); [PERSON].P. );  
[PERSON] (Q.Z.) 1  
[PERSON]  
1 Huawei Technologies Duesseldorf GmbH, Riesrasse 25, 80992 Munich, Germany;  
[PERSON].S. ); [PERSON].P. );  
[PERSON] (Q.Z.) 1  
[PERSON]  
1 Huawei Technologies Duesseldorf GmbH, Riesrasse 25, 80992 Munich, Germany;  
[PERSON].S. ); [PERSON].P. );  
[PERSON] (Q.Z.) 1  
[PERSON]  
1 Huawei Technologies Duesseldorf GmbH, Riesrasse 25, 80992 Munich, Germany;  
[PERSON].S. ); [PERSON].P. );  
[PERSON] (C.P. );  
[PERSON] (Q.Z.) 1  
[PERSON]  
1 Huawei Technologies Duesseldorf GmbH, Riesrasse 25, 80992 Munich, Germany;  
[PERSON].S. ); [PERSON].P. );  
[PERSON] (Q.Z.) 1  
[PERSON]  
1 Huawei Technologies Duesseldorf GmbH, Riesrasse 25, 80992 Munich, Germany;PAM-4. In [8], 248 Gb/s twin single-side band (SSB) discrete multi-tone (DMT) transmission is demonstrated using dual-drive (DD) MZM, while in [9], 100 Gb/s double side band (DSB) DMT enabled again by a DD-MZM is shown. In all aforementioned experiments, digital signal processing (DSP) at the receiver side is essential to recover the performance of the system and compensate for chromatic dispersion. However, in all aforementioned experiments [4; 5; 6; 7; 8; 9], a relatively high-cost and performance transmitter (i.e., either a MZM or a DD-MZM) and external laser sources are used to enable the transmission over SSMF. To reduce the cost and complexity of the system, low-cost, small footprint, and low-power dissipation lasers such as directly-modulated lasers (DMLs) together with PAM-4 format have been investigated to increase the data-rate in short reach links in the O-band due to the high chromatic dispersion tolerance provided by the SSMFs in that region [10]. In this context, 56 Gb/s PAM-4 has been demonstrated in [11] at 1310-nm wavelength using a non-linear compensation scheme for back-to-back transmission. Moreover, authors in [12] use the end-to-end frequency response to pre-compensate bandwidth limitations and a Volterra equalizer to post-compensate nonlinear signal distortion for a 1308.45-nm DMLs achieving 112 Gb/s PAM-4 transmission over 20-km SSMF. However, the drawback of DMLs is that they allow for lower modulation bandwidths, and they can introduce nonlinear signal distortion due to modulation dynamics of the laser. Therefore, the use of electro-absorption modulated lasers (EMLs) is preferable, as the latter provides higher modulation bandwidths and can be used as low-cost and low form-factor transmitters. In this direction, 100-GHz bandwidth EMLs have been reported in [13] for 116 Gb/s signal generation at 1550 nm, and 96 Gb/s/\(\lambda\) PAM intensity modulation/direct detection (IM/DD) transmissions employing an EML and digital equalization is reported in [14] for maximum distances up to 4 km SSMF. In our previous work [15], we demonstrated a record transmission of 56 Gb/s PAM-4 over maximum 20-km of SSMF using a 1550-nm EML at the transmitter side and FFE in conjunction with maximum likelihood sequence equalization (MLSE) at the receiver to compensate for CD, which becomes a limiting factor in such systems [11; 12; 13; 14; 15; 16]. In this work, we demonstrate a record transmission of 56 Gb/s PAM-4 over a transmission distance of 30 km. To achieve that, we optimize our transmitter and receiver implementations; i.e., we use a higher bandwidth digital-to-analogue converter at the transmitter and a higher sensitivity photo-diode with an integrated trans-impendence amplifier (TIA) at the receiver side. This enables the signal transmission over 20-km SSMF without the need for erbium-doped fiber amplifier (EDFA) employment, in contrast to [15], significantly improving the sensitivity requirements, and over 30-km by additionally pre-compensating for the end-to-end transfer function of the system. At the receiver side, we employ DSP consisting of a finite impulse response (FIR) filter that compensates partially for the accumulated CD, cascaded with an MLSE that is used to remove the residual inter-symbol-interference (ISI).