一种水声通信Turbo均衡中的软迭代信道估计算法

 Turbo均衡技术是水声相干通信克服信道多径、消除码间干扰(ISI)的有效工具。Turbo均衡实际使用时需要对时变、多径信道进行良好的估计。为了提高信道估计的效果,该文基于时变横向滤波和相位旋转信道模型,提出一种水声通信Turbo均衡中的软迭代信道估计算法。该算法采用快速自优化最小均方算法得到各数据符号处的横向滤波器系数矢量并与二阶锁相环联合优化计算。通过仿真比较,该算法明显优于硬迭代信道估计算法,且相位估计性能优于其他文献中的软迭代信道估计算法。在海上试验中,水声通信距离5 km,方向近似垂直,接收阵起伏周期10 s,起伏幅度5 m左右,在此情况下进行数据采集。将该算法用于对海试数据的单通道Turbo均衡处理,实现无误码输出,验证了所提算法在软迭代信道相位估计方面的优势。     

Doubly iterative receiver for block transmissions with EM-based channel estimation

Cyclic-prefix code division multiple access (CP-CDMA), multicarrier CDMA (MC-CDMA) and single carrier cyclic-prefix (SCCP) transmission are some schemes that could support the increasing demand of future high data rate applications. The linear and nonlinear equalizers used to detect the transmitted signal are always far from the Maximum-Likelihood (ML) detection bound. The block iterative generalized decision feedback equalizer (BI-GDFE) is an iterative and effective interference cancelation scheme which could provide near-ML performance yet with very low complexity. In order to deploy this scheme, the channel state information (CSI) must be available at the receiver. In practice, this information has to be estimated by using pilot and data symbols. This paper investigates the problem of channel estimation using the Expectation Maximization (EM) algorithm. The BI-GDFE provides the soft information of the transmitted signals to the EM-based algorithm in the form a combination of hard decision and a coefficient so-called the input-decision correlation (IDC). The resultant receiver becomes a doubly iterative scheme. To evaluate the performance of the proposed estimation algorithm, the Cramér-Rao Lower Bound (CRLB) is also derived. Computer simulations show that the bit error rate (BER) performance of the proposed receiver for joint channel estimation and signal detection can reach the performance of the BI-GDFE with perfect CSI.     

Soft-output demodulator in space-time-coded continuous phase modulation

Space-time coding has shown great promise for digital transmission in wireless communication links, especially when the channel response is known at the receiver. Space time coding combined with continuous phase modulation (CPM) can offer better tradeoffs in bandwidth and power efficiency. Because of the memory inherent in CPM, channel estimation is often harder than for linear modulations. We present an adaptive soft algorithm that performs joint channel estimation and data detection for space-time CPM systems. Properly designed pilot symbols are inserted at the very beginning to give good initial estimates of the channels. This soft receiver is further applied to the interleaved space-time CPM system to yield better performance with moderate complexity through iterative processing. Simulation results show that the receiver can often achieve near-coherent performance in quasistatic fading as well as in time-varying fading.     

Performance of LDPC coded FMT systems over frequency selective fading channel

This paper proposes the Low Density Parity Check （LDPC） coded Filtered MultiTone （FMT） systems with high-order modulation for the high data rate reliable transmission over frequency selective fading channel. For the purpose of accomplishing soft input soft output iterative decoding of LDPC codes, a new soft decision metric generation method is proposed, which obviates the need of the noise variance estimation, for M-PSK/M-QAM-type high-order modulation over frequency selective fading channel. Computer simulation indicates that, there is no performance loss with our new metric, but the complexity of implementation is reduced, and that the LDPC codes are effective to improve the Bit Error Rate （BER） of FMT in frequency selective fading channel.     

Turbo编码V-BLAST MIMO-OFDM系统中的联合迭代判决反馈信道估计与检测

针对Turbo编码V-BLAST MIMO—OFDM系统，提出了一种联合迭代判决反馈信道估计与检测方案，该方案将Turbo迭代译码与最小二乘（LS，least square）信道估计相结合，充分利用Turbo迭代译码后的信息位和校验位软值信息来改善信道估计性能。仿真结果表明，该方案不仅纠正了低信噪比时的差错传播问题，还使得整个系统的信道估计性能得到进一步提高，且适合于非常恶劣的信道环境。     

Doubly Iterative Equalization of Continuous-Phase Modulation

In this paper, a doubly iterative receiver is proposed for joint turbo equalization, demodulation, and decoding of coded binary continuous-phase modulation (CPM) in multipath fading channels. The proposed receiver consists of three soft-input soft-output (SISO) blocks: a front-end soft-information-aided minimum mean square error (MMSE) equalizer followed by a CPM demodulator and a back-end channel decoder. The MMSE equalizer, combined with an a priori soft-interference canceler (SIC) and an a posteriori probability mapper, forms a SISO processor suitable for iterative processing that considers discrete-time CPM symbols which belong to a finite alphabet. The SISO CPM demodulator and the SISO channel decoder are both implemented by the a posteriori probability algorithm. The proposed doubly iterative receiver has a central demodulator coupled with both the front-end equalizer and the back-end channel decoder. A few back-end demodulation/decoding iterations are performed for each equalization iteration so as to improve the a priori information for the equalizer. As presented in the extrinsic information transfer (EXIT) chart analysis and simulation results for different multipath fading channels, this provides not only faster convergence to low bit error rates, but also lower computational complexity.     

基于CPM的部分带干扰抑制技术

提出了一种空时编码慢跳频连续相位调制（CPM）系统用于抗部分带噪声干扰。由于CPM信号的内在记忆性,传统的检测方法复杂度较高,结合Laurent分解和差分预编码方法部分消除了CPM信号的内在记忆性,提出了一种软输入软输出的线性接收机,联合信道译码进行迭代检测,大大降低了接收机的复杂度并且提高了系统的抗干扰性能。最后,利用迭代干扰状态信息（JSI）估计,进一步改善了系统的抗干扰性能。     

一种用于短波通信的新的频域差分解调器

在短波通信中,传输信号遭受延时`扩展、时变干扰以及接收机加性高斯噪声的影响。常规的差分解调器在恶劣的短波信道环境下性能很差。该文提出一种新的频域差分解调器,通过盲相位估计来消除延时扩展造成的相位误差的影响。仿真了常规差分解调器和新的差分解调器在恶劣短波信道环境下的性能对比。结果表明在快速衰落短波信道环境下,新的频域差分解调器的性能优于传统的差分解调器。     

OFDM/OQAM系统中联合迭代信道估计和信号检测

与基于复数域空间正交条件的传统正交频分复用系统(OFDM with Cyclic Prefix, CP-OFDM)有所不同,基于交错正交调制的正交频分复用系统(OFDM/Offset QAM, OFDM/OQAM)满足实数域空间严格正交条件。因此在多径衰落信道条件下,CP-OFDM系统中的信道估计方法会导致OFDM/OQAM系统严重的字符间干扰和载波间干扰。该文结合OFDM/OQAM系统结构特点,提出了一种基于迭代信道估计和信号检测算法。该算法通过信道估计器和接收到的信号互相交换信息,消除导频序列中的字符间干扰和载波间干扰,提高信道估计和信号检测的准确度。仿真分析结果表明,经过一定次数迭代处理后,OFDM/OQAM迭代信道估计性能趋近于理想信道估计性能。     

Iterative receivers for multiuser space-time coding systems

Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called “turbo multiuser detection” technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. An iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals. Computer simulations demonstrate that the proposed iterative receiver techniques provide significant performance improvement over conventional noniterative methods in both single-user and multiuser STC systems. Furthermore, the performance of the proposed iterative multiuser receiver approaches that of the iterative single-user receiver in both STBC and STTC systems     

BICM-OFDM中利用叠加导频的迭代信道估计

提出了一种利用叠加导频的迭代信道估计算法。将导频信号叠加在发送信号上,利用一阶统计量进行初始信道估计。利用比特交织编码调制（BICM）中迭代译码的硬判决反馈信息,进行迭代信道估计。该算法在不牺牲信息速率的前提下,有效提高了系统带宽利用率。在BICM-OFDM系统中的仿真结果表明,与传统叠加导频算法相比,文章提出的算法可有效提高信道估计精度和系统性能。     

Data-aided channel estimation for turbo-PIC MIMO detectors

We consider data-aided channel estimation for multiple input multiple output (MIMO) systems when iterative parallel interference cancellation (PIC) is performed for signal detection. We compare some data-aided channel estimation methods based on expectation maximization (EM) algorithm or on hard estimated transmit symbols. In particular, we propose a modified EM-based approach and show that when few iterations are to be performed, it provides considerable performance improvement.     

MIMO-OFDM-CDM系统低复杂度Turbo检测

提出了将码分复用(CDM)模块看作系统等效信道的一部分,从而在MIMO-OFDM-CDM系统中引入Turbo迭代检测的措施。使用Turbo迭代检测可以通过解码器和解调器之间软信息的多次迭代提高译码的性能。根据CDM模块的特点,提出了将等效多天线之间的干扰视为高斯噪声,从而降低Turbo迭代检测算法复杂度的措施。仿真结果表明,在系统中使用Turbo迭代检测可以在有限次迭代后达到比传统算法更低的误码率。     

An iterative receiver for turbo-coded pilot-assisted modulation infading channels

We introduce an iterative joint channel and data estimation receiver that exploits both the power of pilot-symbol assisted modulation (PSAM) and turbo coding for fading channels. The key innovation is a low-complexity soft channel estimator which divides a processing block into overlapped cells and performs maximum a posteriori (MAP) sequence estimation and MMSE filtering based on the received signal and extrinsic information delivered by the soft channel decoder. Simulation results show that for turbo-coded PSAM systems under time-variant fading the proposed receiver offers significant performance gains over a non-iterative receiver and two other cancellation schemes     

Soft‐output MMSE V‐BLAST receiver with MMSE channel estimation under correlated Rician fading MIMO channels

For wireless multiple‐input multiple‐output (MIMO) communications systems, both channel estimation error and spatial channel correlation should be considered when designing an effective signal detection system. In this paper, we propose a new soft‐output MMSE based Vertical Bell Laboratories Layered Space‐Time (V‐BLAST) receiver for spatially‐correlated Rician fading MIMO channels. In this novel receiver, not only the channel estimation errors and channel correlation but also the residual interference cancellation errors are taken into consideration in the computation of the MMSE filter and the log‐likelihood ratio (LLR) of each coded bit. More importantly, our proposed receiver generalizes all existing soft‐output MMSE V‐BLAST receivers, in the sense that, previously proposed soft‐output MMSE V‐BLAST receivers can be derived as the reduced forms of our receiver when the above three considered factors are partially or fully simplified. Simulation results show that the proposed soft‐output MMSE V‐BLAST receiver outperforms the existing receivers with a considerable gain in terms of bit‐error‐rate (BER) performance. Copyright © 2011 John Wiley & Sons, Ltd.     

WCDMA中联合迭代信道估计与解码的研究

首先介绍了基于WCDMA系统中RAKE接收机的信道估计方法，然后将Turbo原理引入信道估计，针对WCDMA系统提出两种迭代信道的估计方法。通过使用RAKE接收机或解码之后的软信息来改善整个WCDMA系统的性能，对WCDMA系统中联合运用信道估计与解码技术进行了研究。由于联合信道估计与解码技术可以利用通信系统不同模块之间的“软信息”交换，从而有效地提高了信道估计准确程度和解码器的性能。仿真结果表明，利用解码之后的软信息来修正信道估计的方法较传统的RAKE接收机中的信道估计方法性能有较大改善，在误码率为10^-2左右时，提高大约1dB。     

Double turbo equalization of continuous phase modulation with frequency domain processing

In this paper, a doubly-iterative linear receiver, equipped with a soft-information aided frequency domain minimum mean-squared error (MMSE) equalizer, is proposed for the combined equalization and decoding of coded continuous phase modulation (CPM) signals over long multipath fading channels. In the proposed receiver architecture, the front-end frequency domain equalizer (FDE) is followed by the soft-input, softoutput (SISO) CPM demodulator and channel decoder modules. The receiver employs double turbo processing by performing back-end demodulation/decoding iterations per each equalization iteration to improve the a priori information for the front-end FDE. As presented by the computational complexity analysis and simulations, this process provides not only a significant reduction in the overall computational complexity, but also a performance improvement over the previously proposed iterative and noniterative MMSE receivers.     

OFDM系统中基于软信息处理的自适应双模半盲信道估计

本文提出了一种新的基于软信息迭代处理的自适应双模半盲信道估计算法.该算法构造了新的置信度量函数获取译码和软映射重构的反馈信号的置信度,和置信阈值比对后在判决反馈和盲估计两种估计模式之间自适应选择,这样估计的信道频响可以提高软信息迭代接收的性能.仿真结果表明,本文提出的算法能综合两种估计模式的优势,限制传统判决反馈模式的错误传播,在没有分散导频辅助估计的条件下有效跟踪时变信道,达到好的系统性能.     

A low-complexity noncoherent iterative space-time demodulator

Turbo coded unitary space-time modulation (USTM) can provide large coding gain as compared to uncoded USTM. Because the noncoherent space-time maximum a posteriori demodulator is very complicated, in this letter, we propose a new low-complexity noncoherent iterative space-time demodulator for the USTM constructed from pilot symbol-assisted modulation. The proposed demodulator utilizes both hard and soft decisions from the turbo decoder to simplify the computational task as well as produce reliable soft outputs. Several examples demonstrate that this demodulator has both low complexity and good error performance.     

OFDM系统中基于Turbo迭代的WLMS联合信道估计和检测算法

利用Turbo迭代原理,该文提出了一种迭代的维纳LMS联合信道估计和检测算法,该算法利用软映射和软解映射算法提供的软值信息,实现检测译码模块和信道估计模块之间的信息交换,通过WLMS信道估计和跟踪算法实现信道响应的逐符号更新。仿真结果表明,通过2次迭代,系统误码率性能已经基本收敛,而且在系统误码率低于10-3 时,与理想信道估计性能相比,联合估计和检测算法性能信噪比损失仅为0.5-0.8dB。