## 4 Custom A-IFOF Transceiver Preliminary Evaluation and Integration to Deployed MNO's Infrastructure
Taking into account that the EVM measured at the electrical Tx output was 3% for the QPSK-OFDM format and 3.9% for the QAM16-OFDM signals, the short reach optical part of the setup, including the EML, the 7 km SSMF and the 14 GHz photoreceiver, degraded the 204 MHz signal's performance by 3.8% (for the QPSK-OFDM) and 3.7% (for the QAM16-OFDM) and the 398 MHz signal's EVM by 4.3% (for the QPSK-OFDM) and 4.5% (for the QAM16-OFDM), indicating the absence of strong limiting effects related to the active electro-optic module's response or to the fiber transmission. By extending the fiber spool link from a 7 km to 25 km fiber link, the obtained EVM penalty was slightly increased by less than 2.5%. This added EVM penalty mainly originated from the lower received optical power that was linked with the fiber loss, since the dispersion-induced power fading can be neglected for this low IF carrier frequency [34]. The second part of the present experimental study focused on the performance evaluation of a DL scenario by exploiting the deployed fiber-wireless fronthaul topology. Figure 5 illustrates an EVM bar diagram of the QPSK-OFDM and QAM16-OFDM modulated radio for both the 204 MHz and 398 MHz bands after 25 km of fiber transmission and OTA transmission over a 3 m horizontal distance using V-band radio equipment. The introduction of the active V-band radio part and the link caused an increase in the EVM performance of our link by less than 1.1% for the 204 MHz band, whilst the EVM performance was degraded by less than 2.5% for the wider version of radio bands at the 398 MHz band. The above EVM penalties are mainly associated with the IF/mmWave radio boards that included power amplifiers (PAs) and complex frequency translation stages. It should be also mentioned  
Figure 4: (a) EVM values for 204 MHz Bandwidth (BW) after 7 km and 25 km fiber transmission, (b) Constellation diagrams for 204 MHz BW after 7 km and 25 km fiber transmission, (c) Constellation diagrams for 398 MHz BW after 7 km and 25 km fiber transmission and (d) EVM values for 398 MHz BW after 7 km and 25 km fiber transmission. that the higher order QAM OFDM waveforms that were occupying wider bandwidths suffered from severe distortion that was reflected into their higher EVM values, while the presence of nonlinear distortion was also evident in all of the 16-QAM constellation diagrams. Nevertheless, the transmission in all of the cases was successful according to the 3 GPP specifications [33]. The next step of the presented work was to extend the measurements that are described above by also employing the receiver side of the developed A-IFoF transceiver. As a further step, by integrating the real-time transceiver over an existing mobile infrastructure the acquired measurements go beyond the EVM-based physical performance evaluation of the deployed setup by demonstrating the delivery of real-world services. The employed experimental setup is illustrated in Figure 6. The existing mobile infrastructure of Greece's largest mobile network operator (COSMOTE) was exploited, which (among other features) includes an evolved packet core (EPC) and a small cell. Between these two network units, the FPGA-based converged A-IFoF-mmWave setup was interposed. For the generation of the downlink/uplink (DL/UL) complex OFDM signals, two pairs of DACs and ADCs were employed for their digital up- and down-conversion at 1.5 GHz IF. RF baluns were also used for the conversion of the DACs/ADCs' differential inputs/outputs to single-ended, resulting in the generation of 350 mVpp RF signals at the outputs of the RFSo, C. A digital amplification stage at the ADC units offered the flexibility of receiving voltage swings varying between 150 mVpp and 1 Vpp without affecting the reception's performance. Moreover, the presented experimental setup was customized so as to emulate three different optical-wireless network layouts.