## 4 Simulation Modeling
### Externally Modulated Laser (EML) Chirp Simulation Model  
Since in the previous sub-section the adequate matching of the EML's static behavior was achieved, the next step was to provide a fully-matched simulation model of the EML including also an approximation of its chirping effect. To this end, an analog optical IFoF link was simulated using: (a) a fiber simulation model that its response was matched with a commercial available SMF [59] including all the known parameters such as losses, dispersion, dispersion slope, etc., and (b) the EML model with the known static response and the under investigation chirp response. To goal of this study was to approximate the EML's chirping effect by matching the performance of this simulated link with the performance of an experimentally-evaluated optical IFoF link that employed the actual fabricated EML and SMF transmission, as presented in [41]. It should be noted that, in the experimentally-measured link the EML static response, the fiber characteristics, the power of the signal at various stages of the link as well as the noise figure of the electronics were all known values and the only unknown factor was the chirping effect of the EML. Figure 3a shows the simulated setup employed for the analog IFoF fronthaul link. A number of m (m = 4,6) QPSK or 16-QAM data signals were electrically generated and each of them up-converted to a different IF frequency by the multiplication with the respective local oscillator (LO). Pulse shaping was implemented on the data signals by using root raised cosine (RRC) shaping filters with roll-off factor equal to 0.2. The IF-up converted signals were SC multiplexed and inserted into the EML in order to modulate a continuous wave (CW) at 1550 nm. The IFoF signal exiting the EML was fed into a  
Figure 2: Matching of the experimentally-measured and simulated (**a**) power versus volts transfer functions of the externally modulated laser (EML), and (**b**) S21 response of the EML. SMF for transmission to the Rx. In order to investigate the role of the SMF link, a fiber length of 7 km was employed as in the experimental link presented in [41]. The Rx comprised a PD with responsivity of 0.7 A/W that converted the optical IFoF signal to the respective electrical SC multiplexed-IF signal and a low-noise TIA with 20 dB gain. After down-conversion and pulse shaping, the resultant baseband signals were sent to the EVM tester for the performance evaluation. To achieve the performance matching of the simulated and the experimentally-verified IFoF link presented in [41], we considered a dispersion coefficient of 17 ps/(nm\(\cdot\)km) at the reference wavelength of 1550 nm and a dispersion slope of 0.08 ps/(nm\({}^{2}\cdot\)km), as it is the case for a typical SMF [59]. For the same purpose, we approximated the EML's frequency chirp effect by including in our EML model, a voltage-dependent chirp factor that varies from 3.5 to \(-4\), similar to experimentally observed values presented in [60]. Figure 3b shows the simulated and experimentally-measured EVM values for a four-band SC multiplexed transmission of QPSK and 16 QAM data signals at a symbol rate equal to 1 Gbd. The IF frequencies of the four SCs were equal to 0.625 GHz, 1.875 GHz, 3.125 GHz, and 4.375 GHz, respectively, while the fiber length was equal to 7 km. As it is observed, the performance of the simulated link followed closely the performance of the experimentally evaluated fronthaul link. For both modulation orders the EVM slightly increased for the higher IF bands, while the use of 16-QAM slightly deteriorated the performance of the link when comparing with the QPSK results. Finally, Figure 3c presents the EVM measurements for a six-band SC signal modulated with QPSK format at 1 Gbd symbol rate. As it can be seen, the simulated and experimental performance were quite similar, both showing an increasing EVM for the last two IF frequencies after 7 km fiber transmission.