Transmitter IQ Research Articles

Robust, wide-range, and precise transmitter IQ impairment monitoring scheme for coherent digital subcarrier multiplexing systems.

In this Letter, we present a robust, wide-range, and precise monitoring scheme for transmitter (Tx) impairments in coherent digital subcarrier multiplexing (DSCM) systems. The proposed scheme employs frequency-domain pilot tones (FPTs) to compensate for frequency offset (FO), polarization aliasing, and carrier phase noise, thus isolating Tx impairments from channel distortions. It then implements 4 × 4 real-valued MIMO to compensate for Tx impairments by equalizing symmetric subcarriers. Tx impairment monitoring is derived from the equalizer coefficients. By considering the phase shift caused by Tx impairments, a wide-range and precise monitoring of Tx impairments including IQ skew, IQ phase, and gain imbalances is achieved. We experimentally validated our approach using a 48-GBaud, four-subcarrier, dual-polarization coherent DSCM system. The results confirm the method's capability for a wide-range, robust, and precise Tx impairment monitoring in coherent DSCM systems, maintaining performance even in the presence of ultra-fast polarization variation.

Paired-subcarrier equalization for phase noise and transmitter IQ skew in digital subcarrier multiplexing system

The digital subcarrier multiplexing (DSCM) transmission scheme is expected for future ultra-large baud rate transmission. However, the phase noise and transmitter (Tx) IQ skew tolerance are decreased due to the narrow sub-band transmission and conjugated interference from symmetric subcarrier. In this paper, we propose a paired-subcarrier equalization scheme to jointly mitigate the phase noise and Tx IQ skew. We use a phase locking loop (PLL) embedded 4 × 4 MIMO equalizer to simultaneously realize polarization demultiplexing, phase noise and Tx IQ skew compensation. The 4 × 4 MIMO can deal with the paired-subcarrier interference in the DSCM transmission. Besides, since the inner subcarrier suffers smaller interference from its symmetric subcarrier, we estimate the phase noise by inner subcarriers and share the phase noise information with other subcarriers to reduce the overall complexity. Through simulations of 100-GBaud 64-QAM DSCM coherent optical fiber transmission consisting of eight 12.5-Gbaud subcarriers and experiment of 10-GBaud four-subcarriers PM-16QAM transmission, we find that the PLL embedded equalizer for DSCM scheme exhibits better skew and phase noise compensation ability compared with other equalizers. Additionally, we compare the performance of single-carrier and DSCM schemes with the proposed equalizers in simulation. The influence of phase noise and Tx IQ skew on DSCM transmission can be largely relaxed.

Low-complexity algorithms for coherent optical systems with transceiver IQ imbalance.

The next-generation coherent optical communication relies on higher symbol transmission rates with higher modulation cardinality. The imbalance between the in-phase (I) and the quadrature (Q) arms in such systems leads to deteriorated performance if not corrected. In this work, we propose a reference constellation adaptation-based single-tap, novel carrier phase recovery algorithm, tolerant to the transmitter IQ imbalance and a pilot-free geometric parameter extraction-based receiver IQ imbalance correction method for MQAM signals. The proposed scheme is numerically investigated through extensive simulations for 32 GBaud polarization multiplexed (PM)-16QAM, and 64QAM modulation formats and experimentally verified for 32 GBaud PM-16QAM signal. The proposed pilot-free scheme is applicable to any modulation format and offers lower computational complexity than traditional algorithms.

Experimental Comparison of Single-Carrier and Digital Subcarrier Multiplexing Transmissions in a W-Band 200 Gb/s Fiber-Wireless System Considering Transmitter IQ Imbalance and Skew Mitigation

Experimental Comparison of Single-Carrier and Digital Subcarrier Multiplexing Transmissions in a W-Band 200 Gb/s Fiber-Wireless System Considering Transmitter IQ Imbalance and Skew Mitigation

Transmitter IQ Imbalance Mitigation and PA Linearization in Software Defined Radios

Transmitter IQ Imbalance Mitigation and PA Linearization in Software Defined Radios

Experimental Demonstration of Simultaneously Precise Tx and Rx Skew Calibration for Coherent Optical Transceiver

High precise IQ skewcalibration of the coherent optical transceiver is crucial for high baud rate and high-order modulation formats optical transmission system. In this paper, a novel high precise optical coherent transceiver calibration method based on the specially designed interleaved in-phase (I) and quadrature-phase (Q) multi-tone signals is proposed. This method can simultaneously characterize the transmitter (Tx) and receiver (Rx) side IQ skew by just one measurement without using any additional device or training process. Because of the non-overlapping characteristic of these interleaved I and Q signals in the frequency domain, the IQ crosstalk in the coherent optical transceiver can be effectively avoided, contributing to high-precision calibration. Since the calibration process only focuses on the phase frequency response characteristic, the dynamic range of IQ skew calibration can easily reach dozens of picoseconds. Theoretical analysis and simulation results indicate that the measurement accuracy of the Tx skew and Rx skew is within ±0.2 ps in the proposed scheme, and the dynamic range of IQ skew calibration can reach ±40 ps. With the help of this IQ skew calibration method, the transmission performance improvement is experimentally verified in both single-polarization 30 GBaud 32QAM signal and dual-polarization 30 GBaud 16QAM signal transmission systems.

Widely Linear Filtering for Multiimpairment Compensation in Dispersion Managed mQAM Modulated Optical Systems

We propose a blind joint equalization algorithm for M-QAM signals based on a widely linear filtering approach. The proposed scheme jointly compensates for receiver IQ imbalance, receiver IQ skew, polarization mixing, carrier recovery, followed by transmitter IQ imbalance and skew. We first investigate the proposed scheme&#x2019;s tolerance to each of these impairments through numerical simulations for 32 GBd PM-16QAM and PM-64QAM signals and compare its performance with the conventional digital processing algorithms with and without IQ imbalance compensation. The proposed joint transceiver equalizer outperforms the conventional algorithms with a <i>Q²</i> improvement of greater than 1 dB, and with improved tolerance to IQ imbalance. Further, with the proposed algorithm, we experimentally demonstrate the improvement in <i>Q²</i> value with respect to conventional DSP for both PM-16QAM and PM-64QAM signals. We also show that the MSE convergence of the proposed joint equalizer is much faster than conventional DSP algorithms.

Compensation of Transmitter IQ Imbalance in Multi-User Hybrid Beamforming Systems

In this paper, we consider the uplink (UL) communication of a massive multi-user (MU) multiple-input multiple-output (MIMO) hybrid beamforming system (HBFS) in which the transmitting user-equipments are affected by IQ imbalance (IQI). We first show that if the transmitter IQI is not compensated, it will result in a finite ceiling of the UL achievable sum-rate at high signal-to-noise ratio, which only depends on the transmitter IQI matrices and is independent of the propagation channel and the choice of the hybrid combining matrices. This justifies the need for transmitter IQI compensation in massive MU-MIMO HBFS. Therefore, we propose a novel zero-forcing based transmitter IQI compensation algorithm to be implemented at the base station which effectively mitigates the undesired effects of transmitter IQI and is applicable for any channel model and any choice of the number of RF chains. For uncorrelated Rayleigh fading channel, we derive an approximate closed-form expression of the UL sum-rate achieved by the massive MU-MIMO HBFS with the proposed transmitter IQI compensation. Finally, numerical results are presented which confirm the effectiveness of the proposed transmitter IQI compensation algorithm in mitigating the undesired effects of transmitter IQI in different channel environments.

Joint Estimation of RF Impairments, Channel, and Low Complexity Iterative Equalization Technique for High Mobility SC-FDMA/OFDMA Uplink Systems

In this paper, we consider the problem of joint estimation and equalization of doubly selective channels and all radio frequency impairments, namely transmitter IQ imbalance (TIQI), receiver IQI (RIQI) and carrier frequency offsets (CFO) in the uplink of single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA) systems with direct conversion transceiver architecture and high mobility users. We combine the effect of TIQI, RIQI and DSC to a new effective channel and propose the use of a Hermite polynomial based basis expansion model for approximating it to avoid the identifiability issue associated with this complete parameter estimation problem. As the maximum likelihood solution to this joint estimation problem requires complex multi-dimensional searches, we propose a novel subspace nulling based iterative estimation technique that effectively reduces the multi-user interference and replaces the multi-dimensional search with single-dimensional searches. Further, without much compromise on the performance, we also derive a low complexity version of the proposed technique. Additionally, we derive the Cramer-Rao bound for the joint estimation of CFO and effective channel. Moreover, we propose two equalization techniques for iteratively equalizing the mirror subcarrier pairs. The proposed techniques do not put any restriction on the carrier assignment scheme and provide good performance even at high mobile speeds.

Compensation of Limited Bandwidth and Nonlinearity for Coherent Transponder

Coherent optical transponders are widely deployed in today’s long haul and metro optical networks using dense wavelength division multiplexing. To increase the data carrying capacity, the coherent transponder utilizes the high order modulation format and operates at a high baud rate. The limited bandwidth and the nonlinearity are two critical impairments for the coherent in-phase quadrature transmitter. These impairments can be mitigated by digital filters. However, to accurately determine the coefficients of these filters is difficult because the impairment from the limited bandwidth and the impairment from nonlinearity are coupled together. In this paper, we present a novel method to solve this problem. During the initial power-up, we apply a sinusoidal stimulus to the coherent IQ transmitter. We then scan the frequency and amplitude of the stimulus and monitor the output power. By curve-fitting with an accurate mathematical model, we determine the limited bandwidth, the nonlinearity, the power imbalance, and the bias point of the transponder simultaneously. Optimized coefficients of the digital filters are determined accordingly. Furthermore, we utilize a coherent IQ transponder and demonstrate that the limited bandwidth is improved by the finite impulse response filter, while nonlinearity is mitigated by the memoryless Volterra filter.

Corrigendum to “Novel transmitter IQ imbalance compensation algorithm using training symbols in PDM CO-OFDM system” [Opt. Commun. 426 (2018) 607–611

Corrigendum to “Novel transmitter IQ imbalance compensation algorithm using training symbols in PDM CO-OFDM system” [Opt. Commun. 426 (2018) 607–611

Detection and compensation of power imbalance, modulation strength, and bias drift in coherent IQ transmitter through digital filter.

A novel method to simultaneously detect power imbalance, modulation strength, and bias drift of coherent IQ transmitter during the initial power-up is presented. This is achieved by sweeping gain scaling factor of finite impulse filter in a digital domain and monitoring the combined output power. Furthermore, by dithering gain scaling factor of finite impulse filter, the power imbalance is measured with live traffic. Those impairments can be compensated accordingly. For example, the power imbalance is compensated through adjustment of gain setting of a radio frequency amplifier. This novel method works for multiple channels over C band, and the built-in photodiode of coherent transmitter provides sufficient accuracy.

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.

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.

Blind Transmitter IQ Imbalance Compensation in M-QAM Optical Coherent Systems

We investigate transmitter IQ imbalance compensation based on the blind adaptive source separation (BASS) method in a dual-polarization M-QAM optical coherent system. The robustness of the BASS method against the residual carrier frequency offset (CFO) is numerically investigated and compared to that of the Gram-Schmidt or-thogonalization procedure (GSOP). We further validate experimentally the proposed method with 10 Gbaud optical 4-QaM and 16-QAM signals at 30° and 10° phase imbalance, respectively, with simulated impairments. More specifically, in the presence of 5 χ 10−6 residual CFO (normalized to the sample rate), the optical signal-to-noise ratio penalty reduction of the BASS method compared to the GSOP method is 1 dB for 4-QAM at a bit error ratio (BER) of 2 χ 10−3 and 2 dB for 16-QAM at a BER of 10−3. In contrast to the GSOP, which requires an independent block, the BASS method can be integrated into an equalizer, simplifying the operation and allowing parallel implementation.

Novel Training Symbol Structure for Transmitter IQ Mismatch Estimation for PDM Coherent Optical OFDM System

In this paper, two novel training symbol structures are proposed to mitigate transmitter IQ mismatch and channel distortion for polarization division multiplexed (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. Accordingly, two corresponding estimation methods are proposed. Compared with the conventional estimation approach, the advantage of the first method is that IQ mismatch and channel distortion are estimated independently, and the benefit of the second method is that the overhead of training symbols is reduced. The theoretical analysis is validated by numerical Monte Carlo simulations of PDM CO-OFDM system. Under the simulation conditions: the phase mismatch 15°, the amplitude mismatch 3 dB, the transmission bit rate 100 Gb/s, the standard signal-mode fiber link 480 km, and bit error rate 1e-3, 24 dB optical signal-to-noise ratio (OSNR) is necessary for the first method, and 28 dB OSNR for the second one.

Self-interference cancellation in full-duplex wireless with IQ imbalance

To achieve full-duplex wireless communication, the most essential issue is to eliminate in-band self-interference caused by simultaneous transmission and reception. In this paper, we investigate the impact of transmitter IQ imbalance on digital signal processing (DSP)-assisted analog self-interference cancellation, where two transmitters are used for transmission and cancellation, respectively. Our result implies that the transmitter IQ imbalance results in residual self-interference after conventional DSP-assisted analog cancellation, which still has a much higher power compared to the desired signal. Then, a baseband cancellation signal generated by widely linear (WL) filtering is proposed to effectively handle the transmitter IQ imbalance, and the optimal WL filter and the adaptive algorithm to obtain them can be derived. The impact of receiver IQ imbalance on the proposed adaptive algorithm is analyzed, and we show that the algorithm is capable of obtaining the same optimal WL filter despite receiver IQ imbalance. In addition, for practical implementation, we propose an adaptive algorithm based on the augmented complex least mean squares (ACLMS) method to obtain the WL filter, where no information about the transmitter and receiver IQ imbalances is needed. The steady state behaviors of the proposed adaptive algorithms are analyzed and the performance bounds are derived. Numerical simulation results confirm the validity and superior performance of the proposed ACLMS algorithm in various practical scenarios of transmitter and receiver IQ imbalances. Also, the tolerance of the proposed algorithm to other transceiver impairments was investigated using simulations.

Joint Compensation of Transmitter and Receiver IQ Imbalance in OFDM Systems Based on Selective Coefficient Updating

In this paper, a selective coefficient updating (SCU) approach at each branch of the per-tone equalization (PTEQ) structure has been applied for insufficient cyclic prefix (CP) length. Because of the high number of adaptive filters and their complex adaption process in the PTEQ structure, SCU has been proposed. Using this method leads to a reduction in the computational complexity, while the performance remains almost unchanged. Moreover, the use of set-membership filtering with variable step size is proposed for a sufficient CP case to increase convergence speed and decrease the average number of calculations. Simulation results show that despite the aforementioned algorithms having similar performance in comparison with conventional algorithms, they are able to reduce the number of calculations necessary. In addition, compensation of both the channel effect and the transmitter/receiver in-phase/quadrature-phase imbalances are achievable by these algorithms.

A High Modulation Bandwidth, 110 GHz Power-DAC Cell for IQ Transmitter Arrays With Direct Amplitude and Phase Modulation

This paper studies the maximum Baud rate and the scalability to the W-Band of the mm-wave power-DAC transmitter architecture. Proof-of-concept implementations of a single DAC lane and of a 2×2 IQ transmitter array are reported in 45 nm SOI CMOS. The DAC cell achieved 29 GHz OOK and 29 GHz BPSK modulation bandwidth and 2×44 Gb/s BPSK+OOK data rates for carriers in the 100-110 GHz range. The corresponding energy efficiency is 7.5 pJ/bit at an output power of 12 dBm. For the 2×2 IQ array, an EVM of 9.0% is estimated over a 12 GHz bandwidth, from large signal power and S-parameter phase measurements.

Widely linear digital self-interference cancellation in direct-conversion full-duplex transceiver

This paper addresses the modeling and cancellation of self-interference in full-duplex direct-conversion radio transceivers, operating under practical imperfect radio frequency (RF) components. First, detailed self-interference signal modeling is carried out, taking into account the most important RF imperfections, namely, transmitter power amplifier nonlinear distortion as well as transmitter and receiver IQ mixer amplitude and phase imbalances. The analysis shows that after realistic antenna isolation and RF cancellation, the dominant self-interference waveform at the receiver digital baseband can be modeled through a widely linear transformation of the original transmit data, opposed to classical purely linear models. Such widely linear self-interference waveform is physically stemming from the transmitter and receiver IQ imaging and cannot be efficiently suppressed by classical linear digital cancellation. Motivated by this, novel widely linear digital self-interference cancellation processing is then proposed and formulated, combined with efficient parameter estimation methods. Extensive simulation results demonstrate that the proposed widely linear cancellation processing clearly outperforms the existing linear solutions, hence enabling the use of practical low-cost RF front ends utilizing IQ mixing in full-duplex transceivers.