A cross-correlation based signal detector with two-step down-converter robust to finite image rejection

A spectrum sensing technique based on cross correlation that can be applied to cognitive radios for detection of weak signals is described. It is known that by cross-correlating the outputs of two signal paths, which have independent noise and correlated signals, the sensitivity of the detector can be enhanced. An analog approach to cross-correlation is employed, which avoids the requirement for high dynamic range analog-to digital converters and digital FFT cores. Each signal path uses an integrated receiver to down-convert the desired portion of the spectrum to baseband for correlation. The use of two-step image reject down-converters is proposed to alleviate flicker noise and DC offset related degradation that arises in direct-conversion receivers. The impact of finite image rejection in the receivers is addressed and a solution based on offset local oscillator frequencies is proposed. The effectiveness of the approach is validated through simulation.