Particle Filters for Joint Blind Equalization and Decoding in Frequency-Selective Channels

This paper introduces new algorithms for joint blind equalization and decoding of convolutionally coded communication systems operating on frequency-selective channels. The proposed method is based on particle filters (PF), recursively approximating maximum a posteriori (MAP) estimates of the transmitted data without explicitly determining channel parameters. Further elaborating on previous works, we assume that both the channel order and the noise variance are unknown random variables, and develop a new formulation for PF weight propagation which allows these quantities to be analytically integrated out. We verify via numerical simulations that the proposed methods lead to near optimal performance, closely approximating that of algorithms that require exact knowledge of all channel parameters.     

非线性卫星信道下的粒子滤波盲均衡方法

针对卫星信道中高功率放大器产生非线性失真的问题,本文提出了一种基于粒子滤波技术的盲均衡法。该算法优势在于不需要对非线性信道线性化处理,而是利用带权值的离散随机样本点来对期望分布进行近似,通过将非线性模型建模成状态空间模型,对信道参数进行跟踪和符号序列估计。仿真结果表明,算法实现了对放大器非线性幅度和相位特性的粒子滤波估计,并对符号序列进行了盲恢复,在误比特率为 较截断Volterra均衡有1.5 dB左右的性能增益;通过增加粒子数目和平滑长度能一定程度上提高算法性能,但在复杂度与性能折中考虑下,不能无限增加粒子数目。      

改进的分布式粒子滤波盲均衡算法

针对多节点合作接收系统中的盲均衡问题,提出了一种基于一致优化的分布式粒子滤波盲均衡改进算法。该算法采用多个接收机组成无融合中心的分布式网络,使用分布式粒子滤波合作估计共同的发送符号序列,克服了单接收节点受信道影响大、误码率高的问题。为了保证粒子滤波中每个节点产生共同的粒子集和粒子权重,采用基于交替方向乘子法的一致优化算法获得联合似然函数,并与最大一致协议相结合,从而使每个节点获得相同的最佳重要性函数和粒子权重。理论分析与仿真结果表明,该算法只需经过有限次的一致迭代就可以达到集中式合作盲均衡的性能。全分布式的多节点合作获得了空间分集增益,降低了系统误码率。      

OFDM系统中低复杂度联合译码和信道估计方法研究

本文针对OFDM(Orthogonal Frequency Division Multiplexing)系统提出了一种低复杂度联合译码和信道估计算法的接收机。接收机首先通过导频对信道进行估计，然后使用Turbo码译码输出判决信息对信道参数进行稳健的估计。仿真结果表明，即使系统误码率大于10^-2，经过3次迭代估计，系统性能提高仍有0．5-1dB；在高信噪比下，可显著地降低系统的误码底限。本文同时还给出了译码迭代次数对系统性能的影响。     

OFDM联合信道估计与均衡技术研究

OFDM系统在时变信道中会受到子载波间干扰,单独进行信道估计和信号检测的策略对于提高接收机的抗干扰能力有限,将信道估计和信号检测统一考虑则可更有效地抵抗子载波间干扰。针对此问题,基于迭代SAGE算法提出了一种新的联合信道估计与符号检测算法,为了减低算法的复杂度,引入BEM信道建模方法。仿真评估了BEM算法的归一化均方误差性能,验证了BEM建模的有效性,显示该算法的误码率优于基于BEM算法的线性均衡检测算法和基于MMSE的干扰对消算法,而且该算法只通过少数几次迭代便可达到收敛域,较好地克服了子载波间干扰的问题。     

Iterative MAP Equalization and Decoding in Wireless Mobile Coded OFDM

Orthogonal frequency division multiplexing (OFDM) system suffers extra performance degradation in fast fading channels due to intercarrier interference (ICI). Combining frequency domain equalization and bit-interleaved coded modulation (BICM), the iterative receiver is able to harvest both temporal and frequency diversity. Realizing that ICI channels are intrinsically ISI channels, this paper proposes a soft-in soft-out (SISO) maximum a posteriori (MAP) equalizer by extending Ungerboeck's maximum likelihood sequence estimator (MLSE) formulation to ICI channels. The SISO MAP equalizer employs BCJR algorithm and computes the bit log-likelihood ratios (LLR) for the entire received sequence by efficiently constructing a trellis that takes into account of the ICI channel structure. A reduced state (RS) formulation of the SISO MAP equalizer which provides good performance/complexity tradeoff is also described. Utilizing the fact that ICI energy is clustered in adjacent subcarriers, frequency domain equalization is made localized. This paper further proposes two computational efficient linear minimum mean square error (LMMSE) based equalization methods: recursive q-tap SIC-LMMSE equalizer and recursive Sliding-Window (SW) SIC-LMMSE equalizer respectively. Simulations results demonstrate that the iterative SISO RS-MAP equalizer achieves the performance of no ICI with normalized Doppler frequency f_dT_s up to 20.46% in realistic mobile WiMAX environment.     

一种粒子滤波的盲多用户检测快速算法

该文在时间观察状态空间模型基础上,针对粒子滤波计算复杂的问题,构造了一种在同步快变平衰落信道下计算复杂度低的粒子滤波的DS-CDMA盲多用户检测快速算法。该算法通过对超过门限数目的粒子分类,以不同类别粒子的概率差值作为粒子滤波所需粒子数目是否足够的判断准则,自动调节不同情况下所需的粒子数目,并对不同类别粒子概率差值下盲多用户检测性能进行分析。仿真结果表明,该文算法可以在保证盲多用户检测性能的前提下有效地降低运算复杂度,且检测性能与概率差值成正比。     

一种快变信道中的正交频分复用系统均衡算法

当信道参数随时间的快速变化时,正交频分复用通信系统(OFDM)子载波间的正交性遭到破坏,出现了载波间的相互干扰(ICI),传统的单抽头频域均衡不再适用。虽然可采用最小均方误差(MMSE)均衡来补偿信道失真,但其计算量太大。为此,常用的方法是:先对接收信号进行ICI消除,恢复载波间的正交性,然后再进行单抽头频域或均衡。现有文献对ICI的分析均在频域进行,在此基础上提出的ICI消除与均衡算法存在计算量大或频谱利用率低的缺点。本文对ICI的产生机理和性质进行了时域和频域两方面的分析,利用现有OFDM标准中的空闲子载波信息,提出了一种ICI消除与均衡算法。理论分析和计算机仿真结果表明:该算法具有ICI消除效果好、计算量小和频谱利用率高等优点。     

Blind Frequency Ambiguity Resolution in OFDM Systems Using a Linear Precoder

This paper presents a technique of the blind frequency ambiguity resolution for orthogonal frequency division multiplexing systems. The basic idea behind this method is a linear precoding scheme applied to data in predefined subcarriers before the inverse discrete Fourier transform to deploy a correlation structure, thus the integer part of the carrier frequency offset can be estimated after the discrete Fourier transform via cross correlation operations among these subcarriers. Performance analysis is carried out to gain insight into the relationship between the false estimation probability and system parameters. Allocation of these predefined subcarriers is optimized to avoid false estimation. Finally, numerical simulations are conducted to demonstrate the efficiency of the proposed method and the performance comparison with the existing estimators.     

粒子滤波重采样及在盲均衡中的应用

提出了一种新的粒子滤波重采样算法——降序二分法,这种算法的主要思想是在分层重采样算法的基础上,寻找权重最大点的过程用折半二分法。另外,把这种算法应用于信道的盲均衡中,为了易于采样,重要性函数采用先验概率密度,用信道的均值代替信道的真实值,仿真结果表明,这种算法的平均性能优于之前的重采样算法,且均衡的同时能够完成对信道的辨识。     

基于粒子滤波的Turbo盲均衡

粒子滤波是一种基于贝叶斯估计的算法,在信道盲辨识和盲均衡问题上具有快收敛、抗深衰信道等优势。Turbo盲均衡在低信噪比条件下有较好的误码性能。为了在深衰信道下使通信具有良好的误码性能,对粒子滤波盲均衡算法进行改进,改进算法的重要性采样函数利用了粒子的先验信息,得到一种软输入软输出的粒子滤波盲均衡算法。依据Turbo盲均衡的框架结构实现了一种基于粒子滤波的Turbo盲均衡算法,该算法利用信道编码带来的编码增益,提高了均衡和信道辨识的性能。仿真结果表明相比粒子滤波盲均衡算法本文提出算法的误码率性能提高1dB左右,误帧率性能则提高了3dB以上,经分析可知在信道系数估计较为准确的条件下,系统数据帧几乎没有误码。      

OFDM系统中的盲信道估计

本文从OFDM信号的矩阵表示出发,分析比较了OFDM系统中现有的各种盲信道估计方法。OFDM盲信道估计方法分为两类,一类是统计型方法,它利用了发送信号和接收信号的统计特性;另一类是确定型方法,它利用了发送调制信号的固有特性。一般而言,统计型方法的计算量较小,但是估计精度不高且估计的实时性不好;而确定型方法的估计精度较高,实时性较好,但是其计算量较大。计算机仿真表明,这些盲信道估计方法的性能受信道参数尤其是多普勒频率影响很大,盲信道估计的实用化有待进一步研究。     

联合均衡和多重深度TCM译码技术研究

针对ATSC DTV系统,提出一种多重深度TCM译码作为软判决的联合均衡及译码方案,从而降低误差传播,提高了均衡器的性能.新方案仅增加了译码输出单元,复杂度较深度为1时增加并不大.当采用译码深度为1的TCM译码输出作为反馈,比硬判决反馈时的均衡器收敛门限降低了6 dB.当最大的译码深度增加到6时,均衡器收敛的门限进一步降低了3 dB,从而比硬判决时均衡器的收敛门限降低9 dB.     

联合OFDM技术的常数模盲均衡仿真研究

针对传统盲均衡技术理论上消除码间干扰需要无限长均衡器抽头系数的缺陷,丈中采用OFDM技术与盲均衡技术进行结合,提出了一种联合OFDM技术的常数模盲均衡算法。利用OFDM抗多径衰落性能,再利用有限长度均衡器对信号进行均衡接收,有效地提高了通信质量。计算机仿真证明了这一方法的有效性。     

MIMO OFDM系统中的Turbo子载波均衡

在MIMO OFDM系统中,为了对抗同天线干扰及由于保护间隔不足而引起的码间干扰和载波间干扰,该文给出了一种基于MMSE的Turbo子载波均衡器。在该算法中,软输入软输出(SISO)的子载波均衡器与软输入软输出(SISO)解码器通过迭代进行软信息交换。仿真结果表明,与非迭代的子载波均衡器相比,该文给出的Turbo子载波均衡器能够有效利用时间和空间分集,使系统性能得到了改善。     

Blind Sequential Monte Carlo Joint Tracking of Channel State and Frequency Offset in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation scheme, which has been adopted for several wireless standards. In order to fully exploit the benefits of an OFDM system, estimation of the channel state information must be performed. Moreover a well-known problem of OFDM is its sensitivity to frequency offset between the transmitted and received carrier frequencies. This frequency offset introduces inter-carrier interference in the OFDM signal. In this paper we address the problem of jointly tracking the channel and frequency offset based on a Sequential Monte Carlo filtering approach. The proposed algorithm works in a decision-directed way, thus does not require the use of pilot symbols, providing a worth-mentioned increase in the useful data rate. Through simulations we demonstrate the efficiency of this approach against a similar approach where the Extended Kalman Filter is used. Moreover our method is compared with two recently proposed pilot-based methods.     

Instantaneous Mixture Channel Selection for Blind Equalization Using Cumulant Features in MIMO Systems

Cognitive radio combined with multiple-input multiple-output (MIMO) communications provides high data rate, efficiency and high reliability. One of the most important challenges in MIMO communication is combating MIMO multipath channel. MIMO blind equalizers and channel estimators combat MIMO multipath channels without the use of training or pilot sequences. First, the multipath channel is converted into instantaneous mixture channel (IMC), using second-order statistics of the data. Then using higher-order statistics, these mixtures are separated. However, proper selection of IMC is a major challenge. In this paper, a novel blind algorithm for choosing the best IMC is proposed. The proposed algorithm is based on the cumulant value of the received signal.     

Blind Joint Estimation for OFDM Time-Frequency Parameters

A novel blind joint estimation of orthogonal frequency division multiplexing (OFDM) time-frequency parameters is conceived in low signal-to-noise ratio (SNR) and multipath channel conditions. The oversampling factor is estimated by employing a new-designed cosine roll-off filter whose cut-off frequency is considered as the approximation of the sampling signal’s spectrum. The useful data length is estimated by means of the improved varying time-delay autocorrelation, and its final estimated value is defined as a power of 2, which has the minimum Euclidean distance with the peak position. The estimated useful data length is further utilized to refine the previous estimation of the oversampling factor. The bandwidth of OFDM signals is estimated by adopting the spectrum approximation method based on the estimated time-parameters. The simulation results demonstrate that the proposed method is robust to noise and multipath channel conditions without any requirement of a prior knowledge.