An adaptive vector quantized flexible precoder for a dynamiccontinuous frequency selective mobile channel

Bandwidth efficient techniques are required to satisfy the demand for future public mobile radio communication systems. Precoding is a nonlinear pre-equalization technique that provides equalization equivalent to a decision-feedback equalizer (DFE) while not being affected by coding. Precoding is shown to be better than conventional equalization over a digital mobile channel that is dynamic in its characteristics. A novel precoding vector quantization scheme is applied to the prediction filter in the precoder and is shown to provide comparable results to the LMS adaptation algorithm while reducing the overheads     

Performance of a hybrid decision feedback equalizer structure forCAP-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     

Sorted spectral factorization of matrix polynomials in MIMO communications

Sorted spectral factorization of matrix polynomials is studied. Such type of factoring Hermitian matrix polynomials is the key step in calculating the optimum receive filter matrices in spatial/temporal decision-feedback equalization as well as the optimum transmit filter matrices in spatial/temporal Tomlinson-Harashima-type precoding schemes. Contrary to other approaches, we inherently consider asymptotic rather than finite-length results for transmission over MIMO channels with intersymbol interference. It is shown how the different types of factorizations can be transformed onto a prototype factorization task, which in turn can be solved by first performing an unsorted factorization and then determining the optimal processing order. An easy-to-use iterative algorithm for unsorted spectral factorization and the adjustment of the optimized order in DFE and precoding are explained. Numerical simulations cover the impact of sorted and unsorted spectral factorization on the performance of DFE and precoding schemes.     

Combined coding and precoding for PAM and QAM HDSL systems

A system is described for high-speed digital subscriber line (HDSL) applications. It uses trellis coding combined with Tomlinson (1971) precoding. Theoretical performance results are provided for uncoded pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) assuming an ideal decision feedback equalizer (DFE) that includes an optimum feedforward filter. The similarities and differences in behavior are explained. It is shown that combined coding and Tomlinson precoding can achieve the equalization performance of the DFE and full coding gain of the trellis code. Simulation results are given to show that about 3.4 dB additional margin is obtained by using a four-dimensional eight-state trellis code. The receiver complexity is increased by it is quite manageable     

Frequency-domain precoding for single carrier frequency-division multiple access - [topics in radio communications]

At present there is considerable interest in the use of single carrier frequency-division multiple access. This interest is justified by the inherent single carrier structure of the SC-FDMA scheme, which is more robust against phase noise and has a lower peak-to-average power ratio than orthogonal frequency-division multiple access. This consequently makes it more attractive for uplink transmission from low-cost devices with limited transmit power. SC-FDMA commonly makes use of frequency domain linear equalization in order to combat the frequency selectivity of the transmission channel. Frequency domain decision feedback equalization, composed of a frequency domain feed forward filter and a time domain feedback filter, outperforms LE due to its ability to cancel precursor echoes. Although these solutions suffer from error propagation, results show that DFE still offers a significant performance gain over conventional LE for uncoded SC-FDMA. In this article we show how precoding can be used on the uplink of the LTE standard to overcome the frequency selective nature of the radio channel. We propose a frequency domain implementation of Tomlinson- Harashima precoding and investigate the bit error rate and the PAPR performance for SCFDMA using ZF and MMSE THP.     

Local ML detection for multicarrier DS-CDMA downlink systems with grouped linear precoding

Abstract-A multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) downlink system with linear precoding over a group of subcarriers is considered. This scheme preserves user orthogonality independently of the underlying frequency-selective channel, collects the channel diversity and enables low-complexity decoding. In this context, we examine a local maximum-likelihood (LML) detection technique that searches for the maximum-likelihood (ML) solution in the neighborhood of the output provided by the minimum mean-squared error (MMSE) detector. By exploiting the soft information of the MMSE detector output and the precoder structure, we introduce useful criteria to reduce the computational complexity of the LML search. Simulations illustrate that the LML-MMSE detector with minimum neighborhood size yields considerable BER improvement with respect to MMSE, and outperforms a block decision-feedback equalization (DFE) approach at comparable complexity.     

Performance Analysis of a Pre-BLAST-DFE Technique for MISO Channels With Decentralized Receivers

The performance of a pre-Bell Laboratories Layered Space-Time Architecture (BLAST)-decision feedback equalization (DFE) technique with Tomlinson-Harashima precoding and decentralized receivers, operating over multiple-input single-output (MISO) frequency-selective fading channels, is evaluated. First, we derive the probability of symbol error for Tomlinson-Harashima precoding operating over single-input single-output channels with intersymbol interference. We then generalize this result to MISO frequency-selective fading channels with decentralized receivers by using the QR decomposition technique. The effect of optimal ordering of the decentralized receivers, which minimizes the total transmit energy, is investigated. We obtain a closed-form expression for the probability density function of the squared diagonal elements of the upper right triangular matrix belonging to the optimal QR decomposition when two transmit antennas and two receivers are used. We also provide simulations to corroborate the analytical results.     

Inflated Lattice Precoding, Bias Compensation, and the Uplink/Downlink Duality: The Connection

We review the uplink/downlink duality between decision-feedback equalization (DFE) and (nonlinear) precoding at the transmitter side. Thereby the effect of bias compensation in DFE is addressed, and its connection to the concept of "inflated lattice preceding" (ILP)-a tool which has been proven to be the key to achieve the capacity of channels with interference known at the transmitter-is elucidated. Concentrating on signal-to-interference plus noise ratios and hence error rates, pairs of schemes dual to each other are identified. It is shown that bias compensation in DFE and the scaling in ILP are dual operations and cancel each other     

A wide-band radial basis function decision feedbackequalizer-assisted burst-by-burst adaptive modem

The performance of radial basis function-based decision feedback equalized (RBF DFE) burst-by-burst adaptive quadrature amplitude modulation (AQAM) is presented for transmissions over dispersive wide-band mobile channels. This scheme is shown to give a significant improvement in terms of the mean bit error rate (BER) and bits per symbol (BPS) performance compared to that of the individual fixed modulation modes. The structural equivalence of the RBF DFE to the optimal Bayesian equalizer enables it to potentially outperform the conventional Kalman-filtered AQAM DFE scheme     

Coding for intersymbol interference channels-combined coding andprecoding

This paper describes a new coding scheme for transmission over intersymbol interference (ISI) channels. This scheme, called ISI coding, combines trellis coding with precoding (used to combat ISI). Like the recently introduced precoding scheme of Laroia, Tretter, and Farvardin (LTF), the ISI coder makes it possible to achieve both shaping and coding gains over ISI channels. By combining coding and precoding, however, the ISI coder makes the “precoding loss” independent of the number of coset partitions used to generate the trellis code. At high rates (large signal-to-noise ratio (SNR)), this makes it possible to approach the Shannon capacity of an ISI channel. The V.34 (formerly V.fast) international modem standard for high-speed (up to 28.8 kb/s) communication over voice-band telephone lines uses the version of the ISI coder described in Section IV of this paper     

Optimal precoding for orthogonalized spatial multiplexing in closed-loop MIMO systems

In this paper, we propose a new precoding algorithm for orthogonalized spatial multiplexing (OSM) systems over flat-fading multiple-input multiple-output (MIMO) channels. The OSM scheme was recently introduced for closed-loop MIMO systems which allows single symbol decodable maximum likelihood detection. To further improve the performance of the OSM system, we propose a new precoding method by maximizing the minimum Euclidean distance between constellation points in the effective channel. In order to efficiently identify the parameters of a precoder which maximizes the minimum distance, we introduce a partitioning approach. Through analysis, it is shown that one real value parameter and two bits are required for feedback information for precoding in 16-QAM systems. Simulation results demonstrate that our algorithm provides 9 dB and 7.5 dB gains at a bit error rate (BER) of 10^-4 over the conventional OSM systems for 4-QAM and 16-QAM, respectively. We also confirm that the performance of the proposed scheme is the same as that of the optimum closed-loop MIMO systems in terms of the minimum distance. Consequently, our precoding algorithm significantly improves the system performance with a small increase of feedback amount.     

An Adaptive Combined DFE/RSSE Structure for Unknown Channel with Severe ISI

This paper presents an adaptive combinedDFE/RSSE structure for transmission using a largeconstellation size over a slowly time-varying channelwith severe ISI. This structure combines theconventional decision-feedback equalization (DFE) withthe reduced state sequence estimation (RSSE).Analytical and simulation results carried out for 8PSKsignals indicate that the proposed scheme outperformsboth the conventional DFE and RSSE techniques.     

Pole-zero decision feedback equalization with a rapidly convergingadaptive IIR algorithm

A decision feedback equalizer (DFE) containing a feedback filter with both poles and zeros is proposed for high-speed digital communications over the subscriber loop. The feedback filter is composed of a relatively short FIR filter that cancels the initial part of the channel impulse response, which may contain rapid variations due to bridge taps, and a pole-zero, or IIR, filter that cancels the smoothly decaying tail of the impulse response. Modifications of an adaptive IIR algorithm, based on the Steiglitz-McBride (1965) identification scheme, are proposed to adapt the feedback filter. A measured subscriber loop impulse response is used to compare the performance of the adaptive pole-zero DFE, assuming a two-pole feedback filter, with a conventional DFE having the same number of coefficients. Results show that the pole-zero DFE offers a significant improvement in mean squared error relative to the conventional DFE. The speed convergence of the adaptive pole-zero DFE is comparable to that of the conventional DFE using the standard least mean square (LMS) adaptive algorithm     

多波束移动卫星系统的自适应开环预编码方法

对于卫星移动通信系统,由于卫星与地面终端之间的相对运动以及星地间传输延迟,传统的基于理想信道信息的预编码方法是不适用的。针对这一问题,提出了一种基于开环信道估计的预编码方法。卫星端利用开环获取的部分信道信息实现多波束联合预编码,并导出了系统传输速率的闭合解析表达式。此外,为了克服强干扰环境下多波束预编码系统性能恶化问题,提出了一种自适应预编码传输方法。卫星发射机依据开环获得的慢时变用户位置信道信息和信道统计量信息,自适应地选择预编码方法或传统频率复用方法,实现最优的系统性能。理论分析和仿真结果表明,与传统的干扰抑制方法相比,所提方法能实现更优的系统性能,同时也克服了传统预编码方法的局限性。     

Orthogonal superimposed training design for block transmission over frequency‐selective fading channels

Channel estimation for single‐carrier block transmission over frequency‐selective fading channel using superimposed training is addressed. A novel affine precoding model based on orthogonal polyphase sequence set is designed to decouple channel estimation from symbol detection. The orthogonal constraints on the training and precoding matrices ensure the separation of superimposed signals and accurate channel estimation with less training overheads as compared with time‐multiplexed scheme. Simulation results show that the proposed scheme exhibits good performance and outperforms another data‐dependent superimposed training scheme, especially for compact constellations or channel with long delay‐spread. Copyright © 2014 John Wiley & Sons, Ltd.     

Transmitter-Based Equalization for Wireless LAN's

In a high-speed mobile communications network, it is desirable tooff-load as much hardware complexity as possible from mobile terminalsto the base station. A system architecture is presented where allequalization-related computations, requiring most hardware resources,can be performed at the base station. A conventional decision-feedbackequalizer (DFE) is used to equalize the received transmissions, and amodified Tomlinson–Harashima precoding method (Tx-THP) is appliedto pre-equalize the transmitted data sequence. The Tx-THP scheme canalso be extended to differentially coherent detection, to suit thesystems where perfect frequency synchronization is not available. Amodified DQPSK receiver is presented to process the extendedconstellations. To avoid the side effect of severe noise multiplication,an alternative receiver structure is developed, incorporating adiscrete-time phase tracker to allow the reduction of the constellationsize prior to the correlation operation. With a 2nd-order estimatorfilter, this receiver approaches the performance of coherent detectionfor Tx-THP.     

Near‐Optimum Blind Decision Feedback Equalization for ATSC Digital Television Receivers

This paper presents a near‐optimum blind decision feedback equalizer (DFE) for the receivers of Advanced Television Systems Committee (ATSC) digital television. By adopting a modified trellis decoder (MTD) with a trace‐ back depth of 1 for the decision device in the DFE, we obtain a hardware‐efficient, blind DFE approaching the performance of an optimum DFE which has no error propagation. In the MTD, the absolute distance is used rather than the squared Euclidean distance for the computation of the branch metrics. This results in a reduction of the computational complexity over the original trellis decoding scheme. Compared to the conventional slicer, the MTD shows an outstanding performance improvement in decision error probability and is comparable to the original trellis decoder using the Euclidean distance. Reducing error propagation by use of the MTD in the DFE leads to the improvement of convergence performance in terms of convergence speed and residual error. Simulation results show that the proposed blind DFE performs much better than the blind DFE with the slicer, and the difference is prominent at the trellis decoder following the blind DFE.     

统计和瞬时CSI混合的3D MIMO预编码

本文针对分别具有统计信道状态信息（Channel State Information,CSI）和瞬时CSI的用户,研究了两类用户在三维多输入多输出（Three-Dimension Multiple-Input Multiple-Output,3D MIMO）系统中的下行链路传输模式,解决了混合利用统计和瞬时CSI的下行预编码的设计问题。利用3D MIMO信道的克罗内克积的性质,提出了一种用于最小化基站总发射功率的扩展迫零预编码的方法,并分别求解其最优水平预编码矩阵和最优垂直预编码矩阵。仿真结果表明,该方案具有更低的复杂度和良好的速率。      

A precoding scheme for DFT-based OFDM to suppress sidelobes

In spectrum pooling scenario, the spectral leakage of DFT-based OFDM signal can be divided into in-band-out-of-subband (IBOSB) radiation and out-of-band (OOB) radiation. A precoding scheme is proposed to suppress the IBOSB sidelobes. The precoding design is based on the generalized eigenvalue problem. Simulation results demonstrate that the proposed scheme can suppress the sidelobes significantly in contrast to the uncoded schemes. At the same time, the bit-error-rate (BER) performance of the precoded system over multipath fading channel is improved due to the frequency diversity.     

Grassmannian precoding MU-MIMO scheme

In this article, a Grassmannian precoding multiuser multiple-input multiple-output （MU-MIMO） scheme for downlink transmission is proposed. The proposed MU-MIMO scheme will perform scheduling and precoding simultaneously at the base station, to obtain both the multiuser diversity gain and the precoding gain, to maximize the system capacity. The precoding method is related to Grassmannian precoding, which extends the point-to-point single-user Grassmannian precoding to point-to-multipoint multiuser Grassmannian precoding. It provides further significant system capacity enhancement than the single user MIMO （SU-MIMO） system and also outperforms the block dia~onalization （BD） algorithm under the same simulation environment.