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Detection technique and front-end RF tunable filter for cognitive radio systems


Cognitive radio (CR) can effectively increase the spectral efficiency. However, there is an inevitable interference problem between the primary licensed users and the unlicensed CR devices. A detection technique utilizing the local oscillator (LO) leakage signal coming from a nearby mobile terminal is proposed and evaluated for short-range wireless communication systems, such as an ultra-wide band (UWB) system. Due to the low-level LO leakage, the detection sensitivity is a great concern. Local oscillator phase noise of the detector can degrade the detection sensitivity. Spectrum broadening of the LO phase noise interaction with an adjacent channel signal through a multiplier is analyzed and the degradation in detection performance is evaluated. Spectrum broadening is applied to the cognitive radio (CR) device operation in UHF bands. A strong TV signal transmitted by a nearby TV station can interact with LO phase noise of a CR receiver, and induce serious in-band interference which possible degrades the receiver performance. In order to suppress strong TV signals, a wide dynamic range low-power CMOS-based RF tunable filter with image cancellation is presented using a balanced passive complex mixer and frequency dependent loads. The tunable RF notch filter with 17 dB attenuation and 1.6 dB insertion loss is achieved using parallel LC tank loads. A tunable RF bandpass filter is implemented with capacitive termination, and achieves 18 dB maximum attenuation and 1.8 dB insertion loss

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