A Study on the Robust OFDM System based on the Cognitive Radio Technology

Abstract

Cognitive radio is an effective technology for a flexible use of the radio spectrum. The interference between the primary and the secondary system, however, becomes a critical problem for cognitive radio system. When the secondary system operates in the adjacent channels of the primary system, the adjacent channel interference (ACI) inevitably occurs due to drastically changing environment between the two systems. For the realizable operation, the secondary system should overcome performance degradation by the interference from the primary system. On the other hand, it is not allowed that the interference from the secondary system affects primary system. In this thesis, we propose a cognitive radio based orthogonal frequency division multiplexing (CR-OFDM) system, which has the capability of overcoming ACI adaptively to the channel condition. This new approach is characterized by applying the adaptive frequency/time spreading and power control of the subcarriers at the borders of the secondary user’s spectrum. For protecting the secondary system, we, first, propose frequency spreading according to the amount of the ACI coming from nearby primary users, which improves the power gain of data under severe interference. In order not to give an excessive interference the primary system, we then propose that the power of the subcarriers is stepwise reduced from bandwidth edges and the reduced subcarrier is transmitted repeatedly as the amplitude of power reduction. By the computer simulation, it has been proved that the proposed CR-OFDM system outperforms the conventional OFDM system having adaptive guard bands in both the throughput and BER performance in the presence of ACI from primary systems. And we confirmed that the performance of the primary user is maintained without degradation in the coexistence environment of the primary and secondary system.