# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 4 Optical Transmission: Key Challenges and Proposed Directions
### Capacity Scaling: What to Expect in the F6G
To further understand this need, Figure 19 illustrates the progress over time of the downstream rate for the various PON standards. Figure 19 also designates the main advantages of each PON standard over its predecessor. These advances in bit rate from standard to standard are a result of the advances in the transceiver operational parameters, such as the modulation format, baud rate, operational wavelength, and number of wavelengths. It goes without saying that as the access part of the network is the most sensitive to cost and power consumption, direct detection technology still qualifies to increase the overall rate. Regarding the access rate evolution beyond 50 Gb/s, several options for realizing 100G-PON have been demonstrated. In the first option, which was presented by Huawei in BT's Innovation 2017 exhibition, the system exploited a ValkyrieBay chassis equipped with the unique Xena Loki-100G-55-1P 5-speed dual-media test module in order to generate Ethernet traffic through the 100G PON. The main purpose of this test was the verification of the overall implementation [76]. More specifically, the system exploited WDM, as four wavelengths were employed to carry 4 \(\times\) 25G data channels over a 10 km feeder fiber. This implementation is highly scalable as it can support different rates: 25G, 50G, or 100G. The second option is a 100G PON Prototype developed by ZTE [77], which is an integration test platform featuring many capabilities as it can simulate various modulation formats, like Non-Return-to-Zero (NRZ), Duobinary, PAM4, and DMT (Discrete Multitone), whilst various DSP algorithms can be employed. Moreover, it considers four 25G wavelengths supporting the following four combinations: Asymmetric single-wavelength uplink 10G/downlink 25G, symmetric single-wavelength uplink/downlink 25G, asymmetric four-wavelength uplink 40G/downlink 100G, and symmetric four-wavelength uplink/downlink 100G. The feasibility of this prototype was verified at MWC 2017 in Barcelona. The third 100G PON solution was developed by Nokia Bell Labs and validated in Vodafone's Competence Centre in Eschborn, Germany [78]. This option is based on a single wavelength using a combination of 25G optics and advanced DSP techniques to reach 100 Gb/s. As the 100G PON (or even beyond 100G) commercialization is planned for the second half of  
Figure 19: Evolution of bit rate in the downstream direction for various PON standards and the road towards 100G PON (or even beyond 100G). the decade, the next PON standard is expected to be the result of a significant amount of work and conveyed knowledge among academia, operators, vendors, and standardization organizations in order to adopt the most cost and energy efficient solution. One of the main aspects of the adoption of PON technology is the maximization of spectral efficiency. Ideal candidates for achieving this target are Nyquist wavelength division multiplexing PON (Nyquist-WDM-PON) and orthogonal frequency division multiplexing WDM PON (WDM-OFDM-PON). Both solutions can (a) provide a finer granularity compared with Time Division Multiplexing (TDM)-PON schemes, (b) enable the bandwidth assignment on an as-needed basis, and (c) maximize spectral efficiency as they have a rectangular spectrum. In particular, Orthogonal Frequency Division Multiple Access (OFDMA) PON is a well-established technology in wireless and fixed access systems such as WiMAX, long-term evolution (LTE), Asymmetric Digital Subscriber Line (ADSL), and Wi-Fi, and can be efficiently incorporated into the PON context to provide significant advantages [79, 80, 81]. Next, OFDM can be used both for coherent and direct detection schemes. This is important because when the coherent scheme is adopted, the overall performance can be greatly boosted, and on the other hand, when a DD scheme is considered, low-cost components, such as Directly Modulated Laser (DML), Vertical Cavity Surface Emitting Laser (VCSEL), and low-bandwidth electronics along with DMT, can be used, suppressing the overall costs.