In narrowband Internet of Things (NB-IoT) networks, latency reduction is a significant issue especially regarding IoT applications with strict latency requirements. The signalling overhead constitutes the major factor of latency increase. 3GPP made several attempts to decrease the signalling overhead for massive machine type communications (mMTC) in NB-IoT networks. Among that attempts, early data transmission (EDT) is one of the latest efforts that reduces the signalling overhead by allowing data transmission without a connection establishment. Although EDT can support the transmission of multiple data packets besides one, it cannot deal with the randomness of data packet arrivals at buffered IoT devices. In this work, a new data transmission scheme is proposed that reduces signalling overhead by employing piggybacking. Specifically, the proposed scheme concerns buffered IoT devices, which transmit the first data packet of the buffer by participating in the contention-based random access (RA) and the following data packets on the reserved data channels without taking part in RA again, according to the proposed scheme. The effectiveness of the proposed scheme is evaluated with simulation experiments versus the number of IoT devices and mean interarrival time of data packets at each IoT device. Simulation results show that the proposed scheme reduces dropped packets, service delay and enhances throughput.

Energy conservation is crucial for clustered wireless sensor networks (WSNs) due to the bounded energy at sensor nodes. Energy conservation is achieved mainly through load balancing, which is realized with reclustering in frequent time intervals called rounds. Although in clustering the round duration is typically fixed throughout the network lifetime, the use of a dynamic round duration based on energy exhaustion at sensor nodes has also been proposed. Even though many attempts have been made to determine the round duration dynamically, all of them focus on WSNs. However, in mobile WSNs (MWSNs), apart from energy exhaustion, packet loss due to node mobility is also an important issue to be considered in the case of dynamic round duration. To investigate the problem of the calculation of the dynamic round duration in MWSNs, in this article, we evaluate the applicability of an existing recluster triggering scheme initially proposed for WSNs, which is adaptive to energy consumption at cluster heads (CHs). We also propose a new scheme, the lost ratio reclustering (LRRC), which provides adaptivity to the percentage of mobility-induced packet losses at each CH. Furthermore, the case of reclustering in fixed time intervals is also considered. Simulation results show the effectiveness of the LRRC scheme in increasing the packet delivery ratio (PDR) and reducing the clustering energy overhead.

Communication in Body Area Networks (BANs) involves extremely weak signals because of safety regulations. Human mobility adds one more layer of complexity as it has an effect on path loss depending on the activity. In this paper, we improve the quality of service (QoS) by searching for the lost nodes. Specifically, an Emergency Phase (EP) is added after RAP1 of IEEE 802.15.6-2012 superframe. The connected nodes transmit rescue beacons to reach distressed nodes, i.e. nodes that are disconnected. If a distressed node receives a rescue beacon, it participates to the current EP. The packets are buffered and relayed to the hub by the connected nodes. Our results show that when EP is enabled it is feasible to reach more nodes.

In wireless mobile sensor networks (WMSNs), the majority of clustering algorithms employ random waypoint (RWP) to consider mobility in nodes. In this work we evaluate the performance of clustering under RWP and random walk (RW) derivatives mobility models through simulations, since a particular mobility model cannot represent the mobility behavior of all nodes. The performance evaluation is conducted for two clustering approaches; the non-overlap and overlap clusters. In the first, a non-cluster-head node belongs exactly to one cluster, while in the second to more than one cluster. The latter is quite realistic and scarcely examined in the literature. Simulation results show that the performance of a clustering algorithm is strongly affected by the node mobility model selection and by many aspects of the model.

High level of security is essential in wireless communications multimedia applications. The last few years there has been an increase in research interest in the potential of the radio channel's physical properties to provide communications security. These research efforts investigate fading, interference, and path diversity to develop security techniques for implementation in wireless systems. In this paper, we propose an enhancement to existing physical layer security schemes, taking advantage of the characteristics of the OFDM technique. An OFDM symbol includes the pilot sub-carriers, essential for the pilot channel estimation process performed at the legitimate receiver. In this work we propose the positions of the sub-carriers to change on every OFDM symbol following a probability distribution known only to the legitimate transmitter and legitimate receiver. An eavesdropper receiver, does not have access to the information of the pilot sub-carriers positions so, it performs blind channel estimation. The theoretical analysis is based on the information theoretic problem formulation and is confirmed by simulations. The performance metrics used are the secrecy capacity, the outage probability, the channel mean square error and the bit error rate. The proposed scheme is very simple and robust, strengthening security in multimedia applications.

ABSTRACT This paper presents a distributed medium access control (MAC) protocol for low data rate ultra‐wideband (UWB) wireless sensor networks (WSNs), named LA‐MAC. Current MAC proposal is closely coupled to the IEEE 802.15.4a physical layer and it is based on its Impulse‐Radio (IR) paradigm. LA‐MAC protocol amplifies its admission control mechanism with location‐awareness, by exploiting the ranging capability of the UWB signals. The above property leads to accurate interference predictions and blocking assessments that each node in the network can perform locally, limiting at the same time the actions needed to be performed towards the admission phase. LA‐MAC is evaluated through extensive simulations, showing a significant improvement in many critical parameters, such as throughput, admission ratio, energy consumption, and delay, under different traffic load conditions. Copyright © 2010 John Wiley & Sons, Ltd.

Since 1990s, the investigations of aerospace communication segment have not only been concerned with satellites, but increasingly with lower altitude repeaters flying in the stratosphere. They are the so-called high-altitude platforms (HAPs) with important advantages with respect to satellites in terms of reduced cost of implementation, deployment, and launch. However, HAPs are characterized by a reduced coverage, as compared with satellites. Nevertheless, in recent literature, HAPs are not regarded as competitors of the satellite technology. On the contrary, the emphasis is on the effective and seamless integration among heterogeneous aerospace segments (GEO, LEO, and HAP) and aerospace segments with terrestrial wireless networks in order to globally extend the broadband wireless connectivity. This paper is focused on the role of HAPs in providing global connectivity in future communication systems and services. Potentialities, enabling technologies, and challenges are presented from the perspective of the integrated terrestrial/HAPs/satellite communications infrastructure.

Due to the AMC (Adaptive Modulation and Coding) mechanism adopted in the 802.16e physical layer, each connection's spectrum requirements depend on its SNR (Signal-to-Noise Ratio). Meanwhile, according to the OFDMA functionality, the limited transmission power of every terminal is divided among the simultaneously allocated subcarriers, i.e. a terminal's uplink SNR is reversely proportional to the number of occupied subcarriers. In this framework, the present paper introduces a Radio Resource Management (RRM) algorithm capable of minimizing bandwidth demands via adequately manipulating the subcarriers' assignment. The number of allocated subcarriers is not dictated solely by the connections' bitrate requirements, but it is particularly chosen so as to optimize the bits per symbol ratio via toggling between different AMC schemes.

In order to overcome the inherently challenging conditions of the mobile environment, the physical layer of 802.16e has utilized an Adaptive Modulation and Coding (AMC) scheme, capable of compensating for any SNR (Signal-to-Noise Ratio) fluctuations. As a result, the actual bandwidth demands of each data flow are not solely defined by its datarate requirements, but they are also highly dependent on the terminal's channel conditions. In this respect, the present paper introduces an efficient CAC (Connection Admission Control) algorithm, capable of optimizing the network revenue while guaranteeing the required QoS, under the bandwidth-limited and time-varying conditions of the IEEE 802.16e architecture. The proposed algorithm makes virtue of the terminals' SNR patterns so as to maximize the accuracy of the resource reservation predictions.

Ultra-Wideband (UWB) is a key solution for wireless connectivity, characterized by ultralow power consumption and a good degree of robustness to interference. Evidence of its importance, is its recent use in the IEEE 802.15.4a standard. UWB technology with joint consideration of directional antennas can benefit when compared to classical omnidirectional antennas from the energy conservation viewpoint, which is of fundamental concern when it comes to wireless sensor networks (WSNs). However, exploiting directionality requires new approach in the design of a medium access control (MAC) protocol to be applied. In this work, idle nodes continuously rotate their receiving beams over 360 degrees until a predefined preamble trailer is detected. The resulting scheme is a directional ultra-wideband MAC protocol, named DU-MAC, which deals effectively with the problem of deafness and the problem of determination of neighbors' location. Simulation based studies would demonstrate the effectiveness of our protocol in many critical parameters.

In this paper, a distributed on-demand routing protocol for Low Earth Orbit (LEO) satellite systems, named multiservice on-demand routing (MOR), is proposed and evaluated. The proposed protocol adjusts the routing procedure to the QoS requirements of different traffic classes. The performance of the MOR protocol is compared to the unique proposal for traffic class dependent routing in the literature and the good characteristics of the proposed scheme are corroborated by ample simulation experiments, where significant gains in performance are witnessed.

In this paper, we propose a lightweight distributed routing algorithm, called Distributed Load-Aware Routing (DLAR). The proposed protocol adopts a distributed approach to handle the complexity of the satellite system and at the same time provides a hop-by-hop mechanism for splitting traffic load in order to alleviate the problem of congestion that occurs near polar regions. The performance of DLAR is assessed through extensive simulations and compared to the performance of centralized routing schemes proposed so far in the literature. Simulation results document and confirm the positive characteristics of the proposed protocol.

Sensor networks are gaining more and more interest in the research community, due to their unique characteristics. The most important problem, though, is energy consumption, which limits the lifetime of the network. Clustering has been proposed as a technique for organizing sensors in order to prolong their lifespan. Although many clustering algorithms have been proposed, very few of them are especially designed for the ultra-wideband (UWB) technology. UWB is a key solution for wireless connectivity characterized by ultralow power consumption and a good degree of robustness to interference. Based on these principles, we propose a novel energy-aware clustering algorithm for UWB sensor networks. Simulation based studies would demonstrate the superior performance that it is achieved by the proposed algorithm.

Recently, Multi-User Selection Diversity (MUSDiv) for single-carrier systems has been under extensive study on account of the enhancement it provides to system performance with minimum feedback requirements. However, its application to multichannel systems is considered straightforward and thus, it has not been thoroughly examined. In this paper, the performance of MUSDiv is investigated when applied to the spread-spectrum multi-carrier multiple-access system, where the scheduling has to be performed for all the available channels and self interference must also be considered. Specifically, based on the absolute and normalized Signal-to-Noise-Ratio (SNR) scheduling algorithms, two algorithms are presented, modified and optimized, so that they can be applied on a subband instead of a single-channel basis. Moreover, we propose a new scheduling scheme which constitutes a trade-off between the previous schemes, concerning fairness and capacity performance. The new algorithms are related to the symbol Signal-to-Noise-plus-Interference-Ratio (SNIR) instead of SNR and two interference models were devised to this end. Closed-form expressions for the system capacity are extracted for each case, which are compared with simulation results. The research is also extended to the case of non-identical user power profiles among the available subcarriers. The channel state information required to utilize multi-user selection diversity is already necessary for the most common combining schemes, so no more feedback is actually needed.

In this paper, a location-assisted on-demand routing (LAOR) protocol for low earth orbit (LEO) satellite systems is proposed and evaluated. This protocol can be viewed as a variant of the well-known ad-hoc on-demand distance vector (AODV) routing protocol, tailored to the requirements imposed by the characteristics of LEO satellite systems' topology. The LAOR protocol is assessed for different link-cost metrics and compared to centralized routing protocols proposed in the literature thus far. Simulation studies further document and confirm the positive characteristics of the proposed protocol.

Ultra-wideband (UWB) transmission is an emerging wireless communication technology with unique potential characteristics, such as high-rate, low-power transmission, immunity to multipath propagation and precise positioning capability. This technique, based mainly on the Impulse-Radio (IR) paradigm, has gained interest significant for future broadband wireless access. The unique characteristics not only pose unprecedented challenges, but also offer great flexibility in the design process of efficient protocols for Wireless Multimedia Sensor Networks (WMSNs), i.e., networks of interconnected sensing devices capable of retrieving and relaying multimedia content, such as video and audio streams, still images, and scalar sensor data, over the wireless medium. In this paper we discuss the applicability of the UWB transmission technique in WMSNs and we identify potential challenges in this area. Finally, we identify the cross-layer dependencies between the specified physical layer and the higher layers of the communication stack and investigate open research issues.

In this paper we propose an UWB-based location-aided MAC protocol for wireless sensor networks, named LA-MAC. Current MAC proposal is closely coupled to the IEEE 802.15.4a physical layer and it is based on the Impulse-Radio (IR) paradigm. LA-MAC exploits the precise position location provided by the UWB transmission technique in order to assess the channel conditions more accurately and enhance the network performance by wisely allowing simultaneous data transmissions to occur. The produced interference estimation model in conjunction with the distributed admission control scheme, leads to better interference predictions and blocking assessments and thus improves channel spatial reuse efficiency and data throughput. The proposed MAC is evaluated and compared to traditional MAC protocols, like the 802.15.4 that does not take advantage of the UWB peculiar characteristics, showing a significant improvement in terms of network throughput, energy consumption and average MAC delay.

An ad-hoc network is a network whose particular characteristics include absence of any kind of infrastructure, dynamic topology and limited resources. Up to day, several routing protocols have been proposed to face such a hostile environment. These solutions usually provide to the sources of data flows with single valid paths towards their destinations. However, it has been proven that maintaining multiple paths can be favorable to the network, either using the paths at the same time (for load balancing) or using an alternative path, when the primary one fails. It is also a way to meet the Quality of Service requirements demanded by some particular applications. In this paper, we propose a novel routing mechanism based on Dynamic MANET On-demand (DYMO) routing protocol, in order to enable the establishment of more than one node disjoint routes towards the same destination. Simulation results show that there is a significant gain in end-to-end delay for data packets and a reduction in the number of the transmitted routing packets.

In this paper we propose a handover method for multiservice non-geo satellite systems. The proposed algorithm can be used for satellite handover as well as for cell handover when a fixed amount of resources is allocated to each cell. Specifically, we consider the case in which the satellite system provides services to users of different QoS requirements. The first type of users require the minimization and, if possible, the elimination of the probability of forcing a connection in progress to termination during a handover. On the other hand, the users of the second category do not have any specific handover requirements. The new method is evaluated through extensive simulations and is proved to meet the requirements of the first type of users while at the same time maximizes the system utilization for both types of users.

Wireless local area networks own their popularity mainly to the robust characteristics of distributed medium access schemes. However, as new user applications diverge from the traditional data-centric model, the introduction of distributed medium access methods that achieve high efficiency in terms of both throughput and fairness becomes of utmost importance. IEEE 802.11 is the most widely used access scheme for wireless networks. In this paper, we address a shortcoming of IEEE 802.11 as indicated by previous studies, namely the inability of the protocol to adapt to different working conditions. We study and evaluate a low complexity mechanism that allows a network employing the IEEE 802.11 protocol to adapt its operating parameters according to the offered load. Simulation studies further document and confirm the high throughput and fairness performance of the proposed scheme.

The integration of a satellite system with a HAPS segment appears very suitable to provide communication services, including Internet access, for a large set of applications. In fact, the satellite capability to provide wide coverage and broadband access can be enhanced by the use of cost-effective, mobile/portable and low-power consuming user terminals, when HAPS acts as an intermediate repeater. Moreover, also TCP-based applications, which suffer from long latency introduced by the satellite link and in general by errors, can get benefits in terms of end-to-end performance. In this frame, this paper deals with the introduction, on board the HAPS, of an efficient CAC scheme in order to guarantee an optimal utilization of the precious radio resources. In particular, we propose an innovative TCP driven CAC algorithm, which shall take into account not only the QoS requirements, but also TCP statistics obtained through a proxy installed on the HAPS. Results show that the overall system performance in terms of both average throughput and blocking probability is significantly improved.

Analytical expressions concerning the capacity and bit error rate (BER) of multiple-input multiple-output systems with space-time block coding (STBC) are derived. Two fading environments are examined, log-normal and Rician channels. A tight closed-form upper bound is presented for the BER of systems operating in log-normal fading environments in addition to an upper bound for the capacity of this type of systems. The latter bound applies to systems that operate under Rician fading as well. The analytical results were validated against ample numerical simulations for three STBC schemes and three phase-shift-keying modulations. The proposed bounds proved to be a tractable way to evaluate the system performance when no closed-form expression for the probability density function or the moment generating function is known.

The performance of a DMT system, which transmits information over channels with varying characteristics through the frequency zones, is improved when the subchannels of the system are loaded with variable data rate. In this study, we meet the bit loading problem of the subcarriers of a DMT system. We propose two new optimum-loading algorithms with low computational complexity. These algorithms assign integer number of bits successively until a target bit rate is fulfilled. We compare them with several existing algorithms in terms of transmitting energy versus data rate and complexity versus system's characteristics i.e. number of subcarriers and target data rate. Simulations prove that without sacrificing optimality both our proposals present a lower complexity level. Copyright © 2006 John Wiley & Sons, Ltd.

Systems that provide ground movement management at airports, maintaining ground safety and increasing air traffic capacity, are called A-SMGCS (advanced surface movement guidance and control systems). However, common A-SMGCS systems, based on surface movement radars (SMR), are affected by limitations in their coverage due to reflections or shadows from objects on the airport surface. Hence, the use of a complementary system is necessary to act as a "gap-filler", i.e. to provide accurate ground movement information for these problematic areas. This paper aims to present a novel cost-effective video-based system to act as a "gap-filler" for existing A-SMGCS systems. The proposed system consists of a network of intelligent digital cameras, which are able to detect the presence of an aircraft or vehicle in specific locations within their fields of view, and then provide this information to a multiple hypothesis tracking (MHT) algorithm to extract target tracks.

Motivated by the suitability of the Weibull distribution to model multipath fading channels, the second order statistics and the spectral efficiency (SE) of L-branch SC receivers are studied. Deriving a novel closed-form expression for the probability density function (pdf) of the SC output SNR, the average level crossing rate (LCR), the average fade duration (AFD) and the Shannon's average SE, at the output of the SC, are derived in closed-forms. Our results are sufficiently general to handle arbitrary fading parameter and dissimilar branch powers.

Abstract Next Generation Mobile Systems will implement one's dream of anywhere, anytime communications. Providing global coverage and mobility, satellite systems will play an important role in order for this to be achieved. The design options of satellite systems include several choices. The basic ones are the choice of orbit, processing and switching, intersatellite links, and link layers technologies. In addition to the network architecture, the protocols must be developed for efficient use of the satellite radio resources. The most important protocol design issues concentrate on Medium Access Control (MAC) techniques, routing and traffic management. In this paper some of the special features of satellite systems are considered. Emphasis is given in MAC protocols design, as well as in research programmes that led to their development. Finally, an adaptive hybrid access scheme that combines CDMA technology and random access techniques is presented. This new scheme is called CDMA/PRMA and from the results obtained it is a very promising access technique for next generation satellite systems. Copyright © 2002 John Wiley & Sons, Ltd.

Wireless mobile networks are characterized by stochastic topology variations. Sudden movements of mobile hosts in and out of each other's range impose limitations on the system performance. Routing protocols have been proposed for producing stable routes. All these protocols address the issue of route stability rather than connection stability. This paper proposes a new protocol for enhancing stability of connections rather than routes, without increasing the related overhead. The new protocol is based on the employment of a number of disjoint paths in each route discovery phase. Its performance is evaluated by simulations in the Java programming language that present adaptability to increased mobility.

This paper deals with the performance evaluation of various path selection strategies that are suitable for non-GEO satellite systems. The modified Dijkstra (1959) algorithm is used for different link cost functions. These various cost functions result to different performance in terms of system utilization, rerouting failure probability and delay parameters. In particular four metrics have been examined for the establishment of paths. The first two are based on the propagation delay and the traffic at the network whereas the other two are based on new, special factors that characterize the operation of inter-satellite links (ISLs). The scenarios are tested through extended real-time simulations and fruitful conclusions are derived at the end of the study.

In this paper, we derive an analytical framework for the performance evaluation of an M-ary orthogonal scheme based on differential encoding/decoding of the Walsh/Hadamard chips prior/after spreading. This technique makes feasible nonpilot-assisted detection over fast fading environments such as the land mobile satellite (LMS) channel. Our results show that differential M-ary orthogonal signaling presents very good performance at Doppler frequency shifts much higher than the symbol rate. Amplitude statistics are considered to be Rayleigh, but may be easily extended to more general models based on the analytical derivation presented.

This paper presents a DS/CDMA system based on M-ary orthogonal signaling and Reed-Solomon (RS) coding for voice and data cellular communications. The proposed system is analysed under Rician or Rayleigh fading, either frequency selective or nonselective depending on the transmission bandwidth. Lognormal shadowing and a two-slope path loss channel model are considered. Numerical together with simulation results and comparison with previous work verify that the system under consideration consists a strong candidate for cellular communications.

An extended comparison and a classification of the control algorithms for rearrangeable switching networks is tried in this study. Besides of the failure probability P/sub f/ a new evaluation criterion is introduced, the mean number of rearranges R, the network performs in order to satisfy incoming calls. A simulation comparison for eight control algorithms concerning Clos-type networks and networks with 2/spl times/2 elements is attempted with very helpful results for the network design. Furthermore, some deadlock conditions discovered in one of these algorithms are completely recovered. The comparison is carried out for both evaluation criteria (R/sub m/ and P/sub f/) and it is shown that the new criterion leads to a deep insight of the systems, necessary for a successful network design.

The impact of innovative chip waveforms on the performance of a direct sequence code-division multiple-access (DS/CDMA) packet mobile radio network is investigated, assuming either Rayleigh or Nakagami-m frequency selective channels and a differential phase shift keying (DPSK) receiver with selection diversity. A constant improvement of the bit-error rate (BER) and the throughput is presented throughout the study.

In this paper a direct sequence spread-spectrum multiple access communication system is studied for a frequency selective mobile radio channel. The reverse link (mobile to base station) is analysed, assuming Rayleigh fading channel and K asynchronous users, with M orthogonal sequences per user. The analysis is based on the consideration of the quadrature components of the signal and noise. The proposed receiver is a modified RAKE receiver that improves the performance of the system at medium signal to noise ratios (SNR), giving the possibility of applying this system in a high noise cellular mobile radio environment.

A voice/data cellular system is studied. Analytical methods are used based on generating functions techniques. An extended simulation analysis was also carried out, defining the main performance parameters and their impact on system behaviour. The limits of analytical study are investigated and solutions through simulation methods are proposed.>

Two-dimensional traffic models for cellular mobile systems are formulated and the main performance measures are evaluated. System analysis in its general form is rather complex but a solution is always attained in closed form or by numerical techniques.>