Cognitive Radio is an innovative technology that allows unlicensed (secondary users) users to opportunistically access channels licensed to other users (primary users). A key procedure in the context of opportunistic access is the spectrum sensing, performed by secondary user to determine whether the channel is idle or busy. However, decisions regarding the channel state can be corrupted by fading conditions, leading to wrong decisions regarding the channel state. Collaborative spectrum sensing schemes have been proposed in the literature as a possible way to mitigate the effects of fading. In these schemes, local decisions or observations about the channel state are combined to reach a global decision. Even though collaborative spectrum sensing in general leads to a higher performance, correlated shadowing may reduce the benefits of collaboration. In this paper we investigate the performance of different combining rules in collaborative spectrum sensing in correlated shadowing environment. Our results show that the incremental performance gain achieved by adding more users in the collaboration scheme tends to reduce when the number of users in the collaboration grows, indication that there is a limit in the performance of collaborative spectrum sensing.
Electric power systems have undergone substantial changes in their operation. The higher penetration of renewable resources, demand response capability, and generators operating via droop control at the distribution level are the main features resulting in the microgrid concept. Microgrids must operate connected or islanded from the main grid, ensuring reliability and quality in the supply of energy in both operating scenarios. In this sense, the secondary control becomes essential in the system's resilience, since it is responsible for restoring the frequency and voltage within acceptable values. This study proposes a unified frequency and voltage secondary controls for microgrids operating in islanded mode. For this sake, a modification in the load flow algorithm considering a Jacobian matrix takes place, enabling a sensitivity analysis to give the adjustments in the set point of generators. The help of the Levenberg-Marquardt method improves the convergence in the modified load flow. All generators are continuously considered in this process, regarding their capabilities and relative control sensitivities concerning the operation point restoration. The proposed methodology is validated in a modified IEEE-37 node test feeder, showing the efficacy of the centralised secondary control under different scenarios of renewable generation penetration and load levels.
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