Conventional Decision Feedback Equalizer Research Articles

Robust equalization for Single-Carrier Underwater Acoustic Communications Based on Parameterized Interference Model

There are several sources of artificial and naturally generated impulsive interference in marine environment which could severely degrade the acoustic communication performances. This letter tackles the equalization for single-carrier modulated systems with impulsive interference based on parameterized model, and a two-step equalization algorithm is proposed. In the first step, an iterative interference impulse estimation/cancellation can be performed and initial symbol estimation is obtained by conventional decision feedback equalization. In the second step, a joint channel, impulse, and symbol estimation for single-carrier modulation is applied to enhance the performance. The proposed algorithm is evaluated using partial-band partial-block-duration interference. Simulation results show that the proposed joint estimation algorithm has significant performance advantages over conventional algorithm in various signal-to-interference ratio levels.

ML-Assisted Equalization for 50-Gb/s/λ O-Band CWDM Transmission Over 100-km SMF

We propose and demonstrate a bidirectional Vanilla recurrent neural network (Vanilla-RNN) based equalization scheme for O-band coarse wavelength division multiplexed (CWDM) transmission. Based on a 4 × 50-Gb/s intensity modulation and direct detection (IM/DD) system, we demonstrate the significantly better bit error rate (BER) performance of the Vanilla-RNN scheme over the conventional decision feedback equalizer (DFE) for both Nyquist on-off keying (OOK) and Nyquist 4-ary pulse amplitude modulation (PAM4) formats. It is shown that the Vanilla-RNN equalizer is capable of compensating for both linear and nonlinear impairments induced by the transceiver and the single-mode fiber (SMF). As a result, up to 100-km and 75-km SMF transmission can be achieved for OOK and PAM4 transmission, respectively. Furthermore, through the comparison with other equalization schemes, including the linear equalizer, 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> -order Volterra equalizer, and Volterra+DFE, it is demonstrated that the Vanilla-RNN equalizer achieves the best BER performance. In the meantime, it also exhibits lower implementation complexity when compared to Volterra-based schemes. Our results show that the Vanilla-RNN scheme is a viable solution for realizing simple and effective equalization. This work serves as an exploration and offers useful insights for future implementations of reach-extended O-band CWDM IM/DD systems.

A 112-Gb/s PAM-4 Low-Power Nine-Tap Sliding-Block DFE in a 7-nm FinFET Wireline Receiver

Practical realization of decision feedback equalizers (DFEs) has to date been limited to at most two taps in 100-Gb/s long-reach (LR) wireline applications due to significant power, area, and timing costs. This article presents a systolic many-tap low-complexity sliding-block decision feedback equalizer (SB-DFE) that overcomes the implementation challenges of conventional DFEs with no performance loss. A nine-tap configuration is demonstrated in a 112-Gb/s analog-to-digital converter (ADC)-digital signal processing (DSP) four-level pulse amplitude modulation (PAM-4) LR wireline receiver implemented in 7-nm FinFET. The architecture partitions the received signal into overlapping but computationally independent blocks thereby breaking the feedback loop of the DFE and allowing logic pipelining. Unlike existing feedback-breaking techniques, the computational overhead of the SB-DFE can be made arbitrarily small for any tap count—indeed, we show the practicality of SB-DFE implementations exceeding 30 taps. Optimized pipeline cuts are employed to minimize the latency through the SB-DFE while maintaining timing margin. The nine-tap SB-DFE is paired with a five-precursor tap feedforward equalizer (FFE) and compared to a two-tap-DFE 15-tap-FFE reference DSP implemented in the same receiver. A bit error rate of 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times $ </tex-math></inline-formula> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−12</sup> is measured over a 36-dB loss channel—at least an order-of-magnitude reduction compared to the reference DSP. Power is reduced by 0.33 pJ/b. DSP gate area is reduced by 30%. Noise tolerance is improved by 0.2-mV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RMS</sub> . Error-free operation is demonstrated on an RS(544,514) KP4 forward error correction (FEC)-encoded link even when the DFE tap values are manually stressed. Techniques for further reduction in complexity are described.

Experimental Demonstration of 55-m / 2-Gbps Underwater Wireless Optical Communication Using SiPM Diversity Reception and Nonlinear Decision-Feedback Equalizer

Underwater wireless optical communication (UWOC) is considered as an enabling technology with a mass of potential applications. The silicon photomultiplier (SiPM) exhibits a bright prospect for UWOC thanks to the traits of low-light detection capability, low-voltage operation, and superior operability. However, the performance of the SiPM-based UWOC system is severely degraded by the dead-time caused nonlinear response. In this paper, to mitigate the dead-time induced nonlinear distortion and explore the achievable capacity of the newly developed SiPM, we propose and experimentally demonstrate a 55-m/2-Gbps UWOC system by virtue of SiPM diversity reception and nonlinear decision-feedback equalizer (NDFE). The performance of NDFE is superior to that of the conventional decision-feedback equalizer (DFE), and NDFE with a pruning factor of 5 declares similar performance as that without pruning strategy, while the number of the nonlinear equalizer can be reduced by &#x007E;31.8&#x0025;. Significant performance improvement is also obtained by the proposed scheme under different turbidity waters. The measured data rate is pushed from 1 Gbps to 2 Gbps with a receiver sensitivity as low as &#x2212;41.96 dBm, which to the best of our knowledge is the largest data rate ever achieved using the off-the-shelf SiPM among the reported UWOC works. In accordance with the receiver sensitivity and the model of optical propagation in the water channel, the maximum attainable distance/data rate is predicted to be 147 m/1 Gbps and 128 m/2 Gbps with the proposed scheme. The research results are promising for long-reach and high-speed UWOC.

A new flip-flop based on discrete-time parametric amplifier

This paper introduces a low power flip-flop based on the discrete-time parametric amplifier, with improved rise (fall) time, setup-time, hold-time, and area. Post-layout simulation result of the proposed flip-flop in a 180 nm CMOS technology with 3.33 GHz frequency illustrates about 21% of the power consumed by CML flip-flop and about 30% improvement in the clock-to-q delay. Also, the simulation shows improved rise (fall) time, lower data-to-q delay compared to dynamic CML and Footless flip-flop, lower area than Charge Steering flip-flops without the need for additional return-to-zero to non-return-to-zero conversion circuitry. When using this flip-flop as a slicer in a decision feedback equalizer (DFE) circuit of a serial link receiver of 1 m Nelco backplane channel, the post-layout simulation results show that the DFE circuit based on the parametric flip-flop has about 50% improvement of power consumption in comparison with the conventional CML DFE circuit in identical conditions.

Linear Approximation Signal Detection Scheme in MIMO-OFDM Systems

In this paper, a linearly approximate signal detection scheme is proposed in multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) systems. The huge MIMO-OFDM system, which uses many transmit antennas and high order modulation, requires a detection scheme at the receiver with very low complexity for practical implementation. In the proposed detection scheme, one N × N MIMO-OFDM system is divided into N / 2 2 × 2 MIMO-OFDM systems for linear increase of complexity. After the partial zero-forcing (ZF), decision feedback equalizer (DFE) and QR decomposition-M algorithm (QRD-M) are applied to each 2 × 2 MIMO-OFDM system. Despite nonlinear detection schemes, the overall complexity of the proposed algorithm increases almost linearly because the DFE and the QRD-M are applied to 2 × 2 MIMO-OFDM systems. Also, the value of M in the QRD-M is fixed according to position of the center point in constellation for efficient signal detection. In simulation results, the proposed detection scheme has higher error performance and lower complexity than the conventional ZF. Also, the proposed detection scheme has very lower complexity than the conventional DFE, with slight loss of error performance.

A Time-Based Receiver With 2-Tap Decision Feedback Equalizer for Single-Ended Mobile DRAM Interface

A time-based (TB) receiver (RX) with a 2-tap TB decision feedback equalizer (DFE) is proposed for mobile DRAM interface. The TB RX consists of a voltage-to-time converter (VTC), a TB DFE, and a time comparator. The VTC converts the RX input voltage to a time difference between two VTC outputs by using the difference in clock-to-Q delays between two latches with different input offset voltages. The TB DFE inserts an additional delay to one of the two VTC outputs and bypasses the other VTC output to increase the time opening. The time comparator makes a decision with the first arriving edge of the two outputs of the TB DFE. While the feedback loop delay must be less than 1 UI for proper operation in the conventional voltage-based DFE, the TB DFE allows the feedback loop delay up to 1.43 UI in this paper. A transmitter (TX) transmits a single-ended signal of 200-mV swing by using an n-over-n voltage-mode driver. The transceiver in a 65-nm CMOS process achieves a 12.5 Gb/s with a 0.8-V supply through a 15-inch FR-4 channel of 14-dB loss. The TX and RX chip consume 4.3 and 3.4 mA, respectively. The energy efficiency is 0.49 pJ/b.

An Energy Efficient VLSI Architecture of Decision Feedback Equalizer for 5G Communication System

Decision feedback equalizer (DFE) is widely used to mitigate the inter-symbol interference. In gigabit applications, the complexity of time domain DFE grows exponentially with feedback filter (FBF) order. In this paper, we have derived low complexity, high throughput architectural designs for time domain DFEs, which meets the requirements of 5G communication systems. These designs are based on the concept of pre-speedup the conventional DFE architecture. The number of FBF coefficient combinations to be stored in the lookup table (LUT) and the hardware complexity of proposed designs are reduced by almost $(M/2)^{-1}$ with $M$ being the number of levels in quadrature-amplitude modulation (QAM). The proposed design achieves the throughput nearly 2.15 Gb/s for 16-QAM. A new strategy for updating the FBF coefficients is proposed to improve the convergence rate and steady state error performances without extra hardware overhead. This is based on storing the past decisions in separate LUT, whose contents are updated time to time using parallel error multiplexor. In addition, the bit-error-rate performance of proposed designs is evaluated for different speedup factors in AWGN and Rayleigh fading channels. From the implementation results, it is found that the area, power, and logic utilization of the proposed designs are significantly reduced. For example, an $8^{\mathrm{ th}}$ -order FBF of proposed adaptive DFE for 16-QAM occupies times 1.19 less area, 1.04 times less area-time product, with nearly consumes 2.48 times less power and; utilizes 1.93 times less slice LUTs and 2.04 times less flip-flops as compared with the best existing scheme.

Finite-length predictive decision feedback equalizer design for multipath channels with large delay spread

Decision feedback equalizers (DFEs) have been widely used to mitigate the effect of intersymbol interference. Most previous studies have focused on conventional DFEs (CDFEs), and relatively little research has addressed predictive DFEs (PDFEs). A finite-length minimum-mean-squared-error predictive DFE (MMSE–PDFE) was developed herein in the presence of multipath channels with large delay spread. We found that the MMSE–PDFE may have lower computational complexity and achieve a better symbol error rate performance than the existing MMSE–CDFE. Therefore, the proposed MMSE–PDFE may offer a viable alternative to the MMSE–CDFE. Computer simulations were conducted to verify our results using highly dispersive multipath channels.

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.

A wide band QAM receiver based on DFE to reject ISI in wireless time varying fading channel

Quadrature Amplitude Modulation (QAM) system with high data rate suffers from Inter-Symbol-Interference (ISI) in frequency-selective wireless channel. To reduce this ISI signal, Decision Feedback Equalizer (DFE) is used. When the channel impulse response is time varying, the channel's coefficients need to be adaptively estimated. Adaptive implementation of the forward filter on the conventional DFE can track the time variation of the channel coefficients, but its tracking capability depends on the covariance matrix of the receiver vector and the speed of channel variation. In this paper, wide band QAM receiver is proposed with new arrangement of the DFE filters. Data diversity at the receiver is achieved by using N antennas at its input. The forward filter of the DFE will be used as a Maximum-Ratio-Combiner (MRC) as well as a channel compensator. By first removing the ISI signal from the received vector, the tracking performance of the forward filter will be enhanced. The new performance of the proposed receiver will be compared with the conventional DFE receiver. The tracking performance of the forward filter using LMS and RLS algorithms in the new arrangement is compared with that of the conventional DFE receiver.

Single-Carrier Space Time Transmission Diversity with Decision Feedback Equalization

In order to suppress inter-symbol interference (ISI), the Frequency domain decision feedback equalization (FD-DFE) algorithm is proposed in this paper for single carrier frequency domain equalization (SC-FDE) combined with Alamouti space time block coding (STBC) systems over frequency-selective channels. We use the cyclic prefix (CP) and two transmit antennas to achieve the significant diversity gain. The feedback equalization part is designed by Mean Square Error (MSE) criterion and the coefficients of filters are all in the frequency domain for the given channel frequency response. The outage and diversity analysis of system is also made to measure the equalization performance. Finally, the simulation results show that the proposed algorithm has better performance than the conventional liner equalization (LE) and hybrid structure decision feedback equalization (HDFE) schemes, and the computational complexity of our method is acceptable.

A DFE Receiver With Equalized VREF for Multidrop Single-Ended Signaling

In this brief, decision-feedback equalization (DFE) receiver for multidrop single-ended signaling is described. The proposed DFE receiver adopts a reference voltage having equalization information to remove channel intersymbol interference. It uses a smaller number of DFE tap elements than the conventional DFE receiver for the same level of equalization, resulting in reduced power consumption and silicon area, particularly for multi-input/output interface. The proposed DFE receiver was fabricated in a 0.13- μm CMOS process, whose evaluation results indicate a reliable operation up to 3.40-Gb/s data rate with reduced power consumption.

Enhancement of New Channel Equalizer Using Adaptive Neuro Fuzzy Inference System

The rapidly increasing need for information communication requires higher speed data transmission over the existing channels. The data rate over these channels is limited mainly by Inter Symbol Interference (ISI). Channel equalizers are used to reduce the effect of ISI. In this paper, a new equalizer based on Adaptive Neuro-Fuzzy Inference System is presented. The performance of the proposed equalizer is evaluated for both linear as well as non-linear channels in terms of bit-error rate for different noise powers. Simulation results show that the proposed equalizer has better Bit Error Rate (BER) performance compared to multi-layer perceptron and least mean square equalizers. However, its BER performance is slightly poorer than that of radial basis function network and optimal Bayesian equalizer but is better in terms of structural complexity. Keywords: Channel equalizer, Hybrid learning algorithm, Intersymbol interference, Membership function, optimal Bayesian equalizer. The problem of equalization is treated in two different ways. Firstly, it may be interpreted as an inverse filtering problem, so that the combination of the channel and equalizer must behave like ideal channel .The conventional equalizers such as Linear, Decision-feedback, and Fractionally- spaced equalizers are based on this approach. But their performance is poor especially when the channel response is non-linear. Secondly, the problem of equalization may be considered as a classification problem. All equalizers using neural networks are based on the latter approach.. The capabilities of neural networks for equalization of simple channels are described by Gibson, Siu and Cowan. Multi-layer perceptron (MLP) equalizers are superior to conventional linear and decision feedback equalizers in terms of probability of error, but their practical applications are severely restricted due to difficulties such as very long training time, indeterminate nature of training time, and lack of a methodology for architecture selection. Radial basis function network (RBFN) equalizer has received a great deal of attention because of its structural simplicity and more efficient learning compared to MLP. Moreover, there exists structural equivalence between RBFN equalizer and optimal symbol spaced Bayesian equalizer . However, the structure of RBFN equalizer depends on channel length, and thus requires a large number of centers in the hidden layer, which increases the computational complexity. In this paper, we propose a new channel equalizer based on adaptive neuro-fuzzy inference system, i.e., an adaptive fuzzy inference system whose parameters are adapted using neural network learning algorithms. Herein hybrid learning algorithm is used for optimizing parameters of ANFIS. The performance of the proposed equalizer is evaluated in terms of bit-error rate (BER) for different noise powers in the channel. Both linear as well as non-linear channels are considered for performance evaluation. The BER of proposed equalizer is compared with conventional least mean square (LMS) equalizer, MLP equalizer, RBFN equalizer, and optimal Bayesian equalizer. Simulation results show that proposed equalizer has better BER performance compared to MLP and LMS equalizers, but slightly poorer than RBFN and optimal Bayesian equalizers. However, the proposed equalizer has smaller structure than RBFN equalizer, thus provides a better compromise between the performance and structural complexity.

Efficient Computation of the Feedback Filter for the Hybrid Decision Feedback Equalizer in Highly Dispersive Channels

The hybrid decision feedback equalizer (DFE) is a combined time-frequency domain implementation of the conventional time-domain DFE that is able to provide a good trade-off between performance and computational complexity in single carrier transmission over severely frequency-selective channels. In the hybrid DFE the implementation of the feedforward filter is done in the frequency domain, while the feedback filter (FBF) is implemented in the time-domain. The computation of the coefficients for the two filters is usually done in the same domain where they are implemented. A method for frequency-domain computation of the FBF is proposed in the paper. As is known, the key operation in the computation of the FBF is the spectral factorization. In the paper it is proposed to adopt the (cepstral) method for spectral factorization due to Kolmogoroff, which can be efficiently implemented by using the fast Fourier transform (FFT). The application of the method is considered for highly dispersive channels. By using simulations we show that for this type of channels the performance of the proposed method is virtually the same as that obtained by using time-domain approaches. The advantage of the proposed approach is that the efficient FFT gives a substantial reduction of complexity compared to time-domain methods.

A new soft decision feedback equalizer in underwater acoustic communications

The tap coefficients of the decision-feedback equalizer (DFE) are associated with the multipath spread of the channel. For the underwater acoustic communication channel, the impulse response often covers tens to hundreds of symbols, requiring at least tens of taps in the feedback filter. Once the incorrect decisions are fed back, the error propagation will be severe. In this paper, we propose a new soft decision equalizer. The expected output symbols of the equalizer will be weighted according to their reliability, and summed up, then the result is fed back. This approach can alleviate the effect of those incorrect symbols. For evaluating the performance of the proposed method, a sea test in shallow water was carried out. The experimental results show that its BER decreases by 50% without channel codec, compared with the conventional DFE. Furthermore, the mean square error can also reduce about 2 dB when the error propagation is severe.

Bidirectional equalization for underwater acoustic communication

The bidirectional decision feedback equalizer (BiDFE) that combines the outputs of a conventional decision feedback equalizer (DFE) and backward DFE can improve the performance of the conventional DFE by up to 1-2 dB based on simulations. In this letter, the BiDFE concept is extended to multichannel time reversal communications involving a DFE as a post-processor. Experimental data collected in shallow water (10-20 kHz) show that the performance can be enhanced by 0.4-1.8 dB in terms of output signal-to-noise ratio. In particular, a larger improvement (e.g., 1.8 dB) is achieved for time-varying channels where the channel diversity in opposite directions is more profound.

SC-FDMA 수신기를 위한 잡음 백색화 판정궤환 등화기

이 논문에서는 SC-FDMA 수신기를 위한 잡음 백색화 판정궤환 등화기를 제안하였다. SC-FDMA 방식은 다중경로의 영향을 제거할 수 있는 OFDMA 방식의 장점을 유지하면서 OFDMA의 단점인 높은 PAPR 문제를 해결할 수 있는 장점을 가지고 있다. 또 SCFDMA 방식은 기본적으로 단일 반송파 전송방식이지만, 주파수 영역에서 채널 등화기를 구현함으로써 OFDMA 수신기와 마찬가지로 등화기의 복잡도를 크게 감소시킬 수 있다. 뿐만 아니라 시간 영역 신호가 디지털 변조된 복소심볼이므로 시간 영역에서 추가적으로 비선형 등화인 판정궤환 등화를 함으로써 주파수 영역의 선형 등화에 의해 잔류해 있는 심볼간 간섭을 더 제거할 수 있다. 한편 지금까지 제시된 SC-FDMA용 판정궤환 등화기는 판정기에 입력되는 신호에 포함된 잡음이 백색화되어 있지 않기 때문에 최적판정을 수행하고 있지 않다. 이 논문에서는 이러한 문제를 해결하기 위해 선형 잡음 백색화 필터를 판정기 앞에 연결하여 판정궤환 등화기의 성능을 더 향상 시킬 수 있는 방법을 제시한다. 또 컴퓨터 모의실험을 통해 기존의 등화기와 제안된 등화기의 비트 오율 성능을 비교한다. In this paper, we propose a noise whitening decision feedback equalizer for single carrier frequency division multiple access (SC-FDMA) receivers. SC-FDMA has the same advantage as that of orthogonal frequency division multiple access (OFDMA) in which the multipath effect can be removed easily, and also solves the problem of high peak to average power ratio (PAPR) which is the main drawback of OFDMA. Although SC-FDMA is a single carrier transmission scheme, a simple frequency domain linear equalizer (FD-LE) can be implemented as in OFDMA, which can dramatically reduce the equalizer complexity. Moreover, some residual intersymbol interference in the output of the FD-LE can be further removed by an additional nonlinear decision feedback equalizer (DFE) in time domain, because the time domain signal is a digitally modulated symbol. In the conventional DFE, however, the noise is not white at the input of the decision device and correspondingly the decision is not optimum. In this paper, we propose an improved DFE scheme for SC-FDMA systems where a linear noise whitening filter is inserted before the decision device of the conventional DFE scheme. Through computer simulations, we compare the bit error rate performance of the proposed DFE scheme with the conventional equalizers.

Minimum SER Block Precoding and Equalization for Frequency-Selective Fading Channels

In this paper, we study the joint transmitter and receiver design problem over frequency-selective fading channels. Motivated by a conventional minimum mean square error decision feedback equalizer (MMSE-DFE), we present a simple approximate maximum-likelihood (ML) decision feedback equalizer at the receiver. We then perform precoding in a downlink scenario at the transmitter side to minimize the analytical symbol error rate (SER) of the proposed equalization scheme. Simulation results indicate that the proposed precoder achieves substantial performance gains over previously reported techniques.

Efficient bad equalizer in two-ray multipath fading channels

An efficient bidirectional arbitrated decision feedback (BAD) equalizer is presented in single-carrier block transmission system in the two-ray multipath fading channels, where the output from the bidirectional equalizers are combined together directly using maximal ratio combining (MRC) rule to improve the signal-to-noise ratio (SNR) before demodulation. The computational complexity of the BAD equalizer presented is linear with the channel length, which is the same as conventional decision feedback equalizer (DFE) and is significantly lower than that of conventional BAD equalizer as well as the maximum likelihood (ML) algorithm. While the performance of the new scheme depends on the specific channel characteristics, it is shown by simulation results that the performance of the new BAD can surpass the one of DFE dramatically in the minimum or non-minimum phase Two-ray multipath fading channels.