宽带移动通信系统中基于梳状导频的信道估计

本文主要研究了OFDM系统中基于梳状导频的空时信道估计方法,并提出一种新的信道估计算法,即实现了空时二维信道估计。基于梳状导频的空时信道估计的算法分为两步:第一步是利用导频信号对导频位置的信道进行估计,第二步是信道插值,得到所有频域位置的信道信息。同时还研究了基于LS准则的信号估计以及基于线性插值的信道插值。     

基于802.16e的信道估计算法研究

常用的空时译码方法在很大程度上依赖于准确的信道状态信息,使得信道估计成为正确进行空时编码译码的先决条件。结合虚拟导频的设计和时频二维多项式拟合算法,研究了一种适用于快速移动场景下的虚拟导频信道估计算法。该算法能够利用时频二维信道信息的相关性,对信道进行拟合,从而提升信道估计的性能。分别在车载60km以及车载120km信道模式下进行仿真,结果表明,较现有的信道估计算法,在快速移动场景下虚拟导频信道估计算法具有更好的均方误差性能。     

MIMO-OFDM中一种新的信道估计算法

根据不同发射天线发射的导频移位后应相互正交的原则,运用最小二乘推导出了基于MIMO-OFDM信道估计模型的时域信道估计解。针对该时域信道估计解需要矩阵求逆,运算量大的缺点,研究了基于空时频三维导频符号的设计方法,该方法使得时域信道估计解不需要矩阵求逆,可大大降低信道估计的复杂度,并且可以使信道估计误差最小。仿真结果表明,提出的基于空时频导频的信道估计方法相比传统LS算法和文献算法,MSE和BER性能均有较大的提高。     

基于STBC-SCFDE系统的信道估计算法研究

针对频率选择性衰落信道下2发射天线的空时分组编码-单载波频域均衡(STBC-SCFDE)系统,分别设计了2种新颖的时域多天线导频序列和频域多天线导频序列,相应地提出了一种时域导频信道估计算法和一种频域导频信道估计算法,并分别进行了信道估计的MSE性能分析以及计算复杂度对比.仿真验证表明,相比传统经典多天线信道估计算法,所提2种信道算法在中低信噪比下均能实现良好的信道估计性能,为未来无线通信系统的信道估计开辟了新的解决途径.     

一种基于OFDM电力线通信系统信道估计方法

通过实际测量给出低压电力线的信道模型,在此基础之上提出了利用循环前缀对信道进行估计的方法,即在梳状导频模式时,使用导频获得导频子信道频率响应后,对其在频域方向进行插值得到数据子信道响应,在其后没有导频的序列的数据周期内,使用循环前缀对信道进行估计,从而起到改善系统误比特性能的目的。     

认知无线电NC-OFDM系统中基于压缩感知的信道估计新方法

提出了一种基于压缩感知(CS,compressive sensing)理论的不连续子载波正交频分复用(NC-OFDM,non-contiguous orthogonal frequency division multiplexing)系统信道估计新方法,全面研究了认知无线电NC-OFDM系统CS信道估计的理论框架、导频图案的设计、信道估计算法,并依据CS测量矩阵设计理论提出了测量矩阵互相关最小化的导频图案优化准则。仿真结果表明,同已有的NC-OFDM系统信道估计方法相比,CS信道估计能够在多种禁用子载波场景下,使用较少导频获得很好的信道估计性能。     

一种结合空时编码的导频辅助信道估计方法

多输入多输出-正交频分复用(MIMO-OFDM)作为一种高速率无线通信的有效方式,其信道估计技术具有非常重要的位置。将OFDM技术与基于发射分集的空时格码相结合,构造了一个基于空时格码的OFDM模型。基于MIMO-OFDM系统的导频设计准则及特性,提出了一种空时编码OFDM系统中基于导频的辅助信道估计方法。仿真实验结果表明信道估计的性能接近理想信道情况下的性能。     

基于线性插值的RF-导频插入DDO-OFDM系统的仿真分析与实验验证

RF-导频（RF-pilot）插入的直接探测光正交频分复用（DDO-OFDM）系统通过调节载信比（CSPR）可以有效对系统传输性能进行优化,是高速短距离局域网传输的备选方案之一。在该系统下,利用导频子载波处信道估计结果,提出一种基于线性插值与时域平均的信道估计方法,用于实现非导频子载波处信道响应的估计。本文方法可以实时跟踪信道变化,有效抑制噪声影响。仿真中,13个导频子载波用于信道估计,采用线性插值与时域平均的方法估计出非导频子载波处的信道响应,传输4-QAM映射的20 Gbit/s信号在光纤中传输400 km。与背靠背相比,在误码率（EBR）为1×10^-3时,几乎没有光信噪比（OSNR）损伤。通过原理验证性实验,验证了本文方法在RF-pilot插入DDO-OFDM系统中用于信道估计的可行性。     

OTFS系统中基于最小多普勒间干扰的导频设计

正交时频空(OTFS)调制系统能够改进时变频选信道的错误概率,但是目前只是简单地采用矩形导频。文中在OTFS系统中一种基于冲激的信道估计模型的基础上,优化其导频图案。在固定导频开销的约束下,最小化多普勒间干扰,也即是最大化被估计的信道平均功率。经论证,该优化问题可以转化为一个经典的背包问题,并可以通过贪心算法求解。仿真结果表明,文中经过优化的导频图案与原矩形导频图案相比,相同的导频开销下,具有更低的均方误差和误码率。     

WiMAX系统中导频和信道估计

WiMAX系统中物理层多址方案采用了正交频分多址接入(OFDMA),并根据上、下行链路的不同特点,定义了多种导频图案。当OFDMA和多输入多输出(MIMO)技术结合时,导频图案也需要变化来支持多天线。WiMAX系统中MIMO-OFDMA有五种导频模式,包括下行部分使用子信道(DL-PUSC)、下行完全使用子信道(DL-FUSC)、下行可选完全使用子信道(DL-OFUSC)、上行部分使用子信道(UL-PUSC)和上行可选部分使用子信道(UL-OPUSC)。通过分析时域LS、频域LS和基于FFT的信道估计方法下的仿真结果,可以得出每种模式下的最优信道估计方案。     

Space-time-frequency coded OFDM over frequency-selective fading channels

This paper proposes novel space-time-frequency (STF) coding for multi-antenna orthogonal frequency-division multiplexing (OFDM) transmissions over frequency-selective Rayleigh fading channels. Incorporating subchannel grouping and choosing appropriate system parameters, we first convert our system into a set of group STF (GSTF) systems. This enables simplification of STF coding within each GSTF system. We derive design criteria for STF coding and exploit existing ST coding techniques to construct both STF block and trellis codes. The resulting codes are shown to be capable of achieving maximum diversity and coding gains, while affording low-complexity decoding. The performance merits of our design are confirmed by corroborating simulations and compared with existing alternatives.     

A link adaptation scheme optimized for wireless JPEG 2000 transmission over realistic MIMO systems

This paper proposes a Joint Source Channel Coding solution optimized for a wireless JPEG 2000 (JPWL ISO/IEC 15444-11) image transmission scheme over a MIMO channel. To ensure robustness of the transmission, channel diversity is exploited with a Closed-Loop MIMO-OFDM scheme. This relies on the Channel State Information (CSI) knowledge on the transmitter side, which allows the MIMO channel to be decomposed into several hierarchical SISO subchannels. In the proposed scheme, the JPWL codestream is divided into hierarchical quality layer passing through the SISO subchannels. With the CSI, a global and optimal method for adjusting all the system parameters of each SISO subchannel is provided. Accordingly, adaptive modulation, Unequal Error Protection (UEP), Unequal Power Allocation (UPA) and source coding rate is provided for each quality layers. The major strength of this work is to provide an optimal method that parameterizes several variables. These have an effect on the rate-distortion trade-off under bitrate, Quality of Service (QoS) and power constraints. Finally, the proposed work allows flexible and reactive coding of a JPWL codestream adapted to the instantaneous channel status. The performance of this technique is evaluated over a realistic time-varying MIMO channel provided by a 3D-ray tracing propagation model. A significant improvement in the quality of the image is demonstrated.     

Linear constellation precoding for OFDM with maximum multipath diversity and coding gains

Orthogonal frequency-division multiplexing (OFDM) converts a frequency-selective fading channel into parallel flat-fading subchannels, thereby simplifying channel equalization and symbol decoding. However, OFDM's performance suffers from the loss of multipath diversity, and the inability to guarantee symbol detectability when channel s occur. We introduce a linear constellation precoded OFDM for wireless transmissions over frequency-selective fading channels. Exploiting the correlation structure of subchannels and choosing system parameters properly, we first perform an optimal subcarrier grouping to divide the set of subchannels into subsets. Within each subset, a linear constellation-specific precoder is then designed to maximize both diversity and coding gains. While greatly reducing the decoding complexity and simplifying the precoder design, subcarrier grouping enables the maximum possible diversity and coding gains. In addition to reduced complexity, the proposed system guarantees symbol detectability regardless of channel s, and does not reduce the transmission rate. Analytic evaluation and corroborating simulations reveal its performance merits.     

Block coding capacity of high bit rate digital subscriber lines bythe structured channel signaling technique

Self near-end crosstalk (NEXT) is assumed to be the dominant source of impairment, and the subscriber loop configuration is governed by carrier serving area (CSA) design rules. The structured channel signaling (SCS) technique decomposes the physical channel into multiple parallel independent signaling subchannels by exploiting the combined eigenstructure of the channel and the correlation of the (NEXT) interference. Computer performance evaluation studies reveal two distinct patterns. For a given loop configuration, as the block length increases, the coding gain usually increases, and for a fixed block code length, the coding gain degrades as the loop length (including bridged taps) increases. For loops at the extreme range of a CSA, block codes of at least 20 symbols are required to achieve a performance commensurate with that of a decision feedback equalizer (DFE) composed of an optimal nine-tap minimum mean-square error (MMSE) feedforward filter and an ideal feedback canceler     

Complex-field coding for OFDM over fading wireless channels

Orthogonal frequency-division multiplexing (OFDM) converts a time-dispersive channel into parallel subchannels, and thus facilitates equalization and (de)coding. But when the channel has s close to or on the fast Fourier transform (FFT) grid, uncoded OFDM faces serious symbol recovery problems. As an alternative to various error-control coding techniques that have been proposed to ameliorate the problem, we perform complex-field coding (CFC) before the symbols are multiplexed. We quantify the maximum achievable diversity order for independent and identically distributed (i.i.d.) or correlated Rayleigh-fading channels, and also provide design rules for achieving the maximum diversity order. The maximum coding gain is given, and the encoder enabling the maximum coding gain is also found. Simulated performance comparisons of CFC-OFDM with existing block and convolutionally coded OFDM alternatives favor CFC-OFDM for the code rates used in a HiperLAN2 experiment.     

Space-time coding over correlated fading channels with antenna selection

In a previous paper by Bahceci et al., antenna selection ' for multiple-antenna transmission systems under the assumption that the subchannels between antenna pairs fade independently was studied. In this paper, the performance of such systems when the subchannels experience correlated fading is considered. It is assumed that the channel-state information (CSI) is available only at the receiver, the antenna selection is performed only at the receiver, and the selection is based on the instantaneous received signal power. The effects of channel correlations on the diversity and coding gain when the receiver system is a subset of the antennas are quantified. Theoretical results indicate that the correlations in the channel do not degrade the diversity order, provided that the channel is full rank. However, it does result in some performance loss in the coding gain.     

QoE and energy efficiency aware resource allocation for OFDM systems in group mobility environments

The global information and communication technology industry is a fast growing contributor to the electrical energy consumption, especially for the base stations, accounting for a substantial amount of the energy use. Inherently, a resource allocation strategy including subchannels blackout would reduce energy consumption. In this paper, energy efficient resource allocation algorithms are addressed for the OFDM system in group mobility environments, suffering from high intercarrier interference. We first propose the subchannels blackout scheme to save energy implicitly without performance degradation by turning off certain subchannels when transmitting signals. Then, resource allocation scheme in combination with subchannels blackout scheme is developed, consisting of intergroup subchannels allocation and inner‐group subchannels blackout. Its advantage is twofold. (i) Energy consumption is reduced obviously; (ii) intercarrier interference is decreased and channel quality is enhanced simultaneously. However, the original transmit rate decreases with the decrease of active resources. We also prove under subchannels blackout scheme, achieved throughput and perceptual quality of experience (QoE) are quasiconcave in energy saving percentage, which reflects the number of blackout subchannels. We then present two energy efficient resource allocation algorithms. Both algorithms focus on the optimal solution by using an iteration method. The difference lies in the objective. One tries for energy consumption minimization above the satisfactory QoE level, but the other aims to maximize QoE perceived by users. Numerical results confirm the theoretical findings and demonstrate the promising energy‐saving capability with satisfying QoE of the proposed resource allocation schemes. Copyright © 2013 John Wiley & Sons, Ltd.     

Uniform channel decomposition for MIMO communications

Assuming the availability of the channel state information at the transmitter (CSIT) and receiver (CSIR), we consider the joint optimal transceiver design for multi-input multi-output (MIMO) communication systems. Using the geometric mean decomposition (GMD), we propose a transceiver design that can decompose, in a strictly capacity lossless manner, a MIMO channel into multiple subchannels with identical capacities. This uniform channel decomposition (UCD) scheme has two implementation forms. One is the combination of a linear precoder and a minimum mean-squared-error VBLAST (MMSE-VBLAST) detector, which is referred to as UCD-VBLAST, and the other includes a dirty paper (DP) precoder and a linear equalizer followed by a DP decoder, which we refer to as UCD-DP. The UCD scheme can provide much convenience for the modulation/demodulation and coding/decoding procedures due to obviating the need for bit allocation. We also show that UCD can achieve the maximal diversity gain. The simulation results show that the UCD scheme exhibits excellent performance, even without the use of any error correcting codes.     

OFDM系统的联合收发机设计

Orthogonal Frequency Division Multiplexing (OFDM) is an effective technique to deal with a frequency selective channel since it can convert the channel into some flat fading subchannels. However, very different output SNR values of the sub-channels will lead to poor bit error performance when a linear equalizer and Equal Bit Allocation (EBA) are adopted in OFDM systems. So, we proposed three novel nonlinear Joint Transceiver (JT) schemes based on Zero-Forcing (ZF) criterion and Minimum Mean Square Error (MMSE) criterion respectively, which can transform all subchannels of an OFDM system into subchannels with identical channel gain. Thus, EBA is equivalent to the Optimum Bit Allocation (OBA) for these subchannels. Numerical analysis helps us to obtain the theoretical approximate BER values of the JT scheme. Simulation results verify the numerical analysis and confirm that the performance of our proposed JT scheme greatly outperforms the traditional linear equalizer with EBA at moderate and high SNR values.     

Power allocation and control for multicarrier systems with softdecoding

We consider the application of multicarrier modulation in a wireless cellular network in order to enable high-data rate communication and alleviate the multipath induced intersymbol interference (ISI). In this scenario, power control becomes crucial in enhancing the spectral and power efficiency. A conventional approach of maintaining the same link quality for all the subchannels, in other words, disregarding any possible post-demodulation processing, is considered first. This approach appears to have increasing power consumption as the number of subchannels increases. It also deteriorates the power control stability and convergence properties in a multicell network. We attribute this phenomenon to lack of frequency diversity exploitation, and thus, we propose to use channel coding and soft decoding as vehicles to profit from the (frequency) diversity advantage in addition to the coding advantage. Based on the soft decoding performance bound, a power allocation and control algorithm is proposed. It is shown through simulations that the proposed algorithm improves the power efficiency as the number of subchannels increases. It also provides a better convergence property and is able to “detect” and eliminate ill-conditioned subchannels. The advantages of using multicarrier modulation are thus reassured. Besides these enhancements, the proposed algorithm is simple and feasible in that it consists of only the traditional closed-loop power control algorithm and a target signal-to-interference ratio (SIR) reassignment at the receiver. Detailed channel information feedback from receiver to transmitter is not required