Decision Feedback Equalizer Structure Research Articles

Design and verification of DFE re-decision algorithm for PAM6 high-speed transceiver based on FPGA

In this paper, a decision feedback equalizer (DFE) re-decision algorithm for a 6-level pulse-amplitude-modulation (PAM6) high-speed transceiver is proposed. PAM6 is a two-dimensional (2D) modulation. The four outer points are not used in the 2D constellation diagram to reduce the output power. Although the four outer points will not be modulated, they may be encountered during demodulation. This paper considers the above situation and theoretically analyzes the DFE module under the additive white Gaussian noise (AWGN) channel. Compared with the traditional pipelined DFE structure, the proposed algorithm can help solve the four outer points in PAM6. A serializer/deserializer (SerDes) simulation system is built based on a field programmable gate array (FPGA), and the performance of the proposed algorithm is carried out on the simulation system. According to this proposed algorithm, the symbol error rate (SER) of the PAM6 SerDes system can be reduced, and the system performance can be improved. When the DFE tap coefficient is 0.50 and the signal-to-noise ratio (SNR) is 22.0 dB, the proposed algorithm can additionally correct 8.18 % of erroneous symbols. When the DFE tap coefficient is 1.00 and the SNR is 22.0 dB, the proposed algorithm can additionally correct 17.30 % of erroneous symbols.

Swarm Intelligence Based MMSE Frequency Domain Equalization for MIMO Systems

The automatic upgradation of equalizer weights in channel equalization demands a low-complexity, highly accurate estimation of recovery at the minimum possible time. The low-complexity frequency domain equalization improves the minimum mean square error (MMSE) of the equalization process. Adding the superiority of particle swarm optimization (PSO) to the equalizer coefficient selection process enhances the MMSE. This work proposes frequency-domain channel equalization along with a modified PSO (MPSO) as an adaptive algorithm for equalizer weight selection in MIMO systems. The simulation results validate the performance with the time domain linear and decision feedback equalizer structures for BPSK and QAM systems. The parameters are carefully selected by analyzing MMSE thoroughly under timevarying channel conditions.

A Distributed Arithmetic based realization of the Least Mean Square Adaptive Decision Feedback Equalizer with Offset Binary Coding scheme

A Least-Mean-Square (LMS) based Adaptive Decision Feedback Equalizer (ADFE) structure using Distributed Arithmetic (DA) is presented. The filtering and weight-updating operations of the Feed Forward Filter (FFF) and Feedback Filter (FBF) of the ADFE have been recast using DA framework in order to obtain efficient multiplierless realization. Unlike, the DA based realization of the fixed coefficient filter, in case of adaptive filters, the updating of partial-products from time to time is a difficult task. We proposed an efficient technique which uses an auxiliary memory to perform the weight-update operation. This memory is updated from time to time using a register which stores the newest sample of the filter input. The proposed architecture is hardware efficient in the sense that it uses no multiplier units and fewer adders compared to existing architecture. For instance, for an ADFE with FBF length is equal to 8, with proper choice of parameters in the DA structure, the proposed architecture uses around 20% less chip area and power compared to most recent architecture existing in the literature. Simulation results show that the convergence characteristics of the proposed DA based LMS ADFE is almost similar to conventional multiply-accumulate (MAC) based realization.

A Novel Decision Feedback Equalization Structure for Nonlinear High-Speed Links

As the degree of nonlinearity in high-speed links becomes more and more serious, traditional decision feedback equalization (DFE) which is based on linear time-invariant assumption can no longer eliminate the inter-symbol interference effectively. In this paper, the shortcomings of traditional DFE structure for nonlinear high-speed links are first analyzed. Then, the multi-bit response (MBR) method is proposed to accurately construct the bit stream responses of the nonlinear high-speed links. Lastly, a novel DFE structure based on the MBR method is presented to improve the signal quality of the nonlinear high-speed links. The accuracy of the proposed method is verified by the simulation based on a nonlinear high-speed link model. Compared with traditional DFE, the proposed DFE structure can significantly improve the signal quality of the nonlinear high-speed links. As the degree of nonlinearity increases, the advantage of the proposed DFE becomes more prominent.

Diffusive MIMO Molecular Communications: Channel Estimation, Equalization, and Detection

In diffusion-based communication, for molecular systems, the achievable data rate depends on the stochastic nature of diffusion, which exhibits a severe inter-symbol-interference (ISI). Multiple-input multiple-output (MIMO) multiplexing improves the data rate at the expense of an inter-link interference (ILI). This paper investigates training-based channel estimation schemes for diffusive MIMO (D-MIMO) systems and corresponding equalization methods. Maximum likelihood and least-squares estimators of mean channel are derived, and the training sequence is designed to minimize the mean square error (MSE). The numerical validations in terms of MSE are compared with Cramer–Rao bound derived herein. Equalization is based on decision feedback equalizer (DFE) structure as this is effective in mitigating diffusive ISI/ILI. Zero-forcing, minimum MSE, and least-squares criteria have been paired to DFE, and their performances are evaluated in terms of bit error probability. D-MIMO time interleaving is exploited as an additional countermeasure to mitigate the ILI with remarkable performance improvements. The configuration of nano-transceivers is not static but affected by a Brownian motion. A block-type communication is proposed for D-MIMO channel estimation and equalization, and the corresponding time-varying D-MIMO MC system is numerically evaluated.

An improved adaptive DFE structure based on ISI detection

An adaptive decision feedback equalizer (DFE) in 0.18 µm CMOS process for 2.5 GB/s data rate is proposed in this paper which is comprised of three taps. Adaptive DFE circuit is used for the purpose of automatic inter-symbol interference cancellation in cords with different lengths. The status of the high frequency components and low frequency components (LFC) of the signal adjust the coefficient of each tap continuously in online manner. In order to extract the clock, the clock and data recovery circuit is interfused within the DFE. Using the proposed DFE, the status of eye diagram varies from the almost closed to wide open. Total power consumption of the receiver circuit is approximately 50 mW and it occupies the area about 350 µm × 350 µm. Simplicity and less hardware requirements are features of the designed circuit in comparison with the similar circuits.

An Improved FD-DFE Structure for Downlink VLC Systems Based on SC-FDMA

Visible light communication (VLC) technology is considered to be an attractive solution for future wireless indoor multimedia communication. To reduce the peak-to-average power ratio, single-carrier frequency division multiple access (SC-FDMA) with time domain asymmetrical clipping is applied for the downlink multi-user VLC system. An improved frequency-domain decision feedback equalizer (FD-DFE) structure is proposed for the SC-FDMA -based VLC system in this letter. Different kinds of subcarrier distributions are considered in the analyses and simulations. Thanks to the clipping noise information on the even subcarriers, which can reduce the decision error, the proposed algorithm provides a lower bit error rate and better transmission performance compared with traditional schemes.

Iterative Equalization with Decision Feedback based on Expectation Propagation

This paper investigates the design and analysis of minimum mean square error (MMSE) turbo decision feedback equalization (DFE), with expectation propagation (EP), for single carrier modulations. Classical non iterative DFE structures have substantial advantages at high data rates, even compared to turbo linear equalizers - interference cancellers (LE-IC), hence making turbo DFE-IC schemes an attractive solution. In this paper, we derive an iterative DFE-IC, capitalizing on the use of soft feedback based on expectation propagation, along with the use of prior information for improved filtering and interference cancellation. This DFE-IC significantly outperforms exact turbo LE-IC, especially at high spectral efficiency, and also exhibits various advantages and performance improvements over existing variants of DFE-IC. The proposed scheme can also be self-iterated, as done in the recent trend on EP-based equalizers, and it is shown to be an attractive alternative to linear self-iterated receivers. For time-varying (TV) filter equalizers, an efficient matrix inversion scheme is also proposed, considerably reducing the computational complexity relative to existing methods. Using finite-length and asymptotic analysis on a severely selective channel, the proposed DFE-IC is shown to achieve higher rates than known alternatives, with better waterfall thresholds and faster convergence, while keeping a similar computational complexity.

Recursive myriad-mean filters: Adaptive algorithms and applications

In this paper, a new class of recursive hybrid filtering structures is proposed for impulsive noise removal; the so-called recursive myriad-mean (RMyM) filters. More precisely, the output of the RMyM filter can be thought of as the sum of two independent weighted M-filters: the nonlinear weighted myriad acting on a subset of input samples and the linear weighted mean acting on a subset of filter’s previous outputs. The uncoupled structure of the proposed filters takes into account the benefits of both weighted M-estimators: the robustness against impulsive noise of the myriad operator and the desired spectral response induced by the linear feedback. Least mean absolute (LMA) based adaptive algorithms are developed for designing these filtering structures under the equation error formulation framework. The results of extensive simulations are shown to evaluate both the behavior of the adaptive algorithms as well as the performance of the proposed recursive filters against impulsive noise. Additionally, taking into account the uncoupled structure of the proposed recursive filters, a decision feedback equalizer (DFE) based on the RMyM filter is proposed, where its performance is compared to those yielded by various conventional DFE structures, under different conditions of impulsive noise.

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.

The novel feed forward and decision feedback equalizer structures and improved variable step algorithm

Skin effect and dielectric loss in super-long cable will cause nonlinear attenuation at different signal frequency, and in addition, coupling noise and thermal noise also cause signal distortion at the receiver end. These factors seriously affect the signal transmission speed in the super-long cable. Especially, in the field of exploration of shale gas and bed methane, the transmission cable is also used to transport high-precision synchronization pulse signal, and the synchronization pulse must reach the microsecond accuracy, which is used for data phase calibration. A synchronization signal is a high frequency signal, which suffers more severe attenuation and noise interference. At the receiving end, the sync pulse signal will be drowned in the noise environment, and so it is difficult to restore the original signal.#br#Although fiber can achieve a high transfer rate, but the fiber cable cannot transmit power energy; in addition, the tensile strength and heat resistance of the fiber are much worse than copper cable, these weaknesses limit its application in such industry. Therefore, an effective balancing algorithm is necessary to overcome the propagation effects and interference in a super-long copper cable. However, conventional equalization techniques have well-balanced effect for the short-range communications, but for the long-distance communication, they often have poorly balanced results. In order to solve the above problem and improve the long cable signal transmission speed, this paper presents a new balanced portfolio structure; the new structure uses feed-forward equalizer (FFE) as the pre-stage, and decision-feedback equalizer (DFE) as the post stage to form a new structure. The combination structures can effectively utilize the flexibility of FFE and overcome the problem of error diffusion in DFE. By mathematical modeling and simulation, this paper gives the best combination factors. Furthermore, based on the improved structure, a new convergence algorithm is proposed, which uses the arc tangent function combined with three error converge factors to form a converging function, and it has the advantages of fast convergence and steady-state error. Simulation results show that the FFE-DFE combination equalizer has low computational complexity, fast convergence, and strong channel tracking capability; in addition, it can speed up the data processing speed, and better respond to the real variation of the channel. Simulation results show also that the performance is improved by 50% by eliminating inter-symbol interference and noise.#br#The real circuit board based on the new algorithm have been tested in the East China Petroleum Bureau, the test results show that the algorithm can rectify 160 dB signal distortion, and the transmission speed can reach 5 Mbps in 6 dB signal to noise ratio.

Stochastic Gradient Pursuit for Adaptive Equalization of Sparse Multipath Channels

In this paper, a new heuristic algorithm for the sparse adaptive equalization problem, termed as stochastic gradient pursuit, is proposed. A decision-feedback equalization structure is used in order to effectively mitigate the effect of long multipath channels. Diverging from the commonly used approach of sparse channel identification, we exploit the sparsity of the inverse problem under the compressive sensing perspective. Also, an extension to the case where the sparsity order parameter is unknown, is developed. Simulation results verify that the proposed schemes exhibit faster convergence and improved tracking capabilities compared to conventional and other sparse aware equalization schemes, offering at the same time a reduced computational complexity.

A RAKE Receiver With an ICI/ISI Equalizer for a CCK Modem

In this paper, we first derive the theoretical performance of a complementary code keying (CCK) code on an additive white Gaussian noise (AWGN) channel and over a multipath channel. To derive the error performance, we use the weight and cross-correlation distributions of the CCK code for optimal and suboptimal decoding, respectively, based on union bound. In addition, we propose a RAKE receiver for a CCK modem, which is suitable for a multipath environment with a large delay spread. The RAKE receiver principle is acceptable for modest multipath because it can coherently combine multipath components to provide signal-to-noise ratio (SNR) enhancement. However, as the delay spread is larger and the data rate of systems goes higher, intersymbol interference (ISI) generated due to multipath environments are increased. To handle the increasing ISI, the CCK modem needs an equalization technique to remove the ISI, together with RAKE processing. Thus, our proposed system is based on a channel matched filter (CMF) with a decision feedback equalizer (DFE). The CMF is applied for RAKE processing, whereas the DFE structure is used for ISI cancellation. In our system, ISI is calculated and removed by using a decoded CCK codeword.

A DFE Structure Using Quadrature Components of 8-VSB Signals

In this paper, we present a decision feedback equalizer (DFE) that uses the quadrature components of 8-VSB signals in ATSC systems to mitigate multipath distortion efficiently. Unlike the conventional DFE structure for 8-VSB signals in which a real-valued feedback filter is used with real-valued decisions, the proposed DFE uses a complex-valued feedback filter with a complex-valued feedback sample generator. Due to the difficulty in analyzing a recursive system, we present a limited performance analysis on the proposed algorithm that a well-modeled feedback filter successfully cancels multipath channels with quadrature leakages in the absence of noise. For the performance comparison with conventional DFEs in realistic environments, we uses simulation methods.

Joint preprocessing and feedback strategies for perfectly reconstructing equalizers

In this paper we consider the transmission of discrete-valued data via a communication channel that is subject to (additive) noise with a known upper bound on its magnitude but otherwise completely unrestricted and unknown behavior. We consider a discrete-time setup and extend previous equalization strategies for perfect reconstruction by allowing linear preprocessing of the data and/or linear feedback from the receiver to the transmitter. We are interested in the characterization of general conditions that allow perfect reconstruction of the discrete data with any given (possibly nonzero) delay (and under all possible realizations of channel noise and a limit on the power of transmission) when linear preprocessing of the data and/or linear feedback from the receiver is employed. In particular, we obtain necessary and sufficient conditions for perfect reconstruction under either linear power-limited preprocessing or linear power- limited preprocessing along with linear feedback. We prove that in order to improve the conditions for perfect reconstruction, it is necessary that the feedback and preprocessing systems are unstable. We also consider the case when a Decision Feedback Equalizer (DFE) structure is imposed at the receiver and provide necessary conditions for improvements in the perfect reconstruction in terms of l <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> norms of appropriate maps. In addition, a procedure that results in parametric l <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> optimization is developed to design a DFE to improve the maximum tolerable noise bound.

FRESH-DFE: A New Structure for Interference Cancellation

This paper proposes a new structure of decision feedback equalizer that exploits the cyclostationary properties of digitally modulated signals to mitigate interference. In the proposed structure, the forward filter of the conventional DFE is replaced by a cyclic filter. It is assumed that the desired and the interference signals use some mutually different signaling attributes, for example symbol rates, centre frequency etc. The resulting structure is evaluated in the presence of up to six strong interfering signals, a scenario that is typically found in wireless cellular systems. The proposed structure provides performance gains for some modulation formats but it reduces to the conventional DFE for other signal formats.

Interpolated decision feedback equalization for long intersymbol interference channels

The decision feedback equalizer (DFE) is widely used in communication systems combating intersymbol interference effect. In wireline communications, the channel response is usually long and the computational complexity of the DFE is often high. This is particularly true when the precoding is applied in which the feedback signal value becomes continuous rather than discrete. In this paper, an interpolated DFE (IDFE) structure is proposed to solve the problem. The proposed IDFE consists of a finite-impulse-response (FIR) feedforward filter and an interpolated FIR feedback filter. Thanks to the interpolation operation, the computational complexity of the DFE can be greatly reduced. The least-mean-squared (LMS) algorithm is applied to train the tap weights yielding an adaptive IDFE. Closed-form expressions for optimal filter coefficients, mean squared errors, and signal-to-noise ratios are derived. The convergence behavior of the adaptive IDFE is also analyzed. Simulation results show that while the computational complexity is significantly reduced, the performance of the proposed IDFE is similar to the conventional DFE. The computational saving can be as high as 76% for a high-speed digital subscriber line application.

Adaptive equalization of time-varying MIMO channels

This paper addresses the problem of adaptive multiple-input multiple-output (MIMO) equalization of time-varying channels (TVC). The proposed technique is based on Kalman filtering and decision feedback equalization. The time-varying channels can be estimated and tracked using a Kalman filter type of algorithm. An equalization technique for MIMO channels is derived based on a novel decision feedback equalizer structure. Moreover, we present a method to estimate the measurement and state noise variances used by the Kalman filter because they are crucial parameters in obtaining a reliable performance. The performance of the algorithm is investigated in simulations using realistic channels generated based on the COST 207 model. The results show that the algorithm performs well even in low SNR conditions or in difficult channel conditions.

Receivers with chip-level decision feedback equalizer for CDMA downlink channels

For the code division multiple access (CDMA) downlink channel, we investigate the decision feedback equalizer (DFE) at chip-level to effectively suppress multiple-access interference (MAI) when the spreading sequences are orthogonal. The structure of the receiver with the chip-level DFE has been investigated and the minimum mean-square error solution has been derived. Due to the inherent structure of the chip-level DFE, some iterative techniques with hard and soft decisions have been proposed. It is shown that the proposed receivers with the chip-level DFE can provide satisfactory performance. In comparison with the adaptive chip-level linear equalizer, the number of users can be doubled by using adaptive chip-level DFE at a bit-error rate of 10/sup -3/. Throughout the paper, we assume that scrambled orthogonal codes are used for spreading sequences.

Performance of a hybrid decision feedback equalizer structure for CAP-based DSL systems

We study the performance of a class of derision feedback equalizer (DFE) structures for high-speed digital transmission systems. We first present mathematical formulation of minimum mean-square error (MMSE) and the optimum tap coefficients for various finite-length phase-splitting equalizers over the loop in the presence of colored noise, such as near-end crosstalk (NEXT) and far-end crosstalk (FEXT). The performance of the equalizers is also analyzed in the presence of narrowband interference and the channel reflections introduced by bridged taps. The hybrid-type DFE (H-DFE) is presented as a practical equalizer structure for these applications. The results of analysis show that the H-DFE has advantages in the performance and/or in the implementation complexity as compared with the existing DFE structures. An additional advantage of the H-DFE is in the transmission systems that employ the precoding technique. The precoding for the H-DFE allows the system to track small changes in the channel.