In this paper, we propose distributed hybrid active-passive reconfigurable intelligent surfaces (H-RIS) for Terahertz (THz) wireless communication systems in the presence of hardware impairments and$\alpha -\mu $small-scale fading. At first, the end-to-end channel is characterized by a gamma distribution approximation by means of the Lyapunov-central limit theorem. Based on this, analytic expressions of the outage probability and the ergodic capacity of the system are derived. Also, the scaling-law for the received signal-to-noise ratio (SNR) is analyzed in association with the H-RIS scheme, in comparison with fully active RIS and fully passive RIS counterparts. Next, the transmit power is optimized to achieve maximum energy-efficiency in practical scenarios where only partial channel state information (CSI) is available. Furthermore, impacts of hardware impairments, RIS phase-shift errors, as well as numbers of active and passive RIS elements on the system performance are evaluated. Finally, simulations are provided to validate the accuracy of the theoretical analysis.

In this article, we study a reconfigurable intelligent surfaces (RISs)-assisted Terahertz (THz) wireless systems with hardware impairments, where$\alpha \!-\!\mu $small-scale fading is considered for THz links in accordance with a recent measurement campaign. First, we propose an accurate closed-form approximation of a weighted sum of cascaded nonidentical$\alpha \!-\!\mu $variates based on the Gauss-Laguerre quadrature and a moment-matching method. This approximate approach facilitates analysis of the RIS-THz system over$\alpha \!-\!\mu $fading channels. To demonstrate, we derived closed-form expressions of the outage probability (OP), the ergodic capacity (EC), and the energy efficiency (EE) of the system based on the proposed approximation. Second, we approximately characterize the end-to-end channel of the RIS-THz system when the number of RIS elements is large in scenarios with or without the presence of phase-shift errors. Based on this statistical characterization, the closed-form expressions of the OP, the EC, and the EE of the large-size RIS-THz system are obtained. Furthermore, we devise a low-complexity algorithm that jointly optimizes the transmit power and RIS element activation (i.e., ON/OFF RIS) to maximize the EE in the RIS-THz systems. This algorithm adopts an iterative dynamic programming approach for a maximum subarray problem (i.e., Kadane’s algorithm). Finally, simulations are provided to validate the accuracy of the theoretical analysis as well as demonstrate the efficacy of the devised algorithm.

We study the performance of the secondary relay system in a power-beacon (PB) assisted energy harvesting cognitive relay wireless network. In our system model, a secondary source node and a relay node first harvest energy from distributed PBs. Then, the source node transmits its data to the destination node with the help of the relay node. Also, fading coefficients of the links from the PBs to the source node and relay node are assumed independent but not necessarily identically distributed (i.n.i.d) Nakagami- m random variables. We derive exact expressions for the power outage probability and the channel outage probability. Based on that, we analyze the total outage probability of the secondary relay system. Asymptotic analysis is also performed, which provides insights into the system behavior. Moreover, we evaluate impacts of the primary network on the performance of the secondary network with respect to the tolerant interference threshold at the primary receiver as well as the interference introduced by the primary transmitter at the secondary source and relay nodes. Simulation results are provided to validate the analysis.

In this paper, we investigate the effect of of the primary network on the secondary network when harvesting energy in cognitive radio in the presence of multiple power beacons and multiple secondary transmitters. In particular, the influence of the primary transmitter's transmit power on the energy harvesting secondary network is examined by studying two scenarios of primary transmitter's location, i.e., the primary transmitter's location is near to the secondary network and the primary transmitter's location is far from the secondary network. In the scenario where the primary transmitter locates near to the secondary network, although secondary transmitter can be benefit from the harvested energy from the primary transmitter, the interference caused by the primary transmitter suppresses the secondary network performance. Meanwhile, in both scenarios, despite the fact that the transmit power of the secondary transmitter can be improved by the support of powerful power beacons, the peak interference constraint at the primary receiver limits this advantage. In addition, the deployment of multiple power beacons and multiple secondary transmitters can improve the performance of the secondary network. The analytical expressions of the outage probability of the secondary network in the two scenarios are also provided and verified by numerical simulations.