一种基于信道状态信息的最优软判决译码

针对衰减信道卷积码的译码,给出了当信道附加噪声非固定时,计算最优软判决度量的表达式。仿真结果表明,在适当的信噪比条件下,采用该方法比硬判决的信噪比提高2dB。     

衰落信道下卷积码的软判决译码

本文将交织、信道输出软信息、信道特征软信息相结合,研究Rayleigh衰落信道下卷积码的Viterbi软判决译码,在利用和不利用信道状态信息(CSI)两种情况下给出软判决的支路度量计算公式和性能仿真结果,计算了卷积码的性能上界;研究了解调器输出量化门限(量化级数目、量化间距)对译码性能的影响.结果与硬判决译码及AWGN信道的相关结论进行了比较.     

衰落信道下多级编码方案的软判决译码

研究Rayleigh衰落信道下,软判决译码对三种不同映射方案多级编码系统性能的影响.基于信道容量规则,构造卷积码为分量码的8ASK多级编码系统,在多阶段译码中提出利用信道状态信息的软判决支路度量计算公式,并探讨系统的误码传播特性,研究结果可为衰落信道编码调制方案的最佳设计度量提供参考.     

AWGN和Rayleigh信道下的自适应多级编码系统设计

基于信道容量规则和分组分割,构造自适应于AWGN和Rayleigh两种信道的多级编码系统,为移动信道编码调制系统的设计度量提供参考。提出利用信道状态信息的软判决支路度量计算公式,分析分组分割方案的自适应特性,比较软、硬判决译码对系统性能的影响,结果表明本文提出的自适应系统比未编码系统获得可观的编码增益。     

Performance of soft metrics for convolutional coded asynchronous fast FHSS-MA networks using BFSK under Rayleigh fading

Performance of robust soft decoding metrics is examined for use in convolutional coded asynchronous fast frequency-hop spread-spectrum multiple-access networks using binary frequency-shift keying under Rayleigh fading channels. For comparison, maximum-likelihood metrics based on a Gaussian approximation are derived. Significant gains are observed compared to hard decision decoding by using appropriate soft metrics.     

Forward Error Correction Coding for Fading Compensation in Mobile Satellite Channels

Fading in mobile satellite communications severely degrades the performance of data transmission. The channel is modeled with nonfrequency selective Rice and Rayleigh fading. Also, stored channel simulation is used for hardware data transmission. FEC coding with Viterbi decoding of convolutional codes, and Berlekamp-Massey decoding of Reed-Solomon codes, are used to compensate for the fading. In addition to interleaving, channel state and erasure information improve the performance of the decoder. The BER after decoding is calculated for specific codes on several channels and for different transmission schemes. Using very simple channel state and erasure information gives 2-7 dB additional coding gain. These gains have been verified by hardware data transmission on synthetic fading channels and stored mobile satellite channels.     

OFDM系统中改进的16QAM软判决解调算法

针对OFDM信号在多径衰落信道中传输的特点,提出了一种改进的16QAM解调软信息计算方法,通过在均衡时利用信道信息改善信号总体的信噪比水平,并且使软判决结果包含更多的信道信息,从而提高软判决译码的性能。仿真结果表明,所提出的方法能有效地改善系统在多径衰落信道环境下的性能,且易于工程实现。     

移动信道下采用软判决译码的自适应多级编码系统

基于信道容量规则和分组分割,构造自适应于AWGN和Rayleigh两种信道特征的多级编码系统,分量码选用删除型卷积码,系统性能与未编码系统和BCH码为分量码的系统作比较.在多阶段译码中提出利用信道状态信息的软判决支路度量计算公式,比较软、硬判决译码对系统性能的影响,为移动信道编码调制系统的最佳设计度量提供参考.     

Diversity and Coding for FH/MFSK Systems with Fading and Jamming--Part I: Multichannel Diversity

The performance of diversity and/or coding is evaluated for FH/MFSK signaling over Rayleigh fading channels in the presence of jamming. The effects of partial-band tone and partial-band noise jamming on uncoded and coded systems are considered. The results indicate that FH/MFSK signaling with diversity provides satisfactory performance for jammed fading channels. For coded FH/MFSK signaling over fading channels, noise jamming may be more effective than tone jamming. The amount of improvement resulting from the use of diversity in conjunction with coding depends upon many factors, including the nature of the channel, the degree of channel state information available at the decoder, the type of decoding, and the modulation alphabet size.     

Layered MAP algorithm for MIMO ISI channels

The layered maximum a posteriori （L-MAP） algorithm has been proposed to detect signals under frequency selective fading multiple input multiple output （MIMO） channels. Compared to the optimum MAP detector, the L-MAP algorithm can efficiently identify signal bits, and the complexity grows linearly with the number of input antennas. The basic idea of L-MAP is to operate on each input sub-stream with an optimum MAP sequential detector separately by assuming the other streams are Gaussian noise. The soft output can also be forwarded to outer channel decoder for iterative decoding. Simulation results show that the proposed method can converge with a small number of iterations under different channel conditions and outperforms other sub-optimum detectors for rank-deficient channels.     

Maximum-likelihood differential detection of uncoded and trelliscoded amplitude phase modulation over AWGN and fading channels-metricsand performance

This paper derives metrics for maximum-likelihood differential detection of uncoded and trellis coded MPSK and QAM transmitted over Rayleigh and Rician fading channels. Receiver structures based on these metrics are proposed and their error probability performance analyzed and/or simulated. The results represent a generalization of the notion of multiple symbol differential detection, previously introduced by the authors for MPSK over an AWGN, to the fading channel and other modulations. For the coded cases, ideal interleaving/deinterleaving is assumed and furthermore the presence or absence of channel state information. An interesting side result is that for a constant envelope modulation transmitted over a fading channel with unknown but rapidly-varying phase error (the other extreme to the slowly-varying phase error case normally assumed for differential detection), under certain practical assumptions, it is shown that the optimum receiver is of the limiter-discriminator type     

Rice-Lognormal衰落信道中不同译码度量特性的比较

本文推导出了Ｒｉｃｅ－Ｌｏｇｎｏｒｍａｌ衰落信道中没有信道状态信息和具有信道状态信息两种情况下的最大似然译码度量,并同传统欧氏距离度量的错误概率进行了比较,数值计算结果表明：对于ＴＣ－ＭＰＳＫ方案,信道衰落特征（ｆＤＴ）对度量特性打电话几乎可以忽略。最后,讨论了Ｒｉｃｅ－Ｌｏｇｎｏｒｍａｌ衰落信道中的最小交织深度。     

Performance of parallel and serial concatenated codes on fading channels

The performance of parallel and serial concatenated codes on frequency-nonselective fading channels is considered. The analytical average upper bounds of the code performance over Rician channels with independent fading are derived. Furthermore, the log-likelihood ratios and extrinsic information for maximum a posteriori (MAP) probability and soft-output Viterbi algorithm (SOVA) decoding methods on fading channels are developed. The derived upper bounds are evaluated and compared to the simulated bit-error rates over independent fading channels. The performance of parallel and serial codes with MAP and SOVA iterative decoding methods, with and without channel state information, is evaluated by simulation over independent and correlated fading channels. It is shown that, on correlated fading channels, the serial concatenated codes perform better than parallel concatenated codes. Furthermore, it has been demonstrated that the SOVA decoder has almost the same performance as the MAP decoder if ideal channel state information is used on correlated Rayleigh fading channels.     

Soft-Decision Metrics for Coded Orthogonal Signaling in Symmetric Alpha-Stable Noise

This paper derives new soft-decision metrics for coded orthogonal signaling in impulsive noise, more specifically symmetric alpha-stable noise. For the case of a known channel amplitude and known noise dispersion, exact metrics are derived both for Cauchy and Gaussian noise. For the case that the channel amplitude or the dispersion is unknown, approximate metrics are obtained in closed-form based on a generalized-likelihood ratio approach. The performance of the new metrics is compared numerically for a turbo-coded system, and the sensitivity to side information of the optimum receiver for Cauchy noise is considered. The gain that can be achieved by using a properly chosen decoding metric is quantified, and it is shown that this gain is significant. The application of the results to frequency hopping ad hoc networks is also discussed.     

Optimal Joint Detection/Estimation in Fading Channels With Polynomial Complexity

The problem of sequence detection in frequency-nonselective/time-selective fading channels, when channel state information (CSI) is not available at the transmitter and receiver, is considered in this paper. The traditional belief is that exact maximum-likelihood sequence detection (MLSD) of an uncoded sequence over this channel has exponential complexity in the channel coherence time. Thus, for slowly varying channels, i.e., channels having coherence time on the order of the sequence length, the complexity appears to be exponential in the sequence length. In the first part of this work, it is shown that exact MLSD can be computed with only polynomial worst case complexity in the sequence length regardless of the operating signal-to-noise ratio (SNR) for equal-energy signal constellations. By establishing a relationship between the aforementioned complexity and the rank of the correlation matrix of the fading process, an understanding of how complexity of the optimal MLSD receiver varies as the channel dynamics change is provided. In the second part of this paper, the problem of decoding turbo-like codes in frequency-nonselective/time-selective fading channels without receiver CSI is examined. Using arguments similar to the ones used for the MLSD case, it is shown that the exact symbol-by-symbol soft-decision metrics (SbSSDMs) implied by the min-sum algorithm can be evaluated with polynomial worst case complexity in the sequence length regardless of SNR for equal-energy signal constellations. Finally, by simplifying some key steps in the polynomial-complexity algorithm, a family of fast, approximate algorithms is derived, which yield near-optimal performance     

EM‐based iterative receiver for coded MIMO systems in unknown spatially correlated noise

We present iterative channel estimation and decoding schemes for multi‐input multi‐output (MIMO) Rayleigh block fading channels in spatially correlated noise. An expectation‐maximization (EM) algorithm is utilized to find the maximum likelihood (ML) estimates of the channel and spatial noise covariance matrices, and to compute soft information of coded symbols which is sent to an error‐control decoder. The extrinsic information produced by the decoder is then used to refine channel estimation. Several iterations are performed between the above channel estimation and decoding steps. We derive modified Cramer–Rao Bound (MCRB) for the unknown channel and noise parameters, and show that the proposed EM‐based channel estimation scheme achieves the MCRB at medium and high SNRs. For a bit error rate of 10⁻⁶ and long frame length, there is negligible performance difference between the proposed scheme and the ideal coherent detector that utilizes the true channel and noise covariance matrices. Copyright © 2006 John Wiley & Sons, Ltd.     

Markov characterization of channels with soft decision outputs

It is shown that a methodology based on hidden Markov models is applicable to the modeling of slowly varying Rayleigh fading channels with additive Gaussian noise and soft decision outputs. The fading is considered to be frequency nonselective, and ideal demodulation is considered throughout. To prove the validity of robustness of the modeling technique, various results that show good agreement between the simulated channels and the models found are presented. Two soft decision statistical distributions, namely, the soft burst and soft burst interval distributions, are defined and compared. To illustrate the accuracy of the models obtained, the simulation and model outputs are compared for a convolutional encoder with Viterbi decoding and various degrees of interleaving     

Performance evaluation of superorthogonal turbo codes in AWGN andflat Rayleigh fading channels

Turbo codes are parallel concatenated codes whose performance in the additive white Gaussian noise (AWGN) channel has been shown to be near the theoretical limit. In this paper, we describe a low-rate superorthogonal turbo code that combines the principles of low-rate convolutional coding and that of parallel concatenation. Due to the bandwidth expansion, this code outperforms the ordinary turbo code both in AWGN and especially in fading channels. Thus, superorthogonal turbo codes are suited mainly for spread-spectrum applications. For the purposes of iterative decoding, we concisely describe the connection between the optimal maximum a posteriori symbol estimation and suboptimal soft-output decoding based on sequence estimation. The suboptimal decoder produces outputs that can directly be used as additive metrics at successive decoding iterations, without the need for estimating channel noise variance. Simulation results in AWGN and flat Rayleigh fading channels are also presented, along with analytical upper bounds of bit- and frame-error probabilities     

Soft Decoding Assisted SNR Estimation Under Block Fading Channels for Orthogonal Modulations

The performance of the coded orthogonal modulation (OM) system under slow fading channels heavily depends on the estimation of the signal-to-noise ratio (SNR), including the fading amplitude and the noise spectral density. However, a relatively long packet of pilot symbols is often required to guarantee the accuracy of the SNR estimation, which makes it impractical in some situations. To address this problem, this paper proposes an iterative SNR estimation algorithm using the soft decoding information based on the expectation-maximization algorithm. In the proposed method, a joint iterative loop between the SNR estimator and decoder is performed, where the extrinsic information generated by the soft decoder is employed to enhance the estimation accuracy and the SNR estimated by the estimator is used to generate the soft information to the decoder. Also, no pilot symbols are needed to estimate the SNR in the proposed estimator. The Cramer–Rao lower bound (CRLB) of fully data-aided (FDA) estimation is derived to works as the final benchmark. The performance of the proposed algorithm is evaluated in terms of the normalized mean square errors (NMSEs) and the bit error rates (BERs) under block fading channels. Simulation results indicate that the NMSE of the proposed estimator reaches the CRLB of the FDA estimator and outperforms that of the approximate ML (ML-A) estimator proposed by Hassan et al. by 4.1 dB. The BER performance of coded OM system with the proposed estimation algorithm is close to the ideal case where the channel fading and the noise spectral density are known at the receiver.     

Décodage à sortie souple des réseaux de points

We present a universalmse algorithm for lattice decoding in dimensions up to 1024 for both Gaussian and Rayleigh fading channels. This algorithm can be applied to decode high diversity multidimensional rotations. The decoding is performed by a decision feedback equalizer and provides soft output which allows a concatenation of the lattice codes with other type of error-correcting codes. The problem of selecting a good rotation is also considered, and we show that a high dimensional random rotation exhibits very good performance on a Rayleigh fading channel.