Usage of cognitive radio (CR) in ARQ feedback systems

Cognitive Radio (CR) seeks to identify channels that are empty or infrequent by detecting the spectrum environment so that the available spectrum can be used more efficiently, and to allocate appropriate channels to secondary users. In this thesis, simulation studies on detection and evaluation of communication resource for cognitive secondary users in Automatic Repeat Request (ARQ) systems have been carried out. The frequencies of the primary systems can be used by the secondary systems in two different ways: underlay and overlay. While there is an option to create a certain level of interference on primary users according to the underlay method, in the overlay model, it is aimed to use the detected empty channels without allowing any interference on the primary users. In this thesis, CR applications for Basic ARQ and Hybrid ARQ (HARQ) systems with both methods are examined. By listening to the ARQ control signals of the primary system, the CR indirectly has the opportunity to obtain information about the state of the channel and its interference on the primary system. Thus, the secondary user can determine the action strategy for the purpose of accessing the spectrum. In the simulations performed for the secondary use of primary ARQ and HARQ systems, it has been observed that a significant amount of resources can be provided to the secondary user while maintaining the primary user throughput performance.