Interpolation-based precoding with limited feedback for MIMO-OFDM systems

The channel state information (CSI) at the transmitter can significantly improve the performance of multiple-antenna systems. However, providing full knowledge of CSI at the transmitter may not be affordable in many practical cases. Thus, exploiting the partial channel knowledge to improve system performance seems to be attractive. An interpolation based limited feedback precoding scheme (ILFP) for MIMO-OFDM systems is proposed. In this scheme, both the transmitter and the receiver store the codebook of precoding matrices constructed offline using two-variable joint vector quantisation. Considering the correlation between OFDM subcarriers, they are divided into subcarrier clusters. At the receiver, precoding is carried out on the clusters, and then the precoding information is conveyed to the transmitter by limited bits of feedback. At the transmitter, the precoding matrices for each subcarrier are obtained by interpolation according to the feedback. Simulation results show that the proposed scheme outperforms the existing schemes.     

Adaptive modulation for limited diversity fading channels

In this study, the authors propose a novel technique for adaptive modulation over limited diversity fading channels with channel state information at the transmitter. Limited diversity channels such as those encountered in indoor orthogonal frequency division multiplexing systems are characterised by the fact that achievable diversity orders are limited by the channel and not by code-free distances. The authors first propose a novel analysis technique for the performance of coded modulation on limited diversity block fading channels with different modulation sets on each block. The authors then propose adaptive modulation techniques for maximising the throughput at a fixed bit error probability and also for minimising the bit error probability at a fixed rate. Lastly, the authors show simulation results that support the arguments presented in the paper.     

Iterative low-complexity multi-user detection for asynchronous multi-sequence signalling based bit-interleaved coded modulation code division multiple access systems

An uplink direct sequence spread spectrum communications systems employing a multi-sequence model over a quasi-static frequency-selective fading channel is considered. In analogy with bit-interleaved coded modulation (BICM) technique, a group of bits at the output of a bit-wise interleaver is mapped uniquely to a complex signalling vector belonging to an orthogonal plane sequence modulation signal space, which is constructed over a set of expanded signature sequences. This transmission system provides not only bandwidth efficiency offered by additional signal planes but also time diversity resulting from the BICM technique. It is observed that at high system traffic load error performance could degrade substantially due to user cross-correlations, multi-access asynchronism as well as channel frequency selectivity. The authors employ a 'turbo principle' receiver, consisting of a soft interference cancellation scheme, soft demappers and maximum a posteriori decoders, to avert this capacity loss by exploiting the serially concatenating structure at the transmitter. After simple mathematical manipulation, a soft space-time linear minimum mean-square error multi-user detector could even be explored on the basis of per signal plane per user. Both analytical performance-bound and computer simulation of the proposed framework in terms of bit-error rate (BER) are revealed. Further, performance comparisons with convolutionally coded and conventional bandwidth-efficient coded direct sequence code division multiple access systems under the same system conditions are illustrated. The authors have also investigated the impact of labelling maps on the BER performance.     

Transmitter pre-filtering for the downlink of a time-division-duplex/direct sequencecode division multiple access system over time-varying multipath fading channels

In this paper, we investigate an optimised transmitter pre-filtering technique for downlink time-division-duplex (TDD) code division multiple access (CDMA) communications, which employs the conventional matched filter (MF) detector at the mobile receivers. The proposed pre-filtering technique eliminates the multiple-access interference and intersymbol interference (MAI/ISI) effects by applying a very simple transmission scheme that combines a signal transformation with a cyclic prefix strategy under a power constraint condition. Two constrained pre-filtering transformations are suggested depending on the information required at the mobile unit. An open-loop transmitter pre-filtering is first formulated; however, this solution does not consider the properties of the noise at the mobile receiver. A second solution is then presented via a closed-loop transmitter pre-filtering that includes an optimum gain for a given transmit and noise power. Some associated issues such as system efficiency, computational complexity and channel estimation errors are also addressed. Simulation results show that the proposed transmitter pre-filtering scheme can be used to increase the system performance and capacity. In addition, its performance is compared with another similar transmit pre-processing scheme in order to evaluate the performance improvement by the proposed algorithm.     

Adaptive pseudo-noise code acquisition scheme using automatic censoring for DS/SS communication in frequency-selective Rayleigh fading channel

An adaptive pseudo-noise code acquisition scheme based on automatic multipath signal cancellation for mobile communication systems in frequency-selective Rayleigh fading channel is proposed. The proposed system combines a serial strategy and an automatic censored cell-averaging constant false alarm rate (CFAR) detector based on ordered data variability. This system does not require any a priori information about the number of interferences caused by the presence of multipath signals in the reference channel. The mean acquisition time and the detection performances of the proposed processor are evaluated and compared with those of the conventional adaptive acquisition scheme based on fixed-censoring point detector. It is shown that the considered scheme outperforms significantly the conventional one. The effects of various channel parameters on the acquisition performance, namely the number of resolvable paths, the partial correlation length and the signal-to-noise ratio are also investigated.     

利用CPM实现自适应OFDM系统研究

在正交频分复用(OFDM)系统中应用自适应技术能有效地提高系统性能。正交频分复用-连续相位调制(OFDM-CPM)通过CPM相关相位状态引入记忆,从而获得比传统OFDM更好的误码性能。文章提出了一种利用单一的CPM调制方式实现自适应OFDM-CPM系统的新方案。仿真结果表明,选取最佳的CPM参数组合,通过改变CPM的参数来实现自适应OFDM-CPM调制,能够获得比传统调制方式要低的门限值,同时系统硬件实现更为简单。     

Optimisation of limited feedback design for heterogeneous users in multi-antenna downlinks

The authors study the problem of limited channel information feedback in multiple-input single-output (MISO) broadcast channels serving heterogeneous users. The heterogeneous users have different feedback channel capacities because of their physical constraints and limitations. Our objective is to design an optimised limited feedback strategy for multiple users by using a set of multi-resolution codebooks such that, under varying and quantised channel information feedback, the multiuser system can maximise its achievable sum rate. By exploiting the receive antenna combining technique, the authors further generalise the proposed scheme to the multiuser beamforming case with multi-antenna users. Finally, the authors verify the proposed scheme by numerical methods. Simulation results show that the sum rate performance for heterogeneous users can be effectively improved by using the proposed limited feedback scheme, which is optimised according to only statistic channel information of users.     

Spectrum etiquettes for terrestrial and high-altitude platform-based cognitive radio systems

Two spectrum etiquettes are developed for the use on the downlink of coexisting high-altitude platform (HAP) and terrestrial fixed broadband systems that are intended for future application with cognitive radio-based user terminals with directional antennas. The spectrum etiquettes are based on the interference to noise ratio and carrier to interference plus noise ratio levels at the receiver of an incumbent user. Antenna beamwidths and multiple modulation scheme levels determine the parameter settings for coexistence performance. It is shown that coexistence performance can be improved by exploiting the surplus transmitter power of the terrestrial system, thereby enabling the incumbent terrestrial system to accommodate additional interference arising from a newly activated HAP system.     

A fast dual boundary element method for 3D anisotropic crack problems

In the present paper a fast solver for dual boundary element analysis of 3D anisotropic crack problems is formulated, implemented and tested. The fast solver is based on the use of hierarchical matrices for the representation of the collocation matrix. The admissible low rank blocks are computed by adaptive cross approximation (ACA). The performance of ACA against the accuracy of the adopted computational scheme for the evaluation of the anisotropic kernels is investigated, focusing on the balance between the kernel representation accuracy and the accuracy required for ACA. The system solution is computed by a preconditioned GMRES and the preconditioner is built exploiting the hierarchical arithmetic and taking full advantage of the hierarchical format. The effectiveness of the proposed technique for anisotropic crack problems has been numerically demonstrated, highlighting the accuracy as well as the significant reduction in memory storage and analysis time. In particular, it has been numerically shown that the computational cost grows almost linearly with the number of degrees of freedom, obtaining up to solution speedups of order 10 for systems of order 10⁴. Moreover, the sensitivity of the performance of the numerical scheme to materials with different degrees of anisotropy has been assessed. Copyright © 2009 John Wiley & Sons, Ltd.     

Nanoscale communication with molecular arrays in nanonetworks

Molecular communication is a promising nanoscale communication paradigm that enables nanomachines to exchange information by using molecules as communication carrier. Up to now, the molecular communication channel between a transmitter nanomachine (TN) and a receiver nanomachine (RN) has been modeled as either concentration channel or timing channel. However, these channel models necessitate exact time synchronization of the nanomachines and provide a relatively low communication bandwidth. In this paper, the Molecular ARray-based COmmunication (MARCO) scheme is proposed, in which the transmission order of different molecules is used to convey molecular information without any need for time synchronization. The MARCO channel model is first theoretically derived, and the intersymbol interference and error probabilities are obtained. Based on the error probability, achievable communication rates are analytically obtained. Numerical results and performance comparisons reveal that MARCO provides significantly higher communication rate, i.e., on the scale of 100 Kbps, than the previously proposed molecular communication models without any need for synchronization. More specifically, MARCO can provide more than 250 Kbps of molecular communication rate if intersymbol time and internode distance are set to 2 μs and 2 nm, respectively.     

Low-complexity iterative equalisation and decoding for wireless optical communications [optical wireless communications]

A low-complexity scheme of iterative equalisation and decoding by combining a recursive systematic convolutional code and a pulse-position modulation is proposed here. A graph- based equalisation for intersymbol interference (ISI) known at both transmitter and receiver is considered. By representing the memory channel with ISI as the factor graph and applying sum-product (SP) algorithm to this graph, a posteriori probability (APP) of the desired symbol necessary to implement iterative equalisation and decoding is derived. A partial response precoding is used to reduce the span of ISI from a possible infinite number of two baud periods. This precoding scheme makes the factor graph of memory channel cycle-free, and SP algorithm for combating ISI converges to an optimum detection. Numerical results show that the proposed low-complexity strategy has almost the same performance as the optimum turbo equalisation.     

Optimal power and subcarriers allocation in downlink OFDMA system with imperfect channel knowledge

Most of existing work on resource allocation in TDMA and OFDMA systems assumes the availability of perfect channel state information (CSI) at the transmitter, which is rarely possible due to feedback delay and channel estimation error. In this paper, we study the effect of feedback delay and channel estimation error on margin adaptive resource allocation in a downlink OFDMA system. By using convex optimization framework, we find an optimal solution to the problem. First, we study the individual effect of feedback delay and channel estimation error on resource allocation by considering them exclusively. Then, we consider the simultaneous presence of feedback delay and channel estimation error and study their combined effect on resource allocation. We derive an explicit close form expression for the users’ transmit power and propose an algorithm for power and subcarriers allocation for each of these three scenarios. The algorithms have polynomial complexities and solve the problem with zero optimality gaps. Simulation results show that the system performance is very sensitive to feedback delay and is affected significantly by imperfect channel estimation. Our proposed algorithms highly improve the system performance in the availability of only imperfect CSI at the transmitter.     

Cross-entropy optimisation of multiple-input multiple-output capacity by transmit antenna selection

Radio channel capacity can be increased dramatically using a multiple-input multiple-output (MIMO) scheme, but at the expense of hardware complexity. An efficient approach for complexity reduction is antenna subset selection at the transmitter and/or receiver. A novel transmit antenna selection algorithm is presented using the cross-entropy optimisation method to maximise channel capacity. In contrast with the existing work, the proposed algorithm guarantees a result to within 99% of the true optimum (i.e. the maximal capacity with selected transmit antennas) with substantially low complexity. The simulation results indicate that the proposed algorithm is independent of the relationship between the selected transmit array size and receive array size. The proposed scheme has the potential to make practical MIMO systems with high performance simpler to implement.     

虚拟时间反转镜PDS水声通信双向均衡方案

虚拟时间反转镜（Virtual Time Reversal Mirror,VTRM）引导了空间聚焦和信道均衡,将其应用在Pattern时延差编码（Pattern Time Delay Shift Coding,PDS）水声通信体制,构成VTRM-PDS系统,可增强抗多途干扰的能力。但当通信节点间存在缓慢运动,即信道缓慢时变时,在一帧信号中,信道均衡的效果会随时间逐渐降低,产生失配现象。提出了双向均衡方案,在没有信道先验知识的前提下,通过前向和后向信道均衡,充分利用探测信号和信道信息,提高了信道均衡效果。湖试结果证明了该方案的优越性。     

Information theoretic capacity of Gaussian cellular multiple-access MIMO fading channel

Higher spectral efficiency can be achieved by exploiting the space dimension inherent to any wireless communication system using multiple receiver and multiple transmitter antennas (MIMO). There are several results that provide closed form solutions for acellular system with a single antenna at each base station and each user terminal. Results are also available for the single cell case with MIMO. A cellular system with multiple antennas at the transmitter and the receiver nodes has not been investigated to obtain a closed form solution for the capacity limit. The main information theoretic theorems are not directly applicable to this system because of the form of the channel matrix of such a system. In this paper we extend the well known Wyner's model to a MIMO cellular system. It is observed that the achievable rate is bound by an upper limit and lower limit corresponding to two extreme fading conditions: channel with Rayleigh fading and with no fading. The analytical results are verified using Monte Carlo simulations. The analysis provides the insight that for a cellular system, increasing the number of transmitting antennas is not beneficial to increase the achievable rate, and this is reflected in the results obtained.     

多径瑞利衰落信道下AOAM系统最佳信噪比门限值的研究

自适应调制根据瞬时信道信噪比选择适当的调制方式，能够改善系统的BER性能以及平均吞吐量性能。在常用于高速移动环境的六径瑞利衰落信道模型下，研究自适应调制系统的最佳信噪比门限值，使系统的平均吞吐量最大，同时保证了目标BER性能，得到的最佳门限值可以用于ITU—RM．1225 Vehicular Test A模型的自适应调制系统。     

Two-dimensional binary halftoned optical intensity channels [optical wireless communications]

The capacity of two-dimensional (2D) optical intensity channels in which transmit images are constrained to be binary-level has been considered. Examples of such links exist in holographic storage, page-oriented memories, optical interconnects, 2D barcodes as well as multiple-input/multiple-output wireless optical links. Data are transmitted by sending a series of time-varying binary-level optical intensity images from transmitter to receiver. Neither strict spatial alignment between transmitter and receiver nor independence among the spatial channels is required. The approach combines spatial discrete multitone modulation developed for spatially frequency selective channels with halftoning to produce a binary-level output image. Data are modulated in spatial frequency domain as dictated by a water pouring spectrum over the optical transfer function as well as channel and quantisation noise. A binary-level output image is produced by exploiting the excess spatial bandwidth available at the transmitter to shape quantisation noise out of band. A general mathematical framework has been presented, in which such systems can be analysed and designed. In a pixelated wireless optical channel application, halftoning achieves 99.8% of the capacity of an equivalent unconstrained continuous amplitude channel using lmegapixel arrays.     

水声DS-CDMA通信中多相位假设码片速率自适应判决反馈均衡算法研究

研究了直接序列扩频码分复用(DS-CDMA)通信中基于多相位假设的码片速率自适应判决反馈均衡算法.水声信道的衰落和多普勒频移会严重影响扩频信号的相关特性,需要在做解扩频之前进行多普勒频移补偿和信道自适应均衡.将空间分集-多普勒频移补偿-自最佳自适应判决反馈均衡算法应用到DS-CDMA通信中,提出一种基于多相位假设的码片速率自适应判决反馈均衡算法,并用实际的千岛湖实验数据和仿真数据对算法性能进行了分析.结果表明算法以增加计算复杂性为代价显著提高了DS-CDMA通信性能,在复杂快变的多途和多普勒频移条件下可以保证低的误符号率,在低信噪比条件下能够稳定工作,总体性能良好.     

Eigenspace-based blind pattern optimisations of steerable antenna array for interference cancellation

A novel adaptive algorithm for an array using directional elements called a hybrid smart antenna system is proposed. The algorithm controls the element patterns on the basis of an objective function composed of eigenvalues of a covariance matrix. A high and stable array output signal-to-interference-plus-noise ratio is achieved by improving both the received powers and the spatial correlation coefficient between incident waves, without prior knowledge such as directions-of-arrival, channel state information or training signals. The characteristics of the proposed algorithm are theoretically and numerically clarified for a simple case involving two incident waves. Convergence with least mean squares algorithm is found to be as fast as that with recursive least squares algorithm in this system. Also, simulation for statistical performance evaluation is carried out in comparison with a conventional system. Furthermore, a method to implement the proposed eigenspace control algorithm without having to solve the eigenvalue problem is shown.     

Adaptive Fuzzy Strong Tracking Extended Kalman Filtering for GPS Navigation

The well-known extended Kalman filter (EKF) has been widely applied to the Global Positioning System (GPS) navigation processing. The adaptive algorithm has been one of the approaches to prevent the divergence problem of the EKF when precise knowledge on the system models are not available. One of the adaptive methods is called the strong tracking Kalman filter (STKF), which is essentially a nonlinear smoother algorithm that employs suboptimal multiple fading factors, in which the softening factors are involved. Traditional approach for selecting the softening factors heavily relies on personal experience or computer simulation. In order to resolve this shortcoming, a novel scheme called the adaptive fuzzy strong tracking Kalman filter (AFSTKF) is carried out. In the AFSTKF, the fuzzy logic reasoning system based on the Takagi-Sugeno (T-S) model is incorporated into the STKF. By monitoring the degree of divergence (DOD) parameters based on the innovation information, the fuzzy logic adaptive system (FLAS) is designed for dynamically adjusting the softening factor according to the change in vehicle dynamics. GPS navigation processing using the AFSTKF will be simulated to validate the effectiveness of the proposed strategy. The performance of the proposed scheme will be assessed and compared with those of conventional EKF and STKF