The IEEE 802.11ad and IEEE 802.11ay standards define the medium access control (MAC) operation for millimeter-wave communications. During the beamforming training (BFT) period, the stations compete for the channel adopting a modified version of slotted aloha, where stations mandatorily transmit and can retry in a finite number of slots. This change makes the probability of a station accessing a given slot dependent on the previous ones. Additionally, stations halt the competition for BFT, if the number of consecutive collisions reaches a predefined collision threshold (CT) value. In this paper, we characterize the probability of a station interrupting the competition for BFT due to consecutive collisions. To address the characterization of the interruption for the BFT competition, we model the individual collision distribution and the distribution of a station reaching the CT over multiple BFT periods. The proposed model is validated through simulation and its performance is compared to other approximated IEEE 802.11ad models, confirming the accuracy of the proposed approach.

This paper explores the impact of different classification and feature engineering techniques in radar-based posture detection systems. The research focuses on an experimental setup involving a radar sensor used to classify human postures and the absence of human activity. The methodology includes the detection of the absence of human activity in the first stage, followed by feature engineering extraction through Variational Auto-Encoders (VAEs), and subsequent data reduction via either t-distributed Stochastic Neighbor Embedding (t-SNE) or Principal Component Analysis (PCA) if human activity is detected and posture classification is required. Five solutions differing in terms of methodology and features' dimensionality reduction strategies are presented and compared in terms of classification accuracy and computation time. The main findings reveal that all five solutions achieve classification accuracy exceeding 90%, with variations observed in computation times, showing the effectiveness of adopting VAEs to derive classification features from raw data. The research highlights a trade-off between data preservation and classification accuracy and/or computation time, demonstrating that feature extraction and data reduction techniques are a valuable enhancement to posture detection and classification.

This paper studies the efficiency of Radio-frequency fingerprinting (RFF) on IEEE 802.15.4 networks using properties extracted by machine learning techniques. As a test case, we used the measurements of 33 wireless sensor devices to create a dataset with 640 complex samples from the signals' synchronization header of the frames transmitted by all sensors in two moving scenarios. This paper evaluates using an autoencoder (AE) neural network (NN) for extracting the RFF features. It addresses the challenge of identifying the performance of different AE NN architectures using varying numbers of features obtained from the AE latent state (LS). We compare the relative performance of the AE NN with varying types of NN (combining dense, convolutional, and recurrent layers) with five different LS dimensions (between 4 and 128), as well as two classifiers: a multi-layer perceptron (MLP) and a random forest (RF). An optimization for the encoder's LS, which uses the weights of the AE and a classifier NNs on an additional NN training phase, is also proposed. We show that a larger LS size does not always lead to better classification accuracy and that the AE loss is a bad predictor for the classifier performance. The best F1-score achieved was 93%, measured using the MLP classifier and the optimized one-dimensional convolutional AE with an LS dimension of 8, or 86% using the same configuration without the optimization. The RF classifier is much faster to train and still achieves an 88% F1-score for the optimized Gated Recurrent Unit (GRU) AE with an LS dimension of 16.

The pressing need to support connectivity to an ever-increasing number of devices has motivated the development of new channel access schemes, which achieve throughput gains with low channel access latency. In this paper, we characterize the performance achieved by an uplink grant-free channel access scheme adopting a spreading-based NOMA scheme. Modeling the network as a stochastic geometry problem and assuming flat fading, we characterize the distribution of the received power from each interferer, the aggregate interference, and the signal-to-interference-plus-noise ratio (SINR) for a given transmitter when Zadoff-Chu (ZC) sequences are adopted to spread the data in orthogonal frequency division multiplexing (OFDM) symbols. The probabilities of false-alarm and detection are derived and compared with simulation results, validating the accuracy of the proposed model. Finally, we provide results of the distribution of the number of successfully decoded transmissions depending on the number of competing nodes, the detection threshold, the networks’ density, and ZC sequence lengths, showing essential insights that can be taken into consideration to design new medium access control (MAC) schemes capable of regulating the uplink channel access of multiple nodes adopting spreading-based NOMA schemes.

This paper investigates rate-splitting multiple access (RSMA) networks with multiusers assisted by aerial intelligent reflecting surfaces (AIRS). To improve the sum-rate of the system, the UAV’s trajectory and phase-shift vectors are optimized, in which the mobility scenarios with static and dynamic users are explored. In particular, long short-term memory (LSTM)-based frameworks for predicting the UAV’s trajectory and the phase-shift of the reflecting elements of AIRS are proposed. For more insight, a third model is created by combining information from the static and dynamic scenarios. Furthermore, to improve the transmit beamforming at the BS, an algorithm based on alternating optimization (AO) under the assumptions of imperfect successive interference cancelation (SIC) is presented. Training progress and testing results are provided to demonstrate the efficiency of the proposed models. In addition, numerical simulations are presented to verify the performance gains in terms of sum-rate. The simulation results show that the UAV performs better in trajectory prediction and phase-shift when different investigated scenarios are not combined.

In this paper, we develop a 3GPP-inspired hardware implementation for the out-of-band Integrated Access and Backhaul (IAB) network, which promises to be a solution to both coverage extension and capacity boosting in 5G and beyond networks. The emulated gNodeB transmits an FR1 5G downlink signal as standardized by the 3GPP, which has been tested by means of commercial equipment. By employing Ettus x310 SDR board, Pasternack’s 60 GHz Transmitter (Tx) waveguide module, and MatlabTMsoftware, we design and develop an easy-to-use out-of-band mmWave Layer 2 (D&F) protocol. The proposed protocol decodes an FR1 5G signal as input at 3.5 GHz, which is retransmitted to the UE as an FR2 5G signal. In the Layer 2 protocol, the least square (LS) estimator implemented considers the channel state information reference signal (CSI-RS). We review and evaluate the key performance indicators (KPIs) of the proposed Layer 2 protocol in real non-line-of-sight (NLOS) environments and a comparison between the gNode-to-UE link has been carried out. Our results indicate that the performance of the proposed Layer 2 protocol is similar to the obtained with the Anritsu equipment demonstrating the right functionality of the developed algorithms. Experimental results evidence the superiority of the proposed D&F protocol over the gNodeB-to-UE link (direct link communication).

This work characterizes the influence of different spatial sampling resolutions on the prediction of vehicular mobility. By assuming different spatial quantization areas over a region the vehicles move through, we characterize the distribution of trajectories for a fixed number of samples using the data available in a dataset of GPS positions sampled in an urban scenario. Both spatial resolution and the number of GPS samples per trajectory are analyzed, concluding that similar distributions of the trajectories can be obtained when the unitary dimension of the spatial area is approximately 0.05 km2and the trajectories last approximately 5 minutes. This is of particular importance to adequate the spatio-temporal sampling variables to the dynamics of the vehicular motion, thus avoiding over-sampling or sub-sampling in the spatial and temporal domains. The paper also proposes a deep-learning approach based on recurrent neural networks to predict future positions of a vehicular trajectory, showing the influence of spatial sampling to predict single and multiple future positions of the trajectory. The accuracy and the computation time of the prediction process are evaluated, showing how the magnitude of the prediction error is influenced by the adopted spatial sampling resolutions.

Context awareness using Radio Frequency (RF) sensing systems has attracted increased attention due to its importance in future communication systems. In this work, we investigate the importance of multiple features in the classification of different RF human mobility scenarios. Considering a practical RF system operating in the 76-81 GHz band, we focus on different types of features computed from the RF time of flight. The importance of the features is then evaluated through its score computed with four methodologies: the Mutual Information (MI), the Analysis of Variance (ANOVA), the Recursive Feature Elimination (RFE), and the Feature Weighting algorithm (RelieF). The results indicate that the methodologies do not rank the multiple features coherently and some categories of features have lower importance. Moreover, when adopting a k-nearest Neighbor classifier the RFE methodology is shown to be advantageous as it achieves the highest accuracy with the smallest number of features.

In this paper, we derive the outage probability in directional millimeter-wave communications, capturing the effect of beamforming. In the proposed scenario, multiple transmitters are spatially distributed according to a spatial Poisson Point Process, which can cause interference to a pair of transmitter/receiver communicating nodes. The analysis considers a general distance-based path loss with Rayleigh and Rician fading channels and a sectored antenna model. Assuming that nodes are uniformly distributed over a circular or annular area centered at the receiver, we derive the outage probability due to the aggregate interference caused by multiple transmitters. The outage probability is based on the Signal-to-interference-plus-noise ratio (SINR) derived for the transmitter/receiver pair. Several simulations confirm the effectiveness of the derived results for different wireless channels and beamwidth values, highlighting the effect of directional communications on the outage probability.

In this paper, we characterize the aggregate interference power in directional millimeter-wave communications, capturing the effect of beamforming. The analysis considers a general distance-based path loss with Rayleigh and Rician fading channels and a sectored antenna model. Moreover, the nodes are uniformly distributed over a circular or annular area centered at the receiver. The main contribution of the paper is the derivation of the moment generating function (MGF) of the aggregate interference power. The MGF is adopted in a moment-based approximation to a Gamma distribution and used as a model of the aggregate interference power. Several simulations confirm the effectiveness of the proposed approximation for different scenarios, highlighting the effect of directional communications on the aggregate interference power.

This work presents an effective tool to predict the future trajectories of vehicles when its current and previous locations are known. We propose a Long Short-Term Memory (LSTM) Recurrent Neural Network (RNN) prediction scheme due to its adequacy to learn from sequential data. To fully learn the vehicles’ mobility patterns, during the training process we use a dataset that contains real traces of 442 taxis running in the city of Porto, Portugal, during a full year. From experimental results, we observe that the prediction process is improved when more information about prior vehicle mobility is available. Moreover, the computation time is evaluated for a distinct number of prior locations considered in the prediction process. The results exhibit a prediction performance higher than 89%, showing the effectiveness of the proposed LSTM network.

This paper derives a model that computes the optimal carrier sensing range adopted by transmitters of a Carrier Sense Multiple Access (CSMA) network when multiple wireless networks coexist in the same band. To investigate the optimal carrier sensing range, we define a utility function that considers the impact of the carrier sensing threshold in the medium access probability and the transport capacity of a CSMA transmitter. Consequently, the definition of the carrier sensing range is a tradeoff between the avoidance of low channel access probability due to low carrier sensing thresholds and the avoidance of a high number of concurrent transmissions (collisions) per unit area due to high carrier sensing thresholds. Considering the aggregate power sensed by a CSMA node, we derive a method to compute the optimal carrier sensing range. The proposed solution is adopted in two different scenarios, considering the cases when the coexisting networks are spatially overlapped or non-overlapped. Extensive simulation results indicate that the optimal carrier sensing range is accurately approximated for the two scenarios, and higher optimal carrier sensing ranges are obtained for the overlapped scenario. The proposed method is a useful tool to find the optimal carrier sensing range when the spatial distribution of the coexisting networks is known.

Assuming a cooperative non-orthogonal multiple access (NOMA) system with rate fairness in a scenario with multiple users and arbitrary relays, this paper investigates adaptive power allocation (PA), relay selection (RS), and user pairing (UP) policies. Specifically, two adaptive PA optimization problems are formulated, one at the base station (BS) and another at the selected relays. Closed-form expressions for the power allocation factors are derived as well as an algorithm that provides the optimal solution at the BS. In order to show the superiority of the proposed study, our results are compared with other benchmark schemes in terms of outage probability, Jain's fairness index, and average sum rate.

The detection of abnormal sequences of SIP messages in real-time is crucial to avoid SIP signaling-based attacks. In this paper, we propose a deep learning approach to detect signaling patterns of multimedia sessions established with the Session Initiation Protocol (SIP). The approach is based on a recurrent neural network (RNN). We study the performance of different Long Short-term Memory (LSTM) RNN architectures, which are trained using a SIP signaling dataset of trustworthy SIP dialogs captured by a SIP server. The trained RNNs are then used to detect the SIP dialogs in real-time. After characterizing the dataset adopted for the training, validation, and testing, we present the experimental results obtained for the different RNN architectures, showing that the classification probability of trustworthy SIP dialogs exceeds 93% in the test stage. Finally, we present two methodologies to detect abnormal SIP dialogs, i.e., not contained in the trustworthy training dataset. After a detailed analysis of the skewness and kurtosis computed with the numerical RNN outputs, we show that they can be used as classification features. The first method is based on a K-means unsupervised classifier, while the second one is based on a semi-supervised threshold-based classifier. Experimental results show that the threshold-based classifier achieves 99.45% of detection probability, showing the effective utility of the proposed methodology to detect abnormal SIP sequences in a short period of time.

Residual self-interference (SI) is primarily a key challenge when designing In-Band Full-duplex (IBFDX) wireless systems. Channel estimation errors are one of the major causes of the residual SI. The SI channel is composed by multiple fading taps which makes the characterization of the residual SI more challenging as multiple copies of the transmitted signal, with variable delays and gains, are eventually aggregated at the receiver. In this paper, we derive an approximation for the distribution of the residual SI power in multi-tap delay fading channels. In particular, we show that under specific conditions the multi-tap fading channel can be represented by a summation of non-identical independent gamma distributions. In a further step, we approximate the summation of the gamma distributions using the Welch-Satterthwaite equation, obtaining a closed form expression for the distribution of the residual SI power. The accuracy of the theoretical approach is evaluated through simulation results. The similarity comparison between simulated data and the proposed model indicates a high accuracy of the adopted approximation when considering low fading uncertainty associated to the taps and low estimation errors. On the other hand, the accuracy of the approximation slightly decreases for higher uncertainty fading scenarios and for higher estimation errors. However, as a final remark, we highlight that the results computed with the model are close to the simulated ones and for most of the applications the model's error can be negligible.

In this paper, we present a novel method to estimate the distribution of the residual self-interference (SI) power of an in-band Full-duplex (IBFDX) system. Considering the residual SI distribution caused by the uncanceled SI in a post-mixer scheme due to a single propagation tap, we propose a framework capable of estimating the residual SI distribution. The estimator relies on the computation of two estimated parameters, which takes advantage of using the method of moments of the residual SI distribution in a straightforward manner. Additionally, we assess the accuracy of the proposed estimators, considering different sample set lengths adopted in the estimation process. The results testify the quality of the estimation method and its ability to achieve high accuracy, even for a reduced number of samples. This work is of practical importance, particularly when the residual self-interference is mainly due to a line-of-sight component, where the proposed technique can use a small set of samples to predict the residual interference in an accurate way.

In this paper we propose a modulation classification scheme based on deep learning and convolutional neural networks. The proposed solution is based on I/Q amplitude samples that are classified into a set of 24 modulations, including analog and digital modulations. We investigate the adoption of different features in the training data. The features are computed using I/Q amplitude samples and include statistical information obtained in the time-domain and frequency-domain. Given the high amount of features considered in the training data, we investigate the possibility of projecting the features' data into a set of uncorrelated variables through the Principal Component Analysis procedure. We evaluate the classification performance for the different modulations and signal-to-noise ratio (SNR) values. Moreover, we also quantity the performance of the classifier when the features' data is projected into a subset of the uncorrelated variables. Finally, we compare the performance of the classifier with other works already available in the literature, assessing the effective performance gains of the proposed solution.

In this paper we propose a method to estimate the most likely region a vehicle will transverse when the current position and a priori spatial-temporal trajectory data of multiple vehicles are known. The proposed solution is based on a hidden Markov chain that models the trajectory of each vehicle. The estimation of the vehicle trajectory relies on Viterbi algorithm, which identifies the most likely trajectory as a new vehicle's location is known. The proposed modeling and estimation methodology is evaluated using real mobility traces sampled from multiple taxis traveling in the city of Porto in Portugal. The estimation performance of the most likely region a vehicle will cross is between 32% and 89%, depending on several factors that include the probability of trajectory's occurrence and the number of previous locations considered in the estimation methodology (Markov order). Finally, we discuss the advantages and limitations of the proposed method.

Recently, low-power wide-area networks (LPWANs) have attracted great interest due to the need of connecting more and more devices to the so-called Internet of Things (IoT). LoRa networks are LPWANs that allow a long-range radio connection of multiple devices operating in nonlicensed bands. In this article, we characterize the performance of LoRa's uplink communications where both physical layer (PHY) and medium access control (MAC) are taken into account. Motivated by recent works that consider the possibility of decoding multiple frames at the same time, we characterize the performance of the PHY layer through the probability of decoding multiple frames that were transmitted with the same spreading factor. The MAC performance is evaluated by considering that the interarrival time of the frames generated by each LoRa device is exponentially distributed. A LoRaWAN operating scenario is considered, where the transmissions of LoRa Class A devices are influenced by path loss, shadowing, and Rayleigh fading. The numerical results obtained with the modeling methodology are compared with simulation results, and the validation of the proposed model is discussed for different traffic load levels, different PHY-layer conditions, and different capture thresholds. The contribution of this article is primarily focused on studying the average number of decoded LoRa frames for the different capture conditions, being a general model when compared to the works published so far. Moreover, given the different research initiatives to develop innovative multicapture LoRa schemes, we believe that the proposed model is particularly useful to foresee LoRa's PHY/MAC performance in such innovative scenarios.

In this paper, the problem of coexistence interference caused by unlicensed networks is addressed. An analytical framework is developed to characterize the aggregate interference caused to a fixed licensed user when the coexisting interferers belong to different unlicensed networks. The distribution of the interference is analyzed considering the stochastic nature of the path loss, as well as fast fading and shadowing effects. The unlicensed users are deployed according to a homogeneous Poisson Process, and it is shown that the distribution of aggregate interference can be approximated by an α-μ distribution. Our results show the impact of node densities, transmitted power levels, and access channel probabilities on the distribution of the aggregate interference. Numerical and simulated results are compared to assess the accuracy of the proposed analysis.

Recently, Low Power Wide Area Networks (LPWANs) have attracted a great interest due to the need of connecting more and more devices to the so-called Internet of Things (IoT). LoRa networks are LPWANs that allow a long-range radio connection of multiple devices operating in non-licensed bands. In this work, we characterize the performance of LoRa's Uplink communications where both physical layer (PHY) and medium access control (MAC) are taken into account. By admitting a uniform spatial distribution of the devices, we characterize the performance of the PHY-layer through the probability of successful decoding multiple frames that were transmitted with the same spreading factor and at the same time. The MAC performance is evaluated by admitting that the inter-arrival time of the frames generated by each LoRa device is exponentially distributed. A typical LoRaWAN operating scenario is considered, where the transmissions of LoRa Class A devices are affected by path-loss, shadowing and Rayleigh fading. Numerical results obtained with the modeling methodology are compared with simulation results, and the validation of the proposed model is discussed for different levels of traffic load and PHY-layer conditions. Due to the possibility of capturing multiple frames simultaneously, we consider the maximum achievable performance of the PHY/MAC LoRa scheme according to the Signal-to-interference-plus-noise ratio (SINR). The contribution of this work is primarily focused on studying the average number of successfully received LoRa frames, which establishes a performance upper bound due to the optimal capture condition considered in the PHY-layer.

Full-duplex wireless communication systems are capable of transmitting and receiving signals in the same band. An important performance indicator of full-duplex systems is the amount of uncanceled self-interference, a.k.a. residual self-interference. This work investigates the distribution of the residual self-interference (SI) power in an analog post-mixer canceler. First, we focus on the impact of the SI channel coherence time on the distribution of the residual SI power and we analyze in detail the impact of the phase-noise caused by the up/down-conversion oscillators. The results achieved in this study confirm that the coherence time of the SI channel strongly impacts the distribution of the residual SI power. Considering a SI Rayleigh channel, we show that the residual SI power only follows a Gamma distribution in the limit case, when the coherence time tends to infinity (time-invariant channel). This work is the first step to understand the stochastic properties of the residual SI power, which is an important feature when digital-domain cancellation is jointly adopted with analog-domain cancellation to account for the estimated effects of the propagation channel.

Decentralized PHY/MAC techniques for the uplink of Multi-Packet Reception (MPR) schemes are of great interest in future wireless communication systems (e.g., IEEE 802.11ax). In MPR schemes the average number of successful received packets depends on the radio propagation effects, the number of nodes simultaneously transmitting and the capture threshold that characterizes the PHY-layer MPR technology. In this paper we adopt a MPR capture model capable of describing the average number of successful packets simultaneously received. The capture model is then used in an optimal reservation-based medium access control (MAC), designed to operate in a decentralized way through a random access policy. This work describes the PHY/MAC cross-layer design, as well as the rationale required to understand its optimality. While the main objective of the proposed MAC is to achieve the optimal number of simultaneous transmissions that maximize the PHY-layer performance, a cost due to the MAC reservation period must also be considered in the optimization process. A model of the PHY/MAC performance is derived, which allows the optimization of the parameters that regulate the nodes' random access. Different simulation results are described and compared with numerical results obtained with the proposed PHY/MAC model. The paper ends up discussing possible solutions to implement the proposed methodology in practical wireless networks formed by a high number of competing nodes.

In-band Wireless Full Duplex communication systems are capable of transmimng and receiving in the same band by canceling the self-interference (SI) power originated during the transmission. However, several issues may degrade its performance, including the nature of wireless SI channel. In this work we study the distribution of the residual SI power of an in-band full-duplex analog post-mixer canceler, which represents the amount of uncanceled SI. The impact of the fading channel is evaluated on the distribution of the residual SI power. The results achieved confirm that the type of fading channel strongly impacts the distribution of the residual SI power. This is a first step to understand the stochastic properties of the residual SI power, which is an important feature when digital-domain cancellation is jointly adopted with analog-domain cancellation to account with the effects of the propagation channel.

In this work, target localization problem in adverse indoor environments is addressed, where most (if not all) links are non-line-of-sight (NLOS). Localization accuracy in such environments is highly affected by multipath, which makes the problem very challenging. Hence, in order to enhance the localization accuracy, received signal strength (RSS) and time of arrival (TOA) integrated measurements, are considered here. Nevertheless, the derived joint maximum likelihood (ML) problem is highly non-convex and has no closed-form solution; thus, some approximations are required to solve it. We show that, for small noise power, the ML estimator can be tightly approximated by another (non-convex in general) one, given in a form of a generalized trust region sub-problem (GTRS). Hence, exact solution of the derived estimator can be readily obtained by merely a bisection procedure. The proposed algorithm is compared with the state-of-the-art (SOA) RSS/TOA algorithms, as well as its RSS-only and TOA-only complements. Our simulations validate the effectiveness of the proposed approach, outperforming the SOA algorithms in all considered scenarios, and show the benefit of the measurement fusion.

This paper deals with the cross-layer design of a decentralized medium access control (MAC) scheme to coordinate the access of multiple transmitters adopting a Multi-Packet Reception (MPR) physical (PHY) layer. Knowing that in MPR systems the average number of packets successfully decoded depends on the number of simultaneous transmissions, we propose an integrated PHY/MAC cross-layer design that explores the advantages of MPR communications in a decentralized network. The joint performance of the PHY/MAC cross-layer scheme is characterized to allow the maximization of the average number of packets successfully decoded. While the main objective of the proposed MAC is to randomly grant access to the channel in order to achieve the optimal number of simultaneous transmissions, the decentralized MAC introduces a cost due to the time required to coordinate the multiple transmitters. In a first step we characterize the throughput achieved by the cross-layer scheme, by modeling the performance of the PHY-layer and the random MAC scheme. The formal characterization of the throughput is then used to optimize the duration of the first stage of the MAC scheme. In this way, the number of transmitters is regulated to optimize the cross-layer operation, taking into consideration the features of the MPR PHY-layer and the maximum performance achieved with the proposed MAC design. Several results obtained through simulation are presented, which validate the numerical results obtained with the proposed characterization. Finally, the results confirm the advantages of the proposed PHY/MAC design.

Vehicles need to locate other vehicles and network infrastructure elements on unmanned autonomous vehicle (UAV) systems. Human passengers also need to locate and be located by the vehicles, preferentially using a portable device, such as a smartphone. This paper analyses the accuracy of several localization algorithms in the remote location of entities running WiFi access points, using measurements collected in moving vehicles using a new application developed by us. The algorithms analysed include closed form estimators and one based on second order cone programming (SOCP) relaxation, which exhibits the best accuracy and is capable of estimating the path loss exponent and the transmission power. Although, due its lower complexity, the Levenberg-Marquardt algorithm was better suited for the stand-alone Android prototype application. The results show that real-time accurate positioning of static/slow moving remote entities is possible, even though the accuracy degrades when the measuring vehicle's speed increases.

In this paper, we characterize the wireless interference of a mobile ad hoc network, where the nodes move according to the random waypoint model. The interferers are assumed to be located within an interference region that is defined as a circular region centered in a fixed node located at a given point of the mobility scenario. The main contribution of this paper is the characterization of the aggregate interference caused to the fixed node by mobile interferers located within the interference region. The distribution of the interference is analyzed taking into account the stochastic nature of the path loss due to the mobility of the nodes, as well as fast fading and shadowing effects. The derivation of the characteristic function of the aggregate interference is used in two different estimators, which successfully characterize the interference using only a small set of samples. The theoretical approach is validated through simulations, which confirm its effectiveness. Finally, we assess the accuracy of the proposed estimators, demonstrating the practical value of this paper.

In this paper we characterize the radio frequency (RF) spectrum opportunities available in a common GSM (Global System for Mobile Communications) channel to support the operation of a cognitive radio network (CRN). In a first step, we describe the technical details involved to sample the channel using a Software Defined Radio (SDR) device. Adopting a simple Energy-based detector, we identify the two energy regions where the GSM system is active or inactive. Based on the output of the detector, we show that the distribution of the durations of busy and idle periods are approximated by geometric distributions. Finally, we validate a theoretical model for the distribution of the service time. The validation results indicate that the service time can be successfully represented by a discrete Generalized Pareto (dGP) distribution, which is confirmed by the Kolmogorov-Smirnov test. Because the throughput of the CRN is represented by the inverse of the service time, the proposed analysis provides an upper bound for the networks' throughput, indicating the maximum throughput that can be attained when a single SU transmits over a GSM cellular channel. The results presented in the paper are validated with real data, confirming the accuracy of the proposed service time model.

It is well-known that Massive MIMO systems (Multiple-input multiple-output) have high potential for future wireless broadband systems. Massive MIMO (m-MIMO) relies on spatial multiplexing, and for that reason the base station needs a precise channel knowledge at uplink and downlink. Pilots can be used for channel state information (CSI) estimation, but common estimation processes imply a matrix inversion which can be a heavy computational process for m-MIMO system where the number of antennas used in the communication is very high. To alleviate computational requirements, reduce latency and to improve battery life capacity of mobile devices matrix inversion operations should be avoided. Having in mind these constrains, a new channel estimation method based on Zadoff-Chu (ZC) sequences is presented here, that achieves similar or better performance than least squares (LS) or minimum mean-Square Error (MMSE) channel estimators. It is also presented a set of performance results that sustain our assumption.

In this paper we propose two decentralized medium access control (MAC) schemes to coordinate the access of multiple transmitters adopting a Multi-Packet Reception (MPR) physical (PHY) layer. The proposed MAC schemes operate over a MPR-based PHY-layer and are divided in two stages. In the first stage the nodes indicate their willingness to transmit, while in the second one they jointly transmit. The MAC schemes differ by considering a Single-Packet Reception (SPR) or a MPR PHY-layer in the first stage. We analyze the throughput achieved by each MAC in a theoretical way. Several results obtained through simulation are also presented, which validate the numerical results obtained with the theoretical model. The results demonstrate the effectiveness of the proposed MAC schemes, and allow us to draw valuable conclusions about the design of distributed MPR systems.

In recent schemes proposed to the physical layer of wireless systems the receiver node has the capability of simultaneously receiving multiple transmissions, which is commonly referred to as a Multi-Packet Reception (MPR) scheme. Assuming that the transmitters are randomly located in the far-field region and a generic signal-to-interference-plus-noise ratio (SINR) threshold-based criterion is used as the packet capture model, we derive the probability of successful reception of a packet considering path loss, shadowing and fading effects. Our approach relies on the characterization of the aggregate interference caused by the transmitters, which is used in the capture model in a simplified way. The probability of successful reception of a packet is then used to approximate the average number of packets simultaneously received. The proposed analysis is evaluated for different scenarios and compared with several results obtained through simulation. The simplicity of the model, as well as its accuracy, makes it a useful tool to assist the design of future medium access control mechanisms for MPR wireless systems.

In this paper we propose a decentralized medium access control (MAC) scheme to coordinate the access of multiple transmitters adopting a multi-packet reception (MPR) physical (PHY) layer. MPR receivers are able to decode multiple transmissions, and depending on the technique adopted at the PHY-layer, there is an optimal number of simultaneous transmissions that maximize the number of packets successfully decoded in a simultaneous way. The proposed MAC scheme admits an MPR PHY-layer and is divided in two stages. In the first one the nodes indicate their willingness to transmit, while in the second one they jointly transmit. Adopting a generic model for the PHY-layer, we characterize the throughput achieved by the proposed MAC when both MAC and PHY layers are considered. The formal characterization of the throughput is then used to optimize the duration of the first stage of the MAC scheme in order to optimize the throughput. In this way, the number of transmitters is regulated to optimize the cross-layer operation, taking into consideration the features of the MPR PHY-layer and the maximum performance achieved with the proposed MAC design. Several results obtained through simulation are presented, which validate the numerical results obtained with the proposed characterization. The proposed scheme is a first step to develop MAC protocols specifically designed to distributed MPR systems that operate in a cross-layer and improve the exploitation of current and future MPR techniques proposed for the PHY-layer.

FTN (Faster Than Nyquist) signaling allows capacity gains but requires substantially complex equalization schemes, even for an ideal AWGN (Additive White Gaussian Noise) channel. Extending conventional FTN receivers for severely time- dispersive channels is not an option, since the receiver complexity becomes prohibitively high. In this paper we address the design of a receiver for FTN signaling over severely time-dispersive channels. To cope with the severe ISI (Inter-Symbol Interference) associated to the combined effects of FTN signaling and the time-dispersive channel we consider an iterative FDE scheme (Frequency-Domain Equalization) combined with a HARQ (Hybrid Automatic Repeat reQuest) especially designed taking into account the characteristics of FTN signals. We also present a simple, yet accurate model for the performance evaluation, considering multiple retransmissions per block. Our performance results show that we can have significant throughput gains, while maintaining essentially the receiver complexity of conventional, Nyquist-rate signaling schemes, even for severely time-dispersive channels.

This paper addresses a Cognitive Radio Mobile Ad Hoc Network (CRMAHN) where the licensed users (Primary users - PUs) are mobile nodes. In this scenario, the non-licensed users (Secondary Users - SUs) may opportunistically use the PUs' band when it is available. Because the availability of the band is determined by the spectrum sensing adopted by each SU, this work characterizes the transmission capability of the SUs (Sensing Capacity) through the characterization of the cases when the SUs sense the spectrum as being vacant. Admitting that PUs are mobile, the SUs must declare the band as being used when the PUs within their sensing region use the band. Contrarily to other works, our main contribution is the assumption that PUs may also be detected active when they are located outside the SUs' sensing region, which may frequently occur in CRMAHNs. This effect, known as Spatial False Alarm (SFA), is taken into account through the probability of detection of the PUs located outside the SU's sensing region, which is derived in this work. Finally, it is shown that SFA degrades the sensing capacity of CRMAHNs, when the number of mobile PUs increases and/or the level of activity of the PUs also increases.

Prefix-assisted direct-sequence code-division multiple access (DS-CDMA) is a viable transmission technique for high-data-rate wireless broadband systems, capable of coping with highly dispersive channels at the uplink. Packet combining (PC) ARQ with an iterative block decision feedback equalization (IB-DFE) technique can be used to cope with errors. However, extracting the IB-DFE's packet error rate (PER) for MAC-layer simulations can be time consuming. Most works that model the DS-CDMA behavior with PC ARQ use simplified asymptotic PER models in the study of the system performance, which are not precise for low signal-to-noise ratios and for a low number of mobile terminals (MTs). In this paper, the MAC performance is modeled using a discrete-time Markov chain that relies on a PER model. As an example, this paper describes a PER analytical model for an uncoded prefix-assisted DS-CDMA system with PC ARQ that considers the following inputs: the number of MTs accessing the channel, the MTs' number of transmissions, and the channel realizations for a given bit energy over noise ratio. The results show that the model portrays with better accuracy the simulations' results, compared with an alternative asymptotic model.

Radio Spectrum sensing has been a topic of strong research in the last years due to its importance to Cognitive Radio (CR) systems. However, in Cognitive Radio Networks (CRNs) with multiple Primary Users (PUs), the Secondary Users (SUs) can often detect PUs that are located outside the sensing range, due to the level of the aggregated interference caused by that PUs. This effect, known as Spatial False Alarm (SFA), degrades the performance of CRNs, because it decreases the SUs' medium access probability. Adopting Energy-based spectrum sensing (EBS) in each SU, this work starts to characterize the interference caused by multiple PUs located outside the sensing area. The interference formulation is then used to derive the probabilities of detection (PD) and false alarm (PFA), and closed form expressions are presented. Several results show that the probabilities PDand PFAare successfully validated through simulation. Finally, the work shows that the SFA can be almost neglected, depending on the path loss factor and on the number of samples collected by the energy detector to decide the spectrum's occupancy state.1

Most studies on performance of IEEE 802.11 distributed coordination function (DCF) have proved that the binary exponential backoff (BEB) algorithm suffers from low throughput, long transmission delays and low transmission reliability, for a high traffic load. In this paper, we propose a two-phase contention window (TPCW) access mechanism that enhances the aggregate throughput and transmission reliability while decreasing the medium access delay. In addition, this scheme aims at being computationally simple. This work characterizes the performance of the TPCW mechanism for different parameterization values. An analytical model is proposed for the TPCW access mechanism, which characterizes the frame transmission reliability, the total transmission delay and the aggregate throughput. The validity of our analytical model is verified through extensive simulations. By comparing the performance results, we conclude that the proposed scheme can significantly enhance IEEE 802.11 in terms of frame transmission reliability, total transmission delay and aggregate throughput.

Random access approaches in Low Earth Orbit (LEO) satellite networks are usually incompatible with the Quality of Service (QoS) requirements for multimedia traffic, especially when hand-held terminals must operate with a very low signal-to-noise ratio. This paper proposes the Satellite Random Network Diversity Multiple Access (SR-NDMA) protocol that handles multimedia traffic under this context through the combination of a random and scheduled access scheme. The protocol uses a multi-packet receiver, for Single Carrier with Frequency Domain Equalization (SC-FDE) in the uplink, that gradually reduces the packet error rate with additional transmissions. The paper proposes analytical performance models for the throughput, delay and energy efficiency - as long as terminals have finite queues. System parameters are defined to enhance the energy efficiency while satisfying the QoS requirements for limited queue and bit-rate constraints. Results show that the proposed system is energy efficient and provides enough QoS to support multimedia services such as video telephony.

Traditionally, Low Earth Orbit (LEO) satellite networks use hard handovers when a Mobile Terminal (MT) moves between the footprints of two satellites. Such approach introduces a transitory disconnection interval, which may limit the capacity to provide demanding real-time requirements. A soft- handover approach that applies distributed packet combining on a satellite diversity scenario is proposed in this paper. Considering inexpensive MT hardware and low transmission powers, the paper analyses the performance gains and the difficulties that are associated to different Doppler and time-of-arrival shifts when the MT signal is broadcast to two satellites during the short handover period where both satellite's footprint overlap. A Single-Carrier with Frequency Domain Equalization (SC-FDE) transmission technique is considered in this paper. It is shown that it is possible to have an inter-satellite handover almost without quality of service (QoS) degradation for a Satellite Network Diversity Multiple Access (S-NDMA) scheme.

Traditionally, an unsuccessful packet reception, either due to channel conditions or interference from multiple sources (collisions), is discarded and followed by a new reception of the same packet. Network diversity multiple access (NDMA) handles collisions by using time diversity multipacket reception: the base station forces terminals to transmit P copies of each packet when P terminals collide. This rigidity of having P copies is unsuitable for poor propagation conditions. It might be desirable to have more than P copies of a given packet, adopting a technique for single packet reception called hybrid-ARQ. Hybrid-ARQ effectively decreases the packet error rate. This paper proposes such mechanism for multi-packet reception, called hybrid-ARQ NDMA (H-NDMA), a gated slotted random access protocol, where the access mechanism forces the terminals involved in a collision with reception errors to transmit more than P times. Analytical models for the throughput, delay and energy per useful packet are proposed. The energy performance is analyzed for bounded average delay constraints. The proposed system's performance is evaluated for a single-carrier with frequency domain equalization (SC-FDE) scheme, and compared with the classical NDMA protocol. The comparison highlights that H- NDMA is scalable for an increasing number of terminals and network load.

This paper presents a Medium Access Control (MAC) protocol solution designed to properly handle collisions when in the presence of a multi-packet detection receiver for Single-Carrier (SC) modulations with Frequency-Domain Equalization (FDE). It is considered an iterative frequency-domain receiver that jointly performs equalization, multi-packet separation and channel decoding operations, for up to Qmaxmobile terminals transmitting in one slot. In this work, it is proposed and evaluated a p-persistent Network Diversity Multiple Access (NDMA) random MAC protocol designed to cope with a total number of mobile terminals J, for a maximum decoding capability of Qmaxsimultaneous packets. An accurate analytical model is presented to optimize two different scenarios: in the first one, a saturated network is considered and it is determined the packet transmission probability that maximizes the uplink throughput; the second represents a non-saturated network and the goal is to compute the optimal transmission probability associated to each mobile terminal that minimizes the packet transmission delay. In the end, analytical results obtained through physical and MAC layer simulations are discussed.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Hybrid Automatic Retransmission reQuest (H-ARQ) and time diversity multipacket reception approaches, such as Network Diversity Multiple Access(NDMA), improve the system performance by requesting additional retransmissions and combining all the signals received together. However, the high round trip delay time associated to satellite networks introduces limitations in the number of retransmission requests that may be issued by the terminals to fulfill the Quality of Service (QoS) requirements. This paper considers the design of hybrid protocols combining H-ARQ and NDMA for satellite networks with demand-assigned traffic. The satellite NDMA (S-NDMA) protocol is presented and analytical models are proposed for its performance. Energy efficient QoS provisioning is also analyzed. The proposed system's performance is evaluated for a Low Earth Orbit (LEO) network with a Single-Carrier with Frequency Domain Equalization (SCFDE) scheme, and compared to H-NDMA. Results show that the proposed system is energy efficient and can provide enough QoS to support high demand services such as video telephony.

Multipacket Reception (MPR) techniques employing either Direct Sequence Code Division Multiple Access (DS- CDMA) or Network Diversity Multiple Access (NDMA) have great potential to deal with packet collisions from multiple Mobile Terminals (MTs). However, their performance evaluation in prac- tical systems is difficult because lengthy simulations are needed to obtain the packet error rates for specific channel conditions. This paper considers MPR receivers based on the Iterative Block Decision Feedback Equalization (IB-DFE) concept. An analytical tool that computes the Bit Error Rate (BER) and Packet Error Rate (PER) for DS-CDMA is proposed, where its theoretical results have a good accuracy. This analytical approach, that provides a simple and relatively fast way to compute the BER and PER performance for DS-CDMA, is then employed for a comparison with NDMA.

This paper presents the URBAN XOR1protocol, an XOR-based flat routing mechanism developed for vehicular ad hoc networks (VANETs) formed in urban scenarios. The paper firstly describes the URBAN_XOR routing principle, which requires reduced knowledge about the set of nodes present in the VANET in order to provide traffic forwarding. Basically, the URBAN XOR protocol introduces the concept of local visibility, prioritizing the insertion of closer neighbors (in number of hops) in the routing tables, and simplifying the management of the frequent network mobility found in VANETs. Then, the performance of the proposed URBAN_XOR protocol is compared through simulation with other topology-based and position-based protocols, characterizing its performance in terms of path availability ratio, end-to-end delay, path length and path duration. The results reveal that URBAN_XOR contributes for the overall network stability, reducing the end-to-end delay due to its ability of generating shorter paths. At the same time, URBAN_XOR exhibits path availability similar to other topology-based protocols, but exhibiting better path duration times.

Diversity Combining (DC) and Multipacket Detection (MPD) are efficient techniques that cope with packet errors due to severe propagation conditions and/or collisions. To evaluate their network performance on a network simulator, an accurate characterization of the Bit Error Rate (BER) and Packet Error Rate (PER) can successfully replace the behavior of the PHY layer. This choice is better in terms of computational time than implementing a full PHY layer to be jointly simulated with the upper layers. In this paper, an analytical model is proposed to compute the BER and PER of DC, MPD and hybrid techniques, considering a Single-Carrier with Frequency-Domain Equalization (SC-FDE) scenario. The paper also considers two frequency-domain detection methods: a simple linear receiver and a powerful iterative receiver. The proposed model is simple and versatile, since it can support nonhomogeneous transmitting powers, and different channel conditions concerning retransmissions and channel randomization techniques. Several simulations show that the proposed model provides accurate BER and PER results for a wide range of scenarios.

The Quality of Service (QoS) requirements for modern broadband wireless systems can be very high, with small error rates and delays. Packets with errors are usually discarded and need to be retransmitted, leading to performance degradation. An alternative to simple retransmissions like Automatic Repeat reQuest (ARQ) schemes, that can minimize the degradation is to combine the signals associated to different transmission attempts, usually called diversity combining techniques. This paper considers the use of Single-Carrier (SC) modulations with Frequency-Domain Equalization (FDE) and low-complexity soft combining ARQ schemes. The time diversity combining technique presented in this paper, allows packets associated to different transmission attempts to be combined in a soft way so as to improve the performance. This technique is employed in a Time Division Multiple Access (TDMA) scheme. An accurate analytical model is proposed for the evaluation of the uplink packet delay in non-saturated traffic condition, and for the system's throughput (goodput) considering a generic packet arrival process. Physical (PHY) layer (packet error rates) and system-level (goodput and packet delay considering both PHY and Medium Access Control (MAC) layers) results are presented and validated by simulations for two distinctive arrival processes: Poisson and Geometric.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Network Diversity Multiple Access (NDMA) handles collisions by combining a multipacket detection scheme with time diversity. In NDMA, the base station (BS) forces mobile terminals (MTs) to transmit J copies of each packet when J MTs collide. Diversity combining is limited to J copies of the packets, not allowing it to adapt to severe errors due to channel noise. This paper considers a multipacket detection scheme recently proposed, which reduces the packet error rate (PER) when more than J copies of the packets are available. In this paper, an Hybrid-ARQ NDMA access mechanism is proposed. The access mechanisms forces MTs with reception errors during a collision resolution epoch to transmit more than J times. Analytical models are proposed for the goodput and delay, considering Poisson traffic. The proposed system's performance is evaluated for a Single-Carrier with Frequency Domain Equalization (SC-FDE) scheme, and compared to classical NDMA.

Traditionally, a packet with errors, either due to channel noise or collisions, is discarded and needs to be retransmitted, leading to performance losses. Two different approaches are considered to cope with lost packets, a low-complexity packet combining ARQ scheme (Automatic Repeat reQuest) and a multipacket detection scheme. These approaches are employed on a SC-FDE scheme (Single-Carrier with Frequency Domain Equalization). Suitable receiver structures are described, employing packet combining, multipacket detection and hybrid schemes. An accurate analytical tool is presented for predicting the performance of the different schemes.

Current applications and services for ad hoc net works heavily rely in advertisements, which are transmitted using broadcast. In the current Medium Access Control (MAC) schemes, namely in IEEE 802.11, broadcast co-exists with unicast frames. While the transmission scheme used for unicast frames is considered reliable, the IEEE 802.11 does not offer any possibility of retransmission for broadcast frames. This work presents a novel reliable MAC scheme, which is designed to maximize the network throughput. In our scheme all nodes adopt a common optimal contention window, which is based on a common view of the channel. Contrary to IEEE 802.11 we use the same contention window for both unicast and broadcast transmissions. Thus the conditional collision probability is independent of the traffic type. Departing from the optimal throughput for a saturated network, we estimate the number of nodes, which is a prime parameter to regulate the medium access control. We show that each node is suitable to estimate the number of competing nodes by using both its own medium access probability and the idle slot probability observed from the channel. Several simulation results evaluate the reliability and the throughput of our proposal, for different amounts of unicast/broadcast traffic. The results are compared to the well known IEEE 802.11 MAC, indicating that even without retransmitting the broadcast frames, the probability of successful transmission can be doubled. Moreover, the throughput of our approach outperforms IEEE 802.11 MAC and the results show it can be boosted up to 100%.

In this paper we present a performance analysis of XOR1-based flat routing protocols in high mobility conditions, considering a vehicular ad hoc network (VANET) formed in a highway scenario. First, we describe an XOR-based protocol that incorporates several adaptations of the existing XOR-based routing algorithms for wired networks, in order to cope with the network mobility. Then we propose an improved version of it, XORi, which modifies the protocol's information gathering process to accommodate the specific dynamic nature of VANETs topology. Finally, we evaluate the performance of XOR-based protocols with other topology-based routing protocols. Simulation results allow us to characterize the performance of this class of protocols through the comparison of the packet delivery ratio, end-to-end path delay and average number of path hops2. When a moderate density of nodes is considered, simulations show that XOR-based algorithms achieve almost the same packet delivery rate as link state algorithms, such as OLSR, while for high density of nodes XOR-based algorithms scale better in terms of delay when compared to source routing algorithms, such as DSR3.

In the last years, there has been an increasing interest in developing and testing Medium Access Control (MAC) protocols for ad hoc networks based on optimal methods. Several solutions do not take into account the throughput-fairness tradeoff, presenting high throughput performance but under-performing in terms of medium access fairness. This work presents a novel MAC scheme, which is designed to maximize the network throughput-fairness performance. In our scheme, all nodes adopt a common optimal contention window, which is based on a common view of the channel. Our proposal improves the access fairness because the contention window is similar to all nodes, and its behavior does not depend on previous MAC states. Departing from the optimal throughput for a saturated network, we devise a scheme to estimate the number of nodes, which is a prime parameter to regulate the medium access control. We show that each node is suitable to estimate the number of competing nodes by using both its own medium access probability and the idle slot probability observed from the channel. Several simulation results evaluate the throughput-fairness performance of our proposal with several state-of-the-art MAC protocols, such as AOB, Idle Sense, GDCF and the well known IEEE 802.11. The results indicate that our approach presents the highest value of medium access fairness among the simulated protocols, meaning that its throughput is closer to the optimal theoretical throughput obtained for a fair network.

In the last years, there has been an increasing interest in developing and testing Medium Access Control (MAC) protocols for ad hoc networks based on optimal methods. The solutions already proposed in several works are generally hard to implement because they require extensive message passing among the nodes. Moreover, a relaxation for the optimal solution is generally needed to achieve an implementable algorithm from the optimization problem solution. These disadvantages decrease the performance of the optimal-based algorithms, often translated into a less than expected throughput. This is the main reason why several MAC protocols, such as IEEE 802.11, use simple heuristics to control the access of the nodes to the medium. Generally, the adoption of heuristics in MAC protocols turns its formal performance analysis very difficult. The motivation of this work is to assess the validity of a formal method that analyzes the throughput of heuristic-based MAC protocols. This work starts to introduce the method that characterizes the node's medium access satisfaction. This method is used to compare the heuristics used to control the medium access. We describe and compare several heuristics by applying the formal analysis, which is later validated through simulations. The method here described can be applied at MAC design stage, since the designer can evaluate the performance of a given heuristic by comparison with others.

A frequent subject in forums, academia and industry is the evolution of the Internet in terms of routing. Basically, the scalability in the Default Free Zone related to (1) the growing rate of the routing tables and (2) the convergence of the routing system, are pointed by routing experts as the main concerns of the current mechanism. Several approaches have emerged, but they normally require a mapping system to translate from identifiers to locators (IP). We contribute with this discussion by introducing our XOR-based Flat Routing mechanism for Internet-like topologies. Essentially, the proposed mechanism routes directly on top of flat ASes identifiers, eliminating the need for mapping systems. In this work we propose a mechanism for building the routing tables over the XOR-based scenario in conjunction with a reachability service developed using the concepts of Landmark and Bloom filters. The proposal is evaluated using our developed emulation tool under five different Internet-like topologies ranging from 512 to 8192 nodes.

This paper considers the use of low-complexity diversity combining hybrid ARQ schemes (Automatic Repeat reQuest) on a TDMA (Time Division Multiple Access) system. Its low complexity makes it particularly interesting for battery-powered nodes of a wireless sensor network (WSN). TDMA is commonly used for WSNs applications that require high throughput or constrained packet delay. This paper analyzes the energy per useful packet, packet delay and goodput performances of the TDMA system. Analytical models were used to define a constrained energy optimization problem, where the minimum transmission power is calculated based on the goodput and delay constraints. Analytical results were validated through simulations.

Lost packets, either due to errors or collisions, are usually discarded and need to be retransmitted, leading to performance losses. In this paper we compare the performance of two different approaches to cope with lost packets. One alternative is a multi-packet detection approach able to separate up to Qmaxmobile terminals transmitting in one slot. The other alternative is a low-complexity soft combining ARQ scheme (Automatic Repeat reQuest) employed in a TDMA (Time Division Multiple Access) architecture. This paper evaluates the uplink non-saturated packet delay and goodput for a Poisson traffic generator. Our analytical results are validated by simulation using joint PHY (physical layer) and MAC (Medium Access Control) behavior.

This paper considers the use of a code combining hybrid ARQ scheme (Automatic Repeat reQuest) on a TDMA (Time Division Multiple Access) system. A multi-rate turbo-code is proposed to implement the H-ARQ scheme, allowing improved performances. This paper analyzes the energy per packet successfully received, the packet delay and goodput performances for the TDMA system on a HAP (High Altitude Platform). Analytical models were used to define a constrained energy optimization problem, where the minimum transmission power is calculated based on the goodput and delay constraints. Analytical results were validated through simulations for a typical HAP Network uplink channel.

This paper proposes the Mobile MH-MAC (Multimode Hybrid - Medium Access Control) protocol, an extension of MH-MAC with enhanced support for mobility on wireless sensor networks. It provides an asynchronous mode for energy efficiency during idle (unconnected) periods, and a synchronous mode for high throughput periods. Mode change is controlled by the application in a cross-layer approach. MMH-MAC was specially designed to tolerate asynchronous mobile nodes in the neighborhood of synchronous nodes, providing fast transition and interference mitigation mechanisms. Current MAC protocols rely on a passive method to associate a mobile node to a static cluster, triggered by the reception of a beacon. We propose an active approach that allows a mobile node to start sending data packets shortly after entering into data range of a static node, with negligible data loss in existing synchronous data connections. MMH-MAC was implemented in TinyOS and tested on TelosB motes and TOSSIM simulator.

Nonlinear operations are usually regarded as something undesirable that should be avoided in digital communication systems since they lead to performance degradation, namely spectral widening and decrease in power efficiency, for a given modulations. In this paper we show that the adoption of appropriate nonlinear devices can allow an improvement on the performance of some modulation schemes, either for OQPSK-type schemes (Offset Quadrature Phase Shift Keying) or TC-OQAM (Trellis Coded Offset Quadrature Amplitude Modulation). We present an analytical characterization of these signals after a bandpass memoryless nonlinear device. It is shown that an encoder followed by a nonlinear amplifier can be viewed as a serial concatenated coding scheme and the coding gain of such scheme can be improved relatively to the linear transmission scheme.

Several routing protocols for Mobile Ad Hoc Networks (MANETS), including the well known Ad hoc On-Demand Distance Vector Routing (AODV) and Optimized Link State Routing (OLSR), propose the use of periodic messages (Hello messages) to detect neighbor nodes. After receiving the first Hello message from one of its neighbors, a node starts the link sensing task by setting up a sensing timer. Each time a new Hello message is received from the same neighbor, the sensing timer is restarted and the link duration is prolonged. If the sensing timer expires, it indicates a long time interval without receiving an Hello message and, consequently, the link is considered broken. The transmission frequency of the Hello messages and the expiration value of the sensing timer truly depends on node's mobility: if the nodes are moving quickly and the Hello messages are rarely transmitted, the neighbor nodes can be in communication range but they are not detected; in the same scenario, if the expiration value of the sensing timer is too high, a link is sensed broken too late. In this paper, we consider a MANET under the Random Waypoint mobility model. We investigate the relationship between the transmission frequency of the Hello messages and the sensing timer expiration value with the network node's mobility. We formally deduce the probability of link existence after β periods of transmission of the Hello message. The probability is later used to define the sensing timer expiration value, considering a given probability that the Hello message transmission fails. Finally, we evaluate our study through both numerically analysis and simulations, which confirms the effectiveness and accuracy of our approach.

This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency- Domain Equalization). We consider iterative frequency-domain receivers that jointly perform equalization, multi-packet separation and channel decoding operations, for up to Qmaxmobile terminals transmitting in one slot. In this paper we propose an accurate analytical model for a p-persistent random access MAC protocol, designed to handle a total number of mobile terminals J above Qmax. It evaluates the uplink non-saturated packet delay and calculates the optimal configuration for the packet transmission probability associated to each mobile terminal that minimizes the delay. Our analytical results are validated using physical and MAC layer simulations.

This paper considers the use of SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization) with low-complexity soft combining ARQ schemes (Automatic Repeat reQuest). With our technique, packets associated to different transmission attempts are combined in a soft way, allowing improved performances. Its low complexity makes it particularly interesting for the uplink of wireless systems. This paper proposes an accurate analytical model for a TDMA (Time Division Multiple Access) scheme where packet combining ARQ is applied. It evaluates the uplink non-saturated packet delay and goodput for a generic message arrival process. Our analytical results are validated using physical and MAC layer simulations.

This paper deals with multi-packet detection for SC modulations (Single-Carrier) with FDE (Frequency-Domain Equalization). We consider iterative frequency- domain receivers that jointly perform the equalization, multi-packet separation and channel decoding operations. Our receivers allow a high throughput provided that the number of mobile terminals that is trying to access a given slot is equal to or below the multi-packet detection capacity Qmax. We propose a new MAC protocol that allows a total number of mobile terminals J above Qmaxby controlling the number of mobile terminals contending to transmit. The paper evaluates the uplink saturated system throughput and proposes an optimal configuration for the packet transmission probability associated to each mobile terminal. Our analytical results are validated using physical and MAC layer simulations.

This paper introduces two important enhancements to the IEEE 802.11 medium access control (MAC) protocols which are not considered in the current IEEE 802.11 MAC protocol: the channel state between the transmitter and the receiver and the node queue state. Our objective is to characterize the impact of these parameters on medium access regulation, and ultimately rely on them in order to enhance the sum throughput compared to the legacy 802.11x MAC protocols. We consider the scenario of several nodes attempting to access an access point (AP). Each node is characterized by: (i) a link quality to the AP which is essentially mapped to the PHY-layer rate that can be supported; (ii) a queue length depending on the packet arrival process at that node. We make the contention window dependent on queue size (backlog) and PHY-rate. The key idea is that a node with large PHY transmission rate and large queue size should tend to use smaller contention window, so that it gets higher chances for accessing the channel. We suggest heuristic queue and link state-aware rules for defining the contention window. We demonstrate with numerical arguments a significant enlargement in the capacity region under our modified access control policies and substantial performance improvement in terms of sum throughput for this system, compared to legacy IEEE 802.11 protocols.

The traditional approach to cope with collisions is to discard the packets involved in it and to ask for their retransmission. However, since the signal associated to a collision has important information concerning the packets involved, we can efficiently resolve collisions with proper retransmissions. In this paper we consider severely time-dispersive channels and we propose a technique that allows efficient packet separation. We employ SC-FDE schemes (single-carrier with frequency-domain equalization) and we propose an iterative frequency-domain multi-packet detection. Since our technique requires uncorrelated channels for different retransmissions, we also propose an SP technique (Shifted Packets) for retransmissions in fixed channels. Since the total number of transmissions is equal to the number of packets involved in the collision (even when the channel remains fixed for the retransmissions), our technique allows high throughputs. The analysis of our detection technique combined with the NMDA (network-assisted diversity multiple access) MAC (medium access control) protocol shows significant throughput and delay improvements for low Eb/N0values compared to a TDMA (time division multiple access) approach, where collisions are avoided. This technique is particularly appealing for the uplink of broadband wireless systems, since we consider SC-FDE schemes and the complexity is concentrated in the receiver. By employing the SP scheme we can use the same channel for the retransmissions, with only a small performance degradation.

Usually, packets involved in a collision are lost, requiring their retransmission. However, the signal associated to collisions has important information concerning the packets involved. In fact, with proper retransmissions we can efficiently resolve collisions. In this paper we propose a frequency-domain multi- packet detection technique for SC-FDE schemes (Single- Carrier with Frequency-Domain Equalization) that allows an efficient packet separation in the presence of successive collisions. This technique allows high throughputs, since the total number of transmissions is equal to the number of packets involved in the collision, even when the channel remains fixed for the retransmissions. Since we consider SC-FDE schemes and the complexity is concentrated in the receiver, this technique particularly appealing for the uplink of broadband wireless systems.

This paper proposes MH-MAC, a new MAC protocol for wireless sensor networks capable of handling applications that generate infrequent huge peaks of traffic. Existing protocols are not adapted to this kind of applications. Asynchronous protocols are energy efficient for the long inactive periods, but fail to cope with the bandwidth and latency requirements of the traffic peaks when more than two nodes are sending data to a common sink. Synchronous protocols that support contention free slots provide good throughput for handling the load peaks, but consume unnecessary energy maintaining clocks synchronized for very long idle periods. MH-MAC is a multimode hybrid protocol that can be configured by the application to run in asynchronous mode or in synchronous mode, with or without contention, providing the best possible trade-off. MH-MAC is a single-hop MAC, which supports multi-hop applications through a cross-layering API. The paper includes simulation results with the energy consumption, latency and throughput for the operation modes of MH-MAC, showing the asynchronous-synchronous trade-offs and the state transition overhead.

The strong research on the performance of the IEEE 802.11 standard seen lately contributes to a better understanding of the network and as a starting point for the design of future networks (specially in the ad-hoc area). Oliveira et al. proposed a model to study the IEEE 802.11 DCF delay assuming saturated or non-saturated broadcast and unicast traffic. Oliveira et al. assumes that all nodes generate frames having the same length. This paper extends Oliveira et al. supporting variable frame's length and both unicast basic and RTS/CTS multirate transmission schemes. The 802.11 MAC average time overhead per frame is deduced. Several simulations scenarios are described to validate the theoretical analysis. The validation results confirm that the MAC transmission delay is strongly dependent on the amount of broadcast traffic in the network. For high transmission data rates the MAC delay increases when the broadcast traffic also increases. However when high data rates are not possible the broadcast traffic increase can have the reverse effect.

The existing models for IEEE 802.11 DCF networks only consider unicast frames, ignoring the existence of broadcast traffic. In a real scenario, the stations are most of the times non-saturated and unicast and broadcast frames exist. These specific characteristics influence the service time which consequently affects the queue behaviour. In this paper, the authors model the total frame's delay for IEEE 802.11 DCF networks in presence of both unicast and broadcast traffic. Our theoretical analysis proposes a model able to compute the time between the instant when a new frame is inserted on the transmission queue and the instant when its transmission finishes. The time needed to serve a frame (service time) is formally deduced from the IEEE 802.11 transmission procedure, conceptually using the view of one network's node. The authors use an M/M/1/K queue model in order to model each frame's queueing delay. The authors validate the total frame's delay using several simulations and present some results. These are analysed for different scenarios of broadcast/unicast network loads and different number of nodes.

Most studies on performance of IEEE 802.11 DCF networks do not contemplate the existence of broadcast traffic. They model network behaviour in presence of unicast traffic alone. In a real scenario, broadcast frames exist, and they will influence the overall network behaviour. This work presents a new traffic generalized model for CSMA/CA saturated single-hop networks able to describe the network behaviour in presence of both unicast and broadcast frames. Interesting statistics like station transmission probability, average time needed to complete a frame transmission, and network aggregate throughput are deduced from the model. The paper compares the performance of our model with other models proposed only for unicast traffic. Our model is validated for broadcast and unicast traffic through simulations, using an IEEE 802.11 DCF network scenario. Results are presented and analysed for different scenarios of broadcast/unicast network loads, different number of nodes and different frame data lengths