Iterative Joint Tone-Interference Cancellation and Decoding for MIMO-OFDM

In this paper, multi-input–multi-output orthogonal frequency-division multiplexing (MIMO-OFDM) is considered in the presence of multipath fading and multitone interference (MTI). The MIMO-OFDM system makes joint use of channel coding and orthogonal space–frequency block coding (OSFBC) on the transmit side and iterative processing on the receiver side for robustness and improved performance against the fading and MTI effects of the channel. The new iterative receiver is implemented by either an optimal a posteriori probability (APP) space–frequency detector or a soft-information-aided minimum-mean-squared-error (MMSE) combiner at its front end and a soft-input–soft-output channel decoder at its back end. An approximate error performance analysis is provided for the OSFBC-OFDM system under maximum-likelihood decoding to illustrate the interference mitigation efficiency of the system. Then, the two iterative receivers are compared in terms of their computational complexities and bit error rate (BER) performances. As depicted in the BER graphs, both iterative receivers provide an improvement in performance after only a few detection/decoding iterations. It is also shown that despite its suboptimality, the MMSE receiver has the potential to achieve a BER performance close to that of the APP detector at a significantly lower cost.      

Iterative Joint Detection, Decoding, and Channel Estimation in Turbo-Coded MIMO-OFDM

An iterative receiver for a multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) system is considered to jointly decode the transmitted bits and estimate the channel state. The receiver consists of a list detector, a turbo decoder, and a channel estimator that is based on the space-alternating generalized expectation-maximization (SAGE) algorithm. This paper proposes a way to improve the convergence of the iterative detection and decoding by using a priori information to also recalculate the candidate list, aside from the log-likelihood ratios (LLRs) of the coded bits. A new list parallel interference cancellation (PIC) detector is derived to approximate an a posteriori probability (APP) algorithm with reduced complexity and minimal losses of performance. Furthermore, the organization of spectrally efficient decision-directed (DD) SAGE channel estimation under a constrained number of detector-decoder iterations is optimized by computer simulations, and the SAGE algorithm itself is modified for nonconstant envelope constellations. The list recalculation is shown to improve convergence. It is also shown that the list PIC detector with good initialization outperforms the K-best list sphere detector (LSD) in the case of small list sizes, whereas the complexities of the algorithms are of the same order. Despite the low preamble density and fast-fading channel, the proposed iterative receiver shows robust performance.     

RS码与LDPC码的联合迭代译码

针对RS码与LDPC码的串行级联结构,提出了一种基于自适应置信传播(ABP)的联合迭代译码方法.译码时,LDPC码置信传播译码器输出的软信息作为RS码ABP译码器的输入;经过一定迭代译码后,RS码译码器输出的软信息又作为LDPC译码器的输入.软输入软输出的RS译码器与LDPC译码器之间经过多次信息传递,译码性能有很大提高.码长中等的LDPC码采用这种级联方案,可以有效克服短环的影响,消除错误平层.仿真结果显示:AWGN信道下这种基于ABP的RS码与LDPC码的联合迭代译码方案可以获得约0.8 dB的增益.     

Iterative Multiuser Detection and Decoding for DS-CDMA System With Space-Time Linear Dispersion

This paper considers a $Q$-ary orthogonal direct-sequence code-division multiple-access (DS-CDMA) system with high-rate space-time linear dispersion codes (LDCs) in time-varying Rayleigh fading multiple-input–multiple-output (MIMO) channels. We propose a joint multiuser detection, LDC decoding, $Q$-ary demodulation, and channel-decoding algorithm and apply the turbo processing principle to improve system performance in an iterative fashion. The proposed iterative scheme demonstrates faster convergence and superior performance compared with the V-BLAST-based DS-CDMA system and is shown to approach the single-user performance bound. We also show that the CDMA system is able to exploit the time diversity offered by the LDCs in rapid-fading channels.      

On the Suboptimality of Iterative Decoding for Turbo-Like and LDPC Codes With Cycles in Their Graph Representation

Performance of Multicarrier CDMA With Successive Interference Cancellation in a Multipath Fading Channel A high-capacity, low-complexity, and robust system design for a successive interference cancellation (SIC) system is developed and analyzed. Multicarrier code-division multiple access (MC-CDMA) is used to suppress multipath and to overcome the multipath channel estimation problem in single-carrier SIC systems. In addition, an optimal power control algorithm for MC-CDMA with SIC is derived, allowing analytical bit-error rate expressions to be found for an uncoded system. Low-rate forward error-correcting codes are then added to the system to achieve robustness. It is found that the capacity of the coded system approaches the additive white Gaussian noise capacity for SIC, even in a fading multipath channel with channel estimation error. This indicates that MC-CDMA is very attractive for systems employing SIC.     

Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.      

MIMO系统中一种自适应的迭代检测译码算法

针对常用的非穷尽列表形式后验概率检测算法直接采用恒定且较大的列表长度,导致列表冗余度大的问题,该文提出了一种自适应长度的列表球形译码算法(Adaptive Size List Sphere Decoding, ASLSD)。在算法中通过更新检测半径和设置停止条件,使检测列表长度可随信噪比和迭代次数自适应变化。而且通过将列表操作与LSD (List Sphere Decoding)检测相结合,避免了符号序列在不同半径下的重复检测。仿真表明,在较小性能损失的前提下,该算法可以大大减小所需检测列表的长度,进而有效降低接收机的复杂度。     

基于置信传播和波束搜索的LDPC联合译码算法

当置信传播算法BP（Belief Propagation）应用于LDPC短码时,由于传递消息的不独立性,使得译码性能与最大似然译码有较大差距。考虑到LDPC码分布的稀疏性,以及在BP失败译码中仅有少量错误位的统计事实,提出了一种波束搜索算法,把它级联于BP算法来对失败译码进行后处理。在付出一定计算复杂性的代价下,以期提高整体的译码性能。在波束搜索算法的实现中,译码伴随式的信息以及码字位的可靠性信息被融合进对搜索个体的代价函数中。它的目标是在给定的波束宽度下,寻找到失败译码对应的错误样式,从而纠错的目的。仿真结果表明对LDPC短码,此联合算法在译码性能与计算复杂度之间取得较好的折衷。     

基于生成矩阵的LDPC/Turbo码的构造及内外迭代译码新技术

提出了一种新的LDPC／Turbo码（简称L／T码）,及采用内外迭代译码新技术的简便实用的译码器。L／T码将原来Turbo编码中的两个子编码器分别用RSC码和LDPC码来代替,并由一个交织器连接。解码采用软输入／软输出的最大后验概率算法迭代执行,利用外信息的传递,每个子译码器都可以充分利用另一个在上一步计算中得到的结果。仿真在加性白高斯信道下进行,并将其结果和传统方法进行了比较。     

MIMO-OFDM系统中一种新的联合迭代信道估计与符号检测算法

该文提出了一种MIMO-OFDM系统中基于MMSE准则的联合迭代信道估计和符号检测算法。联合迭代算法的性能取决于初始信道估计器的精度和迭代算法。首先提出了一种基于时频变换(TFT)的信道估计算法;其次提出了利用初始信道估计值与检测符号在MMSE准则下进行联合迭代信道估计和符号检测的算法。分析和仿真结果显示,在准静态和快时变信道条件下,新的联合迭代信道估计和符号检测算法有效地提高了系统性能。     

Adaptive Parallel and Iterative QRDM Detection Algorithms for MIMO Multiplexing Systems

QR-decomposition with M-algorithm (QRDM) achieves quasi-ML performance in multiple-input multiple-output (MIMO) multiplexing systems. Nevertheless, QRDM performs avoidable computations because of its systematic search strategy and its failure to consider the channel and noise conditions. Another drawback is that QRDM is sequential, which limits pipelining capabilities. In this paper, we propose quasi-ML adaptive parallel QRDM (APQRDM) and adaptive iterative QRDM (AIQRDM) algorithms based on set grouping. In set grouping, the tree-search stage of the QRDM algorithm is divided into partial detection phases (PDPs). These are processed in parallel and iteratively in the proposed APQRDM and AIQRDM algorithms, respectively. Therefore, when the tree-search stage of the QRDM algorithm is divided into G PDPs, the latency of the proposed APQRDM algorithm and the hardware requirements of the proposed AIQRDM algorithm are reduced by a factor of G compared to the QRDM algorithm. Moreover, simulation results show that in 4?×?4 MIMO system, and at E _b /N ₀ of 12?dB, APQRDM decreases the average computational complexity to approximately 43% that of the conventional QRDM. Also, at E _b /N ₀ of 0?dB, AIQRDM algorithm reduces the computational complexity to about 54%, and the average number of metric comparisons to approximately 10%, compared to the conventional QRDM.     

Performance Comparison of MLSE and Iterative Equalization in FEC Systems for PMD Channels With Respect to Implementation Complexity

Advanced electronic signal processing for high-bit-rate lightwave systems has become available recently. Adaptive receiver concepts are proposed to mitigate dynamic changes in the optical channel response. In particular, two different approaches are under discussion: independent equalization and decoding, e.g., by maximum-likelihood sequence estimation followed by a separate error-correction decoder, and a combined approach by iterative decoding of the channel correlations and the error-correction code. The authors compare the two concepts with respect to their mitigation capabilities in the presence of polarization mode dispersion. Special attention is paid to hardware implementation aspects, since the application of the concepts is severely limited by memory and signal processing requirements     

A 0.18-$muhbox m$CMOS Analog Min-Sum Iterative Decoder for a (32,8) Low-Density Parity-Check (LDPC) Code

Current-mode circuits are presented for implementing analog min-sum (MS) iterative decoders. These decoders are used to efficiently decode the best known error correcting codes such as low-density parity-check (LDPC) codes and turbo codes. The proposed circuits are devised based on current mirrors, and thus, in any fabrication technology that accurate current mirrors can be designed, analog MS decoders can be implemented. The functionality of the proposed circuits is verified by implementing an analog MS decoder for a (32,8) LDPC code in a 0.18-mum CMOS technology. This decoder is the first reported analog MS decoder. For low signal to noise ratios where the circuit imperfections are dominated by the noise of the channel, the measured error correcting performance of this chip in steady-state condition surpasses that of the conventional floating-point discrete-time synchronous MS decoder. When data throughput is 6 Mb/s, loss in the coding gain compared to the conventional MS decoder at BER of 10^-3 is about 0.3 dB and power consumption is about 5 mW. This is the first time that an analog decoder has been successfully tested for an LDPC code, though a short one     

Combine LDPC Codes Over GF(q) With q-ary Modulations for Bandwidth Efficient Transmission

Low-density parity-check (LDPC) codes are playing more and more important role in digital broadcasting standards due to their excellent error correction performance. In this paper, we study the combination of LDPC codes over GF(q) with q-ary modulations for bandwidth efficient transmission over AWGN channel and consider the design of the codes. Specifically, we develop the concept of quasi-regular codes, and propose an improved Monte Carlo method to optimize the quasi-regular codes. To justify the performance of our proposed scheme, simulation results are presented and analysed.     

一种新的谐波检测算法及其对比研究

本文通过详细的数学推导证明了瞬时无功功率理论的数学本质是三维时变向量空间中的正交变换和反变换,在不同正交向量坐标基下,可以检测任意次、任意序的有功或无功电流。通过与传统FFT算法相比,证明二者在数学本质上是一致的,低通滤波器和积分器是二者实时性能差异的根源。据此文中提出了一种采用LPF的FFT改进算法用于谐波提取。     

比特交织编码调制(迭代译码)系统标识映射的对称性研究与应用

该文从群论的角度分析了比特交织编码调制(迭代译码)(BICM(-ID))系统中标识映射的对称性。首先给出了标识映射对称性的定义,并指出,二进制标识映射的对称性是BICM(-ID)系统的固有特性,该对称性同构于m阶超立方的对称群。然后基于BICM(-ID)的对称性,提出一种改进的二进制交换算法(IBSA)。该算法的搜索空间为标识映射对称群陪集的代表系。因此,与传统的二进制算法相比,IBSA的搜索效率得到了提高。最后,2维16阶星座图的仿真结果表明,在40000次的运行过程中,IBSA能提高4%的搜索效率;32阶相移键控星座图的仿真结果表明,该算法至少可提高3.5%的搜索效率,单个映射的计算时间缩短了约4000倍。