A novel image rejection architecture for quadrature radio receivers

In radio communications, a bandpass-to-lowpass transformation is needed to demodulate the received signal down to baseband. One crucial question in this context is how to effectively attenuate the image band signal. For this purpose, inphase/quadrature (I/Q) signal processing is widely utilized in today\´s radio receivers. In this paper, a novel structure for obtaining an image-free baseband observation of the received bandpass signal is presented. The starting point is to approximate the needed 90° phase difference between the I and Q branch signals using a simple time delay of one quarter of the carrier cycle. For narrowband signals, this approach can be used directly to attenuate the inherent "self-image". By using an interference canceller-type of compensation technique, this concept is here generalized to cover also wideband multichannel signals. Furthermore, a closed-form expression to explicitly characterize the obtainable image attenuation is derived. Efficient implementation structures for digital radios utilizing periodically nonuniform subsampling are presented, and the validity of the proposed approach is further illustrated through simulation and design examples.