A Robust Cognitive Radio Based Adaptive Frequency/Time Spreading for OFDM Systems

The cognitive radio (CR) technology is an effective technology for a flexible use of the radio spectrum. However, the interference between the primary and the CR users becomes a critical problem for the CR system. In this paper, we propose a cognitive radio based adaptive frequency/time spreading for the orthogonal frequency division multiplexing (OFDM) systems. This new approach is characterized by the adaptive frequency spreading and time spreading, and the adaptive power control of the subcarriers at the borders of the CR user´s spectrum. Specifically, we propose a frequency spreading scheme according to the amount of the adjacent channel interference (ACI) coming from nearby primary users, which improves the power gain of data under severe interference. On the other hand, in order not to give an excessive interference the primary system, we also propose to reduce the power of the subcarriers near the edges of the bandwidth by spreading the signal in time. By the computer simulation, it has been proved that the proposed CR based OFDM system is very robust against ACI in comparison with the conventional OFDM system having adaptive guard bands. We also confirmed that the performance of the primary user is maintained with only a nominal degradation in the coexistence environment of the primary and CR system.