Markus Allén

Joint Mitigation of Nonlinear RF and Baseband Distortions in Wideband Direct-Conversion Receivers

Software-defined radio technology is experiencing more and more attention in modern communication and radar systems. The main practical challenge in deploying such technology is related to achieving sufficient linearity and spurious-free dynamic range in the RF front-end, especially in low-cost mass-product devices. This paper focuses on the analysis and digital mitigation of nonlinear distortion in software-define radio devices, building on wideband multicarrier/multiradio direct-conversion receiver principle where a wide collection of radio frequencies is in-phase/quadrature down-converted as a whole. A complete behavioral model for the total nonlinear distortion of the whole receiver chain is first derived, taking into account the third-order nonlinear distortion effects in all individual components, namely, RF low-noise amplifier, in-phase/quadrature mixer, and baseband in-phase/quadrature amplifiers. Stemming from this modeling, an adaptive digital feed-forward linearization structure is then developed to efficiently mitigate the joint nonlinear distortion of the whole receiver. The effectiveness of this approach is verified through extensive simulations and actual RF system measurements with a commercially available software-defined radio platform, which clearly outperforms the existing state-of-the-art methods that do not jointly consider RF and baseband nonlinearities.

Modeling and mitigation of nonlinear distortion in wideband A/D converters for cognitive radio receivers

This article discusses the reduction of nonlinearities in analog-to-digital (A/D) converters using digital signal processing (DSP). Also modeling of certain essential nonlinearities is considered in detail. The main focus is on wideband radio receivers, such as the emerging cognitive radio applications, where a collection of signals at different frequency channels is converted to digital domain as a whole. Therefore, the overall dynamic range can easily be in the order of tens of dBs and thus even mild nonlinear distortion can cause strong carriers to block weaker signal bands. In this article, a mathematical model for clipping distortion due to improper input signal conditioning is derived through Fourier analysis. Additionally, stemming from the analysis an adaptive DSP-based post-processing method for reducing the effects of clipping and integral nonlinearity (INL) in A/D converters is presented with illustrative examples using both computer simulations and laboratory radio signal measurements.

Digital post-processing for reducing A/D converter nonlinear distortion in wideband radio receivers

This article addresses the reduction of analog-to-digital (A/D) converter nonlinearities in radio receivers using digital signal processing (DSP). The main focus is on wideband A/D conversion where a collection of different waveforms at different frequency channels is digitized as a whole. The overall dynamic range in such composite signal can easily be in the order of tens of dBs, especially in the emerging cognitive radio type developments, and the nonlinear distortion due to strong carriers can easily block the weaker signal bands. In this article, DSP-based post-processing is proposed and demonstrated for reducing the effects of A/D converter integral nonlinearities (INL), stemming from unintentional deviations in the quantization intervals, as well as clipping due to improper input conditioning in wideband radio receiver context.

Frequency-Agile Multiband Quadrature Sigma-Delta Modulator for Cognitive Radio: Analysis, Design and Digital Post-Processing

A quadrature ΣΔ analog-to-digital converter (ADC) is a promising solution for intermediate frequency digitizing software defined cognitive radio (CR) receivers because of, e.g., multiband capability and power efficiency. However, inherent coefficient mismatches between the in-phase and quadrature rails can severely damage the performance of such receiver by creating mirror-frequency interference (MFI). In this article, a novel frequency-agile and reconfigurable transfer function design, allowing digital post-compensation of the MFI, is proposed. The design is based on a novel closed-form transfer function model for higher-order quadrature ΣΔ modulators (QΣΔMs) under implementation inaccuracies, proposed herein. By doing the compensation in digital domain, it is possible to take into account all error-sources of the receiver chain at once, including, e.g., a quadrature mixer before the ADC. This capability is obtained by preserving the mirror-band signal information and using the noise transfer function of a QΣΔM to remove quantization noise from therein. This is demonstrated in a multiband scenario aimed for CR receivers, where a number of frequency channels can be received and detected in parallel. Practical examples of the transfer function analysis under implementation inaccuracies and the post-compensation performance are given with a two-stage QΣΔM, having stage-orders of four, allowing eighth-order noise shaping.

Digitally-enhanced wideband analog-digital interfaces for future cognitive radio devices

The cognitive radio concept has gained widespread interest to avoid spectral scarcity in next generation mobile communications systems. However, there are major challenges from the radio transceiver electronics point of view. This paper addresses the analog-to-digital converter saturation problem due to highly-varying signal conditions in wideband cognitive radio receivers. Two different post-processing approaches are proposed here to mitigate this nonlinear distortion. Their performance is compared to current state-of-the-art methods using laboratory radio signal measurements with commercially available hardware.

Digital linearization of direct-conversion spectrum sensing receiver

Reliable spectrum sensing ability is a key factor in cognitive radios. However, there are many aspects that impact the sensing reliability. One important aspect is impairments in the cognitive radio receiver hardware. Received signals tend to have high dynamic range which drives the receiver to the nonlinear zone. This may cause nonlinear distortion falling to the sensing band and therefore either triggers a false alarm or missed detection. This paper specifically focuses on the digital compensation of sensing receiver LNA nonlinearities which are typically the most significant sources of nonlinearity. The proposed method is able to notably remove nonlinear distortion from the received signal and thus spectrum sensing algorithms become more reliable. With the help of simulations, this is shown not only for a classical energy detector but also for a cyclostationary feature detector.

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

This article addresses the design, analysis, and parameterization of reconfigurable multi-band noise and signal transfer functions (NTF and STF), realized with multistage quadrature ΣΔ modulator (QΣΔM) 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 QΣΔM 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 QΣΔMs with arbitrary number of stages. Concrete examples are given with three-stage QΣΔM, 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 QΣΔMs can indeed offer these properties together with robust operation without risking stability of the modulator.

Simultaneous Transmission and Spectrum Sensing in OFDM Systems Using Full-Duplex Radios

This paper studies the idea and performance of cyclostationary spectrum sensing in cognitive full-duplex radios, when secondary transmission and spectrum sensing are done simultaneously at the same channel. The paper starts by briefly introducing the ideas of cognitive full-duplex radio and the cyclostationary spectrum sensing in the presence of self-interference. The idea of changing the cyclic features of the secondary signal is then proposed and its impact on the spectrum sensing under self-interference is analysed. The cyclic features can be changed by changing the length of the cyclic prefix of the OFDM signal or by changing the amount of subcarriers. The effects of both approaches are evaluated with comprehensive performance simulations. It is shown that changing the cyclic features of the secondary signal can provide significant improvements in the sensing results of the primary signal, and that in general, reliable inchannel spectrum sensing while transmitting is feasible. This can enable enhance coexistence mechanisms, e.g., for LTE-Unlicensed technology at ISM band.

Iterative signal processing for mitigation of analog-to-digital converter clipping distortion in multiband OFDMA receivers

In modern wideband communication receivers, the large input-signal dynamics is a fundamental problem. Unintentional signal clipping occurs, if the receiver front-end with the analog-to-digital interface cannot respond to rapidly varying conditions. This paper discusses digital postprocessing compensation of such unintentional clipping inmultiband OFDMA receivers. The proposed method iterativelymitigates the clipping distortion by exploiting the symbol decisions. The performance of the proposed method is illustrated with various computer simulations and also verified by concrete laboratory measurements with commercially available analog-to-digital hardware. It is shown that the clipping compensation algorithm implemented in a turbo decoding OFDM receiver is able to remove almost all the clipping distortion even under significant clipping in fading channel circumstances. That is to say, it is possible to nearly recover the receiver performance to the level, which would be achieved in the equivalent nonclipped situation.

Reference Receiver Enhanced Digital Linearization of Wideband Direct-Conversion Receivers

This paper proposes two digital receiver (RX) linearization and in-phase/quadrature (I/Q) correction solutions, where an additional reference RX (ref-RX) chain is adopted in order to obtain a more linear observation, in particular, of the strong incoming signals. This is accomplished with reduced RF gain in the ref-RX in order to avoid nonlinear distortion therein. In digital domain, the signal observed by the ref-RX is exploited in linearizing the main RX. This allows combining the sensitivity of the main RX and the linearity of the lower gain ref-RX. The proposed digital processing solutions for implementing the linearization are feedforward interference cancelation and nonlinearity inversion, which are both adapted blindly, without a priori information of the received signals or RX nonlinearity characteristics. The linearization solutions enable flexible suppression of nonlinear distortion stemming from both the RF and analog baseband components of different orders. Especially, wideband multicarrier RXs, where significant demands are set for the RX linearity and I/Q matching, are targeted. Using comprehensive RF measurements and realistic base-station scale components, an RX blocker tolerance improvement of 23 dB and a weak carrier signal-to-noise-and-distortion ratio gain of 19 dB are demonstrated with combined linearization and I/Q correction.