Mixed-Signal Architectures for Spectrum Sensing

The radio spectrum is subject to temporal and geographic variations and measurements indicate low utilization below 6GHz. In response, the Federal Communications Commission sent a notice of proposed rulemaking to facilitate cognitive radio use in the licensed digital television bands. Cognitive radios identify unused spectrum segments for data transmission while minimizing interference with licensed radios. Spectrum sensing is the enabling technology and detects the presence of a signal within a frequency band. Mixed-signal architectures offer potential power savings by performing signal detection in the analog domain, significantly reducing the analog-to-digital converter (ADC) sampling rate. The integrating mixer is a novel mixed-signal architecture for spectrum sensing that is based on the short-time Fourier transform (STFT). The architecture consists of a folded double balanced mixer with capacitive loads that implements current-domain windowing in the first stage while downconversion mixing and integration is implemented in the second stage. A load capacitor array enables integration with programmable time constant. A prototype was designed and fabricated in IBM's 0.13Âľm CMOS process. The measured results indicate an average dynamic range (DR) of 24.2dB over a 2.2GHz bandwidth (BW) with a power dissipation of 1.55mW. The integrating mixer architecture is extended by utilizing binary-weighted load capacitors, which integrates the STFT signal and acts as the sampling capacitors for a successive approximation register (SAR) ADC. An array of folded mixers are utilized to remove current restrictions on the selection of the window function, which improves side-lobe reduction and fall off in the frequency domain. A prototype was designed and fabricated in IBM's 0.13Âľm CMOS process. The measured results indicate an average DR of 26.8dB over a 1.25GHz BW. The integrated SAR ADC achieves a peak signal-to-noise-and-distortion ratio of 45.4dB at a sampling frequency of 200kHz for an effective number of bits of 7.25. The power dissipation is 0.88mW for a full quadrature implementation. In comparison to recent spectrum sensing implementations, the integrating mixer prototypes achieve the lowest power dissipation while obtaining a DR that falls within the reported range. The prototypes are well suited for integration within low power cognitive radio transceivers that target portable IEEE 802.22 applications.Ph.D