Wideband Direct-conversion Research Articles

A novel IQ mismatch compensation method in wideband direct-conversion receivers.

Direct-conversion receivers (DCRs) have been widely used in recent years due to their small size and low power consumption. However, the mismatch between the in-phase (I) and the quadrature (Q) branches will seriously affect the performance of the DCRs. This paper proposes a novel blind compensation method to suppress the interference introduced by IQ mismatch. Based on the Hilbert transform, our proposed method can obtain the orthogonal signal of the I-channel signal by utilizing the Weaver architecture. Compared with traditional compensation methods, the main difference of the proposed method is that it ignores prior information, training sequences, and additional hardware circuits. Furthermore, the complexity of the proposed blind compensation method is low because no iterative operations are involved in the compensation process. The simulation results show that the proposed method has an excellent compensation performance, especially in wideband applications.

A 250-GHz Wideband Direct-Conversion CMOS Receiver Adopting Baseband Equalized Low-Loss Resistive Passive Mixer

A 250-GHz Wideband Direct-Conversion CMOS Receiver Adopting Baseband Equalized Low-Loss Resistive Passive Mixer

A Frequency-Domain I/Q Imbalance Calibration Algorithm for Wideband Direct Conversion Receivers Using Low-Cost Compensator

The amplitude and phase imbalances of the in-phase (I) and quadrature (Q) branches inherent in the direct conversion receiver structure cause the generation of image frequency interference signals. In this paper, a frequency-domain I/Q imbalance calibration algorithm is proposed for wideband direct conversion receivers. The I/Q imbalance model is rebuilt by applying the infinitesimal method and FFT algorithm. The mathematical expressions for the exact computation of the I/Q imbalance parameters are derived based on the frequency-domain statistical properties of the baseband signal. The two phase parameters, frequency-dependent I/Q imbalance (FD-IQI) and frequency-independent I/Q imbalance (FI-IQI), are separated according to the parity properties of the imbalance parameter. To avoid the interference from the transmitter, the receiver impairment estimation is performed using a frequency offset (FO) DC training signal, and a low-cost real-valued compensation (RVC) filter is introduced to correct the impairments of the received signal. The performances of the proposed calibration model are evaluated through simulations and experiments. The simulation results show that the image rejection ratio (IRR) is improved to 80-120 dBc and can also exceed 40 dBc at high noise levels. The experimental results based on the CX9261A evaluation board show that the average IRR of the multi-tone signal is increased by 24.99 dB, and the IRR of the wideband signal is increased by 19.08 dB.

Complex Properness Inspired Blind Adaptive Frequency-Dependent I/Q Imbalance Compensation for Wideband Direct-Conversion Receivers

Direct-conversion receivers (DCRs) have been adopted in wideband communication systems owing to their simple structure and low cost, however, their operation is affected by amplitude and phase mismatches between their analog inphase (I) and quadrature (Q) branches, as well as the discrepancy of low-pass filter coefficients between these two channels. In this paper, a blind adaptive frequency-dependent I/Q imbalance compensator is proposed, which exploits the complex properness (second-order circularity) of ideal constellation mappings to provide more enhanced insight into the problem setting within the proposed compensator. This serves as a basis for a novel full second-order performance assessment framework, which is established through a joint consideration of the weight error covariance and complementary covariance in both the transient and steady-state stages. This conjoint analysis is further shown to facilitate accurate quantification of the overall mirror-frequency interference attenuation capability of the proposed compensator. Simulation results in an orthogonal frequency division multiplexing (OFDM) transmission system demonstrate the excellent performance of the proposed compensator.

DISTORTION COMPENSATION FOR WIDEBAND DIRECT RF DIGITIZATION RECEIVER

This paper studies the effect of distortions due to low noise amplifiers (LNA) nonlinearity and the compensation techniques on the multichannel wideband direct RF digitization conversion receivers. The distortion models have been analytically derived and verified by both simulation and the device measurements. The state-of-the-art techniques to reduce and compensate the LNA’s distortions in both RF and digital processing domains have been discussed in detail. This work also proposes a modified algorithm using a sub-channel reference to compensate the distortions and demonstrates the effectiveness the proposed technique using on four channel QPSK wideband receiver. Results show that the proposed linearization substantially improve the SFDR of the received signal by 20 dB (30 dB) for nearby (far-away) distortion.

Dual-Stage Co-Site RF Interference Canceller for Wideband Direct-Conversion Receivers Using Reduced Observation Chain

In this paper, a dual stage interference cancellation architecture is proposed to cancel the wideband interference for direct-conversion receiver in co-site radio environment. A single tap all-analog canceller and a reduced observation chain (ROC) based digital canceller are deployed at the radio frequency stage and digital stage, respectively. First, in order to avoid the saturation of the receiver, the all-analog canceller that contains an analog vector modulator and an analog correlator is used to cancel the direct coupling interference with low hardware complexity. Then, the ROC-based digital canceller is employed to cancel the multipath components. In the digital canceller, the ROC is used to obtain a digital domain copy of the transmitted signal with a reduced sampling rate. Furthermore, a parallel interference coupling model which jointly considers multipath wireless channel, nonlinearity and in-phase/quadrature (I/Q) imbalance of the direct-conversion receiver, as well as a widely linear least mean squares-based coefficients estimation algorithm are developed to track the time-varying interference coupling channel. Simulation and experimental results confirm that the proposed scheme can effectively suppress the co-site interference under multipath, nonlinearity, and I/Q imbalance effect, and significantly reduce the implementation complexity at the cost of tiny interference cancellation performance loss.

A wideband current amplifier with DC-offset cancellation utilizing chopper modulation

A wideband current amplifier with DC-offset cancellation utilizing chopper modulation is proposed. Combining series–series feedback with backgate driven choppers, a wideband current amplifier with DC-offset cancellation is realized. As consumed a power of 1.495 mW under a supply voltage of 1.8 V, measured bandwidth of the current amplifier is 120 MHz, which leads the current amplifier can be adopted to compose the analog baseband of a wideband direct-conversion receiver with bandwidth of 240 MHz (twice of the baseband bandwidth). Measured current gain of the current amplifier is 15 dB. Measured output DC-offset without and with DC-offset cancellation are − 70.8 dBV and − 91 dBV, respectively. Thus, DC-offset reduction of 20 dB is achieved.

Augmented Real-Valued Time-Delay Neural Network for Compensation of Distortions and Impairments in Wireless Transmitters.

A digital predistorter, modeled by an augmented real-valued time-delay neural network (ARVTDNN), has been proposed and found suitable to mitigate the nonlinear distortions of the power amplifier (PA) along with modulator imperfections for a wideband direct-conversion transmitter. The input signal of the proposed ARVTDNN consists of Cartesian in-phase and quadrature phase ( I/Q ) components, as well as envelope-dependent terms. Theoretical analysis shows that the proposed model is able to produce a richer basis function containing both the desired odd- and even-order terms, resulting in improved modeling capability and distortion mitigation. Its actual performance has been validated through extensive simulations and experiments. The results show that the compensation and hardware impairment mitigation capabilities of the ARVTDNN are superior to the existing state-of-the-art real-valued focused time-delay neural network (RVFTDNN) by 3-4 dB for the adjacent channel power ratio and by 2-3 dB in terms of the normalized mean square error. Other important features of the proposed model are its reduced complexity, in terms of the number of parameters and floating-point operations, and its improved numerical stability compared to the RVFTDNN model.

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.

Self-IQ-Demodulation Based Compensation Scheme of Frequency-Dependent IQ Imbalance for Wideband Direct-Conversion Transmitters

A low cost frequency-dependent (FD) I/Q imbalance self-compensation scheme is investigated in this paper. The direct conversion transmitters are widely used in wireless systems. However, the unwanted image-frequencies and distortions are inevitably introduced into the direct conversion system. This problem is even severer in wideband systems. Therefore, the accurate estimation and compensation of I/Q imbalance is crucial. The current compensation method is based on external instruments or internal feedback path which introduces additional impairments and is expensive. This paper proposes a low cost FD I/Q imbalance self-IQ-demodulation based compensation scheme without using external calibration instruments. First, the impairments of baseband and RF components are investigated. Further, I/Q imbalance model is developed. Then, the proposed two-step self-IQ-demodulation based compensation scheme is investigated. In the first step of the scheme, the local oscillator (LO) related I/Q impairments parameters are estimated. Then in the second step, the overall FD I/Q imbalance parameters are estimated by utilizing the transmitter LO. To realize this self-IQ-demodulation algorithm, this paper introduces minor modifications to the current power detector circuit. Afterwards, the estimated parameters are applied to the baseband equivalent compensator. This sophisticated algorithm guarantees low computation complexity and low cost. The compensation performance is evaluated in laboratory measurement.

A Programmable 0.7–2.7 GHz Direct $\Delta \Sigma$ Receiver in 40 nm CMOS

This paper presents a wideband direct ΔΣ receiver for the 0.7-2.7 GHz frequency range. The architecture embeds a wideband direct-conversion RF front-end into a continuous-time feedback ΔΣ modulator, which initiates the analog-to-digital conversion of the selected channel already at the RF nodes. A feedback-type architecture enables simultaneous filtering of nearby interfering signals. The inductorless 40 nm CMOS receiver supports programmable ΔΣ modulator coefficients and RF channel bandwidths up to 20 MHz. The receiver consumes 90 mW from a 1.1 V supply, and it provides a peak SNDR of 46 dB, NF of 5.9-8.8 dB, and an IIP3 of -2 dBm.

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.

Wideband SAW-Less Receiver Front-End With Harmonic Rejection Mixer in 65-nm CMOS

A wideband direct-conversion receiver front-end featuring a new harmonic rejection technique is demonstrated in 65-nm CMOS. The circuit consists of a two-stage low-noise amplifier, the first stage with capacitive feedback, a harmonic rejection mixer using 25% and 50% duty cycle local oscillator signals, and a third-order channel-select filter with configurable bandwidth. The receiver front-end is intended for surface-acoustic-wave-less cellular applications, and its performance was measured at 900- and 1800-MHz bands. The average harmonic rejection over GSM and LTE channel bandwidths is between 60 and 70 dB. Peak harmonic rejection exceeds 80 dB. The noise figures (NFs) are 3.3 and 3.9 dB for the complete receiver front-end in low band and high band, respectively, with an S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> below -15 dB from 500 MHz to 2.5 GHz. The 1-dB received signal compression points with a blocker present at 20/80 MHz offset for low/high band are 0 and +2 dBm, respectively. The NF with 0-dBm blocker is 13 dB. For low band, the in-band IIP3 and IIP2 are -14.8 and > 49 dBm, respectively, and, for high band, -18.2 and > 44 dBm. The circuit worst case consumes 80 mW of power.

A Time-Domain Joint Estimation Algorithm for CFO and I/Q Imbalance in Wideband Direct-Conversion Receivers

Carrier frequency offset (CFO) and in-phase and quadrature-phase (I/Q) imbalance are two of the common front-end impairments in low-cost communication devices. It is known that CFO can cause significant performance degradation in multi-carrier modulation (MCM) systems. Also, the existence of the I/Q imbalance usually reduces the accuracy of CFO estimation. In this paper, we propose a new data-aided scheme for the joint estimation of CFO and I/Q imbalance using simple matrix formulation. The proposed algorithms utilize only the periodicity of the generalized periodic pilot (GPP). They do not need to know the channel impulse response and the exact values of the training sequence. Moreover, our method has a low complexity and its performance compares favorably with the existing methods.

A Mutual Distortion and Impairment Compensator for Wideband Direct-Conversion Transmitters Using Neural Networks

This paper presents a one-step solution for transmitter nonlinearity estimation and linearization control in the presence of I/Q modulator imperfections for wideband direct-conversion transmitters. These transmitters include power amplifiers with frequency-dependent nonlinearities and modulator imperfections. With the proposed two-hidden-layer feedforward neural network, traditional two-step characterization and specially designed training signals are not required in the parameter estimation stage; and, estimation can be done without interrupting the operation of the transmitter. The measurement results and comparisons of the proposed neural network with the existing state-of-the-art methods show the superior performance in the presence of extreme RF impairments.

A CMOS Wideband RF Front-End With Mismatch Calibrated Harmonic Rejection Mixer for Terrestrial Digital TV Tuner Applications

A wideband direct-conversion RF front-end for Advanced Television Systems Committee terrestrial digital TV (DTV) tuner applications is realized in a 0.18-μm CMOS technology. In order to effectively solve the critical local oscillator (LO) harmonic mixing problem in an ultra-wideband frequency environment of 48-860 MHz, the combination of a mismatch calibrated harmonic rejection mixer (HRM) and a simple preceding integrated third-order passive RF tracking filter with an external inductor is utilized to obtain 60-80 dB of harmonic rejection for all odd-order harmonic mixing within the DTV spectrum. In addition, an efficient novel calibration algorithm for the HRM is proposed in order to simplify the compensation process. The RF front-end also includes a broadband noise-canceling low-noise amplifier, an attenuator to cover the wide dynamic range, an LO multiphase generator, and peripheral circuits such as an I <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> C serial interface for digital control. The implemented CMOS RF front-end achieves a total gain of 40 dB, output third-order intercept point of 30 dBm, and noise figure of 5.5 dB while consuming a low power of 140 mW from a 1.8-V supply voltage.

Digitally assisted equalization of third-order intermodulation products in wideband direct conversion receivers

An effective linearization technique capable of equalizing IM3 products resulting from an arbitrary out-of-band blocking scenario in a wideband direct conversion receiver is presented. IM3 products are regenerated in the RF analog domain of a low-power mixed-signal feedforward path and are used to cancel analogous signal terms in the original receiver at digital baseband via adaptive equalization. The composite SAW-less receiver achieves an improvement in effective IIP3 from −7.1 to +5.3 dBm under worst-case UMTS Region 1 blocking when the feedforward path is active.

Equalization of Third-Order Intermodulation Products in Wideband Direct Conversion Receivers

This paper reports a SAW-less direct-conversion receiver which utilizes a mixed-signal feedforward path to regenerate and adaptively cancel IM3 products, thus accomplishing system-level linearization. The receiver system performance is dominated by a custom integrated RF front end implemented in 130-nm CMOS and achieves an uncorrected out-of-band IIP3 of -7.1 dBm under the worst-case UMTS FDD Region 1 blocking specifications. Under IM3 equalization, the receiver achieves an effective IIP3 of +5.3 dBm and meets the UMTS BER sensitivity requirement with 3.7 dB of margin.

A Compact Wideband Front-End Using a Single-Inductor Dual-Band VCO in 90 nm Digital CMOS

As CMOS scales down and grows more expensive, area-aware RF front-end design becomes appropriate. A wideband front-end is presented that uses an inductorless LNA and downconversion section up to 6 GHz. Frequency synthesis is realized using a single-inductor dual-band 3.5 and -10 GHz VCO. In-depth analysis describes the operation of the 4-port oscillator, and compares phase noise to that of a classical VCO. The front-end is realized in 90 nm digital CMOS. The LNA achieves a noise figure of 2.7 dB with an average IIP3 of -2 dBm. The dual-band VCO achieves a phase noise of -122 dBc/Hz and -128 dBc/Hz at 3.9 GHz and 10 GHz, respectively, at 2.5 MHz offset. Both circuits are embedded in a wideband direct-conversion front-end consuming less than 60 mW from a 1.2 V supply.

Circularity-Based I/Q Imbalance Compensation in Wideband Direct-Conversion Receivers

Communication receivers that utilize I/Q downconversion are troubled by amplitude and phase mismatches between the analog I and Q branches. These mismatches are unavoidable in practice and reduce the obtainable image frequency attenuation to the 20-40-dB range in practical receivers. In wideband multichannel receivers, where the overall bandwidths are in the range of several megahertz and the incoming carriers located at each other's mirror frequencies have a high dynamic range, the image attenuation of the analog front-end (FE) alone is clearly insufficient. In this paper, two novel blind low-complexity I/Q imbalance compensation techniques are proposed and analyzed to digitally enhance the analog FE image attenuation in wideband direct-conversion receivers. The proposed algorithms are grounded on the concept of circular or proper complex random signals, and they are, by design, able to handle the often overlooked yet increasingly important case of frequency-dependent I/Q mismatches. The first technique is an iterative one, stemming from adaptive filtering principles, whereas the second one is a moment-estimation-based block method. The performance of the algorithms is evaluated through computer simulations, as well as real-world laboratory signal measurement examples in practical multicarrier receiver cases. Based on the obtained results, the proposed compensation techniques can provide very good compensation performance with low computational resources and are robust in the face of different imbalance levels and dynamics of the received signals, as well as many other crucial practical aspects such as the effects of the communications channel and carrier synchronization.