# Low Complexity DSP for High Speed Optical Access Networking +
## 1 Introduction  
Today's fastest standardized passive optical network (PON) is the 25 Gb/s per lane Ethernet PON [1]. Driven by bandwidth hungry applications such as high definition video services and virtual reality, a higher lane rate PON is under study by Telecommunication Standardization Sector of the International Telecommunications Union (ITU-T) [2]. An alternative approach is to increase the lane count by ultra-dense wavelength division multiplexing (UWDM) with central office consolidation and extended geographic coverage. On this topic, the Institute of Electrical and Electronics Engineers (IEEE) 802.3 cs super PON standard under development supports a 50 km reach and a larger than 1:64 split ratio but requires a wavelength router in the field. As the attractiveness of PONs is very sensitive to cost and power consumption, it is highly desirable that high speed PONs leverage the optical component ecosystem of today's mature 10 Gb/s PONs, where intensive efforts have been made by the research community [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Higher spectrally efficient modulation schemes and advanced digital signal processing (DSP) must be employed to achieve such targets. Examples include chirp managed non-return-to-zero (NRZ) [3], electrical/optical Duobinary [4, 5, 6, 7], four-level pulse amplitude modulation (PAM-4) [3, 7, 8, 9, 10, 13] and PAMswith even more amplitude levels [15], as well as more complex multi-carrier [5; 9; 11] or multi-band modulation schemes [12]. Notably, PAM-4 has been chosen as the format for high speed Ethernet standards [16] for data center (DC) applications due to its implementation simplicity. High-volume PAM-4 chips are commercially available. However, at >25 Gb/s bit rates, implementing PAM-4 with simple linear equalizers [7; 8; 17; 18] to support a transmission distance of 20 km over a single-mode fiber (SMF)--as required in access networks--using only 10-G optoelectronics [7], is extremely challenging, unless higher bandwidth transmitters or receivers (e.g., 25 Gb photodiodes [8]), or a dedicated chromatic dispersion compensation module [4] are adopted. It has been shown that reach extension is possible by mitigating system and component nonlinearities using DSP without changing the optical infrastructure [18; 19]. It is worth mentioning that, alternatively, there are research studies proposing to employ coherent-lite technologies [20; 21; 22; 23]. Coherent reception enables a much higher achievable transmission capacity as well as longer transmission distances but requires a higher number of devices. However, the work presented in this paper is limited to simple direct detection (DD) applications using PAM-4 at the C-band. Although previous DD PAM-4 systems with improved capacity [24; 25] have been demonstrated, they required the use of sophisticated neural network (NN) nonlinear equalization [24], or an O-band where chromatic dispersion is much smaller [25]. Moreover, none of the previous DD PAM-4 demonstrations considered a simpler nonlinear equalization architecture with noise suppression. This work proposes a novel 40 Gb/s lane rate PON architecture that includes a high-performance DSP unit, 10-G optoelectronics and at the same time eliminates the need for a power-hungry radio frequency (RF) driver from the transmitter. Thanks to the adoption of shared bi-directional optical amplifiers (OAs) in the optical line terminal (OLT) site, as standardized in the 10-G lane PON, the optical launch power can be maintained at a high level in order to guarantee a high optical power budget. The technical challenge is that the nonlinearity attributed to signal-signal beating upon square-law detection must be tackled and traded off with the carrier-to-signal power ratio (CSPR) of the modulation scheme. Our novel DSP approach uses a nonlinear adaptive DSP with zero overheads at the receiver side to effectively tackle the nonlinearity issue. The nonlinear DSP can be further simplified and flexibly reconfigured, guaranteeing a low-cost and low-power receiver.