# DSP-Based 40 GB/s Lane Rate Next-Generation Access Networks +
## 1 Introduction  
The continuous growth of bandwidth demand from residential users and enterprises urgently pushes telecommunication operators to upgrade the capacity of access networks to higher data rates. Among various access technologies that connect end users to metropolitan or regional networks, the passive optical network (PON) is the most efficient one, as it does not require any active components such as switches in the optical distribution network (ODN), instead, a passive power splitter or a wavelength router is usually deployed in the remote node. Figure 1 shows the standardizationroadmap of PONs released by FSAN in November 2016. The roadmap not only shows the evolving history of PONs, but also predicts the future PONs beyond today's standardized NG-PON2 which adopts time/wavelength division multiplexing (TWDM) technology capable of delivering 40 Gb/s transmission over four wavelengths [1]. The future access networks have to offer more features, such as system reconfigurability, flexibility, and a high availability of network equipment. Among these features, high capacity and long reach are of great importance, because of the significant bandwidth demand from emerging high-speed mobile services such as 5G Infrastructure Public Private Partnership (5G PPP) which anticipates to cope with a 1000-fold increase in data traffic, as well as the Internet of things (IoT). Low-cost mobile front-haul will become one of the major drivers for optical access networks. To meet the emerging service requirements, the roadmap also indicates that alternative ODNs such as wavelength-routed ODN (namely WDM-PONs) may also be considered, especially for green field deployments. Intensive research has been performed on upgrading today's TWDM-PON from its 10 Gb/s lane data rate into 25 Gb/s or 40 Gb/s. Much effort has been made to delivering beyond 10 Gb/s data by using existing 10-G, optics in order to upgrade the capacity of PONs in a cost and energy-efficient manner. Example demonstrations include 25 Gb/s, 40 Gb/s, or 50 Gb/s electrical Duobinary [2; 3; 4; 5; 6], optical Duobinary [3; 5; 7], and four-level pulse amplitude modulation (PAM-4) [2; 4; 5; 6; 8]. More complex multi-band or multi-carrier modulation schemes [9; 10] have also been demonstrated. These schemes have shown the feasibility of supporting 25 Gb/s or 40 Gb/s data rates using 10-G transmitters [2; 3; 7; 8] and/or 10-G receivers [2; 3], and thus they can offer low-cost solutions. The measurements use either optical domain dispersion compensation [2; 3], or simple digital signal processing (DSP) for dispersion compensation [4; 5; 6; 7; 8]. DSP is vital for enabling a practical low-cost realization of high-speed next-generation (NG) PON, without changing the optical infrastructure. In addition, DSP-based schemes have relatively simple transceiver structure, and thus they are potentially cost- and energy-efficient solutions. Despite various NG-PONs beyond the 10 Gb/s lane rate having been demonstrated, a fair complete comparison between different modulation schemes under a wide range of DSP configurations has been lacking. Electrical/optical Duobinary and PAM-4 represent relatively simple and efficient schemes for practical implementations. By examining and comparing the achievable performance and DSP complexity between 40 Gb/s lane rate electrical/optical Duobinary and PAM-4 NG-PONs, the choice of the optimum modulation scheme and DSP configuration would be identified for different  
Figure 1: FSAN standard roadmap released in 2016.  
application scenarios. This includes different cases with different fiber distances, and whether advanced or low-complex DSPs should be considered. The more complex multi-band or multi-carrier schemes may find superior advantages in long-reach PONs [9]; however, here we focus on the maximum achievable performance for simple single carrier schemes.