# The 6G Ecosystem as Support for IoE and Private Networks: Vision, Requirements, and Challenges
## 4 The Requirements of 6G
### Spectrum  
The realm of 6G spectrum management can be distinguished into two primary categories of challenges: the efficient reuse of spectrum and the adoption of either novel spectrum bands or innovative transmission technologies. For the reuse part, techniques similar to 5G can be used, sharing temporally underutilized spectrum to maintain the availability and reliability [5], or communications can be conducted using cognitive radio (CR) techniques [73] and, in this case, the devices can access a shared or underused spectrum using interference detection and allocation management mechanisms. Another innovative technique of spectrum sharing is symbiotic radio (SR), which supports and bolsters intelligent and diverse wireless networks while enhancing the effectiveness of spectrum sharing in a mutualistic and competitive way [91; 92]. The efficiency of THz signals can be enhanced through spectrum reuse and sharing strategies. Among the techniques available for spectrum sharing and reuse, CR stands as a notable example. When considering spectrum sharing, temporally underutilized or unlicensed spectrum upholds availability and dependability. A novel SR technique has emerged to facilitate intelligent and diverse wireless networks.  
The promise is that 6G will be based on THz communications, meaning high-frequency communication and a high data rate. However, the THz signal could be either a pulse or continuous type, but 6G transmission needs a continuous wave THz signal, which is very difficult to generate and comes at the expense of greater complexity in antenna/transmitter design. This presents a paradox as generating THz signals is costly, whereas 6G asserts to provide low-cost communication services.  
However, other communication techniques are being considered: VLC uses light-emitting diodes (LEDs) to achieve high-frequency (HF) bands [93]. Nonetheless, VLC has some constraints in terms of coverage and noise interference [94], whereby its application within confined spaces devoid of interference from other light sources. Molecular communication [64; 65], whose signals are biocompatible, has low energy consumption because needs low power to obtain the signal as well as for transmission, and can support high data rates [95]. In quantum communication, based on the application of quantum principles, information is encoded and stored in a quantum state (utilizing photons or quantum particles) and cannot be retrieved or copied without causing changes to the data, making it a major challenge for them to be accessible by unauthorized entities. This approach offers notable benefits, including robust security, high data rates, and efficient long-range transmission capabilities. However, this technique is still at an embryonic stage [95]. Overall, 6G offers an intelligent eco-system enabling a wireless propagation environment with active signal transmission and reception and an all-spectrum reconfigurable dynamic spectrum access [66]. A comprehensive hyperspectral and full-spectral system is anticipated, encompassing a spectrum ranging from visible light, microwave, mmWave, and terahertz frequencies to laser-based free-space optical communication.