Performance of blind channel estimation algorithms for space-frequency block coded multi-carrier code division multiple access systems

The problem of blind channel estimation for downlink space-frequency block coded multi-carrier code division multiple access (SFBC MC-CDMA) schemes is considered. For these schemes, the authors first develop a system model for complex modulated signals, which reduces the multichannel estimation problem to a single-input single-output problem. Then, they present an intuitive subspace-based channel estimation method along with the corresponding necessary and sufficient conditions under which the channel estimate is unique (within a complex scalar). Their studies highlight two interesting properties of SFBC MC-CDMA systems: (i) there is no antenna order ambiguity (also known as permutation ambiguity) even though only one spreading code is assigned to each user; (ii) channel identifiability is guaranteed, regardless of the channel zeros location. They also establish the unbiasedness of the channel estimates and derive closed-form expressions for the mean-square-error of the estimates as well as the corresponding Cramer-Rao bound (CRB). In the derivation of the CRB, they suggest a novel approach which assumes the knowledge of only the spreading code of desired user. This approach results in a tighter bound than the CRB derived based on the knowledge of all users' signatures.     

Blind channel estimation for multiple input multiple output uplink guard-band assisted code division multiple access systems with layered space frequency equalisation

Single carrier (SC) code division multiple access (CDMA) with block transmission has been shown to be more effective while utilising a low-complexity equaliser to combat frequency-selective fading channels, when compared with conventional direct sequence CDMA technology. It also has lower peak-to-average power ratio and lower frequency sensitivity compared with multicarrier CDMA. The authors propose two blind channel estimation methods for uplink multiple input multiple output SC-CDMA systems with block transmisssion-one is the subspace-based method and the other is the so-called autocorrelation contribution method (ACM). Both the methods provide close performance to the case with perfect channel knowledge at high signal-to- noise ratio (SNR) without any training data required. It is shown that ACM yields a better performance than the subspace method at a lower SNR, and a similar performance at a high SNR, with the advantages of avoiding rank determination and noise power estimation as in the subspace method. In addition, the authors integrate layered space frequency equalisation with blind channel estimation, which provides improved performance over the conventional linear equalisation, by employing successive interference cancellation.     

Synchronous optical code-division multiple access systems with constant multi-user interference

A synchronous optical code-division multiple access system with balanced encoder instead of on-off-keying (OOK) encoder is proposed. In the system, the multi-user interference is maintained constant during a frame, and the receiver estimates the interference frame by frame instead of bit by bit. The data rate of the proposed system is no longer severely limited by the electronic processing time in estimating the interference, and the structure of the proposed receiver is simpler than those in OOK systems. In the receiver, both linear and non-linear threshold estimators are introduced. Numerical results show that the proposed system has smaller bit error rate than OOK systems, especially when the number of training bits is increased.     

Timing and frequency offset estimation scheme for the uplink of OFDMA systems

Timing and frequency synchronisation for the uplink of OFDMA systems is discussed. The uplink synchronisation procedure is presented and a novel timing and frequency offset estimation scheme is proposed. The timing and frequency offsets are estimated by identifying the differential phases of the training subcarriers in frequency and time dimensions, respectively. The frequency offset is estimated ahead of the timing offset, after which intercarrier interference compensation is carried out based on the estimated frequency offset. Finally, the timing offset is estimated after eliminating the frequency offset's influence. The principle of best linear estimation is applied. Both the case of a single user and that of multiple users simultaneously accessing the network are considered. In contrast to other methods, the proposed scheme has moderate complexity and allows flexible subcarrier assignment schemes. The analyses and simulation results demonstrate the accuracy of the proposed scheme in the uplink channels.     

Robust phased array receiver for multipath code-division multiple access communication systems

The problem of estimating information symbols transmitted through non-Gaussian multipath CDMA wireless channels with a phased array is considered. First, a robust detector based on the minimax robust estimation theory is proposed. This proposed robust technique has a decreased sensitivity of the estimates with respect to an actually unknown distribution of random noises and interferences. Then, the estimation accuracy of information signals is analysed by Monte Carlo simulation of this direction-of-arrival estimationbased robust detector in multipath scenario.     

Carrier frequency offset estimation and correction for OFDMA uplink

The paper develops a new method for estimating and correcting the carrier frequency offsets (CFO) of orthogonal frequency division multiple access (OFDMA) systems at the uplink receiver. Multiple CFO of the different users in the OFDMA system may violate the orthogonality among subcarriers and induce inter-carrier interference (ICI) and inter-user interference (IUI). The IUI provides some extra challenges to CFO estimation not addressed in OFDM systems with only a single CFO. By significantly reducing the IUI, the paper presents an improved CFO estimator which can provide a higher accuracy than the existing approaches. After having estimated the CFO, the author also proposes a new method for CFO correction or compensation at the uplink receiver. Through simulations, it is shown that the new CFO estimator and the correction method outperform the existing approaches.     

OFDM系统符号定时精同步研究

基于时域的帧同步估计算法在通过高斯信道和多径衰落信道时,其时间测度由于循环前缀的影响有峰值平台,难于准确的判定帧到达的时刻。文中提出了一种低复杂度的符号定时估计算法,利用简化的时间测度函数检测峰值来实现同步。分析了该算法的性能,并在各种信道中进行了性能分析,给出了了仿真结果。结果表明,该算法具有较高的精度和较低的计算复杂度。     

Precoding in downlink multi-carrier code division multiple access systems using expectation maximisation algorithm

The presence of multiple access interference and hardware complexity of the mobile terminal are two major burdens for multi-carrier code division multiple access schemes. Both burdens can be overcome in downlink by precoding the transmitted signal using the knowledge of the channel state information. In multiuser precoding techniques, the transmitter is optimised to combat channel impairments through the use of new spreading sequences that are obtained by solving an optimisation problem based on some criterion. Among these optimisation problems, the problem based on maximum likelihood is hard and complex to solve. In the proposed precoding scheme, the well-suited expectation maximisation algorithm is used to solve this problem. The proposed precoding technique is simulated and its performance is analysed and compared with some other precoding and detecting techniques. The results show that the proposed scheme considerably outperforms the previous precoding schemes.     

Distributed algorithm for optimal sequence and power allocation in uplink code division multiple access systems

An alternative approach is presented for centralised algorithms that design optimal sequences and powers under quality of service constraints in the uplink of a code division multiple access systems. The authors propose a distributed algorithm, where each user designs its optimal codeword in such a way to transmit minimum power, based on certain feedback information sent from the base station. The authors define the user cost function, which is the user power written as a convex function of user codewords for a defined signal-to-interference plus noise ratio target. For the constrained optimisation problem, optimal codewords are designed based on feasible direction method and the optimal user powers are the minimum values of the user cost functions. For the optimal configuration, the matched filter employed at receiver will have the same performance as the minimum mean squared error filter. Even if the optimal user powers are unique, the optimal codewords do not correspond to a unique allocation, but rather to an entire class of codewords that can be related by unitary transformations. The algorithm works properly in the presence of multiple access interference with white or coloured additive noise and requires no ordering. The proposed algorithm is analysed and illustrated with numerical examples obtained from simulations.     

Joint rate and power adaptation for radio resource management in uplink wideband code division multiple access systems

The benefits of adaptive joint power control and rate allocation for uplink transmission in a wideband code division multiple access cellular system are investigated. Closed-loop power control (CLPC), to adaptively adjust the transmit power, has the effect of maintaining a target signal-to-interference ratio and bit error rate (BER) performance. On the other hand, rate adaptation requires less transmit power, although the BER performance may be poorer. The authors differentiate the power update interval from the data rate update interval, analyse and evaluate the performance of two joint rate/power adaptation algorithms in a fading environment: optimal spreading factor-power control (OSF-PC) and greedy rate packing-power control (GRP-PC). Numerical results show that GRP-PC exhibits superior throughput performance compared with other three adaptation schemes. CLPC alone exhibits throughput and BER performances comparable to those of the OSF-PC scheme, but consumes a significantly higher amount of transmit power. Rate adaptation only is not efficient in enhancing throughput, but its power consumption is minimal.     

Effect of encoder--decoder mismatch due to wavelength and time misalignments on the performance of two-dimensional wavelength--time optical code-division multiple access systems

We examine the effects of encoder and decoder mismatch due to wavelength and time chip misalignments on the bit-error rate (BER) performance of two-dimensional (2D) wavelength--time optical code-division multiple access systems. We investigate several instances of misalignment in the desired user encoder and decoder as well as in the interfering user encoders. Our simulation methodology can be used to analyze any type of 2D wavelength--time code family as well as probability distribution for misalignment. For illustration purposes, we consider codes generated by use of the depth-first search algorithm and a Gaussian distribution for the misalignment. Our simulation results show that, in the case of a misalignment in either wavelength or time chip, the variance of the distribution for the misalignment must be below 0.01 for the corresponding degradation in the BER system's performance to be less than 1 order of magnitude compared with that when there is no mismatch between the encoders and decoders. The tolerances become even more strict when misalignments in both wavelength and time chips are considered. Furthermore, our results show that the effect of misalignment in wavelength (time chips) is the same regardless of the number of wavelengths (time chips) used in the codes.     

High mobility orthogonal frequency division multiple access channel estimation using basis expansion model

Owing to the loss of subcarrier orthogonalities in high-speed applications, the use of conventional frequency-domain-based channel estimation in high mobility orthogonal frequency division multiple access (OFDMA) systems such as mobile WiMax may give rise to an unacceptable high channel estimation error floor. To alleviate this problem, the authors develop some basis expansion model (BEM)-based estimation schemes for the OFDMA uplink. Specifically, the authors express the time-varying channel as a superposition of a small number of complex exponential basis functions spanning the entire Doppler range, and then formulate least square (LS) and linear minimum mean square error (LMMSE) algorithms to estimate the basis coefficients for two different types of pilot patterns. The authors also derive the respective Cramer-Rao lower bounds for these estimators. It has been shown that the time domain BEM using a pilot scheme where pilots are placed over time axis will give better performance under a high Doppler scenario. Lastly, using simulation results, the proposed algorithms have been found to have better estimation accuracy over current frequency domain estimation techniques. This is in addition to the advantage that the proposed algorithms have in general a lower computational complexity.     

滤波器组多载波系统的高精度符号定时估计

针对滤波器组多载波(FBMC)系统的传统定时同步算法精度较低的问题,研究了其发送信号统计特性,从理论上量化分析了受数据符号重叠影响部分和非重叠部分的同步训练数据的特性,在此基础上提出了一种适用的高精度符号定时估计算法.该算法通过传输一串长度大于滤波器重叠因子的连续相同的训练符号,使得时域合成信号具有多重延迟冗余特性.仿...     

一种适用于慢衰落信道OFDM系统的信道估计算法

针对慢衰落信道OFDM系统,提出了一种新的低复杂度的且能对抗定时误差的信道估计器.这种信道估计器是基于导频间隔I=2的特殊梳状导频结构设计的.基于工程实现的考虑,采用了最小平方(LS)算法进行导频信道估计,同时利用了线性内插对非导频信道进行估计.分析了定时误差对非导频子载波估计的影响,并提出了一种相当简单但十分有效的修正算法,仿真结果有力的证明了此算法能有效的抵抗定时误差对非导频信道插值估计的影响.     

VMIMO-OFDM系统精频偏估计算法的研究

在对多种可应用于VMIMO-OFDM系统的频偏估计方法的性能和效率进行分析的基础上,针对基于VMIMO-OFDM的无线传感器网络中的精频偏估计问题,提出了一种在频域进行频偏估计的算法.同时,针对基于VMIMO-OFDM的无线传感器网络应用于瑞利衰落严重的环境时不同的导频子载波上频偏估计的误差差异较大的问题,提出了一种基于信道统计信息(CSI)加权的精频偏估计算法,该方法对各个子载波上的频偏估计结果进行加权平均,可使估计误差最小化.理论分析和仿真结果均表明,新提出的算法在高斯信道和多径信道中均有更优的性能.     

MIMO-OFDM系统中一种新的信道估计方法

提出了多输入多输出-正交频分复用(MIMO-OFDM)系统中一种新的信道估计方法.根据最小均方误差的准则,推导出了求解信道频率响应的方程,方程求解的运算量主要集中在一个矩阵的求逆上.对每一个符号,此矩阵是固定的,所以矩阵的求逆仅需计算一次,整个求解过程的运算量较低.计算机仿真给出了这种方法的归一化均方误差和误码率性能.仿真结果表明,本文方法在多普勒频移为60Hz和360Hz下,其归一化的均方误差值相当,表明本文方法可以在不同的移动环境中良好工作.在不同的数据调制方式和不同的多普勒频移下,本文方法的误码率性能优于最小平方算法,略差于线性最小均方误差算法,但复杂度大大降低.     

适用于OFDM系统的改进的信道估计算法

针对实际系统中的信道冲激响应(CIR)泄漏问题,提出了一种适用于梳状导频正交频分复用(OFDM)系统的改进的最小二乘(LS)信道估计算法.该算法基于分数抽头延时信道近似(FTCA)的参数化信道模型,此信道模型采用一个抽头间隔为分数倍采样间隔的抽头延时线结构的有限冲激响应(FIR)滤波器表示.此算法首先通过FTCA信道模型来近似实际信道,然后再估计该信道模型中的某些参数来获取实际的信道信息;该算法利用了FTCA信道模型中所有的延时抽头,从而很好地抑制了CIR泄漏.仿真与分析表明,FTCA信道模型能够有效地近似实际信道,同时降低信道自相关矩阵的维数,从而提高信道估计的性能.     

基于无迹卡尔曼滤波的MIMO-OFDM系统信道估计算法

基于对现有MIMO-OFDM系统信道估计技术及其理论的分析,提出了一种基于无迹卡尔曼滤波（UKF）的信道估计算法。该算法不需要信道的任何先验统计信息,从频域出发,首先利用最小二乘（LS）算法得到导频处信道的粗略估计值,再在时域运用路径捕获法和UKF算法得到信道的精确估计值。仿真结果表明,与LS算法、最小均方（LMS）算法等相比,该算法在较低运算复杂度下可以获得信道估计性能的明显改善。     

V-BLAST MIMO-OFDM系统的一种迭代信道估计方案

研究了频率选择性衰落信道条件下Turbo编码V-BLAST MIMO-OFDM系统的信道估计方法,提出了V-BLAST MIMO-OFDM系统的一种新的迭代信道估计方案。该方案首先利用发送的正交导频序列得到导频处的初始信道估计,然后利用导频符号辅助方法对信道进行内插,得到数据处的初始信道估计,经Turbo迭代纠错译码后,将得到的信息位和校验位软值信息反馈给信道估计器,并利用期望最大化(EM)迭代信道估计方法对信道进行更新。仿真结果表明,随着迭代次数的增加,系统的信道估计性能显著提高。