# TDM-PON PAM Downstream Transmission for 25 Gbit/s and Beyond
## 1 Introduction  
The future of access networks is currently motivating research and standardization activities on Time Division Multiplexing Passive Optical Network (TDM-PON) solutions beyond 10 Gbit/s. After the IEEE 10G-EPON [1] and the International Telecommunication Union (ITU-T) XGS-PON (10 Gbit/s symmetrical bit rate capable PON) [2] norms, a new standard has been proposed in the ITU-T roadmap based on TDM-PON with a single channel per stream, which is referred to as High-Speed-capable PON (HS-PON) [3]. Different throughputs are investigated within the HS-PON framework but 25 Gbit/s and 50 Gbit/s seem to be serious candidates for future systems. The evolution towards higher data bit rates is mainly driven by the new mobile interfaces for 5 G, where 25 Gbit/s could be needed soon for either backhauling or high/low layer functional split interfaces. HS-PON could thus allow easier convergence between residential, business, and mobile networks [4]. Another technique to achieve higher bit rates for optical access networks is to use Wavelength Division Multiplexing (WDM). The Next Generation PON 2 (NG-PON2) standard gives specifications for this technique with 10 Gbit/s per wavelength [5]. Nevertheless, NG-PON2 technology is not deployed in access networks and here we focus on future TDM-PON in coexistence with legacy technologies deployed for residential. Modulation formats other than the standard Non-Return-to-Zero (NRZ) are being considered within the HS-PON study group. Multilevel formats such as Duo-Binary [6; 7; 8] and Pulse Amplitude Modulation with 4 levels (PAM4) [9; 10; 11], currently used in data center communication [12] and Visible Light Communications (VLC) [13], are good candidates to achieve 25 Gbit/s while keeping 10 GHz optics. State-of-the-art PAM4 reports devoted to optical access networks have, however, been limited in most cases to offline transmissions. Indeed, \(-14.5\) dBm of sensitivity (24.5 dB optical budget) for 28 Gbit/s PAM4 was demonstrated over 20 km [10]. A report on real-timePON applications demonstrated a 40 Gbit/s downstream PAM4 transmission in C band with 10 and 20 km reach and 26.5 dB and 24.5 dB optical budgets but with rather costly optics (Mach-Zehnder modulator, erbium-doped fiber amplifiers), high-sampling-rate Analog-to-Digital and Digital-to-Analog Converters (ADC and DAC), and equalization [14]. As far as the state of the art for 50 Git/s multilevel transmissions is concerned, the huge majority of assessments are again based on offline transmissions. For instance, 41.85 Gbit/s downstream PAM8 achieving a \(-13\) dBm receiver sensitivity with 10 G optics and Digital Signal Processing (DSP) was demonstrated over 20 km of fiber [15]. Another work showed a receiver sensitivity of \(-21.4\) dBm with 50 Gbit/s Electrical Duo-Binary (EDB) over up to 20 km in O band without DSP [6]. Those new modulation formats for the access network have additional requirements compared to the NRZ. For instance, PAM4 requires a linear driver to maintain equal-amplitude spacing between the four optical levels. However, the most complicated element of the PAM4 system design is the decoder that requires logical gates and limiting amplifiers to adjust the decision threshold. Although the Duo-Binary decoder is simpler with fewer logical gates and decision thresholds, encoding is more complicated especially for Optical Duo-Binary (ODB), which requires a DC biased Mach-Zehnder modulator. Finally, the NRZ format remains the simplest one for PON implementation. Channel equalization is a key solution to enhance the performances of those modulation formats, and is thus the subject of intense studies for either direct detection [16] or coherent detection [17]. Machine learning and deep learning [18] are now being introduced into PON research for efficient design of the equalization procedure. The choice of the modulation format will depend on the difficulties related to the implementation.