Quantum error correction (QEC) is essential for realizing scalable quantum computation. However, when evaluating its benefits, most analyses assume idealized components, overlooking the imperfections inherent in realistic fault-tolerant (FT) implementations. In this paper, we investigate the performance of QEC schemes taking into account that quantum gates and measurements are themselves error-prone. We derive bounds for the failure probability of Shor-style FT-QEC schemes using limited structural information, such as the number of flag qubits and quantum gates. Our analysis separates and quantifies two key contributors to the failure rate: decoding errors and residual errors arising from circuit-level faults. The derived bounds highlight fundamental limitations in Shor-style FT-QEC performance and quantify how circuit imperfections degrade error correction capabilities, under the assumption of depolarizing noise.

Reliable quantum computation requires fault-tolerant protocols to prevent errors from propagating during syndrome extraction in quantum error correction. We present a novel fault-tolerant syndrome extraction technique for CSS codes, which we refer to as the cut-cat state scheme. While each ancilla qubit interacts non-fault-tolerantly with a pair of data qubits, we introduce additional cat stabilizer measurements to identify and correct the resulting hook errors. Our approach maintains the key benefit of cat-based extraction, i.e., parallelized data qubit interactions, while reducing the number of simultaneous qubits required by more than half. Compared to flag-based state-of-the-art protocols, the cut-cat scheme offers a notable advantage in terms of two-qubit gate count as the code distance increases.

Quantum computers are highly vulnerable to noise, necessitating the use of error-correcting codes to protect stored data. Errors must be continuously corrected over time to counteract decoherence using appropriate decoders. Therefore, fast decoding strategies capable of handling real-time syndrome extraction are crucial for achieving fault-tolerant quantum computing. In this paper, we introduce the bubble clustering (BC) decoder for quantum surface codes, which serves as a low-latency replacement for MWPM, achieving significantly faster execution at the cost of a slight performance degradation. This speed boost is obtained leveraging an efficient cluster generation based on bubbles centered on defects, and avoiding the computational overhead associated with cluster growth and merging phases, commonly adopted in traditional decoders. Our complexity analysis reveals that the proposed decoder operates with a complexity on the order of the square of the number of defects. For moderate physical error rates, this is equivalent to linear complexity in the number of data qubits.

We introduce a massive multiple access scheme designed to meet different trade-offs between reliability, scalability, and latency. To maximize the number of successfully decoded users, the scheme builds upon coded random access, incorporating both grant-free and grant-based procedures, along with a massive acknowledgment phase conducted at the base station. The main design premise is the establishment of two distinct latency deadlines: the first one guaranteeing high reliability (e.g., between 99% and 99.99%), and the second one enforcing ultra-high reliability, even above 99.9999%. This dual-latency approach, supplemented with massive MIMO, enables the system to support a higher number of active users per frame while meeting stringent reliability requirements. Throughout the paper, we present a theoretical analysis and derive performance bounds to guide and support effective system design. The approach opens the door for the development of critical services that bridge the gap between massive machine-type communication (mMTC) and ultra-reliable and low-latency communication (URLLC), providing a more flexible and efficient framework for next-generation systems.

In the implementation of quantum information systems, one type of Pauli error, such as phase-flip errors, may occur more frequently than others, like bit-flip errors. For this reason, quantum error-correcting codes that handle asymmetric errors are critical to mitigating the impact of such impairments. To this aim, several asymmetric quantum codes have been proposed. These include variants of surface codes like the XZZX and ZZZY surface codes, tailored to preserve quantum information in the presence of error asymmetries. In this work, we propose two classes of Calderbank, Shor and Steane (CSS) topological codes, referred to as cylindrical and Möbius codes, particular cases of the fiber bundle family. Cylindrical codes maintain a fully planar structure, while Möbius codes are quasi-planar, with minimal non-local qubit interactions. We construct these codes employing the algebraic chain complexes formalism, providing theoretical upper bounds for the logical error rate. Our results demonstrate that cylindrical and Möbius codes outperform standard surface codes when using the minimum weight perfect matching (MWPM) decoder.

Cell-free massive MIMO (CF-mMIMO) networks leverage seamless cooperation among numerous access points to serve a large number of users over the same time/frequency resources. This paper presents a novel multi-task learning approach aimed at mitigating inter-user interference and enhancing spectral efficiency, particularly in scenarios where the number of users far exceeds the available orthogonal pilots. Our proposed method entails the design and unsupervised training of a deep neural network (DNN), employing a custom loss function specifically tailored to perform joint power control and pilot assignment. Numerical results demonstrate that our algorithm outperforms existing power control and pilot assignment strategies in terms of achievable network throughput, minimum user rate, and per-user energy consumption.

This paper elaborates on the idea of building grant-free channel access schemes from non-orthogonal multiple access ones, and proposes an explicit such scheme based on sparse code multiple access (SCMA). In the designed protocol, SCMA codebooks and pilots are chosen by users in a fully uncoordinated fashion, with multiple pilots associated with the same codebook to aid codebook detection. A modified two-stage SCMA decoder is proposed, where a low-complexity collision resolution algorithm, working on a super-constellation, and an SCMA message passing detector are iteratively applied. Numerical results, integrated by analysis in the high signal-to-noise ratio regime, highlight a potential for the proposed scheme in the context of massive uncoordinated machine-type uplink.

Surface codes are versatile quantum error-correcting codes known for their planar geometry, making them ideal for practical implementations. While the original proposal used PauliXor PauliZoperators in a square structure, these codes can be improved by rotating the lattice or incorporating a mix of generators in the XZZX variant. However, a comprehensive theoretical analysis of the logical error rate for these variants has been lacking. To address this gap, we present theoretical formulas based on recent advancements in understanding the weight distribution of stabilizer codes. For example, over an asymmetric channel with asymmetryA= 10 and a physical error ratep→ 0, we observe that the logical error rate asymptotically approachespL→ 10p2for the rotated [[9, 1, 3]] XZZX code andpL→ 18.3p2for the [[13, 1, 3]] surface code. Additionally, we observe a particular behavior regarding rectangular lattices in the presence of asymmetric channels. Our findings demonstrate that implementing both rotation and XZZX modifications simultaneously can lead to suboptimal performance. Thus, in scenarios involving a rectangular lattice, it is advisable to avoid using both modifications simultaneously.

One of the main challenge for an efficient implementation of quantum information technologies is how to counteract quantum noise. Quantum error correcting codes are therefore of primary interest for the evolution towards quantum computing and quantum Internet. We here analyze the performance of surface codes, one of the most important class for practical implementations, on both symmetric and asymmetric quantum channels. We derive approximate expressions, confirmed by simulations, to evaluate the performance of surface codes and of XZZX codes, and provide a metric to assess the advantage of codes with respect to uncoded systems. Our findings allow to characterize the performance by means of analytical formulas of surface codes, like, for example, the [[13, 1, 3]], the [[23, 1, 3/5]], the [[33, 1, 3/7]], and the [[41, 1, 5]] surface codes.

In this paper, new coded random access schemes for massive IoT, capable of accommodating a non-instantaneous feedback from the receiver, are developed. The proposed schemes are based on the introduction of waiting slots between any two consecutive replicas transmitted by the same active device. The waiting window can be exploited by the device to receive acknowledgment messages that, otherwise, would consume uplink resources with a consequent performance degradation. The achievable performance of the developed schemes is investigated, by analysis and simulation, over a realistic wireless channel model and realistic signal processing at the base station, showing negligible losses with respect to previously proposed coded random access systems with idealized instantaneous feedback.

In next generation Internet-of-Things, the overhead introduced by grant-based multiple access protocols may engulf the access network as a consequence of the unprecedented number of connected devices. Grant-free access protocols are therefore gaining an increasing interest to support massive access from machine-type devices with intermittent activity. In this paper, coded random access (CRA) with massive multiple input multiple output (MIMO) is investigated as a solution to design highly-scalable massive multiple access protocols, taking into account stringent requirements on latency and reliability. With a focus on signal processing aspects at the physical layer and their impact on the overall system performance, critical issues of successive interference cancellation (SIC) over fading channels are first analyzed. Then, SIC algorithms and a scheduler are proposed that can overcome some of the limitations of the current access protocols. The effectiveness of the proposed processing algorithms is validated by Monte Carlo simulation, for different CRA protocols and by comparisons with developed benchmarks.

Grant-free access schemes are candidates to support future massive multiple access applications owing to their capability to reduce control signaling and latency. As a promising class of grant-free schemes, coded random access schemes can achieve high reliabilities also with uncoordinated transmissions and therefore in presence packet collisions. In this paper, an analysis tool for coded random access, based on density evolution, is proposed and exploited for system design and optimization. In sharp contrast with the existing literature, where such tools have been developed under simplified channel assumptions, the proposed tool captures not only MAC layer features, but also the physical wireless fading channel and a realistic physical layer signal processing based on multiple antennas and randomlychosen orthogonal pilots. Theoretical results are validated by comparison with symbol-level Monte Carlo simulations.

The design of highly scalable multiple access schemes is a main challenge in the evolution towards future massive machine-type communications, where reliability and latency constraints must be ensured to a large number of uncoordinated devices. In this scenario, coded random access (CRA) schemes, where successive interference cancellation algorithms allow large improvements with respect to classical random access protocols, have recently attracted an increasing interest. Impressive performance can be potentially obtained by combining CRA with massive multiple input multiple output (MIMO). In this paper we provide an analysis of such schemes focusing on the effects of imperfect channel estimation on successive interference cancellation. Based on the analysis we then propose an innovative signal processing algorithm for CRA in massive MIMO systems.

An information-theoretic approach to irregular repetition slotted ALOHA (IRSA) is proposed. In contrast with previous works, in which IRSA analysis is conducted only based on quantities that are typical of collision models such as the traffic, the new approach also captures more fundamental quantities. Specifically, a suitable codebook construction for the adder channel model is adopted to establish a link with successive interference cancellation over the multi-packet reception channel. This perspective allows proving achievability and converse results for the average sum rate of IRSA multiple access schemes.

Massive multiple access schemes, capable of serving a large number of uncoordinated devices while fulfilling reliability and latency constraints, are proposed. The schemes belong to the class of grant-free coded random access protocols and are tailored to massive multiple input multiple output (MIMO) base station processing. High reliability is obtained owing to an intra-frame spatial coupling effect, triggered by a simple device access protocol combined with acknowledgements (ACKs) from the base station. To provide system design guidelines, analytical bounds on error floor and latency are also derived. The proposed schemes are particularly interesting to address the challenges of massive machine-type communications in the framework of next generation massive multiple access systems.

Networks of randomly distributed nodes appear in various fields, including forestry and wireless communications, and can often be modeled, using stochastic geometry theory, as Poisson point processs (PPPs). In these contexts, estimation of nodes density is important for monitoring and optimizing the network. Originally, this problem has been addressed in forestry where the trees are the nodes and, assuming these are distributed according to an infinite two-dimensional homogeneous PPP, the spatial density can be estimated by measuring the distances from one reference tree to its neighbors. However, in many other scenarios, nodes could result invisible with some probability, for example depending on distance. In this paper, we derive the Cramér-Rao bounds and new estimators for the node spatial density, taking into account a limited capability in sensing neighbors. As an example, we provide estimators of the spatial density of transmitting devices in wireless networks with links affected by thermal noise, path loss, and shadowing.

Owing to the extremely narrow beams, a main issue in optical deep space communications is represented by miss-pointing errors, which may severely degrade the system performance and availability. In this paper, we address pointing losses in the case in which both the receiver and the transmitter are affected by angular errors. Pointing losses are evaluated through two approaches. The first approach is deterministic and only requires knowledge of a maximum angular error. The second approach requires knowledge of the angular error statistical distribution and tackles the problem from an outage probability viewpoint. These tools are then applied to analyze the impact of pointing losses in deep space optical links in which both terminals suffer from miss-pointing effects. The antenna gains are first optimized to maximize the effective system gain. The optimum antenna gains are then applied to evaluate maximum achievable ranges and to perform link design by means of optical link budgets.

An important modulation technique for Internet of Things (IoT) is the one proposed by the low power long range (LoRa) alliance. In this paper, we analyze the M-ary LoRa modulation in the time and frequency domains. First, we provide the signal description in the time domain, and show that LoRa is a memoryless continuous phase modulation. The cross-correlation between the transmitted waveforms is determined, proving that LoRa can be considered approximately an orthogonal modulation only for large M. Then, we investigate the spectral characteristics of the signal modulated by random data, obtaining a closed-form expression of the spectrum in terms of Fresnel functions. Quite surprisingly, we found that LoRa has both continuous and discrete spectra, with the discrete spectrum containing exactly a fraction 1/M of the total signal power.

Data originating from devices and sensors in Internet of Things scenarios can often be modeled as sparse signals. In this paper, we provide new source compression schemes for noisy sparse and non-strictly sparse sources, based on channel coding theory. Specifically, nonlinear excision filtering by means of model order selection or thresholding is first used to detect the support of the non-zero elements of sparse vectors in noise. Then, the sparse sources are quantized and compressed using syndrome-based encoders. The theoretical performance of the schemes is provided, accounting for the uncertainty in the support estimation. In particular, we derive the operational distortion-rate and operational distortion-energy of the encoders for noisy Bernoulli-uniform and Bernoulli-Gaussian sparse sources. It is found that the performance of the proposed encoders approaches the information-theoretic bounds for sources with low sparsity order. As a case study, the proposed encoders are used to compress signals gathered from a real wireless sensor network for environmental monitoring.

One of the key issues in the acquisition of sparse data by means of compressed sensing is the design of the measurement matrix. Gaussian matrices have been proven to be information-theoretically optimal in terms of minimizing the required number of measurements for sparse recovery. In this paper, we provide a new approach for the analysis of the restricted isometry constant (RIC) of finite dimensional Gaussian measurement matrices. The proposed method relies on the exact distributions of the extreme eigenvalues for Wishart matrices. First, we derive the probability that the restricted isometry property is satisfied for a given sufficient recovery condition on the RIC, and propose a probabilistic framework to study both the symmetric and asymmetric RICs. Then, we analyze the recovery of compressible signals in noise through the statistical characterization of stability and robustness. The presented framework determines limits on various sparse recovery algorithms for finite size problems. In particular, it provides a tight lower bound on the maximum sparsity order of the acquired data allowing signal recovery with a given target probability. Also, we derive simple approximations for the RICs based on the Tracy-Widom distribution.

The increasing availability of inexpensive software defined radios (SDRs) allows nowadays to implement cognitive radio (CR) functionalities in large scale networks such as the Internet-of-Things and future 5G systems. In this work, we focus on the spectrum sensing functionality that must take into account the front-end impairments of low-cost devices. Based on the noise model of a real SDR dongle, we address the problem of robust signal detection in the presence of noise power uncertainty and non-flat noise power spectral density (PSD). In particular, we analyze the receiver operating characteristic (ROC) of different known detectors in the presence of such front-end impairments, to understand the performance attainable in a real-world scenario. Based on the analysis, we propose two frequency-domain detectors that are proven to outperform previously proposed spectrum sensing techniques such as, e.g., eigenvalues-based tests.

Precise localization and tracking of moving objects is of great interest for a variety of emerging applications including the Internet-of-Things (IoT). The localization and tracking tasks are challenging in harsh wireless environments, such as indoor ones, especially when objects are not equipped with dedicated tags (noncollaborative). The problem of detecting, localizing, and tracking noncollaborative objects within a limited area has often been undertaken by exploiting a network of radio sensors, scanning the zone of interest through wideband radio signals to create a radio image of the objects. This paper presents a sensor network for radio imaging (sensor radar) along with all of the signal processing steps necessary to achieve highaccuracy objects tracking in harsh propagation environments. The described sensor radar is based on the impulse radio (IR) ultrawideband (UWB) technology, entailing the transmission of very short duration pulses. Experimental results with actual UWB signals in indoor environments confirm the sensor radar's potential in IoT applications.

Data originating from many devices and sensors can be modeled as sparse signals. Hence, efficient compression techniques of such data are essential to reduce bandwidth and transmission power, especially for energy constrained devices within machine to machine communication scenarios. This paper provides accurate analysis of the operational distortion-rate function (ODR) for syndrome-based source encoders of noisy sparse sources. We derive the probability density function of error due to both quantization and pre- quantization noise for a type of mixed distributed source comprising Bernoulli and an arbitrary continuous distribution, e.g., Bernoulli- uniform sources. Then, we derive the ODR for two encoding schemes based on the syndromes of Reed-Solomon (RS) and Bose, Chaudhuri, and Hocquenghem (BCH) codes. The presented analysis allows designing a quantizer such that a target average distortion is achieved. As confirmed by numerical results, the closed-form expression for ODR perfectly coincides with the simulation. Also, the performance loss compared to an entropy based encoder is tolerable.

This letter provides tight upper bounds on the weak restricted isometry constant for compressed sensing with finite Gaussian measurement matrices. The bounds are used to develop a unified framework for the guaranteed recovery assessment of jointly sparse matrices from multiple measurement vectors. The analysis is based on the exact distribution of the extreme singular values of Gaussian matrices. Several joint sparse reconstruction algorithms are analytically compared in terms of the maximum support cardinality ensuring signal recovery, i.e., mixed norm minimization, MUSIC, and OSMP based algorithms.

We derive the probability that all eigenvalues of a random matrix M lie within an arbitrary interval [a, b], ψ(a, b) Pr{a λmin(M), λmax(M) b}, when M is a real or complex finite-dimensional Wishart, double Wishart, or Gaussian symmetric/Hermitian matrix. We give efficient recursive formulas allowing the exact evaluation of ψ(a, b) for Wishart matrices, even with a large number of variates and degrees of freedom. We also prove that the probability that all eigenvalues are within the limiting spectral support (given by the Marčenko-Pastur or the semicircle laws) tends for large dimensions to the universal values 0.6921 and 0.9397 for the real and complex cases, respectively. Applications include improved bounds for the probability that a Gaussian measurement matrix has a given restricted isometry constant in compressed sensing.

Signal compression is essential for energy and bandwidth efficient communication and storage systems. In this paper, we provide two practical approaches for source compression of noisy sparse and non-strictly sparse (compressible) sources. The proposed schemes are based on channel coding theory to construct a source encoder that decreases the number of transmitted bits while preserving the fidelity of the reconstructed signal at the receiver by exploiting its sparsity. In addition, a model order selection scheme is proposed to detect the nonzero elements of sparse vectors embedded in noise, or to find a nonlinear sparse approximation of compressible signals. As illustrated by numerical results, our approach provides a lower distortion-rate function compared to previously known methods. For example, the proposed schemes achieve a lower distortion, about 2 orders of magnitude, compared to compressed sensing, for the same rate.

M-health services are expected to become increasingly relevant in the management of emergency situations by enabling real-time support of remote medical experts. In this context, the transmission of multiple health-related video streams from an ambulance to a remote hospital can improve the efficacy of the teleconsultation service, but requires a large bandwidth to meet the desired quality, not always guaranteed by the mobile network. In order to deliver the multiple streams over a single bandwidth-limited wireless access channel, in this paper we propose a novel optimization framework that enables to classify the available video sources and to automatically select and adapt the best streams to transmit. The camera ranking algorithm jointly works with a cross-layer adaptation strategy for multiple scalable streams to achieve different objectives and/or tradeoffs in terms of number and target quality of the transmitted videos. The final goal of the optimization is to dynamically adjust the overall transmitted throughput to meet the actual available bandwidth, while being able to provide high quality to diagnostic video sequences and lower quality to less critical ambient videos. Numerical simulations considering a realistic emergency scenario with long term evolution advanced (LTE-A) connectivity show that the proposed content/context-aware solution is able to automatically select the best sources of information from a visual point of view and to achieve optimal end-to-end video quality for both the diagnostic and the ambient videos.

In the context of compressed sensing, we provide a new approach to the analysis of the symmetric and asymmetric restricted isometry property for Gaussian measurement matrices. The proposed method relies on the exact distribution of the extreme eigenvalues for Wishart matrices, or on its approximation based on the Tracy-Widom law, which in turn can be approximated by means of properly shifted and scaled Gamma distributions. The resulting probability that the measurement submatrix is ill conditioned is compared with the known concentration of measure inequality bound, which has been originally adopted to prove that Gaussian matrices satisfy the restricted isometry property with overwhelming probability. The new analytical approach gives an accurate prediction of such probability, tighter than the concentration of measure bound by many orders of magnitude. Thus, the proposed method leads to an improved estimation of the minimum number of measurements required for perfect signal recovery.

A common approach for estimating the number of wireless sources is to adopt model order selection based on information theoretic criteria (ITC). In this paper we study the generalized information criterion (GIC) and propose a design method for setting the penalty in practical situations, where the sample size is finite. The design is based on the maximum probability of correct model selection, that can be approximated using the statistic of the ratio between the largest eigenvalue and the trace of a white central Wishart matrix. For this metric we provide the exact distribution and a new approximation.

In this paper, a random access scheme is introduced, which relies on the combination of packet erasure correcting codes and successive interference cancellation (SIC). The scheme is named coded slotted ALOHA. A bipartite graph representation of the SIC process, resembling iterative decoding of generalized low-density parity-check codes over the erasure channel, is exploited to optimize the selection probabilities of the component erasure correcting codes through a density evolution analysis. The capacity (in packets per slot) of the scheme is then analyzed in the context of the collision channel without feedback. Moreover, a capacity bound is developed, and component code distributions tightly approaching the bound are derived.

Spectrum sensing is an essential functionality in cognitive radio (CR) systems allowing us to discover spectrum opportunities and enabling primary user (PU) protection. Wideband spectrum sensing (WS) improves the awareness of the surrounding radio environment by jointly monitoring multiple frequency bands. In this paper we propose a WS approach based on the observation of a frequency domain representation of the received signal and the adoption of model order selection (MOS) to identify the occupied frequency components. We provide a general formulation of the problem valid for any kind of spectral representation and then focus on the case in which discrete Fourier transform (DFT) is used. This choice is motivated by the fact that DFT blocks are available in many wireless systems, such as OFDM receivers and recently proposed software radio architectures. We provide analytical expressions for the maximum probability of correct selection of the occupied sub-bands valid for MOS approaches encompassed within the generalized information criterion (GIC). We then propose a method for designing the selection algorithm to balance overestimation and underestimation. Numerical results show that the MOS scheme derived for DFT can be successfully applied also when more accurate frequency representations, such as multitaper (MT) spectrum estimates, are adopted.

Wideband spectrum sensing (SS) allows cognitive radios (CRs) to reach, by monitoring large portions of spectrum, a better awareness of the surrounding radio environment. In this paper, we formulate wideband SS as a model order selection problem. This approach consists in the adoption of information theoretic criteria (ITC) to identify the occupied frequency components in a frequency domain representation of the observed signal. We provide a general formulation of the problem and then focus on the case in which discrete Fourier transform (DFT) is used as spectral representation. Finally, we propose consistent ITC for which we provide analytical expressions for the maximum probability of detection.

A coded modulation scheme for the Poisson channel is investigated. The scheme relies on the serial concatenation of an outer low-density parity-check (LDPC) code over an order-q finite field and q-ary pulse position modulation (PPM). Due to the matching between code and modulation symbols, no iterative message exchange between the decoder and the modulator is required. The PPM capacity limit serves as a reference to evaluate the efficiency of the proposed scheme in the asymptotic setting via density evolution. A simplified form of the Gallager random coding bound (RCB) is also developed and used as a reference for the finite-length performance of the coded modulation scheme. The optimization via density evolution is performed on a surrogate (erasure) channel, yielding excellent iterative decoding thresholds for a wide range of channel parameters. The proposed coded modulation technique performs close to the theoretical bounds not only asymptotically, but also for moderate block lengths. It turns to represent a viable solution for deep-space direct detection optical links, for which the Poisson channel is adopted as a model.

In this paper we propose a wireless sensor network (WSN) designed for landslides monitoring and risk management. The WSN is self-organizing, has fault tolerance capabilities, and its behavior is driven by the events to be monitored, to guarantee fast deployment, robustness in harsh environments, and very long lifetime. Data collected by sensors are delivered through the network to a remote unit (RU) for on-line analysis and alerting. The WSN has been installed on a landslide located in Torgiovannetto (Italy) for an experimental campaign of several months where performance metrics, such as path statistics and battery levels, have been collected. These metrics demonstrate the effectiveness of the network protocols to manage self-organization, node failures, low link quality and unexpected battery depletion. With negligible human intervention during the pilot experiment the WSN revealed a very high level of robustness, which makes it suitable to monitor landslides in critical scenarios.

Novel non-coherent impulse communications receivers are proposed to alleviate excessive noise collection in clustered multipath channels. To this aim, a stop-and-go strategy based on energy detection in the autocorrelation receiver or in the energy detection receiver is employed. This strategy enables the selective collection of useful signal portions only, allowing the integration interval to be kept large without noise penalty. To implement this strategy, a blind method is employed, using model order selection based on information theoretic criteria, which optimizes the performance of the proposed receiver and does not require the estimation of channel parameters. The bit error probability of the proposed stop-and-go receivers is evaluated, and our results highlight the considerable performance gain at the expense of a small increase in complexity.

In multi-antenna systems the use of multiple antennas at one end or both ends of the link produces multiple channels. A useful, although ill-defined, metric for such a link is the number of independent channels provided. In this paper, we discuss several candidate metrics and compare their utility in the Rayleigh fading, single-input multiple-output case. We show that most of the metrics available in the literature have limitations and can exhibit non-physical behaviour. In order to improve on their performance, we develop two novel measures for the number of independent channels based on the statistical construction of the channel and channel capacity. These two measures are then extended to multiple-input multiple-output systems, Rician channels and arbitrary channel models.

M-health services are expected to become increasingly relevant in the management of emergency situations, enabling real-time support of remote medical experts. In this context, the transmission of health-related information from an ambulance to a remote hospital is a challenging task, due to the variability and the limitations of the mobile radio link. In particular, the transmission of multiple video streams can improve the efficacy of the tele-consultation service, but requires a large bandwidth to meet the desired quality, not always guaranteed by the mobile network. In this paper we propose a novel cross-layer adaptation strategy for multiple SVC videos delivered over a single LTE channel, which dynamically adjusts the overall transmitted throughput to meet the actual available bandwidth, while being able to provide high quality to diagnostic video sequences and lower (but fair) quality to less critical ambient videos. After having introduced a realistic LTE uplink scenario, including an advanced resource allocation strategy, we show through numerical simulations that the proposed solution is capable to achieve an optimal end-to-end video quality for both the diagnostic and the ambient videos.

The finite-length design of non-binary low-density parity-check (LDPC) codes for the q-ary erasure channel under maximum a posteriori (MAP) decoding is addressed. A low-complexity MAP decoding algorithm is reviewed for which a code design strategy is proposed. In particular, it is illustrated how a judicious code design permits to find a trade-off between performance in terms of codeword error rate (CER) and decoding complexity. As an example, the performance curve of a short (400, 200) code on the memoryless 4-ary erasure channel tightly approaches the Singleton bound at least down to a CER of 10-8.

In this paper, tight bounds on the block error probability of linear block codes over order-q finite fields for the q-ary erasure channel, under maximum-likelihood (ML) decoding, are developed. Upper bounds are obtained for uniform parity-check ensembles, sparse parity-check ensembles, general parity-check ensembles (e.g., Gallager regular nonbinary low-density parity-check ensembles), and for any given linear code with known distance spectrum. The tightness of the upper bounds is confirmed both by the comparison with simple lower bounds and, for Gallager low-density parity-check ensembles, by extensive Monte Carlo simulations. Exploiting the derived bounds, it is shown how already for short blocks and small q>2 sparse ensembles attain block error probabilities close to those of idealized maximum distance separable (MDS) codes, down to low error probabilities, whereas in the same regime binary codes show visible losses with respect to the Singleton bound. Thanks to the accurate performance estimates, the developed bounds can support the design of near-optimum erasure correcting codes with short and moderate lengths.

Two classes of turbo codes constructed on high-order finite fields are introduced. The codes are derived from a particular protograph sub-ensemble of the (2,3) regular low-density parity-check (LDPC) code ensemble. The first construction results in a parallel concatenation of two non-binary, time-variant accumulators. The second construction consists of the serial concatenation of a non-binary time-variant differentiator with a non-binary time-variant accumulator, and provides a highly structured flexible encoding scheme for (2,4) LDPC codes. A cycle graph representation is also provided. The proposed codes can be decoded efficiently either as LDPC codes (via belief propagation decoding on their bipartite graphs) or as turbo codes (via the forward-backward algorithm applied to the component code trellises) by means of the fast Fourier transform. The proposed codes provide remarkable coding gains (more than 1 dB at a codeword error rate 10-4) over binary LDPC and turbo codes in the moderate-short block length regime.

Low-rate non-binary low-density parity-check (LDPC) codes for coherent and blockwise non-coherent additive white Gaussian noise (AWGN) channels are developed. The proposed construction is based on the concatenation of non-binary outer LDPC codes with inner binary codes. In case the binary codes are chosen to be Hadamard or Reed-Muller (RM) codes, the complexity of the decoding scheme is considerably reduced. An asymptotic analysis of the concatenation with help of composite capacity considerations anddensity evolution (DE) is provided, from which guidelines on the choice of both inner and outer codes are devised. Finite length designs presented in this work confirm the excellent performance of the proposed codes.

This paper analyzes the asymptotic exponent of the weight spectrum for irregular doubly-generalized LDPC (D-GLDPC) codes. In the process, an efficient numerical technique for its evaluation is presented, involving the solution of a 4 × 4 system of polynomial equations. The expression is consistent with previous results, including the case where the normalized weight or stopping set size tends to zero. The spectral shape is shown to admit a particularly simple form in the special case where all variable nodes are repetition codes of the same degree, a case which includes Tanner codes; for this case it is also shown how certain symmetry properties of the local weight distribution at the CNs induce a symmetry in the overall weight spectral shape function. Finally, using these new results, weight and stopping set size spectral shapes are evaluated for some example generalized and doubly-generalized LDPC code ensembles.

In many applications erasure correcting codes are used to recover packet losses at high protocol stack layers. The objects (e.g. files) to be transmitted often have variable sizes, resulting in a variable number of packets to be encoded by the packet-level encoder. In this paper, algorithms for the (on-line) flexible design of parity-check matrices for irregular-repeat-accumulate codes are investigated. The proposed algorithms allow designing in fast manner parity-check matrices that are suitable for low-complexity maximum-likelihood decoding. The code ensembles generated by the algorithms are analyzed via extrinsic information transfer charts. Numerical results show how the designed codes can attain codeword error rates as low as 10-5without appreciable losses w.r.t. the performance of idealized maximum-distance separable codes. Finally, we apply the proposed codes to the upcoming aeronautical communication standard, showing large performance improvements and proving the efficiency and the flexibility of the developed method.

A novel protograph-based construction of low-density parity-check (LDPC) codes for the relay channel is proposed, which provides an enhanced unequal error protection property named unequal diversity. The focus is on quasi-static fading channels and on the high-code-rate (R>1/2) regimes, for which (according to the Singleton bound) no full diversity can be achieved. In the proposed construction, some nodes (and the corresponding codeword fragment) associated with the code graph enjoy the diversity provided by the relay, whereas the remaining nodes do not experience any diversity. The proposed approach can be thus tailored to transmit information blocks with different priority levels. An extrinsic information transfer (EXIT) analysis is developed, which allows an accurate performance prediction over the considered channel model, and more in general over block-fading channels.

In cooperative spectrum sensing networks the sensing nodes are often assumed to have the same noise power level. However, the different secondary users (SUs) could experience different temperatures, have receiver chains with different characteristics or even with a completely different architecture. Therefore, it is unlikely that they experience exactly the same noise power. In this paper we study the problem of cooperative spectrum sensing in cognitive radio (CR) networks, focusing on the case where the receivers experience different levels of noise power (uncalibrated receivers). We propose the independence test and compare it with the popular sphericity test. The independence test is in fact the generalized likelihood ratio (GLR) when the SUs are uncalibrated. We address in particular the threshold setting problem under a Neyman-Pearson framework. In order to reduce the complexity of the analysis, we approximate the test metrics as beta distributed random variables (r.v.s), by using a moment-matching approach. We provide simple and analytically tractable expressions for the computation of the probability of false alarm and for setting the decision threshold. Numerical simulations show that these approximated forms match very well the empirical distributions, allowing a very precise estimation of the probability of false alarm. In cooperative spectrum sensing with uncalibrated receivers the independence test is shown to be robust to strong imbalances of the noise power level.

Short non-binary irregular repeat-accumulate (IRA) codes based on well-known Hamiltonian and Hypohamiltonian graphs with large girth are presented. The mapping of the code coordinates on the graph edges is discussed for Hamiltonian graphs, and two encoding methods on Hypohamiltonian graphs are introduced. The performance of the presented codes on order-256 finite fields (F256) is provided for both the additive white Gaussian (AWGN) channel and the binary erasure channel (BEC) under iterative (IT) decoding. For the latter case, the performance under maximum likelihood (ML) decoding is also presented, to illustrate that the proposed codes not only attain performances close to the random coding bound, but also show limited losses when decoded iteratively.

An accurate extrinsic information transfer (EXIT) analysis is developed for protograph low-density parity-check (LDPC) codes over the block fading channel (BFC). The analysis is thus exploited for analyzing the behavior of distributed protograph LDPC ensembles over block fading relay channels. A novel protograph-based construction of low-density parity-check (LDPC) codes for the relay channel is proposed, which provides an enhanced unequal error protection (named unequal diversity, UD) property. The focus is on quasi-static fading channels and on the high-code-rate (R >; 1/2) regimes, for which (according to the Singleton bound) no full diversity can be achieved. In the proposed construction, some nodes (and the corresponding codeword fragments) associated with the code graph enjoy the diversity provided by the relay, whereas the remaining nodes do not experience any diversity. The proposed approach can be thus tailored to transmit information blocks with different priority levels.

In this paper we investigate the effect of spatial coupling applied to the recently-proposed coded slotted ALOHA (CSA) random access protocol. Thanks to the bridge between the graphical model describing the iterative interference cancellation process of CSA over the random access frame and the erasure recovery process of low-density parity-check (LDPC) codes over the binary erasure channel (BEC), we propose an access protocol which is inspired by the convolutional LDPC code construction. The proposed protocol exploits the terminations of its graphical model to achieve the spatial coupling effect, attaining performance close to the theoretical limits of CSA. As for the convolutional LDPC code case, large iterative decoding thresholds are obtained by simply increasing the density of the graph. We show that the threshold saturation effect takes place by defining a suitable counterpart of the maximum-a-posteriori decoding threshold of spatially-coupled LDPC code ensembles. In the asymptotic setting, the proposed scheme allows sustaining a traffic close to 1 [packets/slot].

We describe an optimization technique for multiuser resource allocation assuming adaptive modulation and coding (AMC) in OFDMA radio downlink communications. The resource allocation process is based on the dual solution of a weighted sum-rate maximization problem. In particular, to realistically address IP-based video and data communications, the set of considered constraints include the number of bits actually present in the different transmission buffers. Furthermore, we propose an innovative AMC solution, based on M-QAM modulation and LDPC codes, jointly working with the scheduler and capable to maximize the throughput while guaranteeing a given set of performance requirements. A simple weight adaptation algorithm is introduced to properly tune the trade off between opportunistic transmissions and fairness among users. Several simulation results are presented to validate the proposed approach and a comparison with more traditional schemes is provided. In particular, we show that a good trade off between user fairness and opportunistic exploitation of the radio channel can be reached with the proposed strategies, improving the communication quality for both real-time video and non real-time data applications.

This paper investigates efficient maximum-likelihood (ML) decoding of low-density parity-check (LDPC) codes over erasure channels. A set of algorithms, referred to as pivoting algorithms, is developed. The aim is to limit the average number of pivots (or reference variables) from which all the other erased symbols are recovered iteratively. The suggested algorithms exhibit different trade-offs between complexity of the pivoting phase and average number of pivots. Moreover, a systematic procedure to design LDPC code ensembles for efficient ML decoding is proposed. Numerical results illustrate that the designed LDPC codes achieve a near-optimum performance (very close to the Singleton bound, at least down to a codeword error rate level 10-8) with an affordable decoding complexity. For one of the presented codes and algorithms, a software implementation has been developed which is capable to provide data rates above 1.5 Gbps on a commercial computing platform.

Subset diversity (SSD) techniques, which select and combine the signals from a subset of the available diversity branches, are important for practical wireless systems. This paper characterizes the performance loss, or signal-to-noise ratio (SNR) penalty, of one SSD system with respect to another. Both ideal and non-ideal channel estimation are considered, and the analysis is valid for the important case of arbitrary two-dimensional signal constellations. Expressions are given for the asymptotic SNR penalty, for both small and large SNR, for all the comparisons considered. Additionally, we develop bounds and approximations to quantify the performance of one system in terms of another for all SNRs of interest. Furthermore, for some signal constellations, we derive the exact SNR penalty of a non-ideal system with respect to an ideal system, as well as the exact penalty associated with two non-ideal systems with varying degrees of estimation energy. The SNR penalty enables the assessment of system sensitivity to channel estimation energy, combining architecture, and signal constellation.

Transmitted-reference (TR) modulation schemes have generated interest in the context of ultrawide bandwidth (UWB) communications in order to avoid complex channel estimation and synchronization. In these schemes, the length of the integration interval must be carefully chosen to achieve optimal performance. Difficulties arise since this parameter is related to the channel characteristics and to the signal-to-noise ratio (SNR). In this paper, we propose a blind method for the integration time determination, that adopts a model order selection strategy based on information theoretic criteria (ITC). Observing the received signal, without a-priori information about the channel and the SNR, the proposed technique finds an integration time closer to the channel ensemble optimum integration time, i.e., the integration time obtained a-posteriori for the considered channel model as the value that minimizes the average bit error probability (BEP) of the TR scheme for each SNR.

Using the EXIT chart approach, a necessary and sufficient condition is developed for the local stability of iterative decoding of multi-edge type (MET) doubly-generalized low-density parity-check (D-GLDPC) code ensembles. In such code ensembles, the use of arbitrary linear block codes as component codes is combined with the further design of local Tanner graph connectivity through the use of multiple edge types. The stability condition for these code ensembles is shown to be succinctly described in terms of the value of the spectral radius of an appropriately defined polynomial matrix.

A random access scheme for the collision channel without feedback is proposed. The scheme is based on erasure correcting codes for the recovery of packet segments that are lost in collisions, and on successive interference cancellation for resolving collisions. The proposed protocol achieves reliable communication in the asymptotic setting and attains capacities close to 1 [packets/slot]. A capacity bound as a function of the overall rate of the scheme is derived, and code distributions tightly approaching the bound developed.

In many applications erasure correcting codes are used to recover packet losses at high protocol stack layers. The objects (e.g. files) to be transmitted often have variable sizes, resulting in a variable number of packet to be encoded by the packet-level encoder. In this paper, algorithms for the (on-line) flexible design of parity-check matrices for irregular-repeat-accumulate codes are investigated. The proposed algorithms allow designing in fast manner parity-check matrices that are suitable for low-complexity maximum-likelihood decoding. The code ensembles generated by the proposed algorithms are analyzed via extrinsic information transfer charts. Numerical results show how the designed codes can attain codeword error rates as low as 10-5without appreciable losses w.r.t. the performance of idealized maximum-distance separable codes. The application of the proposed techniques to the upcoming aeronautical communication standard is investigated, proving the efficiency and the flexibility of the approach.

Two classes of turbo codes over high-order finite fields are introduced. The codes are derived from a particular protograph sub-ensemble of the (dv=2,dc=3) low-density parity-check code ensemble. A first construction is derived as a parallel concatenation of two non-binary, time-variant accumulators. The second construction is based on the serial concatenation of a non-binary, time-variant differentiator and of a non-binary, time-variant accumulator, and provides a highly-structured flexible encoding scheme for (dv=2,dc=4) ensemble codes. A cycle graph representation is provided. The proposed codes can be decoded efficiently either as low-density parity-check codes (via belief propagation decoding over the codes bipartite graph) or as turbo codes (via the forward-backward algorithm applied to the component codes trellis). The forward-backward algorithm for symbol maximum a posteriori decoding of the component codes is developed and simplified by means of the fast Fourier transform. The proposed codes provide remarkable gains (~1 dB) over binary low-density parity-check and turbo codes in the moderate-short block regimes.

In this work we perform an asymptotic analysis of estimated noise power (ENP) energy detector (ED) to derive the condition for the existence of the SNR wall phenomenon. We prove that an ED with noise estimation does not exhibit the SNR wall if the variance of the estimate reduces when the observation time increases. In the absence of SNR wall, we show that the maximum slope of the design curves (SNR vs. observation time for an arbitrary target probability of false alarm (Pfa) and probability of detection (Pd)), equal to -5 dB/decade for the ideal ED, can be reached also by an ENP-ED. Finally, we derive analytical expressions for the design curves when maximum likelihood (ML) noise power estimation is adopted, and we prove that, asymptotically, the signal-to-noise ratio (SNR) penalty with respect to ideal ED is of 1.5 dB, when the number of noise-only samples is equal to the number of observed samples.

In this work we describe an optimization technique for multiuser resource allocation and adaptive modulation and coding (AMC) in OFDMA radio downlink communications. In particular, we propose an innovative AMC solution, based on M-QAM modulation and LDPC codes, jointly working with the scheduler and capable to maximize the achieved throughput while guaranteeing a given set of BER/FER requirements. Simulation results are presented to validate the proposed approach and a comparison with more traditional schemes is provided.

In this paper, coded slotted ALOHA (CSA) is introduced as a powerful random access scheme to the MAC frame. In CSA, the burst a generic user wishes to transmit in the MAC frame is first split into segments, and these segments are then encoded through a local a packet-oriented code prior to transmission. On the receiver side, iterative interference cancellation combined with decoding of the local code is performed to recover from collisions. The new scheme generalizes the previously proposed irregular repetition slotted ALOHA (IRSA) technique, based on a simple repetition of the users' bursts. An interpretation of the CSA interference cancellation process as an iterative erasure decoding process over a sparse bipartite graph is identified, and the corresponding density evolution equations derived. Based on these equations, asymptotically optimal CSA schemes are designed for several rates and their performance for a finite number of users investigated through simulation and compared to IRSA competitors. Throughputs as high as 0.8 are demonstrated. The new scheme turns out to be a good candidate in contexts where power efficiency is required.

In this paper we present a distributed spectrum sensing technique based on a ring-formulation of the cognitive radio (CR) nodes in a network. The CR nodes in a network form a ring based on a particular criteria and distributes the local spectrum sensing decisions along the ring in a sequential manner to the successive CR nodes. Considering this method, we eliminate the requirement for all the CR nodes to send/broadcast its local decisions to all the other CR nodes as in the traditional distributed detection method. Moreover, in our method all the CR nodes in the ring will have the spectrum sensing information from all the other nodes in the ring unlike the traditional sequential distributed-sensing technique (without the ring formation). We also consider the temporal behavior of the primary user modeled as a Poisson-Pareto burst process, and present two distributed sensing techniques based on the 'ring-around' strategy for the energy based local detection method. We provide closed-form solutions for the detection and false alarm probabilities for the ring-around detection methods and present numerical results for Rayleigh fading signals with AWGN.

In this paper, packet-level coding is considered in the framework of H.264/AVC video transmission. Two distinct solutions are proposed and compared in different realistic communication scenarios. The first is based on classical Reed-Solomon (RS) codes applied at the RTP layer, while the second on modern LDPC codes implemented at the UDP-Lite layer. An end-to-end Quality of Experience (QoE) evaluation is presented, in terms of achieved peak signal-to-noise power ratio (PSNR). Our numerical results show that, in low-latency video applications across communication channels introducing errors and erasures, the adoption of a packet-level coding scheme becomes essential to guarantee a satisfactory quality. The solution based on LDPC codes exhibits better performances in presence of severe packet loss rates.

An uncertain knowledge of the noise power level can severely limit the energy detector (ED) spectrum sensing capability. In some situations this uncertainty can cause signal-to-noise ratio (SNR) penalties or even the rise of the SNR wall phenomenon. In this paper we analyze the performance of the ED with estimated noise power (ENP), addressing the threshold design and giving the conditions for the existence of the SNR wall. We derive analytical expressions for the design curves (SNR vs. observation time for a target performance) for the ENP-ED. Then we apply our analysis to cognitive radio (CR) systems where energy detection is used for fast sensing. For example it is shown that the SNR penalty with respect to ideal ED is of 5 log10(1+λ/λ) dB, when the time dedicated to noise power estimation is a multiple λ of the ED observation interval.

A deterministic method to design degree distributions for low-density parity-check codes over the binary erasure channel is proposed. This method consists of matching the first and high-order derivatives of the extrinsic information transfer (EXIT) function of the variable node set to the corresponding derivatives of the inverse EXIT function of the check node set, in order to reduce the gap between the two curves in the EXIT chart. A sufficient condition for a check-concentrated distribution to achieve derivative matching up to some order is first obtained, and then a deterministic design algorithm, enabled by the Fourier-Budan theorem, is developed exploiting this sufficient condition. A comparison with other deterministic design techniques is also provided, revealing the potential of the proposed algorithm.

In this paper, the asymptotic growth rate of the weight distribution of irregular doubly generalized LDPC (D-GLDPC) codes is derived. The analysis yields a compact expression which accurately approximates the growth rate function for the case of small linear-weight codewords. This paper generalizes existing results for LDPC and generalized LDPC (GLDPC) codes. Ensembles with smallest check or variable node minimum distance greater than 2 are shown to have good growth-rate behavior, while for other ensembles a fundamental parameter is identified which discriminates between an asymptotically small and an asymptotically large expected number of small linear-weight codewords. Also, in the latter case it is shown that the growth rate depends only on the check and variable nodes with minimum distance 2. An important connection between this new result and the stability condition of D-GLDPC codes over the BEC is highlighted. Such a connection, previously observed for LDPC and GLDPC codes, is now extended to the case of D-GLDPC codes. Finally, it is shown that the analysis may be extended to include the growth rate of the stopping set size distribution of irregular D-GLDPC codes.

Ensemble design of low-density parity-check (LDPC) codes and their generalizations is usually performed via numerical optimization techniques, such as differential evolution, in which a threshold analysis tool is always necessary. Threshold analysis of unstructured doubly-generalized LDPC (D-GLDPC) code ensembles over the binary erasure channel (BEC) can be performed via extrinsic information transfer (EXIT) chart, exploiting the information functions and split information functions of the check and variable component codes, respectively. In this paper, multi-type information functions of linear block codes are introduced as an extension of the concept of information functions, when the bit positions are assumed to be associated with different types. It is shown how multi-type information functions (together with their split counterparts) can be exploited within an EXIT analysis approach to perform threshold analysis over the BEC of multi-edge type D-GLDPC code ensembles. The proposed technique for threshold analysis captures D-GLDPC codes based on protographs as a special case.

The use of small sensors that can communicate through wireless links and that are positioned over a wide area can allow to gather precise data about the occurrence of determined phenomena. Depending on the application scenario, the phenomenon to be detected can interest all sensors (global phenomenon (GP)), or just a subset of the deployed sensors (local phenomenon (LP)). One of the most interesting phenomena that can be monitored by wireless sensor networks is fire. In this paper a wireless sensor network configuration for fire detection applications is presented. A chain network is considered where in order to reduce the transmissions and then minimize the power consumptions each node makes a local decision about target absent/present considering its own observation and also the decision made by the previous node. Fusion rules for both the global and local phenomenon scenario are proposed, to minimize the error probability at the last stage of the chain.

This paper analyzes the asymptotic exponent of both the weight spectrum and the stopping set size spectrum for a class of generalized low-density parity-check (GLDPC) codes. Specifically, all variable nodes (VNs) are assumed to have the same degree (regular VN set), while the check node (CN) set is assumed to be composed of a mixture of different linear block codes (hybrid CN set). A simple expression for the exponent (which is also referred to as the growth rate or the spectral shape) is developed. This expression is consistent with previous results, including the case where the normalized weight or stopping set size tends to zero. Furthermore, it is shown how certain symmetry properties of the local weight distribution at the CNs induce a symmetry in the overall weight spectral shape function.

In this paper we consider cooperative spectrum sensing to detect incumbent spectrum users (ISU) in cognitive radio (CR) networks. We propose a time-divisional cooperative periodic spectrum sensing (TD-CPSS) technique and analyze the detection performance based on the blind energy based detection scheme. The CR detects the presence of the ISU by means of TD-CPSS and opportunistically uses the spectrum for secondary communications. The proposed technique saves energy at the local CR nodes due to periodic sensing and at the same time maintains the minimum required detection probability by optimizing the sensing period. The detection probability together with the false alarm probability are derived for the TD-CPSS technique based on the additive noise at the sensing node and the temporal statistics of the ISU transmissions. In our model, we consider additive white Gaussian noise (AWGN) for local sensing and the Poisson-Pareto spectral occupancy model for the temporal behavior of the ISU transmissions. We also provide expression for the required time period for the proposed sensing technique which attains the minimum required detection probability whilst minimizing the energy consumption considering the noise and temporal statistics.

In a receive diversity system the use of multiple antennas at one end of the link produces multiple channels. A useful, although ill-defined, metric for such a link is the number of independent channels provided. In this letter we discuss several candidate metrics and compare their utility. We show that most of the metrics available in the literature have limitations and can exhibit non-physical behaviour. In order to improve on their performance, we develop two novel measures for the number of independent channels based on the statistical construction of the channel and channel capacity.

One of the key operations in communication systems is frame synchronization, which is similar in many aspects to code acquisition in the context of code division multiple access systems. We focus on frame synchronization for binary PSK signals in the presence of additive white Gaussian noise and phase offset due to imperfect carrier phase estimation. We derive optimum and low-complexity suboptimum synchronization techniques, showing large improvements with respect to frame synchronization based on noncoherent correlation. Extensions to higher order modulations and fading channels are also discussed.

Resource allocation promises significant benefits in wireless networks. In order to fully reap these benefits, it is important to design efficient resource allocation algorithms. Here, we develop relay power allocation (RPA) algorithms for coherent and noncoherent amplify-and-forward (AF) relay networks. The goal is to maximize the output signal-to-noise ratio under individual as well as aggregate relay power constraints. We show that these RPA problems, in the presence of perfect global channel state information (CSI), can be formulated as quasiconvex optimization problems. In such settings, the optimal solutions can be efficiently obtained via a sequence of convex feasibility problems, in the form of second-order cone programs. The benefits of our RPA algorithms, however, depend on the quality of the global CSI, which is rarely perfect in practice. To address this issue, we introduce the robust optimization methodology that accounts for uncertainties in the global CSI. We show that the robust counterparts of our convex feasibility problems with ellipsoidal uncertainty sets are semi-definite programs. Our results reveal that ignoring uncertainties associated with global CSI often leads to poor performance, highlighting the importance of robust algorithm designs in practical wireless networks.

Multiple-input multiple-output (MIMO) systems are being considered as one of the key enabling technologies for future wireless networks. However, the decrease in capacity due to the presence of interferers in MIMO networks is not well understood. In this paper, we develop an analytical framework to characterize the capacity of MIMO communication systems in the presence of multiple MIMO co-channel interferers and noise. We consider the situation in which transmitters have no channel state information, and all links undergo Rayleigh fading. We first generalize the determinant representation of hypergeometric functions with matrix arguments to the case when the argument matrices have eigenvalues of arbitrary multiplicity. This enables the derivation of the distribution of the eigenvalues of Gaussian quadratic forms and Wishart matrices with arbitrary correlation, with application to both single-user and multiuser MIMO systems. In particular, we derive the ergodic mutual information for MIMO systems in the presence of multiple MIMO interferers. Our analysis is valid for any number of interferers, each with arbitrary number of antennas having possibly unequal power levels. This framework, therefore, accommodates the study of distributed MIMO systems and accounts for different spatial positions of the MIMO interferers.

In this paper, a method for the asymptotic analysis of generalized low-density parity-check (GLDPC) codes and doubly generalized low-density parity-check (D-GLDPC) codes over the binary erasure channel (BEC), based on extrinsic information transfer (EXIT) chart, is described. This method overcomes the problem consisting of the impossibility to evaluate the EXIT function for the check or variable component codes, in situations where the information functions or split information functions for component codes are unknown. According to the proposed technique, GLDPC codes and D-GLDPC codes where the generalized check and variable component codes arerandomcodes with minimum distance at least 2, are considered. A technique is then developed which finds the EXIT chart for the overall GLDPC or D-GLDPC code, by evaluating the expected EXIT function for each check and variable component code. This technique is finally combined with the differential evolution algorithm in order to generate some good GLDPC and D-GLDPC edge distributions. Numerical results of long, random codes, are presented which confirm the effectiveness of the proposed approach. They also reveal that D-GLDPC codes can outperform standard LDPC codes and GLDPC codes in terms of both waterfall performance and error floor.

The growth rate of the weight distribution of irregular doubly-generalized LDPC (D-GLDPC) codes is developed and in the process, a new efficient numerical technique for its evaluation is presented. The solution involves simultaneous solution of a 4 × 4 system of polynomial equations. This represents the first efficient numerical technique for exact evaluation of the growth rate, even for LDPC codes. The technique is applied to two example D-GLDPC code ensembles.

This paper investigates efficient maximum-likelihood (ML) decoding algorithms for low-density parity-check (LDPC) codes over erasure channels. In particular, enhancements to a previously proposed structured Gaussian elimination approach are presented. The improvements are achieved by developing a set of algorithms, here referred to as pivoting algorithms, aiming to limit the average number of reference variables (or pivots) from which the erased symbols can be recovered. Four pivoting algorithms are compared, which exhibit different trade-offs between the complexity of the pivoting phase and the average number of pivots. Numerical results on the performance of LDPC codes under ML erasure decoding complete the analysis, confirming that a near-optimum performance can be obtained with an affordable decoding complexity, up to very high data rates. For example, for one of the presented algorithms, a software implementation has been developed, which is capable to provide data rates above 1.5 Gbps on a commercial computing platform.

We investigate high spectral efficiency wireless multiple-input multiple-output (MIMO) systems in fading environments. We assume frequency flat fading, channel state information at both the transmitter and receiver sides, and linear preceding based on singular value decomposition (SVD). For this MIMO SVD scenario, the optimal solution in terms of achievable rate requires water-filling to optimally allocate the power to the different channel eigenmodes. Alternatively, reduced complexity power allocation methods can be employed, where the allocation is based on statistical expectations of functions related to the singular values of the channel gain matrix. In this paper we study these power allocation methods, by using the exact distribution of an arbitrary (ordered) eigenvalue of Wishart matrices, with the probability density function of the ¿thlargest eigenvalue given as a sum of terms xße-x¿. We derive expressions for the achievable rate for both zero-outage and non-zero-outage strategies. We show that, often, the low-complexity methods have performance very similar to water-filling methods.

The design of moderate-length erasure correcting low-density parity-check (LDPC) codes over correlated erasure channels is considered. Although the asymptotic LDPC code design remains the same as for a memoryless erasure channel, robustness to the channel correlation shall be guaranteed for the finite length LDPC code. This further requirement is of great importance in several wireless communication scenarios where packet erasure correcting codes represent a simple countermeasure for correlated fade events (e.g., in mobile wireless broadcasting services) and where the channel coherence time is often comparable with the code length. In this paper, the maximum tolerable erasure burst length (MTBL) is adopted as a simple metric for measuring the code robustness to the channel correlation. Correspondingly, a further step in the code construction is suggested, consisting of improving the LDPC code MTBL. Numerical results conducted over a Gilbert erasure channel, under both iterative and maximum likelihood decoding, highlight both the importance of the MTBL improvement in the finite-length code construction and the possibility to tightly approach the performance of maximum distance separable codes.

A class of doubly-generalized low-density parity-check (D-GLDPC) codes, where single parity-check (SPC) codes are used as variable nodes (VNs), is investigated. An expression for the growth rate of the weight distribution of any D-GLDPC ensemble with a uniform check node (CN) set is presented at first, together with an analytical technique for its efficient evaluation. These tools are then used for detailed analysis of a case study, namely, a rate-1/2 D-GLDPC ensemble where all the CNs are (7, 4) Hamming codes and all the VNs are length-7 SPC codes. It is illustrated how the VN representations can heavily affect the code properties and how different VN representations can be combined within the same graph to enhance some of the code parameters. The analysis is conducted over the binary erasure channel. Interesting features of the new codes include the capability of achieving a good compromise between waterfall and error floor performance while preserving graphical regularity, and values of threshold outperforming LDPC counterparts.

With the increasing proliferation of different communication devices sharing the same spectrum, it is critical to understand the impact of interference in heterogeneous wireless networks. In this paper, we put forth a mathematical model for coexistence in networks composed of both narrowband (NB) and ultrawideband (UWB) wireless nodes, based on fundamental tools from stochastic geometry. Our model considers that the interferers are spatially scattered according to a Poisson field, and are operating asynchronously in a wireless environment. We first determine the statistical distribution of the aggregate interference for both cases of NB and UWB emitters. We then provide error probability expressions for two dual configurations: 1) a NB victim link subject to the aggregate UWB interference, and 2) a UWB victim link subject to the aggregate NB interference. The results show that while the impact of a single interferer on a link is often negligible due to restrictions on the transmitted power, the aggregate effect of multiple interferers may cause significant degradation. Therefore, aggregate interference must be considered to ensure coexistence in heterogeneous networks. The proposed analytical framework shows good agreement with physical-level simulations of the system.

Ultra-wide-band (UWB) signals are suitable for underlay communications, over a frequency band where, possibly, other systems are active. Such coexistence of UWB and other systems is possible if the mutual interference has a small impact on their respective performance. This paper aims to present recent results on the interference and coexistence among UWB systems and other conventional narrow-band (NB) systems. Specifically, we consider a point-to-point UWB (NB) under the interference generated by a finite number of NB (UWB) radio transmitters. We consider channels including additive white Gaussian noise and multipath fading both for the victim and the interfering links, and different receiver architectures. While our main focus is on UWB systems based on impulse radio, wide-band systems employing carrier-based direct-sequence spread-spectrum and orthogonal frequency-division multiplexing are also considered.

Diversity techniques play a key role in modern wireless systems, whose design benefits from a clear understanding of how these techniques affect system performance. To this aim we propose a simple class of bounds, whose parameters are optimized, on the symbol error probability (SEP) for detection of arbitrary two-dimensional signaling constellations with diversity in the presence of non-ideal channel estimation. Unlike known bounds, the optimized simple bounds are tight for all signal-to-noise ratios (SNRs) of interest. In addition, these bounds are easily invertible, which enables us to obtain bounds on the symbol error outage (SEO) and SNR penalty. As example applications for digital mobile radio, we consider the SEO in log-normal shadowing and the SNR penalty for both maximal ratio diversity, in the case of unequal branch power profile, and subset diversity, in the case of equal branch power profile, with non-ideal channel estimation. The reported lower and upper bounds are extremely tight, that is, within a fraction of a dB from each other.

This paper considers the analysis of optimum combining systems in the presence of both co-channel interference and thermal noise. We address the cases where either the desired-user or the interferers undergo Rician fading. Exact expressions are derived for the moment generating function of the SINR which apply for arbitrary numbers of antennas and interferers. Based on these, we obtain expressions for the symbol error probability with M-PSK. For the case where the desired-user undergoes Rician fading, we also derive exact closed-form expressions for the moments of the SINR. We show that these moments are directly related to the corresponding moments of a Rayleigh system via a simple scaling parameter, which is investigated in detail. Numerical results are presented to validate the analysis and to examine the impact of Rician fading on performance.

In this paper, a simple and effective tool for the design of low-density parity-check (LDPC) codes for iterative correction of bursts of erasures is presented. The design method consists of starting from the parity-check matrix of an LDPC code and developing an optimized parity-check matrix, with the same performance over the memoryless erasure channel, and suitable also for the iterative correction of single erasure bursts. The parity-check matrix optimization is performed by an algorithm called pivot searching and swapping (PSS) algorithm. It executes permutations of carefully chosen columns of the parity-check matrix, after a local analysis of particular variable nodes called stopping set pivots. This algorithm can be in principle applied to any LDPC code. If the input parity-check matrix is designed to achieve a good performance over the memoryless erasure channel, then the code obtained after the application of the algorithm provides a good joint correction of independent erasures and single erasure bursts. Numerical results are provided in order to show the algorithm effectiveness when applied to different categories of LDPC codes.

Random matrices play a crucial role in the design and analysis of multiple-input multiple-output (MIMO) systems. In particular, performance of MIMO systems depends on the statistical properties of a subclass of random matrices known as Wishart when the propagation environment is characterized by Rayleigh or Rician fading. This paper focuses on the stochastic analysis of this class of matrices and proposes a general methodology to evaluate some multiple nested integrals of interest. With this methodology we obtain a closed-form expression for the joint probability density function of k consecutive ordered eigenvalues and, as a special case, the PDF of the lscrthordered eigenvalue of Wishart matrices. The distribution of the largest eigenvalue can be used to analyze the performance of MIMO maximal ratio combining systems. The PDF of the smallest eigenvalue can be used for MIMO antenna selection techniques. Finally, the PDF the kthlargest eigenvalue finds applications in the performance analysis of MIMO singular value decomposition systems.

The iterative decoding threshold of low-density parity-check (LDPC) codes over the binary erasure channel (BEC) fulfills an upper bound depending only on the variable and check nodes with minimum distance 2. This bound is a consequence of the stability condition, and is here referred to as stability bound. In this paper, a stability bound over the BEC is developed for doubly-generalized LDPC codes, where variable and check nodes can be generic linear block codes, assuming maximumaposteriorierasure correction at each node. It is proved that also in this generalized context the bound depends only on the variable and check component codes with minimum distance 2. A condition is also developed, namely, the derivative matching condition, under which the bound is achieved with equality. The stability bound leads to consider single parity-check codes used as variable nodes as an appealing option to overcome common problems created by generalized check nodes.

Diversity techniques will play a key role in next generation wireless communication systems, thus system design will benefit from a clear understanding of how these techniques affect system performance. To this aim we propose simple bounds, optimized within a given class, on the symbol error probability (SEP) in the presence of non-ideal channel estimation for arbitrary two-dimensional signaling constellations. Unlike known bounds, the optimized simple bounds are tight for all signal-to-noise ratios (SNRs). In addition, these bounds are easily invertible, which enables us to obtain bounds on the symbol error outage (SEO) and SNR penalty. As an example application for digital mobile radio, we consider the SEO in log-normal shadowing for both maximal ratio combining with unequal branch power profile and subset microdiversity. The reported lower and upper bounds are extremely tight, that is, within a fraction of a dB from each other.

The research of closed form expressions for the PDF of the lscrthordered eigenvalue of a Wishart matrix has received a great attention in the past years owing to its applications in the performance analysis of multiple input multiple output (MIMO) systems in fading environments. Although several closed form expressions for this PDF were obtained in the past years, to the authors' knowledge, no one was very friendly for further analysis. We propose a methodology to obtain the PDF for the lscrthlargest eigenvalue whose expression is given as a sum of termsxbetae-xdelta. This expression is easily usable to obtain closed form results for the performance of many MIMO systems, such as, for instance, MIMO beamforming, and MIMO with singular value decomposition (SVD). The methodology is valid for both uncorrelated and correlated central Wishart, allowing the investigation of MIMO systems with uncorrelated and correlated Rayleigh fading.

We consider strategies for the lossy transmission of a zero mean Gaussian source over a 2times2 MIMO channel with Rayleigh fading. The source is represented either using a single description or a multiple description code, depending on each strategy characteristic. Performance is evaluated using normalized expected distortion at the receiver, as a function of outage probability. The first strategy employs repetition coding over the two transmit antennas for the transmission of a single description representation of the source. The second strategy uses a time-shared approach to the two transmit antennas, allowing for the transmission of a multiple description representation of the source. The third and fourth strategies are based on, respectively, the Alamouti scheme and spatial multiplexing, and both of these strategies are used for the transmission of a single description representation of the source. The results show that the spatial multiplexing strategy is able to achieve the lowest distortion, and also that it is possible, with the Alamouti strategy, to obtain similar performance at a lower complexity. We finally consider the outage rates of the different strategies and observe that if a system is designed to maximize the outage rate, the corresponding distortion observed at the receiver will not be minimized.

This paper focuses on the stochastic analysis of Wishart matrices, which appear in many problems related to the performance analysis multiple-input-multiple-output (MIMO). We propose a general methodology to evaluate some multiple nested integrals of interest With this methodology we obtain a closed-form expression for the joint probability density function (pdf) of k consecutive ordered eigenvalues and, as a special case, the pdf of the lthordered eigenvalue of Wishart matrices.

The distribution of the eigenvalues of Wishart matrices and Gaussian quadratic forms is of great interest in communication theory, especially in relation to multiple-input multiple-output (MIMO) systems. In this paper we present some new results on the joint distribution of an arbitrary subset of the ordered eigenvalues of Wishart matrices, using the tensor operator T (.), which was first introduced in . We obtain both the joint probability distribution function (p.d.f.) of the eigenvalues and the expectation of arbitrary functions of the eigenvalues, including the moments, for the case of both ordered and unordered eigenvalues. These expressions are extremely compact and easy to handle. Application to MIMO systems are discussed.

A cognitive radio can sense its environment and adapt some of its features, such as carrier frequency, transmission bandwidth, transmission power, and modulation, thus allowing dynamic reuse of the available spectrum. Due to their high degree of adaptability to environmental variations, cognitive radios are expected to utilize packet-based transmission with variable-length frames. Packet-based transmission requires the receiver to perform frame synchronization, an important enabling step that allows adaptation in cognitive radios. However, proper metrics to characterize the performance of frame synchronization for transmission of variable-length frames are currently unavailable. To address this issue, we put forth two performance metrics, namely the expected duration to complete frame synchronization and the probability of correct acquisition within a given duration. We then develop analytical expressions for these important metrics. This paper advances our understanding of frame synchronization for the continuous transmission of variable-length frames and for bursty transmission.

In this paper, a novel methodology for designing structured generalized LDPC (G-LDPC) codes is presented. The proposed design results in quasi-cyclic G-LDPC codes for which efficient encoding is feasible through shift-register-based circuits. The structure imposed on the bipartite graphs, together with the choice of simple component codes, leads to a class of codes suitable for fast iterative decoding. A pragmatic approach to the construction of G-LDPC codes is proposed. The approach is based on the substitution of check nodes in the protograph of a low-density parity-check code with stronger nodes based, for instance, on Hamming codes. Such a design approach, which we call LDPC code doping, leads to low-rate quasi-cyclic G-LDPC codes with excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel.

In modern wireless systems employing diversity techniques, combining all the available diversity branches may not be feasible due to complexity and resource constraints. To alleviate these issues, subset diversity (SSD) systems have been proposed. Here, we develop a framework for evaluating the symbol error probability for antenna SSD, where the signals from a subset of antenna elements are selected and combined in the presence of channel estimation error. We consider independent identically distributed Rayleigh fading channels and use an estimator structure based on the maximum likelihood (ML) estimate which arises naturally as the sample mean of Nppilot symbols. The analysis is valid for arbitrary two-dimensional signaling constellations. The expressions give insight into the performance losses of non-ideal SSD when compared to ideal SSD. Due to estimation error, these losses occur in branch combining as well as in branch selection. However, our analytical results show that the practical ML channel estimator still preserves the diversity order of an ideal SSD system with Ndbranches. Finally, we investigate the asymptotic signal-to-noise ratio penalty due to estimation error.

In this paper, we derive the asymptotic statistics of mutual information for multiple-input multiple-output (MIMO) Rayleigh-fading channels in the presence of spatial fading correlation at both the transmitter and the receiver. We first introduce a class of asymptotic linear spectral statistics, calledcorrelants, for a structured correlation matrix. The mean and variance of MIMO mutual information are then expressed in terms of the correlants of spatial correlation matrices in the asymptotic regime where the number of transmit and receive antennas tends to infinity. In particular, using Szego's theorem on the asymptotic eigenvalue distribution of Toeplitz matrices, we give examples for special classes of correlation matrices with Toeplitz structure-exponential(orKac-Murdock-Szego),tridiagonal,andconstant(orintraclass) correlation matrices.

In this paper, a novel extrinsic information transfer (EXIT) analysis is presented for protograph-based and multi- edge type low-density parity-check (LDPC) codes. A protograph defines a subset of an LDPCC ensemble (identified by the degree distributions of the bipartite graph), introducing further constraints about the edge connections. For many codes belonging to this class, the conventional approach based on EXIT charts cannot be applied. The proposed EXIT analysis takes into account edge connections, permitting the decoding convergence evaluation for protograph-based LDPC codes, allowing the design of highly-structured capacity approaching LDPC codes.

This paper analyzes the performance of optimum combining systems in the presence of both co-channel interference and thermal noise, addressing the case where the desired- user undergoes Rician fading. Exact expressions are derived for the moment generating function of the SINR which apply for arbitrary numbers of antennas and interferers. Based on these, we obtain expressions for the symbol error probability with M-PSK. We also derive exact closed-form expressions for the moments of the SINR, and show that they are directly related to the corresponding moments of a Rayleigh system via a simple scaling parameter. Numerical results are presented to validate the analysis, and to examine the impact of Rician fading.

In this paper, a family of low-density parity-check (LDPC) degree distributions, whose decoding threshold on the binary erasure channel (BEC) admits a simple closed form, is presented. These degree distributions are a subset of the check regular distributions (i.e. all the check nodes have the same degree), and are referred to as p-positive distributions. It is given proof that the threshold for a p-positive distribution is simply expressed by [lambda'(0)rho'(1)]-1. Besides this closed form threshold expression, the p-positive distributions exhibit three additional properties. First, for given code rate, check degree and maximum variable degree, they are in some cases characterized by a threshold which is extremely close to that of the best known check regular distributions, under the same set of constraints. Second, the threshold optimization problem within the p-positive class can be solved in some cases with analytic methods, without using any numerical optimization tool. Third, these distributions can achieve the BEC capacity. The last property is shown by proving that the well-known binomial degree distributions belong to the p-positive family.

Relay power allocation has been shown to provide substantial performance gain in wireless relay networks when perfect global channel state information (CSI) is available. In this paper, we consider a more realistic scenario, where such global CSI is subject to uncertainty, and we aim to design robust power allocation protocols for both the coherent and noncoherent amplify-and-forward relay networks. The problem formulation is such that the output signal-to-noise ratio is maximized under both the aggregate and individual relay power constraints. Our previous results show that these optimization problems can be formulated as quasiconvex optimization problems, and are solved using the bisection method via a sequence of conic feasibility problems. We extend these results to the case of uncertain global CSI, and design robust relay power allocations using the robust optimization methodology. For simple ellipsoidal uncertainty sets, the robust counterparts of these optimization problems are semi-definite programs and can be solved efficiently via interior-point methods.

Relay power allocation has been shown to provide substantial performance gain in wireless relay channels when perfect global channel state information (CSI) is available. In this paper, we show that by using a class of conic optimization theory, we can solve the relay power allocation problem for amplify-and-forward (AF) relay networks in a straightforward manner. The problem formulation is such that the achievable rate with perfect global CSI is maximized under both the aggregate and individual relay power constraints for coherent and noncoherent AF relay networks. Numerical results quantify the performance gain using the optimal relay power allocation for both the coherent and noncoherent AF relay networks.

In this paper, the stability condition for low-density parity-check (LDPC) codes on the binary erasure channel (BEC) is extended to generalized LDPC (GLDPC) codes and doubly-generalized LDPC (D-GLDPC) codes. It is proved that, in both cases, the stability condition only involves the component codes with minimum distance 2. The stability condition for GLDPC codes is always expressed as an upper bound to the decoding threshold. This is not possible for D-GLDPC codes, unless all the generalized variable nodes have minimum distance at least 3. Furthermore, a condition called derivative matching is defined in the paper. This condition is sufficient for a GLDPC or D- GLDPC code to achieve the stability condition with equality. If this condition is satisfied, the threshold of D-GLDPC codes (whose generalized variable nodes have all minimum distance at least 3) and GLDPC codes can be expressed in closed form.

In this paper, we investigate the performance of optimum combining multiple antenna systems in the presence of Rician-faded interferers. We assume that the desired user is subjected to Rayleigh fading but interferers undergo Rician fading. Unlike other approaches addressing optimum combining with Rician fading, we consider the effect of both interference and thermal noise. Our methodology is based on the evaluation of the moment generating function of the signal-to-interference-plus-noise (SINR) ratio at the output of the combiner. Our results are exact, and allow the calculation of the symbol error probability (SEP) for M-PSK signals with arbitrary number of antennas and interferers.

Future space missions will put severe constraints on communication links in terms of data rates, bandwidth occupancy, complexity, and performance. The requirements imposed by the new missions and their consequences on channel code design are presented in the first part of the paper. All relevant issues, including code rates, frame lengths, modulation formats, performance metrics, complexity, and others, are discussed. In the second part of the paper, long erasure correcting codes are presented and their properties explained. These codes operate at the upper layers of the space link protocol and constitute an attractive new frontier for zero packet loss in future space communications.

We present new tight bounds for evaluating the performance of sync word-based frame synchronization algorithms in the periodically embedded case. We consider antipodal signaling with coherent detection over additive white Gaussian noise and both optimal and suboptimal search techniques. Our bounds are very close to results obtained through simulation and tend asymptotically (for increasing signal-to-noise ratios) to the exact performance.

This paper investigates the performance of adaptive M-ary quadrature amplitude modulation (QAM) with antenna subset diversity. We consider a slow adaptive modulation (SAM) technique that adapts the constellation size to the slow variation of the channel due, for example, to shadowing. The proposed SAM technique is more practical than conventional fast adaptive modulation (FAM) techniques that require adaptation to fast-fading variations. Our results show that the SAM technique can provide a substantial increase in throughput with respect to fixed schemes while maintaining an acceptable low bit-error outage. We also compare SAM and FAM techniques, showing that the throughput of SAM can be, in many practical cases, close to that of FAM, despite the fact that SAM is less complex and requires a lower feedback rate. For example, using a set of possible modulations {4,16,64}-QAM with dual-branch maximal ratio combining reception, 5% outage at a bit-error probability of 10-2and a median signal-to-noise ratio of 22 dB, SAM is capable of improving the mean spectral efficiency of fixed schemes from about 1.9 to 4.7 b/s/Hz, which is close to the 5.5 b/s/Hz achieved by FAM

In this paper, a novel methodology for designing structured generalized low-density parity-check (G-LDPC) codes is presented. The proposed design results in quasi-cyclic G-LDPC codes for which efficient encoding is feasible through shift-register-based circuits. The structure imposed on the bipartite graphs, together with the choice of simple component codes, leads to a class of codes suitable for fast iterative decoding. A pragmatic approach to the construction of G-LDPC codes is proposed. The approach is based on the substitution of check nodes in the protograph of a low-density parity-check code with stronger nodes based, for instance, on Hamming codes. Such a design approach, which we call low-density parity-check (LDPC) code doping, leads to low-rate quasi-cyclic G-LDPC codes with excellent performance in both the error floor and waterfall regions on the additive white Gaussian noise channel.

Frame synchronization is classically treated in literature for equiprobable data symbols. In this case the optimum metric has been derived both in the case of periodically and aperiodically embedded synchronization words and the corresponding performance has been evaluated. In this paper we evaluate the performance of sequential frame synchronization for non- equiprobable data symbols, assuming no a-priori information about data distribution. Results show that the performance may sensibly differ from the case of equiprobable data symbols and strongly depends on the chosen synchronization word.

In a multiuser scenario, we study the capacity of multiple-input/multiple-output (MIMO) communication systems in the presence of multiple MIMO co-channel interferers. We assume that transmitters have no information about the channel status and that all links undergo Rayleigh distributed fading; no restrictions are made to the transmission power levels or on the number of interferers and antennas, so that many possible cases can be studied, including distributed MIMO. In order to be able to cover all possible scenarios, we generalize the known determinant representation of hypergeometric functions with matrix arguments to the case when the argument matrices have eigenvalues with arbitrary multiplicity. Possible extensions and numerical results are then sketched.

We study sequential frame synchronization based on markers (or sync words), where the received samples are observed over a window of length equal to the marker length. A decision variable is derived over the observation window. A marker is declared if the decision variable exceeds a threshold; otherwise the observation window is time-shifted by one sample. This detection strategy can be characterized by the probabilities of false alarm and of correct detection, usually represented in terms of the receiver operating characteristic (ROC). However, once the detector (and hence its ROC) is given, it is crucial from a design perspective to select an optimal operating point on the ROC. In this paper we propose a general framework to connect the ROC to the performance of frame synchronization in terms of the probability of correct acquisition in an arbitrarily long time period, valid both for constant and variable length frames. This framework can also be used to compare different marker detectors for frame synchronization.

This paper proposes a simple co-decoding scheme for a Writing on Dirty Paper scenario, where the transmitted signal is affected by interference known by the transmitter and thermal noise. We focused on rate-1/2 codes, suitable for low SNR regions, but the scheme is easily generalizable to different code rates. In particular our approach involves a proper combination of an LDPCC and a vector quantizer based on a convolutional code and directly allows for iterative softinformation exchanges between the respective decoders. We show that it is possible to reach BER performance within 1.2-1.3 dB from the correspondent LDPCC's in AWGN channels. Moreover, the obtained performance are completely unaffected by the amount of interference.

In this work we deal with Low-Density Parity-Check (LDPC) codes under iterative message passing decoding algorithm, over channels introducing bursts of erasures. The burst erasure channel model we consider in this paper can be seen as an erasure channel based on a hidden Markov chain (HMC-EC). In order to characterize the channel, in the first part of the paper the expression of mutual information is recalled for any erasure channel with memory and with i.i.d. and equiprobable input symbols. In the second part of the paper an optimization algorithm is proposed which is able to heavily improve LDPC iterative decoder performance. This algorithm can be in principle applied to any given LDPC code. Simulation results relative to both random and IRA / eIRA codes are shown comparing the performance before and after the application of our optimization algorithm.

In this paper, we derive the asymptotic symbol error probability (SEP) of antenna subset diversity (SSD), where the signals from a subset of antenna elements are selected and combined in the presence of channel estimation error. The analysis is valid for arbitrary two-dimensional signaling constellations. We investigate the asymptotic SNR penalty, or performance loss between this system and an ideal system, caused by estimation error. We also compare this SNR penalty to that of a SSD system operating with perfect selection, but imperfect combining. We consider independent identically distributed (i.i.d.) Rayleigh fading channels and use an estimator structure based on the maximum likelihood (ML) estimate which arises naturally as the sample mean of Np pilot symbols. In both cases our analytical results show that the practical ML channel estimator still preserves the diversity order of an ideal SSD system with Nd branches.

We present a framework for the analysis of frame synchronization based on synchronization words (SWs), where the detection is based on the following sequential algorithm. The received samples are observed over a window of length equal to the SW; over this window, a metric (e.g., correlation) is computed; an SW is declared if the computed metric is greater than a proper threshold, otherwise the observation window is time-shifted one sample. We assume a Gaussian channel, antipodal signaling, equally distributed data symbols, and coherent detection, where soft values are provided to the frame synchronizer. We state the problem starting from the hypothesis testing theory, deriving the optimum metric [optimum likelihood ratio test (LRT)] according to the Neyman-Pearson lemma. When the data distribution is unknown, we design a simple and effective test based on the generalized LRT (GLRT). We also analyze the performance of the commonly used correlation metric, both with "hard" and "soft" values at the synchronizer input. We show that synchronization can be greatly improved by using the LRT and GLRT metrics instead of correlation and that, among correlation-based tests, sometimes hard correlation is better than soft correlation. The obtained closed-form expressions allow the derivation of the receiver operating characteristic (ROC) curves for the LRT and GLRT synchronizers, showing a remarkable gain with respect to synchronization based on correlation metric.

In this paper, we analyze the capacity of multiple-input multiple-output (MIMO) Rayleigh-fading channels in the presence of spatial fading correlation at both the transmitter and the receiver, assuming the channel is unknown at the transmitter and perfectly known at the receiver. We first derive the determinant representation for the exact characteristic function of the capacity, which is then used to determine the trace representations for the mean, variance, skewness, kurtosis, and other higher-order statistics (HOS). These results allow us to exactly evaluate two relevant information-theoretic capacity measures - ergodic capacity and outage capacity - and the HOS of the capacity for such a MIMO channel. The analytical framework presented in the paper is valid for arbitrary numbers of antennas, and generalizes the previously known results for independent and identically distributed or one-sided correlated MIMO channels to the case when fading correlation exists on both sides. We verify our analytical results by comparing them with Monte Carlo simulations for a correlation model based on realistic channel measurements as well as a classical exponential correlation model

In this letter, we derive a closed-form symbol-error probability expression for adaptive antenna array with optimum (or, equivalently, linear minimum mean-square error) combining. We consider coherent detection of M-ary phase-shift keying signals in the presence of unequal-power interferers and thermal noise. The analysis is based on our new results on the eigenvalues distribution of central Wishart matrices with correlation.

This paper evaluates the performance of wideband communication systems in the presence of narrowband interference (NBI). In particular, we derive closed-form bit-error probability expressions for spread-spectrum systems by approximating narrowband interferers as independent asynchronous tone interferers. The scenarios considered include additive white Gaussian noise channels, flat-fading channels, and frequency-selective multipath fading channels. For multipath fading channels, we develop a new analytical framework based on perturbation theory to analyze the performance of a Rake receiver in Nakagami-m channels. Simulation results for NBI such as GSM and Bluetooth are in good agreement with our analytical results, showing the approach developed is useful for investigating the coexistence of ultrawide bandwidth systems with existing wireless systems.

In this letter, we derive tight invertible bounds on the bit-error probability (BEP) for the coherent detection of M-ary quadrature amplitude modulation with Gray code bit mapping in Rayleigh fading channels. These bounds enable us to easily obtain tight lower and upper bounds on the bit-error outage (BEO), i.e., BEP-based outage probability, in a log-normal shadowing environment. As examples of applications, these bounds are used to investigate the BEO and mean spectral efficiency for slow adaptive modulation.

We investigate the performance of BPSK and QPSK with coherent detection and matched filtering in the presence of both time and phase asynchronous cochannel interfering signals. More precisely, we analyze the role played by different channel statistics on the distribution of the decision variable at the output of the matched filter. The results show that the Gaussian approximation is accurate not only in the (obvious) case of a large number of interferers, but also when the desired signal is subject to fading, whatever the number of interferers is. For example, when the desired signal is subject to Rayleigh fading, even in the presence of only one unfaded interferer the Kullback-Leibler distance between the exact distribution of the decision variable and that obtained with the Gaussian approximation on the interference is lower than 0.01 [nats] for all cases of practical interest.

In this paper, we investigate the performance in terms of symbol error probability (SEP) of multiple-input-multiple-output (MIMO) systems with high spectral efficiency. In particular, we consider the coherent detection of M-PSK signals in a flat Rayleigh-fading environment. We focus on spectrally efficient MIMO systems where, after serial-to-parallel conversion, several substreams of symbols are simultaneously transmitted by using an antenna array, thereby increasing the spectral efficiency. The reception is based on linear minimum mean-square-error (MMSE) combining, eventually followed by successive interference cancellation. Exact and approximate expressions are derived for an arbitrary number of transmitting and receiving antenna elements. Simulation results confirm the validity of our analytical methodology.

Abstract — In this paper we study frame synchronization based on sync word in the general case when the frame size is unknown and possibly variable on a frame by frame basis. We assume a Gaussian channel with binary symbols and coherent decoding, where soft values are provided to the frame synchronizer. Ac-cording to the hypothesis testing theory, the optimum likelihood ratio test (LRT) is derived using the Neyman-Pearson lemma. Then, the performance of the test is analytically assessed. The obtained closed form expressions allow also the derivation of the receiver operating characteristic (ROC) curves for the LRT synchronizer, showing that a remarkable gain is obtained with respect to synchronization based on correlation metric. I.

In this paper, we investigate the performance of adaptive M-ary quadrature amplitude modulation with antenna subset diversity. We consider a slow adaptive modulation (SAM) technique which adapts the constellation size to the slow variation of the channel due, for example, to shadowing. Our results show that the SAM technique can provide substantial increase in throughput with respect to fixed schemes while maintaining an acceptable low bit error outage. We also compare SAM with a fast adaptive modulation (FAM) technique, which tracks fast fading variations, showing that the throughput of SAM is close to that of FAM despite the fact that SAM is less complex and requires a lower feedback rate to the transmitter.

We present a framework for evaluating the bit error probability of N/sub d/-branch diversity combining in the presence of non-ideal channel estimates. The estimator structure is based on the maximum likelihood (ML) estimate and arises naturally as the sample mean of N/sub p/ pilot symbols. The framework presented requires only the evaluation of a single integral involving the moment generating function of the norm square of the channel-gain vector, and is applicable to channels with arbitrary distribution, including correlated fading. Our results show that the diversity order of a system with practical channel estimation matches that of an ideal system operating in the same correlated fading environment, regardless of the number of pilot symbols used in the estimation process.

In this paper, we firstly derive tight invertible bounds on the bit error probability (BEP) for coherent detection of M-QAM in Rayleigh fading channels. These bounds enable us to easily obtain tight lower and upper bounds on the bit error outage (BEO). As examples of applications, these bounds are used to investigate the BEO in a log-normal shadowing environment. Moreover, using the bounds on the inverse BEP, the analysis of the mean spectral efficiency for slow adaptive modulation is assessed.

Future wireless video transmission systems would consider OFDM (orthogonal frequency division multiplexing) as basic modulation technique due to its robustness and low complexity implementation in the presence of frequency selective channels. Recently, adaptive bit loading techniques have been applied to OFDM showing good performance gains in cable transmission systems. In this paper adaptive loading techniques are applied to an OFDM wireless system in the 5 GHz band for efficient layered multimedia traffic transmission. A low complexity scheme is introduced and compared with other solutions to realize unequal error protection both at coding and modulation level. The large impact of this technique in terms of video quality is evaluated for MPEG-4 video transmission.

This paper investigate the performance of time hopping (TH) pulse position modulation (PPM) systems in the presence of narrowband interference. In particular, we derive closed-form expressions for the bit error probability (BEP) of a TH-PPM system with independent asynchronous tone interferers with arbitrary amplitudes and frequencies. Different scenarios are taken into consideration with fading on the interferer and/or on the useful signal. Simulation results show that the assumption of tone interferers is a good approximation for narrowband interferers. It is shown that our analytical results are useful in assessing the possible coexistence of TH-PPM systems with existing wireless systems.

We investigate convergence properties of both the steepest descent (SD) and least mean-square (LMS) algorithms applied to a multiple-input-multiple-output system in a Rayleigh fading environment with correlated fading. For a given value of the adaptation step, we evaluate the probability that the algorithms are stable. Then, we compare two stable strategies to choose the adaptation step of SD and analyze their speed of convergence. Our results are valid for an arbitrary number of transmit and receive antennas for the uncorrelated fading, and for an arbitrary number of receive antennas less or equal to the number of transmit antennas for the case of correlated fading.

This correspondence presents results of code search for convolutional codes over block-fading channels (BFCs). Search criteria are based on a union bound approach which exploits the concept of generalized transfer function (GTF) of the error trellis diagram. A new asymptotic bound is derived, and the performance of the codes found using our search criterion is compared analytically and by numerical simulation with already existing codes.

We derive a simple closed-form upper bound on the symbol error probability for coherent detection of M-ary phase-shift keying using antenna arrays with optimum combining, in the presence of multiple uncorrelated equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. The new bound, based on Laguerre polynomials, is valid for an arbitrary number of antenna elements as well as arbitrary number of interferers, and it is proven to be asymptotically tight. Comparisons with Monte Carlo simulation are also provided, showing that our bound is useful in many cases of interest.

We derive an exact bit-error probability (BEP) expression for coherent detection of binary signals with optimum combining in wireless systems in the presence of multiple cochannel interferers and thermal noise. A flat Rayleigh fading environment with space diversity, uncorrelated equal-power interferers, and additive white Gaussian noise is considered. The approach is to use the chain rule of conditional expectation together with the joint probability density function (pdf) of the eigenvalues of the interference correlation matrix. This joint pdf is related to the Vandermonde determinant. Let N/sub A/ denote the number of antennas and N/sub I/ the number of interferers. We consider both the cases of an overloaded system, in which N/sub I//spl ges/N/sub A/, and an underloaded system, in which N/sub I/

We investigate the capacity distribution of spatially correlated MIMO channels. In particular, we consider MIMO systems with arbitrary correlation among the transmitting antennas or among the receiving antennas in frequency-flat Rayleigh fading environments. We derive a simple expression for the distribution of the eigenvalues of a Wishart matrix including correlation, and then a closed-form expression for the characteristic function (CF) of MIMO system capacity. Using the exact expression of the CF, the probability density function and the cumulative distribution function can be easily obtained, thus enabling the exact evaluation of the outage and mean capacity of spatially correlated MIMO channels.

We investigate the performance, in terms of symbol error probability (SEP), of multiple-input-multiple-output (MIMO) systems in a flat Rayleigh fading environment with coherent detection of M-PSK signals. The reception is based on linear minimum mean square error (MMSE) combining, followed by successive interference cancellation. Compared to conventional vertical-Bell Laboratories layered space-time (V-BLAST) MIMO systems, we assume a low complexity strategy that does not include the ordering phase. To reduce the difference in terms of SEP on the sub-streams, we consider a MIMO scheme where the symbols transmitted by the different antennas have different energy. Our methodology is completely analytical, is valid for an arbitrary number of receive and transmit antennas (provided that N/sub R//spl ges/N/sub T/) and provides a very fast evaluation of MIMO performance. Results show that the unequal power transmission scheme gains about 3-4 dB over the conventional equal-power transmission case.

It is well known that MIMO systems offer the promise of achieving very high spectrum efficiencies (many tens of bits/Hz) in a mobile environment. The gains in MIMO capacity are sensitive to the presence of spatial and temporal correlation introduced by the radio environment. In this paper we examine how MIMO capacity is influenced by a number of factors, e.g.: a) temporal correlation, b) various combinations of low/high spatial correlations at either end, c) combined spatial and temporal correlations, In all cases we compare the channel capacity that would be achievable under independent fading. We investigate the behaviour of "capacity fades", examine how often the capacity experiences the fades, develop a method to determine level crossing rates and average data durations and relate these to antenna numbers.

We derive a closed form expression of the symbol error probability (SEP) for coherent detection of M-ary PSK signals using an array of antennas with optimum (or, equivalently, linear MMSE) combining in the presence of unequal power interferers and thermal noise. This result is based on a new expression for the eigenvalues distribution of central Wishart matrices with correlation.

This letter addresses the optimum energy partition between the midamble and data fields, given the total energy per burst, in channel estimation for joint detected time-division code-division multiple-access systems. We show that, given the lengths of the data and midamble, in general, the optimal solution requires different amplitude levels. We also derive the burst structure leading to the optimum energy partition with equal amplitude symbols, and the performance degradation by using traditional choices. The analysis is validated by simulations.

An exact expression for the symbol-error probability (SEP) for coherent detection of M-ary phase-shift keying using an array of antennas with optimum combining in a Rayleigh fading environment is derived, based on the theory of orthogonal polynomials. In particular, performance analysis in the presence of multiple uncorrelated equal-power cochannel interferers and thermal noise is considered, starting from problems related to the eigenvalues distribution of complex Wishart matrices. We give an effective technique to derive the SEP involving only one integral with finite integration limits. The result is general and valid for an arbitrary number of receiving antennas and/or cochannel interferers. Based on our efficient method, new results that are useful for the design of wireless systems are obtained.

We derive an upper bound and investigate some approximations on the symbol error probability (SEP) for coherent detection of M-ary phase-shift keying, using an array of antennas with optimum combining in wireless systems in the presence of multiple uncorrelated equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. Our results are general and valid for an arbitrary number of antenna elements as well as an arbitrary number of interferers. In particular, the exact SEP is derived for an arbitrary number of antennas and interferers; the computational complexity of the exact solution depends on the minimum number of antennas and interferers. Moreover, closed-form approximations are provided for the cases of dual optimum combining with an arbitrary number of interferers, and of two interferers with an arbitrary number of antenna elements. We show that our bounds and approximations are close to Monte Carlo simulation results for all cases considered in this paper.

This paper addresses the problem of finding the inverse symbol-error probability (SEP) expression for coherent detection of M-ary phase-shift keying with multichannel reception and maximal ratio combining in Rayleigh fading. To this aim, we derive upper and lower bounds on SEP that are simply invertible and uniformly tight for all values of signal-to-noise ratio. This enables us to obtain tight bounds on the inverse SEP and on the symbol-error outage (SEO), i.e., SEP-based outage probability. As an example of application to digital mobile radio, the SEO in a log-normal shadowing environment is analyzed.

In this paper, we investigate the capacity distribution of spatially correlated, multiple-input-multiple-output (MIMO) channels. In particular, we derive a concise closed-form expression for the characteristic function (c.f.) of MIMO system capacity with arbitrary correlation among the transmitting antennas or among the receiving antennas in frequency-flat Rayleigh-fading environments. Using the exact expression of the c.f., the probability density function (pdf) and the cumulative distribution function (CDF) can be easily obtained, thus enabling the exact evaluation of the outage and mean capacity of spatially correlated MIMO channels. Our results are valid for scenarios with the number of transmitting antennas greater than or equal to that of receiving antennas with arbitrary correlation among them. Moreover, the results are valid for an arbitrary number of transmitting and receiving antennas in uncorrelated MIMO channels. It is shown that the capacity loss is negligible even with a correlation coefficient between two adjacent antennas as large as 0.5 for exponential correlation model. Finally, we derive an exact expression for the mean value of the capacity for arbitrary correlation matrices.

We present new exponential bounds for the Gaussian Q function (one- and two-dimensional) and its inverse, and for M-ary phase-shift-keying (MPSK), M-ary differential phase-shift-keying (MDPSK) error probabilities over additive white Gaussian noise channels. More precisely, the new bounds are in the form of the sum of exponential functions that, in the limit, approach the exact value. Then, a quite accurate and simple approximate expression given by the sum of two exponential functions is reported. The results are applied to the general problem of evaluating the average error probability in fading channels. Some examples of applications are also presented for the computation of the pairwise error probability of space-time codes and the average error probability of MPSK and MDPSK in fading channels.

This paper addresses the design of bit-interleaved pragmatic space-time codes (BI-PSTC) over block fading channels (BFC). BI-PSTC are obtained with the serial concatenation of a convolutional encoder, a bit-interleaver and a space-time mapper, whereas the decoder is based on a Viterbi algorithm which uses suitably defined branch metrics. Here, a method for good BI-PSTC construction, which exploits the concept of generalized transfer function for space-time codes over BFC, is proposed. Codes for QPSK constellations achieving a considerable coding gain over known space-time codes are then derived. Different spectral efficiencies and fading rates (taken into account by the BFC model) are investigated.

We present new exponential bounds for the Gaussian Q-function or, equivalently, of the complementary error function er f c(.). More precisely, the new bound is in the form of the sum of exponential functions that, in the limit, approaches the exact value. Then, a quite accurate and simple approximated expression given by the sum of two exponential functions is reported. Moreover, some new simple bounds for the inverse er f c(.) are derived. The results are applied to the general problem of evaluating the average error probability in fading channels. An example of application to the computation of the pairwise error probability of space-time codes is also presented.

The closed form expression of the probability density function of the disturbance due to a quaternary PSK (QPSK) cochannel interferer is obtained. Then the bit error rate performance of QPSK in the presence of multiple cochannel interferers and additive white Gaussian noise (AWGN) is analyzed. In particular, a closed form expression is given to evaluate the bit error probability of QPSK with multiple cochannel interference. It is shown that in order to determine the minimum signal-to-interference ratio required to obtain a fixed bit error probability, care must be given to the number of active interferers: in fact, strong differences are found when varying the number of interfering cochannel signals, using the same signal-to-total interference ratio. The comparison with respect to the Gaussian approximation is also discussed.

In this paper, we derive an exact expression for the symbol error probability (SEP) for coherent detection of M-ary PSK signals using array of antennas with optimum combining in a Rayleigh fading environment. The proposed analytical framework is based on the theory of orthogonal polynomials and we give an effective technique to derive the SEP involving only one integral with finite integration limits. The result is general and valid for an arbitrary number of receiving antennas or co-channel interferers.

We derive a simple closed-form upper bound on the symbol error probability for coherent detection of M-ary PSK using an array of antennas with optimum combining. We assume multiple equal-power cochannel interferers and thermal noise in a Rayleigh fading environment. The new bound applies for an arbitrary number of antenna elements as well as arbitrary number of interferers, and it is proved to be asymptotically tight. Based on the simplicity of the bound, the signal-to-noise ratio penalty due to cochannel interference is evaluated. Comparisons with simulation is also provided, showing that our bound is useful in a large number of practical interesting cases.

In a digital mobile radio system with fast fading superimposed on slow fading, the symbol error probability (SEP) alone is not sufficient to describe the link quality. In this case, a reasonable performance measure related to the slow channel variations is the outage probability (OP). This paper addresses the problem of evaluating SEP-based OP for multichannel reception with maximal ratio combining (MRC). To this end, we derive upper and lower bounds on the OP from upper and lower bounds on the inverse SEP respectively. As an example of application to digital mobile radio, the SEP-based outage probability in a log-normal shadowing environment is analyzed.

We analyze the error performance of coherent binary keying with optimum combining in interference and noise using the joint probability density function of the eigenvalues of the interference correlation matrix. A flat Rayleigh fading environment with space diversity, equipower interferers, and additive white Gaussian noise is considered. Let L denote the diversity order and N the number of interferers. The interference correlation matrix follows a Wishart distribution when N/spl ges/L, and a pseudo-Wishart distribution when N<L. We treat both the cases of N/spl ges/L and N

In this paper we introduce bit-interleaved pragmatic space time codes (BI-PSTC) and evaluate their performance over correlated fading channels taken into account by a block fading channel (BFC) model. More precisely, the BI-PSTC proposed here are obtained with the serial concatenation of a convolutional encoder, a bit-interleaver and a space-time mapper. A simple decoder based on a Viterbi algorithm with suitably defined branch metrics can be used, but iterative decoding can be also applied. In this paper we also discuss the diversity achievable and the performance of BI-PSTC obtained by using the classical optimal convolutional codes for AWGN channel and QPSK constellations. Performance evaluation shows that a considerable gain over known space-time codes can be achieved.

Future wireless video transmission systems will consider OFDM (orthogonal frequency division multiplexing) as the basic modulation technique due to its robustness and low complexity implementation in the presence of frequency selective channels. Adaptive bit loading techniques have been applied to OFDM showing good performance gains in cable transmission systems. In this paper adaptive loading techniques are applied to a HIPERLAN2-like wireless system at 5 GHz for efficient multimedia traffic transmission. Moreover, the classical water-filling algorithm is extended to the multi-layer case. The semi-analytical results obtained show a large improvement respect to the non-adaptive case, also taking different channel state information up-date rates into account. The impact of this technique in terms of video quality is also evaluated for MPEG-4 video transmission.

The most recent development in the field of mobile wireless ad-hoc networks is constituted by the Bluetooth wireless technology which allows users to make effortless, wireless, instant and low cost connections between various communication devices. Even if Bluetooth has not been specifically designed for WLAN, the possibility to establish low cost wireless connections makes this technology attractive also for the world of WLAN. In this paper we assess by means of a simulative approach, the performance of a Bluetooth based WLAN adopting a token ring MAC protocol in realistic channel conditions.

Robust video transmission over wireless channels is very critical due to channel impairments and to their effect on the compressed bitstream. Error detection is a clue point as, if errors are detected, their concealment may be performed. In spite of that, data assisted error detection is often not sufficient to guarantee error detection is correctly performed in any case. A joint source-channel error detection approach is thus proposed in the paper, where standard compatibility is kept, as the source decoder needs not to be modified in order to exploit the proposed technique. This technique may be useful in any case of packet transmission where data assisted error detection is particularly difficult to be performed for some packet partition and compatibility to a standard is required. With the proposed technique, information about the source coded bitstream may be exploited in order to reduce the redundancy added and the amount of data to be discarded. In particular, the case of MPEG-4 video transmission is analyzed in the paper.

The irregular low-density parity-check codes (LDPCC) have been proposed by Richardson, Shokrollahi and Urbanke (see IEEE Trans. on Inf. Theory), in many cases outperforming the best known turbo-codes. These results have been obtained by allowing the degree of nodes (variable or check) of a LDPCC to vary according to some distribution. We give new distributions to design high rate irregular LDPCC, more precisely rates 7/8, 8/9 and 15/16, motivated by applications to space data communication. Then, we investigate the performance of these new codes over AWGN channels, for different codeword lengths. The error rates obtained are always less than 1 dB from the channel capacity, and in most cases better than the results obtained by turbo-codes with the same parameters. Finally, we investigate, by means of an original method, the minimum distance of irregular LDPCC. The results show that irregular LDPCC have quite low minimum distance, giving rise, similarly to turbo-codes, to the "error floor" phenomenon.

We derive the exact symbol error probability for coherent detection of MPSK signals with optimum combining in the presence of multiple uncorrelated equal power co-channel interferers and thermal noise in a Rayleigh fading environment. The expression is general and valid for arbitrary numbers of receiving antennas or co-channel interferers. The complexity of the analytical model depends on the smaller of the number of antennas and the number of interferers.

A pragmatic approach to space-time codes (STC) over block fading channels (BFC) is proposed. The new approach consists in using common convolutional codes to obtain STC, simplifying the encoder and the decoder. It is shown that pragmatic space-time codes (P-STC) achieve good performance, similar to that of the best known STC, and that they are suitable for systems with different spectral efficiencies and fading velocity (taken into account by the BFC model). To design P-STC we propose a search algorithm based on a new formulation of the pairwise error probability and the error enumerating function for geometrically uniform STC over BFC.

Considering the limitations due to channel impairments on the transmission of MPEG-4 video over error prone channels, a technique to apply unequal error protection (UEP) for the MPEG-4 video bitstream is proposed, where UEP is performed through rate compatible punctured convolutional codes (RCPC), with rates chosen according to the perceived importance of bits. The data partitioning tool available in the MPEG-4 standard is exploited, in order to provide a stronger protection for the most significant partitions. The main problem in the application of such a scheme is the fact that packets, like partitions, are not the same length, thus the UEP scheme should be dynamically changed for each packet and the knowledge of each partition length is required. In order to cope with this problem, proportional unequal error protection (P-UEP) is proposed: as the length of each field is not known at the receiver, a proportional scheme may be used, given the length of the packet. The possibility of considering different code rates for the protection of different frames is also proposed: I frames and P frames may be coded with different rates according to their different structure and sensitivity to channel errors.

Provides an abstract of articles to be presented in a forthcoming issue.

The performance of a time-division multiple-access mobile radio system adopting slow frequency hopping is analyzed by following an original procedure that allows the evaluation of thermal noise and fast fading (e.g., Rayleigh) effects on block-coded transmission with nonideal interleaving. Starting from this approach, service coverage can be analytically evaluated when the "slow" disturbances affecting the transmission are fixed. The method proposed is able to take several impairments and parameters into account, such as shadowing and discontinuous transmission. The experimental results confirmed the validity of the theoretical approach.

Channel estimation in the uplink of a TD-CDTMA system can be accomplished by inserting 11 known sequence of symbols at the chip rate, usually in the middle of the burst, and therefore called midamble. In this paper we found the analytical solution to the problem of optimum energy partition, given the total energy per burst, between the midamble and data fields, in joint detected TD-CDMA systems. We show that, given the lengths of the data and midamble, in general the optimal solution requires different amplitude levels. We also show the burst structure in order to reach the optimum energy partition with equal amplitude symbols, and the performance degradation with traditional choices. The analysis is validated, for different channels, by comparison with simulation results of the TD-CDMA radio interlace specified for third generation cellular systems, where gains in terms of signal-to-noise ratio of 0.5-0.9 dB are achieveable.

A semi-analytical approach is proposed to evaluate the performance of TCP-IP based applications on wireless channels. The physical, data link and IP layers are analytically treated; the TCP is simulated. Additive Gaussian noise, slow and fast impairments due to the mobile radio channel are taken into account, with particular attention focused on a careful description of the different time scales to be used for the different processes introduced by the wireless link. It is shown that the fast fading and the spatial correlation due to shadowing can be analytically studied by means of a suitable combination of the link-level bit error rate (BER) vs. signal-to-noise ratio (SNR) curves and the Gilbert-Elliott model. Results are first given for wireless TCP-IP based on generic physical and data link layers; then, the application to IP over GPRS is considered. A validation of the approach is carried out by comparisons with pure simulation at the logical link control (LLC) level.

Richardson, Shokrollahi, Urbanke have proposed irregular low-density parity-check codes (LDPCCs) that outperform, on memoryless channels, the best known turbo-codes. These results have been obtained by allowing the degree of each node (variable or check) of a LDPCC to vary according to some distribution. In this paper we investigate the performance of such new codes over block fading channels (i.e. channels with memory), in terms of bit and codeword error rates adopting the standard decoding algorithm and a modified version which slightly improves performance. Also, a numerical comparison with conventional convolutional codes is carried out. For a code rate 1/2, it results that irregular LDPCCs are convenient only for large codeword size (greater than 500 bits), and that the gain with respect to a constraint length 7 convolutional code decreases considerably with the channel memory.

In a shadowing-free environment, the improvement introduced by slow frequency hopping (SFH) on a TDMA-based mobile radio system can be taken into account by properly re-defining the minimum carrier-to-interference protection ratio. This protection ratio, with SFH, is dependent on the transmission system, channel model, traffic and frequency reuse parameters. The approach is used in order to analytically investigate the up-link capacity of a SFH mobile radio system, by taking into account a complete scenario, i.e. shadowing, fast fading, power control, antenna diversity, discontinuous transmission and forward error correction (FEC) with non-ideal interleaving and sectorization.

The combined use of adaptive antennas and decision feedback equalization (DFE) is analyzed in a realistic propagation scenario at millimeter waves, taking the direction of arrivals (DOA's) of the received paths into account. The joint antennas and DFE scheme, with one forward filter for each antenna and a single feedback filter (FBF), can be viewed as a spatial and temporal DFE (ST-DFE). The performance of this solution is compared with the cascade of adaptive antenna used for beamforming and DFE. It is found that ST-DFE achieves better performance since it combines the beamforming capability of the antenna array with the equalization properties of the DFE, with great advantages especially when rays arrive from similar angles. The mean square error (MSE) is analytically derived for infinitely long filters in a quasi-static environment with multiple rays having different DOAs, and compared (for the two-path model) with simulation results assuming filters with a small number of taps. Finally, service availability through coverage evaluation is developed and compared with that of a coded-orthogonal frequency division multiplexing (C-OFDM) system.

In a shadowing-free environment, the improvement introduced by slow frequency-hopping (SFH) on a time-division multiple-access based mobile radio system can be taken into account by redefining the minimum carrier-to-interference ratio. This protection ratio, with SFH, is dependent on the transmission system, channel model, traffic, and frequency reuse parameters. In this paper, the above-mentioned analysis is used in order to investigate the capacity of a SFH mobile radio system, with reference to both the uplink and downlink, by taking into account a complete scenario, i.e., shadowing, fast fading, power control, antenna diversity, discontinuous transmission, and forward error correction with nonideal interleaving and sectorization. Outage probability is evaluated by a completely analytical methodology for the uplink, whereas the downlink requires a semianalytical approach to take users' positions into account. Comparison with a pure simulative approach is used to validate the results.

A methodology is presented to evaluate analytically the error probability for block codes over block interference channels. The proposed analysis is based on the knowledge of the moments of the bit-error probability over the interference, thus allowing, for instance, fast performance evaluation of block-coded slow frequency hopping (SFH) systems with antenna diversity over fading channels. As an example of application, slow frequency hopping multiple access (SFHMA) systems with nonideal interleaving are analyzed in the presence of fading, cochannel interference, and additive Gaussian noise.

The closed form expressions of the probability density function (PDF), the cumulative distribution function (CDF) and the characteristic function of the disturbance due to a multilevel cochannel interferer are obtained. The analysis assumes L-level amplitude shift keying (L-ASK) modulated signals, nonreturn to zero (NRZ) pulses, transmission over a distortion-free channel and matched filter reception. The distribution for the multiple cochannel interferers case can then be obtained by means of the characteristic function method. As an application example, the closed-form expression of the error-floor for an L-ASK system in the presence of one cochannel interferer is given. The comparison with respect to the Gaussian approximation is also discussed.

A simple modification of the decision feedback equalizer (DFE) slicer is proposed to reduce the effect of error propagation. A comparison of the performance of the modified DFE and conventional DFE is made for specific channels. On these channels, the modified DFE performs only marginally better than the conventional DFE in terms of average error probability, but may offer some advantages in terms of error probability conditioned on specific input sequences and in terms of the distribution of error burst lengths. Some examples are given, concerning binary PAM and multilevel quadrature amplitude modulation (M-QAM) systems.

The bit error rate performance of binary PSK (BPSK) and Gaussian minimum shift keying (GMSK) in the presence of multiple cochannel interferers and additive white Gaussian noise (AWGN) is analysed. In particular, a closed form expression is given to evaluate the bit error probability of BPSK with multiple cochannel interference, whereas GMSK is analyzed by simulation. It is shown that in order to determine the minimum signal-to-interference ratio required to obtain a fixed bit error probability, care must be given to the number of active interferers: in fact, strong differences are found when varying the number of interfering cochannel signals, using the same signal-to-total interference ratio.

An analytical approach is proposed to evaluate the performance of a slow frequency hopping (SFH) mobile radio system. The effects of frequency diversity and interference diversity are investigated by considering the use of error correcting codes, multipath Rayleigh fading and cochannel interference; by means of an analytical procedure and proper definition of carrier-to-interference ratio the role of several parameters such as the number of hopping frequencies and system load are emphasized.

The problem of error control strategies in wireless multimedia networks adopting the ATM protocol is addressed. In particular, we investigate the performance attainable by using a hybrid combination of automatic repeat request (ARQ) techniques and forward error correcting (FEC) codes for class C, connection oriented, non-delay critical and variable bit-rate services. To mitigate the wireless channel impairments we propose an adaptive error control scheme that chooses the control technique for each connection according to its requirements described by the connection specific parameters (e.g. maximum delay and cell loss ratio). The analysis is carried out for an OFDM/QPSK modulation format over a typical indoor radio channel.

A simple procedure for designing the post-detection filter of an MSK-type receiver for continuous phase modulation with modulation index h=0.5 (generalized MSK signals) is proposed: the criterion descends from a useful, general representation of this class of signals. The frequency response of the baseband receiver filter is analytically optimized in closed form whatever the modulation pulse shaping of the transmitted signal, and signal-to-noise ratio at the receiver. The criterion is pointed out through a significant, exact representation of generalized MSK signals, whatever the modulation pulse-shaping waveform, which is assumed to be included in a time range equal to LT, T=symbol time. The main part of intersymbol interference, as appears from the representation of the CPM signal, is cancelled by applying the Nyquist criterion to the most significant function among those describing the signal structure. The numerical results obtained show a performance improvement with respect to the averaged matched filter (AMF) criterion and are very close to the results reported in the literature, deriving from more complex design criteria. The suitability of the design criterion is also tested in the presence of interferers, showing a significant advantage of the proposed filter when considering adjacent channel interference.>