# Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs
## 3 Simulations
The performance is evaluated in the case of DMT modulation signals, considering DSB and SSB transmissions; in both conditions, the generated optical spectra have to occupy less than 25 GHz in order to be transmitted through cascaded WSSs, guaranteeing 25-GHz WDM spacing for fine granularity. For this reason, we tested a DSB DMT signal composed by 256 sub-carriers in 10 GHz range, i.e., the sub-carrier spacing is 39.062 MHz; occupying an optical bandwidth of around 20 GHz; a cyclic prefix (CP) of about 2.1% of the symbol length is also added. On the other hand, the SSB transmission is performed by properly detuning the WSS with respect to the carrier of a DMT signal composed by 256 sub-carriers in 20 GHz range, i.e., the sub-carrier spacing is 78.125 MHz; again, a CP of about 2.1% of the symbol length is added. Figure 3 shows the spectrum of the SSB signal obtained detuning the WSS in order to select half of the 20-GHz DMT DSB spectrum, while preserving the optical carrier. As can be seen, in order to mimic a realistic WSS transfer function with limited extinction, we imposed a maximum out-of-band attenuation of 40 dB. The left side of the optical spectrum is correctly filtered out, while a slight impact on the last subcarriers on the right appears due to the 21-GHz FWHM. Together with CD compensation, the receiver DSP provides digital symbol synchronization, CP removal, sub-carriers phase recovery, demodulation, and error count. In the following section, the transmitted capacity is evaluated performing [PERSON]'s bit loading algorithm [17] with a target BER of 3.8 \(\times\) 10\({}^{-3}\) (corresponding to 7% overhead hard decision FEC limit) for single-channel, single-polarization transmission. Figure 4a-f shows an example of the results obtained for DSB DMT modulation of the tunable VCSEL (\(\alpha\) = 2.75 \(\kappa\) = 8.3 \(\times\) 10\({}^{12}\)) in case of one WSS filtering and 40-dB OSNR. Figure 4a-f reports the received electrical spectrum, the bit loading, the BER per subcarrier and examples of received constellations. Figure 4: (**a–f**) Tunable VCSEL: DSB DMT (10 GHz) for single WSS filtering and 40-dB OSNR. (**a**) Received electrical spectrum; (**b**) Bit loading and corresponding EVM; (**c**) BER per subcarrier; (**d**) Bit-loading SNR; (**e,f**) Examples of received constellations. Figure 3: DMT SSB optical spectrum (red), obtained by a 9.5-GHz detuned WSS (orange) filtering of a DMT DSB signal with 20-GHz electrical bandwidth and 40-GHz optical bandwidth (blue). Figure 5a-f reports the same graphs for the tunable VCSEL SSB DMT transmission in case of one WSS filtering and 40-dB OSNR for a WSS detuning of 9.5 GHz with respect to the VCSEL carrier. By comparing the received spectra, it is evident that SSB has a higher spectral efficiency, exploiting all the available VCSEL bandwidth and filling the whole 25-GHz channel bandwidth with the multicarrier signal. On the other hand, as expected, SSB suffers more of signal-signal beating interference (SSBI) [18] and of VCSEL and WSS bandwidth limitations: in fact, at low and high frequencies, SSB modulation presents lower SNRs with respect to the central frequencies (Figure 5d), and first and last subcarriers can support constellations with a lower level number, i.e., lower bit/symbol (Figure 5b,e,f. DSB modulation, instead, presents a flat SNR over all the subcarriers, supporting 6 or 7 bit per symbol per subcarrier; nevertheless, only half of the available E/O spectrum can be occupied in order to fit the 25-GHz optical spectrum constraint, limiting the maximum achievable capacity. Figure 6 shows the capacities achieved for OSNR ranging from 30 dB to 40 dB in function of the number of crossed WSS in case of DSB DMT (Figure 6a) and in case of SSB DMT (Figure 6b). The results are reported for the two different chirp conditions considered so far.