Decision feedback equalization with quarter-rate clock timing for high-speed backplane data communications

The performance analysis is presented of an adaptive receiver based on a decision feedback equalizer (DFE) that uses a fractionally spaced equalizer (FSE) for channel equalization at 400 kbaud signaling using a quaternary line code (800 kb/s data rate). The minimum mean-square error (MMSE) is used as the performance parameter, where the error is due to the sum of the residual near-end crosstalk (NEXT) and precursor intersymbol interference (ISI). The results of a comprehensive computer simulation study indicate that as a function of the sampling phase, the ratio of the average signal power to MMSE (SNR) is practically constant with a half baud-spaced FSE, whereas with a baud-spaced equalizer the SNR varies 1-3 dB on the lossy loops. A simulation study of an adaptive FSE/DFE receiver demonstrates that the 6 dB of NEXT noise margin is achievable with a 21-tap half baud-spaced FSE and a 60-tap DFE on a 9-kft 26-gauge loop.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

As the data rate has been increased over 10 Gb/s with copper interconnect, the intersymbol interference (ISI) caused from the channel loss should be compensated. While a decision feedback equalizer (DFE), which is widely used in the receiver can compensate the ISI, its ability to enhance the signal-to-noise ratio (SNR) is limited especially for high frequency data patterns (alternating data patterns). Even a DFE with a large number of taps, which has powerful compensation capacity for ISI, can improve only limited amount of SNR. To improve SNR, this brief presents a 1/4th baud-rate continuous-time linear equalizer (CTLE) and a two-tap DFE. The 1/4th baud-rate CTLE recovers data to be adequate for the DFE, and remaining ISI is removed by the DFE. To boost the high frequency gain of the DFE, exclusive or merged adders are introduced, so the proposed equalizer is more tolerant against channel noise. It compensates 21.7-dB channel loss and operates with 11.5-Gb/s data rate as it consumes 25.35 mW from 1.3 V supply in a 110-nm CMOS technology.

Severe intersymbol interference (ISI) is one of the main obstacles for reliable signal reception in ATSC DTV systems. Decision feedback equalizers (DFEs) are commonly used to suppress the ISI. However, DFEs may suffer from error propagation due to incorrect symbol decisions from the symbol slicer. This phenomenon deteriorates the performance even more when the post-cursor ISI is strong. In order to reduce error propagation, we present a novel hybrid equalization scheme for ATSC channels. The proposed scheme consists of an adaptive decision feedback sparsening filter (DFSF), and an iterative maximum a posteriori (MAP) equalizer based on the belief propagation (BP) algorithm. In the first stage, instead of removing all the ISI from post cursors, the DFSF employs a modified feedback filter which leaves the strongest post-cursor ISI taps uncorrected. As a result, a long ISI channel is equalized to a sparse channel having only a small number of nonzero taps. In the second stage, a belief propagation algorithm is applied to mitigate the residual ISI. Since the channel is typically time-varying and suffers from Doppler fading, the DFSF is adapted using the least mean square (LMS) algorithm, such that the amplitude and the locations of the nonzero taps of the equalized sparse channel appear to be fixed. As such, the channel appears to be static during the second stage of equalization which consists of the BP detector. Simulation results demonstrate that the proposed scheme outperforms the traditional DFE in symbol error rate, under both static channels and dynamic ATSC channels.

This paper introduces the decision feedforward equalizer (DFFE), a new low complexity iterative equalization architecture for high speed receivers used on channels with large intersymbol interference (ISI). The decision feedback equalizer (DFE) is one of the preferred receivers for high ISI channels. The high data and symbol rate of current communications systems often require parallel processing receiver implementations [1]. Unfortunately, the complexity of parallel architectures of the DFE grows exponentially with the channel memory [2] and limits their application to low ISI channels. The DFFE avoids the exponential growth of the DFE by using tentative decisions to iteratively cancel the ISI. We show that the DFFE can achieve performance similar to the DFE. These benefits make it an excellent choice for high-speed receivers required to operate over highly dispersive channels.

In this work, we base on multi-layered perceptron neural networks with backpropagation algorithm (MLP/BP) to construct multi-input multi-output (MIMO) decision feedback equalizers (DFEs). The proposal is used to recover distorted quadrature phase-shift keying (QPSK) signal. From the simulations, we note that the proposed scheme can recover severe distorted signals as well as suppress intersymbol interference (ISI), adjacent channel interference (ACI) and background additive white Gaussian noise (AWGN). The better bit-error-rate (BER) and packet-error-rate (PER) performance as compared to a set of least-mean-square (LMS) DFEs is achieved. The presented MLP/BP-based MIMO DFE under the severe ISI channels with ACI and AWGN can improve over 1.5dB at PER=10-1 and BER=10-3.

A decision-feedback equalizer (DFE) is the basis of a recent development of a quadruple diversity troposcatter modem which can operate up to a data rate of 12.6 Mbit/s in a 99% bandwidth of 15 MHz. In this paper a theoretical approach is developed for the calculation of average bit error rate (ABER), including the effects of intersymbol interference due to multipath and the finiteness of the transversal filters used to realize the DFE. By omitting the intersymbol interference effect, the calculation provides a lower bound which can be used to assess the intersymbol interference penalty for a particular DFE structure. The paper includes calculations of a DFE configuration which has a three tap forward filter with tap spacing equal to one-half a symbol interval. Measured performance results from fading channel simulator tests of a three tap forward filter DFE are presented for data rates from 1.5 to 12.6 Mbit/s and for a wide range of multipath statistical conditions. The results for this DFE configuration show (1) excellent agreement between calculated and measured ABER, (2) a small intersymbol interference penalty when the 2σ multipath spread is less than approximnately one-half the data symbol interval, and (3) successful operation at values of multipath spread up to twice the data symbol interval. In a sequel to this paper, the results of a field test of the DFE modem are presented. These live links test results are consistent with both the calculated and simulator measured data presented here.

In this contribution, we present a multiple-input multiple-output (MIMO) transceiver scheme for high-speed chip-to-chip communication over low-cost electrical interconnects. Linear MIMO pre-equalization at the transmitter is combined with decision feedback equalization (DFE) at the receiver to counteract the adverse effect of inter symbol interference (ISI) and crosstalk (XT). Considering an energy constraint at the transmit side, we derive elegant closed-form expressions for the equalization filters under a minimum mean square error (MMSE) criterion. Numerical analysis shows that the combination of linear MIMO pre-equalization and MIMO DFE allows to significantly improve the reliability of future high-speed off-chip communication.

The paper investigates the application of the multilayer perceptron structure to the packet wise adaptive decision feedback equalization of a M-ary QAM signal through a TDMA indoor radio channel in the presence of intersymbol interference (ISI) and additive Gaussian noise. Since the multilayer neural networks are capable of producing complex decision regions with arbitrarily nonlinear boundaries, this would greatly improve the performance of conventional decision feedback equalizer (DFE), where the decision boundaries of conventional DFE's are linear. Nevertheless, the applications of the traditional multilayer neural networks have been limited to real valued signals. To tackle this difficulty, a neural based DFE is proposed to deal with the complex QAM signal over the complex valued fading multipath radio channel without performing time consuming complex valued backpropagation training algorithms, while maintaining almost the same computational complexity as the original real valued training algorithm. Moreover, this neural based DFE trained by packet wise backpropagation algorithm would approach an ideal equalizer after receiving a sufficient number of packets. Results show that the neural based DFE provides a superior bit error rate performance relative to the conventional mean square DFE, especially in poor signal to noise ratio conditions.

In this work, we base on multi-layered perceptron neural networks with backpropagation algorithm (MLP/BP) to construct soft decision feedback equalizers (DFEs). The proposal is used to recover distorted 16-point quadrature amplitude modulation (16-QAM) signal. For better performance, error control codes (ECC) are applied to enhance the accuracy of the transmitted data. From the simulations, we note that the MLP/BP-based soft DFEs with bit-interleaved TCM can recover severe distorted 16-QAM data as well as suppress intersymbol interference (ISI) and background additive white Gaussian noise (AWGN). As compared with the LMS DFE, the proposed scheme can provide better bit-error-rate (BER) and packet-error-rate (PER) performance.

Interpolated decision feedback equalization for long intersymbol interference channels

In this paper an efficient implementation of decision feed back equalizer (DFE) is carried out using novel memory less distributed arithmetic (NMLDA) filter. In wireless transmission systems, DFEs are used to mitigate the inter-symbol interference (ISI). The ISI is occurred due to multi-path propagation of the transmitted signal. High data rate systems demand higher order filters in DFE architectures which increase complexity in hardware design. In our proposed NMLDA design, we have used multiplexers and enhanced compressor adders in place of memory unit and conventional adders. The proposed design occupies lower area and gives higher throughput, when compared to MAC based filter and all other memory based DA filter architectures. By using proposed NMLDA based DFE, the ISI errors in transmission signal, will be minimized and the performance of the transmission system will be enhanced. We have synthesized the NMLDA of 32-tap, 16-tap, 8-tap and 4-tap filters and implemented them on FPGA device. The proposed design has nearly 70% less number of logical elements than OBC DA and 50% less than MDA and offers better throughput than the existing designs when implemented on Altera Cyclone III EP3C55F484C6.

The nonlinear decision feedback equalizer (DFE) provides significantly lower symbol error rates than a linear equalizer, but for the high speed digital transmission systems which shows more serious inter-symbol interference (ISI) the DFE suffers large steady-state mean square error (MSE) and slow convergence rate. So a new DFE is proposed, which consists of a feedforward filter based on T/2 fractionally-spaced structure and a feedback filter based on baud-spaced structure and uses a dual-mode algorithm. The efficiency of the method is proved by computer simulation.

In recent times, there has been growing interests in integration of voice, data and video traffic in wireless communication networks. With these growing interests, WCDMA has immerged as an attractive access technique. The performance of WCDMA system is deteriorated in presence of multipath fading environment. The paper presents space-time coded minimum mean square error (MMSE) Decision Feedback Equalizer (DFE) for wideband code division multiple access (WCDMA) in a frequency selective channel. The filter coefficients in MMSE DFE are optimized to suppress noise, intersymbol interference (ISI), and multiple access interference (MAI) with reasonable system complexity. For the above structure, we have presented the estimation of BER for a MMSE DFE using computer simulation experiments. The simulation includes the effects of additive white Gaussian noise, multipath fading and multiple access interference (MAI). Furthermore, the performance is compared with standard linear equalizer (LE) and RAKE receiver. Numerical and simulation results show that the MMSE DFE exhibits significant performance improvement over the standard linear equalizer (LE) and RAKE receiver.

Intersymbol interference (ISI) of the radio propagation through multipath fading channels is a major factor that limits the performance of mobile communication systems. Blind decision feedback equalization (DFE) is one of major methods for channel equalization. We propose a new method, based on a Hammerstein filter instead of DFE filters, for enhancing the performance of DFE, hence, blind HDFE. In this paper, we show the bit error rate (BER) performance of blind DFE and blind HDFE, in order to prove the superiority of BHDFE.

High speed transmission of information suffers inevitably by intersymbol interference (ISI) induced by multipath propagation over frequency selective channels. ISI has been a major obstacle preventing the successful implementation of physical layer network coding (PLNC) systems in these environments. Another major obstacle is the high computational complexity of existing algorithms dealing with ISI, which is a determining factor in the design of relay nodes. In this paper, we propose a novel and computationally efficient decision feedback equalizer structure that is utilized at the end node and in conjunction with a simple amplify and forward (AF) technique employed at the relay. We derive the optimal equations for the decision feedback equalizer (DFE) coefficients and evaluate the end-to-end bit error rate performance in frequency selective channels, and provide both analytical and numerical simulation results that demonstrate the feasibility of the proposed approach under realistic channel conditions. The obtained results show that E2E performance is dominated by the channel with most severe frequency selectivity.