## 4 Discussion--Open Issues
Moreover, the adoption of certain technologies that facilitate the transition to the 6G era might come in contrast to other important features, such as security by design. For example, as discussed in this work, O-RAN introduces several potential security risks due to its open and disaggregated architecture. * Coexistence with previous generations of networks. As also anticipated in the 5G era, the full transition to a new generation of networks will gradually take place. Until then, coexistence with well-established protocols is of utmost importance. In this case, one solution that has been proposed is the one in [78], where nested networks are formulated. In this context, small 6G cells can be deployed in areas with increased traffic distribution and can communicate with large 5G cells. However, there are not many works in the literature that, on the one hand, deal with the coexistence of 5G/6G and associated issues (e.g., handover and mobility management, resource allocation, etc. ), and, on the other hand, with interference mitigation mechanisms. To this end, the work in [86] presents an interference analysis for the coexistence of terrestrial networks with satellite services. In this work, extensive simulations are carried out regarding cellular coexistence with low-earth-orbit (LEO) satellites in the 47.2-50.2 GHz band. * Novel channel estimation techniques for THz communications. An efficient channel estimation is of vital importance to help THz communication systems achieve their full potential. Conventional channel estimation techniques that are used in currently established networks, such as least-squares algorithms in the uplink received signals, are practically inefficient for THz systems because of their large computation overhead. * Energy harvesting strategies for low-power consumption networks. Since 6G networks deal with the integration of various low -power devices, there is a need to identify suitable energy-efficient mechanisms for such devices. To this end, energy harvesting is defined as the process of extracting energy from external network sources (e.g., solar energy) that is later either used to power network devices or is preserved for wireless autonomous devices, like wearable biomedical monitoring sensors. * In the same context, ML models can also leverage energy efficiency in network design. To this end, a model can be trained to constantly monitor the network traffic and computational burden of each node in the network. Hence, this makes it possible to implement energy-saving techniques like turning down specific base stations or lowering their power consumption during these periods. * Integration of non-terrestrial networks (NTNs) over 6G interfaces [87]. In this context, NTNs could supplement terrestrial 6G infrastructure by extending coverage to remote and under-served areas, where deploying traditional terrestrial networks is challenging or economically impractical. In the same framework, the potential of stringent IoT services worldwide is leveraged. In this context, a promising secure transmission technology rooted in information theory is wireless covert communication, which can conceal the presence of transmission so as to eliminate possible attack threats [88].