Cognitive radio (CR) network is the effective technique for the optimum utilization of the spectrum. The hybrid cognitive network (HCN) has been implemented to increase the utilization of the spectrum, by using both (overlay/underlay) modes of spectrum sharing. In overlay mode, cognitive radio (CR) opportunistically uses the spectrum, whereas in underlay mode, the CR uses the spectrum simultaneously with primary user (PU) with transmit power limitation. The HCN tries to use advantages of both modes of spectrum sharing. Its performance is based on the accuracy of the method used to find the PU status. Hence, in this paper, "clustering-based spectrum access scheme selection and optimal relay link selection for hybrid overlay/underlay cognitive radio networks" have been proposed to improve the performance of the hybrid cognitive radio network. In this proposed method, the scheme selection is done in two levels. In the first level, spectrum sharing scheme is selected based on the cluster in which the SU is present, and in the second level, scheme is selected based on the PU status. The cluster-based cooperative spectrum sensing method has been used to find the status of the PU. Compared with cooperative spectrum sensing, a cluster-based cooperative spectrum sensing has a huge effect on sensing accuracy. While the underlay scheme is used by SU, best relay link has been selected by using particle swarm optimization technique. Thus, the best scheme selection and relay selection improve the performance of hybrid cognitive radio network. The simulation result shows that the proposed method improves the throughput of the HCN.

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Terahertz (THz) communication is developing into an important technology for next 6G networks owing towards the ultra-wide bandwidth it provides. Precoding is a crucial approach in THz communication to get around the THz signals’ severe path loss and support the needed coverage. The primary problems of the prevalent THz precoding approaches for upcoming 6G networks are carefully investigated in this research. To be more precise, we first show how the main distinctions between millimeter-wave and THz channels are made clear. From there, we highlight the main difficulties with THz precoding, including the beam split effect and high power consumption.In THz massive MIMO systems, where the directional beams will split into several substantial directions at different sub-carrier frequencies, recent hybrid precoding approaches relying on frequency-independent phase-shifters are unable to handle the beam split effect. In THz mMIMO systems, the beam split effect will cause a significant array gain loss across the whole bandwidth, which has not been thoroughly studied.As a result, delay-phase precoding and hybrid precoding are suggested in this work. Then, after carefully examining its range of time delayers and antenna elements and making a association with frequency (milli meter wave and SubTHz have been examined), we move on to its design. The suggested delay-phase precoding approaches outperform the other wideband and narrowband precoding techniques already in use. To compare these typical THz precoding techniques and draw some conclusions for their usage in upcoming 6G networks, present simulation results of spectral efficiency.