MIMO decision feedback equalization from an H/sup /spl infin// perspective

We approach the multiple input multiple output (MIMO) decision feedback equalization (DFE) problem in digital communications from an H/sup /spl infin// estimation point of view. Using the standard (and simplifying) assumption that all previous decisions are correct, we obtain an explicit parameterization of all H/sup /spl infin// optimal DFEs. In particular, we show that, under the above assumption, minimum mean square error (MMSE) DFEs are H/sup /spl infin// optimal. The H/sup /spl infin// approach also suggests a method for dealing with errors in previous decisions.     

Computationally efficient methods for blind decision feedback equalization of QAM signals

This paper investigates computationally efficient methods for blind decision feedback equalization (DFE) that reduce the complexity and power requirements of blind equalization algorithms while maintaining their steady-state characteristics for quadrature amplitude modulation (QAM) signals. These include the power-of-two error (POT), selective coefficient update (SCU), and frequency-domain block (FDB) methods. A novel radius-directed stop-and-go (RSG) method is introduced, which selectively adjusts the equalizer tap coefficients based on the equalizer output radius. In addition, a new activation/de-activation method based on the equalizer output radius is utilized to control the feedback equalizer (FBE) of the DFEs. Simulation studies and analysis are provided for empirically derived cable and microwave channels and Ricean fading channels.     

Decision-feedback equalization of pulse-position modulation onmeasured nondirected indoor infrared channels

We examine the performance of two decision-feedback equalizers (DFEs) for pulse-position modulation (PPM) on measured nondirected indoor infrared channels with intersymbol interference. PPM offers high average-power efficiency, but on ISI channels, unequalized PPM suffers severe performance penalties. We have previously examined the performance of the maximum-likelihood sequence detector (MLSD), and found that it yields significant improvements. However, the MLSD often requires such large complexity and delay that it may be impractical. We investigate suboptimal, reduced-complexity equalization techniques for PPM, providing a performance analysis of zero-forcing chip-rate and symbol-rate DFEs. Our results show that a symbol-rate DFE provides performance that closely approaches that of the optimal MLSD     

On the performance of the electrical equalization technique in MMF links for 10-gigabit ethernet

In this paper, the electrical equalization technique utilized in multimode fiber (MMF) links for 10-Gigabit Ethernet is reported. This paper presents and compares three kinds of equalizers: linear equalizer, decision feedback equalizer (DFE), and a nonlinear finite-impulse-response DFE (NL-FIR-DFE) based on the analysis of nonlinear operations of direct modulation and detection in MMF links. Computer simulations reveal that NL-FIR-DFE exhibits superior performance in comparison with normal DFE. The equalization performance for an MMF channel with bandwidth 500 MHz-km as well as 160 MHz-km is demonstrated. It is demonstrated that with direct modulation of cost-effective vertical-cavity surface-emitting laser (VCSEL) or Fabry-Perot laser, the transmission distance for installed MMF with a bandwidth of 500 MHz-km at 10 Gb/s can be extended to 300 m with an appropriate offset restricted mode launch condition by utilizing normal DFE and to more than 650 m for NL-FIR-DFE. Moreover, it is shown that the transmission distance for an MMF with a bandwidth of 160 MHz-km can reach 300 m with more than seven times bandwidth improvement by using NL-FIR-DFE.     

基于最大比特速率准则的FMT频域均衡算法

针对严格采样滤波多音调制(Filtered Multi-Tone modulation,FMT)系统符号间干扰严重的问题,通过推导严格采样FMT系统的矩阵表示式,提出了一种新的频域均衡算法,该算法以最大化比特速率为目标来确定均衡器的系数。仿真结果表明,在ATTC(Advanced Television Technology Center)E型信道条件下,新算法的可达比特率(Achievable Bit Rate,ABR)性能比最小均方误差准则的线性均衡和判决反馈均衡分别提高了40%和70%,误码率性能在BER为10-4则分别改善了3 dB和5 dB;而在WLAN信道条件下,新算法的ABR性能分别提高了45%和75%,误码率性能在BER为10-4则分别改善了6 dB和10 dB。     

Reduced complexity decision feedback equalization for digitalsubscriber loops

Decision feedback equalizers (DFEs) are widely used in modern local network digital transmission systems to remove the intersymbol interference caused by slowly decaying pulse tails. A gradient descent algorithm for adapting a coefficient to model the slowly decaying portion of the tail is described. An equalization strategy is described that exploits prior knowledge of the nature of the subscriber loop channel, together with the new adaptation algorithm, to give reduced complexity DFE structures. The use of this algorithm in FIR and IIR equalizer structures is described. The use of this algorithm in FIR and IIR equalizers is quantitatively compared to a conventional DFE in terms of performance and implementation complexity. An analysis is presented describing the operation of the adaptation algorithm in the presence of noise. Simulation results illustrate the training of the algorithm and its stability in the presence of near-end crosstalk noise     

Realizable MIMO decision feedback equalizers: structure and design

We present and discuss the structure and design of optimum multivariable decision feedback equalizers (DFEs). The equalizers are derived under the constraint of realizability, that is, causal and stable filters and finite decision delay. The design is based on a known dispersive discrete-time multivariable channel model with infinite impulse response. The additive noise is described by a multivariate ARMA model. Equations for obtaining minimum mean square error (MMSE) and zero-forcing DFEs are derived under the assumption of correct past decisions. The design of a realizable MMSE DFE requires the solution of a linear system of equations in the model parameters. No spectral factorization is required. We derive novel necessary and sufficient conditions for the existence of zero-forcing DFEs and point out the significance of these conditions for the design of multiuser detectors. An optimal MMSE DFE will contain IIR filters in general. Simulations indicate that the performance improvement obtained with an IIR DFE is reduced more than for a (suboptimal) FIR DFE when error propagation is taken into account since the use of IIR feedback filters tends to worsen the error propagation     

Block channel equalization in the presence of a cochannelinterferent signal

Novel algorithms for block equalization of M-ary phase shift keying (PSK) signals transmitted over multipath fading channels in the presence of an interferent cochannel signal are introduced and analyzed. The algorithms exploit the intrinsic statistical properties of cochannel interference (CCI) in order to mitigate its effects. Both linear and decision feedback equalizers (DFEs) are derived under the assumption that the overall channel impulse responses of both the useful and the inteferent signal are known. Simulation results show that: (a) whereas zero-forcing block equalizers yield a large noise enhancement effect, a minimum mean-square block DFE (MMSE-BDFE) can efficiently compensate for the distortion in the useful channel and reduce the effect of CCI at the same time, and (b) the MMSE-BDFEs outperform conventional DFEs, at least in the idealized conditions of our analysis     

Detection of coded modulation signals on linear, severely distortedchannels using decision-feedback noise prediction with interleaving

On linear bandlimited Gaussian noise channels with sufficiently high SNR, channel capacity can be approached by combining powerful coded modulation schemes designed for Nyquist channels with the equalization power of decision-feedback equalization (DFE). However, this combination may not be realized in a straightforward manner, since, in general, DFE requires delay-free decisions for feedback, and in a coded system such decisions are not sufficiently reliable. A technique is proposed that combines periodic interleaving with noise-predictive DFE, so that delayed reliable decisions can be used for feedback. When sufficient delay in the interleavers can be tolerated, this technique can attain the DFE performance. On severely distorted channels, modest delays can be sufficient to obtain respectable gains over linear equalization     

Frequency-Domain Channel Estimation and Equalization for Continuous-Phase Modulations With Superimposed Pilot Sequences

This paper proposes a new frequency-domain channel-estimation and equalization method for continuous-phase modulation (CPM) block transmissions with superimposed pilot signals. Our method provides spectral and power-efficient broadband CPM wireless communications with less complexity than previous methods. The proposed frequency-domain channel estimation uses the superimposed pilot sequence as a reference signal to reduce the throughput loss caused by traditionally multiplexed pilots. The proposed CPM frequency-domain decision feedback equalizer (DFE) eliminates the complexity overhead of conventional decomposition-based CPM receivers.      

Fast adaptive equalization/diversity combining for time-varyingdispersive channels

We examine adaptive equalization and diversity combining methods for fast Rayleigh-fading frequency selective channels. We assume a block adaptive receiver in which the receiver coefficients are obtained from feedforward channel estimation. For the feedforward channel estimation, we propose a novel reduced dimension channel estimation procedure, where the number of unknown parameters are reduced using a priori information of the transmit shaping filter's impulse response. Fewer unknown parameters require a shorter training sequence. We obtain least-squares, maximum-likelihood, and maximum a posteriori (MAP) estimators for the reduced dimension channel estimation problem. For symbol detection, we propose the use of a matched filtered diversity combining decision feedback equalizer (DFE) instead of a straightforward diversity combining DFE. The matched filter form has lower computational complexity and provides a well-conditioned matrix inversion. To cope with fast time-varying channels, we introduce a new DFE coefficient computation algorithm which is obtained by incorporating the channel variation during the decision delay into the minimum mean square error (MMSE) criterion. We refer to this as the non-Toeplitz DFE (NT-DFE). We also show the feasibility of a suboptimal receiver which has a lower complexity than a recursive least squares adaptation, with performance close to the optimal NT-DFE     

基于Kalman滤波的水声混合双向迭代信道均衡算法

水声信道均衡中基于信道估计的均衡方法理论上具有更优的均衡性能,但较高的计算复杂度限制了算法的实际应用。针对这一问题,该文首先基于Kalman滤波和Turbo均衡提出一种迭代Kalman均衡器,实现了基于软符号的迭代信道估计与迭代Kalman均衡,且复杂度较常规方法降低约1个数量级。其次,针对单一均衡算法和单一方向Turbo均衡器存在的误差传递现象,设计了基于迭代Kalman均衡器与改进成比例归一化LMS (IPNLMS)自适应均衡器相结合的混合双向Turbo均衡器,提高了自适应均衡器的收敛速度和均衡性能,并通过双向均衡结构带来的增益改善了符号估计误差传递的现象。理论分析与仿真实验验证了该文算法的有效性。     

Differential modulation techniques for a 34 MBit/s radio channel using orthogonal frequency division multiplexing

The orthogonal frequency division multiplexing (OFDM) technique has been proposed for terrestrial digital transmission systems due to its high spectral efficiency, its robustness in different multipath propagation environments and the ability of avoiding intersymbol interference (ISI). Our studies consider a radio channel bandwidth of 8 MHz and a data rate of 34 Mbit/s.In the case of the OFDM transmission system a coherent 64-QAM requires a channel estimation process and a channel equalization in frequency-selective interference situations [4]. The equalization process can be realized by a multiplier bank at the FFT output in the receiver, a so-called frequency-domain equalizer. Alternatively, a multilevel differential modulation technique, the so-called differential amplitude and phase shift keying (64-DAPSK) considering the phase and simultaneously the amplitude for differential modulation, is proposed and presented in this paper. Differential modulation/demodulation techniques do not require any explicit knowledge about the radio channel properties in the differential channel equalization. It is therefore not necessary to implement a frequency-domain equalizer in an OFDM/64-DAPSK receiver, which reduces the computation complexity. The performance of both modulation techniques has been analysed in the uncoded and coded case referring to Gaussian and frequency-selective Rayleigh fading channels. Simulation results are presented in this paper.The OFDM signal has a non-constant envelope with large instantaneous power spikes possible primarily resulting in an overdriving of the high power amplifier (HPA) at the transmitter. This leads to nonlinear distortion causing intermodulation noise and spectral spreading. Both effects can be limited by introducing an appropriate input backoff (IBO). In this paper the performance of OFDM signals in the presence of nonlinearities is analysed quantitatively.     

ADAPTIVE EQUALIZATION WITH RLS-MLSE ASSISTED BY FEC BLOCK CODING FOR FADING MOBILE RADIO CHANNELS

In this paper, we propose an adaptive maximum-likelihood (ML) sequence estimator with RLS channel estimation, which is assisted by forward error control (FEC) coding. The reliable symbols reconstructed in the FEC decoder are used as the feedback signal to the RLS channel estimator. The scheme is compared with decision feedback equalization (DFE) with RLS algorithm, which is assisted by FEC coding. Computer simulations show that in frequency-selective fast fading mobile radio channels, the proposed scheme performs better at moderate Doppler frequencies. It is suitable for four-phase modulation data transmission at the rate of several 10 kb/s in 900 MHz band or in the 1800 MHz band.     

Adaptive minimum symbol-error-rate decision feedback equalization for multilevel pulse-amplitude modulation

The design of decision feedback equalizers (DFEs) is typically based on the minimum mean square error (MMSE) principle as this leads to effective adaptive implementation in the form of the least mean square algorithm. It is well-known, however, that in certain situations, the MMSE solution can be distinctly inferior to the optimal minimum symbol error rate (MSER) solution. We consider the MSER design for multilevel pulse-amplitude modulation. Block-data adaptive implementation of the theoretical MSER DFE solution is developed based on the Parzen window estimate of a probability density function. Furthermore, a sample-by-sample adaptive MSER algorithm, called the least symbol error rate (LSER), is derived for adaptive equalization applications. The proposed LSER algorithm has a complexity that increases linearly with the equalizer length. Computer simulation is employed to evaluate the proposed alternative MSER design for equalization application with multilevel signaling schemes.     

Upper-bound performance of a wide-band adaptive modem

Adaptive modulation is applied in conjunction with a decision-feedback equalizer (DFE) in order to mitigate the effects of the slowly varying wide-band multipath Rayleigh fading channel in a noise-limited environment. An upper-bound mean bit-error rate and bits per symbol performance is introduced for this scheme by utilizing the pseudo signal-to-noise ratio at the output of the DFE in order to switch the modulation schemes on a burst-by-burst basis     

频选快衰落信道的Turbo均衡算法

针对频选快衰落信道,本文提出卡尔曼滤波信道跟踪、软输出判决反馈均衡及软输入软输出信道解码迭代处理的Turbo均衡算法以充分利用已获得的信息,实现信道估计、信道均衡与信道解码的迭代更新,并克服传统判决反馈均衡器误差传播的缺陷.仿真表明,本算法能有效地跟踪快衰落信道,经两次迭代就可获得较为满意的码间干扰消除效果.     

Iterative equalization with adaptive soft feedback

In this letter, a novel equalization algorithm applying soft-decision feedback and designed for binary transmission is introduced. In contrast to conventional decision-feedback equalization (DFE), iterations are necessary, because a simple matched filter serves as feedforward filter, which collects signal energy, but creates noncausal intersymbol interference. The rule for generating soft decisions is adapted continuously to the current state of the algorithm. In most cases, standard DFE methods are clearly outperformed. For a class of certain channel impulse responses, performance of maximum-likelihood sequence estimation is attained, in principle. The high performance of the scheme is explained using results from neural network theory     

Reuse within a cell-interference rejection or multiuser detection?

We investigate the use of an antenna array at the receiver in frequency-division multiple-access/time-division multiple-access systems to let several users share one communication channel within a cell. A decision-feedback equalizer (DFE) which simultaneously detects all incoming signals is compared to a set of DFEs, each detecting one signal and rejecting the remaining as interference. We also introduce the existence of a zero-forcing solution to the equalization problem as an indicator of near-far resistance of different detector structures. Near-far resistance guarantees good performance if the noise level is low. Simulations show that with an increased number of users in the cell, the incremental performance degradation is small for the multiuser detector. We have also applied the proposed algorithms to experimental measurements from a DCS-1800 antenna array testbed. The results from these confirm that reuse within a cell is indeed possible using either an eight-element array antenna or a two-branch diversity sector antenna. Multiuser detection will, in general, provide better performance than interference rejection, especially when the power levels of the users differ substantially. The difference in performance is of crucial importance when the available training sequences are short     

Decision-feedback equalization of time-dispersive channels withcoded modulation

An extension is presented of the work of V.M. Eyubogin (see ibid., vol.36, p.401-9, Apr. 1988) on decision-feedback equalization (DFE) applied to coded systems with interleaving. The authors study the adaptive implementation of the DFE using recursive-least-squares algorithms (RLS). System performance on time-dispersive channels with nulls as well as on channels with relatively small intersymbol interference is investigated. A reference insertion method is used to improve system performance, and a two-stage processing technique is adopted to use the more reliable decisions from the decoder in equalizer coefficient adaptation. Simulation results show that the RLS-implemented DFE systems with coded modulation can successfully combat severe channel distortion while maintaining the coding gain over corresponding uncoded systems