Generalized eigenvector problem for Hermitian Toeplitz matrices and its application to beamforming

The generalized eigenvector problem (GEP) for Hermitian Toeplitz matrices is studied and some properties related to its eigenvectors and the associated eigenfilters are derived. Zero locations of the eigenfilters are also investigated and all of the results are applied to the maximum SINR (signal-to-interference-plus-noise ratio) beamforming problem based on ULAs (uniform linear arrays), since maximizing output SINR can be formulated as a generalized eigenvector problem where the matrix pair consisting of the desired signal correlation matrix and interference plus noise correlation matrix. Theoretical analysis based on a three-element ULA is provided, supported by simulations.     

Adaptive multiuser interference suppression for CDM satellite systems

This paper deals with the application of linear multiuser detectors in mobile terminals of code division multiplex (CDM) satellite systems. Generally, the interference caused by other users can dramatically reduce the performance of a CDM‐based satellite communication system. Since no spreading information of other users is given and since there are no trainings sequences, a blind adaptive interference detector with low complexity is needed. In order to implement a detector for suppressing the interference from other users, different adaptation algorithms can be applied. Comparing them in terms of complexity, adaptation speed and bit error rate, the LMS (least means squares) algorithm with adaptive step size shows the best performance. However, the implementation of this algorithm in a satellite system still leads to high processing times. For this reason, two new schemes with reduced complexity are presented. A very important point is, that the adaptive algorithms are basically restricted to short spreading codes (code period equals symbol period) whereas CDM‐based satellite systems use long codes (one‐code period spans over multiple symbols). In this paper, a new scheme is presented which can be applied to long codes. By cascading multiple separated interference detectors the adaptation is done every time when the same part of the spreading sequence appears. Copyright © 2005 John Wiley & Sons, Ltd.     

Successive interference cancellation algorithms for downlink W-CDMAcommunications

In this paper, successive intracell interference cancellation (IIC) of the wideband-code division multiple access (W-CDMA) signal at the mobile unit is considered. Three new interference cancellation techniques suitable for the downlink of any CDMA system with orthogonal spreading are proposed. No prior knowledge of users' spreading codes or even their spreading factors are required for interference cancellation. A new term, effective spreading code, has been introduced, which is defined as the interfering user physical code as seen by the desired user within the desired user symbol duration. The mobile receiver estimates the effective spreading codes of the interfering users regardless of their spreading factors using fast Walsh transform (FWT) correlators (instead of the regular correlators) and uses this information to suppress the intracell multiuser interference. Three different interference-suppressing techniques are studied: subtraction; combined interfering signal projection; and separate interfering signal subspace projection. The complexity of the proposed techniques is low compared to conventional interference cancellation techniques. For a W-CDMA system and the IMT-2000 vehicular channel model, a capacity increase of up to 150% of the original (without IIC) system capacity is shown     

多径衰落信道下的扩频码设计与联合检测

在多径衰落信道CDMA系统中，扩频码设计与联合检测为提高系统容量的两个重要手段。在扩频码设计中，通过构造零相关窗码可以消除MAI和ISI，从而提高系统容量。当扩频码相关特性不理想时，通过联合检测也可以消除干扰，提高系统容量。但二者都有一定局限性，对于零相关窗码来说，它的码字资源十分有限，对于联合检测来说，当用户数很大，它的复杂度不能容忍。该文将扩频码设计与联合检测结合起来，提出了一种具有组间零相关窗特性的新型扩频码。在扩频码设计时考虑联合检测，可以增加码字资源，在联合检测中考虑扩频设计，又可以降低联合检测的复杂度。     

Construction of quasi‐cyclic low‐density parity‐check codes with low encoding complexity

Low encoding complexity is very important for quasi‐cyclic low‐density parity‐check (QC‐LDPC) codes used in wireless communication systems. In this paper, a new scheme is presented to construct QC‐LDPC codes with low encoding complexity. This scheme is called two‐stage particle swarm optimization (TS‐PSO) algorithm, in which both the threshold and girth distribution of QC‐LDPC codes are considered. The proposed scheme is composed of two stages. In the first stage, we construct a binary base matrix of QC‐LDPC code with the best threshold. The matrix is constructed by combining a binary PSO algorithm and the protograph extrinsic information transfer (PEXIT) method. In the second stage, we search an exponent matrix of the QC‐LDPC code with the best girth distribution. This exponent matrix is based on the base matrix obtained in the first stage. Consequently, the parity‐check matrix of the QC‐LDPC code with the best threshold and best girth distribution are constructed. Furthermore, bit error rate performances are compared for the QC‐LDPC codes constructed by proposed scheme, the QC‐LDPC code in 802.16e standard, and the QC‐LDPC code in Tam's study. Simulation results show that the QC‐LDPC codes proposed in this study are superior to both the 802.16e code and the Tam code on the additive white Gaussian noise (AWGN) and Rayleigh channels. Moreover, proposed scheme is easily implemented, and is flexible and effective for constructing QC‐LDPC codes with low encoding complexity. Copyright © 2012 John Wiley & Sons, Ltd.     

The Application of Frequency Assignment Techniques in Spreading Code Assignment

The use of spreading codes with a zero correlation zone or a low correlation zone can reduce multiple access interference in code-division multiple-access (CDMA) radio systems. However, problems can arise from the limited number of codewords. In this paper it is shown that this shortage of codewords can be mitigated by a combination of two approaches. Existing constructions of binary and ternary codes for quasisynchronous CDMA are modified to double the number of spreading codewords. Increases in correlation are restricted to certain codeword pairs. A metaheuristic algorithm inspired by those used in frequency assignment is then used to assign these codewords. This is done in such a way that a satisfactory signal-to-interference ratio across the network is obtained. Excellent results are reported for several networks. Interference is lower than when a Gold code is used as a scrambling code. Bit-error-rate improvements are quantified in the case that binary phase shift keying is used.     

Combined code reuse scheme with two‐dimensional OVSF codes assignment algorithm for uplink multi‐user/multi‐rate block spread multi‐cellular CDMA

The two‐dimensional (2D) block spread code division multiple access (CDMA) can avoid the uplink multiple‐access interference with low‐complexity single‐user detection in a slow fading channel and, therefore, is very attractive. In the 2D spreading, orthogonal variable spreading factor (OVSF) is used for spreading; an important problem is how to efficiently assign the limited resource of OVSF codes to users with different data rates, while meeting the requirement of quality of service in a multi‐cell environment. In this paper, it is shown that the code reuse can improve the code reuse efficiency and the proposed code reuse scheme combined with code assignment algorithm can allow flexible multi‐rate uplink transmission. The computer simulation confirms that the proposed code assignment algorithm improves the code reuse efficiency while achieving lower blocking probability than traditional CDMA. Copyright © 2012 John Wiley & Sons, Ltd.     

On the performance of centralized DS-SS packet radio networks withrandom spreading code assignment

The paper presents a random spreading code assignment scheme for enhancing channel efficiency in centralized DS-SS packet radio networks which employ a multiple-capture receiver for each code channel. Compared to the common code case, this approach requires a modest increase in receiver complexity, but the number of distinct spreading codes being used is considerably less than the number of radios in the network. A general theoretical framework for evaluation of collision-free packet performance in each code channel is described, in which the possibility of collision-free transmission is conservatively estimated using a combinatorial method, and the effects of asynchronous multiple-access interference are characterized in terms of the primary and secondary user interferences. At the link level, the capture and throughput performances are evaluated for a proper set of codes, and compared with the results from the common code scheme. It is shown that the use of a random assignment scheme with more than one code results in a higher performance gain, and most of this gain can be achieved with just two distinct spreading codes     

Asymptotic Performance of Reduced-Rank Linear Receivers With Principal Component Filter

This correspondence studies the asymptotic performance of output signal-to-interference-plus-noise ratio (SINR) for reduced-rank linear receivers with principal component filter. We prove that for code division multiple access (CDMA) systems with random spreading codes, when the number of users and the spreading gain go to infinity with their ratio being fixed, the output SINR converges to a fixed constant with probability 1, which is consistent with the conjecture made in Honig and Xiao, "Performance of reduced-rank linear suppression," IEEE Trans. Inf. Theory, vol. 47, no. 4, pp. 1928-1946, May 2001     

Code hopping - direct sequence spread spectrum to compensate for intersymbol interference in an ultra-wideband system

We propose code hopping - direct sequence spread spectrum (CH-DSSS) with binary phase shift-keying (BPSK) modulation to compensate for intersymbol interference (ISI) in an Ultra-wideband system. The central idea is that code hopping (CH) affects ISI amplitudes that are produced by both the cross- and the auto-correlation properties of spreading codes for codewords; not just by the latter as occurs without CH. We also propose a low complexity CH pattern search algorithm to find good CH patterns. To evaluate the performance of the CH-DSSS system, a bit error rate (BER) expression is derived for a Rake receiver by applying the Beaulieu series method. Computational results show that significant gains can be obtained by CH for both the average BER and the outage probability (e.g. a 6 dB gain in outage probability for a non-line of sight channel).     

Asymptotic analysis of optimum and suboptimum CDMA downlink MMSE receivers

In this paper, we investigate the performance of two linear receivers for code-division multiple-access (CDMA) downlink transmissions over frequency-selective channels, the users having possibly different powers. The optimum minimum mean-square error (MMSE) receiver is first considered. Because this receiver requires the knowledge of the code vectors attributed to all the users within the cell when these vectors are time varying, its use may be unrealistic in the forward link. A classical suboptimum receiver, consisting in a chip rate equalizer followed by a despreading with the code of the user of interest, is therefore studied and compared to the optimum MMSE receiver. Performance of both receivers is assessed through the signal-to-interference-plus-noise ratio (SINR) at their outputs. The analytical expressions of these SINRs depend in a rather nonexplicit way on the codes allocated to the users of the cell, and are therefore not informative. This difficulty is dealt with by modeling the users code matrix by a random matrix. Because the code matrices used in the forward link are usually isometric, the code matrix is assumed to be extracted from a Haar-distributed random unitary matrix. The behavior of the SINRs is studied when the spreading factor and the number of users converge to /spl infin/ at the same rate. Using certain results of the free probability theory, we establish the fact that the SINRs converge almost surely toward quantities that depend only on the complex amplitudes of propagation channel paths. We then use the expressions of these SINR limits to discuss the influence of the various parameters on the performance of the receivers.     

浅海水声信道中原模图LDPC码的设计及性能分析

原模图低密度奇偶校验码相较于传统LDPC码,具有结构简单、迭代译码门限低、易于扩展及线性编译码复杂度等优点。针对强多途、长时延、窄带宽的浅海水声信道,该文研究了PG-LDPC码的设计及性能特征,提出一种码型设计方案,并采用基于原模图度分布的外部信息转移图算法,对所设计PG-LDPC码的纠错性能进行分析及预测。仿真与实验结果表明,与(3,6)随机规则LDPC码相比,所提的PG-LDPC码在低、高信噪比区域均有良好的纠错性能。     

On (n, n-1) convolutional codes with low trellis complexity

We show that the state complexity profile of a convolutional code C is the same as that of the reciprocal of the dual code of C in case that minimal encoders for both codes are used. Then, we propose an optimum permutation for any given (n, n-1) binary convolutional code that will yield an equivalent code with the lowest state complexity. With this permutation, we are able to find many (n, n-1) binary convolutional codes which are better than punctured convolutional codes of the same code rate and memory size by either lower decoding complexity or better weight spectra     

Simple MAP decoding of first-order Reed-Muller and Hamming codes

A maximum a posteriori (MAP) probability decoder of a block code minimizes the probability of error for each transmitted symbol separately. The standard way of implementing MAP decoding of a linear code is the Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm, which is based on a trellis representation of the code. The complexity of the BCJR algorithm for the first-order Reed-Muller (RM-1) codes and Hamming codes is proportional to n/sup 2/, where n is the code's length. In this correspondence, we present new MAP decoding algorithms for binary and nonbinary RM-1 and Hamming codes. The proposed algorithms have complexities proportional to q/sup 2/n log/sub q/n, where q is the alphabet size. In particular, for the binary codes this yields complexity of order n log n.     

Dynamic signature assignment for direct sequence CDMA systems

Multiple access interference (MAI) is one of the major impediments in the reverse link of wide band CDMA systems, due to the synergy of the users' spreading codes, transmission delays and the channel characteristics. We propose to dynamically assign the users' spreading codes and transmission delays, i.e., to assign the user signature, in order to minimize mutual cross-correlations at the receiver. This dynamic signature assignment (DSA) approach helps to avoid (as opposed to combating) MAI at the base station. In assigning the signatures, we present a low-complexity iterative algorithm which utilizes channel information and is able to minimize signal-to-interference-plus-noise (SINR) ratio at receivers. Computer simulation results are presented which show a potential 2-3 fold capacity increase over conventional systems     

循环码研究中的一种几何方法

本文采用几何的方法对循环码进行了研究。证明了BCH码的校验矩阵是用循环变换的特征向量作基底时的一种表示形式,从而把循环码的研究纳入到线性系统理论研究的框架之中。     

Large-System-Based Performance Analysis and Design of Multiuser Cooperative Networks

In this paper, the performance of the cooperative multiuser direct-sequence code-division multiple-access (DS-CDMA) system is analyzed in the asymptotic regime where both the spreading codes and the number of users grow unboundedly large with the same ratio. Assuming that each terminal is paired with another user which, in addition to transmitting its own data, estimates and relay the information transmitted from its partner, a simple approximate signal-to-interference-plus-noise ratio (SINR) expression is derived that is independent from the spreading codes and explicitly accounts for the effects of the multiple-access interference (MAI) and the relay noise. The so-obtained SINR expression is then computed based entirely on the available local information and without any knowledge about the interfering users. The results obtained above are then used to optimally design the cooperative system. In particular, it is shown how the amount of cooperation between each collaborating pair can be adjusted to simultaneously achieve a preassigned target SINR for both users. Based on the local information, the globally optimal amount of the relay power is obtained that maximizes the achieved SINR at the access point. It is shown that increasing the relay power does not necessarily result in improving the quality of reception at the access point and, to maximize each user's SINR, its relay power should be carefully adjusted based on the environmental parameters such as the interpartner channel link and the powers of MAI and the relay noise. The connection between the cooperative and the conventional multiuser systems is also studied and simulations are used to demonstrate the validity of the analytical results.      

迭代子空间跟踪和结构约束的自适应波束形成算法

自适应波束形成(adaptive beam-forming:ABF)方法通过对阵列数据进行加权处理来获得最大的输出信干噪比,对采样协方差矩阵依赖性较大.在小快拍和通道随机响应误差条件下,采样协方差矩阵的估计值与真实值通常存在较大差异,严重恶化了基于线性约束最小均方误差准则的自适应波束形成方法的干扰抑制性能.针对该问题,本文基于子空间投影类波束形成方法的思想,提出采用迭代子空间跟踪和结构约束的自适应波束形成方法.该方法首先利用clearing技术在训练样本集上依次跟踪主特征矢量并构成信号子空间;然后根据子空间投影类波束形成权矢量的结构特性计算自适应加权矢量.仿真结果表明本文方法能有效提高自适应波束算法的输出信干噪比.     

A novel transmitter design for GLSFBC–OFDM–CDMA communication systems

Relying on mutual orthogonality between subcarriers of different users in orthogonal frequency-division multiple access (OFDMA) systems and mutual orthogonality between spreading codes in code-division multiple-access (CDMA) systems, a novel transmitter design is proposed for group layered space–frequency block code (GLSFBC)–OFDM–CDMA communication systems over frequency-selective fading channels. The proposed method is based on a three-level design of user codes: the top level (based on OFDMA) deals with group interference and intersymbol interference (ISI), the middle level (based on space–frequency block coding) results in space–frequency diversity, and the lower level (based on CDMA) handles multiuser interference. The new approach only needs one receive antenna to distinguish multiple users and suppress group interference simultaneously, so the complexity of the receiver decreases remarkably. Simulation results confirm the validity of the proposed technique.