Downlink/uplink identification of time-division duplexing-OFDMA systems for cognitive radios

When coexisting with dual-link primary systems, secondary systems in cognitive radios should first distinguish between the primary downlinks and uplinks in order to efficiently explore their respective spectrum opportunities. Because of the assumptive prior knowledge about the time-frequency locations of primary downlinks and uplinks, this procedure is usually not considered in the design of cognitive radios. In this paper, a cooperative method is proposed for the downlink/uplink identification of time-division duplex-based orthogonal frequency-division multiple access systems. In this method, the power level of the primary link is extracted as the key feature, which also contributes to the subsequent cognitive behaviours. The effects of the primary and secondary systems and the effects of the detection parameters on the identification accuracy are all analysed in detail. The simulation results show that the proposed method can identify the primary links precisely and quickly with low complexity.