08-09-2017, 12:01 PM
Cognitive radio and cooperative networks are increasingly seen as revolutionary technologies that efficiently utilize the radio spectrum. In this study, the Code Division Multiple Access Scheme (SE-CDMA) spectrally coded (propagation time) is proposed as a flexible and adaptive technique for the physical layer of cognitive retransmission networks. All primary and secondary users transmit their signals to a common base station (BS) and then the BS selects one of the secondary cognitive users based on feedback information for retransmitting the primary user (PU) data. To study the efficiency of the system, two scenarios are considered. In the first scenario, the secondary users transmit their information using the SE-CDMA technique and the PU is considered narrowband. In the second scenario, both primary and secondary users use the SE-CDMA technique. The performance of the system is evaluated in terms of the PU error probability, the probability of interruption of the primary and secondary users and comparing theoretical results with simulations. In addition, the performance of the proposed method is compared to the cognitive retransmission scheme based on the orthogonal frequency division multiple access.