Realizing the potential of low-Earth orbit (LEO)-based space-air–ground integrated networks requires mitigating challenges associated with LEO satellites, such as limited on-board resources, extended downlink (DL) coverage, temporal variability, and dynamic traffic demand. To mitigate these challenges, an efficient resource allocation (RA) system for beam-hopping can be designed. In this article, we address two practical issues in multidimensional RA for beam-hopping LEO under mixed channel conditions. First, we propose a new clustering algorithm for user equipment cell associations. Second, we decompose the RA problem into three subproblems: 1) time-frequency; 2) power; and 3) dynamic RA, and systematically address each subproblem to develop a comprehensive RA approach. We introduce a heuristic approach for time-frequency RA, achieving a good balance between performance and computational complexity. We extend the proposed algorithm to a practical dynamic RA scenario, where the channel parameters are unknown random variables throughout the next time slots, which makes the optimization problem an intrinsically stochastic problem. In addition, throughput balancing for several time slots becomes a dynamic process. Then, we use sequential convex approximations for power allocation and dynamic programming for dynamic RA. Numerical results show that the proposed algorithms outperform baseline methods in delay, capacity (the theoretical maximum data rate), and throughput (the actual data rate achieved).

In this paper, accounting for the effect of noise, we propose a moment-based estimation method for the Gamma-Gamma fading parameters in free-space optical (FSO) links and analyze its estimation performance. The proposed method iteratively estimates the parameters of the Gamma-Gamma fading channel by minimizing the weighted sum of the moment differences, where these differences are obtained by employing from the first to fifth order moments. In particular, by updating each weight of the moment differences for every iteration, the proposed method simultaneously can give more accurate estimates of the parameters. Through computer simulations over a wide range of FSO channel conditions, we demonstrate that the proposed method outperforms the existing moment-based method in noisy environment, especially in low signal-to-noise ratio scenarios and for large values of the Rytov variances.

In this paper, we propose a novel algorithm for blind estimation of a linear scrambler, which is a synchronous scrambler or a self-synchronous scrambler, in direct sequence spread spectrum systems and analyze its estimation performance. We first examine the statistics considering the linearity of the scrambled data and the repetition property of the spreading code. Based on this, we then propose an improved estimation algorithm for the feedback polynomial of the linear scrambler constituting a search process and a verification process of feedback polynomial candidates to determine the correct feedback polynomial of the scrambler. To validate the proposed algorithm, we show through computer simulations that the proposed algorithm outperforms the conventional algorithm in terms of estimation performance.

Pb-free solders based on Sn-Ag-Cu are currently applied to electronic devices. This article investigates the thermomechanical reliability of three different compositions of Pb-free solder joints, Sn-1.0Ag-0.5Cu, Sn-1.2Ag-0.5Cu, and Sn-3.0Ag-0.5Cu in a harsh environment. First, we perform a temperature cycling test (TCT) and a thermal shock test (TST) at temperatures from –55 to 125 °C. In addition, we conduct a combined thermal shock and vibration test (TSVT) using a HALT chamber, at –65 to 150 °C and an acceleration of up to (15 ± 1) Grms, considering the automotive underhood environment. We then analyze the mean lifetimes for each test of Pb-free solder joints dependent on the Ag contents. Finally, we present the lifetime ratios of three different Pb-free solders between TCT and TST, and show that SAC1205 has a similar mean lifetime to SAC305 within about 0.34% in combined TSVT. The results of this article could provide a reference for selecting suitable Pb-free solders in industrial use over harsh environments.

While the performance analysis of linear zero-forcing (ZF) and minimum mean-square error (MMSE) receivers has received much attention, there has been no exact closed-form distribution of signal-to-interference-plus-noise ratio (SINR) for MMSE multiple-input multiple-output (MIMO) systems in the arbitrary fading environments for over the past 20 years. In this paper, an exact and general distribution of SINR for MMSE MIMO systems is presented under uncorrelated Rayleigh fading environments. We start our analysis by finding regular patterns from the probability density function (PDF) of SINR for small dimensions, and using the regular patterns, we extend the PDF to arbitrary dimensions. From the exact and general PDF of SINR for MMSE MIMO systems, the cumulative distribution function and the moment-generating function are derived in terms of the incomplete gamma function and the confluent hypergeometric function of the second kind, respectively. As applications, the error probability, outage probability, and achievable sum rate are investigated by using the distribution of SINR for MMSE MIMO systems. Most notably, the achievable sum rates for ZF and MMSE MIMO systems are defined by using the Meijer G-function, which are the first closed-form expressions in the literature.

It is important to understand the properties of spatial correlation matrices in multiple-input multiple-output (MIMO) systems. In this paper, we derive new bounds for the maximum and minimum eigenvalues of spatial correlation matrices characterized by the exponential model. The new upper bounds for the maximum and minimum eigenvalues are tighter than the previously known bounds. Moreover, the new lower bound for the minimum eigenvalue, which has not yet been derived in the literature as a function of the number of antennas, is also tight. In order to predict the behavior of these bounds, we investigate the gap between the lower and upper bounds. These bounds are directly applicable to the analysis of wireless communication scenarios, such as uniform planar arrays and ill-conditioned channels, which are expected to be widely used for massive MIMO systems.

This letter provides a new expression for the probability of the phase angle between two vectors perturbed by Gaussian noise with in-phase/quadrature-phase (I/Q) unbalance, in order to simplify the analysis of the error probabilities of M-ary phase-shift keying (MPSK). The error probabilities, symbol-error rates, or bit-error rates for MPSK with I/Q balance or unbalance, can be presented straightforwardly from the derived expression. Because the newly derived result is provided in terms of the conventional first-order Gaussian Q-function and the joint Gaussian Q-function with a correlation coefficient dependent on M, it readily allows rapid evaluation for various cases of practical interest.

Closed-form expressions for the exact bit error rate performance of an arbitrary rectangular QAM with Gray code mapping for diversity reception in frequency non-selective slow m-distributed Nakagami fading is derived and analyzed. The maximal-ratio combining (MRC) diversity technique with independent or correlated fading has been considered. Also, the exact BER expression of rectangular QAM for MRC diversity in the presence of cochannel interference is presented. Numerical results demonstrate the error performance improvement by employing MRC diversity reception. The new expressions presented are applicable in many cases of practical interest.

Quadrature amplitude modulation (QAM) is an attractive technique to achieve high rate transmission without increasing the bandwidth. A great deal of attention has been devoted to the study of bit error rate (BER) performance of QAM, and approximate expressions for the bit error probability of QAM have been developed in many places in the literature. However, the exact and general BER expression of QAM with an arbitrary constellation size has not been derived yet. We provide an exact and general closed-form expression of the BER for one-dimensional and two-dimensional amplitude modulations, i.e., PAM and QAM, under an additive white Gaussian noise (AWGN) channel when Gray code bit mapping is employed. The provided BER expressions offer a convenient way to evaluate the performance of PAM and QAM systems for various cases of practical interest. Moreover, simple approximations can be found from our expressions, which are the same as the well-known approximations, if only the dominant terms are considered.