Ali Abdi

A space-time correlation model for multielement antenna systems in mobile fading channels

Analysis and design of multielement antenna systems in mobile fading channels require a model for the space-time cross correlation among the links of the underlying multiple-input multiple-output (MIMO) channel. In this paper, we propose a general space-time cross-correlation function for mobile frequency nonselective Rice fading MIMO channels, in which various parameters of interest such as the angle spreads at the base station and the user, the distance between the base station and the user, mean directions of the signal arrivals, array configurations, and Doppler spread are all taken into account. The new space-time cross-correlation function includes all the relevant parameters of the MIMO fading channel in a clean compact form, suitable for both mathematical analysis and numerical calculations/simulations. It also covers many known correlation models as special cases. We demonstrate the utility of the new space-time correlation model by clarifying the limitations of a widely accepted correlation model for MIMO fading channels. As another application, we quantify the impact of nonisotropic scattering around the user, on the capacity of a MIMO fading channel.

A parametric model for the distribution of the angle of arrival and the associated correlation function and power spectrum at the mobile station

One of the main assumptions in Clarkes classic channel model is isotropic scattering, i.e., uniform distribution for the angle of arrival (AOA) of multipath components at the mobile station. However, in many mobile radio channels we encounter nonisotropic scattering, which strongly affects the correlation function and power spectrum of the complex envelope at the mobile receiver. We propose the use of the versatile von Mises (1918) angular distribution, which includes and/or closely approximates important distributions like uniform, impulse, cardioid, Gaussian, and wrapped Gaussian, for modeling the nonuniform AOAs at the mobile. Based on this distribution, the associated correlation function and. power spectrum of the complex envelope at the mobile receiver are derived. The utility of the new results is demonstrated by comparison with the correlation function estimates of measured data.

A new simple model for land mobile satellite channels: first- and second-order statistics

We propose a new shadowed Rice (1948) model for land mobile satellite channels. In this model, the amplitude of the line-of-sight is characterized by the Nakagami distribution. The major advantage of the model is that it leads to closed-form and mathematically-tractable expressions for the fundamental channel statistics such as the envelope probability density function, moment generating function of the instantaneous power, and the level crossing rate. The model is very convenient for analytical and numerical performance prediction of complicated narrowband and wideband land mobile satellite systems, with different types of uncoded/coded modulations, with or without diversity. Comparison of the first- and the second-order statistics of the proposed model with different sets of published channel data demonstrates the flexibility of the new model in characterizing a variety of channel conditions and propagation mechanisms over satellite links. Interestingly, the proposed model provides a similar fit to the experimental data as the well-accepted Loos (1985) model but with significantly less computational burden.

Sum of gamma variates and performance of wireless communication systems over Nakagami-fading channels

Capitalizing on the Moschopoulos (1985) single gamma series representation of the probability density function (PDF) of the sum of gamma variates, we provide a PDF-based approach for the performance analysis of maximal-ratio combining and postdetection equal-gain combining diversity techniques as well as cochannel interference of cellular mobile radio systems over Nakagami (1960)-fading channels with arbitrary parameters. Aside from putting under the same umbrella many of the past results obtained via characteristic function (CF) or moment generating function (MGF)-based approaches, the proposed approach also allows the derivation of additional performance measures, which are harder to analyze via CF or MGF-based approaches.

On the estimation of the K parameter for the Rice fading distribution

We study the statistical performance of two moment-based estimators for the K parameter of Rice fading distribution, as less complex alternatives to the maximum-likelihood estimator. Our asymptotic analysis reveals that both estimators are nearly asymptotically efficient, and that there is a compromise between the computational simplicity and the statistical efficiency of these two estimators. We also show, by Monte Carlo simulation, that the fading correlation among the envelope samples deteriorates the performance of both estimators. However, the simpler estimator, which employs the second and the fourth moments of the signal envelope, appears to be more suitable for real-world applications.

The Ricean K factor: estimation and performance analysis

In wireless communications, the relative strength of the direct and scattered components of the received signal, as expressed by the Ricean K factor, provides an indication of link quality. Accordingly, efficient and accurate methods for estimating K are of considerable interest. In this paper, we propose a general class of moment-based estimators which use the signal envelope. This class of estimators unifies many of the previous estimators, and introduces new ones. We derive, for the first time, the asymptotic variance (AsV) of these estimators and compare them with the Cramer-Rao bound (CRB). We then tackle the problem of estimating K from the in-phase and quadrature-phase (I/Q) components of the received signal and illustrate the improvement in performance as compared with the envelope-based estimators. We derive the CRBs for the I/Q data model, which, unlike the envelope CRB, is tractable for correlated samples. Furthermore, we introduce a novel estimator that relies on the I/Q components, and derive its AsV even when the channel samples are correlated. We corroborate our analytical findings by simulations.

Estimation of Doppler spread and signal strength in mobile communications with applications to handoff and adaptive transmission

Summary Estimation of signal strength, a measure of channel quality, and Doppler spread which is proportional to the mobile speed, are important for handoff algorithms and optimal tuning of system parameters to changing channel conditions in adaptive transmission systems. This paper provides a survey of existing techniques for estimating the statistical parameters of the mobile channel. We discuss the current state of the art in estimation of the received signal strength, mobile velocity, and other related statistical channel parameters, illustrate their performance, and compare existing techniques. The sensitivity of these schemes to modeling error owing to the presence of line of sight, directional reception, and noise is characterized analytically and illustrated by simulations. Copyright  2001 John Wiley & Sons, Ltd.

Space-time correlation modeling of multielement antenna systems in mobile fading channels

For the analysis and design of multielement antenna systems in mobile fading channels, we need a model for the space-time cross correlation among the links of the multiple-input multiple-output (MIMO) channel. We propose a general space-time cross correlation function for narrowband Rayleigh fading MIMO channels, where various parameters of interest such as angle spreads at the base station and the user, the distance between the base station and the user, mean directions of the signal arrivals, array configurations, and Doppler spread are all taken into account. The new space-time cross correlation function includes all the relevant parameters of the MIMO narrowband Rayleigh fading channel in a clean compact form, suitable for both simulation and mathematical analysis. It also covers many known correlation models as special cases. We demonstrate the utility of the new space-time correlation model by clarifying the limitations of a widely-accepted correlation model for MIMO fading channels.

On the utility of gamma PDF in modeling shadow fading (slow fading)

Shadow fading causes random fluctuations of the envelope mean at the mobile. The lognormal distribution is widely accepted for this phenomenon. In this paper we argue that, based on theoretical results and measured data, gamma distribution does the job as well. Then we show that by using the gamma distribution and in contrast with the use of the lognormal distribution, we can obtain easy-to-use and closed-form composite PDFs for fading channels, which in turn extremely simplify analytic calculations.

Performance comparison of three different estimators for the Nakagami m parameter using Monte Carlo simulation

Nakagami distribution has proven useful for modeling the multipath faded envelope in wireless channels. The shape parameter of the Nakagami distribution, known as the m parameter, can be estimated in different ways. In this contribution, the performance of the inverse normalized variance, Tolparev-Polyakov, and the Lorenz estimators have been compared through Monte Carlo simulation, and it has been observed that the inverse normalized variance estimator is superior to the others over a broad range of m values.