Multistage Quadrature Sigma-Delta Modulators for Reconfigurable Multi-Band Analog-Digital Interface in Cognitive Radio Devices

Abstract

This article addresses the design, analysis, and parameterization of reconfigurable multi-band noise and signal transfer functions (NTF and STF), realized with multistage quadrature modulator (Q M) concept and complex-valued in-phase/quadrature (I/Q) signal processing. Such multi-band scheme was already proposed earlier by the authors at a preliminary level, and is here developed further toward flexible and reconfigurable A/D interface for cognitive radio (CR) receivers enabling efficient parallel reception of multiple noncontiguous frequency slices. Owing to straightforward parameterization, the NTF and the STF of the multistage Q M can be adapted to input signal conditions based on spectrum-sensing information. It is also shown in the article through closed-form response analysis that the so-called mirror-frequency-rejecting STF design can offer additional operating robustness in challenging scenarios, such as the presence of strong mirror-frequency blocking signals under I/Q imbalance, which is an unavoidable practical problem with quadrature circuits. The mirror-frequency interference stemming from these blockers is analyzed with a novel analytic closed-form I/Q imbalance model for multistage Q Ms with arbitrary number of stages. Concrete examples are given with three-stage Q M, which gives valuable degrees of freedom for the transfer function design. High-order frequency asymmetric multi-band noise shaping is, in general, a valuable asset in CR context offering flexible and frequency agile adaptation capability to differing waveforms to be received and detected. As demonstrated by this article, multistage Q Ms can indeed offer these properties together with robust operation without risking stability of the modulator.

A Background Calibration Technique Based on Limit Cycles for Reconfigurable Sigma Delta Modulators

“…An important commercial aspect of mobile communication systems is the efficient usage of the available spectrum by cognitive radio techniques [16]. The key for this view of cognitive radio is a reconfigurable RF frontend providing the required flexibility.…”

Wideband Quadrature Sigma-Delta A/D Conversion for Cognitive Radio - Reconfigurable Design and Digital Mirror-Frequency Suppression

“…This has made reconfigurable ΣΔ modulators very interesting option for CR receivers [2]. Furthermore, with the knowledge of transmission frequency and bandwidth the QΣΔM STF and NTF can be straightforwardly reconfigured for dynamic spectrum access [4], [8]. At the same time, these complex-valued transfer functions can be designed to exploit noncontiguos frequency bands, making it posssible to divide the transmissions between multiple available spectrum holes [4], [8].…”

“…Especially, a wideband direct-conversion receivers have gathered considerable amount of attention in this perspective [2]. For digitization, e.g., lowpass, bandpass and quadrature bandpass ΣΔ A/D converters (ADCs) have been considered [2]- [4]. ΣΔ modulators have several benefits exactly in this kind of scenario, being, e.g., power efficient and able to include part of the receiver selectivity inside the ADC [3].…”

“…Particularly, this capability can be exploited by designing the modulator response for reception of multiple noncontiguous frequency bands or a single, wider, band. In addition, the transfer functions can be straightforwardly reconfigured, making the QΣΔM a very appealing solution for CR hardware platforms [4], [7], [8].…”

“…This is why the mitigation schemes making the QΣΔM more robust, such as [4], [10]- [12], are taking care only of one part of the problem. By doing the compensation in digital domain it is possible to take also the MFI created by, e.g., the down-converting I/Q mixer into account.…”