46 nm and 1539.25 nm with channel spacing of 100 GHz. Each EML was driven by a different electrical multi-IF band OFDM signal resulting in an aggregate data rate of 12 Gb/s per \(\lambda\), exactly as the one used in the reference link of Figure 3a and equivalent to the experimental demonstration of the EML-based FiWi link [37]. The modulation format of 64-QAM was adopted for all SCs for each of the four OFDM IF up-converted signals. Table 1 provides a detailed description of the employed format parameters for each WDM channel as it has been already presented for the single-\(\lambda\) reference \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Modulation & OFDM 64-QAM \\ IF bands & 4 \\ IF center frequencies & 4.4, 5, 5.6, 6.2 (GHz) \\ Bandwidth per IF band & 0.5 GHz \\ Sub-Carriers per IF band & 256 \\ Sub-Carrier band-rate & 1.953 MBaud \\ Data rate per IF & 3 Gb/s \\ Channel guard band & 100 MHz \\ Bandwidth per \(\lambda\) & 2.3 GHz \\ Data rate per \(\lambda\) & 12 Gb/s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Format parameters of the multi-intermediate-frequency (IF) band signal. link. The outputs of the four EMLs were multiplexed by a 4:1 optical MUX into a 4 \(\lambda\)-WDM stream for fiber propagation from the BBU towards the cascade of four ROADMs across half of the stadium periphery, as shown in Figure 1. The data rate of the WDM signal serving half of the stadium was equal to 48 Gb/s (\(4\lambda\times 12\) Gb/s), resulting in a total data rate of 96 Gb/s for the proposed network, where two hotspot links were incorporated. Four ROADMs were employed for serving half of the stadium traffic, each of them being capable to add/drop the four wavelength carrying data from the BBU to the hotspot area. The ROADMs were placed at equidistant locations around the periphery of the stadium in a bus topology. The distance of the neighboring ROADMs was chosen to be equal to 80 m considering the actual diameter of Toumba's stadium [67]. In order to investigate the worst case scenario, for the longest reach ROADM of the downlink bus (ROADM4), we have evaluated the performance of a WDM signal that crossed through the first three ROADMs and all its WDM channels were being dropped by the last ROADM. The worst case scenario corresponds to a single-origin capacity problem, which may be obtained during a football match due to the gathering of the crowd in the fan zone of the stadium (possibly being placed on the longest reach RRH). The filtering response of the ROADMs was matched with the transfer functions derived by the physical-layer designs, presented in Section 4.3. After the optical signals were dropped by the last ROADM, they were inserted into the Rx of the fourth RRH comprising an array of four photo-receivers (PD + TIA) for o/e conversion and amplification. For validation purposes of the optical infrastructure, the outputs of the photo-receivers were sent to the evaluation unit in order to be down-converted, pulse shaped, and decoded, emulating the signal processing operations that would normally be performed by the wireless users' equipment. The decoded signals were then captured by the OSC and EVM tester. VOAs were used for controlling the received optical power entering the PD. Moreover, we evaluated the performance of the WDM signal when each of its wavelength-specific channels was consecutively dropped by the different ROADMs of the downlink optical bus.