非相干能量检测UWB接收机误码率性能优化

 针对基于OOK(On-off Keying)调制的非相干能量检测UWB接收机误码率性能较差,本文通过二维参数优化,理论上证明了存在积分长度与有效信噪比的最佳参数组合使得误码性能最优,并通过训练序列来估计信噪比和搜索积分长度,在系统实现上给出了简单可靠的求解方法.其中,在系统误码率性能较高区域内,利用高斯分布近似最优判决门限精确解的方法存在较大误差,本文对该近似方法的适用性进行了深入的讨论,利用最小二乘方法对高斯近似方法进行了修正,并给出了易于工程应用的多项式系数.仿真结果表明,本文获得的最优参数组合相对于传统方法,较好地改善了接收机性能;对高斯分布近似的修正方法也有效的减小了误码率性能损失.     

非理想信道估计下迫零接收的空间相关多输入多输出空分复用系统的误码率分析

分析了非理想信道估计和发射空间相关信道下,采用迫零接收的多输入多输出空分复用无线通信系统的误码率性能.基于随机矩阵理论,导出了闭合形式的M-PSK和M-QAM误码率近似解析表达式,并通过仿真验证了表达式的有效性.所获得的表达式形式简单、便于数值计算,可以用于系统性能评估.     

一种适用于大气弱湍流信道的极化纠错编码调制方案

针对服从对数正态分布的大气弱湍流信道模型,基于高斯近似的方法计算分析信道的极化现象,提出了一种适用于大气弱湍流信道的极化编码调制方案,并对其性能进行了仿真分析.不同湍流强度下采用串行抵消译码算法时的仿真结果表明,相比于递归构造方案,文中所提方案在达到1e-5误码率时可获得大约1.0dB到1.4dB的编码增益提升.对极化码分别采用开关键控（OOK）调制与脉冲位置调制（PPM）时的仿真结果表明,采用2-PPM的极化编码调制方案可有效提升无线光通信链路在弱湍流条件下的译码纠错性能.     

多输入多输出衰落信道的最小互信息盲均衡

提出了多输入多输出衰落信道的基于广义高斯分布近似的最小互信息盲均衡器.采用输出信号的广义高斯分布近似,基于互信息最小化目标函数自适应调整均衡器的系数.比较了基于广义高斯分布近似和非线性变换的两种最小互信息盲均衡算法.仿真实验表明基于广义高斯分布近似的方法比非线性变换方法有更大的星座图距离,更快的收敛速率和更好的误码性能.     

双向多中继协同通信系统的性能分析

针对频率非选择性瑞利衰落信道,研究了双向多中继协同通信模型的系统性能。首先,基于模型的信号传输过程分析了系统的信道容量;然后,根据接收信噪比的累积分布函数推导了系统中断概率的闭合上界表达式;进一步在MPSK调制方式下,通过计算在高信噪比条件下的矩母函数,推导了平均误码率的闭合表达式。仿真结果验证了理论推导的正确性,并且表明双向多中继模型的中断概率和平均误码率两项性能指标都优于双向单中继模型。     

多业务蜂窝CDMA系统的干扰与容量分析

基于码分多址接入（CDMA）的第三代移动通信系统将支持语音、视频、email,FTP,Web浏览等多种业务传输，该文提出了一种多业务蜂窝CDMA系统上行链路的干扰特性与容量的一般分析方法，考察了在不同的业务混合条件下，分别采用高斯分布与对数正态分布来近似干扰的统计特性时，系统的中断性能与容量关系。理论分析与仿真结果表明，采用对数正态分布来近似干扰的统计特性可以获得更加精确的分析结果。此外，该文还研究了阴影衰落、功率控制误差和非理想扇区天线这些实际因素对系统干扰与业务容量的影响。     

短波宽带信道模型时延功率谱剖面拟合的简化

针对短波宽带信道模型仿真的难点,提出利用对数正态分布概率密度函数拟合模型中的时延功率谱剖面函数,并对对数正态分布、高斯分布与伽马分布的概率密度函数拟合精度进行了比较.仿真结果表明,对数正态分布比高斯分布的精度更高,且比伽马分布易于实现.     

分布式天线系统中的系统下行误符号率分析

首先建立了包含快衰落、阴影衰落和路径损耗的复合衰落信道模型,对分布式天线采用覆盖式（BT）传输策略,推导出给定移动台位置的小区下行点对点链路误符号率表达式.并在高信噪比条件下,通过给出移动台在小区的任意分布图,推导出小区下行平均误符号率的近似闭合表达式.仿真结果表明,所推导的近似表达式可很好地反应系统的实际性能.     

双伽马湍流信道下多阶脉冲位置调制误码率性能分析

基于双伽马(Gamma-Gamma)大气湍流信道,研究了采用脉冲位置调制(PPM)技术的无线光通信系统的误码率(BER)性能。推导出平均EBR的闭合表达式,该表达式适用于任意湍流强度和任意调制阶数,与数值积分相比,可显著提高计算速度。对于给定的平均EBR,对比分析了在不同湍流条件下PPM调制阶数与平均发射功率需求间的关系。结果表明,在无湍流高斯信道和弱湍流信道下,增加调制阶数可以减小平均发射功率需求;然而随着湍流的增强这种作用逐渐减小,在强湍流区,甚至出现了调制阶数越高,平均发射功率需求越大的现象。因此,在实际系统设计时,应考虑可能的湍流环境,再选择合适的调制阶数。     

协同通信系统中差分调制在Rayleigh衰落信道中的误码性能分析

根据源-中继-目的节点链路的等效信噪比模型,推导了检测传送和选择检测传送两种协议下目的节点瞬时信噪比的概率密度函数和特征函数,基于此推导了协同DMPSK调制近似误符号率的闭合表达式.推导结果不含积分表达式,方便计算,为系统设计和功率分配提供了理论基础.最后通过仿真验证了推导结果的正确性.     

无线光中继通信在弱湍流时的性能估计方法

对数正态信道是描述弱湍流条件下无线光通信衰落的有力数学工具.然而精确的对数正态信道性能表达式带有无法化简的多重积分,难以揭示信道参数和性能的联系,不利于性能估计.针对该问题,Wilkinson和Fenton近似被广泛应用于对数正态信道的性能分析,但Wilkinson和Fenton近似的应用范围却缺乏标准,这导致了系统设...     

Downlink System Capacity Analysis in Distributed Antenna Systems

This paper focuses on the downlink system capacity in distributed antenna systems (DAS). Due to the complexity of actual wireless environments, a composite channel model is established which takes into account three factors, i.e., path loss, lognormal shadowing and Rayleigh fading. Based on the channel model, the probability density function (PDF) of the output signal-to-noise ratio (SNR) is derived. To facilitate the analysis, the distribution of the output SNR is approximated by a lognormal distribution. After that, by making use of selective diversity (SD) scheme for distributed antennas, an approximate analytical expression of the capacity for a mobile station (MS) over a given position is derived. Furthermore, considering the distribution of MSs in the system, a closed-form expression of the system capacity is obtained. Numerical results show that the closed-form expression can evaluate the system capacity performance of DAS very accurately.     

Bounds on the Distribution of a Sum of Correlated Lognormal Random Variables and Their Application

The cumulative distribution function (cdf) of a sum of correlated or even independent lognormal random variables (RVs), which is of wide interest in wireless communications, remains unsolved despite long standing efforts. Several cdf approximations are thus widely used. This letter derives bounds for the cdf of a sum of 2 or 3 arbitrarily correlated lognormal RVs and of a sum of any number of equally-correlated lognormal RVs. The bounds are single-fold integrals of readily computable functions and extend previously known bounds for independent lognormal summands. An improved set of bounds are also derived which are expressed as 2-fold integrals. For correlated lognormal fading channels, new expressions are derived for the moments of the output SNR and amount of fading for maximal ratio combining (MRC), selection combining (SC) and equal gain combining (EGC) and outage probability expressions for SC.     

Performance of MRC receiver over Hoyt/lognormal composite fading channel

In this article, mixture gamma (MG) distribution is used for analysing the performance of L-Hoyt/lognormal composite fading channel. To overcome small-scale fading effect, micro-diversity using maximum ratio combining (MRC) is used at the receiver. Due to mathematical complexity, performance analysis of composite (L-Hoyt/lognormal) fading models is not present in closed form. The analytical expressions for the performance measure are derived in the form of received signal-to-noise ratio (SNR). The impact of system parameters on the energy detector (ED) performance is studied in terms of receiver operating characteristic (ROC) and area under the ROC curve (AUC). Further, the detection threshold parameter is optimised by minimising the total probability of error for L-Hoyt/lognormal channel. The accuracy of the proposed closed-form expressions is validated by comparing all the results with the Monte-Carlo/exact simulations.     

强度相干成像中符合计数方法的优化与仿真

为提高强度相干成像中符合计数方法的探测信噪比,实现对中高轨卫星的清晰成像。根据HBT效应和光电转换半经典模型建立了强度相干的符合计数简化模型,分析了设备时间测量误差、符合窗函数和观测光谱形状对符合计数探测信噪比的影响,并根据观测条件对符合窗函数进行了优化。利用蒙特卡洛方法仿真了符合窗函数对探测信噪比的影响。仿真结果表明:利用最优符合窗测量目标的频谱模值时,其探测信噪比较传统方法提高了39.2倍。通过对符合窗的优化能够有效提高强度相干成像探测信噪比,提高对暗弱目标的强度相干成像质量。     

Influence of smoothing window length on electromyogram amplitudeestimates

A systematic, experimental study of the influence of smoothing window length on the signal-to-noise ratio (SNR) of electromyogram (EMG) amplitude estimates is described. Surface EMG waveforms were sampled during nonfatiguing, constant-force, constant-angle contractions of the biceps or triceps muscles, over the range of 10%-75% maximum voluntary contraction. EMG amplitude estimates were computed with eight different EMG processor schemes using smoothing length durations spanning 2.45-500 ms. An SNR was computed from each amplitude estimate (deviations about the mean value of the estimate were considered as noise). Over these window lengths, average ± standard deviation SNR's ranged from 1.4±0.28 to 16.2±5.4 for unwhitened single-channel EMG processing and from 3.2±0.7 to 37.3±14.2 for whitened, multiple-channel EMG processing (results pooled across contraction level). It was found that SNR increased with window length in a square root fashion. The shape of this relationship was consistent with classic theoretical predictions, however none of the processors achieved the absolute performance level predicted by the theory. These results are useful in selecting the length of the smoothing window in traditional surface EMG studies. In addition, this study should contribute to the development of EMG processors which dynamically tune the smoothing window length when the EMG amplitude is time varying     

Window designs for DFT-based multicarrier systems

We consider window designs for discrete Fourier transform (DFT) based multicarrier transceivers without using extra cyclic prefix. As in previous works of window designs for DFT-based transceivers, a postprocessing matrix that is generally channel dependent, is needed to have a zero-forcing receiver. We show that postprocessing is channel independent if and only if the window itself has the cyclic-prefixed property. We design optimal windows with minimum spectral leakage subject to the cyclic-prefixed condition. Moreover, we analyze how postprocessing affects the signal-to-noise ratio (SNR) at the receiver, which is an aspect that is not considered in most of the earlier works. The resulting SNR can be given in a closed form. Join optimization of spectral leakage and SNR are also considered. Furthermore, examples demonstrate that we can have a significant reduction in spectral leakage at the cost of a small SNR loss. In addition to cyclic-prefixed systems, window designs for zero-padded DFT-based transceivers are considered. For the zero-padded transceivers, windows that minimize spectral leakage can also be designed.     

Comments on "Is accurate recording of the ECG surface Laplacian feasible?"

The authors comment on the work of D.B. Geselowitz and J. Ferrara (ibid., vol. 47, p. 1138, 2000) who concluded that the signal-to-noise ratio (SNR) of the ECG surface Laplacian (SL) is very low. Averaged over all recording channels on the anterolateral chest and averaged over all six subjects, when the noise level is estimated during the window of the P-R segment, our experimental results indicated that the potential ECG, regular Laplacian ECG, and diagonal Laplacian ECG have SNR of 105.70±22.49, 33.73±9.86, and 46.83±12.84, respectively. This is in contrast with the magnitude of SNR reported in the text of the above paper, which was too low becaase of their choice of a residue window containing noise as well as signal. Geselowitz and Ferrara reply to these comments     

融合失配处理和LMS滤波的雷达通信一体化OFDM信号距离旁瓣抑制技术

随着5G乃至未来6G无线通信技术的发展,无线通信设备数量呈现爆炸式增长趋势。与之矛盾的是,电磁频谱环境日趋拥堵,接近枯竭的传统通信频段已无法满足激增的业务需求。在此背景下,面向雷达与通信的频谱共享的一体化信号引起了工业界和学术界的极大关注。然而,在匹配滤波框架下,一体化信号无法兼顾雷达和通信性能。通信信息势必会在雷达模糊函数中产生高旁瓣和伪峰。为此,部分学者基于正交频分复用（Orthogonal Frequency Division Multiplexing, OFDM）共享信号,提出将高旁瓣和伪峰外推至雷达观测窗口外的失配处理方法,用以兼顾雷达模糊性能。然而,该方法会产生信噪比损失,且信噪比损失随观测窗口增大而增大。鉴于此,本文提出融合失配处理和最小均方（Least Mean Square, LMS）滤波的算法。通过LMS和失配处理的深度融合,可突破信噪比损失与观测窗口宽度之间的约束,进而能在不减小观测范围的条件下降低信噪比损失,或在相同信噪比损失下大幅提升观测范围。      

A Nakagami-N-gamma Model for Shadowed Fading Channels

Wireless channels are subject to short term fading and shadowing. Such shadowed fading channels are described using a Nakagami-lognormal process, with the Nakagami-m (short term fading) and lognormal distributions (shadowing). This approach does not result in a closed form solution for the density function of the signal-to-noise ratio (SNR) making wireless systems analysis difficult. It was suggested that a gamma or an inverse Gaussian distribution can be used in place of the lognormal distribution providing an analytical framework. The match of these two distributions to the lognormal was less than ideal. Invoking shadowing as multiplicative process, the distribution of the product of N gamma variables is proposed in place of the lognormal pdf resulting in the Nakagami-N-gamma model. It is shown that this model leads to simple solutions to the density and distribution functions as well as error rates for coherent phase shift keying modems. The outage probabilities and error rates based on the Nakagami-lognormal (NL) and Nakagami-N-gamma (NNG) models were compared. Results showed excellent match at levels of shadowing generally observed in wireless systems. While values of N as low as 3 was sufficient for low values of m and weak to moderate shadowing, values of N in the range of 7–9 provided better match for higher levels of shadowing and higher values of m. By varying N, it is also possible to get the NNG pdf to move closer to the NL pdf making the new model an ideal one for the shadowed fading channels with its flexibility and availability of analytical expressions.