A novel MIMO cognitive transceiver (CTR) for LTE-downlink communication system is devised in this work. We consider the cognition concept from the perspective of providing a highly reliable communication to the mobile user anytime anywhere. Rather than handling spectrum allocation (the common perspective of cognitive radio), we consider the channel estimation as the reconfiguration parameter of the proposed CTR. The developed cognitive transceiver is capable of selecting the best channel identification scheme between the conventional least squares (LS) estimator and the recently proposed maximum likelihood (ML) estimator. the proposed CTR is also able to toggle between the conventional pilot-assisted or data-aided (DA) mode and the non-data-aided with pilot (NDA with pilot) mode that relies on both reference and data symbols to track the channel variations. The decision rules of the new CTR that identify the best combination couple of pilot-use and channel-identification modes are drawn after running extensive and exhaustive LTE-downlink link-level simulations. The proposed CTR outperforms all static transceivers, in terms of link-level performance, for any given operating conditions such as SNR, mobile speed, channel type, and channel quality indicator (CQI). The new proposed CTR offers significant link-level throughput gains against the LS channel estimator working in a pilot-assisted mode in most operating conditions and the improvement gains can reach as much as 100% at low SNR and high mobility!

This paper investigates the impact of radio frequency (RF) cochannel interference (CCI) on the performance of dual-hop free-space optics (FSO)/RF relay network. The considered FSO/RF system operates over mixed Malaga-M/composite fading/shadowing generalized-K (GK) channels with pointing errors. The H-transform theory, wherein integral transforms involve Fox's H-functions as kernels, is embodied into a unifying performance analysis framework that encompasses closed-form expressions for the outage probability, the average bit error rate (BER), and the ergodic capacity. By virtue of some H-transform asymptotic expansions, the high signal-to-interference-plus-noise ratio (SINR) analysis culminates in easy-to-compute expressions of the outage probability and BER.

In this paper, we address the problem of direction of arrival (DOA) estimation from turbo-coded square-QAM-modulated transmissions. We devise a new code-aware direction finding concept, derived from maximum likelihood (ML) theory, wherein the soft information provided by the soft-input soft-output decoder, in the form of log-likelihood ratios, is exploited to assist the estimation process. At each turbo iteration, the decoder output is used to refine the ML DOA estimate. The latter is in turn used to perform a more focused receiving beamforming thereby providing more reliable information-bearing sequences for the next turbo iteration. In order to benchmark the new estimator, we also derive the analytical expressions for the exact Cramer-Rao lower bounds (CRLBs) of code-aided (CA) DOA estimates. Simulation results will show that the new CA direction finding scheme lies between the two traditional schemes of completely non-data-aided and data-aided (DA) estimations. Huge performance improvements are achieved by embedding the direction finding and receive beamforming tasks into the turbo iteration loop. Moreover, the new CA DOA estimator reaches the new CA CRLBs over a wide range of practical SNRs thereby confirming its statistical efficiency. As expected intuitively, its performance further improves at higher coding rates and/or lower modulation orders.

In this paper, the asymptotic performance of multiuser multiple-antenna (MU-MIMO) relay networks employing opportunistic scheduling and operating in the presence of Rayleigh fading and co-channel interference is investigated. Notwithstanding the system complexity, due to the newly found complementary moment generating function (CMGF) transform, an exact expression for the capacity, when the antenna counts at the source and the user number are allowed to grow unbound, is obtained. The large scale analysis embodies popular observations, so far intuitively or empirically disclosed, through analytically insightful new formulas. An interesting aspect of this analysis comes from an altered view of multiuser diversity in the context of cellular systems. Previously, multiuser diversity capacity gain has been known to grow as 0(ln ln(K)), from selecting the maximum of K exponentially-distributed powers. Because interference aware scheduling is considered, we find instead that the gain is 0(ln(K1/Q)) where Q is the number of interferers. Simulation results indicate a rather fast convergence to the asymptotic limits with the system's size, thereby demonstrating the practical importance of the scaling results.

The ergodic capacity of multiuser multiple-antenna amplify and forward relay networks with co-channel interference is investigated. Notwithstanding the system complexity, the proposed capacity analysis framework was made possible owing to the newly found complementary moment generating function transform. First, the exact expression for the ergodic capacity, under interference-aware opportunistic scheduling and Rayleigh fading, is derived for finite antenna and user counts. In addition, using extreme value theory, closed-form expressions for the asymptotic ergodic rates are derived for an unlimited number of users or/and antennas at the source. The large-scale analysis embodies, through new insightful formulas, common popular observations so far disclosed intuitively or empirically. Capitalizing on these novel scaling laws, we have been able to derive new closed-form expressions for capacity losses due to intercell interference at the relay and all users. Simulation results suggest a rather fast convergence of the capacity gains to the new asymptotic limits, thereby demonstrating the practical importance of our novel scaling results.

Abstract In this paper, we design a new single‐input multiple‐output context‐aware cognitive transceiver (CTR) that is able to switch to the best performing modem in terms of link‐level throughput. On the top of conventional adaptive modulation and coding, we allow the proposed CTR to make best selection between three different pilot‐utilization modes: conventional data‐aided (DA) or pilot‐assisted, non‐DA (NDA) or blind, and NDA with pilots, which is a newly proposed hybrid version between the DA and NDA modes. We also enable the CTR to make best selection between two different channel identification schemes: conventional least‐squares (LS) and newly developed maximum‐likelihood estimators. Depending on whether pilot symbols can be exploited or not at the receiver, we further enable the CTR to make the best selection among two data detection modes: coherent or differential. Owing to exhaustive link‐level simulations on the downlink of a long‐term evolution system, we draw out the optimal decision rules in terms of the best combination triplet of pilot‐use, channel‐identification, and data‐detection modes that yield the best link‐level throughput as function of channel type, mobile speed, signal‐to‐noise ratio, and channel quality indicator. The proposed CTR offers a link‐level throughput gains improvement as high as 700%compared with DA LS for VehB channel type at a mobile speed of 100 km/h in the low signal‐to‐noise ratio region. For VehA channels, its throughput gain improvements can reach 114%. For PedA and PedB channels, the proposed CTR provides throughput enhancements of about 66%and 330%, respectively. Moreover, realistic simulations at the system‐level of the long‐term evolution‐HetNet network suggest that the new context‐aware CTR outperforms the conventional transceiver (i.e., pilot‐assisted LS‐type channel estimation with coherent detection) by as much as 50% and 60% gains in average and cell‐edge (i.e., five percentile) throughputs, respectively, in the high‐clustering case with type‐B channels. In the low clustering scenario, the average and cell‐edge throughput gain improvements offered by the proposed CTR exceed 80% and 90% for type‐B channels. Copyright © 2016 John Wiley & Sons, Ltd.

This paper tackles the problem of code-aided (CA) timing recovery in turbo-coded square-QAM transmissions. Owing to a simple recursive construction process, some hidden properties of Gray-coded (GC) square-QAM constellations are demystified and the code bits' a priori log- likelihood ratios (LLRs) are explicitly incorporated in the log-likelihood function (LLF). Then, by splitting the underlying LLF into the sum of two analogous terms, we derive for the very first time the closed-form expressions for the exact Cramer-Rao lower bounds (CRLBs) of the underlying turbo synchronization problem. Computer simulations will show that the new closed-form CRLBs coincide exactly with their empirical counterparts evaluated previously using exhaustive Monte-Carlo simulations. They will also show unambiguously the remarkable performance improvements of the CA scheme against the traditional non-data-aided (NDA) one. Over a wide range of practical SNRs, the new CA CRLBs reach those of the completely data-aided (DA) scheme in which all the transmitted symbols are perfectly known to the receiver.

This paper tackles the problem of the direction of arrival (DOA) estimation in turbo-coded systems. We derive for the first time the closed-form expressions for the Cramér-Rao lower bounds (CRLBs) of the code-aided (CA) DOA estimates from arbitrary square-QAM modulated signals. We succeed in factorizing the likelihood function of the system into two analogous terms linearizing thereby all the derivation steps of the Fisher information (FI) element. Simulation results demonstrate that the CRLB for the CA DOA estimates lies between its counterparts in non-data-aided (NDA) and data-aided (DA) estimation schemes. Moreover, the DOA CA CRLB improves by decreasing the coding rate highlighting thereby the potential gain in estimation performance stemming from the proper exploitation of the decoder output.

In this paper, we analyze the ergodic capacity of a two-hop multiple-antenna amplify and forward (AF) system, where the relay is subject to co-channel interference (CCI) while the destination is corrupted by additive white Gaussian noise (AWGN) only. A novel integral transform, called the complementary moment generating function transform (CMGF), is proposed as a unified tool to compute the ergodic capacity. When both the relay and destination perform maximum ratio combining (MRC), we derive a new analytical exact expression for the ergodic capacity. It is shown that the ergodic capacity is better improved by increasing the number of antennas at the relay Nrthan that at destination Nd. Unfortunately, the system shows an incapability of canceling interference even if Nrand/or Ndgrows large.

This paper investigates spectrum-sharing cognitive amplify-and-forward (AF) relay networks employing the maximum ratio transmission/maximum ratio combining (MRT/MRC) scheme at the multiple-antenna source-destination pair. It derives closed-from expressions for the ergodic capacity as well as for the symbol error rate (SER) lower bound and its asymptotic value when considering Nakagami-m fading and interference constraints on N primary receivers.

In this paper, we design a new single-input multiple-output (SIMO) context-aware cognitive transceiver (CTR) that is able to switch to the best performing modem in terms of link-level throughput. On the top of conventional adaptive modulation and coding (AMC), we allow the context-aware CTR to make best selection among three different pilot-utilization modes: conventional decision-aided (DA) or pilot-assisted, non-DA (NDA) or blind, and NDA with pilot which is a newly proposed hybrid version between the DA and NDA modes. We also enable the CTR to make best selection between two different channel identification schemes: conventional least-square (LS) and newly developed maximum-likelihood (ML) estimators. Depending on whether pilot symbols can be properly exploited or not at the receiver, we further enable the CTR to make best selection among two data detection modes: coherent or differential. Owing to extensive and exhaustive simulations on the downlink (DL) of a long-term evolution (LTE) heterogeneous network (HetNet), we are able draw out the decision rules of the new CTR that identify the best combination triplet of pilot-use, channel-identification, and data-detection modes to achieve the best link-level throughput at any operating condition in terms of channel type, mobile speed, signal-to-noise ratio (SNR), and channel quality indicator (CQI). Realistic extensive simulations at the system level suggest that the new context-aware CTR outperforms the conventional transceiver (i.e., pilot-assisted LS-type channel estimation with coherent detection) by as much as 40 and 45% gains in average and cell-edge (i.e., 5-percentile) total throughput per macro-area with 10 pico-cells each, respectively.

We explore the effect of imperfect knowledge on multipoint downlink traffic engineering (TE) (with dynamic point selection DPS) computed centrally and jointly with power control (PC) optimization versus distributed PC techniques. Instead of considering a single metric that shows qualities in a single dimension, we observe both performance and resilience by observing how many users get quality of experience (QoE) — or otherwise, how many users would complain that they do not receive the expected service. We identify that centralized traffic engineering provides the best performance (highest class of service) with distributed power control in a practical (imperfect) network. Although this has a clear drawback as it comes with a low resilience in face of bad service admission. We conclude that fifth generation networks will need to be able to dynamically change optimizing strategies to face different loading of the networks.

In this paper, we propose a new signal-to-noise-ratio (SNR) maximum likelihood (ML) estimator over time-varying single-input multiple-output (SIMO) channels, for both data-aided (DA) and non-data-aided (NDA) cases. Unlike the classical techniques which assume the channel to be slowly time-varying and, therefore, considered as constant during the observation period, we address the more challenging problem of instantaneous SNR estimation over fast time-varying channels. The channel variations are locally tracked using a polynomial-in-time expansion. In the DA scenario, the ML estimator is developed in closed-form expression. In the NDA scenario, however, the ML estimates of the per-antenna SNRs are obtained iteratively, with very few iterations, using the expectation-maximization (EM) procedure. Our estimator is able to accurately estimate the instantaneous SNRs over a wide range of average SNR. We show through extensive Monte-Carlo simulations that the new estimator outperforms previously developed solutions.

This paper evaluates the ergodic capacity of secondary dual-hop cognitive amplify-and-forward (AF) relay networks (CRNs) subject to arbitrary Nakagami-m fading. Considering a spectrum-sharing environment, we assume that the transmit power of secondary users is solely governed by the interference power constraint at N primary users. A closed-form expression for the ergodic capacity of the secondary AF CRN is derived for the first time and shown to be affected by the distance ratio of the interference link (from the secondary transmitter to the closest primary receiver) to the relaying link (between the secondary transmitter and the secondary receiver). This new ergodic capacity expression is validated and assessed by insightful simulations against different key CRN parameters.

Relay deployment performance, such as site planing and dimensioning, highly depends on the capacity of the wireless relay link. However, latter's performance can be severely harmed due to cochannel interference (CCI), particularly with the current aggressive reuse of the available radio spectrum. In this paper, we present an analytical framework for amplify-and-forward (AF) relay capacity that can be used for the deployment of two-hop cellular relay networks operating over generalized fading channels in the presence of cochannel interference. Nakagami-m and generalized-K distributions are used to model the relay and interference links in multipath and composite multipath/shadowing environments, respectively. Multiple antenna AF relaying systems employing transmit beamforming and maximum ratio combining (TB/MRC) are also investigated in the presence of CCI. The derived analytical results provide an efficient mathematical tool for the fast evaluation of the ergodic capacity of the system as well as for the investigation of the impact of key parameters such as fading, shadowing, number of antennas and CCI on the system's ergodic capacity.

An analytical characterization of the ergodic capacity of interference-limited multihop wireless networks with amplify-and forward (AF) relaying is presented. In our analysis, we consider that transmissions are performed over Nakagami-m fading where channel state information is only known at the receiving terminals. We derive an exact expression for the ergodic capacity by exploiting the moment generating function (MGF) of the inverse signal-to-interference ratio (SIR). The result is applicable for arbitrary numbers of interfering signals at the receiving terminals and can be efficiently evaluated. Furthermore, considering the special case of dual-hop transmission, we propose a more refined characterization where the high-SIR capacity is expanded as an affine function. The zero-order term or the power offset for which we find insightful closed-form expressions, is shown to play a chief role in understanding the impact of interference and power on the system's capacity. Finally, some simulation results sustaining our analysis are provided.

Integrating relaying techniques into cellular communications sheds new light on higher capacity and broader coverage. However, applying relaying techniques in practice has to take into account important issues such as co-channel interference (CCI). In this work, a generalized framework for the ergodic capacity analysis of dual-hop fixed-gain amplify and forward (AF) relaying systems in the presence of interference is presented. New expressions for the ergodic capacity are derived considering transmissions over independent but not necessarily identically distributed Nakagami-m fading channels in the presence of a finite number of co-channel interferers. Our results establish that the ergodic capacity is dominated by the source-relay interference power and that it improves slowly with the average signal-to-noise ratio (SNR) increasing. It slightly deteriorates, however, with a larger Nakagami-m fading parameter for interference channels. Furthermore, our results offer an analytical insight into the key impact of relay placement on performance. Our new ergodic capacity expressions could therefore provide a very practical/low-cost performance optimization tool for relayed-communication system designers.

This paper derives for the first time the stochastic Cramer-Rao lower bounds (CRLBs) for direction of arrival (DOA) estimates of any linearly-modulated signals, in presence of additive white circular complex Gaussian noise (AWCCGN). The transmitted symbols are assumed to be completely unknown at the receiver side. The channel is slowly time-varying and assumed to introduce a constant distortion phase during the observation interval. Simulation results show that the CRLBs hold almost the same for all the modulation schemes. We also show that, contrarily to uniform lineair array (ULA) systems, the knowledge of the channel distortion phase does not bring any additional information to the achievable performance in uniform circular array (UCA) systems.

Closed-form error analysis of dual-hop fixed-gain amplify-and-forward (AF) relaying systems for transmissions over Nakagami fading channels has, so far, been tractable only with integer fading parameters. For the general Nakagami-m fading with arbitrary m values, the exact closed-form error analysis is more challenging. This paper goes toward solving this problem by deriving a unified error analysis framework that embraces several general modulation schemes. The obtained Error Probability (EP) expressions involve common functions which can be efficiently evaluated using standard numerical softwares. This work represents a significant improvement on previous contributions, not only by unifying the error probability expressions pertaining to dual-hop fixed gain relaying over integer Nakagami-m fading, but also by extending them to the general arbitrary Nakagami-m fading. Simulation results sustaining our analysis are provided, and the impacts of various parameters on the overall AF relaying system performance are investigated.

In this paper, we introduce the Inverse Gaussian (IG) fading distribution to model the envelope and the power of the received signal in radio propagation fading channels and free-space optical (FSO) channels. The joint distribution of the IG process and its derivative and the marginal distribution of the derivative are developed. The obtained formulas are then used to derive the second-order statistics of the received signal envelope and the channel capacity under IG and Nakagami-IG (NIG) fading distributions. Numerical results sustaining our analysis are provided, and the impacts of various parameters on the system performance are investigated.

In this paper we derive a new method of DOA estimation for ULA configurations using the annihilating filter technique. The new method is non-data-aided (NDA) and does not therefore impinge on the whole throughput of the system. The noise components are assumed spatially and temporally white. The transmitted signals are also assumed to be temporally and spatially white (across the transmitting sources). The new method is compared in performance to the root-MUSIC algorithm, a powerful DOA estimation technique for ULA configurations. Simulations will show that the new method performs well over a wide SNR range. The main advantage of the new method is that it succeeds in accurately estimating the DOAs for fast moving sources or for short data snapshots, and even from a single snapshot where the root-MUSIC method fails completely.

In this paper, we present a new time delay estimator for multipath environments using the importance sampling (IS) method. The new technique allows finding the maximum of the compressed likelihood function in an efficient manner. The main idea consists in generating realizations of a random variable distributed according to a function that approximates the actual compressed likelihood function and then computing the mean of the plausible realizations. We avoid eigen-decomposition operation that is widely used in the conventional high-resolution methods. We show through computer simulations that the new algorithm provides accurate estimates for closely spaced unknown time delays. Moreover, the method does not suffer from lack of convergence and initialization problems that arise with other iterative implementations of the maximum likelihood estimator.

In this paper, we derive for the first time analytical expressions for the Cramer-Rao lower bounds (CRLBs) of timing recovery estimators from binary phase shift keying (BPSK) and square quadrature amplitude modulation (QAM) transmissions. The bounds are derived in the presence of additive white Gaussian noise (AWGN). Moreover the carrier phase and frequency are considered as unknown nuisance parameters. Our new analytical expressions reveal that the CRLBs do not depend on the corresponding time delay parameter and that they do not change widely from one modulation order to another. They also corroborate previous works that computed them empirically and provide a meaningful tool for their quick and easy evaluation.

We address the problem of direction of arrival (DOA) finding for uniform linear arrays (ULAs). We derive a new and very simple method for estimating the DOA of a single source based on the covariance matrix of the received signal. The new method is non-data-aided (NDA) and does not therefore impinge on the whole throughput of the system. The noise components are assumed spatially and temporally white. The new method is derived in closed form and it exhibits exactly - over a wide practical SNR range - the same performance of the popular root-MUSIC algorithm, a powerful DOA estimation technique for ULA configurations. Therefore, the new estimator offers a way for a rapid and very easy DOA evaluation; making it very attractive for practical implementation as compared to the root-MUSIC algorithm that relies on the heavy operation of eigen decomposition.

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In this paper, we present a new non-data-aided (NDA) maximum likelihood (ML) time delay estimator based on importance sampling (IS). We show that a grid search and lack of convergence from which most iterative estimators suffer can be avoided. It is assumed that the transmitted data are completely unknown at the receiver. Moreover the carrier phase is considered as an unknown nuisance parameter. The time delay remains constant over the observation interval and the received signal is corrupted by additive white Gaussian noise (AWGN). We use importance sampling to find the global maximum of the compressed likelihood function. Based on a global optimization procedure, the main idea of the new estimator is to generate realizations of a random variable using an importance function, which approximates the actual compressed likelihood function. We will see that the algorithm parameters affect the estimation performance and that with an appropriate parameter choice, even over a small observation interval, the time delay can be accurately estimated at far lower computational cost than with classical iterative methods.

In this paper, we derive for the first time analytical expressions for the inphase/quadrature (I/Q) non-data-aided (NDA) Cramér-Rao lower bounds (I/Q NDA CRLBs) of the direction of arrival (DOA) estimates from square quadrature amplitude (QAM)-modulated signals corrupted by additive white circular complex Gaussian noise (AWCCGN) with any antenna configuration. Yet the main contribution embodied by this paper consists in deriving for the first time analytical expressions for the NDA Fisher information matrix (FIM) and then for the stochastic CRLB of the NDA DOA estimates in the case of square QAM-modulated signals. It will be shown that in the presence of any unknown phase offset (i.e., non-coherent estimation), the ultimate achievable performance on the NDA DOA estimates holds almost the same irrespectively of the modulation order. However, the NDA CRLBs obtained in the absence of the phase offset (i.e., coherent estimation) vary, in the high SNR region, from one modulation order to another.

In this paper, infinite integrals involving the product of Bessel functions of different arguments are solved in closed-form. The obtained solutions form a framework for the error probability analysis of wireless amplify and forward (AF) systems with an arbitrary number of variable-gain relays operating over independent but not necessarily identical Nakagami-m fading channels. Here we show that the error probability can be described by generalized hypergeometric functions, namely, Gauss's and Lauricella's multivariate hypergeometric functions. This work represents a significant improvement over previous contributions and extends previous formulas pertaining to dual-hop transmissions over identical Nakagami-m fading channels. Numerical examples show an excellent match between simulation and theoretical results.

In this paper, infinite integrals involving the product of Bessel functions of different arguments are solved in closed-form. These solutions provide novel expressions in the error rate analysis for wireless amplify and forward (AF) systems with an arbitrary number of variable-gain relays. Here we show that the error probability can be described by generalized hypergeometric functions, namely Lauricella functions. This work represents a significant improvement on previous contributions and extends previous formulas pertaining to dual-hop transmissions over identical Nakagami-m fading channels. Numerical examples show an excellent match between simulation and theoretical results.

In this paper we investigate the end-to-end performance of dual-hop fixed gain relaying systems under asymmetric fading conditions. In such conditions, the links associated with the relay network undergo different fading phenomena. In our analysis, the source-relay and the relay-destination links are subject to fading-only or composite multi-path/shadowing environments. An outage analysis of the considered system is performed and the proposed expressions are applicable to general fading and shadowing conditions. Based on the newly derived analytical expressions of the cumulative distribution function (CDF) of the end-to-end signal to noise ratio (SNR), the average bit error probability of arbitrary rectangular QAM modulations is also computed. Some representative numerical examples of the system performance in terms of the above metrics are provided and discussed. For instance, it is confirmed analytically that the relaying system exhibits better performance when the first hop undergoes better channel conditions.

In this paper we investigate the end-to-end performance of dual-hop relaying systems over non identical-Nakagami-m fading channels. Our analysis considers channel state information (CSI-) assisted relays that just amplify and retransmit the information signal also known as "non-regenerative" relays. New closed-form expressions for the average bit error probability (ABEP) are derived. The proposed expressions apply to general operating scenarios with distinct Nakagami-m fading parameters and average signal to noise ratios (SNRs) between the hops. When the fading parameter is an odd multiple of one half, the ABEP is expressed in terms of hypergeometric functions. When m takes any real non integer value, the obtained results involve the fourth Appell's hypergeometric function. For an arbitrary fading parameter, an analysis of such a scheme is performed using the well known moment-based approach.

In this paper, we derive for the first time analytical expressions for the exact Cramér-Rao lower bounds on the variance of unbiased non-data-aided (NDA) signal-to-noise ratio (SNR) estimators of square QAM-modulated signals. The channel is assumed to be constant over the observation interval and the received signal is supposed to be corrupted by additive white Gaussian noise (AWGN). The derived expressions corroborate previous attempts to numerically compute the considered CRLBs. It will be shown that the NDA CRLBs differ widely from one modulation order to another especially at moderate SNR levels.

In this paper, we develop a new method for signal-to-noise ratio (SNR) estimation when multiple antenna elements receive linearly-modulated signals in complex additive white Gaussian noise (AWGN) spatially uncorrelated between the antenna elements. We also derive extensions of other existing moment-based SNR estimators to the single-input multiple-output (SIMO) configuration. The new SIMO SNR estimation technique is non-data-aided (NDA) since it is a moment-based method and does not rely, therefore, on the a priori knowledge or detection of the transmitted symbols; it does not require the a priori knowledge of the modulation type or order. The new method is shown by Monte Carlo simulations to clearly outperform the best NDA moment-based SNR estimation methods in terms of normalized root mean square error (NRMSE) over QAM-modulated transmissions, namely the M2M4method and the estimators referred to, in this paper, as the GT and the M6methods, even when we extend them to the SIMO configuration.

In this paper, we derive analytical expressions for the exact Cramer-Rao lower bounds (CRLBs) for the joint estimation of the carrier frequency, the carrier phase, the noise power and the signal amplitude of square quadrature amplitude (QAM) modulated signals. The channel is assumed to be slowly timevarying so that it can be assumed constant over the observation interval. The signal is assumed to be corrupted by additive white Gaussian noise (AWGN). The closed-form expressions for the corresponding modified Cramer-Rao lower bounds (MCRLBs) are also derived in this paper.

This paper addresses the stochastic Cramer-Rao lower bound (CRLB) for the non-data-aided (NDA) direction of arrival (DOA) estimation of square quadrature amplitude (QAM)-modulated signals when the transmitted symbols are supposed to be completely unknown to the receiver. These signals are assumed to be corrupted by additive white circular complex Gaussian noise (AWCCGN). The channel is supposed to be slowly time-varying so that it can be assumed constant over the observation interval. The main contribution of this paper consists in deriving an explicit expression for the Fisher information matrix (FIM) in the case of a single square QAM modulated waveform and an analytical expression for the stochastic CRLB of the NDA DOA estimates. It will be shown that the achievable performance on the DOA estimates hold almost the same irrespectively of the modulation order.

In this paper, we address the problem of non-data-aided SNR estimation in wireless SIMO channels. We derive the per-antenna ML SNR estimator using the expectation-maximization (EM) algorithm under constant channels and additive white Gaussian noise (AWGN). The new method is valid for any arbitrary constellation. It is NDA and, therefore, does not impinge on the hole throughput of the system. We obtain two non linear vector equations which are tackled by a less complex approach based on the EM algorithm. The noise components are assumed to be spatially uncorrelated over all the antenna elements and temporally white with equal power. Besides, in order to evaluate our EM-ML SNR estimator, we derive the Cramer-Rao lower bound (CRLB) in the DA case. Monte Carlo simulations show, that our new estimator offers, a substantial performance improvement over the SISO ML SNR estimator due to the optimal usage of the mutual information between the antenna branches, and that it reaches the derived DA CRLBs. To the best of our knowledge, we are the first to derive the ML per-antenna SNR estimators as well as the CRLBs in the NDA and the DA case, respectively, both over SIMO channels.

A novel distribution referred to as N*generalized K, constructed as the product of N statistically independent, but not necessarily identically distributed, generalized K random variables (RVs) is introduced. Statistical characterization of the proposed distribution is carried out via the derivation of closed-form expressions for its moment generating, probability density, cumulative distribution and moment functions. Using the obtained formulas, we present new closed-form expressions for the outage probability, the bit error probability and the average channel capacity of a digital communication system operating over the N cascaded generalized K distributed channels.

Considering orthogonal frequency division multiplexing (OFDM) transmissions, we derive analytical expressions for the Cramer-Rao bounds for the subcarrier signal-to-noise ratio (SNR) estimates. The channel coefficients of the different subcarriers are assumed to be constant over the observation interval and the received signal is assumed to be corrupted by additive white Gaussian noise (AWGN). We will show that exploiting the mutual information between the different tones improves the achievable performance of subcarrier SNR estimators.

In this paper, we derive analytical expressions for the inphase/quadrature Cramer-Rao lower bounds (I/Q CRLB) for the non-data-aided (NDA) signal-to-noise ratio (SNR) estimation of square quadrature amplitude modulated (QAM) signals. The channel is supposed to be slowly time-varying so that it can be assumed constant over the observation interval. The signal is assumed to be corrupted by additive white Gaussian noise (AWGN). We will demonstrate that the shape of the bounds differs greatly as the modulation order changes.

This paper presents an outage analysis of wireless systems operating in gamma-shadowed Nakagami-faded environments where the desired signal also suffers from co-channel interference. The interfering signals are also subject to fading and shadowing. Based on the obtained signal to interference ratio (SIR) probability density function (pdf), closed- form expressions for the outage probability are obtained in both cases of statistically identical interferers and multiple interferers with different parameters. The effects on the aforementioned performance metric of the reuse distance and of the combined fading, shadowing and co-channel interference are analyzed subsequently. The newly derived closed-form expressions for the outage probability allow as to assess the effects of the different channel and interference parameters easily.

Composite multipath fading/shadowing environments are frequently encountered in different realistic scenarios. These channels are generally modeled as a mixture of Nakagami-m multipath fading and log-normal shadowing. The resulting composite probability density function (pdf) is not available in closed form, thereby making the performance evaluation of communication links in these channels cumbersome. In this paper, we propose to model composite channels by the G-distribution. This pdf arises when the log-normal shadowing is substituted by the inverse-Gaussian one. This substitution will prove to be very accurate for several shadowing conditions. In this paper we conduct a performance evaluation of single-user communication systems operating in a composite channel. Our study starts by deriving an analytical expression for the outage probability. Then, we derive the moment generating function of the G-distribution, hence facilitating the calculation of average bit error probabilities. We also derive analytical expressions for the channel capacity for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate.

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.

This paper presents an analytical framework for performance evaluation of mobile radio systems operating in composite fading/shadowing channels in the presence of colocated co-channel interference. The desired user and the interferers are subject to Nakagami fading superimposed on gamma shadowing. The paper starts by presenting generic closed-form expressions for the signal-to-interference ratio (SIR) probability density function (pdf). From this pdf, closed-form expressions for the outage probability, the average bit error rate and the channel capacity are obtained in both cases of statistically identical interferers and multiple interferers with different parameters. The newly derived closed-form expressions of the aforementioned metrics allow us to easily assess the effects of the different channel and interference parameters. It turns out that the system performance metrics are predominantly affected by the fading parameters of the desired user, rather than by the fading parameters of the interferers.

In this paper, we consider composite multipath/shadowing fading channels characterized by Nakagami- m fading with inverse Gaussian shadowing, and Rice fading with inverse Gaussian shadowing. These channel models are very convenient for land mobile satellite systems and communications with low mobility or stationary users. From an analytical standpoint, one major advantage of these models is that they lead to closed-from expressions for the envelope probability density function (pdf) and the cumulative density function (CDF). Moreover, in this paper, we derive mathematically tractable new expressions for fundamental channel statistics such as the level crossing rate (LCR) and the average fade duration (AFD). Also, we validate our analytical results by providing several numerical examples showing the LCR and the AFD as a function of the Nakagami-m factor, Rice factor, and shadowing spread.

Composite multipath/shadowing fading environments are frequently encountered in different realistic scenarios. These channels are generally modeled as a mixture of Nakagami- m multipath fading and log-normal shadowing. The resulting composite probability density function (pdf) is unfortunately not in closed-form, thereby making the performance evaluation of communication links in these channels cumbersome. In this paper, we propose to model composite channels by the very general G-distribution. This pdf arises when the log-normal shadowing is substituted by the Inverse-Gaussian one. This substitution will prove to be very accurate for several shadowing conditions. In this paper we conduct an exhaustive performance evaluation of communication systems operating in these channels. Our study starts by deriving a closed-form expression for the outage probability. Then, we derive the moment generating function of the G-distribution, hence facilitating the calculation of average bit error probabilities. We also derive closed-form expressions for the channel capacity for three adaptive transmission techniques, namely, (i) optimal rate adaptation with constant power, (ii) optimal power and rate adaptation, and (iii) channel inversion with fixed rate. The different expressions that will be provided are of great importance in assessing the performance of communication systems in composite channels.

In this paper, we propose a novel frequency scheduling strategy for an OFDM switched-beam system. We deal with dynamic frequency allocation based on the distance of the user from the serving base station (BS) or on the power of the channel response received by the user. The key idea is to reduce the interference among the neighboring cells. To do so, we consider a time-space-frequency allocation scheme where users are assigned with the appropriate set of subcarriers according to their serving beam at any given time. We evaluate the performance of the proposed frequency allocation scheme when Kalman filtering is used for joint channel estimation and beam selection. A system-level simulator is developed which computes the signal to interference ratio (SIR) for a reference mobile user with different resource allocation strategies. Previously obtained link-level throughput vs. signal to noise ratio (SNR) results are then translated into a system-level cumulative distribution function (cdf) of the user throughput. Link-level frame error rate (FER) results suggest that the proposed Kalman-based OFDM switched-beam system offers high performance over slow fading channels. System-level SIR results show that the proposed frequency scheduling scheme reduces significantly the interference. Our scheme enhances the system throughput compared to other allocation schemes.

This paper focuses on the problem of angular spread (AS) estimation at the base station in a macro-cellular system when a line-of-sight (LOS) component is potentially present. We will limit our study to low-complexity methods prone to practical implementation. The paper demonstrates the limitations of the well-known low complexity AS estimation method, spread root-MUSIC. As supported by simulations, it introduces a lower complexity "look-up table" (LUT) based approach that compares advantageously with spread root-MUSIC from the point of view of complexity and performance.

This letter investigates the capacity of generalized-k fading channels. This very general model describes accurately composite multipath/shaodwing fading channels which are widely encountered in real-world environments. We derive closed-form expressions for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate. The analytical expressions obtained match perfectly the results obtained by computer simulations. These expressions provide a good tool to assess the spectral efficiency of the aforementioned adaptive transmission techniques over composite channels.

This paper investigates the capacity of generalized- K fading channels. This very general model describes accurately composite multipath/shadowing fading channels which are widely encountered in real-world environments. We derive closed-form expressions for three adaptive transmission techniques, namely, i) optimal rate adaptation with constant power, ii) optimal power and rate adaptation, and iii) channel inversion with fixed rate. The analytical expressions obtained match perfectly the results obtained by computer simulations. These expressions provide a good tool to assess the spectral efficiency of the aforementioned adaptive transmission techniques over composite channels.

In this paper, we consider a pulse shaping OFDM/BFDM system operating over a doubly dispersive channel. We propose to search the optimal pulse maximizing the signal to interference ratio (SIR). We show that the impact of interference, composed by both inter-symbol interference (ISI) and inter-carrier interference (ICI), occurring in such a system, depends crucially on time frequency localization of transmit and receive pulses. We propose to maximize the SIR, written as a ratio of two quaternary forms, by altering between transmit and receive pulse spaces both generated by Hermite Waveforms. Numerical results show that optimized BFDM systems can outperform conventional OFDM systems with respect to SIR. In fact, biorthogonality offers more freedom to design pulses improving robustness of the multi-carrier system against channel dispersion.

This paper investigates the capacity of log-normal fading channels with receiver channel state information. We provide a closed-form expression for the ergodic capacity of the log-normal fading channel. Since the developed expression involves an infinite series, we show that the error that results from the truncation of this series is insignificant. Relying on the fact that the sum of log-normal Random Variables (RV) is well approximated by another log-normal RV, we further apply the obtained results to find the capacity of correlated log-normal fading channels with Maximum Ratio Combining and Equal Gain Combining. The analytical expressions obtained match perfectly the capacity given by simulations.

This paper investigates non-data-aided (NDA) SNR estimation for QAM transmission over additive white Gaussian noise channels. It proposes a novel class of moment-based SNR estimators. This class is found to be a generalization of the well-known moment-based M2M4 SNR estimation method for PSK modulation. The performance of the proposed estimators is evaluated for M-PSK and rectangular 16-QAM modulation.

This letter proposes a new data-aided carrier frequency offset (CFO) estimation scheme for orthogonal frequency division multiplexing (OFDM) communications suitable for frequency-selective channels. The proposed method is based on the transmission of a specially designed synchronization symbol that generates a particular signal structure between the received observation samples at the receiver. This structure is exploited to derive a closed-form expression of the CFO. The proposed solution offers a wide acquisition range with reduced computational load. Simulations over frequency-selective channels confirm the superiority of the proposed method compared to recent data-aided synchronization algorithms

This article addresses a new topic in OFDM with switched-beam systems, which is the exploitation of the OFDM symbols' structure to jointly perform the beam selection and channel estimation. Unlike previous works which were based on the symmetry of the channel to select the serving beam from the uplink of a MISO/MIMO system, our approach deals with the MISO downlink where the selection process is controlled at the mobile level. The method is based on the use of Kalman filtering with very spectrum-efficient overlapping pilots to separate the beams.

Non-data-aided (NDA) SNR estimation is considered for M-PSK transmission over additive white Gaussian noise channels. An iterative SNR estimator is derived from an envelope- based estimator. The performance of the proposed estimator is compared to existing algorithms. Simulation results shown for QPSK and 8-PSK demonstrate that the new estimator clearly performs as well as the best SNR estimator in each SNR range.

The performance of an OFDM system is highly sensitive to both carrier and sampling frequency offsets. This paper proposes a new joint carrier and sampling frequency offset estimation scheme for OFDM communications suitable for a frequency-selective fading channel. The joint carrier frequency offset (CFO) and sampling frequency offset (SFO) estimation in OFDM is reformulated and resolved as a harmonic retrieval problem. Accurate estimation can be reached even when only one OFDM symbol is used, hence making this method appropriate for offsets with a high level of variability in time. Extension to block processing is presented. Moreover the presented method proved to be robust against large CFO-SFO values, a claim that can not be made for other joint estimation techniques.

In this paper, the blind estimation of wireless CDMA receiver coefficients from the second order statistics of the signals is considered. Although many algorithms have been proposed so far, their performance analysis has always been carried out assuming perfect receiver coefficients estimation and/or under time-invariant conditions. In this article, we present some decision-directed blind algorithms and use a time-varying vector channel simulator to compare their performance with those of many previously proposed algorithms. It is shown that decision-directed chip-level algorithms can operate without the use of training sequences to avoid catastrophic error propagation, and that one should not expect an increase in performance from using least squares instead of least-mean-square. Furthermore the unpracticability of the bit-level algorithm (Gerlach 1992) and of the least significant algorithm (Liu and Zoltowski 1997) under a time varying environment is outlined. The performance of the principal component (Stanford) algorithm (Naguib and Paulraj 1995) is also studied.

We present a new, computationally efficient simulator for time-varying multi-path (fast fading) vector channel that can be used to evaluate the performance of antenna array wireless receivers. The development of the simulator is based on the emulation of the spatio-temporal correlation properties of the vector channel. The channel is modeled as a multi-channel FIR system with time-varying coefficients which are obtained via the application of a space-time correlation shaping transformation on some independent random sequences. The various parts of the new simulator are detailed and channel simulation realizations are presented and commented.

Synthetic microphone signals generated with the image model technique are used to study the effects of room reverberation on the performance of the maximum likelihood (ML) estimator of the time delay, in which the estimate is obtained by maximizing the cross correlation between filtered versions of the microphone signals. The results underscore the adverse effects of reverberation on the bias, variance and probability of anomaly of the ML estimator. Explanations of these effects are provided.

Time delay estimation (TDE) between the signals received by two or more spatially separated microphones can be used as a means for the passive localization of the dominant talker in applications such as audio-conference. However, in a recent study, it has been shown that reverberation can have disastrous effects on TDE performance. In this paper, we develop and evaluate a new cepstral prefiltering technique which can be applied on the microphone signals before the actual TDE in order to obtain a more accurate estimate of the position of a source in a typical reverberant environment. The performance of a TDE system with and without cepstral prefiltering is investigated under controlled conditions via Monte-Carlo simulations. The results clearly demonstrate the beneficial effects of the new cepstral prefiltering technique on TDE performance (i.e., reduction of bias, variance and number of anomalous estimates).

Previously, time-delay estimation (TDE) between two or more microphones has been proposed as a means for the passive localization of a talker in an audio-conference room. The authors present a simulation study of the effects of room reverberation on the performance of the maximum likelihood (ML) estimator of the time delay, which is obtained by maximizing the output of a generalized cross-correlator. To this end, synthetic microphone signals are generated with the image model technique and are used to evaluate the bias, variance and probability of anomaly of the ML estimator as a function of the room reflection coefficients and other external parameters of interest. The results clearly demonstrate the adverse effects of room reverberation on MLTDE performance. Qualitative and quantitative explanations of these effects are provided.>