We present a general and tractable fading model for line-of-sight (LOS) scenarios, which is based on the product of two independent and non-identically distributed κ-μ shadowed power envelopes. Simple closed-form expressions for the probability density function, cumulative distribution function and moment-generating function are derived, which are as tractable as the corresponding expressions derived from a product of Nakagami-m random variables. This model simplifies the challenging characterization of LOS product channels, as well as combinations of LOS channels with non-LOS ones. We leverage these results to analyze performance measures of interest in the contexts of wireless powered and backscatter communications, where both forward and reverse links are inherently of LOS nature, as well as in device-to-device communications subject to composite fading. In these contexts, the proposed model shows a higher flexibility when fitting field measurements with respect to conventional approaches based on product distributions with deterministic LOS, together with a more complete physical interpretation of the underlying propagation characteristics.

This letter introduces a new approach to the problem of approximating the probability density function (PDF) and the cumulative distribution function (CDF) of a positive random variable. The novel approximation strategy is based on the analysis of a suitably defined sequence of auxiliary variables which converges in distribution to the target variable. By leveraging such convergence, simple approximations for both the CDF and PDF of the target variable are given in terms of the derivatives of its moment generating function (MGF). In contrast to classical approximation methods based on truncated series of moments or cumulants, our approximations always represent a valid distribution and the relative error between variables is independent of the variable under analysis. The derived results are then used to approximate the statistics of positive-definite real Gaussian quadratic forms, comparing our proposed approach with other existing approximations in the literature.

Orthogonal frequency division multiplexing (OFDM) signals with rectangularly windowed pulses exhibit low spectral confinement. Two approaches usually referred to as pulse-shaping and active interference cancellation are classically employed to reduce the out-of-band emission (OOBE) without affecting the receiver. This paper proposes a spectral shaping method that generalizes and unifies these two strategies. To this end, the OFDM carriers are shaped with novel pulses, referred to as generalized pulses, that consist of the ones used in conventional OFDM systems plus a series of cancellation terms aimed at reducing the OOBE of the former. Hence, each generalized pulse embeds all the terms required to reduce its spectrum in the desired bands. This leads to a data-independent optimization problem that notably simplifies the implementation complexity and allows the analytical calculation of the resulting power spectral density (PSD), which in most methods found in the literature can only be estimated by means of simulations. As an example of its performance, the proposed technique allows complying with the stringent PSD mask imposed by the EN 50561-1 with a data carrier loss lower than 4%. By contrasts, 28% of the data carriers have to be nulled when pulse-shaping is employed in this scenario.

We provide some comments and subsequent corrections to a paper recently published in this Journal (see ibid., vol. 33, no. 1, pp. 111-119, Jan. 2015). More precisely, we show that the pdf for the κ-μ shadowed fading model given by S.L. Cotton cannot be obtained from the underlying statistical model proposed therein. To support this observation, we present a detailed mathematical analysis as well as some Monte Carlo simulations. We also demonstrate that by simply adopting the underlying statistical model for the κ-μ shadowed fading distribution proposed in an earlier and independent work by J.F. Paris enables the κ-μ shadowed pdf later presented by S.L. Cotton to be obtained.

In this paper, the estimation of the parameters of the Two-Wave with Diffuse Power (TWDP) fading distribution from the observation of the received signal envelope is addressed. Specifically, a moment-based approach for the estimation of the parameters and Δ for this fading model is introduced. We first set the boundaries of the estimation problem by calculating the Cramer-Rao lower Bound (CRB). Then, closed-from expressions that relate the moments of the received signal envelope to the parameters and Δ are used for the estimation. Monte-Carlo simulations show that the estimation error is close to the CRB for a wide range of values of the parameters and Δ.

This paper shows that the proposed Rician shadowed model for multi-antenna communications allows for the unification of a wide set of models, both for multiple-input multiple-output (MIMO) and single- input single-output (SISO) communications. The MIMO Rayleigh and MIMO Rician can be deduced from the MIMO Rician shadowed, and so their SISO counterparts. Other more general SISO models, besides the Rician shadowed, are included in the model, such as the κ-μ, and its recent generalization, the κ-μ shadowed model. Moreover, the SISO η-μ and Nakagami-q models are also included in the MIMO Rician shadowed model. The literature already presents the probability density function (pdf) of the Rician shadowed Gram channel matrix in terms of the well-known gamma- Wishart distribution. We here derive its moment generating function in a tractable form. Closed- form expressions for the cumulative distribution function and the pdf of the maximum eigenvalue are also carried out.

We investigate the dynamic behavior of the stationary random process defined by a central complex Wishart matrix W(t) as it varies along a certain dimension t. We characterize the second-order joint cumulative distribution function (cdf) of the largest eigenvalue, and the second-order joint cdf of the smallest eigenvalue of this matrix. We show that both cdfs can be expressed in exact closed-form in terms of a finite number of well-known special functions in the context of communication theory. As a direct application, we investigate the dynamic behavior of the parallel channels associated with multiple-input multiple-output (MIMO) systems in the presence of Rayleigh fading. Studying the complex random matrix that defines the MIMO channel, we characterize the second-order joint cdf of the signal-to-noise ratio (SNR) for the best and worst channels. We use these results to study the rate of change of MIMO parallel channels, using different performance metrics. For a given value of the MIMO channel correlation coefficient, we observe how the SNR associated with the best parallel channel changes slower than the SNR of the worst channel. This different dynamic behavior is much more appreciable when the number of transmit (NT) and receive (NR) antennas is similar. However, as NT is increased while keeping NR fixed, we see how the best and worst channels tend to have a similar rate of change.

This paper presents a new connection between the generalized Marcum-Q function and the confluent hypergeometric function of two variables, Φ3. This result is then applied to the closed-form characterization of the bivariate Nakagami-m distribution and of the distribution of the minimum eigenvalue of correlated noncentral Wishart matrices, both important in communication theory. New expressions for the corresponding cumulative distributions are obtained and a number of communication-theoretic problems involving them are pointed out.

In multiple-input multiple-output (MIMO) systems the communication channel can be split into s-parallel single-input single-output eigenchannels. The channel gains associated with these eigenchannels depend on the magnitude of the eigenvalues of the complex random matrix that characterizes the MIMO channel. We present a general framework for the characterization of the dynamics of MIMO channels. In addition, we provide new analytical results for the distribution of the largest eigenvalue of two correlated Wishart matrices, which enable a direct evaluation of different system performance metrics such as the probability of two consecutive outages separated by a given time window, the level crossing rate and the average fade duration.

In this paper, we derive exact closed-form expressions for the bivariate Nakagami-m cumulative distribution function (CDF) with positive integer fading severity index m in terms of a class of hypergeometric functions. Particularly, we show that the bivariate Nakagami-m CDF can be expressed as a finite sum of elementary functions and bivariate confluent hypergeometric Φ3functions. Direct applications which arise from the proposed closed-form expression are the outage probability (OP) analysis of a dual-branch selection combiner in correlated Nakagami-m fading, or the calculation of the level crossing rate (LCR) and average fade duration (AFD) of a sampled Nakagami-m fading envelope.

In this paper, we derive an exact closed-form expression for the bit error rate (BER) of orthogonal frequency division multiplexing (OFDM) systems impaired by transmit (TX) and receive (RX) in-phase/quadrature (IQ) imbalances and imperfect channel state information (ICSI), in Rayleigh fading channels. Since the decision metric in this scenario can be expressed as a quadratic form D in Gaussian complex-valued non-circularly symmetric random variables (RVs) with zero mean, we provide a general expression for the calculation of Pr{D <; 0}. In addition, two approximate closed-form expressions for the BER are obtained, which are demonstrated to be extremely accurate for practical values of IQ imbalances. Results show that TX IQ imbalance is in general less detrimental than RX IQ imbalance, and confirm that the correlation between the channel response at the observed subcarrier frequency and the mirror subcarrier has a significant impact in the BER.

The performance of multiple-input multiple-output (MIMO) systems using spatial multiplexing is analyzed under channel prediction errors. We derive exact closed-form expressions for the conditional and average bit error rate (BER) for both fixed and adaptive modulation. We apply our analysis to design a rate adaptation policy that optimally adapts antenna use between beamforming and spatial multiplexing. Our results indicate that the prediction error degrades BER in MIMO systems with spatial multiplexing much more than in MIMO systems with beamforming due to the self-interference that arises from channel coupling. In particular, if interference between eigenchannels is high, spatial multiplexing should not utilize the weakest eigenchannels. In our policy, beamforming is used when prediction error is high to avoid interference, whereas multiplexing is used when it is low to achieve the maximum multiplexing gain. We show that this policy improves performance over prior adaptive policies that have been proposed in the literature.

Multiple input multiple output (MIMO) systems, in conjunction with orthogonal frequency division multiplexing (OFDM), are extensively used in modern communication systems in order to improve throughput and robustness in multipath fading environments. However, for an optimal operation of these techniques, channel state information (CSI) must be available in both the transmitter and receiver sides. In this paper, an exact closed-form expression for the bit error rate (BER) in MIMO-OFDM systems with transmit beamforming and maximal ratio combining (MRC) reception is obtained, under imperfect channel prediction and interpolation, in multipath Rayleigh fading. This expression is used to evaluate the BER performance of the system for different antenna configurations, prediction and interpolation filters, and channel conditions.

A blind method for jointly estimating and correcting time, carrier frequency and sampling clock frequency synchronization in OFDM based systems is proposed. The algorithm is particularlly suitable for low-cost terminal implementations, since it provides a great efficiency in terms of hardware complexity. Its performance has been tested on a real system, taking 3GPP-LTE as target technology and FPGAs as hardware platform. Results show its good performance while keeping computational complexity unincreased.

In this paper we analyze the performance of multiple-input multiple-output (MIMO) multiplexing for BPSK and square M-QAM under Rayleigh fading. We derive exact closed-form expressions of the bit error rate (BER) assuming channel prediction error. Our analysis indicates that prediction error degrades MIMO multiplexing BER much more than MIMO beamforming BER. This is not surprising since prediction error in MIMO multiplexing leads to eigenchannel interference, which has a much greater impact on BER than the imperfect beamforming coefficients that result from such errors. Our numerical results also reveal that non-uniform rate allocation between ordered eigenchannels dramatically improves the BER.

Significant throughput improvements can be obtained in multiple-input multiple-output (MIMO) fading channels by merging beamforming at the transmitter and maximal ratio combining (MRC) at the receiver. In general, accurate channel state information (CSI) is required to achieve these performance gains. In this paper, we analyze the impact of channel prediction error on the bit error rate (BER) of combined beamforming and MRC in slow Rayleigh fading channels. Exact closed-form BER expressions are obtained in terms of elementary functions. Numerical results show that imperfect CSI causes little BER degradation using channel prediction of moderate complexity.

Important throughput improvements in multiple- input multiple-output (MIMO) fading channels can be obtained by merging beamforming at the transmitter and maximal ratio combining (MRC) at the receiver. However, to attain these performance gains it is important to obtain accurate channel state information (CSI). For this purpose a channel estimation technique is used at the receiver: channel prediction to feed back to the transmitter for beamforming, and channel interpolation for MRC at the receiver. In this paper, the impact of imperfect channel prediction on bit error probability (BEP) is analyzed in Rayleigh fading, taking into account the channel response variations over the frame interval. An exact closed-form expression for BEP is obtained, and we evaluate this expression assuming both time-variant and time-invariant channel models. These results indicate that the BEP performance degrades on the order of 1.5 dB due to channel variations.

In the world of wireless communications, OFDM is one of the most used multicarrier techniques today. The main reasons for that are the multipath immunity that provides and the easy implementation of FFT, but OFDM systems are sensitive to timing synchronization errors. This paper shows a novel synchronization algorithm for the IEEE 802.16-2004 standard. The proposed method performs frame detection and determines the beginning of the data frame, by using a combination of auto and cross correlation techniques. Besides, a FPGA implementation of the developed algorithm is achieved in order to evaluate its performance on a real system.

Discrete multitone (DMT) modulation is a suitable technique to cope with main impairments of broadband indoor power-line channels: spectral selectivity and cyclic time variations. To attain higher bit rates, in those subbands with good transmission conditions a high number of bits per symbol must be allocated and, hence, synchronization issues become an important concern. This paper analyzes the performance of a conventional DMT timing recovery scheme designed for linear time-invariant (LTI) channels, like digital subscriber lines, when employed over indoor power-lines. The influence of the channel short-time variations and the sampling jitter on the synchronization scheme performance is assessed. Proper parameterization of the timing recovery scheme is provided on the basis of reducing the impact of channel time variations while maintaining the ability to track the sampling jitter.

Discrete multitone (DMT) modulation is a multicarrier technique that allows the transmission of high speed data over band limited channels. This type of system is very sensitive to synchronization errors when used in digital subscriber loop applications (xDSL), due to the high number of carriers and high density constellations involved. This paper addresses the topic of all-digital timing error correction in a DMT system for xDSL applications, where the timing error correction procedure is based entirely on signal interpolation. An analytical study of the interpolator filter performance is carried out, arriving at an expression for the signal-to-distortion ratio (SDR) at the output of the receiver, as a merit figure. In this derivation, the fixed frequency domain equalizer (FEQ) plays an important role since it compensates for a great part of the distortion introduced by the interpolator. From this study, the design of the optimal interpolator filter in terms of SDR, based on a multirate approach with Kaiser window, is presented. Specific designs for ASIC and for DSP-based implementations are obtained. Performance results are excellent, yielding SDR values above 50 dB for all carriers while keeping the computational cost low.