Feedforward Filter Research Articles

An Enhanced QAM-FBMC Scheme With Interference Mitigation

Cyclic prefixed orthogonal frequency division multiplexing (CP-OFDM) has been used as a waveform for mobile communications, but CP-OFDM has weaknesses in an asynchronous heterogeneous network scenario. To overcome the disadvantage of CP-OFDM, a quadrature amplitude modulation (QAM)-filter-bank-based multi-carrier (FBMC) system is proposed to achieve superior spectrum confinement and better spectral efficiency. However, QAM-FBMC has inevitable performance degradation because of interferences from non-orthogonal filters. In this letter, we propose a decision feedback equalization (DFE)-based interference mitigation scheme for the QAM-FBMC system to cancel a residual interference. The interference from the filters of QAM-FBMC influences to other subcarriers and symbols, and it is modeled as an end-to-end system transfer function with para-Hermitian poly-nomial matrix form, equivalently. By spectral factorization of the system transfer function, the system response is transformed into causal, and the receiver detects the symbols successively. This letter describes the spectral factorization method and formulates the feed-forward and feedback filter of the DFE structure with the spectral factor of transfer function. Simulation results show the superiority of the proposed scheme to the conventional QAM-FBMC receiver scheme.

A Variable Leaky Entropy-Based Whitening Algorithm for Blind Decision Feedback Equalization

This paper addresses the joint entropy maximization algorithm constrained by the variable leaky factor (JEM-VL) aiming at mitigating the feedback filter (FBF) mismatch effects characterizing the operation of the decision feedback equalizer which, at the start of adaptation, swaps positions of feedforward and feedback filters so that the latter acts as a linear all-pole whitener of the received signal. The FBF mismatch is a result of disparity between the FBF setup achieved in the blind mode by observing channel outputs and an excepted FBF setup in the tracking mode which is driven by detected data symbols. Depending on the given signal complexity and inter-symbol interference severity, the FBF filter mismatch is typically manifested by the equalizer convergence instability or even the catastrophic error propagation effects arising at the time of the equalizer structure-criterion switching from the blind to the tracking operation mode. The constraint of superfluous coefficients of the FBF filter by means of the JEM-VL algorithm eliminates the equalizer convergence instability at the time of its switching and, consequently, increases equalization successfulness. The efficiency of the JEM-VL algorithm is verified by simulations using the 64-QAM signal.

The DFF‐PSP Iterative Separation and Theoretical Bound for PCMA with Long Memory

This paper proposes the Decision feedbackfeedforward per-survivor processing (DFF-PSP) iterative separation algorithm and the Monte Carlo integrationbased method to compute the theoretical bound for Paired carrier multiple access (PCMA) Single channel blind separation (SCBS) which is highly complex with long memory. The truncated PSP is employed while the pre- and post- cursors are disregarded to control the complexity. The delayed decisions feedforward and local decisions feedback filters are designed to process the pre- and post- cursors. The Bit-interleaved code modulation iterative decoder (BICM-ID) is combined with the second soft decision of the truncated PSP to obtain a better performance. The mixture signal model is regarded as a Multiple-access channel (MAC) with memory, and the theoretical performance bound is obtained via entropy inequality, Monte Carlo integration and Fano inequality. Simulation results show that the DFF-PSP iterative separation provides the good tradeoff between complexity and performance.

DA based approach for the implementation of block adaptive decision feedback equaliser

Adaptive decision feedback equalisers (ADFEs) are used in wireless transmission systems for mitigating the InterSymbol Interference (ISI) that occurs due to multipath propagation of the transmitted signal. In case of high speed communications which involve rapid-varying channels, fast convergent and low complexity ADFEs are required. Frequency domain block processing of signals is an effective means of handling the increased complexities in such high speed systems. However, block ADFEs being inherently non-causal, the feedback filter (FBF) suffers from the lack of unknown decisions in every block computation. In this study, the authors propose an efficient approach for the computation of these unknown decisions. For this, the authors tried to minimise a cost function based on two criteria mean-absolute difference (MAD) and mean-square difference between the ADFE output and the adder that sums up the feedforward filter (FFF) and FBF. A bank of registers which store all the symbols used in the modulation scheme is also utilised for this purpose. Using the proposed solution, the authors recast block ADFE equations in the frequency domain using distributed arithmetic (DA). The algorithm has a good convergence performance and utilises only few computations compared with the existing schemes.

Asymptotically Optimal Low-Complexity SC-FDE With Noise Prediction in Data-Like Improper-Complex Interference

In this paper, single-carrier (SC) block transmission of improper-complex data symbols is considered over a multiple-input multiple-output (MIMO) frequency-selective channel with data-like cochannel interference (CCI). To exploit the cyclostationarity and impropriety of the desired and the interfering signals, an equalizer is designed that oversamples the received signal and then processes the sampled block by using widely-linear (WL) feedforward (FF) and noise-predicting feedback (NP-FB) filters. For the given equalizer structure, the minimum mean-squared error (MMSE)-optimal WL FF and NP-FB filters have high computational complexity due to the augmented correlation matrix of the data-like CCI in the received signal. Motivated by the asymptotic property of the correlation matrix, we approximate in the frequency domain the matrix by a block matrix with diagonal blocks. This leads to the low-complexity WL design of a frequency-domain FF filter and a causally noise-predicting FB filter. It is shown that this MIMO SC frequency-domain equalizer with noise prediction is asymptotically optimal in the sense that the average mean-squared error converges to that of the MMSE-optimal equalizer as the block length tends to infinity. Numerical results show that the proposed equalizer performs well even with a moderate block length.

Maximum entropy whitening algorithm with adaptive neuron slope for blind DFE

The addressed blind decision feedback equalizer (DFE) reverses the classical order of its feed-forward and feedback filters at the beginning of adaptation to achieve the best equalization of the minimum and maximum phase components of a channel transfer function. Although very effective, this blind equalization approach deals with the feedback filter mismatch at the time of its transformation from the front-end all-pole whitener of the received signal to the decision-directed feedback equalizer placed after the feedforward filter. To eliminate this weakness, the adaptive neuron slope is introduced instead of the fixed one into the stochastic gradient whitening algorithm based on the joint entropy maximization cost. The performance of the innovated algorithm is verified by simulating m-QAM (quadrature amplitude modulation) signals transmission over multipath channels. The algorithm with the adaptive neuron slope achieves a better whitening of the received signal spectrum, and, hence, increases the equalization successfulness.

AFD-DFE using Constraint-Based RLS and Phase Noise Compensation for Uplink SC-FDMA

In this paper, we develop a constraint-based block recursive least-squares (CRLS) for an adaptive frequency-domain decision feedback equalizer (AFD-DFE) in uplink single-carrier frequency division multiple access systems. For the AFD-DFE, both the feedforward and feedback filters are implemented in the frequency domain; therefore, the CRLS complexity can be reduced substantially when compared to its time-domain counterpart by exploiting the matrix structure in the frequency domain. The performance of the CRLS algorithm is better than that of the RLS when applied to the AFD-DFE, with no increase in the computational complexity. Our designed AFD-DFE with CRLS not only enjoys a lower computational complexity when compared to the frequency-domain channel-estimate-based minimum mean square error DFE (MMSE DFE), but its performance is also better than that of the MMSE DFE with decision errors (practical case) and is close to the MMSE DFE with correct decisions (ideal case). Moreover, we iteratively compensate the transmitter and receiver phase noise using its properties in the time and frequency domains. Simulation results demonstrate the robustness of our designed AFD-DFE using CRLS.

An Algorithm for Network Fluctuation Hopping Signal Suppression Based on Disturbance Characteristics Decomposition and Feed-Forward Modulation

In the data communication, a time-frequency hopping resonance signal will be created, and this network fluctuation hopping signal must be suppressed in order to improve the network stability. An algorithm for network fluctuation hopping signal suppression based on disturbance characteristics decomposition and feed-forward modulation is proposed, and the disturbance characteristics decomposition for a network fluctuation hopping signal is done in a Hilbert space. A feed-forward modulation filter is created to get the feed-forward modulation suppression of the network fluctuation hopping signal. The simulation results indicate that this algorithm has good real-time data transmission, which can effectively suppress the resonance signal in the network fluctuation hopping, low loss of information, and has the obvious function to resolve the startup hysteresis, server load, tremble and other problems in the large-scale hybrid networking.

Optimal transient performance under output set‐point reset

SummaryThe purpose of this paper is to propose a new method for the optimization of the output transition in the case of set‐point reset for LTI, non‐minimum phase, possibly non‐hyperbolic plants. Assuming that the plant is stabilized by a proper feedback controller, the problem consists in finding a feedforward linear filter yielding a suitable reference trajectory for the closed‐loop system. The approach situates in the framework of model pseudo‐inversion because the external reference trajectory is computed starting from some desired features of the transient output between the two set points. A significant aspect of the new method is that the transition trajectory is not ‘ad hoc’ exactly prespecified by the designer. Rather, it is implicitly defined by the procedure for the minimization of a suitable multi‐objective quadratic cost functional. As no pre‐actuation is required, the method can be practically implemented on line and also works for the critical class of non‐hyperbolic systems. Copyright © 2015 John Wiley & Sons, Ltd.

Distributed Bi-Directional Training of Nonlinear Precoders and Receivers in Cellular Networks

Joint optimization of nonlinear precoders and receive filters is studied for both the uplink and downlink in a cellular system. For the uplink, the base transceiver station (BTS) receiver implements successive interference cancellation, and for the downlink, the BTS station pre-compensates for the interference with Tomlinson-Harashima precoding (THP). Convergence of alternating optimization of receivers and transmitters in a single cell is established when filters are updated according to a minimum mean squared error (MMSE) criterion, subject to appropriate power constraints. Adaptive algorithms are then introduced for updating the precoders and receivers in the absence of channel state information, assuming time-division duplex transmissions with channel reciprocity. Instead of estimating the channels, the filters are directly estimated according to a least squares criterion via bi-directional training: Uplink pilots are used to update the feedforward and feedback filters, which are then used as interference pre-compensation filters for downlink training of the mobile receivers. Numerical results show that nonlinear filters can provide substantial gains relative to linear filters with limited forward-backward iterations.

An Efficient Distributed Arithmetic-Based Realization of the Decision Feedback Equalizer

A distributed arithmetic (DA)-based decision feedback equalizer architecture for IEEE 802.11b PHY scenarios is presented. As the transmission data rate increases, the hardware complexity of the decision feedback equalizer increases due to requirement for large number of taps in feed forward and feedback filters. DA, an efficient technique that uses memories for the computation of inner product of two vectors, has been used since DA-based realization of filters can lead to great computational savings. For higher-order filters, the memory-size requirement in DA would be high, and so ROM decomposition has been employed. The speed is further increased by employing digit-serial input operation. Two architectures have been presented, namely the direct-memory architecture and reduced-memory architecture where the later is derived using the former. A third architecture has also been presented where the offset-binary coding scheme is employed along with the ROM decomposition and digit-serial variants of DA. Synthesis results on Altera Cyclone III EP3C55F484C6 FPGA show that the proposed DA-based implementations are free of hardware multipliers and use less number of hardware resources compared to the multiply-and-accumulate-based implementation.

Enhanced Tracking for Nanopositioning Systems Using Feedforward/Feedback Multivariable Control Design

This paper proposes a systematic synthesis methodology for a combined feedforward/feedback architecture to control multiple-input, multiple-output nanopositioning systems. Coprime factorization of the open loop model is used to design the reference and feedforward filters of the proposed control scheme to achieve enhanced tracking, eliminate the limitation of the feedback on tracking performance, and increase the bandwidth of the closed-loop system. Two types of coprime factorization, namely inner-outer factorization and normalized coprime factorization are discussed. A case study based on hardware experiment is presented to analyze the proposed control architecture and demonstrate its superiority over feedback-only control. In addition to the no-load case, the performance of the system is also tested with loads on the nanopositioning stage.

Low‐complexity soft‐interference cancellation turbo equalisation for multi‐input–multi‐output systems with multilevel modulations

This study presents a low-complexity soft-interference cancellation equaliser (SICE) for the turbo detection of multiple-input–multiple-output systems operating in time dispersive channels. The SICE contains three time-invariant linear filters: a feedforward filter, a causal feedback filter and an anti-causal feedback filter. The feedforward filter is designed to suppress the intersymbol interference because of channel time dispersion and the multiplexing interference from multiple transmit antennas. The causal (or anti-causal) feedback filter is developed to remove the residual interference caused by the symbols transmitted before (or after) the symbol under detection. The filters are designed by analysing the statistical properties of soft decisions. The performance of the proposed SICE is verified through both extrinsic information transfer (EXIT) chart analysis and computer simulations. The EXIT chart analysis shows that, because of the inclusion of the anti-causal soft decision, the SICE performance approaches the ideal matched filter bound as the iteration progresses. Consequently, the proposed SICE achieves significant performance gains over conventional equalisers.

Improved Plucked String Synthesis Model Considering Tuning and Playing Style

ABSTRACTThis paper describes a plucked string synthesis model including a simplified tuning system and a right-hand playing style. The proposed model based on digital waveguide synthesis is applied to the Gayageum, a traditional Korean plucked string instrument. The Gayageum has 12 twisted silk strings, each supported by a movable bridge called an Anjok, which is placed on the body and is moved sideways along a string to tune it. The simplified tuning system is formulated with parameters related to the tension of the string and location of the Anjok, and these determine the pitch of a specific string. The unique right-hand playing style, a consecutive flick in which a player flicks a string twice in succession by the middle and index fingers, is implemented by a simple feed-forward filter. The proposed synthesis model is evaluated based on the frequency signal-to-noise ratio.

Robust Two Degree-Of-Freedom Control for MIMO System with Both Model and Signal Uncertainties

In the study, a model inversion based two degree-of-freedom (DOF) controller is proposed to achieve optimal robust performance for multi-input-multi-output (MIMO) system with existence of both model and signal uncertainties. The original plant dynamics are modified by a pre-compensator and a post-compensator to obtain a diagonally dominant matrix, where the selected dynamics of the plant are contained within the diagonal entries. The inversion of the diagonal matrix, as a result, reflects the inversion of the plant dynamics within a specified frequency range. The feedback controller is designed using a standard H2 mixed sensitivity approach. The feedforward controller consists of plant inversion and a feedforward filter, which is optimized to balance the trade-off between nominal performance and robust performance under model uncertainty and signal uncertainty. A numerical example is used to demonstrate the improvement of the robust performance when using the optimal feedforward control.

Adaptive Frequency-Domain RLS DFE for Uplink MIMO SC-FDMA

It is well known that, in the case of highly frequency-selective fading channels, the linear equalizer (LE) can suffer significant performance degradation compared with the decision feedback equalizer (DFE). In this paper, we develop a low-complexity adaptive frequency-domain DFE (AFD-DFE) for single-carrier frequency-division multiple-access (SC-FDMA) systems, where both the feedforward and feedback filters operate in the frequency domain and are adapted using the well-known block recursive least squares (RLS) algorithm. Since this DFE operates entirely in the frequency domain, the complexity of the block RLS algorithm can be substantially reduced when compared with its time-domain counterpart by exploiting a matrix structure in the frequency domain. Furthermore, we extend our formulation to multiple-input–multiple-output (MIMO) SC-FDMA systems, where we show that the AFD-DFE enjoys a significant reduction in computational complexity when compared with the frequency-domain nonadaptive DFE. Finally, extensive simulations are carried out to demonstrate the robustness of our proposed AFD-DFE to high Doppler and carrier frequency offset (CFO).

Performance Optimization of MIMO Y Channels: Interference Alignment and Signal Detection

In this letter, we propose a combined scheme of interference alignment and signal detection to optimize the performance of a multiple-input multiple-output (MIMO) Y channel, which consists of three users and a middle relay equipped with nU and nR antennas, respectively. First, an iterative interference alignment algorithm is introduced to optimize both beamforming and combining vectors by minimizing the spillage of the interference signals, and ensuring that the desired signals fall into the orthogonal complement of the interference subspace. Then, a signal detection scheme, which employs feed-forward and feedback filters, is applied at the relay to further deal with system interferences. Simulation results show that the proposed combined scheme for MIMO Y channels produces a significant improvement over the existing schemes.

Two degrees of freedom control of a ball and beam system

In this paper, a two-degree of freedom (2DOF) controller is designed for a ball and beam system. The controller is developed based on the algebraic method. The ball and beam system is one of the most popular laboratory experiments for control education. The controller is designed such that the ball can track a square wave with a certain design specification. The advantages of 2DOF controller are the feedback controller takes care of the uncertainty and the feedforward filter ensures the tracking of the reference command. Though the control method is not new, the application of the technique on such unstable system is of interest. Controlling the ball on the beam is a challenging task because of the instability of the system and the fact that the output, i.e. the ball position, increases almost without limit for a fixed input beam angle. The controller needs to regulate the position of the ball by changing the angle of the beam at the pivot point. It is a difficult control task as the ball moves with an acceleration which is proportional to the tilt angle of the beam. Two types of 2DOF controllers are considered; based on dominant pole design and based on integral time absolute error (ITAE) design. The performances of the resulting controllers are compared. Simulations were run over various frequencies. The results indicate the effectiveness of the designed 2DOF controllers in achieving the design specification.

Impedance-Based Resonance Analysis in a VSC-HVDC System

Resonances can limit power transfer in a voltage-source converter-high voltage dc (VSC-HVDC) system. The objective of this paper is to develop impedance models for the rectifier ac system and the inverter ac system for a VSC-HVDC system. Resonance stability will be examined using Nyquist stability criterion and impedance frequency responses. The impedance models consider the outer control loop and the inner current control loops of VSCs. Impacting factors are then examined. Stability analysis demonstrates that the feedforward filter, line length, and power transfer level have a significant impact on resonances. Time-domain simulation results obtained by Matlab SimPowerSystems are used to validate the analysis.

On the implementation and control of a pneumatic power active lower-limb orthosis

This paper develops a pneumatic power active lower-limb orthosis (PPALO) to be a controlled plant. Due to the use of pneumatic actuators, the PPALO inherently possesses non-smooth nonlinearities, such as asymmetric dynamics, friction, and dead zone. In order to eliminate the influence of these nonlinearities on the pneumatic actuators and the dynamic coupling terms included in the dynamics of the lower-limb orthosis, an inner-loop PI controller with a differential pressure feedback and an outer-loop filter-based iterative learning control (FILC) scheme which consists of an outer PD feedback controller as well as a feedforward filter are used. Finally, a trajectory tracking control experiment is conducted to validate that the proposed method can effectively control the system to track the desired trajectory and reduce the vibration caused by nonlinearities of the pneumatic actuators.