The active reconfigurable intelligent surface (RIS) offers a promising solution to overcome the double-fading attenuation inherent in passive RIS-aided systems. However, this capability can be exploited by adversaries to launch potent pilot spoofing attack (PSA). In this paper, we propose a novel active RIS-aided covert PSA scheme for time-division duplex systems, where a passive eavesdropper manipulates the channel state information (CSI) estimation at the legitimate transceiver during the uplink stage and steers downlink data towards itself during the downlink stage. Crucially, without requiring perfect instantaneous CSI, a practical challenge for eavesdroppers, we maximize the average eavesdropping signal-to-noise ratio (SNR) by jointly designing the RIS reflection coefficients for both stages. To ensure covertness, we integrate an anti-energy ratio detection (ERD) mechanism that constrains the detection probability below a predefined threshold. The resulting non-convex optimization problem is solved via an efficient alternating optimization algorithm combined with penalty methods, handling the rank-1 constraints and statistical CSI uncertainties. Simulations demonstrate that the proposed scheme achieves up to 26 dB SNR gain over passive RIS-aided PSA and reduces ERD detection probability compared to traditional PSA. This work reveals the dual-edged nature of the active RIS: while enhancing security, it introduces new attack schemes demanding advanced countermeasures.

A second-order approximation of the Moment-Generating Function (MGF) for the sum of the product of correlated Rayleigh envelopes (a well-known problem in the literature) is presented in this paper. The MGF is applied to evaluate the outage probability of Reconfigurable Intelligent Surfaces (RIS) in both uncorrelated and correlated environments. Key contributions include the derivation of the MGF approximation and its application to outage probability assessment in different scenarios. The accuracy of the approximation is analyzed under two distinct correlation conditions, demonstrating its reliability in providing outage probability estimates. The results emphasize the effectiveness of the approximation as a practical tool for evaluating the outage probability in RIS-based systems, especially in correlated environments. These findings offer valuable insights for future research and practical implementations of RIS in wireless communication systems, particularly in optimizing system performance in the presence of varying correlation conditions.

Achieving digital fairness through NOMA is a critical challenge in modern 5G/6G wireless systems, particularly for satellite uplinks supporting IoT devices across wide coverage areas. The variation in link budgets across space and time increases the risk of unequal access, allowing only a subset of users to achieve sufficiently high transmission rates. This work examines a coordinated uplink NOMA systems to equalize IoT user rates. It also incorporates the impact of imperfect SIC to reflect practical scenarios. For single-slot NOMA, the optimal SIC ordering to maximize the minimum user rate is determined. For multi-slot NOMA, relevant to satellite scenarios, a user rate equalization algorithm is proposed and analyzed numerically, assessing the trade-off between user rates and receiver complexity. The proposed algorithm addresses multi-slot scheduling constraints with SIC limitations—a critical aspect of fairness in practical systems—by leveraging temporal dynamics to achieve fair resource allocation.

We consider the optimization of a two-hop linear relay channel based on an amplify-and-forward Multiple-Input Multiple-Output (MIMO) relay. The relay is assumed to derive the output signal by applying a Relay Transform Matrix (RTM) applied to the input signal. Assuming perfect channel state information about the channel at the relay and iid transmitted symbols, the RTM is optimized according to two different criteria:i)MIMO information rate;ii)information rate based on Orthogonal Space–Time Block Codes. The two assumptions have been addressed in part in the literature. The optimization problem is reduced to a manageable convex form, whose KKT equations are explicitly solved. Then, a parametric solution is given, which yields the power constraint and the information rate achieved with uncorrelated transmitted symbols as functions of a positive indeterminate. The solution for a given average power constraint at the relay is amenable to awater-filling-likealgorithm, and extends earlier literature results addressing the case without the direct link. The duty cycle of the two-hop relaying process is also addressed in the general form of the achievable rate. Simulation results are reported, which are relevant to a Rayleigh fading MIMO relay channel and the role of the direct link SNR is precisely assessed. Duty cycle optimization is also considered by a numerical example.

The optimization of the Relay Transform Matrix (RTM) in a two-hop relay network with an average relay power constraint and perfect channel state information at the relay is addressed in this paper. The study considers the most general case in terms of number of transmit and receive antennas at the source, relay, and destination, with arbitrary correlation of the received noise at all terminals. The optimization problem is reduced to a manageable convex form, which is solved by linear algebra transformation and the KKT equations. A parametric solution is given, which yields the power constraint and the capacity achieved with uncorrelated transmitted data. The solution is shown to be amenable to a water-filling-like algorithmic implementation, which extends earlier literature results addressing the case without the direct link. Simulation results are reported concerning a Rayleigh relay network where, in particular, the role of the direct link SNR is precisely assessed.

A global Internet of Things is possible by embracing constellations of satellites acting as orbiting gateways in a Direct-to-Satellite IoT (DtS-IoT). By removing the dependency on ground gateways, DtS-IoT enables a direct service on the regions illuminated by the passing-by satellite. After an in-depth overview of relevant experiments and candidate technologies, we discover that specific configurations of the Long-Range (LoRa) network protocol specification are particularly appealing to realize the DtS-IoT vision. Specifically, we profit from the maximum clock drift permitted on LoRa devices to propose the sparse satellite constellations concept. This approach significantly reduces the in-orbit DtS-IoT infrastructure at the expense of latency anyway present in resource-constrained IoT networks. We then introduce a novel algorithm comprising specific heuristics to design quasi-optimal topologies for sparse IoT constellations. Obtained results show that LoRa-compatible DtS-IoT services can already be provided world-wide with 10% and 4% of the satellites required for a traditional dense constellation, in different configurations.

A one-sided spatially-correlated multi-cluster scattering Rayleigh MIMO channel is considered in this work and its outage probability is derived in an analytic form based on Meijer function determinants. First, the spatially-uncorrelated case is addressed and the Moment Generating Function (MGF) of the information rate is expressed in an analytic closed-form. The MGF is then used to obtain the outage probability. A few special cases are addressed to provide a confirmation of the analytic results. Next, the MGF in the one-sided spatially correlated case is derived with the constraint of distinct positive spatial correlation eigenvalues. Numerical results are included to provide confirming evidence of the analytic results. These results are then used to assess the outage probability degradation due to spatial correlation in a selected example.

Sparse ISI channels have been considered for applications in underwater acoustic, wireless multipath, aeronautical, and satellite systems. They are characterized by having a long gain vector with few significant taps. BCJR-based simulation methods used when the channel memory is low become quickly impractical since the number of states grows exponentially large with it. A novel techniques is proposed in this paper based on simple information theoretical considerations and counting arguments, which is applied to the i.i.d. binary antipodal input case. Simulation results are reported illustrating how the information rate can be determined for either short ISI channels and long sparse ISI channels with 50-symbol memory.

The multiple cluster scattering MIMO channel is a useful model for pico-cellular MIMO networks. In this paper, space-time coded transmission over such a channel is considered, where the effective channel corresponds to a product of complex Gaussian matrices. An accurate closed-form approximation to the channel outage capacity using orthogonal space-time block codes has been derived. The result is valid for an arbitrary number of clusters of scatterers and an arbitrary antenna configuration. From the analytical and numerical results, we study the relative outage performance between the multi-cluster MIMO channel and the special case of Rayleigh-fading MIMO channel.

We consider orthogonal space-time block coded transmission over the multiple cluster scattering MIMO channels, where the effective channel equals the product of n complex Gaussian matrices. The considered channel model is typical in modeling pico-cellular MIMO propagations. In this setting, we derived a closed-form approximation to the channel outage capacity. The result is valid for an arbitrary number of clusters n-1 of scatterers and an arbitrary antenna configuration. Numerical results show the usefulness of the proposed approximation in diverse scenarios.

Rician fading multiple-input multiple-output (MIMO) channels with Kronecker-decomposable covariance structure are considered in this study to derive information measures characterizing their achievable rate. Analytic results are obtained in the asymptotic setting corresponding to infinitely many transmit and receive antennas, whose numbers approach a finite limiting ratio. More specifically, the asymptotic mean and variance of the mutual information are derived in this asymptotic regime, and it is shown that the corresponding probability distribution converges to a Gaussian distribution. All these results are based on the replica method, whose mathematical principles are discussed in this study. The ergodic capacity is also addressed by deriving an iterative water-filling algorithm yielding the capacity-achieving input covariance matrix (under an average power constraint). Resorting to the asymptotic Gaussianity of the mutual information distribution, an analytic expression of the outage capacity is derived based on the mean and variance of the mutual information. Finally, numerical results are reported to assess the level of approximation of the analytic results in comparison with those obtained by Monte Carlo simulation.

The performance of wireless sensor networks (WSN) is often assessed without paying close attention to the effects of channel state information recovery, which is assumed to be exactly known. This work focuses on the effects of imperfect knowledge of the channel state information on a WSN whose sensors measure a target parameter and send it to a common fusion center by an amplify-and-forward technique. A baseline estimation rule, consisting in disregarding the presence of noise in the estimated channel gain, is considered. Two other estimation rules are also studied, based on the joint processing of the received samples during the sensing and training intervals: i) a maximum a posteriori rule consisting in maximizing the a posteriori probability of the target parameter; ii) a least-squares rule based on the minimization of the mean-square error of the estimated target parameter. In both cases, conditionally on the target parameter, the received samples are assumed to be jointly Gaussian distributed as far as concerns the derivation of the estimation rule. Numerical results illustrate the merits of the proposed estimation rules by showing the MSE performance with different network parameters.

An asymptotic approach to derive the ergodic capacity achieving covariance matrix for a multiple-input multiple-output (MIMO) channel is presented. The method is applicable to MIMO channels affected by separately correlated Rician fading and co-channel interference. It is assumed that the number of transmit, receive and interfering antennas grows asymptotically while their ratios, as well as the SNR and the SIR, approach finite constants. Nevertheless, it is shown that the asymptotic results represent an accurate approximation in the case of a finitely many antennas and can be used to derive the ergodic channel capacity. This is accomplished by using an iterative power allocation algorithm based on a water-filling approach. The convergence of a similar algorithm (nicknamed frozen water-filling) was conjectured in a work by Dumont et al. Here, we show that, in the Rayleigh case, the frozen water-filling algorithm may not converge while, in those cases, our proposed algorithm converges. Finally, numerical results are included in order to assess the accuracy of the asymptotic method proposed, which is compared to equivalent results obtained via Monte-Carlo simulations.

Linear cooperative spectrum sensing has been proposed for cognitive radio networks which are often limited by the achievable complexity of the devices involved. The method is based on the Gaussian approximation of the summary statistics sent to the fusion center. The goodness of this approximation is investigated in this letter in the case of block Rician fading. The impact of the number of secondary users, sampling times, and the Rician factor are assessed by an analytic method proposed to calculate the false-alarm and the missed-detection probabilities.

The statistics of the mutual information of a separately correlated Rician fading multiple-input multiple-output (MIMO) channel in the presence of multiple-access interference are addressed in this paper. The approach followed is asymptotic in the number of transmit, receive, and interfering antennas, which are all assumed to grow asymptotically large while approaching finite ratios. In this asymptotic regime: i) the mean and the variance of the mutual information are calculated; and ii) the mutual information distribution is shown to converge to the Gaussian distribution, specified completely by the mean and variance, under some mild technical conditions. The asymptotic method adopted relies on two powerful tools developed in the context of theoretical physics: the replica method andsuperanalysis. The former has been already successfully applied in several research studies on MIMO systems. The application of these asymptotic results takes advantage of the fact that, in spite of being developed under the assumption of an asymptotically large number of antennas, they still represent a very accurate approximation even when the number of antennas is limited to a few units, as supported by the ample set of numerical results obtained by Monte Carlo simulations and reported in this paper.

The error performance of different types of receivers over anarbitrarily correlatedRician fading MIMO channel is assessed by evaluating the decision metric and the pairwise error probability. The differences are in the way channel state information at the receiver (CSIR) is recovered. We consider: i) agenie-aidedreceiver, based on the perfect knowledge of CSIR; ii) amismatchedreceiver, based on the use of pilot-aided (imperfect) CSIR in the perfect-CSIR decision metric; iii) anoptimumreceiver, based on perfect channel distribution information at the receiver (CDIR), which performs jointly channel and data estimation. In this paper, the decision metric of the optimum receiver is derived, and an iterative algorithm is proposed to calculate it for sequential decoding schemes. The complexity of this iterative algorithm is analyzed and compare against the complexity of the mismatched receiver. Closed-form expressions are given for the pairwise error probabilities (PEPs) of the three receivers witharbitrarily correlatedRician fading. These expressions are further processed to obtain the asymptotic (in the SNR) PEPs, the diversity order, and the asymptotic power losses. Numerical results are presented to support the validity of this analysis and to assess the impact of power efficiency and CDIR estimation errors on the PEP versus the ratio.∈ 󠁢 /Nₒ

Abstract—Channel estimation inaccuracy is known to affect significantly the error performance of coded communication systems. This applies in particular to broadband MIMO channels, often considered in conjunction with OFDM such as in the IEEE 802.11n and 802.16 standards. The focus of this work is on an 802.11n compliant MIMO-OFDM communication system. Different channel estimation techniques (based on pilot symbol insertion) are considered and their relevant error performance is analyzed. More specifically, genie-aided, mismatched, and optimum channel estimation techniques are studied with special emphasis on the last one as far as concerns the relative error performance versus complexity trade-off in suboptimum implementation. It is shown that the optimum receiver can be implemented by limiting the channel processing to the dominant eigenmodes, in order to reduce the ensuing complexity. The approach followed in this work may be seen as an extension of previous results relevant to the narrowband MIMO channel. I.

The ergodic capacity region of a multiple access separately-correlated Rician fading multiple input and multiple output (MIMO) wideband channel is investigated by using an asymptotic approach. Our channel model is sufficiently general to allow for considering different spatial correlation matrices for each user and delay at the transmitter and the receiver. We provide two algorithms which can be used to maximize the (weighted) rate sums in order to obtain the ergodic capacity region. Both algorithms are based on an asymptotic approximation of the single-user wideband mutual information. Moreover, it is shown that sum rate maximization is based on water- filling, similarly to the well known case where the transmitter has perfect channel state information. It is assumed that the number of transmit and receive antennas grows asymptotically approaching finite ratios while the number of users and the signal-to-noise ratios are kept finite. Numerical simulations show that this asymptotic approach is very accurate even when the number of antennas is as low as a few units.

The mutual information of a wideband Rician fading correlated MIMO channel is approximated by an analytic asymptotic method. The method is applied to derive the optimum input signal covariance matrix and the corresponding ergodic capacity. The numerical accuracy of the method is investigated and the effectiveness of covariance optimization is assessed with respect to the level of spatial correlation and the Rice factor.

We analyze the asymptotic error performance of different receiver schemes based on block space-time codes over a fully correlated Rician fading MIMO channel. The receivers differ in the way they obtain the channel state information at the receiver (CSIR) required to detect the transmitted signal. The baseline is a genie-aided receiver, which has perfect CSIR knowledge without any power or rate expenditure. Alternatively, a mismatched receiver extracts CSIR by pilot symbol insertion and uses it in the perfect CSIR decision metric. Finally, an optimum receiver, avoiding direct CSIR detection, estimates the transmitted data from the whole received sample and from the knowledge of pilot symbols. Key results are the receiver diversity and asymptotic power gain.

The asymptotic probability distribution of the mutual information of a separately correlated Rician fading multiple-input multiple-output (MIMO) channel is addressed. The mean and variance of the mutual information are derived when the number of transmit and receive antennas grows asymptotically large while their ratio approaches a finite constant. This derivation is based on thereplica method, widely used in theoretical physics and, more recently, in the analysis of communication systems (code-division multiple access (CDMA) and MIMO). Though the replica method allows to analyze complex systems in a comparatively simple way, some authors pointed out that its assumptions are not always rigorous. It is shown that the mutual information converges asymptotically to a Gaussian distribution under mild technical conditions, which are tantamount to assuming that the spatial correlation structure has no asymptotically dominant eigenmodes. The accuracy of the asymptotic approach is assessed by numerical results. It is shown that the approximation is very accurate in a wide range of system settings, even when the number of transmit and receive antennas is as small as a few units.

The ergodic capacity region of a multiple access separately-correlated Rician fading MIMO channel is investigated by using an asymptotic approach. It is assumed that the number of transmit and receive antennas grow asymptotically approaching finite values while the number of users and the SNR are kept finite. It is shown by numerical results that this asymptotic approach is very accurate even when the number of antennas is as low as a few units. The ergodic capacity achieving covariance matrices for all users are derived according to the algorithm provided and the corresponding capacity is compared with the mutual information achieved by iid power allocation. Monte-Carlo simulations are also reported in order to verify the accuracy of the asymptotic results.

The mean and variance of the mutual information of a separately-correlated Rician fading MIMO channel are derived in the presence of multi-access interference, when the number of transmit and receive antennas grows asymptotically large. Perfect receive channel-state information is assumed. The results are based on the replica method and superanalysis, powerful tools developed in the context of theoretical physics. The former allows to derive the moment generating function of the mutual information and the latter is required to cope with the coupling between the signal and interference parts. Analytic asymptotic results are compared with Monte-Carlo simulations to assess the accuracy of this method when the number of antennas is small.

The error performance of a fully correlated Rician fading MIMO channel is assessed by evaluating in closed form the decoding metric and the pairwise error probability. Three types of receivers are addressed by this work: i) agenie-aidedreceiver based on perfect knowledge of the channel state information (CSI); ii) amismatchedreceiver exploiting pilot-aided channel matrix detection followed by the use of a perfect-CSI metric; Hi) anoptimumpilot-aided receiver based on perfect channel distribution information at the receiver (CDIR) avoiding the direct use of the channel matrix.

A simple method to derive the ergodic capacity and the corresponding capacity-achieving covariance matrix for a MIMO fading channel with multiuser interference is provided. The method applies when the fading distribution is based on the separately-correlated (Kronecker) Rician fading model (with common receive correlation), as the number of antennas grow asymptotically large. Numerical results are provided to assess the accuracy of the asymptotic analytic method.

Two receiver structures based on pilot symbol-aided channel estimation are considered for the separately-correlated Rician fading MIMO channel. 1) A mismatched receiver, which decodes the received signal by first using maximum-likelihood (ML) or minimum mean-square error (MMSE) estimation of the MIMO channel matrix, and then by assuming that the estimate is exact. 2) An optimum receiver, which does not estimate explicitly the channel matrix but jointly processes the received pilot and data samples assuming known channel distribution. The main focus is on the optimum receiver. First, the optimum detection algorithm for the separately-correlated Rician fading MIMO channel is derived. Then, an iterative implementation suitable for trellis space–time decoding is proposed in order to reduce the algorithm complexity. Numerical results are presented for a 2x2 MIMO system with Rayleigh/Rice correlated/uncorrelated fading and a simple trellis space–time code. These results show that substantial gain is available by using our proposed optimum receiver either in terms of Eb/N0 and system throughput (in both cases accounting for pilot-symbol rate reduction). Finally, the effect of parameter estimation (required by the optimum receiver) is studied and it is shown that this effect is almost unnoticeable in the case considered.

The mean and variance of the asymptotic asymptotic mutual information (with any given input covariance matrix) of the separately-correlated MIMO Rician fading channel are derived as functions of two scalar parameters, which are solutions to of a polynomial equation system. The derivation follows the so-called replica method, widely used in theoretical physics and more recently in communication systems. A rigorous proof of the analytic continuation from the positive integer to the positive real domain is given to support the application of the replica method and replica symmetry is assumed for the choice of the integration saddlepoint. Numerical results are provided to illustrate the accuracy of the analytical method in several settings.

Multiple-input multiple-output (MIMO) flat fading channels with channel state information available at the receiver and channel state distribution available at the transmitter are considered. The ergodic capacity-achieving covariance matrix (ECACM) under an average input power constraint is shown to be not unique in general, and three sufficient conditions for the uniqueness are derived. Then, the ECACM structure is characterized for certain classes of MIMO channels including the following classes of MIMO channels: i) right-permutationally-invariant channels including i.i.d. channels; ii) right-symmetrically-distributed channels; and iii) right-permutationally-invariant and right-symmetrically-distributed channels

We address the design of a receiver for the uplink of a direct sequence code division multiple access wireless communication system, endowed with multiple transmit and receive antennas and affected by multiple-access interference (MAI) for the presence of multiple transmitting users. The receiver is assumed to keep separate the tasks of mitigating the spatial and MAI in order to be implementable in two options: a baseline option accommodating the MAI mitigation interface only and an enhanced option accommodating the spatial interference mitigation interface as well. The MAI mitigating interface is based on standard linear multiuser detectors. The spatial interference mitigating interface is derived according to a linear minimum mean-square error criterion, which assumes the MAI mitigating interface to be given.

We consider a multiuser multiple-input multiple-output (MIMO) communication system using code-division multiple access (CDMA) and multiuser detection to discriminate the different users. Our focus is on the CDMA uplink of a frequency-nonselective Rayleigh fading channel. We study two types of receivers: joint receivers, which address simultaneously both spatial and multiple-access interference; and separate receivers, addressing the two types of interference individually. This approach allows assessing the benefits of adding MIMO processing capabilities to existing multiuser single-input single-output systems. For both receiver types, we analyze solutions based on linear (matched filter, decorrelator, minimum mean-square error) and maximum-likelihood receivers. For all the receivers considered, we provide closed-form expressions (as expectations of given functions) of the resulting pairwise error probabilities. Performance results are obtained in terms of frame-error rate versus Eb/N0, following two different approaches. An analytic approach using large-system asymptotic methods, whereby the system parameters (number of users and antennas, spreading gain) are assumed to grow to infinity with finite limiting ratios. A computer-simulation approach is used to illustrate the differences between asymptotic and simulation results

Multiple-input multiple-output (MIMO) flat-fading channels with channel state information available at the receiver and channel state distribution available at the transmitter are considered. The ergodic capacity-achieving covariance matrix (ECACM) under an average input power constraint is shown to be not unique in general, and three sufficient conditions for the uniqueness are derived. Then, the structure of ECACM is characterized for certain classes of MIMO channels including the following classes of MIMO channels: 1) right-permutationally invariant channels including independent and identically distributed (i.i.d.) channels; 2)right-symmetrically distributed channels; and 3) right-permutationally invariant and right-symmetrically distributed channels. Finally, part of the single-user results are extended to the multiple-access channel.

We consider the use of multiple transmit and receive antennas (MIMO) in a wideband code-division multiple-access system with space-time coding. Using large-system asymptotic analyses, we examine the performance of linear receivers whereby multiuser detection and MIMO decoding are kept separate.

We examine the performance of a reduced-complexity doubly iterative decoder for space-time turbo codes on a quasi-static fading channel. The decoder works by using preliminary soft values of the coded symbols, obtained after a limited number of turbo iterations, to reduce the spatial interference from the received signal. Then, new turbo iterations are performed to improve on the quality of the soft values, and so on. Using a number of approximations, we obtain a receiver interface that achieves a good tradeoff between performance and complexity, and allows the use of turbo space-time codes for a large number of transmit and receive antennas.

We introduce a doubly-iterative decoder for space–time turbo codes on a quasi-static fading channel. The decoder works by using preliminary soft values of the coded symbols, obtained from a limited number of turbo iterations, to reduce the spatial interference from the received signal. After this reduction, new turbo iterations are performed to improve on the quality of the soft values, and so on. Using a number of approximations, we obtain a receiver interface that achieves a good trade-off between performance and complexity, and allows the use of turbo space–time codes for a large number of transmit and receive antennas.

Under the assumption of a frequency-flat slow Rayleigh fading channel with multiple transmit and receive antennas, we examine the effects of imperfect estimation of the channel parameters on error probability when known pilot symbols are transmitted among information data. Three different receivers are considered. The first one derives an estimate of the channel (by using either a maximum-likelihood or a minimum-mean-square error criterion), next uses this estimate in the same metric that would he applied if the channel were perfectly known. The second receiver derives again an estimate of the channel, but uses the maximum-likelihood metric conditioned on the channel estimate. Our last receiver processes simultaneously the pilot and data symbols received. Simulation results are exhibited, showing that only a relatively small percentage of the transmitted frame need he allocated to pilot symbols in order to experience an acceptable degradation of error probability due to imperfect channel knowledge. Algorithms for the recursive calculation of the decision metric of the last two receivers are also developed, for application to sequential decoding of trellis space-time codes.

Abstract In this tutorial paper, we describe iterative receivers that combine a soft decoder for a general space‐time code with a spatial‐interference canceler. The performance of these receivers is analyzed by using EXIT charts, a convenient graphical description that yields quite accurate results. By properly combining the EXIT characteristics of the canceler and of the decoder, the convergence behavior of the iterative algorithms can be understood, and design guidelines derived. The idea here is to observe that both the interference canceler and the decoder can be studied by examining the evolution of the extrinsic information passed along to the connected blocks. Copyright © 2004 John Wiley & Sons, Ltd.

We compare complexities and performances of some suboptimum linear and nonlinear (BLAST) interfaces used for decoding space-time codes used over a Rayleigh fading channel. We observe how the introduction of an interleaver can be beneficial here. We introduce a new simple iterative linear interface, based on hard Viterbi decoding and offering a performance considerably improved with respect to noniterative receivers.

Linear periodically time-varying systems are the basis of many practical systems, ranging from electrical networks with periodically time-varying components to filters which incorporate modulators in the signal path. Despite the pervasive use of digital systems, the study of analog time-varying systems can be considered an important branch of applied mathematics and control, with applications to' signal processing and communications. Given the well-known modulators filter bank description of a linear periodically time-varying system the spectral transformation between the input and the output would follow readily. This paper shows a simple way to find such a description for a continuous-time linear periodically time-varying system within a feedback loop, motivated by applications such as gated phase-locked loops.

We describe a simple technique for the numerical calculation, within any desired degree of accuracy, of the pairwise error probability (PEP) of space-time codes. This method is very general, and applies to the calculation of the probability P(/spl nu/ > x), where /spl nu/ and x are independent random variables whose moment-generating functions (MGF) are known. In particular, it applies to the calculation of the expectation E[Q(/spl radic//spl xi/)], where Q(/spl middot/) is the Gaussian tail function and /spl xi/ is a random variable whose moment-generating function is known.

We study the asymptotic behavior of space-time codes when a linear receiver interface is used in lieu of the maximum-likelihood interface. Specifically, we determine the behavior of pairwise error probabilities with maximum-likelihood decoding and with four types of receiver interfaces: the maximum-likelihood interface, the linear zero-forcing interface, the linear minimum-mean-square-error interface, and the matched-filter interface. An asymptotic analysis is performed by assuming that the number of receiving antennas grows to infinity while the number of transmitting antennas is finite, and that both numbers grow to infinity but their ratio remains constant. We show that with all the interfaces studied here the asymptotic performance of space-time codes is determined by the Euclidean distances between code words. Moreover, the performance of linear interfaces comes close to maximum-likelihood if the number r of receive antennas is sufficiently larger than the number t of transmit antennas. The dependence of error probabilities on Euclidean distance is valid for intermediate signal-to-noise ratios even when the number of antennas is small.

We study the asymptotic behavior of space-time codes when the number of transmit and receive antennas grows to infinity. Specifically, we determine the behavior of pairwise error probabilities with maximum-likelihood (ML) decoding and with three types of receiver interfaces: the ML interface, the linear zero-forcing (ZF) interface, and the linear minimum-mean-square-error (MMSE) interface. Two situations are studied: when the number of receiving antennas grows to infinity while the number of transmitting antennas is finite, and when both numbers grow to infinity but their ratio remains constant. We show that with ML or linear interfaces the asymptotic performance of space-time codes is determined by the Euclidean distances between codewords. Moreover, with the two linear interfaces examined here the number r of receive antennas must be much larger than the number t of transmit antennas to avoid a sizeable loss of performance; on the other hand, when r /spl Gt/ t, the performance of these linear interfaces comes close to that of ML. The dependence of error probabilities on Euclidean distance is valid for intermediate signal-to-noise ratios (SNRs) even when the number of antennas is small. Simulations validate our theoretical findings, and show how asymptotic results may be substantially valid even in a nonasymptotic regime: thus, even for few antennas, off-the-shelf codes may outperform space-time codes designed ad hoc.

We describe a simple technique for the numerical calculation, within any desired degree of accuracy, of the pairwise error probability (PEP) of space-time codes over fading channels. This method applies also to the calculation of E[Q(/spl radic//spl xi/)] for any nonnegative random variable /spl xi/ whose moment-generating function /spl Phi//sub /spl xi//(s)=E[exp(-s/spl xi/)] is known. Its application to the multiple antenna independent Rayleigh-fading channel and to the Rayleigh block fading channel is discussed, and illustrated by two simple examples.

We study the performance of a UNITS downlink, as achieved by a single-user (RAKE) and a multiuser (linear MMSE) receiver with adaptive modulation and multicode in the presence of multipath fading. We advocate an adaptive transmission scheme that varies the number of virtual users and the modulation spectral efficiency in order to optimize the system throughput. Two multipath channel models are considered, exhibiting 3 and 9 paths, respectively. Over the more favorable multipath channel, the multiuser MMSE receiver yields a significant performance enhancement with respect to the RAKE receiver.

Using asymptotic analysis, we study the effect of frequency-flat fading on code division multiple access (CDMA) systems with linear receivers and random spreading sequences. Specifically, we let the number of users grow without bound, while the ratio of number of users to spreading sequence length is kept fixed to a value /spl alpha/. We treat separately the cases of slow fading (nonergodic channel) and of fast fading (ergodic channel). For the former channel, we derive the outage probability, while for the latter we compute the channel capacity. In both cases, multiple classes of users with different qualities of service are dealt with. As /spl alpha//spl rarr//spl infin/, the system throughput tends to the same limit of 1.44 bit/symbol as for the nonfading channel with both single-user matched filter (SUMF) and linear minimum mean-square-error (MMSE) receivers. The outage probability exhibits a floor for all /spl alpha/ with the SUMF receivers, while with MMSE receiver the floor is present only for /spl alpha/>1. We also address the tradeoffs involved in the allocation of available bandwidth between spreading and coding.

This paper studies the performance of the clock transfer scheme for burst-mode communication systems for which data are received during short, equally spaced intervals. Its main focus is on satellite-based time-division multiple-access (TDMA) communication systems with data regeneration and switching onboard the satellite, although the results apply to other TDMA systems as well. The system reference clock is generated onboard from an incoming, very stable ground source, based on a burst-mode demodulator that extracts the clock from a discontinuous modulated carrier due to the bursty nature of TDMA signals. If good enough, this onboard regenerated clock avoids the use of bulky and expensive clocks in the satellite payload and can act as the master clock of the TDMA system.

We derive the performance limits of a radio system consisting of a transmitter with t antennas and a receiver with r antennas, a block-fading channel with additive white Gaussian noise (AWGN), delay and transmit-power constraints, and perfect channel-state information available at both the transmitter and the receiver. Because of a delay constraint, the transmission of a codeword is assumed to span a finite (and typically small) number M of independent channel realizations; therefore, the relevant performance limits are the information outage probability and the "delay-limited" (or "nonergodic") capacity. We derive the coding scheme that minimizes the information outage probability. This scheme can be interpreted as the concatenation of an optimal code for the AWGN channel without fading to an optimal beamformer. For this optimal scheme, we evaluate minimum-outage probability and delay-limited capacity. Among other results, we prove that, for the fairly general class of regular fading channels, the asymptotic delay-limited capacity slope, expressed in bits per second per hertz (b/s/Hz) per decibel of transmit signal-to-noise ratio (SNR), is proportional to min (t,r) and independent of the number of fading blocks M. Since M is a measure of the time diversity (induced by interleaving) or of the frequency diversity of the system, this result shows that, if channel-state information is available also to the transmitter, very high rates with asymptotically small error probabilities are achievable without the need of deep interleaving or high-frequency diversity. Moreover, for a large number of antennas, delay-limited capacity approaches ergodic capacity.

We use results from the asymptotic analysis of code-division multiple access with random spreading as a tool for gaining insight and deriving design guidelines on practical system issues, inspired by the current UMTS/IMT2000 standardization process. In particular, we consider a simple synchronous single-cell system with perfect power control and linear detection, and we examine the following: (1) the optimal tradeoff between coding rate and spreading gain and (2) the comparison of different multirate schemes. Our analysis shows that, for the sake of system spectral efficiency maximization, there exists a threshold E/sub b//N/sub o/ below which the single-user matched filter (SUMF) is optimal (within the limits of our system model). As far as multirate schemes are concerned, we show that multicode and variable-spreading with SUMF detection are equivalent, while the former is uniformly better than the latter with linear minimum-mean-square error detection. Variable-spreading can perform very close to multicode if high-rate users are detected by observing the whole "low-rate" symbol interval. Finally, we compare the capacity regions of the multimodulation and multicode schemes versus the E/sub b//N/sub o/ ratio.

We study the synergy between coded modulation and antenna-diversity reception on channels affected by slow Rician fading. Specifically, we assess the impact of channel-state information (CSI) on error probability. We show that with a good coding and constant envelope modulations (for example, phase-shift keying) scheme the loss in performance when CSI is not available is moderate (around 1.5 dB). Moreover, as the diversity order grows, the channel tends to become Gaussian.

We study optimal constant-rate coding schemes for a block-fading channel with strict transmission delay constraint, under the assumption that both the transmitter and the receiver have perfect channel-state information. We show that the information outage probability is minimized by concatenating a standard "Gaussian" code with an optimal power controller, which allocates the transmitted power dynamically to the transmitted symbols. We solve the minimum outage probability problem under different constraints on the transmitted power and we derive the corresponding power-allocation strategies. In addition, we propose an algorithm that approaches the optimal power allocation when the fading statistics are not known. Numerical examples for different fading channels are provided, and some applications discussed. In particular, we show that minimum outage probability and delay-limited capacity are closely related quantities, and we find a closed-form expression for the delay-limited capacity of the Rayleigh block-fading channel with transmission over two independent blocks. We also discuss repetition diversity and its relation with direct-sequence or multicarrier spread-spectrum transmission. The optimal power-allocation strategy in this case corresponds to selection diversity at the transmitter. From the single-user point of view considered in this paper, there exists an optimal repetition diversity order (or spreading factor) that minimizes the information outage probability for given rate, power, and fading statistics.

Maximization of the energy transfer ratio for time-limited signals over linear channels is considered. The case of linear channels with rational transfer function is addressed and a general procedure for the analytic solution of the maximization problem is outlined. The maximum energy transfer ratio over a fixed time interval is evaluated in some cases of interest. Two performance metrics-the energy transfer ratio and the energy intersymbol interference (ISI) ratio-are evaluated for optimal signals and compared to those of commonly used rectangular and sinusoidal pulses in order to determine the achievable gain.

We study the information outage rate of a block-fading Gaussian channel with strict delay constraint and constant rate transmission, under the assumption that both the transmitter and the receiver have perfect knowledge of the channel. We find the power allocation strategy minimizing the outage rate under a constraint on the average transmitted power. Numerical examples for the Rayleigh fading channel are provided. In particular, an explicit formula for the delay-limited capacity of the 2-block Rayleigh fading channel is derived.

It has been recognized by Zehavi (1992) that the performance of coded modulation over a Rayleigh fading channel can be improved by bit-wise interleaving at the encoder output, and by using an appropriate soft-decision metric as an input to a Viterbi (1990) decoder. The paper presents in a comprehensive fashion the theory underlying bit-interleaved coded modulation, provides tools for evaluating its performance, and gives guidelines for its design.

Following the approach introduced by Cavers and Ho (1992), the performance of component-interleaved multidimensional constellations over the Rayleigh fading channel is evaluated analytically. The error probabilities are approximated by the union bound using an exact expression of the pairwise error probability. Simulation results show that this bound can be used effectively as a design criterion for the selection of high-diversity multidimensional constellation over the Rayleigh fading channel.

Multiuser detection schemes are applied to a regional GEO satellite mobile system based on FDM/TDMA. The study focuses on the system uplink (mobile to satellite) and analyzes the performance of candidate schemes taken from the class of linear multiuser receivers. A new simple MMSE combiner is proposed, which provides performance close to optimal in terms of output MSE and lends itself to a simple adaptive implementation. Results pertaining to FDM/TDMA coded transmission with slow frequency hopping and short-depth interleaving are presented.

Zehavi (1992) showed that the performance of coded modulation over a Rayleigh fading channel can be improved by bit-wise interleaving the encoder output and by using an appropriate soft-decision metric as an input to a Viterbi decoder. The goal of this paper is to present in a comprehensive fashion the theory underlying bit-interleaved coded modulation, to provide tools for evaluating its performance, and to give guidelines for its design.

We address the problem of designing and analyzing the performance of a coded modulation scheme for the fading channel when space diversity is used. Under fairly general conditions, a channel affected by fading can be turned into an additive white Gaussian noise (AWGN) channel by increasing the number of diversity branches. Consequently, it can be expected (and is indeed verified by our analyses and simulations) that a coded modulation scheme designed to be optimal for the AWGN channel also will perform asymptotically well on a fading channel with diversity. This paper presents bounds on the bit-error probability of a system with coded modulation and diversity for space- and time-correlated Rician flat fading. Specifically, we derive a new method which allows evaluation of the pairwise error probability extremely easily, as well as accurately and computationally fast. The accuracy achieved improves considerably on the widely used, but rather loose Chernoff bound. Starting from this analysis, we study the asymptotic behavior of the fading channel with diversity as the number of diversity branches increases, and we address the effects of diversity on coded modulation performance and design criteria, including the effect on interleaver depth (which affects the total delay of the system).

For pt. I see ibid., vol.45, no.6, p.563-572, 1997. We study coded modulation with block differential detection in an arbitrarily correlated Rician fading channel with space diversity. Coded differential q-PSK is included in our analysis as a special case. A metric is chosen that is optimum for perfect interleaving, slow fading, and independent diversity branches. For slow fading, we compare the the cutoff rates of the channels resulting from different choices of block length N and diversity index M. Specifically, we show that block detection with diversity may or may not generate a better coding channel than usual differential detection, according to the code selected and the combination of values of M and N. In particular, for low-diversity orders (M=1,2) and for low-to-medium code rates, differential detection is still an optimal or near-optimal solution, while for high-diversity orders (M/spl ges/2) and medium-to-high code rates (up to uncoded modulation) block detection with N>2 can provide a significant gain. An error floor always exists when fading is fast. It decreases exponentially with the product of code diversity and space diversity, so that the latter emerges as a very effective technique for lowering the error floor of a system affected by fast fading. Performance examples based on actual coding schemes are also shown.

For pt.II see ibid., vol.45, no.6, p.55-67, 1997. In previous work, we have studied the impact of diversity on coded digital communication systems operating over fading channels. In particular, we have shown that diversity may be thought of as a way of making the channel more similar to a Gaussian one. The present paper extends this analysis to fading channels affected by co-channel interference (CCI). Three receiver models are examined, namely, with coherent detection and perfect channel-state information (CSI), with differential; and with pilot-tone detection. We study the effect of diversity on the irreducible error floor caused by CCI and fading, and the asymptotic behavior of the channel as the diversity order increases. Our results show that, when perfect CSI is available, diversity is able to turn asymptotically the channel into a CCI-free additive white Gaussian noise (AWGN) channel with the same signal-to-noise ratio (SNR). On the other hand, differential and pilot-tone detection do not remove interference in the limit. Nevertheless, also with these detection schemes, diversity achieves significant gains when the SNR is large enough. Calculation of the channel cutoff rate provides guidelines for the design of coded systems with CCI in fading environments. A wide range of examples, validated by computer simulation, illustrates our conclusions.

We compare three receivers for coded narrowband transmission affected by fading and co-channel interference. The baseline receiver is based on conventional diversity reception with maximal-ratio combining. A multiuser approach allows us to derive a maximum-likelihood multiuser receiver and its reduced-complexity suboptimal version. Finally, a decorrelating diversity receiver, which seeks a tradeoff between performance and complexity, is studied. Closed-form performance parameters are derived for all the proposed receivers in the case of coded coherent PSK and independent frequency nonselective Rayleigh fading.