High bit rate digital subscriber line transmission with noise-predictive decision-feedback equalization and block coded modulation

In high-speed communications systems the task of channel equalization can be quite demanding. One of the most common and most powerful equalization algorithms is the decision feedback equalizer (DFE). In certain cases though, the DFE is not powerful enough to give the required performance margin on its own, and a forward error correction (FEC) scheme must be employed to give an extra boost in performance. The introduction of the FEC in itself creates problems for the DFE which is compensated for by the use of a precoder. In steady state, this does not affect the overall performance of the system, but in a situation where the channel is subject to change, the precoding scheme runs into difficulty. This paper describes a modification to the conventional fixed precoder to allow the precoder to adapt slowly in a dynamic channel environment. Also shown here is a new method of dealing with a dynamic channel by maintaining an active DFE in the receiver, purely for the purpose of adapting the precoder coefficients during runtime. The technique is applied to digital subscriber line (DSL) transmission at 1.544 Mbit/s, using coded 16-PAM modulation.

High bit-rate digital mobile radio transmission performance in a selective fading environment is discussed. Analysis and computer simulation show the amount of performance improvement by utilizing a preset-type decision feedback equalizer. The equalizer adopts the recursive least-squares (RLS) algorithm for tap coefficient determination. The performance is approximately analyzed by using the zero-forcing criterion. This criterion is separately applied to the minimum and nonminimum phase conditions of the two-wave selective fading model. The improved performance is almost equivalent to that of two-branch selection diversity reception. Finally, hardware implementations of a dynamic selective fading simulator and a decision feedback equalizer are demonstrated.< <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.

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.

The authors present a simulation study evaluating the performance of trellis-coded modulation with combined code/ISI sequence estimation for high bit rate (800 kb/s) transmission on subscriber loops. The receiver contains a fractionally spaced forward filter of a decision feedback equalizer (DFE) as a front end. This is shown to suppress phase synchronized crosstalk very effectively. The performance is further enhanced by the use of trellis code with a large number of states received with an M algorithm sequence-estimating receiver with a smaller number of states. >

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.

It is well-known that Vectored Digital Subscriber Line (DSL) transmission promises significant theoretical data-rate increases for DSL technology; however, Vectored DSL requires full knowledge of the channel. The effectiveness of Vectored DSL transmission in a practical setting, where channel knowledge is subject to error, has yet to be determined. This paper proposes a Generalized Regression Neural Network (GRNN)-based approach to DSL channel estimation by interpolating between a subset of measured or estimated data-points. Furthermore, closed-form expressions for the effect of channel estimation error on he achievable Vectored DSL data-rate are derived, using a Zero-Forcing (ZF) interference canceller for upstream transmission and a Diagonalizing Pre-coder (DP) for downstream transmission. Finally, simulation results are provided to demonstrate the performance loss associated with channel estimation error for Vectored DSL transmission, based on the ANN approach and a linear regression approach.

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.

The author proposed soft decision feedback equalisation for orthogonal frequency division multiplexing (OFDM) transmission over doubly selective channels which is used for turbo equalisation. The doubly selective channel is approximated using the basis expansion model (BEM). The decision feedback equaliser (DFE) is obtained and updated for every sub-carrier which results in a per-tone DFE. On the basis of the Gaussian approximation, expressions for the a priori, the a posteriori and the extrinsic information have been derived for quadrature phase shift keying (QPSK) modulation. For turbo equalisation, where iterative joint channel equalisation and channel decoding is executed, in the first iteration, estimates are based on the feed-forward filter which are fed to the channel decoder. These estimates are not reliable and may incur error propagation and poor turbo equalisation performance. For more reliable estimates in the first iteration, the DFE is initialised as a linear minimum mean square error (LMMSE) equaliser. The per-tone LMMSE soft-output equaliser is also derived, and expressions for the a priori, a posteriori and extrinsic information are obtained viewing the LMMSE equaliser as a special case of the DFE. The extrinsic information in both cases was derived based on the Gaussian approximation. Numerical results for bit-error rate performance assuming the BEM channel model and Jakes' channel model are shown to confirm our findings.

Introduction Indoor infrared wireless systems using diuse channels are considered as the best form in respect of the network exibility. Infrared wireless system using ono keying code-division multiple-access (OOK-CDMA) has been proposed to realize multiple access in indoor infrared wireless communications, and the performance is analyzed on diuse channels [1]. It is shown in [1] that the bit error rate (BER) of indoor infrared wireless systems using OOK-CDMA on diuse channels is degraded excessively owing to the eects of ISI. To mitigate the eects of ISI, some equalization schemes for infrared wireless systems have been discussed [2]-[5]. The performance of the OOK-CDMA with the decision-feedback equalizer (DFE) and that of the OOK-CDMA with the decision-feedforward-andfeedback equalizer (DFFE) have been evaluated, and it is shown that the system with the DFFE has better performance than that of the system with the DFE [4]. The performance of the DS-CDMA with the feedfo

Two adaptive modulation systems to counteract frequency and time selective fading in digital transmission over multipath radio channels are analyzed: a single carrier QAM system with decision feedback equalization (DFE-QAM) and an OFDM system. In order to maintain a target BER, an adaptive modulation level of the single-carrier in the DFE-QAM system, or each subcarrier in the OFDM system, which tracks the channel fluctuations is used. The spectral efficiencies of both systems are compared to the theoretical channel capacity. Various impairments such as inaccurate or delayed channel estimation and a limited-rate feedback channel have been considered.

A 4‐tap time‐domain decision feedback equalizer (TD‐DFE) is presented to implement a multi‐tap DFE in a matched DQ (data)‐DQS (strobe) tree architecture. Traditionally, matched architecture holds an advantage in terms of power noise immunity, but it suffers from low‐speed performance due to the unavailability of decision feedback equalizer (DFE) applications. By adopting the proposed TD‐DFE, both high‐speed operation and power noise immunity can be achieved within the matched architecture. An 8‐DQ parallel receiver with the proposed 4‐tap TD‐DFE, designed in 28 nm CMOS, achieves a data rate of 20 Gb/s with 0.6 UI eye‐opening even with 215 mV power fluctuations.

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.

Signal processing in decision-feedback equalization of intersymbol-interference and multiple-input/multiple-output channels: a unified view