In-phase And Quadrature Research Articles

Channel Compensation for Reciprocal TDD Massive MIMO-OFDM With IQ Imbalance

Massive multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems are sensitive to in-phase and quadrature (IQ) imbalances, which destroys the reciprocity of time-division duplex (TDD) uplink/downlink channels with potentially large performance degradation. In this letter, we investigate the compensation of channel reciprocity for TDD massive MIMO-OFDM systems in the presence of IQ imbalances at both the transmitter and receiver. A frequency-domain compensation scheme is proposed, which uses compensation matrices to precode uplink and downlink transmissions. By disassembling the effective channel model into pairs of mirror subcarriers and utilizing block matrix inversion, the compensation matrices can be calculated with a complexity that is linear in the number of antennas at the base station and mobile station. Simulations verify the effectiveness of the proposed scheme.

Pilot-Based Phase Noise, IQ Mismatch, and Channel Distortion Estimation for PDM CO-OFDM System

In this letter, a pilot-based method is proposed to suppress three non-ideal factors, including channel distortion, transmitter in-phase and quadrature (IQ) mismatch, and laser phase noise for polarization division multiplexed coherent optical orthogonal frequency division multiplexing systems. Compared with the conventional phase noise and IQ mismatch estimation methods which only consider one of the two non-ideal factors, we deduce a novel mathematical model considering both and compensate them with special designed pilots: the channel distortion and IQ mismatch are compensate by the block-type pilot while the comb-type pilot is used to estimate phase noise. The simulation results proved that our method can not only better compensate the non-idealities factors, but also can provide high speed transmission with 120 Gb/s over 400-km transmission compared with the conventional methods.

Integrated polarization multiplexing IQ modulator based on lithium niobate thin film and all waveguide structure

The integrated polarization multiplexing in-phase and quadrature (IQ) modulator based on lithium niobate (LN) thin film and all waveguide structure is designed. It consists of polarization demultiplexer (PDM), polarization multiplexer (PM) and two IQ modualtors (IQM). Each of IQM contains two different couplers and two Mach-Zehnder modulators (MZM). The theory of multimode interference (MMI) is used to design the PDM, PM and IQM. For PDM and PM, the extinction ratios of TE and TM polarized lights reach 23dB and 21dB. Quadrature Phase Shift Keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) are achieved well with the integrated modulator.

Automatic modulation classification of digital modulation signals with stacked autoencoders

Modulation identification of the transmitted signals remain a challenging area in modern intelligent communication systems like cognitive radios. The computation of the distinct features from input data set and applying machine learning algorithms has been a well-known method in the classification of such signals. However, recently, deep neural networks, a branch of machine learning, have gained significant attention in the pattern recognition of complex data due to its superior performance. Here, we test the application of deep neural networks to the automatic modulation classification in AWGN and flat-fading channel. Three training inputs were used; mainly 1) In-phase and quadrature (I-Q) constellation points, 2) the centroids of constellation points employing the fuzzy C-means algorithm to I-Q diagrams, and 3) the high-order cumulants of received samples. The unsupervised learning from these data sets was done using the sparse autoencoders and a supervised softmax classifier was employed for the classification. The designing parameters for training single and 2-layer sparse autoencoders are proposed and their performance compared with each other. The results show that a very good classification rate is achieved at a low SNR of 0 dB. This shows the potential of the deep learning model for the application of modulation classification.

IQ Imbalance Compensation for Generalized Frequency Division Multiplexing Systems

Generalized frequency division multiplexing (GFDM) with a flexible structure is one of the promising candidates for the fifth generation wireless communication system. This letter presents a compensation scheme of in-phase and quadrature (IQ) imbalance caused by radio frequency front-end for GFDM. The motivation behind this letter is to relax the requirement on time-invariant channel in the process of estimating IQ imbalance parameters. A low computational complexity approach is proposed to directly estimate IQ imbalance parameters without any prior knowledge of channel. Meanwhile, the structure of training sequences that avoid channel estimation is derived. Simulation results confirm that the proposed method can reduce the effects of IQ imbalance and provide reliable performance even over severe IQ imbalance environment.

Digital Predistortion Techniques for Finite Extinction Ratio IQ Mach–Zehnder Modulators

We present a digital predistortion (DPD) technique to compensate for in-phase and quadrature (IQ) Mach–Zehnder modulators (MZM) with finite extinction ratio (ER). The proposed method significantly improves the system performance by reducing the impairments introduced by the component imperfections. Our technique is performed in two steps: First, the $\arcsin$ predistorts the nonlinear sinusoidal transfer function of the modulator; second, a constrained optimization method, that utilizes the gradient descent algorithm, predistorts the modulator driving signals, thus mitigating the distortion induced by the finite ER. Since the optimal gradient calculation is rather complex, we introduce a simplified polynomial-based approximation that achieves comparable performance. The DPD gain is assessed via numerical simulations by varying modulation format and ER in an optical back-to-back configuration. The results are presented as required optical signal-to-noise ratio versus the modulator output power ( $\text{P}_{\text{MZM}}$ ) normalized with respect to the MZM input power (continuous wave of the laser, $\text{P}_{\text{CW}}$ ). A considerable improvement is showed when compared to the case without DPD or with the $\arcsin$ module for ERs $\leq$ 20 dB and 32/64QAM.

Modulation-format-free and automatic bias control for optical IQ modulators based on dither-correlation detection.

A novel automatic bias control (ABC) method for optical in-phase and quadrature (IQ) modulator is proposed and experimentally demonstrated. In the proposed method, two different low frequency sine wave dither signals are generated and added on to the I/Q bias signal respectively. Instead of power monitoring of the harmonics of the dither signal, dither-correlation detection is proposed and used to adjust the bias voltages of the optical IQ modulator. By this way, not only frequency spectral analysis isn't required but also the directional bias adjustment could be realized, resulting in the decrease of algorithm complexity and the growth of convergence rate of ABC algorithm. The results show that the sensitivity of the proposed ABC method outperforms that of the traditional dither frequency monitoring method. Moreover, the proposed ABC method is proved to be modulation-format-free, and the transmission penalty caused by this method for both 10 Gb/s optical QPSK and 17.9 Gb/s optical 16QAM-OFDM signal transmission are negligible in our experiment.

Compressed sensing-based time-domain channel estimator for full-duplex OFDM systems with IQ-imbalances

In full-duplex orthogonal frequency-division multiplexing (OFDM) systems with in-phase and quadrature (IQ) imbalances, a time-domain least squares (TD-LS) channel estimator is proposed for estimating both the source-to-destination (intended) and the destination-to-destination (self-interference) channels. To further improve the performance, an adaptive orthogonal matching pursuit (OMP) is proposed and its sparsity is estimated by a threshold method. Finally, the full-duplex interference is removed using serial interference cancellation and sphere decoding is used to complete the maximum likelihood detection. Simulation results demonstrate that in terms of both the mean square error (MSE) and the bit error rate (BER), the proposed adaptive OMP performs better than the TD-LS by exploiting the sparse property of the channel. Additionally, compared to the gradient projection, the proposed adaptive OMP is better in the low and medium signal-to-noise ratio (SNR) regions and marginally worse in the high SNR region.

Impact of IQ imbalance on the performance of QSM multiple‐input–multiple‐output system

Quadrature spatial modulation (QSM) is proposed recently as an efficient multiple-input–multiple-output wireless communication technique. In QSM, spatial multiplexing gain is achieved through modulating a two-dimensional spatial constellation diagram in addition to conventional signal modulation. It was demonstrated that QSM can be designed with single in-phase and quadrature (IQ) transmitter. However, the impact of IQ modulator imperfections, which degrade signal fidelity and overall system performance, has not been studied in the literature. In this study, typical IQ modulator/demodulator is considered for QSM system and the performance of the system is analysed and discussed. In particular, IQ imbalance channel modelling, pair-wise error probability, and average bit error ratio are discussed. Results reveal that IQ imbalance can lead to significant performance degradation of QSM system and should be carefully addressed for any future deployment.

Multiuser MIMO Transmission Aided by Massive One-Bit Magnitude Measurements

This paper proposes a multiuser MIMO system with both full measurements and one-bit magnitude observations, which are acquired by several linear inphase-and-quadrature (IQ) structured radio frequency (RF) chains and massive one-bit envelope chains, respectively. The total circuit power and cost are not increased significantly, since the added one-bit envelope chains have low power and low cost. Channel side information on the one-bit envelope chains is acquired by sharing the IQ-structured RF chains and executing a channel calibration operation. Two multiuser detectors are constructed based on the semidefinite relaxation (SDR) and approximate message passing (AMP). The one-bit magnitudes are interpreted as inequality constraints in the SDR detector, and exploited in a Bayesian manner by the AMP detector. Simulation results show that the one-bit magnitude measurements bring about high MIMO multiplexing and diversity gains, and decrease the transmission power. With the increase of the channel coherence time, more one-bit envelope chains are prone to be equipped, and one-bit magnitude-aided MIMO becomes more and more spectral-and-energy-efficient than the conventional MIMO.

Generalized spatial representation for digital modulation and its potential application

Constellation mapping has provided a great convenience to measure the performance of digital signal modulation in Euclid space. However, traditional in-phase and quadrature (IQ) plane is difficult to express the frequency modulation scheme such as minimum shift keying (MSK) and the time domain modulation such as cyclic code shift keying (CCSK). How to represent the digital signal modulation visually through constellation mapping is an attractive problem. To address this issue, in this paper, the combined frequency and phase modulation are utilized to define a new kind of constellation mapping, where the phase and frequency are quantized to the same elements. The uniform geometric construction for combined phase and frequency modulation is redefined in the 3D cylindrical coordinate system based on frequency (f), in-phase component (I) and quadrature component (Q). In the new coordinates, the quadrature frequency-phase shift keying (QFPSK) is produced by the QPSK with dimensional rotation matrix and denoted by the reduced dual quaternion. Furthermore, the spatial extension from QFPSK to chirp cyclic shift keying (Chirp CSK) is analyzed with bandwidth efficiency and energy efficiency. At last, the QFPSK is combined with the 2D OFDM, yielding the image OFDM system. Experimental results verify the effectiveness of QFPSK in the proposed system with the time-varying wireless channel and frequency selective fading channel respectively.

Detection of tissue coagulation by decorrelation of ultrasonic echo signals in cavitation-enhanced high-intensity focused ultrasound treatment.

BackgroundA noninvasive technique to monitor thermal lesion formation is necessary to ensure the accuracy and safety of high-intensity focused ultrasound (HIFU) treatment. The purpose of this study is to ultrasonically detect the tissue change due to thermal coagulation in the HIFU treatment enhanced by cavitation microbubbles.MethodsAn ultrasound imaging probe transmitted plane waves at a center frequency of 4.5 MHz. Ultrasonic radio-frequency (RF) echo signals during HIFU exposure at a frequency of 1.2 MHz were acquired. Cross-correlation coefficients were calculated between in-phase and quadrature (IQ) data of two B-mode images with an interval time of 50 and 500 ms for the estimation of the region of cavitation and coagulation, respectively. Pathological examination of the coagulated tissue was also performed to compare with the corresponding ultrasonically detected coagulation region.ResultsThe distribution of minimum hold cross-correlation coefficient between two sets of IQ data with 50-ms intervals was compared with a pulse inversion (PI) image. The regions with low cross-correlation coefficients approximately corresponded to those with high brightness in the PI image. The regions with low cross-correlation coefficients in 500-ms intervals showed a good agreement with those with significant change in histology.ConclusionsThe results show that the regions of coagulation and cavitation could be ultrasonically detected as those with low cross-correlation coefficients between RF frames with certain intervals. This method will contribute to improve the safety and accuracy of the HIFU treatment enhanced by cavitation microbubbles.

Detection and alignment of dual-polarization optical quadrature amplitude transmitter IQ and XY skews using reconfigurable interference.

Dual-polarization quadrature amplitude modulation (DP-QAM) is one of the feasible paths towards 100-Gb/s, 400-Gb/s and 1-Tb/s optical fiber communications systems. For DP-QAM transmitter, the time mismatch between the in-phase and quadrature (IQ) or x-polarized and y-polarized (XY) tributary channels is known as the IQ or XY skew. Large uncompensated IQ or XY skew can significantly degrade the optical fiber communications system performance. Sometimes, time-interleaved return-to-zero (RZ) DP signal is preferred with lower nonlinear polarization scattering induced penalty. In this work, detection and alignment of DP-QAM transmitter IQ and XY skews using reconfigurable interference is experimentally demonstrated. For IQ skew detection, a total dynamic range of 26.4 dB is achieved with ~1-dB power change for 0.5-ps skew from well alignment. For XY skew detection, it shows 23.2-dB dynamic range, and ~1.5-dB power change is achieved for 1-ps XY skew. Fast detection algorithm for arbitrary skew is also proposed and experimentally verified. The scheme is compatible with different modulation formats, flexible data sequences, and variable waveforms.

Error compensation of IQ modulator using two-dimensional DFT

It is important to precisely set and keep the phase and amplitude of an rf signal in the accelerating cavity of modern accelerators, such as an X-ray Free Electron Laser (XFEL) linac. In these accelerators an acceleration rf signal is generated or detected by an In-phase and Quadrature (IQ) modulator, or a demodulator. If there are any deviations of the phase and the amplitude from the ideal values, crosstalk between the phase and the amplitude of the output signal of the IQ modulator or the demodulator arises. This causes instability of the feedback controls that simultaneously stabilize both the rf phase and the amplitude. To compensate for such deviations, we developed a novel compensation method using a two-dimensional Discrete Fourier Transform (DFT). Because the observed deviations of the phase and amplitude of an IQ modulator involve sinusoidal and polynomial behaviors on the phase angle and the amplitude of the rf vector, respectively, the DFT calculation with these basis functions makes a good approximation with a small number of compensation coefficients. Also, we can suppress high-frequency noise components arising when we measure the deviation data. These characteristics have advantages compared to a Look Up Table (LUT) compensation method. The LUT method usually demands many compensation elements, such as about 300, that are not easy to treat. We applied the DFT compensation method to the output rf signal of a C-band IQ modulator at SACLA, which is an XFEL facility in Japan. The amplitude deviation of the IQ modulator after the DFT compensation was reduced from 15.0% at the peak to less than 0.2% at the peak for an amplitude control range of from 0.1V to 0.9V (1.0V full scale) and for a phase control range from 0 degree to 360 degrees. The number of compensation coefficients is 60, which is smaller than that of the LUT method, and is easy to treat and maintain.

An Efficient Method for Skew Estimation and Compensation in Coherent Receivers

The performance of coherent optical systems can be seriously degraded when in-phase and quadrature (IQ) components experience misalignments crossing certain critical limits. The critical limits of IQ phase, gain, and skew misalignments are lower at higher modulation formats and in narrowband systems. In this letter, frequency-domain skew-estimation algorithm and time-domain skew-compensation technique are proposed. When compared with other published methods, the new architecture improves timing performance and relaxes requirements for a multiple-input multiple-output time-domain equalizer at low implementation price. We discuss IQ imperfections on both transmitter and receiver sides and provide an efficient compensation procedure.

Performance Evaluation of Mobile Front-Haul Employing Ethernet-Based TDM-PON With IQ Data Compression [Invited

In this paper, we experimentally and numerically evaluate the performance of a common public radio interface (CPRI) over a time division multiplexed passive optical network (TDM-PON) system for accommodating densely deployed small cells in centralized-radio-accessnetwork- based future radio access. A prototype of the CPRI over TDM-PON is developed by using Ethernet-based TDM-PON. Experimental results showed that its synchronization accuracy and latency meet the requirements of a mobile front-haul using CPRI by the implemented delay jitter reduction and low-latency bandwidth allocation techniques. An in-phase and quadrature (IQ) data compression technique is also implemented on the system to enable a higher accommodation of remote radio heads. Experiments confirmed that the compression technique meets open radio equipment interface standards in terms of latency and signal quality degradation. Numerical results showed that the technique with a 0.45 compression ratio increases the number of supportable remote radio heads from 3 to 7 for a mobile front-haul with a 100 μs latency limitation.

Silicon photonic integrated circuit swept-source optical coherence tomography receiver with dual polarization, dual balanced, in-phase and quadrature detection.

Optical coherence tomography (OCT) is a widely used three-dimensional (3D) optical imaging method with many biomedical and non-medical applications. Miniaturization, cost reduction, and increased functionality of OCT systems will be critical for future emerging clinical applications. We present a silicon photonic integrated circuit swept-source OCT (SS-OCT) coherent receiver with dual polarization, dual balanced, in-phase and quadrature (IQ) detection. We demonstrate multiple functional capabilities of IQ polarization resolved detection including: complex-conjugate suppressed full-range OCT, polarization diversity detection, and polarization-sensitive OCT. To our knowledge, this is the first demonstration of a silicon photonic integrated receiver for OCT. The integrated coherent receiver provides a miniaturized, low-cost solution for SS-OCT, and is also a key step towards a fully integrated high speed SS-OCT system with good performance and multi-functional capabilities. With further performance improvement and cost reduction, photonic integrated technology promises to greatly increase penetration of OCT systems in existing applications and enable new applications.

Baseband and RF hardware impairments in full-duplex wireless systems: experimental characterisation and suppression

Hardware imperfections can significantly reduce the performance of full-duplex wireless systems by introducing non-idealities and random effects that make it challenging to fully suppress self-interference. Previous research has mostly focused on analysing the impact of hardware imperfections on full-duplex systems, based on simulations and theoretical models. In this paper, we follow a measurement-based approach to experimentally identify and isolate these hardware imperfections leading to residual self-interference in full-duplex nodes. Our measurements show the important role of images arising from in-phase and quadrature (IQ) imbalance in the transmitter and receiver mixers. We also observe baseband non-linearities in the digital-to-analog converters (DAC), which can introduce strong harmonic components in the transmitted signal that have not been considered previously. A corresponding general mathematical model to suppress these components of the self-interference signal arising from the hardware non-idealities is developed from the observations and measurements. Results from a 10 MHz bandwidth full-duplex system, operating at 2.48 GHz, show that up to 13 dB additional suppression, relative to state-of-the-art implementations, can be achieved by jointly compensating for IQ imbalance and DAC non-linearities.

Calibrated Monolithically Integrated 90 <named-content content-type="math" xlink:type="simple"> <inline-formula> <tex-math notation="TeX">$^{\circ} $</tex-math></inline-formula></named-content> Downconverter for Colorless Operation in the C+L Band

We propose a 90 $^{\circ} $ hybrid-based monolithically integrated coherent receiver suitable for colorless operation in the complete C- and L-band (1530–1625 nm). In-phase and quadrature (IQ) signal components are obtained from a calibrated analog IQ recovery circuit, which adequately combines all the four photocurrents at the output of the 90 $^{\circ} $ hybrid nearly cancelling the baseband interference from the self-beating of adjacent wavelength division multiplexing channels. We numerically demonstrate that the proposed downconverter supports colorless reception of 128 channels with 16-quadrature amplitude modulation at 112 Gbps in the C+L band. Aside from this, this receiver exhibits a noticeable dynamic range, thereby not requiring a variable optical attenuator on the signal path, and it is pretty robust under realistic fabrication errors of the optoelectronic chip.

A Comparative Analysis of Phase-Domain ADC and Amplitude-Domain IQ ADC

A phase-domain analog-to-digital converter (PhADC) is a promising alternative to a pair of amplitude-domain in-phase and quadrature (IQ) ADCs for low power FSK/PSK demodulation, but the fundamental benefits and limitations of the PhADC over the IQ ADC have not been precisely quantified as yet. In this paper, analytical methods are proposed to comprehensively compare the PhADC and the IQ ADC. Phase signal-to-noise ratio (SNR) expressions of the two ADC types are formulated analytically to facilitate a quantitative comparison of them. In comparison with the IQ ADC, the PhADC is a more compact quantization and demodulation solution when interference accommodation is not required. Moreover, considering a flash ADC as an example of the low resolution (3-4 bit) IQ ADC, the PhADC has a lower theoretical energy limit than the flash ADC for a given phase ENOB. IQ offsets and amplitude mismatch impose unique nonlinearities on the PhADC due to the nonlinear amplitude-to-phase conversion. The understanding of this nonlinearity leads to a phase-domain mismatch and offset detection technique. Phase SNR is explicitly related to input noise for both ADCs, and to comparator offsets for the PhADC, respectively. All of the results prove that the PhADC is a promising quantization and demodulation solution.