Antenna-pattern diversity versus space diversity for use at handhelds

This paper investigates diversity for dual-antenna systems operating in indoor environments. First, an approximated equation of the diversity gain is derived for different combining techniques. These theoretical results show that the two-term approximation, as generally used in the literature , is too rough an estimate. Consequently, a new six-term approximation is derived. Next, it is demonstrated by a comparison of theoretical and experimental diversity gain values that, due to mutual coupling between the two antennas in practice, the diversity gain will not approach 0 dB if the distance between the two antennas approaches zero. Finally, it is concluded from measurements at 900 MHz that antenna-pattern diversity is a better choice than space diversity for use at handhelds.     

Recursive receivers for diversity channels with correlated flat fading

This paper addresses the design and performance of time-recursive receivers for diversity based communication systems with flat Rayleigh or Ricean fading. The paper introduces a general state-space model for such systems, where there is temporal correlation in the channel gain. Such an approach encompasses a wide range of diversity systems such as spatial diversity, frequency diversity, and code diversity systems which are used in practice. The paper describes a number of noncoherent receiver structures derived from both sequence and a posteriori probability-based cost functions and compares their performance using an orthogonal frequency-division multiplex example. In this example, the paper shows how a standard physical delay-Doppler scattering channel model can be approximated by the proposed state-space model. The simulations show that significant performance gains can be made by exploiting temporal, as well as diversity channel correlations. The paper argues that such time-recursive receivers offer some advantages over block processing schemes such as computational and memory requirement reductions and the easier incorporation of adaptivity in the receiver structures.     

Precoded distributed space-time block codes in cooperative diversity-based downlink

Cooperative diversity is a rapidly emerging topic for wireless communications, with ad hoc and hybrid/relay networks as two main applications so far. In this paper, we investigate the cooperative diversity concept for MIMO multicell networks, where the processing must be optimized to account for the variability of the channel conditions across the cooperative devices. This can be done via distributed preceding and is realistically based on channel statistics (average gains, correlations, etc.). We give a new approach to the previously coined equal diversity spread principle, through minimization of an approximated SER expression. Next, we focus on a low-complexity approach to minimizing a PEP-based performance measure. Gains are evaluated in a multicell scenario with collaborating base stations.     

Semi-analytic Performance of a Multiaccess MIMO Scheme Using Precoding Vectors

This paper addresses the problem of transmitting data to multiple mobile stations using a decode-and-forward strategy. Precoding vectors are used in relays to cancel out multiple access interference at the mobile stations. The diversity gain of the system has been analytically calculated for the case of two mobile stations. The system performance for arbitrary number of mobile stations is calculated using a semi analytic approach. In this approach the distribution of signal to noise ratio is approximated by a mixture of Nakagami laws using an Expectation-Maximization algorithm. Then the symbol error probability of this mixture is analytically calculated. Simulations confirm the theoretical results showing that the full diversity advantage can be obtained, which is the product of the number of antennas at each relay by the number of relays minus the total number of system constraints.     

Penalty of hybrid diversity for two-dimensional signaling in Rayleigh fading

Lower and upper bound expressions for the penalty of hybrid diversity used with any two-dimensional (2-D) signaling constellation with polygonal decision regions are derived. Using these bounds with well-known results for maximal ratio combining, the error probability performance of 2-D signaling with hybrid selection/maximal ratio combining can be approximated to a high accuracy.     

Route Diversity Effect on 20 GHz Band Radio Relay Links

Rain outage performance on route diversity systems, necessary for the circuit design of millimeter communication systems, is examined by comparing it to that of a tandem system. The analysis is mainly based on the attenuation data measured over about two and a half years at the 20 GHz band on 13 tandem links near Tokyo. It is shown that the joint probability of rain attenuation on two paths can be expressed as a function of the path correlation coefficient on the two paths under consideration, and the evaluation of the rain outage performance can be approximated by the joint probability on pair hops of a diversity route. An estimation method using the path length enlargement effect is proposed for the estimation of the route diversity improvement.     

Approximation of envelope statistics with Nakagami distribution foroutput of noncoherent diversity combiners

A simple relationship for estimating the fading parameter m of the Nakagami distribution is obtained for the output of L-branch noncoherent diversity combiners when the branch signals of unequal mean powers are independently Rayleigh faded. Comparisons between the exact distributions and the Nakagami distribution with approximated ms for L=2 and L=3 show good agreement over wide ranges of relative mean powers between the branch signals     

Nakagami-m Keyhole衰落信道下MIMO系统容量及天线选择

本文全面地分析了在Nakagami-m衰落信道下具有"匙孔"(keyhole)效应时正交块空时编码(STBCs)的多输入多输出 (MIMO)系统的容量以及误比特率(BER)性能,导出了该系统环境下平均容量的紧下边界值,同时得出了系统中断概率的具体表达式及其误码率的近似值.在此基础上分析了Keyhole MIMO信道下的天线选择问题.最后进行了仿真,其结果与理论分析一致.研究表明,在Keyhole MIMO信道中,采用天线选择仍然能够获得与全复杂系统相同的分集增益.     

Toward a unified framework for modeling and analysis of diversity in joint source-channel coding

The study of joint source-channel coding (JSCC) systems faces one major challenge in obtaining an analytical expression for the function that links end-to-end distortion with channel signal-to-noise ratio, the D-SNR curve. In this paper, for certain multimedia systems using practical source and channel codes in a JSCC bit rate allocation design, the D-SNR curve is shown to be well approximated by a set of carefully selected points where the relative contribution of channel errors to end-to-end distortion is small. This approach has the potential advantage that it could be applied to represent performance of many practical systems using JSCC bit rate allocation for which it is shown that the D-SNR function is approximately linear in log-log scales. A unified framework for the modeling, analysis and performance measurement of these systems is proposed by considering a view of diversity more general than its usual interpretation. This view extends that of diversity to include redundant information so coding and diversity gain are still used to characterize performance. Furthermore, the proposed approach is applied to study issues arising from using practical source and channel codes, including the effects on performance of channel codes of different strength or source codes with different compression efficiency.     

On the capacity of log-normal fading channels

In this letter we provide an analytical expression for the moments of the capacity for the log-normal fading channel. Since the developed expression involves infinite series, we show that the error that results from the truncation of these series is insignificant. We also analyze in more details the ergodic capacity by giving a simpler expression for the remainder of the truncated series. Relying on the fact that the sum of log-normal Random Variables (RV) is well approximated by another lognormal RV, we further utilize the obtained results to approximate the capacity of diversity combining techniques in correlated lognormal fading channels. The results that we provide in this letter are an important tool for measuring the performance of communication links in a log-normal environment.     

双时隙多用户线性网络编码协作的中断概率分析

线性网络编码协作(LNCC)是一种将线性网络编码与协作通信相结合的技术,能够提高频带利用率,降低中断概率.该文从理论上分析了双时隙M(M≥2)用户LNCC的紧近似中断概率,并通过Monte Carlo仿真验证了理论分析的正确性.同时,通过分析渐近中断概率,该文给出了双时隙M用户LNCC的分集阶数.为了说明线性网络编码为协作通信带来的额外增益,该文将双时隙M用户LNCC与分集阶数相同的传统M用户放大-转发协作系统进行了性能比较.结果发现,给定相同的有效频谱效率,前者的中断概率要显著低于后者.     

Smooth perturbation on a classical energy function for lossy power system stability analysis

The efforts to find Lyapunov functions for power systems with losses have been until now in vain. Despite that, engineers have been using approximated energy-like functions to obtain good estimates of the critical clearing time (CCT) in transient stability analysis of power systems. These approximated energy-like functions are not Lyapunov functions, and are usually obtained by an integration process followed by an approximation of the integration path. Therefore, the good CCT estimates obtained with these functions are not supported by a sound theory. Nevertheless, it is shown in this paper, for a particular approximated energy-like function, a theoretical approach to support these good estimates. The approximated energy-like function studied in this paper is well known in the literature, and was proposed by Athay et al. in the COA formulation. It is shown that this approximated energy-like function is neither a Lyapunov function in the usual sense, nor an extended Lyapunov function, when the transfer conductances are taken into account. In spite of that, a function attending the requirements of the extension of the Invariance Principle, that is, an extended Lyapunov function, can be obtained by smooth perturbations on that energy-like function. This perturbed function can be used to estimate the attraction area without approximations or conjectures. Indeed, the difference between the proposed extended Lyapunov function and the approximated energy-like function has the order of a smooth perturbation. This fact supports the good CCT estimates that have been obtained using these approximated energy-like functions, and encourages engineers to keep using them for CCT estimates.     

DS-CDMA performance with maximum ratio combining and antenna arrays

The mobile communication channel is very hostile to a DS-CDMA signal and therefore effective techniques are needed to enhance system performance and capacity. Further, since DS-CDMA capacity and performance is limited by the uplink, ways to improve the uplink performance is needed. By implementing antenna arrays, diversity schemes or a combination of antenna arrays and diversity techniques, the uplink performance can be improved substantially. In this study we consider a single cell with a base station at the center with mobiles uniformly distributed around it. As channel model a Nakagami distributed path gain is assumed. This model was chosen for flexibility (e.g., Rayleigh and Rice channel models can be approximated) and also since empirical data suggests that path fading statistics are adequately described by this distribution. At the receiver an array of M antennas is used to discriminate between the users based on their spatial diversity. The fading process at each of the antenna elements is statistically dependent and further improvements can be realized by making use of the independent fading characteristics of the received signal. To make use of this statistical independent information, the performance of a P branch Maximum Ratio Combining (MRC) receiver is also considered. We further investigate the performance of a combination of P clusters of M antennas separated by the coherence bandwidth of the channel, thereby making use of both forms of spatial diversity. A comparison of the three schemes (antenna arrays, MRC diversity and a combination of antenna arrays and MRC diversity) under equal complexity conditions are made under multipath fading conditions. It is shown that the performance and capacity of a MRC diversity receiver outperforms the other two methods when perfect power control is assumed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Performance of 2-D RAKE receiver in a correlatedfrequency-selective Nakagami-fading

The bit error rate (BER) performance and the characteristics of a two-dimensional (2-D) RAKE receiver operating in a correlated frequency-selective Nakagami-fading environment are analyzed. Correlated fading between array elements whose fading statistics are identical across the same RAKE branch, as well as an arbitrary number of RAKE-branches with arbitrary finding statistics, are assumed. We derived an approximated signal-to-noise ratio (SNR) statistics for one RAKE branch with correlated multiple antennas, which is extended to that for multiple RAKE branches with arbitrary fading statistics, i.e., a 2-D RAKE receiver. The receiver's performance and characteristics are analyzed using the cumulative distribution function of the SNR at the 2-D RAKE receiver output and the BER under various conditions, Numerical results show that the improvement In performance of the 2-D RAKE receiver is brought about by the average SNR and diversity gains, which are identified by two parameters specifying the gamma distribution of SNR     

Error analysis of generalized selection combining over relay channel

Cooperative communication is a recently popular concept which allows single-antenna devices to benefit from spatial diversity. The performance analysis of cooperative communication using generalized selection combining (GSC) over independent not necessarily identically distributed Nakagami-m fading channels is presented and compared with that of the conventional maximal ratio combining (MRC) and selection combining (SC) schemes. With the aid of Padé approximants theory, new closed-form expression is derived for the moment-generating function (MGF) of the GSC output signal-to-noise ratio (SNR). MGF is an important tool for researching the system performance. In this paper, the average bit-error probability is accurately approximated using the well-known MGF approach. Numerical results show that the proposed mathematical analysis is accurate and that for the more severe fading cases, the GSC receivers are closer to the optimum MRC receivers.     

Blind Ratio Combining (BRC): An Optimum Diversity Receiver for Coherent Detection With Unknown Fading Amplitudes

We present an optimum diversity receiver called blind ratio combining (BRC) that minimizes the average symbol error probability or maximizes the average output SNR, where the channels' time delays and the random phases are known, while the fading amplitudes are unknown. In contrast to previous works, where efforts were made to find a posteriori probabilities at the receiver, the BRC simply calculates the optimum weights, which depend on the channel's statistics, avoiding continuous channel estimation, and thus, it significantly reduces the system's complexity. In nonidentical multipath fading channels with power delay profile (PDP), the BRC receiver performs between maximal ratio combining (MRC) and equal gain combining (EGC), and keeps its performance comparable - and in some cases superior - to that of generalized selection combining, while for large values of the decay factor, it approaches MRC. Moreover, in the important practical case of exponential PDP - common in RAKE receivers modeling and adopted for the Universal Mobile Telecommunications System spatial channel modeling by the European Telecommunications Standards Institute-3GPP - the optimum weights can be accurately approximated by simple elementary functions. Furthermore, it is proved that the utilization of these weights ensures an error performance improvement over EGC for arbitrary PDPs. The proposed BRC receiver can be efficiently applied in wireless wideband communication systems, where a large number of diversity branches exists, due to the strong multipath effects.     

Envelope Correlation Coefficient for Logarithmic Diversity Receivers Revisited

The analysis of the envelope correlation coefficient for logarithmic diversity receivers, given by de Neumann (de Neumann, 1989) for one Rayleigh fading branch in isotropic scattering environments (two independent real Gaussian branches), is extended in this letter to the general case where a maximum ratio combiner (MRC) operates on M ges 1 independent Rayleigh fading branches, with the same temporal correlation coefficient, in nonisotropic scattering environments. The derived exact closed-form expressions include the results by B. de Neumann in 1989 as special cases. In addition, when M is not so small, theoretical analysis and Monte Carlo simulations show that the envelope correlation coefficient can be accurately approximated by the squared amplitude of the channel correlation coefficient.     

最大比合并分集接收性能的新的分析方法

对Rayleigh衰落信道中最大比合并分集系统的性能进行了分析.基于泊松分布,提出了一种新的误比特率计算式,并提出了一个新的性能上界,去除了级数和运算,进一步简化了性能分析.数值分析表明,提出的误码率计算式与原分析式一致,且性能上界与误码率的准确值近乎重合.运用仿真对理论分析进行了验证,数值和仿真结果表明,提出的性能分析式简单准确有效,可更为迅速的评估最大比分集合并系统的性能.