IQ Imbalance Research Articles

Frequency-domain nonlinear interference cancellation scheme with IQ imbalance in full-duplex systems

在全双工自干扰信道反射径较多的情况下, 针对已有时域自干扰对消方案复杂度高的问题, 提出了一种低复杂度的频域自干扰对消方案. 该方案以全双工零中频架构为基础, 综合考虑了IQ不平衡以及功放非线性对自干扰重建模型的影响, 优化了已有频域自干扰重建模型的准确度. 在此基础上, 利用正交化辅助的连续干扰对消技术实现了低复杂度的自干扰消除. 仿真分析表明, 所提方案相比已有的基于RLS算法的频域自干扰对消方案, 能够在存在IQ不平衡以及功放非线性的情况下实现几乎相同的自干扰对消性能, 但是降低了68.75%的复杂度.

Partial Freezing for MIMO Adaptive Equalizers: Precise Demodulation in Dynamic Environments

We propose partially frozen (PF) multiple-input multiple-output (MIMO) adaptive equalizers (AEQs). By increasing the degrees of freedom (DOFs) of widely used complex 2×2 MIMO AEQs, complex 4×2 and 8×2 MIMO AEQs can cope with various signal impairments that occur within a transmitter and receiver, such as in-phase/quadrature (IQ) skew and IQ imbalance. Although precise signal demodulation with such MIMO AEQs are useful for enhancing the information rate of a transmission even to reach beyond 1 Tb/s, the increase in internal DOFs ultimately comes with a decrease in the speed of tracking polarization fluctuations and an increase in computational complexity, which are problematic for efficient demodulation in a dynamic environment. To solve this, as the speed of fluctuation in the state of polarization (SOP) of the signal is much faster than those of impairments within the transmitter and receiver, PF MIMO AEQs reduce effective DOFs by separating each DOF corresponding to the SOP, transmitter, and receiver, then freezing unnecessary updates of transmitter and receiver DOFs. In this paper, we describe the principle and show the results of numerical simulations and experiments with 128-Gbaud quadrature phase shift keying (QPSK) signals under IQ impairments. PF MIMO AEQs increased the tracking speed of the 8×2 MIMO AEQ, enabling the tracking of polarization fluctuations of over 11 Mrad/s (faster than fluctuation induced by lightning strikes) with less computational complexity than the conventional configuration. This method is a promising solution for high-speed data transmissions in dynamic environments.

Symbol Detection of Ambient Backscatter Communications Under IQ Imbalance

Ambient backscatter communications (AmBC) have been mainly studied under the assumption of perfect inphase/quadrature-phase (IQ) balance, which is hard to achieve in practical AmBC networks. In this paper, we study the symbol detection in AmBC under IQ imbalances. In particular, considering a differential encoding scheme at the AmBC transmitter and an energy difference based detector at the AmBC receiver, we derive the bit error rate (BER) and the near-optimal detection threshold in closed forms. Since the near-optimal detection threshold requires a priori knowledge of the channel state information, the IQ imbalance parameters, the noise power and the transmit power of the ambient source, which is not readily known by the AmBC receiver, we propose a simple but practical method to estimate the near-optimal detection threshold by exploiting the received signal samples at the AmBC receiver. Simulation results validate the accuracy of the derived BER and detection threshold, and reveal that the studied detector is more robust to the amplitude imbalance than to the phase imbalance.

Efficient Carrier Frequency Offset Estimation in Wireless Sensor Networks

The carrier frequency offset (CFO) estimation is a crucial problem in orthogonal frequency division multiplexing (OFDM) systems, especially for the enabling technologies in the Internet of Things, which demand stringent limitations of low cost, low power consumption, and wide ranges, like wireless sensor networks. In this letter, we propose an efficient CFO estimation method for OFDM systems with in- and quadrature-phase (IQ) imbalance, which combines the emerging multitask learning (MTL) with the channel residual energy (CRE) to reduce the average inferring time greatly. The MTL estimator can extract the correlated features between the CFO and the IQ imbalance so as to mitigate the intractable coupling effect. On the other hand, the CRE technique can guide the neural network and loss function design to improve the estimation accuracy and the training efficiency. Numerical results demonstrate that our proposal achieves high estimation accuracy while the inferring time is significantly reduced compared with other methods.

Performance Evaluation of E-Band Transmit-Receive Front-Ends Based on Characterization of Joint Effects of IQ Imbalance and Carrier Phase/Frequency Offset

A comprehensive approach for evaluating the performance of transmit–receive (TX–RX) analog front-ends as regards to the characterization of the joint effects of IQ imbalance and carrier phase/frequency offset is presented in this article. The study consists of a system model, a system-level (SL) simulation, and measurements. An SL discretization approach is implemented for the characterization of the joint effects of nonidealities. The approach uses SL simulation to assist in evaluating the measurement results. It also addresses the challenges associated with accurate performance estimation and assessment related to the nonidealities of the analog front-ends. The performance evaluation studies for the simulation and measurement are analyzed, considering recording time, evaluation period per frame, and averaging versus carrier frequency offset (CFO). The measurements are ensured with coherent and noncoherent local oscillator (LO) configurations concerning E-band simplex data transmission. The experimental studies are carried out based on bandwidth-and frequency-dependent performance evaluations. A correlation between the coherent and noncoherent LO configurations is reported in detail in terms of the evaluation of measured error vector magnitude (EVM) in the presence of the joint effects of IQ imbalance and carrier phase/frequency offset.

Rate Splitting in MIMO RIS-Assisted Systems With Hardware Impairments and Improper Signaling

In this paper, we propose an optimization framework for rate splitting (RS) techniques in multiple-input multiple-output (MIMO) reconfigurable intelligent surface (RIS)-assisted systems, possibly with I/Q imbalance (IQI). This framework can be applied to any optimization problem in which the objective and/or constraints are linear functions of the rates and/or transmit covariance matrices. Such problems include minimum-weighted and weighted-sum rate maximization, total power minimization for a target rate, minimum-weighted energy efficiency (EE) and global EE maximization. The framework may be applied to any interference-limited system with hardware impairments. For the sake of illustration, we consider a multicell MIMO RIS-assisted broadcast channel (BC) in which the base stations (BSs) and/or the users may suffer from IQI. Since IQI generates improper noise, we consider improper Gaussian signaling (IGS) as an interference-management technique that can additionally compensate for IQI. We show that RS when combined with IGS can substantially improve the spectral and energy efficiency of overloaded networks (i.e., when the number of users per cell is larger than the number of transmit/receive antennas).

Joint Compensation of TX/RX IQ Imbalance and Channel Parameters for OTSM Systems

Orthogonal time sequency multiplexing (OTSM), robust to doubly-selective fading channel, performs similar to orthogonal time frequency space (OTFS) system with less modulation/demodulation complexity. Direct conversion OTSM system suffers from hardware impairments such as in-phase and quadrature phase (IQ) imbalance at the transmitter (TX) and receiver (RX). At high frequencies, hardware imperfections are inevitable and should be compensated for better performance. In this Letter, we propose joint estimation of TX and RX IQ imbalance and estimate channel parameters in the delay time domain using pseudo noise based training sequence approach. The mean square error of the estimated channel parameters reduce with increase in training sequence length and power. With matched filtered Gauss-Seidel iterative method of detection of low complexity, it is shown that the compensated system performs close to ideal system under fractional delay and fractional Doppler.

Low-complexity two-stage equalizer for receiver IQ imbalance compensation and polarization demultiplexing for real-time System

Reducing the complexity of adaptive equalizer is significant for realizing real-time hardware systems. In this paper, we propose a low-complexity two-stage equalizer that can be used in real-time systems. The performance of the proposed equalizer was verified through a 112 Gbit/s polarization multiplexing quadrature phase shift keying (PM-QPSK) back-to-back experiment with a receiver IQ impairment emulation module. The results revealed that, as for FIR module, compared with the conventional 2 × 2 MIMO, the number of multipliers in the proposed equalizer can be reduced by approximately 70% by use of 21 taps, while this equalizer is less sensitive to IQ impairment.

Receiver IQ Imbalance and Skew Compensation for High Order Modulation Formats by Frequency Domain 4 × 2 MIMO

A typical coherent front-end may encounter imperfections, such as amplitude imbalance, phase imbalance and skew, between four sampling channels. These receiver (Rx) imperfections, together with traditional chromatic dispersion (CD) compensation, would induce in-phase (I) and quadrature (Q) mixing, which greatly degrades the system performance. In this work, we propose a modulation format independent frequency domain (FD) 4 × 2 multi-inputs and multi-outputs (MIMO) based on decision directed least mean square (DD-LMS) algorithm to solve this problem. Compared with traditional 2 × 2 MIMO, it enables to compensate for all of Rx imperfections. Compared with 4 × 4 MIMO, it can eliminate performance penalty induced by the IQ mixing after optical fiber transmission. Compared with traditional time domain 4 × 2 MIMO, the proposed FD 4 × 2 MIMO has advantages in terms of complexity and parallel processing. We also evaluate the Rx imperfections compensation capability, noise tolerance tolerance, polarization tracking capability, residual CD and transmitter imperfections tolerance of the proposed equalizers. The proposed FD 4 × 2 MIMO algorithms are verified by numerical simulation with different modulation formats and experimentally validated with 40 Gbaud polarization division multiplexed 16-ary quadrature amplitude modulation signal.

Single-pixel optical modulation analyzer based on phase retrieval for dual-polarization IQ modulators.

In-service monitoring and adaptive digital compensation of analog imperfections in optical transponders are vital in the next-generation optical coherent transmission systems employing extremely high-order, high-speed modulation formats. A notable example of such analog impairments is the imbalance of amplitude, phase, and/or timing between the in-phase (I) and quadrature (Q) tributaries in an optical IQ modulator, namely the IQ imbalance. Recently, an IQ-imbalance estimation technique based on phase retrieval without using a coherent receiver, the so-called single-pixel optical modulation analyzer (SP-OMA), has been proposed as an affordable in-service monitoring solution for the frequency-dependent IQ imbalance in a (single-polarization) IQ modulator. In this work, we extend the concept of the SP-OMA to dual-polarization IQ modulators. A novel phase retrieval algorithm with an alternating minimization procedure is proposed for identifying the frequency-dependent IQ imbalances on both polarization channels simultaneously from a single photodetector output. The validity and feasibility of the proposed SP-OMA for a dual-polarization IQ modulator are demonstrated numerically and experimentally with a 63.25-Gbaud DP-16QAM signal.

On the impact of IQI on cooperative NOMA with direct links in the presence of imperfect CSI

Cooperative non-orthogonal multiple access (CNOMA) is considered as a promising technique to improve network coverage, reliability and transmission for future wireless communication networks. Meanwhile, transceivers can suffer from a number of hardware imperfections that will significantly reduce their performance, such as phase noise and in-phase/quadrature-phase imbalance (IQI). In this paper, we investigate the effect of in-phase and quadrature-phase imbalance (IQI) on CNOMA with direct links in the presence of imperfect channel state information (ICSI). The outage probability (OP) and throughput expressions are derived to evaluate the performance behaviors of the CNOMA with direct links under the IQI and ICSI imperfections. Theoretical analyzes are verified by Monte Carlo simulation. The effect of the IQI on the CNOMA with the direct links has been studied with different parameters (image rejection ratio (IRR), power allocation) and compared with conventional NOMA to clearly observe the degrading effects of imperfections on the systems. The simulation results demonstrate that the IQI and ICSI have a negative impact on the outage and throughput performance.

A Novel Calibration Method of DC-Offsets in Direct-Conversion Transmitter

Direct-conversion transmitter based on the zero intermediate frequency architecture has been widely used. However, the direct current offsets (DC-offsets) introduced by the modulator affect the system performance and severely limit the calibration of IQ imbalance errors. This letter focuses on estimating and calibrating DC-offsets of modulators in the transmitter. Based on the frequency difference between the transmitted local oscillator (LO) signal and the received LO signal, the parameters of DC-offsets can be estimated by exploiting a training sequence and a self-closed loop system to achieve digital pre-compensation. The frequency response of the feedback loop and the interference introduced by the receiving channel can be ignored. There is no need to add additional hardware circuits, and the algorithm’s complexity is lower than other methods. In addition, the frequency-independent IQ imbalance errors in the transmit channel are accurately estimated. Finally, the effectiveness of the proposed algorithm is verified by simulation.

Analog Mobile Fronthaul Supporting 128 RF Chain With Nonlinear Compensator for B5G and 6G Ma-MIMO Beamforming

These paper details a fronthaul optical link using delay-division-multiplexing (DDM) scheme with Volterra based nonlinear compensator for beyond fifth generation (B5G) and 6G massive multiple-input multiple-output (Ma-MIMO) beamforming. Based on pre-allocated relative time delays, pre-processed signals can be deaggregated to different RF chain signals following spectral aliasing caused by the digital signal processing (DSP) at remote radio units. The Volterra based nonlinear compensator is used to compensate nonlinear distortion and IQ imbalance of the fronthaul link. By combined the DDM scheme and Volterra technique, we can easily deaggregate signals and mitigate the interference caused by nonlinear distortion and IQ imbalance at the same time. In experiments, the DDM scheme support 128 RF chain signals is demonstrated. With the Volterra compensator, the signal input power of Mach-Zehnder modulator (MZM) has 18 dB margin. IQ imbalance caused by phase misalignment can also be compensated by the IQ Volterra. It can keep the acceptable error vector magnitude (EVM) performance within a ±10° range. After 25-km optical link, the EVM of each RF Chain is underneath 8%. And the total received 128 RF chain signals will require CPRI data rate as high as 840 Gb/s.

On the Impacts of I/Q Imbalance in Analog Least Mean Square Adaptive Filter for Self-Interference Cancellation in Full-Duplex Radios

Frequency-dependent in-phase/quadrature-phase imbalance (IQI) is a critical hardware impairment, which affects the performance of the self-interference (SI) cancellation structure in full-duplex systems. A novel frequency-dependent IQI model is proposed in this paper, providing a theoretical tool to evaluate the impacts of this impairment in the analog domain. The model is then applied to investigate the performance of the analog least mean square (ALMS) loop under both frequency-selective and frequency-independent IQI. The degradation in the SI cancellation performance of the ALMS loop is revealed and its upper bound is derived. Numerical results show that the ALMS loop is resilient to both types of IQI with the maximum level of only 3.5 dB performance degradation even when the image rejection ratio is just 15 dB.

System impact and calibration method of power imbalances for a dual-polarization in-phase/quadrature optical transmitter.

For the latest 400-Gb/s or upcoming 1-Tb/s single-carrier optical fiber communications systems, dual-polarization quadrature amplitude modulation (DP-QAM) based on in-phase (I) and quadrature (Q) optical transmitter is the only possible solution. In a coherent DP-IQ transmitter, the power difference between the I and Q branches or orthogonally polarized (X and Y) channels are known as the IQ or XY power imbalance, respectively. Uncompensated IQ and XY power imbalances are always troublesome and can significantly limit the performance in long-haul transmission. In this work, we first experimentally investigate the impact of IQ and XY power imbalances on various QAM formats and baud rates in high-speed transmission. A DP-16-QAM up to 86 GBd is investigated, aiming for beyond 400 Gb/s applications. Then, a convenient pre-calibration method based on optical interference is proposed and experimentally demonstrated to detect and compensate both the IQ and XY power imbalances. This method can use the existing modulator structure and low-speed photodiode in the DP-IQ transmitter to obtain the optical interference power by injecting a specific coded electrical signal. After adjusting the time skew of each channel, the power imbalance between two channels can be calibrated by minimizing the optical interference power. The results fit well with the given theoretical analysis, and the proposed technique can be adopted to the DP-IQ transmitter with different QAM formats.

Performance analysis of massive MIMO systems with low-resolution ADCs and IQI

In massive multiple-input multiple-output (MIMO) systems, the substantial increase in the number of antennas leads to a surge in hardware cost and power consumption. Meanwhile, the impact of IQ imbalance (IQI) on system performance also tends to be serious. In this paper, closed-form expressions for the achievable rates of maximum-ratio combining (MRC) receivers are derived for uplink massive MIMO systems with both low-resolution analog-to-digital converters (ADCs) and IQI. Based on the derived closed-form expression, the influence of system parameters on the achievable rate is analyzed. The simulation results verify the accuracy of the theoretical results. It is found that low-resolution ADC and IQI will degrade the achievable rate compared with employing high resolution ADCs, but this loss can be compensated for by increasing the number of base station (BS) antennas, so as to significantly increase the energy efficiency.

Global Estimation and Compensation of Linear Effects in Coherent Optical Systems Based on Nonlinear Least Squares

This article proposes a new estimation and compensation approach to mitigate several linear and widely linear effects in coherent optical systems using digital signal processing (DSP) algorithms. Compared to most of the available strategies that employ local estimation and/or compensation algorithms, this approach performs a global impairments estimation and compensation based on nonlinear least squares. The proposed method estimates and compensates for the chromatic dispersion (CD), carrier frequency offset (CFO), in-phase/quadrature (IQ) imbalance, and laser phase noise in two steps. First, it estimates the quasi-static parameters related to the CD, CFO, and both transmitter and receiver IQ imbalance. Second, it estimates both transmitter and receiver lasers’ phases and compensates for all the imperfections by using a zero-forcing equalizer. Simulations show the effectiveness of the approach in terms of statistical performance and computational time. The estimation performance is assessed by computing the Cramér–Rao lower bound, while the detection performance is compared to a modified clairvoyant equalizer.

Low-complexity and blind receiver in-phase/quadrature imbalances compensation and estimation in the frequency domain for high-order modulation formats.

A frequency domain (FD) 4×2 multi-input and multi-output (MIMO) equalizer based on radially directed equalizer is proposed to compensate receiver in-phase/quadrature (IQ) imbalances of M-ary quadrature amplitude modulation signals. This algorithm has a significantly lower complexity compared with a conventional time-domain 4×2 MIMO equalizer. Furthermore, each of imperfection estimations is derived from the converged discrete frequency response of the FD 4×2 MIMO equalizer. The simulation and experimental results indicated that the receiver (Rx) IQ imbalances were fully compensated by the proposed equalizer and precisely estimated by estimators, even for long-haul transmission with Rx IQ imbalances varying over a wide range.

A Novel Average Autoencoder-Based Amplify-and-Forward Relay Networks With Hardware Impairments

In this paper, we propose a novel Average autoencoder (AE)-based amplify-and-forward (AF) relay networks impacted by the I/Q imbalance (IQI) and additional hardware impairments (AHI), where the source and destination nodes are equipped with neural network (NN)-based encoder and decoder, while a conventional AF relay node assists the transmission. The average AE employs multiple small NN-based decoders at the destination node, each decoding a soft probabilistic output that is averaged to obtain the final soft probabilistic output at the destination node. By considering multiple small NN decoders, we reduce the implementation complexity significantly while improving the performance compared to the AE with a single large but NN-based decoder. Within this Average AE framework, we propose a coded modulation design (CMD) with zero-forcing-based IQI compensation that considers the availability of the channel state information (CSI) and IQI knowledge. However, the IQI and CSI need to be estimated separately. Thus, we also propose a CMD with no IQI compensation that requires only the CSI knowledge. Finally, we propose a differential CMD that removes the necessity of both the CSI and IQI knowledge. Under low signal-to-interference-and-noise-ratio regimes, we show that the proposed Average AE outperforms the optimal maximum likelihood detector by considerable margin.

A convergent structure-function substrate of cognitive imbalances in autism.

Autism spectrum disorder (ASD) is a common neurodevelopmental diagnosis showing substantial phenotypic heterogeneity. A leading example can be found in verbal and nonverbal cognitive skills, which vary from elevated to impaired compared with neurotypical individuals. Moreover, deficits in verbal profiles often coexist with normal or superior performance in the nonverbal domain. To study brain substrates underlying cognitive imbalance in ASD, we capitalized categorical and dimensional IQ profiling as well as multimodal neuroimaging. IQ analyses revealed a marked verbal to nonverbal IQ imbalance in ASD across 2 datasets (Dataset-1: 155 ASD, 151 controls; Dataset-2: 270 ASD, 490 controls). Neuroimaging analysis in Dataset-1 revealed a structure-function substrate of cognitive imbalance, characterized by atypical cortical thickening and altered functional integration of language networks alongside sensory and higher cognitive areas. Although verbal and nonverbal intelligence have been considered as specifiers unrelated to autism diagnosis, our results indicate that intelligence disparities are accentuated in ASD and reflected by a consistent structure-function substrate affecting multiple brain networks. Our findings motivate the incorporation of cognitive imbalances in future autism research, which may help to parse the phenotypic heterogeneity and inform intervention-oriented subtyping in ASD.