A novel low‐complexity transmission power adaptation in MC‐CDMA systems with a‐MRC receiver over Nakagami‐m fading channels

In this paper, we propose a novel low‐complexity transmission power adaptation with good bit error rate (BER) performance for multicarrier code‐division multiple‐access (MC‐CDMA) systems over Nakagami‐m fading channels. We first propose a new receiver called ath‐order‐maximal‐ratio‐combining (a‐MRC) receiver with which the receiver power gain for the nth subcarrier is the ath (a?1) power of the corresponding channel gain. Incorporating the a‐MRC receiver, we then propose a new transmission power adaptation scheme where the transmission power is allocated over all the N subcarriers according to the subchannel gains and the transmitter adapts its power to maintain a constant signal‐to‐interference‐plus‐noise (SINR) at the receiver. The proposed scheme has a significant performance gain over the nonadaptive transmission scheme over both independent and correlated fading channels. Moreover, the proposed scheme keeps good BER performance while it is much simpler than the previous power control/adaptation schemes. Copyright © 2008 John Wiley & Sons, Ltd.     

Order statistics and random matrix theory of multicarrier continuous‐variable quantum key distribution

In a multicarrier continuous‐variable quantum key distribution (CVQKD) protocol, the information is granulated into Gaussian subcarrier CVs and the physical Gaussian link is divided into Gaussian sub‐channels. Here, we propose a combined mathematical framework of order statistics and random matrix theory for multicarrier continuous‐variable quantum key distribution. The analysis covers the study of the distribution of the sub‐channel transmittance coefficients in the presence of a Gaussian noise and the utilization of the moment generation function (MGF) in the error analysis. We reveal the mathematical formalism of sub‐channel selection and formulation of the transmittance coefficients and show a reduced complexity progressive sub‐channel scanning method. We define a framework to evaluate the statistical properties of the information flowing processes in multicarrier CVQKD protocols. Using random matrix theory, we express the achievable secret key rates and study the efficiency of the adaptive multicarrier quadrature division‐multiuser quadrature allocation (AMQD‐MQA) multiple‐access multicarrier CVQKD. The proposed combined framework is particularly convenient for the characterization of the physical processes of experimental multicarrier CVQKD.     

Improved auxiliary particle filter‐based synchronization of chaotic Colpitts circuit and its application to secure communication

In this paper, we propose a synchronization scheme based on an improved auxiliary particle filter (IAPF) for chaotic Colpitts circuit and conduct an experimental study on the synchronization performance with application to secure communications. Specifically, with the synchronization scheme, when the chaotic signals generated by an analog Colpitts circuit are transmitted through a nonideal channel, the distorted signals are processed digitally by the novelly designed IAPF at the receiver, in order to obtain the synchronized signals of the transmitter circuit. Experimental results indicate that synchronization can be achieved over both the additive white Gaussian noise channel and the multipath fading channel with low signal‐to‐noise ratio, even if there exist severe circuit parameter mismatches between the transmitter and the receiver. Furthermore, a chaos‐masking secure communication system is constructed and verified over both the additive white Gaussian noise channel and the multipath fading channel, and the bit error rate is evaluated versus different signal‐to‐noise ratios and symbol periods. It is shown that the achievable bit error rate can reach the order of magnitude of 10 ^{− 4} without error correction coding techniques. In addition, security analysis demonstrates that the proposed chaotic secure communication system is resistant to the brute‐force attack. Copyright © 2013 John Wiley & Sons, Ltd.     

Sequence detection and adaptive channel estimation for ISI channels under class-a impulsive noise

In this paper, sequence detection and channel estimation for frequency-selective, intersymbol interference (ISI)-producing channels under Class-A impulsive noise are considered. We introduce a novel suboptimum sequence detection (SSD) scheme and show that although SSD employs a simplified metric, it achieves practically the same performance as maximum-likelihood sequence detection (MLSD). For both SSD and MLSD, a lower bound on the achievable performance is derived, which is similar to the classical matched-filter bound for frequency-selective (fading) channels under Gaussian noise. For channel estimation, we adopt a minimum entropy criterion and derive efficient least-mean-entropy and recursive least-entropy algorithms. For both adaptive algorithms, we analyze the steady-state channel-estimation error variance. Theoretical considerations and simulation results show that in Class-A impulsive noise, the proposed sequence detection and adaptive channel-estimation schemes yield significant performance gains over their respective conventional counterparts (designed for Gaussian noise). Although the novel algorithms require knowledge of the Class-A noise-model parameters, their computational complexity is comparable to that of the corresponding conventional algorithms.     

Generalized frequency division multiplexing–based acoustic communication for underwater systems

The underwater (UW) acoustic channel poses multiple challenges like coloured ambient noise, frequency‐dependent attenuation, and doubly selective fading. The availability of a robust underwater communication mechanism can largely enhance the success of human effort in a multitude of applications, ranging from pollution surveillance to defence and search/rescue operations. In this work, generalized frequency division multiplexing (GFDM), a non‐orthogonal multicarrier scheme, which has recently been studied for terrestrial wireless fading channels, is developed and tested for signalling in UW acoustic communication. UW noise, attenuation, and doubly selective fading channels are modelled with appropriate statistics. The BER performance of proposed system is systematically evaluated under different channel conditions, starting from simple additive white Gaussian noise (AWGN) and Rayleigh fading channels to a horizontally configured UW channel. The performance is also compared with contemporary orthogonal frequency‐division multiplexing (OFDM)– and filter bank multicarrier (FBMC)–based systems.     

Performance Comparison of M-QAM Communications for (S + N) and (S/N) Channel State Estimation Schemes

Channel estimation at the receiver side is essential for adaptive modulation schemes, prohibiting low complexity systems from using variable rate (VR) and/or variable power transmissions. This problem can be solved using variable-rate M-QAM modulation scheme for communications over fading channels in the absence of channel gain estimation at the receiver. It is shown that signal plus noise (S + N) sampling value can serve as a much better criterion compared to signal-to-noise ratio (S/N) for determining modulation order in VR systems. In this way, low complexity transceivers use VR transmissions to improve spectrum efficiency under an error performance constraint. Two kinds of fading channels: Weibull fading and α–μ fading are considered. Spectrum efficiency of (S + N) based systems are compared to that of S/N systems and the advantage of (S + N) scheme over (S/N) scheme is shown. The symbol error rates of two schemes are also studied. As an application, the proposed VR modulation scheme is shown to work with a maximum ratio combining diversity receiver.     

Minimax Noncoherent Detection of BFSK Signals in Nonwhite Gaussian Noise

We derive an optimum receiver for noncoherent detection of binary frequency shift keyed (BFSK) signals in nonwhite Gaussian noise. It will be shown that the achieved symbol error rates are smaller when operating on a nonwhite Gaussian noise channel than on a white Gaussian noise channel. The well-known BFSK receiver designed for white noise proves to be a minimax receiver when used in nonwhite noise. We show that the symbol error probability of the minimax receiver does not depend on the noise characteristics.     

Performance analysis of multicarrier frequency-hopping (MC-FH) code-division multiple-access systems: uncoded and coded schemes

In this paper, we provide multiuser performance analysis of a multicarrier frequency-hopping (MC-FH) code-division multiple-access system as first introduced in the work of Lance and Kaleh. We propose to use a practical low-rate convolutional error-correcting code in this system, which does not require any additional bandwidth than what is needed by the frequency-hopping spread-spectrum modulation. We provide multiuser exact performance analysis of the system for both uncoded and coded schemes in additive white Gaussian noise and fading channels for a single-user correlator receiver. We also derive the performance analysis of the system based on a Gaussian distribution assumption for multiuser interference at the receiver output. Our numerical results first indicate that the coded scheme significantly increases the number of users supported by the system at a fixed bit error rate, in comparison with the uncoded MC-FH scheme. Moreover, it shows that the Gaussian analysis in some cases does not accurately predict the number of users supported by the system.     

Subsymbol equalization for discrete multitone systems

In conventional discrete multitone (DMT) systems, a multitap time-domain equalizer (TEQ) is used to shorten the channel length, so that the bandwidth efficiency reduction due to cyclic extension is relieved. The TEQ, however, tends to introduce spectral s which degrade the achievable signal-to-noise ratio at corresponding subchannels, thereby decreasing the bandwidth efficiency. Furthermore, computationally expensive joint TEQ initialization and optimal delay (introduced by TEQ) selection is necessary. In this paper, a novel subsymbol-equalization scheme is proposed, and is based on the observation that the high-bit-rate twisted-pair channels rarely contain a zero that is close to the unit circle. Although a delay of a fraction of a DMT symbol period is introduced, the proposed subsymbol-equalization scheme eliminates the necessity of both the channel shortening at the receiver and the cyclic extension at the transmitter. Simulation results demonstrate the computational efficiency (no TEQ) and the bandwidth efficiency of the proposed subsymbol-equalization scheme.     

Bit error performance evaluation of double-differential QPSK infaded channels characterized by Gaussian plus impulsive noise andDoppler effects

A Fourier-Bessel series-based analysis that allows real-time symbol-to-symbol error performance evaluation for mobile radio direct-sequence code division multiple-access (DS-CDMA) systems is described. The technique analyzes mobile radio communication aeronautic links and generates average bit error rate (BER) and bit-to-bit patterns. Various communication systems parameters (e.g., modulation scheme, data rate, signal-to-noise ratio, and receive speed) as well as multiaccess interference (MAI) environments (i.e., Rayleigh-faded channels with Doppler frequency shift) may be specified and permit performance comparison. Additive white Gaussian (AWG) and impulsive ϵ-mixture type of noise are also considered to be present for the asynchronous and quasi-synchronous DS-CDMA configurations, with both schemes characterized by the incorporation of double-differential QPSK (DDQPSK) modulation. The obtained results demonstrate that the proposed combination in terms of multiplexing and digital modulation may be a suitable alternative solution for air-to-air and/or air-to-ground aeronautic communications for optimized BER versus signal-to-noise ratio system performance and, hence, for “best” channel capacity versus receiver complexity     

基于稀疏贝叶斯学习的MIMO-OFDM电力线通信系统接收机设计

丰富的脉冲噪声干扰对基于MIMO-OFDM技术的电力线通信系统接收机设计带来了巨大挑战。针对这个问题,提出了一种联合估计电力线信道和脉冲噪声的接收机设计方案。该方案主要利用电力信道多径模型参数在频域上的稀疏性和脉冲噪声在时域上的稀疏性特征,将待估计信道模型参数和脉冲噪声联合视作一个稀疏向量,同时利用MIMO系统的空间相关性,构建了一个基于多测量向量的压缩感知模型,并引入多测量向量稀疏贝叶斯学习理论,设计了一种联合估计MIMO信道模型参数和脉冲噪声的方法。仿真结果表明,与传统的MIMO信道估计与脉冲噪声抑制相互分离的接收机方案相比,新方法在估计性能和误比特率性能上有明显提升。     

相位调制的多用户OFDM-FH通信系统性能分析

 本文研究一种高效的通信方式,即相位调制的OFDM-FH通信系统。为了提高衰落信道下系统误码率性能,接收端采用最大比合并的分集接收技术。利用特征函数方法,推导该系统在加性高斯白噪声信道和瑞利衰落信道时系统误码率性能。给出误比特率与跳频频点数 ,OFDM子载波数N和用户数 等参数之间的关系。理论分析和仿真实验结果表明,在相同参数条件下,基于相位键控调制的OFDM-FH多址接入系统与其它OFDM-FH通信系统比较,具有系统频带宽度占用少、误比特率低等优势。     

Blind turbo equalization in Gaussian and impulsive noise

We consider the problem of simultaneous parameter estimation and restoration of finite-alphabet symbols that are blurred by an unknown linear intersymbol interference (ISI) channel and contaminated by additive Gaussian or non-Gaussian white noise with unknown parameters. Non-Gaussian noise is found in many wireless channels due to the impulsive phenomena of radio-frequency interference. Bayesian inference of all unknown quantities is made from the blurred and noisy observations. The Gibbs sampler, a Markov chain Monte Carlo procedure, is employed to calculate the Bayesian estimates. The basic idea is to generate ergodic random samples from the joint posterior distribution of all unknowns and then to average the appropriate samples to obtain the estimates of the unknown quantities. Blind Bayesian equalizers based on the Gibbs sampler are derived for both Gaussian ISI channel and impulsive ISI channel. A salient feature of the proposed blind Bayesian equalizers is that they can incorporate the a priori symbol probabilities, and they produce as output the a posteriori symbol probabilities. (That is, they are “soft-input soft-output” algorithms.) Hence, these methods are well suited for iterative processing in a coded system, which allows the blind Bayesian equalizer to refine its processing based on the information from the decoding stage and vice versa-a receiver structure termed as blind turbo equalizer     

Single-block differential transmit scheme for broadband wireless MIMO-OFDM systems

In frequency-selective multiple-input multiple-output (MIMO) channel, differential space-time-frequency (DSTF) modulations are known as practical alternatives that are capable of exploiting the available spatial and frequency diversities without the requirement of multichannel estimation at the receiver. However, the encoding nature of the DSTF schemes that expand several OFDM symbol periods makes the DSTF schemes susceptible to fast-changing channel conditions. In this paper, we propose a differential scheme for MIMO-OFDM systems that is able to differentially encode signal within two OFDM symbol periods, and the proposed scheme transmits the differentially encoded signal within one OFDM block. The scheme not only reduces encoding and decoding delay but also relaxes the restriction on channel assumption. The successful differential decoding of the proposed scheme depends on the assumption that the fading channels keep constant over two OFDM symbol periods rather than multiple of them as required in the existing DSTF schemes. We also provide pairwise error probability analysis and quantify the performance criteria in terms of diversity and coding advantages. The design criteria reveal that the existing diagonal cyclic codes can be applied to achieve full diversity. Performance simulations under various channel conditions show that our proposed scheme yields superior performance to previously proposed differential schemes.     

On reducing the rate of retransmission in time-varying channels

For data communications in time-varying channels such as wireless channels, the dynamic channel fluctuations often cause high frame-error rates. When the link layer detects that a frame is in error, conventionally, the frame is dropped and retransmission of the frame is requested. Based on the fact that the erroneous frames still contain useful information, several schemes have been proposed, such as packet combining and incremental redundancy, which retain and utilize the erroneous frames to improve retransmission performance. In this paper, we address two questions: 1) how much information is still useful in the erroneous frame; and 2) how to design a retransmission scheme to make efficient use of such information. We model this scenario (retransmission with an erroneous frame available at the receiver) as communication with side information at the receiver, and for a class of time-varying channels, the compound block interference channels, we derive with information-theoretic arguments the minimum information rate sufficient for retransmission to recover the erroneous frame. Motivated by the theoretical results, we propose an embedded channel coding/modulation structure together with a rate-adaptive retransmission scheme. Performance results indicate significant improvements over existing retransmission schemes in both additive white Gaussian noise and quasi-static Rayleigh fading channels.     

Optimization of Linearly Equalized QAM

A QAM signal transmitted over a channel with linear distortion and additive white Gaussian noise can be linearly equalized at the receiver to eliminate intersymbol interference. If the QAM signal is power constrained and a given symbol error rate is required, we show that it is possible to maximize the bit rate of this system by optimizing the symbol rate and the number of bits/symbol. Ideal linear equalization is assumed at the receiver to overcome the distortion introduced by the channel. As an example, a Gaussian channel is chosen, and the bit rate is maximized for this channel. The QAM maximization is especially useful for channels with slowly decaying channel attenuation characteristics, e.g., the twisted-pair channel.     

Analysis of a multicarrier DS-CDMA code-acquisition system

We present two code-acquisition schemes for a multicarrier direct-sequence code-division multiple-access system, one that uses equal gain combining and the other that uses selection combining. The code-acquisition performance of the two multicarrier systems, as well as that of a single-carrier system, are analyzed in both nonfading and Rayleigh fading channels under the assumption that the receiver is chip-synchronized; the effect of partial-band interference (PBI) on the performance is also included. It is demonstrated that in an additive white Gaussian noise channel, the single-carrier system has a better code-acquisition performance than both multicarrier systems. However, in a Rayleigh fading channel, the code-acquisition performance of a multicarrier system with equal gain combining is better than that of the single-carrier system, while a multicarrier system with selection combining has the same performance as the single-carrier system. Further, the presence of PBI more severely affects the code-acquisition performance of the single-carrier system than those of both multicarrier systems. Finally, the code-acquisition performance of a multicarrier system with equal gain combining is always better than that of the selection combining system     

一种基于不等纠错保护的图像传输方法

对噪声信道上的图像传输方法进行了研究,提出了一种新的基于不等纠错保护的图像传输方法,该方法在编码端利用纠错算术码对SPIHT码流进行不等纠错保护,根据SPIHT码流各个不同重要程度的部分采用不同禁用区间的纠错算术码进行不同程度的差错保护,相比传统的基于不等纠错保护图像传输方法而言,可获得近似连续可变的编码码率;在解码端,采用堆栈序列估计算法进行信道估计后再进行SPIHT解码,重建图像.实验结果表明,与经典的Guionnet不等纠错保护传输方法以及分离编码传输方法相比,所提出的传输方法具有较为明显的性能增益.     

Symbol error probability evaluation for PSK modulation schemes in generalized non‐Gaussian noise channels

The average symbol error probability (ASEP) performance over the faded radio frequency (RF) link when the noise is additive in nature and has generalized Laplacian distribution is evaluated in this paper. The additive white generalized Laplacian noise (AWGLN) distribution can model different impulsive and non‐Gaussian noise environments often encountered in practice and provides a robust alternative to Gaussian distribution. A new expression for evaluating the exact symbol error probability over a multilevel M‐ary PSK‐modulated AWGLN channel is derived. Based on the obtained expression, the ASEP for the single‐hop RF link that models the shadowing and fading conditions over the RF channel by a generalized –K (GK) distribution is derived. Further, the error performance of a decode‐and‐forward relayed and GK‐distributed two‐hop RF link is discussed, and the results are validated through numerical plots.     

Per‐antenna power minimization in symbol‐level precoding for the multibeam satellite downlink

This paper addresses the problem of multiuser interference in the forward downlink channel of a multibeam satellite system. A symbol‐level precoding scheme is considered, to exploit the multiuser interference and transform it into useful power at the receiver side, through a joint utilization of the data information and the channel state information. In this context, a per‐antenna power minimization scheme is proposed, under quality‐of‐service constraints, for multilevel modulation schemes. The consideration of the power limitations individually for each transmitting radio frequency chain is a central aspect of this work, and it allows to deal with systems using separate per‐antenna amplifiers. Moreover, this feature is also particularly relevant for systems suffering nonlinear effects of the channel. This is the case of satellite systems, where the nonlinear amplifiers should be properly driven to reduce the detrimental saturation effect. In the proposed scheme, the transmitted signals are designed to reduce the power peaks, while guaranteeing some specific target signal‐to‐noise ratios at the receivers. Numerical results are presented to show the effectiveness of the proposed scheme, which is compared both with the state of the art in symbol‐level precoding and with the conventional minimum mean square error precoding approach.