Abstract
This paper discusses three the fundamental issues of Cognitive radio: Frequency, Modulation and Power. Frequency is located in 3.1 GHz ∼ 10.6 GHz so that the system can sufficiently utilize UWB spectrum resource and communications channel. OFDM UWB transmission technology is extensively used to mitigate multi-path interference. QPSK and DCM modulation are used to map binary data on signal wave shape on subcarriers. The most important problem is emission power mask level which should not affect primary users' communications. Cognitive Radio that operates in the unused range of licensed spectrum could transmit the same level power as license wireless communications.