IQ Imbalance Research Articles

Specific Emitter Identification Using Convolutional Neural Network-Based IQ Imbalance Estimators

Specific Emitter Identification is the association of a received signal to a unique emitter, and is made possible by the naturally occurring and unintentional characteristics an emitter imparts onto each transmission, known as its radio frequency fingerprint. This paper presents an approach for identifying emitters using convolutional neural networks to estimate the inphase/quadrature (IQ) imbalance parameters of each emitter, using only the received raw IQ data as input. Because an emitter’s IQ imbalance parameters will not change as it changes modulation schemes, the proposed approach has the ability to track emitters, even as they change the modulation scheme. The performance of the developed approach is evaluated using simulated quadrature amplitude modulation and phase-shift keying signals, and the impact of signal-to-noise ratio, imbalance value, and modulation scheme are considered. Furthermore, the developed approach is shown to outperform a comparable feature-based approach, while making fewer assumptions and using fewer data per decision.

Performance Analysis of Kramers–Kronig Receiver in the Presence of IQ Imbalance

We investigate the impact of IQ imbalance of the transmitter on the performance of the direct-detection system utilizing the Kramers–Kronig receiver. We show through theoretical analysis that the amplitude imbalance, timing misalignment, and IQ phase mismatch create double-sideband distortions, which, in turn, serve to violate the minimum phase condition and also lead to the dispersion-induced RF power fading after transmission. The theoretical analysis is verified by using numerical simulations. The results show that the amplitude imbalance and timing misalignment should be less than 2 dB and 2 ps, respectively, when the receiver sensitivity penalty is smaller than 1 dB for the transmission of a 112-Gb/s 16-QAM OFDM signal.

On the Interaction Between Joint Tx/Rx IQI and Channel Estimation Errors in DVB-T Systems

Digital video broadcasting terrestrial (DVB-T) systems use hierarchical modulation, which involve a “high-priority” and a “low-priority” (LP) bit stream. An inherent weakness of this scheme is the severe performance degradation of the less protected LP stream. In addition, the presence of inphase quadrature imbalance (IQI) at both transmitter/receiver front end may significantly reduce the detection accuracy. Here, we propose an improved receiver design for OFDM-based DVB-T systems confronting the effects of both imperfect channel estimation and IQI. First , we propose a specific pilot-symbol arrangement inside the OFDM symbol that facilitates derivation of the posterior distribution of the perfect channel conditioned on its maximum-likelihood (ML) estimate. The latter posterior distribution plays an important role toward an improved receiver design for taking into account channel estimation errors in the presence of IQI at the receiver side. Second , we derive the formulation of an improved and efficient detector under channel estimation errors when IQI is also present only at the receiver side. Third , we generalize our proposed improved detector for the case where IQI is present at both transmitter and receiver sides. The proposed approach is able to reduce the impact of channel uncertainty on the receiver's performance in the presence of IQI. Our results are compared to those achieved by the conventional mismatched ML detector, which uses the imperfect channel as perfect channel and to those obtained by the detector based on the expectation-maximization algorithm. Numerical results over a realistic COST 207 channel model show that significant performance improvement is achieved for bit error rate of the LP stream in the considered DVB-T system.

π-Splicer: Perceiving Accurate CSI Phases with Commodity WiFi Devices

WiFi technology has gained a wide prevalence for not only wireless communication but also pervasive sensing. A wide variety of emerging applications leverage accurate measurements of the Channel State Information (CSI) information obtained from commodity WiFi devices. Due to hardware imperfection of commodity WiFi devices, the frequency response of internal signal processing circuit is mixed with the real channel frequency response in passband, which makes deriving accurate channel frequency response from CSI measurements a challenging task. In this paper, we identify non-negligible non-linear CSI phase errors and report that IQ imbalance is the root source of non-linear CSI phase errors. We conduct intensive analysis on the characteristics of such non-linear errors and find that such errors are prevalent among various WiFi devices. Furthermore, they are rather stable along time and the received signal strength indication (RSSI) but sensitive to frequency bands used between a transmission pair. Based on these key observations, we propose new methods to compensate both non-linear and linear CSI phase errors. We demonstrate the efficacy of the proposed methods by applying them in CSI splicing and indoor distance ranging. Results of extensive real-world experiments indicate that accurate CSI phase measurements can significantly improve the performance of splicing and the stability of the derived power delay profiles (PDPs). Moreover, the estimated distance errors are reduced by 5.7 times on average comparing to the state-of-the-art schemes.

DESIGN OF FREQUENCY INDEPENDENT DIGITAL IQ IMBALANCE COMPENSATOR FOR RECEIVERS

Derivation of IQ imbalance in IQ receivers because of non-idealities of analog front-end has a significant negative effect on received signal quality and serious RF system performance degradation. Thereat the RF designers have an objective to create and improve compact and stand-alone methods of IQ imbalance compensation which provide best possible Image Rejection Ratio (IRR). The DSP-based methods of compensation are the most perspective. In this paper described one of version of autonomous and adaptive frequency independent IQ imbalance compensation algorithm which can be implemented as an IP that which can be used in different SoC for RF applications. The calculation method is based on digital signal statistic estimation with real-time convergence of compensation coefficients to the goal values which provide best image rejection. The results of algorithm’s processing for different input values with different IQ imbalance values are presented with max and min image rejection ratio estimation.

An LMMSE-Based Receiver for Uplink Massive MIMO Systems With Randomized IQ Imbalance

In-phase/quadrature-phase imbalance (IQI) is one of the hardware impairments in uplink massive multiple-input multiple-output systems. In this context, by modeling the IQI parameters as random variables, this letter first analyzes the statistic property of the effective channel in which both the IQI parameters and channel state information are included. By doing so, an LMMSE-based receiver is then investigated for channel estimation and data detection. Furthermore, we provide tightness analysis on asymptotic achievable sum rate for the proposed scheme using results from random matrix theory. Finally, simulation results show that our proposed scheme has small performance loss but with the advantage that estimating IQI parameters can be avoided.

A 2.0 GHz IQ Imbalance Compensator With Programmable Switch Biases in a Passive Mixer

This brief presents a frequency-independent IQ imbalance compensator with low complexity. The proposed compensator is a revised AND-Type passive mixer with eight programmable switch bias generators. The quadrature phase and gain compensation are simultaneously generated by programming these bias voltages. The generated quadrature compensation is approximately first-order linear to the differential and common mode variances of these bias voltages. The compensator is fabricated within a 2.0 GHz direct-conversion transmitter in a 0.13 μm RFCMOS process. The measured image reject ratio adjustment range is 34.8-60.4 dB. All eight 7-step (3-bit D/A converter) bias generators dissipate 1.0 mA at 1.35 V supply.

New metric for IQ imbalance compensation in optical QPSK coherent systems

We report on a simple alternative method for the compensation of quadrature imbalance in optical quadrature phase-shift-keying (QPSK) coherent systems. By introducing a new metric, the phase imbalance can be determined and compensated. The proposed method is theoretically and numerically analyzed. In particular, it is shown that the method exhibits a small bias of estimated phase imbalance value. Thanks to its deterministic property, this bias can be simply compensated by incorporating at the receiver a phase rotator (or phase shift) whose value can be determined based on an analytical analysis. Moreover, the algorithm is also experimentally validated through bit-error-rate and error vector magnitude (EVM) measurements. A good agreement on the performance of the proposed method with that of the Gram–Schmidt orthogonalization procedure is shown in a 20-Gbit/s optical QPSK experiment. The robustness of both methods was verified with up to $$30^\circ $$ phase imbalance by comparing the signal with and without phase imbalance compensation. A 10% reduction in EVM is achieved with our method for a high phase imbalance of $$30^\circ $$ , while the implementation complexity can be reduced owing to the suppression of the use of square-root operators.

IQ Imbalance Compensation and Digital Predistortion for Millimeter-Wave Transmitters Using Reduced Sampling Rate Observations

This paper expounds on the mitigation of the hardware nonidealities exhibited by millimeter-wave (mm-wave) transmitters (TXs) generating ultra-wideband signals. In particular, it tackles the IQ imbalance and nonlinear distortions attributed to the IQ modulator and power amplifier (PA) stages. For that, an offline frequency-domain-based IQ imbalance detection and compensation method using an interleaved multitone test signal with an interleaved I and Q frequency grid is first proposed. Then, the multitone signal settings were modified to enable the reduction of the required sampling rate of the analog-to-digital converter in the TX observation receiver (TOR) needed for IQ imbalance detection. Subsequently, a rearranged pruned Volterra series digital predistortion (DPD) scheme was exploited to separately set the nonlinearity order and linear and nonlinear memory depths for reducing the number of DPD coefficients needed for mm-wave PA linearization. The application of the proposed IQ imbalance detection and compensation method to a commercial IQ mm-wave modulator is stimulated by an interleaved multitone test signal spanning a modulation bandwidth of 4 GHz allowed for the reduction in the normalized mean squared errors (NMSEs) between the input and output signals from −14.0 to −37.7 dB at a complex baseband sampling rate of 8 GS/s and −37.1 and −36.5 dB with the sub-Nyquist complex baseband sampling rates of 1 and 500 MS/s, respectively. With the full and reduced complex baseband sampling rates of the TOR, the subsequent application of the rearranged DPD scheme was used to linearize a PA under test with an output 1 dB compression point of 23 dBm operating at 30 GHz. The PA stimulated by carrier aggregated orthogonal frequency division multiplexing signals with the bandwidths of 320 and 800 MHz is demonstrated the NMSEs of −36.5 and −32.4 dB and the adjacent channel power ratios of 50 and 45 dBc, using 77 and 66 DPD coefficients, respectively.

Novel transmitter IQ imbalance compensation algorithm using training symbols in PDM CO-OFDM system

We systematically deduce the optical fiber channel transmission mathematical model with in-phase/quadrature (IQ) imbalance and channel distortion for polarization division multiplexed (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. With the analysis of model, we propose two novel algorithms using training symbols, which can well realize channel distortion and IQ imbalance estimation. This study shows the proposed algorithms performed significantly better than the commonly used approaches. We examine the two proposed methods with simulation via analyzing the bit error rate in the case of different optical signal to noise ratio (OSNR), transmission distance and IQ mismatch at 80Gb/s and 100Gb/s.

Soft-Information Aided Channel Estimation With IQ Imbalance for Alternate-Relaying OFDM Cooperative Systems

In this letter, we exploit the feature of data redundancy associated with alternate-relaying cooperative systems to develop an iterative channel estimation algorithm in the context of orthogonal frequency division multiplexing (OFDM) transmission. Our attention is also focused on the problem of in-phase/quadrature-phase (IQ) imbalance which is typically associated with OFDM transmission. Analytical analysis indicates that instead of estimating a family of parameters including IQ imbalance occurring at the source, relays, and destination, and channel impulse responses (CIRs) between the source-destination link, and relays-destination links, we can estimate one parameter called the equivalent CIR. In addition, we illustrate how to perform data detection using the estimated parameter. By employing expectation-maximization algorithm, we show that soft information provided by the detector can be combined with pilot symbols in an efficient way to enhance the estimation process. Simulations experiments have confirmed the efficiency of the proposed approach.

Performance Evaluation of Channel Estimation and Data Detection with Phase Noise and IQ Imbalance for MIMO-OFDM Systems

Performance Evaluation of Channel Estimation and Data Detection with Phase Noise and IQ Imbalance for MIMO-OFDM Systems

Performance Analysis of Non-Orthogonal Multiple Access Under I/Q Imbalance

Non-orthogonal multiple access (NOMA) has been recently proposed as a viable technology that can potentially improve the spectral efficiency of fifth generation wireless networks and beyond. However, in practical communication scenarios, transceiver architectures inevitably suffer from radiofrequency (RF) front-end related impairments that can lead to degradation of the overall system performance with in-phase/quadrature-phase imbalance (IQI) constituting a major impairment in direct-conversion transceivers. In this paper, we quantify the effects of IQI on the performance of NOMA-based single-carrier (SC) and multi-carrier (MC) systems under multipath fading conditions. This is realized by first deriving analytic expressions for the signal-to-interference-plus-noise ratio and the outage probability of both SC and MC NOMA systems subject to IQI at the transmitter and/or receiver (RX) side. Furthermore, we derive asymptotic diversity orders for all considered impairment scenarios. Capitalizing on these results, we demonstrate that the effects of IQI differ considerably among NOMA users and depend on the underlying systems' parameters. For example, for a target data rate and power allocation ratio satisfying a given condition, IQI does not affect the asymptotic diversity of SC NOMA systems, whereas the asymptotic diversity of MC NOMA systems, suffering from RX IQI, is always zero. Moreover, it is shown that for both SC and MC NOMA systems, the first sorted user appears more robust to IQI, which indicates that higher order users are more sensitive to the considered impairment.

Joint Digital Self-interference Cancellation in Full-duplex Radios under IQ Imbalance and Transmitter Non-linearity

RF imperfections can significantly degrade the performance of full-duplex wireless communication system by introducing non-idealities and random effects, which make it difficult to cancel the self-interference completely. In this paper, we first address the adverse benefits of both the transmitter non-linearity and the IQ imbalance. Then on the basis of these, a joint digital self-interference cancellation scheme is proposed, in which not only the effect of IQ imbalance and power amplifier non-linearity individually, but also the comprehensive function of them are taken into account. Furthermore, the simulation is implemented in the MATLAB platform using standard WiFi 802.11ac PHYs. The results show that the proposed canceller can eliminate more compared with other cancellation schemes, and the overall self-interference attenuation can attain 108dB, which makes the residual self-interference closer to the noise floor.

Dual PHY Layer for Non-Orthogonal Multiple Access Transceiver in 5G Networks

Non-orthogonal multiple access (NOMA) is a promising multiple access technique, proposed in literature for the fifth generation (5G) mobile networks. The NOMA system model consists of the conventional orthogonal frequency division multiplexing (OFDM), as a pulse shaping technique in conjunction with a variable power domain for various users, allocated in proportion to each user’s channel gain. OFDM technique based on wavelet filter banks, namely wavelet OFDM (WOFDM) has been utilized in digital communication to improve the system robustness to noise and adjacent channel interference, and is therefore anticipated to be adopted for the NOMA technique. WOFDM in NOMA (WNOMA) outperforms OFDM-based conventional NOMA (CNOMA) with reference to interference mitigation, bandwidth efficiency, spectral confinement, and multi-user capacity. Most of the fourth generation (4G) networks are based on OFDM and its variants. Therefore, in this paper, keeping in view the interoperability with the 4G networks and the latency requirements in 5G, a dual physical layer based on conventional OFDM and WOFDM as pulse shaping methods, is proposed for the NOMA transceiver. Performance of WNOMA and CNOMA is analyzed for bit error rate in the presence of channel impairments including additive noise and IQ imbalance and multiuser capacity is also computed. Comparison of various parameters indicates the advantage of adopting WNOMA over its conventional counterpart for relatively poor channel conditions.

A novel transmitter IQ imbalance and phase noise suppression method utilizing pilots in PDM CO-OFDM system

In this paper, we develop a novel pilot structure to suppress transmitter in-phase and quadrature (Tx IQ) imbalance, phase noise and channel distortion for polarization division multiplexed (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. Compared with the conventional approach, our method not only significantly improves the system tolerance of IQ imbalance as well as phase noise, but also provides higher transmission speed. Numerical simulations of PDM CO-OFDM system is used to validate the theoretical analysis under the simulation conditions: the amplitude mismatch 3 dB, the phase mismatch 15°, the transmission bit rate 100 Gb/s and 560 km standard signal-mode fiber transmission. Moreover, the proposed method is 63% less complex than the compared method.

Towards 5G: Performance evaluation of 60 GHz UWB OFDM communications under both channel and RF impairments

Detailed analysis on the impact of RF and channel impairments on the performance of Ultra-Wideband (UWB) wireless Orthogonal Frequency Division Multiplexing (OFDM) systems based on the IEEE 802.15.3c standard, for high data-rate applications using the 60 GHz millimetre frequency band is presented in this paper. This frequency band, due to the large available bandwidth is very attractive for future and 5G wireless communication systems. The usage of OFDM at millimetre-wave (mmWaves) frequencies is severely affected by non-linearities of the Radio Frequency (RF) front-ends. The impact of impairments is evaluated, in terms of some of the most important key performance indicators, including spectral efficiency, power efficiency, required coding overhead and system complexity, Out-Of-Band Emissions (OOBEs), Bit Error Rate (BER) target and Peak Signal-to-Noise Ratio (PSNR). Additionally, joint distortion effects of coexisting Phase-Noise (PN), mixer IQ imbalances and Power Amplifier (PA) non-linearities, on the performance degradation of a mmWave radio transceiver, combined with various multipath fading channels, are investigated. Subsequently, the power efficiency of the system is evaluated by estimating values of the PA Output-Power-Backoff (OBO) needed to meet the requirements for the Transmit Spectrum Mask (TSM) and BER target. Finally, a comparison of the system overall performance between uncoded and coded OFDM systems combined with Quadrature Amplitude Modulations (16 and 64 QAM) and its maximum operable range are evaluated by transmitting a Full HD uncompressed video frame under five different RF impairment conditions over a typical LOS kiosk 60 GHz IEEE channel model.

Mitigation of time-varying distortions in Nyquist-WDM systems using machine learning

We propose a machine learning-based nonsymmetrical demodulation technique relying on clustering to mitigate time-varying distortions derived from several impairments such as IQ imbalance, bias drift, phase noise and interchannel interference. Experimental results show that those impairments cause centroid movements in the received constellations seen in time-windows of 10k symbols in controlled scenarios. In our demodulation technique, the k-means algorithm iteratively identifies the cluster centroids in the constellation of the received symbols in short time windows by means of the optimization of decision thresholds for a minimum BER. We experimentally verified the effectiveness of this computationally efficient technique in multicarrier 16QAM Nyquist-WDM systems over 270 km links. Our nonsymmetrical demodulation technique outperforms the conventional QAM demodulation technique, reducing the OSNR requirement up to ∼0.8 dB at a BER of 1 × 10−2 for signals affected by interchannel interference.

Blind receiver IQ imbalance compensation in DD Nyquist-SCM system

Receiver in-phase/quadrature (IQ) imbalance caused by imperfect analog receiver front end can rapidly degrade system performance in Nyquist-subcarrier-modulation (Nyquist-SCM) direct detection scheme. These imbalances, including IQ skew, IQ gain and phase imbalance, become increasingly important when higher symbol rate, higher modulation order, and lower roll-off pulse shape are used. Here, an adaptive four real-valued widely linear equalizer in an inner two-by-two butterfly structure is used to compensate receiver IQ imbalance and channel linear distortion, which has the same computational complexity with the conventional complex-valued FIR filter. In addition, the effectiveness of adaptive equalizer is validated experimentally with offline processing through a 20-Gbps 16-QAM Nyquist-SCM signal over 40-km standard single mode fiber transmission.

Iterative IQ Imbalance Compensation Receiver for Single Carrier Transmission

It is well known that low-cost RF front-end implementation such as direct conversion transmitter suffers from serious in-phase/quadrature phase (IQ) imbalance. Most existing works in single carrier transmission employ linear compensation methods, such as Least Square and minimum mean square error, to combat the interference caused by IQ imbalance. Actually, for single carrier transmission, it is possible for the receivers to adopt advanced nonlinear compensation methods to improve the system performance under IQ imbalance. In this paper, we design a new iterative decision feedback receiver to compensate the IQ imbalance caused by unideal radio front end for single carrier frequency domain equalization (SC-FDE). Numerical results shows that, compared with the conventional linear method, our proposed iterative IQ imbalance compensation can significantly improve the performance of SC-FDE system under IQ imbalance.