# 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
#### 4.3.5 Towards the Ultimate Capacity Limits of Optical Transmission  
In this sub-section, we briefly elaborate on the main factors that can stress optical transmission toward its practical capacity limits. In order to achieve this, we need to maximize the following three quantities: (a) The bit rate per channel, (b) the number of employed channels per spatial element, e.g., fiber/core/mode, and (c) the number of spatial elements. Given that each channel has a specific bandwidth, the bit rate per channel is maximized by selecting a more spectrally efficient modulation format, e.g., 16 or 32 QAM instead of QPSK. However, there is a clear tradeoff between the spectral efficiency and the physical layer performance, as the higher cardinality modulation formats require a significantly larger SNR in order to attain the same BER. This is clearer when the Shannon  
Figure 30: Data rate per TRx vs. number of multiplexing Degrees of Freedom. Figure 29: Multi-Pb/s TRx block diagram in an UWB-SDM scenario for future 6G networks. Hartley theorem is considered, which calculates the spectral efficiency (SE) in the presence of noise as follows:  
\[SE=\frac{C}{B}=\text{log}_{2}\bigg{(}1+\frac{S}{N}\bigg{)} \tag{1}\]  
where \(C\) is the channel capacity in Gb/s, \(B\) is the bandwidth of the channel in GHz, and S/N is the signal-to-noise ratio. From Equation (1), it is clear that a high SNR is required to transmit multiple b/s/Hz, which cannot be attained within, e.g., a core network, via PM-32 QAM, PM-64 QAM, or even higher-cardinality formats for long distances more than a few hundreds of kilometers [91], mainly due to the accumulation of ASE noise and fiber nonlinearity. In a core network, PM-QPSK or even PM-16 QAM can be exploited to interconnect distant nodes [90], as these modulation formats have relaxed SNR requirements compared with higher-cardinality formats, in order to attain the same BER. Certainly, the spectral efficiency can be boosted using (a) sophisticated DSP modules, which can significantly improve the signal's quality, however with additional computational complexity and added cost/power consumption, and (b) using more sophisticated forward error correction codes, but at the cost of an increased number of redundant bits. Next, increasing the number of transmitted channels within the same spatial element, e.g., fiber, will directly result in an \(N_{ch}\) multiplier. This can be realized by exploiting the low-loss attenuation spectrum of the optical fiber as analyzed in Section 4.3.1. The ultimate limit of UWB is the 365 nm spectrum, which is obviously finite, and after its exploitation, only the third method can be used, which is the space division multiplexing. This method can pack a very large number of spatial elements, in practice much larger than 24, which is the number of the largest SDM system up to now, offering another capacity multiplier, allowing one to attain multi-Pb/s rates within a single transmission link. The aforementioned analysis is summarized in Figure 31, where the three methods are combined to maximize the number of transmitted data in an optical system. Predictions on the Attainable Capacity  
Based on the announced capabilities of submarine cables, we can perform predictions about the transported capacity for the coming years based on data taken from previous decades. For this purpose, Figure 32 illustrates the announced transported capacity of various submarine fiber cables [16] with and without SDM. As is evident, SDM is the key technology that can assist in exceeding the limit of 100 Tb/s and reach 1 Pb/s by 2030. These predictions are fully aligned with the predictions of various research works summarized in Figure 17, where it was also highlighted that the capacity of commercially available systems will reach 1 Pb/s in 2030.