Decision-feedback soft-input/soft-output multiuser detector foriterative decoding of coded CDMA

The optimal decoding scheme for a code-division multiple-access (CDMA) system that employs convolutional codes results in a prohibitive computational complexity. To reduce the computational complexity, an iterative receiver structure was proposed for decoding multiuser data in a convolutional coded CDMA system. At each iteration, extrinsic information is exchanged between a soft-input/soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders. However, a direct implementation of the full-complexity SISO multiuser detector also has the exponential computational complexity in terms of the number of users. This paper proposes a low-complexity SISO multiuser detector based on tentative hard decisions that are made and fed back from the channel decoders in the previous iteration. The computational complexity of the proposed detector is linear in terms of the number of users and can be adjusted according to the complexity/performance tradeoff. Simulation results show that even with this simple feedback scheme, the performance of the coded multiuser system approaches that of the single-user system for moderate to high signal-to-noise ratios (SNRs)     

Turbo greedy multiuser detection

Previously, a novel scheme for iterative multiuser detection and turbo decoding was proposed by Damnjanovic and Vojcic (2000, 2001). In this scheme, multiuser detection and single-user turbo decoding are tightly coupled to maximize the overall gain. The extrinsic probabilities for the coded bits of the interfering users, obtained after each turbo decoding iteration, are used as a priori probabilities in the following multiuser iteration and the extrinsic information for the systematic bits of the desired user is used as a priori information in the next single-user turbo decoding iteration. Turbo decoding of parallel concatenated convolutional codes is carried out in parallel fashion. It has been shown that the proposed detector approaches the multiuser capacity limit within 1 dB in the low signal-to-noise ratio region. However, the main drawback of the scheme is its exponential complexity in the number of users, which is due to the complexity of the maximum a posteriori probability (MAP) multiuser detector. In this paper, we show that the complexity of the scheme can be significantly reduced by replacing the (MAP) multiuser detector with an iterative detector derived from the greedy multiuser detector proposed by AlRustamani and Vojcic (2000). In this paper, we demonstrate that, for both the additive white Gaussian noise and the frequency-nonselective Rayleigh fading, the substantial reduction in complexity of the iterative scheme proposed by Damnjanovic and Vojcic when the greedy detector is utilized introduces a slight degradation in performance     

Multiuser decoding for multibeam systems

An iterative multiuser decoding algorithm for co-channel BPSK/QPSK users in a multibeam system is presented. The approach can be applied to the return link of multibeam satellites and to terrestrial systems with sectored base-station antennas. It allows the reuse of the same spectrum in each beam. The algorithm is based on the extension of turbo-decoding techniques to the iterative decoding of parallel users. Simulation results show one can asymptotically achieve single user performance in a high multiuser interference environment; often this includes some diversity gain. The complexity of the algorithm is approximately O(2^K+2^κ) operations per bit per iteration where K is the number of co-channel users and κ is the constraint length of the forward error correction code     

基于MIMO系统的差分解码算法及性能分析

针对两用户MIMO系统,首先提出了一种基于正交空时分组码的部分差分解码算法,接收端获得一个用户的信道状态信息即可实现对两个用户的解码;然后给出了一种针对两用户MIMO系统的完全差分解码算法,可以在没有任何一个用户的信道状态信息情况下进行解码;最后针对该算法进行了复杂度分析和旋转预编码优化,并通过仿真给出了最优旋转角度。计算机仿真证明该完全差分解码算法有效地降低了解码复杂度。     

一种有效改善系统性能的非正交传输方案

本文主要解决当前功率域单胞元非正交多址接入结构应用层面存在的问题.具体说,针对现有单胞元结构因多用户时频同传形成强干扰而导致边缘用户服务质量差的问题,利用资源正交化的组网策略构建中心用户频带独立正交、边缘用户与其频带合作共享的并行多胞元结构,并进一步利用中心用户可获取边缘用户先验知识的特征将全双工的协作通信机制引入其中,从而有效解决现有单胞元结构在译码复杂度以及用户公平性等方面存在的不足.仿真结果证实了所提方案的有效性.     

Iterative threshold decoding without interleaving for convolutional self-doubly orthogonal codes

A novel iterative error control technique based on the threshold decoding algorithm and new convolutional self-doubly orthogonal codes is proposed. It differs from parallel concatenated turbo decoding as it uses a single convolutional encoder, a single decoder and hence no interleaver, neither at encoding nor at decoding. Decoding is performed iteratively using a single threshold decoder at each iteration, thereby providing good tradeoff between complexity, latency and error performance.     

基于模型驱动辅助MPA的SCMA多用户检测算法

稀疏码分多址接入(Sparse Code Multiple Access,SCMA)作为有应用前景的空口技术,在高吞吐量以及大规模连接中极具优势。针对SCMA通信系统中接收端消息传递算法(Message Passing Algorithm,MPA)计算复杂度较高的问题,提出了基于模型驱动辅助MPA法(Model driven Assisted MPA,MD MPA)的SCMA多用户检测算法。MD MPA在MPA算法迭代过程中节点更新后的信息矩阵和输出的概率矩阵之后添加权重参数,并通过神经网络训练更新参数。经训练所得权重参数可加快算法收敛速度,降低误码率,获得更佳的译码效果。仿真结果表明,MD MPA相较于MPA算法,误码率降低了20%,复杂度降低了33%。     

基于紧缩处理独立分量分析的多用户检测

CDMA系统中随着用户数的增加,独立分量分析(ICA)算法复杂度也随之增加,性能恶化严重.为了降低复杂度,在进行ICA运算时,先对接收数据进行降维预处理,然后对迭代后的数据进行正交化处理.实验结果表明:随着用户数的增加,文中方法性能优于一般的基于独立分量分析的多用户检测的性能.     

协作通信中一种分布式酉空时码

提出了一种基于三角函数酉星座的分布式酉空时码,主要介绍了编码矩阵结构及其低复杂度的译码算法。根据仿真结果可知:提出的酉空时性能差于正交设计、系统设计和循环酉群码的性能,但从译码复杂度看,所提出的译码算法计算复杂度相比其他3种简单很多,具有很大的优势。并且还证明了同样的码字采用最大似然译码算法与所提出的低复杂度算法性能相近,但是计算复杂度相差较大。总的来说,所提出的编码矩阵结构简单,易于构造,且具有较低的译码复杂度。     

On the Performance of Energy-Division Multiple Access with Regular Constellations

In this paper we describe a multiple-access protocol in which different users are assumed to share the same bandwidth and the same pulse. Users employ the same modulation (binary-phase shift keying, quadrature-phase shift keying, and rectangular-phase shift keying are considered) with different transmitted magnitude, and are discriminated on the basis of the corresponding magnitude at receiver location. Conditions for user discrimination are analyzed. The proposed receiver uses successive decoding in order to avoid exponential complexity of maximum-likelihood decoding. Such a scheme, compared to orthogonal multiaccess schemes (e.g. time- or frequency-division multiple access) allows to achieve larger normalized throughput for systems operating in large signal-to-noise ratio range, and may be jointly applied with classical protocols in personal-area networks. Analytical and numerical results, in terms of bit error rate and normalized throughput, are derived for performance evaluation on additive white Gaussian noise channels.     

A low-complexity SISO multiuser detector for iterative decoding of asynchronous CDMA systems with convolutional codes

The optimal decoding scheme for asynchronous code-division multiple-access (CDMA) systems that employ convolutional codes results in a prohibitive computational complexity. To reduce the computational complexity, an iterative receiver structure was proposed for decoding multiuser data in a convolutional coded CDMA system. At each iteration, extrinsic information is exchanged between a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders. A direct implementation of the optimal SISO multiuser detector, however, has exponential computational complexity in terms of the number of users which is still prohibitive for channels with a medium to large number of users. This paper presents a low-complexity SISO multiuser detector using the decision-feedback scheme, of which tentative hard decisions are made and fed back to the SISO multiuser from the previous decoding output. In the proposed scheme, the log-likelihood ratios (LLR) as well as the tentative hard decisions of code bits are fed back from the SISO decoders. The hard decisions are used to constrain the trellis of the SISO multiuser detector and the LLRs are used to provide a priori information on the code bits. The detector provides good performance/complexity tradeoffs. The computational complexity of the detector can be set to be as low as linear in the number of users. Simulations show that the performance of the low-complexity SISO multiuser detector approaches that of the single-user system for moderate to high signal-to-noise ratios even for a large number of users.     

低复杂度的TBCC自适应循环VA译码算法

咬尾是一种将卷积码转换为块码的技术,它消除了归零状态所造成的码率损失,同时避免了截尾带来的性能降低,在短块编码中具有明显优势.针对咬尾卷积码(TBCC)现有译码算法复杂度过大和收敛性问题,提出一种低复杂度的TBCC自适应循环维特比(VA)译码算法.该算法根据信道变化自适应调整译码迭代次数,使咬尾路径收敛到最佳.通过仿真...     

Low complexity resource allocation and scheduling algorithms for downlink SDMA systems

The downlink zero-forcing beamforming strategy in the case of random packet arrivals is investigated. Under this setting, the relevant fairness criterion is the stabilization of all buffer queues which guarantees a bounded average delay for all users. It has been shown that allocating resources to maximize a queue-length-weighted sum of the rates is a stabilizing policy. However, the high complexity of user selection and the feasible rates determination for optimal scheme may prevent the real-time scheduling operation. Two low complexity algorithms are provided taking the channel state, queue state and orthogonality into account. In particular, the authors pick the first user with the largest product between channel gain and queuing length, and select the remaining users to construct candidate user set based on the greedy user selection method or channel orthogonal user selection method. Then, the power and rate allocation for the selected users are implemented based on the modified water-filling method. The complexity of the proposed algorithms is analyzed. The average delay and average throughput are studied in homogeneous scenarios and heterogeneous scenarios, respectively. Simulation results show that the proposed algorithms can take full advantage of the multi-user diversity gain and provide average delay (or throughput) and fairness improvement compared with channel-aware-only schemes.     

Collaborative spreading for the downlink of overloaded CDMA

A new scheme called ‘Collaborative Spreading’ is proposed for the downlink of CDMA to allow the sharing of the same spreading sequence by more than one user. In particular, it addresses the problem of user overloading and maintain the use of the same set of available orthogonal sequences and simple receiver structure. In this scheme, a total of K users are divided into G groups each containing T users which are collaboratively coded to form uniquely decodable composite codewords. These codewords are spread using a single sequence to perform the CDMA function between the groups. At the receiver, a low complexity maximum likelihood (ML) joint detection and decoding is carried out over a small set of allowed composite codewords to recover the desired user's data. Theoretical and simulation performance analysis of the bit error rate (BER) and user capacity are presented in different channel conditions. It is shown that the proposed collaborative spreading is a simple and very effective means for extending the user capacity at the cost of a modest degradation in BER performance compared with non‐overloaded fully orthogonal CDMA. It can achieve higher signal‐to‐interference plus noise ratio (SINR) and higher overloading ratio compared with Orthogonal CDMA/Orthogonal CDMA schemes and other group orthogonal CDMA schemes. Copyright © 2009 John Wiley & Sons, Ltd.     

Iterative interference cancellation for DS-CDMA systems with highsystem loads using reliability-dependent feedback

In this paper, coded transmission over time-variant multipath Rayleigh-fading channels employing direct-sequence code division multiple access (DS-CDMA) is considered. Assuming ideal knowledge of the actual channel state and randomly chosen spreading sequences, we show that iterative multi-user interference suppression based on adapted minimum mean-squared error (MMSE) filters combined with serial successive cancellation and single user decoding can reach near optimum performance within a few iteration cycles. This holds even for doubly loaded systems, i.e., when the number of users K in the system is two times as large as the spreading factor N. Further, we evidence that for sufficiently reliable symbol estimates soft decision feedback can be replaced by hard decision feedback without any performance degradation but with significant savings in complexity     

LTE虚拟MIMO系统中用户正交配对改进算法

第三代合作伙伴计划(3GPP)在长期演进(LTE)上行传输中引入了虚拟多输入多输出(V-MIMO)技术,能够提升频谱效率,改善无线链路传输性能.用户配对是V-MIMO技术必不可少的环节.针对现有的正交配对算法由于不能准确衡量两用户的正交性而导致的性能恶化问题进行了理论分析,并提出更具有一般性的正交性定义.为适应LTE-Advanced上行最大支持4流、下行最大支持8流的特点,结合贪婪思想将正交配对的应用情形由两用户推广至多用户.为了进一步改善系统性能,提出基于范数的贪婪正交配对算法.仿真结果与复杂度分析表明,基于范数的贪婪正交配对算法与穷举的最优算法性能接近,且在计算复杂度上要远低于最优算法,对于V-MIMO技术的发展具有重要意义.     

Antenna and User Subset Selection in Downlink Multiuser Orthogonal Space-Division Multiplexing

Block diagonalization (BD) and successive optimization (SO) are two suboptimal but more practical (compared to dirty paper coding (DPC)) orthogonal linear precoding techniques for the downlink of multiuser MIMO systems. Since the numbers of users supported by BD or SO for a given number of transmit antennas are limited, BD or SO should be combined with scheduling so that a subset of users is selected at a given time slot while meeting the dimensionality requirements of these techniques. On the other hand, receive antenna selection (RAS) is a promising hardware complexity reduction technique. In this paper, we consider user scheduling in conjunction with receive antenna selection. Since exhaustive search is computationally prohibitive, we propose simplified and suboptimal user scheduling algorithms for both BD and SO. For BD, we propose capacity and Frobenius-norm based suboptimal algorithms with the objective of sum rate maximization. Starting from an empty set, each step of proposed algorithms adds the best user from the set of users not selected yet until the desired number of users have been selected. Proposed receive antenna selection works in conjunction with user scheduling to further enhance the sum rate of BD. For SO, a Frobenius-norm based low complexity algorithm is proposed, which maximizes the ratio of the squared Frobenius norm of the equivalent channel (projected to the joint null space of the previously selected users) to the sum of the squared Frobenius norms of the previously selected users’ preprocessed channels. Simulation results demonstrate that the proposed algorithms achieve sum rates close to exhaustive search algorithms with much reduced complexity. We also show that in addition to reduced hardware complexity at the receiver, antenna selection enhances multiuser diversity gain that is achieved with user scheduling.     

A soft decoding scheme for vector quantization over a CDMA channel

The optimal decoding of vector quantization (VQ) over a code-division multiple-access (CDMA) channel is too complicated for systems with a medium-to-large number of users. This paper presents a low-complexity, suboptimal decoder for VQ over a CDMA channel. The proposed decoder is built from a soft-output multiuser detector, a soft bit estimator, and the optimal soft VQ decoding of an individual user. Simulation results obtained over both additive white Gaussian noise and flat Rayleigh fading channels show that with a lower complexity and good performance, the proposed decoding scheme is an attractive alternative to the more complicated optimal decoder.     

一种新的准正交空时分组码的编码研究

准正交空时分组码可以牺牲一定的分集增益和解码的简单性,来避免当天线数目大于2时正交空时分组码码率下降的缺点。基于准正交空时编码的优点,为了进一步提升性能,提出一种新的4天线准正交空时分组码,并与RS码进行级联编码。仿真表明这种新的编码方法可以保证在复杂度不是很高的前提下,相比Jafarkhani码、TBH码和该广义复正交码具有更低的误码率。     

Iterative multiuser detection for turbo-coded FHMA communications

An iterative receiver structure Is proposed for turbo-coded frequency-hop multiple access (FHMA) systems. In FHMA systems, the adjacent channel interference (ACI) is the major contributor of multiple access interference (MAI) if orthogonal hopping patterns are used. The ACI is a function of the tone spacings of the adjacent subchannels and the rolloff factor of the pulse-shaping filter. The calculation of the ACI for a square-root raised-cosine pulse-shaping filter in an FHMA system is presented in this paper. In addition, a low complexity iterative multiuser detector is developed to mitigate the degradation caused by ACI in the FHMA systems. The iterative receiver structure is based on a modified turbo decoding algorithm which makes use of the a posteriori log-likelihood ratio (LLR) information of the systematic bits to obtain the a posteriori information of the turbo-encoded parity bits. Iterations of the receiver/decoder are used as the mechanism to estimate and mitigate the MAI in the FHMA system. The properties of both soft and hard interference suppressors based on the modified turbo decoding algorithm are examined and an efficient recursive implementation is derived. Compared to maximum-likelihood multiuser detection, the proposed system is more practical and its complexity is only a linear function of the number of users. Simulation results show that the proposed iterative receiver structure offers significant performance gain in bandwidth efficiency and the required signal-to-noise ratio (SNR) for a target bit-error rate (BER) over the noniterative receiver structure. Moreover, the single user performance can be achieved when imperfect power control exists