Robust progressive image transmission over OFDM systems using space-time block code

A joint source-channel coding (JSCC) scheme for robust progressive image transmission over broadband wireless channels using orthogonal frequency division multiplexing (OFDM) systems with spatial diversity is proposed for the application environments where no feedback channel is available such as broadcasting services. Most of current research about JSCC focuses on either binary symmetric channels (BSC) or additive white Gaussian noise (AWGN) channels. To deal with fading channels in most previous methods, the fading channel is modeled as two state Gilbert-Elliott channel model and the JSCC is normally aimed at the BER of bad channel status, which is not optimal when the channel is at good status. By using diversity techniques and OFDM, the frequency selective fading effects in broadband wireless channels can be significantly decreased and we show that subchannels in OFDM systems approach Gaussian noisy channels when the diversity gain gets large; as a result, the system performance can be improved in terms of throughput and channel coding efficiency. After analyzing the channel property of OFDM systems with spatial diversity, a practical JSCC scheme for OFDM systems is proposed. Simulation results are presented for transmit diversity with different numbers of antennas and different multipath delay and Doppler spread. It is observed from simulations that the performance can be improved more than 4 dB in terms of peak signal-to-noise ratio (PSNR) of the received image Lena and the performance is not very sensitive to different multipath spread and Doppler frequency.     

A high performance parallel Radon based OFDM transceiver design and simulation

A major goal of the next-generation wireless communication systems is the development of a reliable high-speed wireless communication system that supports high user mobility. They must focus on increasing the link throughput and the network capacity. In this paper a novel, spectral efficient system is proposed for generating and transmitting two-dimensional (2-D) orthogonal frequency division multiplexing (OFDM) symbols through 2-D inter-symbol interference (ISI) channel. Instead of conventional data mapping techniques, discrete finite Radon transform (FRAT) is used as a data mapping technique due to the increased orthogonality offered. As a result, the proposed structure gives a significant improvement in bit error rate (BER) performance. The new structure was tested and a comparison of performance for serial one-dimensional (1-D) Radon based OFDM and parallel 2-D Radon based OFDM is made under additive white Gaussian noise (AWGN), flat, and multi-path selective fading channels conditions. It is found that Radon based parallel 2-D OFDM has better speed and performance than serial 1-D Radon based OFDM.     

基于支持向量机的小波包调制信号识别

基于小波包调制信号和OFDM信号在分数阶傅里叶变换域分布的不同,利用图像成形技术对接收信号的分数阶傅里叶变换域分布图进行处理,探讨了小波包调制信号和OFDM信号的分数阶域分布成形图的特征参数,利用支持向量机做分类器,实现了这两种多载波调制信号的分类识别。计算机仿真结果,验证了算法的性能。     

采样频率偏差对QPSK OFDM系统误码率的影响分析

研究了采样频率偏差在加性高斯白噪声信道、瑞利平坦衰落信道和瑞利频率选择衰落信道下对QPSK OFDM系统误码率的影响。通过分析发现,采样频率偏差引起的OFDM系统子载波间的邻道干扰（ICI）不仅与子载波间的间隔有关,而且还与子载波的位置有关,其不能等效为载波频率偏差引起的ICI。通过对OFDM系统子载波间的ICI进行分析,求得上述三种信道条件下接收信号的特征函数;在基带调制方式为QPSK调制时得到了OFDM系统误码率的理论解析解。计算机仿真结果验证了该误码率理论解的正确性。     

基于辅助数据的OFDM时间同步算法

为了实现OFDM(Orthogonal Frequency Division Multiplexing,正交频分复用)系统在任意载波频偏下的高精度同步,本文提出了一种基于辅助数据的系统定时偏移估计方法.首先,在高斯白噪声信道下,基于不具有特殊结构的辅助数据,推导了最大似然准则下的最佳同步算法;然后针对最大似然法复杂度过高的问题,设计了计算复杂度降低的次优方法;最后,采用蒙特卡罗仿真,在频率选择性瑞利衰落信道下,对得到的新算法进行了评估.实验结果表明,新方法的定时性能显著优于传统算法.     

Performance of multi-processing gain QS-CDMA over AWGN and Rayleigh fading channels

In this paper, we analyze the bit error rate performance of multi-processing gain quasi-synchronous (QS) code division multiple access (CDMA). Analysis is carried out for deterministic spreading sequences over additive white Gaussian noise (AWGN) and frequency selective slowly Rayleigh fading channels. We also find the bit error rate performance of multi-processing gain QS-CDMA for random spreading sequences and compare to that of deterministic spreading sequences. Results show that deterministic Gold codes outperform random codes considerably and perform differently for different rates while random codes give almost the same performance for all rates.     

Performance of multiple access CSK communication system with DF cooperative strategy

A multiple access chaos shift keying cooperative communication (MA-CSK-CC) system over Rayleigh fading channels and in the presence of Gaussian white noise is investigated in depth in this paper. The simplified half-duplex with a single relay decode-and-forward (DF) cooperative strategy is proposed to illustrate and analysis as an example. Exact and approximation bit-error-ratio (BER) performance expressions are derived. Simulation results demonstrate that the proposed system has a prominent advantage of high BER performance over the conventional multiple access CSK systems that have a single path to transmit signal at the same data rate over Rayleigh fading channels with additive Gaussian white noise. Moreover, based on the BER performance analysis, some useful results are also derived such as the relationships between BER versus spread spectrum factors, BER versus the number of users, etc.     

基于IEEE802.11a无线标准的STC-OFDM系统性能研究

提出了使用正交频分复用(OFDM)技术减少码间干扰(ISI)来改善频率选择性衰落信道下空时编码(STC)性能的合理方案。在IEEE802.11a无线标准下对STC与OFDM技术相结合后的STC-OFDM系统进行了理论分析,通过仿真对系统性能进行了评估。仿真结果表明,空时分组编码(STBC)与OFDM技术的结合优于垂直的贝尔实验室分层空时方案(VBLAST)与OFDM技术的结合;FFT点数与子载波数对系统性能有一定的影响;随机交织器有助于显著改善多径衰落信道下系统性能,并能在各种信道上达到合理的鲁棒性。     

宽带MIMO信道下的并行干扰抵消接收机

基于数据块传输模型,将窄带MIMO并行干扰抵消接收机推广到宽带MIMO信道,在理想信道和信道估计下,通过仿真评估了所提出的宽带MIMO并行干扰抵消接收机的性能。仿真结果表明：所建议的方案在宽带MIMO信道下的性能好于窄带MIMO信道下的性能,在宽带MIMO信道下,接收机可获得比窄带下更大的分集增益。     

Control of Continuous-Time Linear Gaussian Systems Over Additive Gaussian Wireless Fading Channels: A Separation Principle

This note is concerned with the control of continuous-time linear Gaussian systems over additive white noise wireless fading channels subject to capacity constraints. Necessary and sufficient conditions are derived, for bounded asymptotic and asymptotic observability and stabilizability in the mean square sense, for controlling such systems. For the case of a noiseless time-invariant system controlled over a continuous-time additive white Gaussian noise channel, the sufficient condition for stabilizability and observability states that the capacity of the channel C must satisfy C > Sigma_{{i;Re(lambdai(A))ges0}} Re(lambda_i(A)), where A is the system matrix and lambda_i(A) denotes the eigenvalues of A. The necessary condition states that the channel capacity must satisfy C ges Sigma _{{i;Re(lambdai(A))ges0}} Re(lambda_i(A)). Further, it is shown that a separation principle holds between the design of the communication and the control subsystems, implying that the controller that would be optimal in the absence of the communication channel is also optimal for the problem of controlling the system over the communication channel.     

On the combined response of clipper‐amplifier cascade for PAPR reduction of OFDM signals

Orthogonal frequency division multiplexed (OFDM) signals have high peak‐to‐average power ratio (PAPR) which results in excessive nonlinear distortion when amplified by power amplifiers (PA) and hence, reduction in system signal‐to‐noise (SNR) and spectral efficiency. Clipping has been one of the most popular and low‐cost technique for reducing the PAPR of OFDM signals. The overall spectral efficiency of an OFDM system that involves clipping depends on the combined response of the cascade of clipper‐amplifier and not only on the response of the clipper. In this article, the performance of clipped and nonlinearly amplified OFDM signals is studied. The combined response of a clipper‐amplifier cascade is obtained using an orthogonalized behavioral model which enables in‐band and out‐of‐band nonlinear distortions to be accurately predicted without using the Gaussian assumption of signal statistics. Furthermore, different clipping characteristics are studied where it is shown that the overall nonlinear distortion, and hence the overall SNR, is highly dependent on the type of clipping characteristics and the clipping ratio. Simulation results are verified by Simulink measurement model which uses measured amplifier characteristics in a “General Amplifier” block. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:656–664, 2014.     

OFDM系统中基于因子图的信道估计算法

为提高正交频分复用(OFDM)系统的信道估计精度,根据频谱资源的无陑信道特性,提出基于因子图的 OFDM 系统信道估计算法,包括二维联合信道估计算法和2个级联的一维信道估计算法。将时变频率选择性衰落信道建模为一阶自回归模型,使信道参数之间的交互信息近似为高斯分布,利用和积算法实现OFDM系统的联合信道估计和符号检测。仿真结果表明,该信道估计算法能够以较低的计算复杂度逼近最优的估计性能。     

衰落信道下SCMA性能仿真研究

本文在一种上行SCMA（稀疏码分多址）系统模型的基础上,研究了衰落信道下SCMA通信系统的误码性能。首先比较了AWGN（高斯）信道以及衰落信道下SCMA和SC-FDMA（单载波频分多址）两种系统的性能;接着分析了快衰落信道、慢衰落信道、平坦衰落信道和频率选择性衰落信道下SCMA系统性能。仿真结果表明,与传统正交多址SC-FDMA相比,衰落信道下SCMA在用户连接数和误码性能上有一定优势。与AWGN信道相比,衰落信道使得SCMA系统误码性能迅速下降。在瑞利平坦衰落信道下,信道衰落的快慢变化导致SCMA系统误码性能最大产生约5dB损失;在频率选择性衰落信道下,多径时延扩散致使SCMA系统误码性能最差。     

一种基于MIMO预编码的多径分离方法

针对选择性MIMO信道多经分离问题,提出了一种基于预编码的收发联合设计方案。该方案中的每路发射矢量及其解码矩阵处于其他时延信道的共同零空间,把选择性信道多径分离为空间正交的若干平坦子信道,从而使平坦信道的预编码研究成果可以直接应用于选择性MIMO信道,解决了传统Z域处理中未消除ISI的问题,复杂度为Z域方法的L(多径数目)倍。仿真结果表明,该算法具有较好容量性能和误码率性能。     

New theoretical framework for OFDM/CDMA systems with peak-limited nonlinearities

A new theoretical framework for the evaluation of the in-band nonlinear distortion effects on the performance of OFDM systems is presented. In contrast to previous works that approximate the nonlinear noise as a Gaussian additive random process, the new framework is based on the properties of the large deviations of a stationary Gaussian process and shot noise theories, which can evaluate the performance of the OFDM system with high accuracy, especially at realistic scenarios where the Gaussian approximation of the nonlinear noise is no longer valid. The approach can be used to evaluate many communication systems with peak-limited nonlin-earities and high PAPR, such as the downlink performance analysis of large ca-pacity DS-CDMA systems.     

OFDM水声通信信道估计技术研究

水声信道是一种极其复杂多变的时—空—频变信道,其信道窄、强多径干扰、信号起伏衰落严重,一直是水下信息可靠高速传输的主要障碍。正交频分复用(OFDM)是近年来数字通信中流行的一种并行传输新技术,其核心思想是将整个可用频带分割成多个正交子信道,将待传输的高速串行码流并行地调制在这些子信道载波上。主要研究利用三种不同的导频图案对OFDM水声通信信道进行估计,并通过仿真的方式来分析不同导频图案下信道估计的性能。     

An adaptive signal-to-noise ratio estimator in mobile communication channels

Representing the reception condition directly, signal-to-noise ratio (SNR) is an important parameter in mobile propagation channels, and therefore is widely used in system performance evaluations and adaptive applications. Hence, this paper puts forward a frequency domain SNR estimator in mobile communications, where we exploit the signal model with the band-limited fading channel and the additive white Gaussian noise. With the above model, the noise power spectrum density can be estimated from the periodogram of channel-plus-noise signals, subsequently leading to our SNR estimation. Moreover, in order to degrade the intrinsic spectrum leakage of fast Fourier transform in periodogram calculation, the leakage expression is derived analytically and then an adaptive process is proposed to make a tradeoff between leakage reduction and noise smoothing. We verify our algorithm by simulations and observe high accuracy in a wide range of velocities and SNRs. Additionally, unlike the conventional work, the proposed estimator is not strictly based on the assumption of specific Doppler spectral shapes except for the requirement of the band-limited channel, hence it is robust to general mobile channels.     

频率选择性衰落信道下的同步参数联合盲估计算法

对于多径频率选择性衰落信道以及低信噪比环境下线性调制信号的同步参数盲估计问题,提出基于循环累积量的载波频偏、初始相偏和符号定时误差前向联合盲估计算法。通过理论推导得出多径频率选择性衰落信道下信号的循环累积量与初始相偏和符号定时误差的数学关系。在此基础上先以较大频率间隔进行粗估计确定频偏范围,再以较高精度遍历检测信号特定循环频率,提高载波频偏估计精度,进而由累积量值估计出初始相偏和符号定时误差,不依赖于信道衰落和加性噪声的分布特性,尤其适用于频偏、相偏、定时误差和信道衰落同时存在的复杂情况。仿真结果表明,该算法 能有效实现低信噪比和多径频率选择性衰落信道下对线性调制信号同步参数的联合盲估计。     

Performance analysis of digital FM demodulators for fading channels in the threshold region

The quasi-optimum digital FM demodulators for fading channels reported earlier reflected the system performance accurately under high signal to noise ratio conditions. In this paper, the prediction of the system performance for low SNR values is considered. A new set of error variance algorithms is developed from the filter algorithm assuming a Gaussian distribution for the state estimation errors and taking the effect of high frequency terms into consideration. Simulation analysis for an FM system with Rician fading channel shows that these algorithms predict the system performance accurately in the threshold region.     

Standard-independent I/Q imbalance estimation and compensation scheme inOFDM

Direct-conversion transceivers are gaining increasing attention due to their low power consumption. However, they suffer from a serious in- and quadrature-phase (I/Q) imbalance problem. The I/Q imbalance can severely limit the achievable operating signal-to-noise ratio (SNR) at the receiver and, consequently, the supported constellation sizes and data rates. In this paper, we first investigate the effects of I/Q imbalance on orthogonal frequency division multiplexing (OFDM) receivers, and then propose a new I/Q imbalance compensation scheme. In the proposed method, a new statistic, which is robust against channel distortion, is used to estimate the I/Q imbalance parameters, and then the I/Q imbalance is corrected in the frequency domain. Simulations are performed to verify the effectiveness of the proposed method for I/Q imbalance compensation. The results show that the proposed I/Q imbalance compensation method can achieve bit error rate (BER) performance close to that in the ideal case without I/Q imbalance in additive white Gaussian noise (AWGN) or multipath environments. Furthermore, because no pilot information is required, this method can be applied in various standard communication systems.