Development of maneuver cooperation strategies is essential for enhancing traffic safety and efficiency. In a Vehicle-To-Everything (V2X) ecosystem, Maneuver Coordination Messages (MCM) allow road users to coordinate trajectory intentions based on decision-making mechanisms so they can effectively solve complex interactions such as unsignalized intersection crossings or lane changes. As these mechanisms are still in the early stages of development, research continues exploring their effectiveness and feasibility. In this paper, a Decentralized Maneuver Coordination Service (D-MCS) implementation is evaluated in real-world conditions where Vulnerable Road Users (VRUs) and emergency vehicles are taken into account, assessing the efficiency of the cooperative algorithm by demonstrating a reduction in execution time to complete the maneuvers. The results of the experiments also demonstrate that the proposed DMCS achieves a significant reduction in maneuver coordination time compared to a centralized MCS and improves traffic flow and VRU safety by allowing vehicles to proactively adjust their maneuvers.

One of the key concerns related to the pervasive integration of artificial intelligence and machine learning (AI/ML) models in vehicular-to-everything (V2X) communication systems pertains to adversarial attacks, which may lead trained models to exhibit undesirable behaviors. As security and user safety are tightly coupled in V2X, ensuring the resilience of AI/ML models against adversaries becomes indispensable. However, addressing adversarial attacks poses a challenging task, requiring appropriate countermeasures to elevate the trustworthiness of the targeted AI/ML models. In this paper, we propose a deep reinforcement learning (DRL)-based approach to defend against two data poisoning attacks, namely label-flipping and policy induction. Extensive evaluation with the aid of an open-source dataset demonstrates that our scheme outperforms benchmark classifiers, achieving significantly superior detection performance in the presence of label-flipping attacks. The effectiveness of our DRL-based approach is also showcased under different adversarial strategies in the policy induction attack.

To facilitate the widespread adoption of Cooperative Intelligent Transport Systems (C-ITS) and applications, deploying infrastructure alongside roadways to support vehicle communication becomes imperative. This infrastructure, leveraging Multi-access Edge Computing (MEC), necessitates communication infrastructure integrating two technologies: cellular networks and C-V2X (Cellular Vehicle-to-Everything), thus embracing a Vehicle-to-Infrastructure-to-Vehicle (V2I2V) approach. This paper presents an analysis of various Road-Side Units (RSUs) devices from different manufacturers to be used in these architectures, focusing on two key aspects. Firstly, their user-friendliness to enable programming for seamless integration with the communication protocol stack executed in the MEC. Secondly, their ability to meet the transmission capacity demands in high-density scenarios.

Emerging vehicle-to-everything (V2X) systems call for a diverse set of novel mechanisms to address vulnerabilities and security breaches. In this context, misbehavior detection approaches aim to detect malicious behavior of rogue V2X entities and possible attacks that may originate from them. In this paper, we introduce a data-driven ensemble framework which jointly leverages clustering and reinforcement learning to detect misbehaviors in unlabeled vehicular data. A rigorous detection assessment using an open-source dataset reveals meaningful performance trends for various attacks. In particular, while the majority of attacks can be effectively detected, detection may be curtailed for certain misbehavior types due to partly inaccurate clustering and erratic activity of the attacker over time. Performance comparison against benchmark detectors reveals the robustness of our approach in the presence of potentially inconsistent or mislabeled training data. The real-time detection capabilities of our framework are also explored in an effort to evaluate its practical feasibility in mission-critical V2X scenarios.

Vehicle-to-everything (V2X) communication is contributing towards the realization of futuristic vehicular networks such as Internet-of-Vehicles (IoV). The IoV is expected to usher in a new direction of intelligence and networking to achieve the goal of intelligent transport systems, which rely on the secure exchange of messages between vehicles and infrastructure. However, the transmission of false/incorrect data by malicious vehicles may cause serious damages on road safety. Therefore, it is crucial to detect safety-threatening incorrect information and mitigate potentially detrimental effects on road users. In this paper, we propose a reinforcement learning (RL)-based misbehavior detection approach for V2X scenarios. In our method, the RL-based detection model processes V2X data broadcast by vehicles as time-series at the roadside units, and classifies incoming data as misbehaving or genuine. We evaluate the proposed RL-based approach for detection of various attack types using an open-source dataset, and compare its performance against recent work in misbehavior detection. Our scheme is able to detect all types of misbehavior with a superior recall of 0.9970 and an F1 score of 0.9845, yielding a significant improvement over the benchmarks. Our research outcomes further reveal that misbehaving vehicles can be detected with a great accuracy of 0.9882 by exploiting real-time V2X information.

The transformative Internet-of-Vehicles (IoV) paradigm comes inadvertently with challenges which involve security vulnerabilities and privacy breaches. In this context, denial-of-service (DoS) attacks may perniciously affect the normal operation of IoV systems by causing extensive periods of network unavailability where legitimate vehicles are prevented from accessing vehicular services. In this paper, we offer an in-depth vulnerability assessment of 5G-enabled IoV systems when DoS attack variants are launched at multiple network domains. We further evaluate the resilience of an IoV-tailored authentication mechanism against DoS attacks under various configurations. A data-driven detection scheme is also proposed to address DoS variants in the radio access network, which take the form of false data injection attacks on the exchanged vehicular information. Our performance assessment with the aid of an open-source dataset reveals that the proposed scheme is able to accurately detect DoS traffic originated from malicious vehicles.

The fifth generation (5G) of communication networks aims to accelerate the adoption of incipient vertical industries which will leverage innovative smart applications and services such as Cooperative, Connected and Automated Mobility. One of the objectives globally within that area is reducing to zero the number of fatal vehicle accidents. Unfortunately, human errors are the main cause of them, where vulnerable road users (VRUs) are involved in half of the cases. A possible approach to reduce accidents is estimating the probability of collision between two vehicles based on their estimated trajectories. These trajectories are usually tracked on-board using sophisticated devices such as cameras and LiDAR. However, VRUs are generally not equipped with such equipment and, ideally, VRUs carry smartphones with active geolocation capabilities based on satellite-based positioning systems. In this paper, we propose a novel vehicular service based on a regression algorithm to predict trajectories by uniquely using Cartesian coordinates. We compare different types of regression techniques in terms of prediction time window, position accuracy and processing time using Weka. Results show that the Alternating Model Tree (AMT) technique can predict the next position with an error of less than 3.2 centimeters, increasing up to 1 meter when predicting the next 5 positions with a period of 1 second between consecutive positions. In this case, a prediction time window of 5 s is processed within 1.25 milliseconds. AMT resulted as the lowest complex and most accurate algorithm in a multiple-step prediction position.

The reliable acquisition of monitoring information is critical for several industrial use cases relying on wireless sensor network deployments. However, missing sensor measurements are typical in industrial systems empowered by cellular connectivity due to the stochastic nature of the wireless channel. In this paper, we propose a sensor data reconstruction scheme that exploits the hidden data dynamics to accurately estimate the missing measurements. Based on an analytical framework for the network model and a closed-form expression for the outage probability, the impact on the reconstruction error performance is thoroughly explored. Considering a dataset with high spatiotemporal correlation in the sensor observations, our proposed scheme is shown to outperform two baseline data recovery methods in terms of reconstruction error for various network configurations. In addition, despite the presence of imperfect cellular connectivity, our proposed scheme exhibits high event-detection accuracy.

In Software Defined Networks (SDN) a Controller installs forwarding rules or queries the status of network devices through a separate control channel using the OpenFlow protocol. Such queries may ask for instantaneous metrics, like port status, packet error rate, or transmitted packets. In turn, the Controller uses these metrics, or context, to condition forwarding rules. As network conditions change over time, forwarding rules are set to expire if not used (or matched) during a configurable idle timeout. Consequently, they are erased from the flow table, forcing a query to the Controller every time a packet for which there is no forwarding rule arrives at the network device. This work presents experimental results showing the delay increase produced by timing out forwarding rules of periodic flows, such as the generated by sensors in IoT deployments. Furthermore, it proposes and experimentally evaluates a pro-active forwarding rule installation mechanism able to reduce such delay by 90%, easing the implementation of context-aware forwarding strategies for IoT.

Selection of the most appropriate communications technology for a smart grid (SG) application is far from trivial. We propose such a feasibility assessment starting from identification of key performance indicators (KPIs) required for peer-to-peer (P2P) energy trading and grid control operations from a communications perspective. A set of cross-disciplinary KPIs, both quantitative and qualitative, are considered from communications, power, business, actor involvement, financial, and demand side management categories. They serve as a general baseline for use cases, as there have been few previous works attempting to capture the essential features of P2P SG operations. The KPIs are briefly identified along with their relations to P2P energy trading and grid control. A straightforward comparison of the quantitative and qualitative KPIs' impact on technology selection is not feasible. This paper addresses the comparison with: 1) a prioritization of the KPIs using the analytic hierarchy process; 2) a comparison of technology solutions evaluated in our previous works against the KPIs' requirements; and 3) a total feasibility evaluation of the solutions against selected KPIs. The prioritization shows latency, reliability, security, scalability, robustness, costs of information and communication technologies (ICT) devices, and costs of ICT deployment are the most important KPIs in enabling P2P energy trading and grid control. Further, the technology feasibility assessment enables identification of the most suitable candidates for an SG application.

The realization of advanced electrical grid functionalities, e.g., phasor measurement unit (PMU) information acquisition in wide-area monitoring systems (WAMS), requires a scalable and reliable underlying communication technology. Cellular networks, e.g., LTE/LTE-A systems, appear as a promising option to facilitate the smart grid evolution. However, the effect of LTE communication constraints on power system state estimation (SE) has not been thoroughly addressed in the literature. In this paper, we investigate the impact of the LTE random access channel (RACH) reliability on the transmitted PMU measurements and, consequently, on the SE accuracy. In particular, we assess the SE performance based on the achieved reliability per PMU attained with i) increasing number of contending devices in the system, i.e., smart meters and PMUs, and ii) varying cell coverage range. Numerical results demonstrate that, under different network and traffic configurations, SE accuracy can be significantly affected by the RACH reliability levels. Useful insights can thus be drawn for a reliability-aware design of a power system state estimator in LTE-based WAMS.

The reliable support of event-driven massive machine-type communication (MTC) requires radical enhancements in the standard LTE random access channel (RACH) procedure to avoid performance degradation due to a high probability of collision in the preamble transmission. In this paper, we investigate the relation between the cell size and the number of preambles generated from a single or multiple root sequences and we study their impact on the achieved reliability. Based on an analytical expression of the RACH reliability per cell, we introduce an interference- and load-aware cell-planning mechanism that efficiently allocates the root sequences among multiple cells and regulates the traffic load to guarantee reliable support of MTC. In addition, we propose a realistic traffic model that accurately captures the event-driven nature of MTC traffic. Finally, a performance evaluation of a power distribution automation scenario with MTC-overload reveals the superior performance of our proposed mechanism in terms of RACH reliability against benchmarking network-deployment schemes.

The Energy Harvesting-aware Distributed Queuing access protocol (EH-DQ) is presented in this paper as a novel Medium Access Control protocol for wireless Machineto-Machine networks with energy harvesting capabilities. EHDQ is theoretically modeled to analyze its performance. A performance comparison with a Time Division Multiple Access (TDMA) and an EH-aware Reservation Dynamic Frame SlottedALOHA (EH-RDFSA) shows its superior performance. While TDMA requires updated network information to maintain a collision-free schedule and EH-RDFSA requires to estimate the number of contenders per frame to dynamically adjust the frame length, EH-DQ uses short and fixed frame lengths and does not require knowledge of the network topology or the number of end-devices in advance.

This paper provides an analysis of energy efficiency of the Transmission Opportunity Power Save Mode (TXOP PSM) in IEEE 802.11ac Wireless Local Area Networks (WLANs). This mechanism allows a device to sleep during transmissions in the channel that are addressed to other devices. This operation is also referred to as microsleep and can significantly reduce the energy consumption of devices during overhearing periods. A key contribution of the analysis presented in this paper is the awareness of the non-negligible time and energy consumption that a device incurs when it switches between awake and sleep states. If the duration of such state transitions is longer than the transmission time, microsleep operation is not possible. This becomes a critical issue as transmission rates increase, thus reducing the transmission times. In this paper, we show that the performance dependence of TXOP PSM on the awake/sleep state transitions can be overcome by using burst transmission inherent to the TXOP operation. Results obtained through theoretical analysis and computer-based simulation show gains of up to 424% in energy efficiency when compared to legacy mechanisms.

This paper proposes a new Reverse Direction (RD) Medium Access Control (MAC) protocol to improve the throughput and energy efficiency of IEEE 802.11 Wireless Local Area Networks (WLANs). The proposed protocol allows a source device to transmit a burst of data frames to the intended destination device in a single channel access opportunity. After the successful reception of each data frame, the destination device may opportunistically respond with a data frame, thus being able to perform multiple RD transmissions. This operation can reduce the overall channel access overhead, hence increasing the efficiency of data transmission between two sender-receiver devices. The results obtained by means of theoretical analysis and computer-based simulation show that the novel RD protocol can outperform existing IEEE 802.11 protocols by yielding gains close to 60%.

In this paper, a novel non-orthogonal multiple access (NOMA)-based device-to-device (D2D) communications framework is proposed. A major novelty of the proposed framework is that it introduces the new concept of ``D2D group" which utilizes NOMA transmission, enabling one D2D transmitter to communicate with multiple D2D receivers simultaneously. Based on the considered framework, a resource allocation optimization problem is formulated, where multiple D2D groups are allowed to reuse the same subchannel. The objective of this work is to maximize the system sum rate by satisfying the signal-to-interference- plus-noise (SINR) constraints of both D2D and traditional cellular users. Note that the formulated problem is non-deterministic polynomial-time (NP) hard in nature, thus a novel resource allocation algorithm based on the many- to-one two-sided matching theory is proposed for obtaining a suboptimal solution. It is proved that the proposed algorithm converges to a stable state within limited number of iterations. Numerical results illustrate that: i) the proposed algorithm is an effective approach for obtaining near- optimal performance with acceptable complexity; and ii) the proposed NOMA-based D2D framework is capable of achieving promising gains over traditional orthogonal multiple access (OMA)-based D2D framework.

The Energy Harvesting-aware Contention Treebased Access (EH-CTA) protocol is presented in this paper as a novel Medium Access Control (MAC) protocol for wireless Machine-to-Machine (M2M) networks where end-devices are equipped with energy harvesters. The protocol is theoretically modeled to analyze its performance. A performance comparison with an EH-aware Dynamic Frame Slotted-ALOHA (EH-DFSA) shows its superior performance. While EH-DFSA requires an estimation of the number of contenders per frame in order to dynamically adjust the frame length, EH-CTA uses short and fixed frame lengths. This ensures scalability and facilitates synchronization in highly dense M2M networks.

Ongoing IEC 61850 standardization activities aim at improved grid reliability through advanced monitoring and remote control services in medium- and low-voltage. However, extending energy automation beyond the substation boundaries introduces the need for timely and reliable information exchange over wide areas. LTE appears as a promising solution since it supports extensive coverage, low latency, high throughput and Quality-of-Service (QoS) differentiation. In this paper, the feasibility of implementing IEC 61850 services over public LTE infrastructure is investigated. Since standard LTE cannot meet the stringent latency requirements of such services, a new LTE QoS class is introduced along with a new LTE scheduler that prioritizes automation traffic with respect to background human-centric traffic. Two representative grid automation services are considered, a centralized (MMS) and a distributed one (GOOSE), and the achievable latency/throughput performance is evaluated on a radio system simulator platform. Simulations of realistic overload scenarios demonstrate that properly designed LTE schedulers can successfully meet the performance requirements of IEC 61850 services with negligible impact on background traffic.

It has been shown in the literature that Network Coding (NC) can boost the performance of wireless networks. However, to really obtain the potential gain of NC, efficient Medium Access Control (MAC) protocols that operate with awareness of the NC functions are necessary. In this paper, we propose a novel NC-aware MAC protocol for IEEE 802.11 wireless networks that combines k-batch transmissions and multiple receiver-initiated reverse direction exchanges involving NC data to boost the overall network performance. The proposed protocol allows any node to transmit a burst of data packets in a single channel access invocation. Then, an intermediate node can transmit an NC data packet when receiving a valid data packet from a source node, without contending for channel access. Both analytical and simulation results presented in this paper show the high throughput and energy efficiency of the proposed protocol with gains ranging from 33% to 298% when compared to existing mechanisms based on the IEEE 802.11 Standard.

In this paper, we facilitate device-to-device (D2D) communications to provide relay assistance to cell-edge user equipments (UEs) with the objective of improving system throughput. We first formulate a joint problem of relay node selection which helps cell-edge UEs find the proper relay nodes, as well as spectrum allocation for D2D links to maximize the system throughput with interference constraints to both D2D and traditional cellular UEs. Furthermore, we propose a distributed algorithm adopting a two-level game model which consists of inner and outer levels to solve the formulated problem. In the inner level, we use the Stackelberg game to select relay nodes for cell-edge UEs, where the relay nodes act as the leaders and the cell-edge UEs act as the followers. In the outer level, the coalition formation game is used to allocate proper spectrum for the D2D links between cell-edge UEs and their relay nodes. The games do not proceed separately, but are dependent on each other, which improves the efficiency of the proposed game model. Simulation results demonstrate that the proposed algorithm outperforms the benchmarks in terms of system throughput.

Machine-to-Machine (M2M) networks allow enddevices to communicate without human intervention. Due to the high density of M2M networks, efficient Medium Access Control (MAC) protocols are required to manage the access to the channel. In this paper, we consider an M2M area network composed of hundreds of end-devices that periodically transmit data to a gateway. We evaluate the performance of two Medium Access Control (MAC) protocols based on Frame Slotted-ALOHA (FSA) with Successive Interference Cancellation(SIC): Intra-frame SIC-FSA and Inter-frame SIC-FSA. By means of computer-based simulations, we have compared the delay and energy performance with respect to conventional FSA. Results show that the average delay can be reduced in a 78% and 94% by using Intra-frame SIC-FSA and Inter-frame SIC-FSA, respectively, and the average energy consumed per end-device is reduced in 18% by using Intra-frame SIC-FSA, while Interframe SIC-FSA decreases the energy consumed per end-device in a 22%.

In this paper, we propose a solution to efficiently handle the initial access contention for highly dense LTE networks, which still represents a challenge in terms of the high number of devices associated to Machine Type Communications. The analysis is based on the fact that, as the radius of a LTE cell is larger, the number of available preambles generated from a single root sequence is reduced. The current access mechanism, based on frame-slotted ALOHA, does not operate efficiently under these conditions, negatively affecting the access delay and collision probability. We show how the access procedure can be improved by implementing tree-splitting algorithms in order to cope with high number of simultaneous arrivals in large cells, above 5 km in radius. The implementation achieves as much as 85% reduction on the access delay and 54% reduction on energy consumption for cells with radius larger than 39 km.

In this paper, we propose an energy-efficient solution for implementing Network Coding (NC) in wireless networks based on the IEEE 802.11 Standard. The proposed mechanism, called GreenCode, allows nodes to duty cycle by switching to a low-power (sleep) state when they overhear coded packet transmissions that will not provide any new information for them. To facilitate the sleep operation, bidirectional transmissions involving both coded and non-coded packets between pairs of sender-receiver nodes are integrated into the operation of GreenCode. Both analytical and simulation results presented in this paper show the high energy efficiency of GreenCode with gains of up to 360% when compared to the existing mechanisms based on the IEEE 802.11 Standard.

This paper describes an experimental implementation of a variation of the Reverse Direction (RD) Medium Access Control (MAC) Protocol (RDP) defined in the IEEE 802.11n using the Wireless Open-Access Research Platform (WARP). The proposed approach, named Bidirectional MAC (BidMAC), allows the receiver of a valid data sequence to perform an RD transmission to the transmitter without contending for the channel. Whereas in RDP the RD transmission must be initiated by the transmitter, in BidMAC it can be dynamically initiated by the receiver according to its traffic requirements. Previous results based on mathematical analyses and computer-based simulations have shown that BidMAC can better balance downlink and uplink transmission opportunities in a Wireless Local Area Network (WLAN) where the Access Point (AP) handles bidirectional data flows for some of its wireless stations (STAs). This paper aims at going one step further and demonstrating that such superior performance can be attained in real environments. Towards this end, an implementation of BidMAC has been carried out in a reference design of WARP compatible with the IEEE 802.11a/g and tested in a proof-of-concept network formed by an AP and two STAs. Experimental results confirm the superior performance of BidMAC when compared to the legacy Distributed Coordination Function (DCF) of the IEEE 802.11 versus the traffic load, packet length, and data rate, yielding gains of up to 60%.*

This paper analyzes the performance of a duty-cycled polling-based access mechanism that exploits the Transmission Opportunity Power Save Mode (TXOP PSM) defined in the IEEE 802.11ac to improve the energy efficiency of Wireless Local Area Networks (WLANs) based on the IEEE 802.11. The basic idea behind the proposed approach, named GreenPoll, is to enable contention free periods, based on polling with beacons, during which wireless stations can save energy by turning off their radio transceivers after exchanging data with the access point. The closed expression of energy efficiency of GreenPoll is formulated in this paper and is used to evaluate the performance of GreenPoll considering important parameters like the traffic load, packet length, data rate, and number of stations in the network. Both analytical and simulation results show the high energy efficiency of GreenPoll with gains of up to 330% and 110% when compared to the legacy Distributed Coordination Function (DCF) and the Point Coordination Function (PCF) defined in the IEEE 802.11, respectively.

We consider a wireless Machine-to-Machine (M2M) area network where a gateway periodically collects data from a group of end-devices equipped with energy harvesters. While the use of energy harvesters ideally provides infinite lifetime, the unpredictable amount of harvested energy may not guarantee that all data transmissions can be done in due time because of temporary energy shortages. We propose in this paper the Energy Harvesting-aware Reservation Dynamic Frame Slotted-ALOHA (EH-RDFSA) protocol as a solution suitable for managing the access of end-devices that transmit bursts of data packets while taking into account the energy availability. We derive a model based on a discrete-time Markov chain to analyze the evolution of the energy available in an end-device and to evaluate the performance of the network. In particular, we compute the data delivery ratio, which measures the ability of the protocol to successfully transmit data to the gateway without depleting the energy reserves of the end-devices, and the time efficiency, which measures the amount of data that can be transmitted in a given period of time. We have validated the accuracy of the analysis by means of computer-based simulations. Results show that the overall performance is influenced by the energy harvesting rate and the amount of data to transmit from each end-device. Finally, we have compared the performance of EH-RDFSA with that of DFSA and Time Division Multiple Access (TDMA) protocols.

Current and future releases of 3GPP standards will include enhancements in LTE-Advanced for machine-type communications (MTC). The main reason for this new trend is the fact that 5G wireless systems will need to enable the coexistence between MTC and human-type communications (HTC). Therefore, novel solutions are needed to efficiently exploit the radio resources for MTC, considering, among others, the need to optimize transmissions for small data packets. With this aim, this paper addresses the use of short-range device-to-device (D2D) communications as enabling technology to efficiently manage the radio spectrum and to reduce the energy consumption of MTC devices. We consider a scenario where MTC devices are grouped in a cluster; among the cluster members, one terminal acts as aggregator in charge for (i) receiving data from neighboring terminals via D2D links and (ii) relaying the aggregated data to the base station via macro-cellular link. The main contribution of this paper is to compare the performances of the two most popular D2D technologies, i.e., WiFi-Direct and LTE-Direct, used to transmit data toward the aggregator. The performance evaluation in terms of latency and energy efficiency has been conducted in a wide set of scenarios by varying the number of clustered devices and the data to upload per single MTC device.

In this paper, we address the challenge of high device density performing simultaneous transmissions by proposing and evaluating a solution to efficiently handle the initial access contention for highly dense LTE networks. We present the implementation of a tree-splitting algorithm in the access procedure of LTE, which is capable to cope with high number of simultaneous arrivals. Based on simulations we show a feasible implementation capable to achieve, under certain network configuration conditions, up to 85% average access delay reduction and 40% reduction on the average energy consumption, while maintaining a consistently low blocking probability, regardless of the number of initial simultaneous access attempts.

The implementation of Network Coding (NC) in IEEE 802.11-based wireless networks presents the important challenge of providing additional transmission priority for the relay nodes responsible for coding. These nodes are able to convey more information in each transmission than those that forward single packets, by combining several received packets in a single coded packet. To transmit data, the nodes execute the IEEE 802.11 Medium Access Control (MAC) protocol, called the Distributed Coordination Function (DCF). Thus, they compete for the access to the wireless channel and get equal transmission opportunities under high congestion. As a result, congested relay nodes will severely limit the performance of the network. In this paper, we investigate a backwards-compatible mechanism, called Reverse Direction DCF (RD-DCF), that allows relay nodes to transmit data upon successful reception of data. We analyze the performance limits of the proposed protocol with and without NC in terms of throughput and energy efficiency. The performance evaluation considers different traffic loads, packet lengths, and data rates. The results of this work show that the proposed RD-DCF+NC protocol can improve throughput and energy efficiency up to 335% when compared to legacy DCF.

The Distributed Coordination Function (DCF) is the mandatory access method for any compliant device in Wireless Local Area Networks (WLANs) based on the IEEE 802.11 Standard. WLAN Access Points (APs) and stations (STAs) contend for the access to the wireless channel in order to transmit data by using a variation of Carrier Sense Multiple Access (CSMA). In doing so, they consume a significant amount of energy for continuously monitoring the channel state. In this paper we investigate backwards-compatible mechanisms to increase throughput and energy efficiency in WLANs during contention periods based on DCF. The first mechanism is called Bi-Directional DCF (BD-DCF) because it allows for bidirectional transmissions between APs and STAs with a single channel access invocation. The second mechanism is called Bi-Directional Sleep DCF (BDSL-DCF) as it allows overhearing STAs to enter the sleep state, i.e. switch off the radio transceiver, during bidirectional transmissions. We analyze the performance limits of the proposed protocols in terms of throughput and energy efficiency considering different values for the data packet length and data rate. The results of this work show that the BD-DCF and BDSL-DCF protocols can improve throughput up to 60% and energy efficiency up to 360% when compared to legacy DCF.

Machine-to-Machine (M2M) area networks must provide connectivity between an M2M gateway and a large number of energy-constrained M2M devices. Attaining high energy efficiency is essential in order to prolong devices lifetime. In this paper, we consider a wireless M2M area network composed of hundreds or even thousands of dormant devices that wake up periodically to transmit data upon request from a gateway. We theoretically analyze the energy efficiency of a Medium Access Control (MAC) protocol that uses a tree-splitting algorithm to resolve the collisions among devices: the Distributed Queuing (DQ) access. Computer-based simulations have been carried out to validate the accuracy of the analytical model and to evaluate and compare the energy consumption of devices using also a basic Contention Tree Algorithm (CTA) and Frame Slotted-ALOHA (FSA). Results show that DQ can reduce energy consumption in more than 35% with respect to CTA and in more than 80% with respect to FSA in dense M2M area networks with devices in compliance with the IEEE 802.15.4 physical layer.

The IEEE 802.11 Distributed Coordination Function (DCF) is the fundamental access method providing asynchronous best-effort services in Wireless Local Area Networks (WLAN). In this standard, the currently employed Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and the Binary Exponential Backoff (BEB) mechanism represent major sources of energy consumption at both the access point and mobile stations of a WLAN. To improve energy efficiency in WLANs, this paper introduces an enhanced DCF protocol incorporating bidirectional transmissions in combination with sleep periods, called Bidirectional Sleep DCF (BDSL-DCF). By following this new scheme, every successfully established connection between a sender and its intended destination can be used to exchange data, hence reducing control overhead and channel contention. Furthermore, this functionality allows those mobile stations not participating in data transmission to activate the sleep mode to conserve energy. Simulation results show that BDSL-DCF can outperform DCF in terms of energy efficiency and throughput, with negligible impact on packet transmission delay.

Attaining very high energy efficiency is one of the big challenges to implement Machine-to-Machine (M2M) networks whose lifetime (without human intervention) must be measured in years. In this paper, we consider a synchronized duty-cycled M2M network composed of a large number of devices that periodically wake up their radio interfaces to transmit data to a coordinator. We theoretically analyze the delay and energy efficiency of two contention-based Medium Access Control (MAC) protocols for this kind of networks. One protocol is based on Frame Slotted ALOHA, and the other is based on a tree-splitting contention resolution algorithm. Computer-based simulations have been carried out to ensure the accuracy of the theoretical models and to evaluate and compare the performance of the two alternatives for M2M applications based on low power Wi-Fi devices.

In this paper, we evaluate the energy-efficiency of LTE for Machine-to-Machine (M2M) communications with small data transmissions. We quantify the minimum resources that can be allocated in LTE for the uplink and we show that the energy-efficiency of the transmissions of small blocks of data strongly depends on the transmission power and the Adaptive Modulation and Coding (AMC) procedure. Indeed, when an aggressive Modulation and Coding Scheme (MCS) is used thanks to good channel quality, the energy-efficiency of LTE decreases for the transmission of small amounts of data, as the physical resource blocks are under-utilized. In addition, we propose a solution to deal with this drawback by selecting an optimal MCS and utilizing the LTE Uplink Power Control (UPC) mechanism. The results presented in this paper are based on open-source ns-3 computer-based simulations.

Emerging Machine-to-Machine (M2M) applications demand small data packet sizes, very low latencies, and ultrahigh energy efficiencies. For all these reasons, Binary Bose-Chaudhuri-Hocquenhem (BCH) codes, which are very simple to implement, could constitute a good option to guarantee the required reliability of M2M transmissions. Nevertheless, existing delay and energy analyses of BCH decoders in the literature neither consider the channel statistics nor the first and second moments of the decoding delay. Therefore, they provide conservative codeword designs that lead to high delays and waste of energy. In this paper, we analyze the first and second moments of the delay and energy performance of Binary Bose-Chaudhuri-Hocquenhem (BCH) codes in realistic channel statistics to show that, if optimized, they can perform very efficiently for M2M transmissions. The results presented in this paper allow for the codeword length optimization for BCH codes given specific delay and energy constraints.

The design of energy-efficient communication strategies is needed to extend the lifetime of Machine-to-Machine low power devices operating in realistic wireless fading and shadowing channels. In this paper, we assess the suitability of using cooperative and duty-cycled retransmissions (CDC-ARQ) in realistic low power networks to improve their energy-efficiency. A CDC-ARQ scheme achieves this goal by maintaining two interchangeable wireless links towards any intended destination and using the currently optimal link in an opportunistic manner. We perform comprehensive performance evaluation using the ns-3 simulator to verify a theoretical framework fitted to the reference IEEE 802.15.4e protocol and its cooperative extension. The ns-3 simulation outcomes confirm prior theoretical findings and indicate optimal operating regions with minimum energy consumption for the shadowed relay system under consideration. We establish that a significant energy improvement can be achieved in a typical three-node scenario when a duty-cycled opportunistic cooperative relaying protocol is used instead of a fixed two-hop relaying system.

Due to propagation and interference effects at the 2.4 GHz band, Machine-to-Machine (M2M) wireless communications based on the IEEE 802.15.4 standard typically need multi-hop communications to connect end devices with a gateway. Unfortunately, multi-hop transmissions pose some challenges that are not trivial to solve and may slow down the deployment of M2M networks. For this reason, the IEEE 802.15.4f Working Group (WG) is currently defining the specifications of a new physical layer operating at the 433 MHz band, which offers better propagation conditions and suffers from lower interference levels. In this paper, we analyze the energy consumption of single-hop and multi-hop communications at both 433 MHz and 2.4 GHz. We use realistic propagation models and accurate energy consumption models to conduct a comprehensive assessment of the energy performance at the two frequency bands. The results presented in this paper show that operating at 433 MHz instead of 2.4 GHz can significantly reduce the number of hops between the end device and the gateway, which can be translated into a reduction of the overall network energy consumption.

Over the last years, cooperative communications have been studied from both fundamental and practical points of view. However, most of the existing studies of cooperative communications rely on math or computer simulation. In this paper, we describe the implementation of a cooperative communication strategy for wireless networks in a testbed based on Click Modular Router. We describe the methodology we have followed to reprogram the drivers controlling the data layer functions of off-the-shelf commercial Wireless Network Interface Cards (NICs). More precisely, we have implemented a Cooperative Automatic Retransmission (C-ARQ) scheme to show that the performance of wireless communications can be boosted by using an intermediate relay when the wireless channel between the source and the destination is in bad conditions. The results presented in this paper are promising, as they show that previous theory and simulation results presented in the literature can become true in a real implementation.

The design of communications protocols that exploit the availability of both cellular and short-range radio access interfaces in a same terminal can be exploited to improve the energy efficiency of wireless communications networks. Devices equipped with a cellular radio interface can act as dynamic gateways to provide energy-constrained devices in its single-hop cluster with cellular connectivity. For this purpose, any device may discover its single-hop neighbourhood in order to select the appropriate gateway candidates. Conventional neighbour discovery algorithms have a great cost in terms of delay and energy consumption, and thus they are not the optimal approach for energy-constrained devices. This paper presents the energy consumption analysis of two distributed neighbour discovery algorithms based on frame slotted-ALOHA, and evaluates their performance in terms of energy consumption. In addition, it describes the conditions that minimize the energy consumption in the discovery process.

The Persistent Relay Carrier Sensing Multiple Access (PRCSMA) Protocol has been proposed in the literature as a Medium Access Control (MAC) protocol suitable for Cooperative Automatic Retransmission Request (C-ARQ) schemes. The improvement in throughput achieved with PRCSMA is already known. In this paper, we analyze the energy efficiency of this protocol to assess application to energy-constrained networks. Towards this aim, a suitable energy model is derived. The model has been validated through computer simulations and shows that a C-ARQ based on PRCSMA can help improve the energy efficiency of wireless communications and thus increase the lifetime of devices.

In this paper we show how a cooperative Medium Access Control (MAC) protocol can help extend the lifetime of Machine to Machine (M2M) networks. A comparison between non-cooperative Automatic Repeat reQuest (ARQ) schemes (retransmissions performed only from the source) and a cooperative retransmission scheme (all the stations in the network are spontaneous helpers and assist the destination during the communication) in energy-constrained networks is also presented in this work. The results shown in this paper are discussed in terms of the trade off between the network lifetime and the amount of data delivered, leading to the conditions where it is possible to optimize the lifetime of this kind of networks.

Mobile Machine-to-Machine (M2M) market is expected to grow dramatically in the upcoming years; predictions agree that the majority of wireless traffic will correspond to this type of communication. M2M devices deployed in proximity to each other might be handled as groups or clusters when sharing the same communication attributes and serving to achieve joint target. Moreover, devices that feature multiple radio interfaces are commonly used nowadays; they can be connected to a cellular access network, e.g., LTE, and simultaneously to a short-range network, also known as capillary network, among surrounding devices, e.g., Wi-Fi and ZigBee. For services such as over-the-air (OTA) firmware update, the same information shall be sent to all the group members over the cellular access network. A Multi-Radio Cooperative Retransmission Scheme for Reliable Multicast Services is presented as a solution to reliably manage reception errors occurred over the cellular network; by carrying out retransmissions over a short-range network it is possible to reduce both the traffic load over the cellular link and the energy consumption in error recovery. Ns-3 simulations were performed with LTE network models to analyze the scheme in terms of energy consumption and prove its usefulness in autonomous, battery driven, devices.

Accurate representation of the physical layer is instrumental for a sound design and optimization of Medium Access Control (MAC) protocols for cooperative wireless networks. However, the vast majority of MAC protocols are designed and analyzed by considering simplified physical layer and channel models, which often lead to too optimistic performance predictions. In particular, even though many experimental activities have showcased the important role played by shadow-fading, most protocols are designed and evaluated by taking into account only the transmission distance (circular coverage model) or only the fast-fading. Motivated by the proved unsuitability of these models, the contribution of this paper is threefold: i) we provide important considerations on how to adequately include the effect of shadowing into the design of MAC protocols for cooperative networks; ii) we provide an analytical framework to determine the subset of active relays in order to meet a given Quality-of- Service (QoS) requirement; and iii) we study, through analysis and simulation, the performance of a promising MAC protocol for cooperative networks, which is called Persistent Relay Carrier Sensing Multiple Access (PRCSMA), by explicitly taking into account the effect of shadowing. Our study shows that shadowing can dramatically change system and protocol performance.

We present in this paper the evaluation of the energy consumption of PRCSMA, an 802.11-based medium access control protocol designed to coordinate the retransmissions from the relays in a wireless network implementing a Cooperative Automatic Retransmission Request (C-ARQ) scheme. A comparison in terms of energy efficiency with non-cooperative ARQ schemes (retransmissions performed only from the source) and with ideal C-ARQ (with perfect scheduling among the relays) is included in this paper. The main results show the conditions under which a C-ARQ scheme with PRCSMA outperforms, in terms of energy efficiency, non-cooperative ARQ schemes and also show that the overhead of the MAC layer cannot be neglected in order to accurately evaluate the performance of a C-ARQ scheme.

In this paper we analyze the performance of a distributed queuing medium access control (MAC) protocol designed to execute cooperative ARQ (C-ARQ) schemes at the MAC layer. Due to the broadcast nature of the wireless channel, a user that receives a data packet with unrecoverable errors can request retransmission from any of the other users in the transmission range of the transmitter that overheard the original transmission. These users can act as spontaneous relays and provide the communication with cooperative diversity gains. Upon retransmission request, the relays have to contend for the access to the channel. The DQCOOP protocol has been proposed in the literature as a high-performance MAC protocol for this kind of scenario. In this paper we theoretically evaluate its performance. The analytical results are supported by computer-based simulation that show the accuracy of the analysis.

We describe in this paper a methodology to facilitate the coexistence of the Distributed Queuing Medium Access Control (MAC) protocol for Ad hoc Networks (DQMAN) with the IEEE 802.11 Standard MAC protocol. By using dual terminals which can operate with the two MAC protocols, it is possible to run a hybrid network with legacy and novel terminals. In addition, it is possible to enable the intercommunication between all the terminals of the network. Computer simulations have been carried out to assess the performance and the feasibility of such dual operation. The main conclusion of the study is that the use of dual terminals allows the integration of DQMAN within legacy networks without degrading the performance of IEEE 802.11 terminals. In addition, the mechanism presented in this paper can be used to enable the coexistence of any cluster-based distributed MAC protocol within IEEE 802.11 networks.

Machine-to-Machine (M2M), an emerging communications paradigm, is a facilitator of data flows between machines used, e.g., in mission-critical applications. Focusing in this paper on embedded multihop M2M implementations with myopic routing, a prime design aim is to jointly minimize energy expenditure and the end-to-end delay whilst not jeopardizing the network's operational reliability. We thus evaluate the achievable energy-delay tradeoff in such embedded multihop wireless networks with focus on channel coding. Supported by analytical as well as numerical evaluation, we show that channel coding has a profound impact on the end-to-end performance of M2M systems. We also detail some viable design guidelines which quantify which channel coder to use under which operating conditions. Finally, we formulate the problem in more general shadowing conditions.

We present in this paper the analysis of an IEEE 802.11 network in the presence of hidden and exposed terminals when a Cooperative ARQ (C-ARQ) scheme is executed at the MAC layer. When spontaneous helpers transmit to assist in a failed transmission, the area exposed to the original transmission increases in comparison to non-cooperative ARQ schemes. In this paper we quantify this effect and we evaluate the relevance of designing efficient protocols to combat the exposed terminal problem.

This paper provides details and performance evaluation of the implementation of the Persistent Relay Carrier Sensing Multiple Access (PRCSMA) protocol for Cooperative ARQ (C-ARQ) scenarios using off-the-shelf wireless cards. The underlying idea of PRCSMA is to modify the basic rules of the IEEE 802.11 MAC protocol to execute a distributed C-ARQ scheme in wireless networks in order to enhance their performance and to extend coverage. For the implementation, we modify the HostAP driver for off-the-shelf air-interfaces. HostAP is a Linux based driver for 802.11b WLAN cards based on Intersil's Prism 2.5. Performance evaluation tests provide proof-of-concept of the efficiency, in terms of mean delay, of the PRCSMA in realistic conditions.

We present in this paper the analytical evaluation of a simple mechanism to reduce the average transmission delay of master stations in a network based on the Distributed Queuing MAC protocol for Ad hoc Networks (DQMAN). When DQMAN is executed, the network is self-organized into dynamic and spontaneous master-slave clusters. Within each cluster, a high performance MAC protocol based on a tree-splitting collision resolution algorithm which uses access minislots is executed. By allowing temporary master stations to avoid contention to get access to the channel, their average packet transmission delay can be effectively reduced compared to that of slaves. This technique provides thus master stations with higher access priority to the channel and, indeed, could be used in any MAC protocol based on access minislots to provide a subset of users with higher priority.

In this paper we describe a new MAC protocol for ad hoc networks called distributed point coordination function (DPCF). It constitutes an extension of the PCF of the IEEE 802.11 standard to operate over infrastructureless networks. It combines both the operation of DCF and PCF. Users get access to the channel through the DCF function. However, once they get access to the channel, they establish a virtual cluster where the PCF coordination function can be executed. The performance of DPCF in single-hop and multi-hop ad hoc networks has been evaluated by means of link-level computer simulation. The results show that DPCF outperforms DCF in terms of throughput and average transmission delay when executed in networks without infrastructure.

The Persistent Relay Carrier Sensing Multiple Access (PRCSMA) has been recently presented in the literature as a novel Medium Access Control (MAC) protocol based on the IEEE 802.11 and modified to cope with the contention among the relays in a Cooperative ARQ (C-ARQ) scheme. We present in this paper a theoretical model to calculate the average cooperation delay of PRCSMA. Computer simulations have been carried out to assess the accuracy of the model.

A throughput analysis of the Persistent Relay Carrier Sensing Multiple Access (PRCSMA) protocol is presented in this paper. The protocol has been designed to execute a cooperative ARQ (C-ARQ) scheme in wireless networks. The model herein presented takes into account the contention time among the relays. This time consists in wasted time due to both backoff deferral periods and collisions occurred between relays attempting to access the channel in a same time slot. Link-level computer simulations have been carried out to assess the accuracy of the model and to evaluate the performance of the protocol.

Several clustering algorithms have been proposed during the last decade within the context of ad hoc and sensor networks. They allow adding some organization to an otherwise disorganized network. Although the virtual organization attained by clustering has been typically designed within the scope of routing algorithms, clusters can also enable the use of conventional highly efficient medium access control (MAC) protocols to support heavy traffic loads. In this paper we adopt a novel approach to calculate the saturation throughput of a single- hop ad hoc network when using a MAC protocol embedded in a passive clustering mechanism based on the Carrier Sensing Multiple Access (CSMA) protocol. The DQMAN protocol is considered as a study case to validate the proposal.

The distributed queuing MAC protocol for cooperative networks (DQCOOP) is presented in this paper as an innovative MAC protocol designed to coordinate the relay retransmissions in a cooperative automatic retransmission request (C-ARQ) scheme for wireless networks. The distributed cooperative ARQ scheme when the DQCOOP is applied at layer two is compared to an ideal non-cooperative ARQ scheme. Computer simulations show that the relay multiple access problem is efficiently handled by the DQCOOP and the cooperative approach outperforms the traditional non-cooperative ARQ scheme in all cases. In addition, the DQCOOP is compared with an IEEE 802.11-based MAC protocol for C-ARQ schemes, demonstrating improved efficiency.

Voice transmission over WLAN and cross-layer optimization are emerging research topics. This paper proposes two novel cross-layer mechanisms for a WLAN system where the offered traffic is of mixed nature (voice and data) while using a distributed MAC protocol. On one hand, the performance of the proposed mechanisms has been analyzed, and a significant system efficiency improvement in terms of throughput and packet delay for data nodes is shown. On the other hand, a remarkable increase in the number of simultaneous possible voice nodes is also achieved. These benefits are obtained by means of a PHY-MAC dialogue and a smart scheduling of the radio resources. Transmissions are scheduled taking into account the measurements of the channel state and the waiting time of packets in the accessing system. The two proposed mechanisms have different performance behavior as they present a trade-off between system enhancement and fairness in resource assignment among the nodes. The obtained results emphasize the advantages of the proposed schemes and the importance of Cross-Layer design in wireless communication systems.

We consider a cooperative network where the communication between a single source-destination pair is assisted by the transmission of relays. In the considered cooperative model, once a node requires cooperation, it broadcasts a claim for cooperation message. Under high mobility conditions or in some special scenarios this is the natural procedure to discover the potential relays. However, when there is a large number of nodes that can potentially serve as a relay, a Multiple Relay Access Control (MRAC) problem turns up and the relay transmissions must be organized in an effective manner. We introduce this problematic and propose three different strategies to manage this severe competitive channel. In these strategies either the source, the destination or both trigger the cooperative process. The methodology employed makes the results and conclusions valid for a wide range of cooperative scenarios, and hence, useful for a wide range of applications.

Cross-layer opportunistic scheduling on DQCA-based WLAN systems yields high performance results. This paper emphasizes the need for accurate channel state information (CSI) and presents an enhanced mechanism that provides frequent CSI updates. The proposed idea consists in introducing special update frames specifically dedicated to channel state estimation. The impact of this mechanism on the performance of the system is evaluated through simulations. The paper shows that there is a trade-off between the overall system performance and the number of update frames that are used. The periodicity of these special frames is a parameter that needs to be carefully selected, depending on the channel time-variation characteristics.

It has been shown that the cooperation among nodes can improve the performance of a network under certain considerations. So far, research focus has been mainly put on cooperation and not on the coordination required to obtain this cooperation. In this paper we discuss how the MAC layer plays a key role in determining the effectiveness of a cooperative orthogonal multiple relay channel. We propose and analyze the performance of a novel MAC protocol based on the legacy IEEE 802.11 standard in a representative and general study case scenario. The main conclusion of the study is that a proper selection of the MAC protocol being used in a cooperative system is highly relevant in order to actually achieve the benefits of this kind of systems

Cross-Layer optimization is an emerging research topic. This paper proposes two novel cross-layer mechanisms to be included in a WLAN system where a near-optimum distributed MAC protocol is used. Their performance in terms of throughput and packet delay is analysed, and a significant system efficiency improvement is shown. This benefit is achieved by means of a PHY-MAC dialogue and a smart scheduling of the radio resources. Transmissions are scheduled taking into account the measurements of the channel state and the waiting time of packets in the accessing system. The two proposed mechanisms have different performance behaviour and thus present a trade-off between system enhancement and fairness. The obtained results emphasize the advantages of the proposed schemes and the importance of cross-layer design in wireless communication systems

This paper proposes and analyses a novel MAC protocol for wireless ad hoc networks that makes use of distributed queues to achieve an efficient use of the radio channel. The protocol includes a dynamic self-configurable master-slave architecture that allows the synchronism between the nodes of the network. Computer simulations demonstrate that the proposed scheme outperforms throughput bounds achieved when using a legacy 802.11 MAC protocol for mobile ad hoc networks.