相关阴影Rician信道上广义矩形MQAM的性能

本文使用矩生成函数方法推导了相关视距(LOS)分量和独立散射分量条件下的多输入多输出阴影Rician衰落信道上采用正交空时分组编码(OSTBC)的广义矩形M进制正交幅度调制(MQAM)的平均误符号率(SEP)的精确闭合表达式.利用该表达式可计算信道衰落参数以及天线间的相关性对广义矩形MQAM平均SEP性能的影响.数值计算结果阐明,天线间的相关性恶化了广义矩形MQAM的平均SEP性能,广义矩形MQAM的平均SEP性能随着信道衰落参数的增大而得到改善.     

Performance Analysis of System with Micro- and Macrodiversity Reception in Correlated Gamma Shadowed Rician Fading Channels

In this paper, the performance of wireless system employing microdiversity to mitigate the effects of short-term fading and macrodiversity to reduce long-term fading (shadowing) effects is studied. The system model assumes implementation of maximal-ratio combining (MRC) at the microlevel and selection combining (SC) at the macrolevel. The received signal envelope follows a Rician distribution and it also suffers gamma shadowing. Novel expressions for the probability density function (PDF), cumulative distribution function (CDF), and moment-generating function (MGF) of the output signal-to-noise ratio (SNR) are obtained. Several useful performance criteria, such as the moments of the output SNR and outage probability are analytically derived. Moreover, the average bit error probability (ABEP) for noncoherent binary differential phase-shift keying (BDPSK) is calculated using the MGF based approach while the ABEP for coherent binary phase-shift keying (BPSK) is studied by averaging the conditional bit error probability over the PDF. Numerical results are graphically presented to show the effects of various system parameters to the system performance, as well as the enhancement due to use of the combination of micro- and macrodiversity. Some of numerical results are complemented by equivalent computer simulated results which validate the accuracy of the proposed analysis. The agreement between the Rician-gamma and Rician-lognormal fading model is also established.     

Error Rates in Generalized Shadowed Fading Channels

Most of the existing models to describe the shadowed fading channels use either the Suzuki or Nakagami-lognormal probability density function (pdf), both based on lognormal shadowing. However, these two density functions do not lead to closed form solutions for the received signal power, making the computations of error rates and outages very cumbersome. A generalized or compound fading model which takes into account both fading and shadowing in wireless systems, is presented here. Starting with the Nakagami model for fading, shadowing is incorporated using a gamma distribution for the average power in the Nakagami fading model. This compound pdf developed here based on a gamma-gamma distribution is analytically simpler than the two pdfs based on lognormal shadowing and is general enough to incorporate most of the fading and shadowing observed in wireless channels. The performance of coherent BPSK is evaluated using this compound fading model.     

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.     

Effect of Microdiversity and Macrodiversity on Average Bit Error Probability in Gamma‐Shadowed Rician Fading Channels

In this letter, we analyze the error performance of a mobile communication system with microdiversity and macrodiversity reception in gamma‐shadowed Rician fading channels for a binary differential phase‐shift keying modulation scheme. Analytical expressions for the probability density function (PDF) and moment‐generating function (MGF) are derived. The average bit error probability can be calculated by averaging the conditional bit error probability over the PDF or using the MGF‐based approach. Numerical results are graphically presented to show the effects of macrodiversity, correlation, number of diversity branches, and severity of both fading and shadowing.     

Data-Aided Fading Estimation Technique for Shadowed Mobile Satellite Fading Channels

A novel data-aided fading estimation technique that employs both pilot anddata symbols is proposed to significantly reduce the bandwidth redundancy ofthe pilot-symbol-aided (PSA) systems using receivers with low complexity andlatency in the shadowed mobile satellite fading channels. The shadowed mobilesatellite fading channels are modeled as the sum of a lognormally distributeddirect component and a Rayleigh distributed multipath component, and the PSAsystem employs 16-ary quadrature-amplitude-modulation (16QAM) fortransmission. Monte Carlo computer simulation has been used to assess thetechnique on the bit-error-rate (BER) performances of the system in the lightshadowed, the average shadowed and the heavy shadowed Rician fadingenvironments. The results have shown that the proposed technique requires avery low bandwidth redundancy to provide satisfactory BER performances, andcan substantially lower the error floors of the PSA systems.     

Sensing Coverage for Wireless Sensor Networks in Shadowed Rician Fading Environments

Wireless Personal Communications - The effect of shadowing and multipath fading on sensing coverage has been studied in this paper. Rician fading is one of the most widely occurring type of...     

Macrodiversity and Microdiversity in Correlated Shadowed Fading Channels

Outage probabilities in shadowed fading channels are evaluated when macrodiversity and microdiversity techniques are implemented. The existence of correlation at the micro and macro levels is taken into account for the computation of outage probabilities. By modeling shadowing using a gamma probability density function, it is shown that an analytical expression for the outage can be expressed as the product of marginal outages summed with weighting factors that depend on the correlation coefficients at the macro level, assuming a maximal ratio combining at the micro level and selection combining (SC) diversity at the macro level. Results demonstrate the potential use of the approach in the analyses of wireless systems in shadowed fading channels.      

Capacity of MRC on Correlated Rician Fading Channels

A new exact explicit expression is derived for the ergodic capacity of maximal ratio combining (MRC) schemes over arbitrarily correlated Rician fading channels. This is used to study the effects of channel correlation on the ergodic capacity. Numerical results reveal that both the phase and the magnitude of correlation have an impact on the ergodic capacity of Rician fading channels. This is in contrast to correlated Rayleigh fading, where the phase of the correlation has no effect on the ergodic capacity. It is also observed that negatively correlated branches in Rician fading may lead to an increase in ergodic capacity beyond that obtained by uncorrelated branches.     

Second‐Order Statistics of System with Microdiversity and Macrodiversity Reception in Gamma‐Shadowed Rician Fading Channels

In this letter, a wireless communication system with microdiversity and macrodiversity reception in gamma‐shadowed Rician fading channels is considered. Exact and rapidly converging infinite‐series expressions for the average level crossing rate and average fade duration at the output of the system are provided. Numerical results are presented graphically to illustrate the proposed mathematical analysis and to examine the effects of the system's parameters on the quantities considered.     

Performance Analysis of Diversity Combining Algorithms in Shadowed Fading Channels

A compound fading model incorporating short term fading and shadowing proposed recently is used to analyze the performance of wireless systems employing microscopic diversity to mitigate the effects of flat fading. This model can account for the presence of different levels of fading and shadowing and provide an analytical solution for the probability density function of the signal-to-noise ratio. Using that model, the performances of MRC and SC diversity combining algorithms were studied. The amount fading (AF) following diversity implementation was calculated and it is seen that the decline in the amount of fading is bound by the level of shadowing present, with the MRC providing a larger decrease in the amount of fading than the SC algorithm. The effect on the error rates was studied using the example of the coherent BPSK modem. Results show that the performances of wireless systems can be analyzed using the compound model for the shadowed fading channels. P.M. Shankar received his M. Sc (1972) in Physics from Kerala University, India, M. Tech (1975) in Applied Optics and Ph. D. in Electrical Engineering (1980) from Indian Institute of Technology, Delhi, India. He was a visiting scholar at the School of Electrical Engineering, University of Sydney, Australia, from 1981 to 1982. He joined Drexel University in 1982 and is currently the Allen Rothwarf Professor of Electrical and Computer Engineering. He is the author of the textbook ‘Introduction to Wireless Systems’, published by John Wiley & Sons, 2002. His research interests are in Fading Channels, Wireless communications, and Statistical signal processing for medical applications.     

Maximal Ratio Combining (MRC) in Shadowed Fading Channels in Presence of Shadowed Fading Cochannel Interference (CCI)

Simultaneous existence of multipath fading and shadowing leads to worsening conditions in wireless channels. This is further compounded by the interference from other base stations operating at the same frequency. The effect of this cochannel interference (CCI) and shadowed fading in error rates is studied when maximal ratio combining is used to mitigate short term fading. The CCI channels were also treated as undergoing shadowed fading. The generalized K distribution was used to model the signal-to-noise ratio of composite shadowed fading channel. The probability density functions of the signal-to-noise ratio taking into account the presence of multipath fading, shadowing and CCI were derived and used for the estimation of error rates. Results demonstrated the existence of degradation in the channel manifested as increased error rates and higher error floors. The improvements in the channel obtained through diversity were also demonstrated. The approach presented here can be easily adapted to the analysis of other diversity schemes in shadowed fading channels.     

Chaos-Coded Modulations Over Rician and Rayleigh Flat Fading Channels

In this brief, we analyze a kind of chaos-coded modulations over both Rician and Rayleigh frequency non-selective uncorrelated fading in the presence of additive white Gaussian noise. We provide bounds both for the case when perfect channel-state information (CSI) is available at the decoder and when there is no CSI. We show that the bounds proposed can be tight enough to give reason of the behavior of these systems in a flat fading channel. We compare the results with a related trellis-coded modulation and show that the degradation in performance can be at least as low as with conventional coded modulation systems.     

Differential Space-Time Modulation Using DAPSK Over Rician Fading Channels

This paper studies differential space-time modulation using diversity-encoded differential amplitude and phase shift keying (DAPSK) for the multiple-input multiple-output (MIMO) system over independent but not identically distributed (inid) time-correlated Rician fading channels. An asymptotic maximum likelihood (AML) receiver is developed for differentially detecting diversity-encoded DAPSK symbol signals by operating on two consecutive received symbol blocks sequentially. Based on Beaulieu’s convergent series, the bit error probability (BEP) upper bound is analyzed for the AML receiver over inid time-correlated Rician fading channels. Particularly, an approximate BEP upper bound of the AML receiver is also derived for inid time-invariant Rayleigh fading channels with large received signal-to-noise power ratios. By virtue of this approximate bound, a design criterion is developed to determine the appropriate diversity encoding coefficients for the proposed DAPSK MIMO system. Numerical and simulation results show that the AML receiver for diversity-encoded DAPSK is nearly optimum when the average received signal-to-noise power ratios are high and the channel is heavily correlated fading and can provide better error performance than conventional noncoherent MIMO systems when the effect of non-ideal transmit power amplification is taken into account.     

阴影Nakagami-m衰落信道条件下分布式天线系统中断分析

This paper presents an outage analysis of distributed antennas system (DAS) suffering from shadowed Nakagami-m fading environment where the desired signal also suffers from cochannel interference. The desired signal and interfering signal are subjected to path loss, multipath and shadowing fading. Based on Wilkinson's method, the signal to interference ratio (SIR) probability density function ( PDF) of fixed DAS is obtained. Some numerical results of outage probability with different parameters are analyzed. The analysis results can provide sufficient precision for evaluating the outage performance of DAS     

An Optimum Blind Receiver for Correlated Rician Fading MIMO Channels

A blind receiver scheme for narrowband separately- correlated Rician block fading coded MIMO systems is described. It is shown that this receiver (which can be interpreted as a limiting case of the optimum pilot-aided receiver) attains the maximum achievable throughput at sufficiently high E_b/N₀ ratio. The influence of the E_b/N₀ ratio and of the channel Rice factor on the receiver performance are studied and it is shown that, as either of these parameters increases, the advantage of the blind versus the pilot-aided receiver becomes more sensible. The results obtained support the intuition that, if the line-of-sight component is sufficiently strong, a blind scheme using only the knowledge of the fading statistics parameters matches or outperforms pilot- aided detection.     

Approximate Outage Analysis of Land Mobile Satellite Systems in Lognormally Shadowed Rician Channels

Among the proposed models for land mobile satellite (LMS) channels, the shadowed Rice model proposed originally by Loo, has found wide applications in different frequency bands. In Loo’s model, it is assumed that the received signal is affected by nonselective Rice fading with lognormal shadowing on the direct component only, while the diffuse scattered component has constant average power level. The resulting composite probability density function (PDF) includes an infinite-range integral and is not available in closed-form, thereby making the performance evaluation of LMS communication links in these channels cumbersome. To bypass this problem, in this paper an approximation method is developed which makes it possible to describe the envelope PDF as a sum of weighted Rice’s PDFs. Therefore, in contrast with Loo’s PDF, the proposed method leads to an easy-to-use, closed-form approximate expression for the PDF and also for the most statistical characteristics, such as cumulative distribution function and moments of the signal envelop in shadowed Rice channels. Based on the derived expressions, the performance analysis of a single receiver operating over lognormally shadowed Rice channel is investigated in terms of the outage probability. Numerically evaluated results show the good accuracy of the proposed approximation method.     

Rician衰落信道下空时分组码的性能分析

基于Alamouti提出的BPSK调制下空时分组码在Rayleigh衰落信道中的码性能原理,推导出高阶(M ary)调制下Rician衰落信道中空时分组码的符号差错率的最小距离球界,并进行计算机仿真分析了两信道下引入空时分组码的多天线系统中发射和接收天线的分集增益,发射天线数量的“地板效应”以及Rician因子K对符号差错性能的影响。     

Statistical Models for Fading and Shadowed Fading Channels in Wireless Systems: A Pedagogical Perspective

A unified analysis of statistical models for describing fading, shadowing, and shadowed fading channels is presented from a pedagogical viewpoint. The different probability density functions such the Rayleigh, Nakagami, gamma, generalized gamma, Weibull, lognormal, Nakagami-lognormal, K distribution, generalized K distribution, and Nakagami inverse Gaussian distribution are presented and the relationships among them are detailed. These density functions are compared in terms of two quantitative measures, namely the amount of fading and outage probability. A general approach to fading and shadowed fading channels using a cluster based approach is also presented to link several of the distributions. It is expected that this overview will be very helpful to students and educators who are engaged in the study of wireless systems and the adverse impact of fading and shadowing in wireless data transmission.