This paper studies the use of Blind Source Separation (BSS) as a replacement for Coded Random Access in diversity ALOHA massive machine-type communications. We propose a scheme for efficiently implementing BSS within the ALOHA framework, while also determining the number of active users in each slot. We experimentally evaluate the achievable performance and highlight the factors that influence the number of detectable users.

This paper theoretically analyzes the usage of directional slotted Aloha schemes for managing the peer-to-peer random access in fifth and sixth generation (5G/6G) systems. To this aim, the physical layer is modeled by accounting for interference and noise, while a Markov chain approach is developed to investigate the network behavior in the presence and in the absence of a separate feedback channel, which provides information concerning the success or not of each transmission attempt. Closed-form expressions for the coverage probability and for the transition matrices with and without feedback are derived to then evaluate the corresponding throughput. The analytical results, which are validated by independent Monte Carlo simulations, are used to estimate the impact of the antenna gain, of the burst length, and of the node density on the achievable performance, as well as to discuss the directional random access benefit/complexity tradeoff.

Multidirectional antennas are one of the enabling technologies for the widespread diffusion of intelligent vehicles, which are expected to fill smart cities in the near future characterized by the fifth generation cellular system (5G). In this context, this paper proposes a simple and versatile design procedure for a feeding network that realizes the beam steering of linear antenna arrays. The developed procedure is intended for a feedline, which must meet some requirements typical in a 5G scenario, such as, for example, easy mass production, low cost, easy integration with other system components and compactness. Precisely, a Blass matrix using the microstrip technology and identical branch line directional couplers is implemented so that the multidirectional antenna system can be entirely printed on a compact and cheap printed circuit board (PCB). However, when the coupling value is fixed in advance some pointing directions may be not realizable. So, this paper proposes an algorithm, which evaluates a desired number of beams in such a way to have a simple design procedure of a feeding network for a multidirectional linear antenna array. The effectiveness of the developed solution is validated by numerical results

This paper presents a method for estimating the time of arrival (ToA) of long term evolution (LTE) signals received on multiple separate transmission bands by the same base station (BS) mast. By exploiting the overall bandwidth occupied by the different signals and the correlation between the corresponding channel impulse responses, a higher precision is achieved with respect to the usually adopted single-band approach whenever the time-correlation among the bands is sufficiently high. The ToA estimation is carried out by generalizing the space-alternating generalized expectation-maximization (SAGE) algorithm to the multi-band context, proving that the availability of multiple bands provides a reduced standard deviation for the estimated ToA, with a limited increase of the computational cost. The main analyzed issue consists in the management of the asynchrony between transmitters belonging to distinct cellular operators, which is addressed by developing a suitable method to combine the contributions provided by the different bands. The method is validated by simulations in dual- and tri-band scenarios, and is further applied to real dual-band signals measured through a portable setup and experimentally acquired from an LTE BS mast covering multiple cells.

This paper presents a novel network architecture for an integrated nanosatellite (nSAT)-5G system operating in the millimeter-wave (mmWave) domain. The architecture is realized adopting a delay/disruption tolerant networking (DTN) approach allowing end users to adopt standard devices. A buffer aware contact graph routing algorithm is designed to account for the buffer occupancy of the nSATs and for the connection planning derived from their visibility periods. At the terrestrial uplink, a coded random access is employed to realize a high-capacity interface for the typically irregular traffic of 5G users, while, at the space uplink, the DTN architecture is combined with the contention resolution diversity slotted Aloha protocol to match the recent update of the DVB-RCS2 standard. To achieve a reliable testing of the introduced functionalities, an accurate analysis of the statistic of the signal to interference-plus-noise ratio and of the capture probability at each mmWave link is developed by including interference, shadowing, fading, and noise. The application of the designed architecture to data transfer services in conjunction with possible delay reduction strategies, and an extension to inter-satellite communication, are finally presented by estimating the resulting loss/delay performance through a discrete-time discrete-event platform based on the integration of Matlab with Network Simulator 3.

Among the recently proposed non-orthogonal multiple access techniques, the Coded Slotted Aloha (CSA) scheme has been identified as one of the most efficient ones. This scheme relies on the subdivision of a packet into segments and of a slot into slices, in order to enable the transmission of different segments in different slices. With respect to SA, the CSA implementation is more sophisticated, since the header of each segment must contain proper pointers specifying the slices where the other segments of that packet lie. Purpose of this paper is to investigate the impact of these pointers on the CSA throughput by properly considering the capture effect at the receiver. To this aim, a capture analysis accounting for the modulation and the header coding rate in the presence of Rayleigh fading is developed. The numerical results, which also involve a realistic millimeter-wave channel, show that the beneficial interference diversity effect introduced by the segmentation process is not significantly reduced by the overhead due to the pointers, thus confirming the practical usefulness of the CSA scheme.

This paper presents a theoretical analysis for estimating the coverage probability in two-dimensional (2D) and three-dimensional (3D) peer-to-peer (P2P) millimeter-wave (mmWave) wireless networks. The analysis is carried out by adopting suitable link state models and realistic propagation conditions, involving path-loss attenuation, angular dispersion, mid- and small-scale fading, which comply with recent channel measurements. The presented framework accounts in detail for the actual shape of the transmitting/receiving antenna patterns and for the spatial statistic that describes the node location, by considering the widely adopted Poisson point process, the uniform distribution, and the random waypoint mobility model. Analytical expressions for the statistic of the received power and simple integral formulas for the coverage probability in the presence of interference and noise are derived. The accuracy of the obtained estimations and of the introduced approximations is checked by independent Monte Carlo validations. As possible applications in the 3D mmWave context, the conceived mathematical theory is used to discuss the impact of the interference model on the reliability of the noise-limited approximation, and to estimate the average link capacity of an interfered P2P communication.

This paper discusses the use of the Space-Alternating Generalized Expectation-maximization (SAGE) algorithm for Time of Arrival (ToA) estimation in Orthogonal Frequency Division Multiplexing (OFDM) systems. This algorithm, which is an extension of the expectation-maximization approach, can exploit the intrinsic properties of a multi-carrier OFDM system to achieve the estimation of a number of parameters of each incoming signal replica, such as its amplitude, ToA, Doppler shift, and angle of arrival. The SAGE method can also be easily tailored to estimate only a desired subset of these parameters. Accordingly, this study focuses on the sole ToA estimation, but extending the SAGE functionalities to dual-band OFDM systems. This extension is achieved by exploiting the availability at the receiver of more than one band from the same transmitter, and combining the derived channel information to increase the bandwidth available for estimation. It will be shown that, in highly correlated scenarios, the performance of the SAGE algorithm for ToA estimation in a dual-band system is comparable to that of a single-band system with equivalent total bandwidth.

This paper investigates the throughput of a Framed Slotted Aloha (FSA) random access scheme, called Segmented FSA (SFSA), in which each packet is encoded and subdivided into segments before the transmission, and where Interference Cancellation (IC) is adopted to improve the success probability in case of collision between segments. The overall system performance is estimated by considering a decoding criterion based on the average information rate that is experienced during each communication. This criterion, which enables to properly consider the capture effect in the presence of interference, noise, and fading, is compared to the classic packet erasure channel model, with the aim of identifying the reliability of the two decoding approaches in different network scenarios. The final purpose of the presented study is to discuss the benefits of the interference diversity mechanism that is triggered by the combination of segmentation and IC in a capture channel.

This paper presents a theoretical model for estimating the uplink capacity of a communication in a three-dimensional(3D) millimeter-wave (mmWave) scenario. The model is developed considering realistic channel conditions, including path-loss attenuation, shadowing, and angular dispersion, which comply with recent measurement campaigns carried out in mmWave environment. The analysis also incorporates the effects of interference and noise, additionally accounting in detail for the actual shapes of the transmitting/receiving antenna patterns. Analytical expressions for the statistics of the interference power are derived, in order to enable a low-complexity evaluation of the capture probability and of the Shannon capacity for an interfered uplink communication. The conceived 3D mathematical framework, whose reliability is verified by Monte Carlo validations, is used to investigate some insights concerning the application of beamforming in dense mmWave scenarios.

This paper presents an experiment using real Long-Term Evolution (LTE) signals to extend positioning from outdoors to indoors. LTE signals are of interest for positioning applications because of their availability indoors, where GNSS signal reception is limited. Different approaches for time of arrival (TOA) extraction are evaluated for their positioning performance, combined with an extended Kalman filter (EKF) for movement tracking. The paper shows that the performance is surprisingly good, with high visibility of cellular signals even in the difficult indoor test environment, and with a positioning error once indoors smaller than 8m in 50% of cases.

Advanced antenna systems enable spatial multiplexing through beamforming, allowing the coexistence of multiple, asynchronous, peer-to-peer communications at the medium access control layer, thus introducing multi-packet communication (MPC). The result of each communication is usually established adopting a receiving threshold on the signal-to-interference ratio (SIR). However, more sophisticated reception criteria would be desirable, since the instantaneous SIR of a packet may change because of the asynchrony of the scenario. This paper investigates this partly unexplored issue moving from the exploration of some analytically tractable situations and then performing a comprehensive investigation carried out by a suitable simulation platform. The aim is to realistically estimate the combined effect of the channel access strategy, of the acknowledgement packet, and of the channel code rate, on the throughput, the drop probability, and the packet delay of an MPC network.

The main concern in nowadays digital communications standards, such as IEEE802.11n, 802.16e (Wi-MAX), 100BaseT Ethernet, and Digital Video Broadcasting (DVB), is the need of adaptive and flexible communication techniques. The request for higher efficiency, both in bandwidth use and power consumption, increases the need for limit-achieving, flexible techniques of channel coding. Low Density Parity Check (LDPC) Codes are very interesting because of their high performances and potential flexibility introduced by puncturing. In this paper, the design of rate-compatible LDPC is addressed. A class of punctured LDPC codes is defined through their equivalent parity check matrices. A formal analysis is provided, based on a simplified approach on the decoding belief propagation algorithm, i.e., considering a Gaussian approximation for message densities under density evolution. A suitable design criterion for the puncturing patterns is then addressed.

This paper presents a mathematical framework for including the angular domain beside the radial one in the theoretical modeling of wireless networks, in which spatial reuse enables the coexistence of multiple single-hop peer-to-peer communications inside a finite region. The proposed model analyzes a scenario where the transmitting sources are uniformly distributed over a disk and the communications are subjected to path-loss attenuation and multipath-fading, considering the actual location of each destination and its antenna system. Different from most of the previous theories in which the coverage probability of a destination is estimated assuming that the destination itself is positioned at the center of the network, in the proposed analysis, the destination location is generic. This generalization, together with the consideration of the spatial channel model and of the actual receiving pattern, allows one to investigate the influence of the angular domain on the statistic of the interference power and on the coverage probability. The conceived theory, which is further verified by Monte Carlo validations, is finally exploited to derive the network transmission capacity, with the purpose to illustrate the possible advantages that may derive from a reliable modeling of the non-isotropic context, in which each destination has to operate realistically.

This paper proposes an algorithm for the estimation and tracking of the direct path (DP) time of arrival (TOA) of the signals received from 4G long term evolution (LTE) cellular base stations (BSs) in a mobile multipath environment. This is important for TOA-based ranging measurements, which may be exploited for positioning applications. A sub-space approach is used for the estimation of the multipath time of arrival, and a state-space approach is exploited for tracking the direct path. The developed framework is applied to real LTE signals collected using a portable experimental setup during a car drive in the town of Rapperswil, Switzerland. The pseudoranges derived from the tracking of the DP TOA are then compared to the ranges from the considered LTE base stations calculated using GPS, demonstrating the effectiveness of the proposed approach.

Video quality assessments represent useful tools not only to evaluate the quality of the received video, but also to enable unequal error protection mechanisms. The computational cost of these quality assessments is however a relevant limitation to their use, mainly in the presence of time constraints, such as in live streaming applications. This paper proposes an algorithm capable to estimate, with a very low computational cost, the distortion due to the loss of a frame on videos encoded using the H.264 Scalable Video Coding (SVC) standard, by adopting one of the most reliable video quality assessments: the Structural SIMilarity (SSIM) index. The results show that the proposed solution allows one to obtain a satisfactory approximation of the exact SSIM-based distortion, simultaneously reducing the time required by the estimation process. The approximated SSIM distortion values are finally applied to an 802.11e distributed network with energy constraints, in order to test the effectiveness of the conceived algorithm in a practical scenario.

This letter characterizes the error performance of realistically modelled orthogonal frequency division multiplexing (OFDM) signals, when their time of arrival has to be estimated in an additive white Gaussian noise channel. In particular, different power distributions on the available sub-carriers of the OFDM signal are considered, and bounds on the corresponding root mean square estimation error (RMSEE) are evaluated. The tools used for such purpose are the widely adopted Cramér-Rao bound and the Ziv-Zakai bound, which is tight in a wide range of signal-to-noise ratio (SNR) values. The presented analysis reveals that, for a given signal bandwidth, a proper power distribution on the OFDM sub-carriers is crucial for achieving a good performance in the low to medium SNR region, where the RMSEE curve exhibits the typical threshold behavior. Moreover, a trade-off between asymptotic and threshold performance is identified, thanks to the adoption of a novel performance figure, which directly describes the threshold RMSEE behavior.

This paper discusses the design requirements for enabling multiple simultaneous peer-to-peer communications in IEEE 802.11 asynchronous networks in the presence of adaptive antenna arrays, and proposes two novel access schemes to realize multipacket communication (MPC). Both presented solutions, which rely on the information acquired by each node during the monitoring of the network activity, are suitable for distributed and heterogeneous scenarios, where nodes equipped with different antenna systems can coexist. The first designed scheme, called threshold access MPC (TAMPC), is based on a threshold on the load sustainable by the single-node, while the second protocol, called signal-to-interference ratio (SIR) access MPC (SAMPC), is based on an accurate estimation of the SIR and on the adoption of low density parity check codes. Both protocols, which are designed to be backward compatible with the 802.11 standard, are numerically tested in realistic scenarios. Furthermore, the performance of the two schemes is compared to the theoretical one and to that of the 802.11n extension in a mobile environment.

This paper presents an approximated, but accurate and fast to compute, mathematical framework for evaluating the capture probability and the number of sustainable communications in a mobile wireless network where spatial reuse enables the coexistence of multiple traffic flows. The proposed study focuses on a particular multi-packet approach, known as multi-packet communication, in which different communicating pairs are simultaneously active. Within this context, the statistics of the received power, of the interference power, and of the signal-to-interference ratio are derived accounting for the antenna system and the network topology in the presence of Nakagami fading. The theoretical results, which are validated by independent Monte Carlo simulations, are used to discuss the impact of mobility on the result of each transmission attempt. The purpose of the developed theory is to provide an alternative analysis, including the spatial domain, for modeling the capture probability and the number of concurrent peer-to-peer communications that can be sustained in a mobile ad-hoc network.

Multiple‐input multiple‐output (MIMO) techniques have received a considerable attention from the research community for their promising performance in faded environments, and particular interest has been given to the physical (PHY) layer implications of the tradeoff between multiplexing and diversity. This study discusses this tradeoff extending the analysis to the medium access control (MAC) layer. To this purpose, a theoretical model for assessing the performance of an 802.11 network that relies on a MIMO‐enabled PHY layer is presented. The proposed model characterises the different MIMO systems in terms of their multiplexing gain and bit error rate performance, considering both the diversity gain of the specific MIMO scheme and the impact of the spatial distribution of the nodes. The results compare, from a MAC layer perspective, the benefits of multiplexing and diversity, providing some analytic insights on the tradeoff between increasing the system's reliability and enhancing the data rate of the single link.

This paper analyzes the performance of an 802.11 network in which spatial reuse enables the simultaneous coexistence of multiple asynchronous communications between different node pairs. This scenario, defined multi-packet communication (MPC), is theoretically investigated by deriving the capture probability for the single communication and by evaluating the network throughput using a Markov chain approach. The mechanisms introduced in the analysis for MPC operations enable the development of an improved version of the 802.11 access scheme, whose performance is evaluated using a reliable simulation platform. The main purpose of this study is to directly compare the throughput obtained from the completely theoretical analysis of an MPC scenario with that obtained from a protocol implementation, discussing some issues related to the accuracy of the collision model in realistic network topologies.

This paper has the objective of identifying the application domain of a particular multi-packet strategy enabled by spatial reuse, which allows the simultaneous coexistence of multiple traffic flows in 802.11 wireless networks and is known as multi-packet communication (MPC). More precisely, this study outlines the differences between MPC and the widely investigated multi-packet reception (MPR) and multi-packet transmission (MPT) techniques, proposing extensions to the existing MPC theoretical models that aim to reliably estimate the throughput, the average successful packet delay, and the drop probability. The presented discussion involves the channel coding rate and the stochastic collision model, which determines the throughput-delay behavior of the network. Moving from the derived results, some open issues are identified as possible topics for the future research in the field of MPC.

The sphere-packing bound represents a suitable tool for assessing the limiting performance of a digital communication system. In fact, most of the modern transmission techniques, adopting capacity approaching codes, such as turbo codes, or Low Density Parity Check (LDPC) codes, may offer a performance that is within 1 dB from the bound, both for Additive White Gaussian Noise (AWGN) and for the fading channel. Unfortunately, the sphere-packing exponent may be difficult to compute, and this may prevent its adoption as a performance estimator. This paper presents and discusses the accuracy of an approximate expression of the bound, that may be used for evaluating the achievable performance of a communication system, without resorting to cumbersome simulations.

This paper analyzes the problem of modeling the collisions in a distributed and heterogeneous fixed wireless network in which spatial reuse enables multi-packet communication (MPC), thus allowing the coexistence of multiple peer to peer communications. The objective is to present a completely theoretical model that provides closed form expressions for the capture probability, and for the statistics of the interference power and of the signal-to-interference ratio (SIR) experienced by a receiving node. The presented theory aims in particular to investigate the impact on the capture probability of the heterogeneity introduced by the network topology and by the use of different antenna systems. All theoretical results are validated by independent Monte Carlo simulations. Finally, this study discusses the possible analogies between the developed and the existing frameworks for homogeneous scenarios, and presents a preliminary application of the developed capture analysis by evaluating the throughput-delay behavior of a homogeneous 802.11 MPC network.

This work presents our effort in developing a complete, real-time, Software-Defined Radio implementation of an OFDM transceiver, compliant with the IEEE 802.11 Physical Layer specifications. All the baseband components in both the transmitter and the receiver are implemented with fast software functions, running on a General Purpose CPU. Real-time operation is achieved on a modern CPU by means of extensive code optimization, mostly using the SIMD instruction sets which are widely available on almost every modern CPU.

This paper analyzes the problem of modeling the collisions in a wireless network supporting multi-packet communication by spatial reuse. Typically, in a multi-packet scenario, the result of a transmission attempt is established using two main approaches: one, more idealized but suitable for theoretical analyses, based on the success probability or on the threshold on the number of sustainable communications, and another one, more realistic, relying on the signal-to-interference ratio (SIR) experienced by the receiving node. The two approaches may lead to considerably different results, since the number of allowed communications is often estimated neglecting many physical aspects. This study investigates this issue by presenting a mathematical model, validated by simulations, which relates the number of active communications with the statistic of the SIR. Moving from the developed framework, this paper discusses how some assumptions usually adopted for analyzing multi-packet networks exploiting spatial reuse may be revised in presence of a more detailed characterization of the spatial domain.

This paper proposes a novel Medium Access Control (MAC) protocol for enabling multiple simultaneous communications in 802.11 networks where the nodes can use adaptive antenna arrays. The presented scheme is backward compatible with the 802.11 standard and is designed for heterogeneous scenarios, where legacy and non-legacy nodes equipped with different antenna systems can coexist. The access rules are based on a local but accurate Signal-to-Interference Ratio (SIR) estimation and on the adoption of Low Density Parity Check (LDPC) codes, which enable each node to reliably estimate the result of the transmission attempt of all active nodes. The proposed scheme is compared to a previously developed one in noisy and multipath-fading environments.

Cooperative MAC protocols have been shown to increase the attainable network throughput and to reduce the outage probability when perfect Channel State Information (CSI) is available either at the source station or at candidate relays. This paper investigates the realistically achievable network performance of relay selection schemes under imperfect CSI knowledge. Novel relay selection algorithms, based on a Markov Decision Process (MDP) framework, are proposed for mitigating the performance degradation introduced by imperfect channel knowledge. We demonstrate that the performance of cooperative MAC schemes crucially depends on the accuracy of the CSI knowledge. Furthermore, it is shown that the Markov framework allows the source station to predict the channel quality and hence to reduce the number of transmission errors, ultimately resulting in an improved network performance.

The performance of a Space-Time Shift Keying (STSK) scheme was shown to degrade in frequency-selective fading channels. Hence, we propose Orthogonal Frequency Division Multiplexing (OFDM) combined with STSK for frequency-selective broadband channels. Furthermore, we consider both an uncoded and a near-capacity coded scenario. Our results show that a STSK system combined with OFDM is capable of overcoming the impairments of dispersive channels, hence approaching the same performance as in a flat- fading channel.

The Space-Time Shift Keying (STSK) framework subsumes diverse Multiple-Input Multiple-Output (MIMO) schemes, offering a near-capacity performance at a reduced complexity. The STSK system's performance crucially depends on the dispersion matrix (DM) set used for encoding the transmitted symbols. We introduce a novel criterion, based on EXtrinsic Information Transfer (EXIT) chart analysis, for selecting capacity-approaching sets from candidate DMs, and a novel Genetic Algorithm (GA) for efficiently exploring the search space formed by the candidate DM sets. Our proposed GA allows obtaining DM sets that enhance the system's performance compared to a random selection, while simultaneously reducing the search algorithm's complexity.

Cooperation has been recently introduced to mitigate the impairments imposed by fading on a communication channel. A cooperative transmission, in which the destination may overhear all transmission attempts, benefits from incremental redundancy techniques, achieving a higher throughput and reducing the packet delay and the outage probability. This paper presents a theoretical framework to assess the limiting performance of efficient coding techniques in Decode-and-Forward cooperative scenarios. In this paper it is shown that suitable designed codes, such as punctured turbo codes, approach the limiting performance in all the studied scenarios. The paper also addresses the design of punctured turbo code schemes capable of performing close to the presented limiting bound. A novel Genetic Algorithm (GA) is introduced for designing incremental puncturing schemes suited to cooperative transmissions. The algorithm uses a recently proposed tri-dimensional (3D) EXtrinsic Information Transfer (EXIT) chart technique to model the decoding behavior of parallel concatenations of non-systematic periodically punctured convolutional codes. It is shown that the design algorithm allows contriving capacity approaching codes for the source and the relay. The paper further shows that the GA aided designed codes closely approach the theoretical results, outperforming previously proposed systematic incremental and complementary schemes.

Abstract This paper describes an experimental setup for measuring the throughput of an 802.11 network that supports Quality of Service (QoS) mechanisms, addressing some issues concerning the comparison between experimental and theoretical results. The setup is an ad hoc network in which the nodes implement the Multiband Atheros Driver for Wireless Fidelity (MADWiFi). There are three main contributions of this study. The first one describes the adjustments to the MADWiFi source code that guarantee the control of the QoS parameters in ad hoc mode. The second contribution aims to provide a practical method for verifying if the backoff generated by a given wireless card is uniformly distributed, as dictated by the 802.11 standard. This represents a key point when theoretical and experimental results must be compared, since many vendors adopt nonuniform backoff distributions in their commercial cards. As a third contribution, this paper presents a theoretical nonsaturated analysis that provides the throughput and the successful packet delay of an 802.11 network supporting QoS in presence of nonuniform backoff. The throughput derived from this analysis using a uniform backoff, which is included in the proposed model as a particular case, is compared to the measurements provided by the deployed setup. Copyright © 2010 John Wiley & Sons, Ltd.

This paper presents a novel Medium Access Control (MAC) protocol for 802.11 networks in which the nodes can adopt smart antenna systems. The presented protocol is designed to satisfy two fundamental requirements: the exploitation of adaptive arrays to increase the network throughput and the backward compatibility of the novel scheme with the 802.11 MAC layer. The developed solution allows the simultaneous coexistence of multiple communications, while maintaining, for the legacy nodes, the same throughput levels achievable in the 802.11 standard. The performance and the backward compatibility of the proposed MAC protocol are tested in multipath-fading environment and considering heterogeneous scenarios, including 802.11 legacy stations together with nodes implementing the novel scheme and equipped with different antenna systems.

Cross-layer designed cooperative Medium Access Control (MAC) protocols for 802.11 wireless networks have been recently shown to achieve a better performance than the Distributed Coordination Function (DCF). This paper analyses the effects of different channel coding techniques on the performance of cooperative MAC protocols. Different forwarding schemes, inspired by Hybrid Automatic Repeat and reQuest (HARQ) techniques, are evaluated, and some guidelines are given to design codes suited to cooperative MAC protocols. Furthermore, the cooperative network's saturation throughput and average packet delay are theoretically evaluated using the sphere-packing bound and extending the Markov models used for 802.11 networks. The model is validated performing bit by bit simulations of the cooperative MAC schemes, showing that the theoretical limiting performance are closely approached by efficient coding schemes.

This paper proposes a PHYsical (PHY) layer de sign that enables Multi-Packet Reception (MPR) in distributed wireless networks using advanced antenna systems. The main objective of this work is to present a backward compatible solution that is able to exploit the capabilities of adaptive arrays, allowing the coexistence of non-legacy and legacy 802.11g nodes. The proposed scheme is designed for an asynchronous scenario, in which a node can access to the medium as long as the number of ongoing communications lies within a given threshold, and considering an heterogeneous network, in which the antenna system can be different for each node. The developed solution is implemented in an hybrid discrete-time discrete-event simulation platform obtained integrating MATLAB and network simulator 2 (ns-2), in order to explore the system performance for different network topologies and in presence of multipath-fading.

Cooperation may be used to exploit the spatial diversity provided by independently faded antenna systems. In a cooperative scenario, incremental redundancy and adaptive modulation may be used to improve the system performance. This paper presents and uses an approximated theoretical framework for designing and evaluating the performance of a decode and forward (DF) cooperative system adopting efficient encoding techniques. The proposed framework may be used to model both incremental redundancy and adaptive modulation. The analytical results are compared with simulation results, to assess their accuracy.

Recent studies have revealed that many vendors implement nonuniform backoff distributions in their commercial 802.11 Network Interface Cards (NICs) in order to obtain a better performance in particular scenarios. Therefore, in many cases, the backoff behavior of the cards can be not adherent to the 802.11 standard specifications. By consequence, the theoretical and the experimental performance of an 802.11 distributed network, including throughput and packet delay, may not coincide because the real backoff behavior of the adopted cards is different from the expected one. This paper presents a practical method for verifying if the backoff generated by a given 802.11 NIC is uniformly distributed, as dictated by the 802.11 standard, or not. The proposed method is based on the comparison between the theoretical and the experimental statistics of the time between two successfully transmitted packets. Thus it does not require intrusive measurements inside the chipset and can be performed using a simple open source software platform.

Recent studies have shown that designing a Medium Access Control (MAC) protocol combined with a cooperative approach may improve the attainable network throughput, despite reducing the mean packet delay. In this paper we design a MAC scheme adopting cooperative physical layer aided cross-layer techniques. We consider a popular cooperative MAC protocol, namely the CoopMAC technique of Liu et al., which is improved by facilitating cooperative signal combining at the destination and employing two relays in the context of a successive relaying technique. The performance of the proposed scheme is evaluated by Monte-Carlo simulations. We demonstrate that a cross-layer design further improves the performance gain provided by the CoopMAC protocol over that of the legacy 802.11 Distributed Coordination Function (DCF), enhancing both the achievable network throughput and the outage probability.

This paper proposes a mathematical framework for evaluating the throughput of an 802.11 distributed wireless network when the system allows multi-packet reception in asynchronous operating mode. Differently from previous studies on multi-packet reception, which assume a certain synchronization between the different sources at the beginning of the respective transmissions, this work theoretically analyzes a fully distributed scenario, where each node can access to the medium in a completely asynchronous way. More precisely, a new transmission can begin in a channel where other nodes are already communicating as long as the current channel load is within a given threshold. The proposed framework consists of a single source, 802.11 specific model, which generalizes a previously presented analytical model, and a novel network model, which is adopted to evaluate the theoretical performance in terms of throughput. The novel model, which includes the synchronous scenario as a particular case, is employed to investigate the influence of the number of allowed communications, of the load threshold, and of the minimum contention window on the network performance. Furthermore, the theoretical analysis is used to evaluate the throughput in presence of Rayleigh fading and noise when error correcting codes are adopted.

This paper proposes a mathematical framework for evaluating the throughput of an 802.11 Distributed Wireless Network (DWN) when the system allows Multi-Packet Reception (MPR) in asynchronous operating mode. Differently from previous studies on MPR, which assume a certain synchronization between the different sources at the beginning of the respective transmissions, this work analyzes a fully distributed scenario, where each node can access to the medium in a completely asynchronous way. More precisely, a new transmission may begin in an already busy channel, as long as the current channel load is within a given threshold. The presented model, which includes the synchronous scenario as a particular case, is employed to investigate the influence of the number of allowed communications, of the load threshold, and of the minimum contention window on the network performance.

This paper compares the theoretical and the experimental throughput of an 802.11 adhoc network implementing quality of service (QoS) mechanisms. The theoretical model provides the throughput of each access category (AC) of each node in saturated and non-saturated traffic conditions. The experimental setup is described in detail to enable the reproduction of the presented measurements. This setup adopts the widely used multiband atheros driver for wireless fidelity (MADWiFi), which, however, provides the QoS option for centralized operations only. Hence, this study describes the adjustments to the MADWiFi source code that guarantee the control of the QoS parameters in adhoc mode.

This paper proposes a mathematical framework for comparing the performance of wireless networks based on the IEEE 802.11 standard, employing switched-beam, directional and adaptive antennas. The analysis adopts detailed models for the antenna system behavior and the propagation environment, taking into account multipath and interference. The purpose of this study is to determine the number of simultaneous communications that may be sustained by adopting advanced antenna systems, and to investigate, by analysis and simulation, the resulting performance improvements, in terms of throughput and packet delay. Both saturated and non-saturated traffic conditions are treated. Suitable 802.11 Medium Access Control (MAC) layer parameter settings, able to exploit smart and directional antenna capabilities for maximizing the throughput, are also discussed.

This paper applies three low complexity methods, previously proposed by the authors, to estimate the distortion deriving from packet losses in error-prone video communication systems. In a first application scenario, the distortion estimated by the encoder is encapsulated in each video packet and used as side information by a modular router, in which the architecture is properly modified for taking into account the distortion impact produced by the loss of each packet. Computational complexity considerations, compared with traditional rate distortion optimization (RaDiO) techniques, are also discussed, showing that the proposed distortion estimation algorithms may be easily used and integrated in a wireless router with limited CPU resources. In the paper a multi-hop scenario is also considered as possible application environment, in which the prioritization and selective discarding policy are based on the estimated distortion and on the loss pattern deriving form previous hop transmissions.

This paper proposes a mathematical framework for evaluating, in saturated and non-saturated traffic conditions, the performance of an 802.11 wireless network in which the nodes are equipped with adaptive antenna systems. The analysis, validated by simulations, adopts detailed models for the antenna behavior and the propagation environment, taking into account multipath and interference. The developed model is used to investigate the proper parameter settings that lead to throughput maximization, providing a simplified method to obtain the optimum contention window when adaptive arrays are employed.

This article proposes three methods to estimate the distortion deriving from packet losses in wireless video communication. The proposed methods take into account the short-term properties of the encoded video sequences. A suitable set of functions is adopted to model the distortion envelope resulting from multiple losses. The estimated performance is compared with the actual distortion, evaluated by decoding the received sequence with a properly designed decoder. Numerical results confirm the accuracy of the proposed models in approximating the actual Mean Square Error (MSE) for a wide range of loss rates. Some applications of the proposed algorithms are presented.

This paper investigates the influence of channel coding on the performance of a distributed wireless network in which the nodes are equipped with multiple antennas. Different coded modulation techniques and antenna systems are taken into consideration and compared, assuming an outdoor environment. More precisely, the proposed study compares the performance of wireless networks adopting convolutional codes, already adopted in the 802.11a physical layer standard, with the performance that may be achieved by adopting more efficient coding techniques. A realistic channel model including Rayleigh fading and Laplacian power azimuth spectrum is adopted, and code performance comparison is performed using both simple switched-beam antenna (SBA) systems and more complex fully adaptive smart antennas (FASAs).

This paper investigates the behavior of a wireless mesh network in which the nodes are equipped with a multiple antenna system. The objective of this study is the comparison of the low-cost switched-beam antennas with the more expensive fully adaptive smart antennas. The analysis and the simulations are performed together with some considerations regarding the advantages and the practical possibilities of employing multi-antenna systems on mesh routers. The analytical and the numerical results are derived employing realistic antenna radiation patterns and a proper spatial channel model.

This paper investigates the performance of a distributed wireless network in which the nodes are equipped with fully adaptive smart antennas. The main goal is the evaluation of the influence that the geometrical configuration of the antenna array has on network throughput and packet delay. Analysis and simulations are performed to study the relationship between the geometry of the antenna array and the network topology. In this paper, the performance degradation due to both the small-channel angular spreads and the speed of mobile nodes is investigated. The results, which hold for the low-rank channel model only, extend to this specific case previous results obtained in ideal channel conditions. The study adopts an access scheme, previously proposed for ad-hoc networks, which is able to exploit the interferer suppression capabilities of smart antennas, together with realistic models for the antenna and the channel behaviors.

This paper proposes three methods to estimate the distortion deriving from packet losses in wireless video communication. The proposed models take into account the short-term properties of the encoded video sequences and the properties of the wireless links. A suitable set of functions is adopted to model the distortion envelope resulting from multiple losses. The performance of the employed models, which depends on the average packet loss rate (PLR), is compared with the actual distortion, evaluated by decoding the received sequence with a properly designed, robust decoder. Numerical results confirm the accuracy of the proposed models, that are able to closely approximate the actual mean square error (MSE) behavior for a wide range of loss rates. Some applications of the proposed algorithms are also presented in the paper.

This paper analyzes the maximum number of simultaneous communications that can be sustained by an ad-hoc network in which the nodes are equipped with fully adaptive smart antennas. The study proposes an analytical model for the evaluation of the performance of ad hoc networks which includes multipath together with some characteristics of the access scheme. The main goal is the analysis of the relationship between the angular spread of the channel and the transmission policy ruled at MAC layer. Moreover, in the paper, it is examined the influence of the spatial channel model on network performance.

The paper addresses the problem of designing an adaptive system for the fading channel, adopting efficient coded modulation techniques. Coded modulation techniques adopting turbo codes and single parity check (SPC) codes are defined and compared. The sphere-packing bound is used as a tool for selecting the proper transmission technique for a given signal to noise range. The same bounding technique is used for comparing the performance of the technique under investigation with the limiting one. In the paper it is shown that a careful choice of encoding parameters allows the SPC based techniques to achieve a very good efficiency, which is very close to the turbo codes one, and is approximately 1 dB away from the theoretical limit.

In this paper, we propose and evaluate compatible puncturing schemes for turbo codes for Forward Error Correction / Automatic Repeat Request (FEC/ARQ) algorithms, in which the data transmission is supposed to go on progressively. The aim is 1) to obtain a better performance w.r.t. hybrid ARQ schemes already proposed in the literature when the user has to cope with some random packet erasures due, e.g., to deep fading conditions on wireless links, or to congestion on wired networks, i.e., when the number of lost data packets (erasures on an erasure channel) is hard to predict; 2) to obtain a performance comparable to one of the most efficient hybrid ARQ schemes proposed in the literature when all the packets are available to the final user, even if seriously corrupted, as on a pure wireless link. To obtain an unbiased comparison of the different schemes, an analytical method based on the sphere-packing bound is employed. To assess the accuracy of the bound, simulation results are also presented.

Partially systematic rate-compatible serial concatenated convolutional codes (SCCCs) are considered. These are generated from a classical rate 1/3 serial concatenated mother code. To obtain rate-compatible SCCCs, the puncturing is limited to inner coded bits, by puncturing both the inner code's parity and systematic bits. It is shown that the performance can be enhanced in the "waterfall" or in the "error-floor" region, by simply spreading the puncturing over the parity and the systematic bits without increasing encoding and decoding complexity.

A new design criterion to obtain well performing rate-compatible serial concatenated convolutional codes (SCCC) families is proposed. To obtain rate-compatible SCCCs, the puncturing is limited to inner coded bits. However, the puncturing is not restricted to inner parity bits, but extended also to inner systematic bits, thus obtaining high rate SCCCs (i.e., beyond the outer code rate). The considerations presented in S. Benedetto, et al., (1998) to find "best" component encoders for SCCCs construction are extended to find the "best" rate-compatible puncturing patterns for a given input decoding delay I. A rate-compatibility restriction to the puncturing rule is used, implying that all the code bits of a high-rate punctured code are used by the lower rate codes. The two main applications of this technique are its use in hybrid ARQ/FEC schemes and to achieve unequal error protection (UEP) of an information sequence.

In this letter, we propose and compare some design criteria for the search of good rate-compatible systematic turbo codes (RCPTC) families. The considerations presented by Benedetto et al. to find "best" component encoders for turbo-code construction are extended to find good rate-compatible puncturing patterns leading to codes with promising performances.

The authors propose an analytical method, based on the sphere-packing bound, which can be used to assess the achievable performance of transmission techniques, adopting efficient block codes, over nonideally interleaved fading channels. The proposed method, which is not subject to the union bound drawbacks, represents a suitable tool for gaining insights into the effects of fading impairments on system performance. The method is used to investigate the performance of codes on block and correlated fading channels, to assess the benefits of channel interleaver, to study the effects of transmission diversity, and to evaluate some hybrid automatic repeat request schemes.

We propose and compare some good rate-compatible serial concatenated convolutional code (SCCC) families. To obtain rate-compatible SCCCs, the puncturing is limited to inner coded bits. However, and this is the novelty proposed, we do not limit the puncturing to inner parity bits only, but we extend it also to inner systematic bits, thus obtaining higher rate SCCCs (i.e., beyond the outer code rate). The two main applications of this technique are its use in hybrid ARQ/FEC schemes and to achieve unequal error protection (UEP) of an information sequence.

The performance of turbo codes is evaluated through analysis and simulation over the Rice multiplicative fading channel. The analysis is conducted extending the results presented by A.M. Viterbi and A.J. Viterbi (see Proc. ISIT'98, p.29, 1998) to the Rice multiplicative fading channel case. Simulation results are obtained using an extended version of the improved Jakes' channel simulator, first presented by M.F. Pop and N.C. Beaulieu (see IEEE Trans. Commun., vol.49, p.699-708, 2001).

In this paper, the achievable performance of a turbo coded system adapted to a block fading channel model is evaluated. This model is suitable for analyzing, for instance, frequency-hopped multiple access (FHMA) systems that operate in a mobile satellite environment characterized by frequency-nonselective slow Rician fading, provided that the spacing between carriers is larger than the coherence bandwidth, resulting in basically uncorrelated blocks. In such systems, coded information is transmitted over a small number of fading channels in order to achieve diversity. The best coded information allocations over a certain number of fading channels are evaluated. Bounds on the achievable performance due to coding are derived using information-theoretic techniques. Moreover, simulation results are obtained and compared with the theoretical ones.

We propose and compare some design criteria for the search of good rate-compatible punctured turbo (RCPT) code families. The considerations presented by S. Benedetto et al. (see IEEE Trans. on Commun., vol.46, no.9, p.1101-5, 1998) to find "best" component encoders for turbo code construction are extended to find the "best" rate-compatible puncturing patterns for a given interleaver length k. This approach is shown to be the optimal one, both in the maximum-likelihood sense (using transfer function bounds) and in the iterative decoding sense (through simulation results). A rate-compatibility restriction to the puncturing rule is used, implying that all the code bits of a high-rate punctured code are used by the lower rate codes. The two main applications of this technique are its use in modified type II ARQ/FEC schemes and to achieve unequal error protection of an information sequence.

In this work, the performance of turbo codes over the Rice multiplicative fading channel is evaluated through analysis and simulation. The analysis is conducted through improved upper bounds on bit error probabilities in conjunction with both the fully interleaved channel and the correlated Rice slow fading channel. The analytical expression of the improved bound is obtained by extending the results of Viterbi and Viterbi (see Proc. IEEE International Symposium on Information Theory, p.29, 1998) to the Rice multiplicative fading channel case. Simulation results are obtained and presented also for intermediate fading rates, ranging between the two extreme multipath fading rates considered in the analysis. Simulations are performed using the improved Jakes' channel simulator to simulate Rayleigh fading channels, and extended to adequately model the multipath Rice fading propagation environment.

We define and adopt a theoretical method, based on the sphere-packing bound, for evaluating and comparing the achievable performance of different hybrid automatic repeat request schemes for the fading channel. The channel state evolution is assumed to be represented by a finite-state Markov chain. To assess the correctness and the accuracy of the analysis, some simulation results, based on the use of turbo codes, are also presented.

This paper investigates the performance of hybrid ARQ schemes for the mobile low Earth orbit (LEO) satellite channel. The proposed schemes adapt their characteristics to the time-varying behavior of the channel. To obtain an unbiased comparison of different ARQ schemes a theoretical viewpoint is adopted. More precisely, the sphere-packing bound is used to evaluate the best possible performance of a hybrid ARQ scheme. To assess the correctness and the accuracy of the analysis, simulation results, based on the use of turbo codes, are also presented.

We define and use a general method for evaluating and comparing the performance of different hybrid ARQ schemes for the fading channel, whose evolution is assumed to be represented by a finite state Markov chain (FSMC). To obtain an unbiased comparison of different ARQ schemes a theoretical viewpoint is adopted. More precisely, the sphere-packing bound is used to evaluate the best possible performance of a hybrid ARQ scheme. To assess the correctness and the accuracy of the analysis, some simulation results, based on the use of turbo codes, are also presented. The results highlight the relative merits and drawbacks of the investigated hybrid ARQ schemes, some of which are under evaluation for the Universal Mobile Telecommunications System (UMTS) standard.

This paper investigates the properties of a method for obtaining Markov models of unspecified order to be applied to narrow-band fading channels with additive white Gaussian noise. The models are obtained by applying the context tree pruning algorithm to experimental or simulated sequences. Fading environments are identified in which the extension from first-order to higher order models is justified. The paper presents, as examples, the evaluation of the covariance function and the packet error distribution.

This paper presents statistical analysis of data obtained by measuring narrowband path loss at DECT (digital enhanced cordless telecommunications) frequency (1.89 GHz) in an indoor environment. Specific goodness-of-fit tests are applied to the data. The tests assess whether the data generating source belongs to a known family of random variables. Results obtained support that local path-loss distribution can be represented as Weibull or Nakagami in most environments. The close resemblance between such distributions and Rice distribution (with Rayleigh as a special case) confirms that Rice/Rayleigh description of fading can be applied to indoor environments. That similarity leads to a general, simple, approximate expression for the cumulative distribution function of the signal-to-interference ratio. The expression can be used without limitations to the number and the parameters of the Rice interferers.

As a powerful coding technique, turbo codes are a prime candidate for improving the reliability of communication over wireless channels, where fading is the main impairment. However, to date, only limited attention has been given to the performance of turbo codes on fading channels, and the influence of multipath fading has been fairly accurately investigated as far as the Rayleigh fading distribution is concerned only. Therefore, it is the purpose of this work to conduct a study of turbo codes performance considering the more general Rice fading case: the study is performed in conjunction with both the fully-interleaved channel and the correlated Rice slow-fading channel.

The effect of the Rayleigh fading channel on data block transmission is investigated. In a packet communication system the correctness or incorrectness of the received blocks is usually strictly related to a comparison of the signal-to-noise ratio (SNR) with a set of thresholds. The possibility of fitting a Markovian model to the quantized fading process is investigated, assuming the states of the model are coincident with the quantization levels (symbols). The two-threshold case is considered and the suitability of a first-order Markov model or even a zero-order Markov model (that means independent and identically distributed symbols) in describing such a process is verified. For this purpose, the parameters of a second-order Markov model are estimated and the table of contingency analysis for Markov chains is applied. It is shown that, in the fast fading case, process symbols can be considered as independent and identically distributed, while in the slow fading case the first-order Markovian model only approximates the real behavior of the fading.

The bit-stream generated by most source coding algorithms, for voice, audio and images, usually consists of bits of different significance and error sensitivity. By exploiting this property, in the design of a wireless communication system, it is possible to obtain significant advantages. First, the network capacity can be increased, with a smoother quality degradation under heavy traffic or severe interference conditions. Secondly, the application of source sensitivity matched FEC (forward error correction) protection schemes, leads to a higher robustness of source bits to channel impairments. We evaluate the usefulness of prioritized, variable-rate speech encoding schemes, for handling congestion over packet switching systems. The system performance is shown to be sensitive to the fraction of information placed in each priority and to the number of priorities. The applicability of source-sensitivity matched error protection schemes, for variable-rate speech transmission on a radio channel, is also investigated.

Packet switching allows one to exploit the intrinsic variable-rate behaviour of real-time information, such as voice and video. Indeed, packet switching permits one to handle in a more flexible way the information bits and to obtain, therefore, a significant increment in system capacity. A relative measure of the capacity increment we obtain over circuit switched systems is the multiplexing gain. This paper presents preliminary results of the multiplexing gain we can achieve when we transmit multi-mode and variable-rate speech on a slotted and framed radio channel. For our evaluation we use an embedded, variable bit-rate speech coding scheme, namely the Mobile Audio Visual Terminal (MAVT) coder, candidate to the new standard MPEG-4. The peculiar characteristics of this coding scheme can be exploited for obtaining a significant multiplexing gain. Moreover, at high loads, a smooth and graceful degradation of speech quality can be obtained by exploiting the embedded property of the encoder. In fact, being that the information bits are classified by priority, we observe a graceful quality reduction during network congestion, if the transmission algorithm is designed to discard first the bits with the minimum impact on the output quality.

We determine the performances of a collision resolution multiple access technique for handling speech and data traffic on a radio network with capture. The proposed access technique decouples the speech access procedure from the data access procedure. It follows that speech access performances do not depend on actual data load. It is well known that capture improves the performances of random access algorithms. However, due to the decoupling and the limited delay requirements, capture could have effect on performances of speech access procedure that depend on the speech model. In the paper we present analytical results of performance evaluations for different capture models. Two speech models are taken into consideration.