# A Vision of 6 th Generation of Fixed Networks (F6G): Challenges and Proposed Directions
## 4 Optical Transmission: Key Challenges and Proposed Directions
### Capacity Increase Employing a Greater Number of Channels
#### 4.3.1 Ultra-Wideband Transmission  
Ultra-Wideband transmission allows for a ten-fold increase in the number of channels compared with the case of C-band transmission only. However, populating the entire low-loss attenuation spectrum of the SMF with channels is not trivial, as to date, optical components such as amplifiers, filters, and commercially available transceivers are mainly focused on C, L, and parts of the S-band. To further understand the current state, Figure 22 illustrates the attainable gain and noise figure of indicative available doped fiber amplifiers for the five bands of an UWB system. As can be observed, in O and E-bands, the technology at the component level has to be further developed in order to unlock the transmission of these frequencies. We expect that future research will be able to provide components with desirable characteristics, such as amplifiers with sufficient gain, low noise figures, and a significantly wider amplification gain. To realize UWB transmission, various types of amplifiers can be employed, such as Doped Fiber Amplifiers (DFAs), Semiconductor Optical Amplifiers (SOAs), and Raman amplifiers. The pros and cons of each method are discussed in [74]. According to our opinion, DFAs are currently the dominant amplification schemes, as they (a) are a well-known technology due to the extensive use of EDFAs in the C-band, (b) allow for modular engineering, introducing amplifiers on an as-needed basis, and (c) provide highly desirable characteristics such as low noise figure, gain flatness, and high output power. The full potential of a UWB (when all 365 nm are fully exploited) is to transmit more than 1000 channels (e.g., with 37.5 GHz channel spacing). However, the true number of transmit ted channels is expected to be lower, due to the current absence of optical components with desirable characteristics, especially in O and E-bands, as well as the catastrophic impact of nonlinear effects such as Nonlinear Interference (NLI) and Stimulated Raman Scattering (SRS), the impact of which scales nonlinearly with the increase in the total injected power in the fiber. In particular, ref. [88] showed that by using the currently available technology, the transmission of 871 channels in all five bands can be feasible. However, the attainable capacity of a UWB system depends also on the modulation format \"carried\" by each channel. For example, if PM-16 QAM is considered, an overall rate of 174 Tb/s can be envisaged. Next, the estimation of the transparent reach strongly depends on the impact of the physical layer effects, which are a function of various system parameters. In particular, the estimation of transparent reach is a more complex procedure, since each band \"sees\" different transmission parameters, such as the attenuation parameter, local dispersion parameter, effective area, etc., and for this reason, the attainable reach in each band (even in different channels of the same band) is different, unless an optimization strategy like [89, 90, 91] is considered, which can ensure similar physical layer performance in all bands. The attainable reach and bit rate are antagonistic target metrics, as when migrating to a higher modulation format, an increased physical layer performance is required in order to attain the same BER. This eventually leads to a lower transparent reach compared with the case of a lower cardinality format, e.g., PM-QPSK. This means that rates of >250 Tb/s can be theoretically achieved in metro, access, and inter/intra data center networks, while in longer networks, such as submarine and regional, a capacity of up to around 100 Tb/s can be achieved. Figure 23 summarizes the relation between transported capacity and transparent reach up to 10,000 km for experimental works over the past decade [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106].