A downlink/uplink identification algorithm for TDD communication systems in cognitive radio content

Cognitive radio has been considered as the most possibly techniques to solve the problems of spectrum underutilization and the spectrum scarcity. To discover the spectrum utilization correctly and quickly, the primary signals are sensed and analyzed by the secondary system with no cooperation from primary system. When the primary system is a communication system, downlink and uplink of the primary system should be distinguished by the cognitive radio since different links have different spectrum access opportunities due to the different transmitters should be sensed and the different receivers should be protected by the secondary system. Thus, the downlink/uplink identification is the first step for secondary access. Especially in TDD communication systems, the link identification meets tough demanding of timing and accuracy. In this paper, we proposed a link identification algorithm for TDD communication systems exploiting the power level features of downlink and uplink. Via the energy detection results of a number of distributed secondary users participating in the identification process, secondary system makes the decision about the current link is downlink or uplink. The effects of the local detection and the coverage of the secondary system on the algorithm performance are also analyzed. The results of the simulation show that, even in the toughest situation where the received downlink signal by SS is the lowest and the received uplink signal is the highest, the proposed algorithm can make the identification with high accuracy in a very short detection time.