Control data for unmanned aerial vehicles (UAVs) are characterized by stringent reliability and latency requirements, thus transmission parameters must be suitably set. In particular, packet retransmission can significantly improve the reliability but risks compromising latency requirements. In this paper an efficient proactive retransmission scheme based on channel classification is proposed. The retransmission occurs without waiting the receiver feedback, and is selectively used for a subset of packets. Differently, classical proactive schemes foresee the retransmission of all packets with a consequent waste of resources. The selection is based on the classification of the channel behavior before the next channel state report and on the aging of the channel state information. A deep recursive neural network is used to classify the UAV channels. Numerical results show that the proposed approach allows to reach a suitable tradeoff between reliability and spectral efficiency compared with different solutions.

Abstract Densification of networks through heterogeneous cells deployment is considered a key technology to satisfy the huge traffic growth in future wireless systems. In addition to achieving the required communication capacity and efficiency, another significant challenge arises from the broadcast nature of wireless channels: vulnerability to wiretapping. Physical-layer security is envisaged as an additional level of security to provide confidentiality of radio communications. Typical characteristics of the wireless channel (noise, interference) can be exploited to keep a message confidential from potential eavesdroppers. In particular, heterogeneous networks (HetNet) have inherent security features: while the legitimate user can benefit of the HetNet architecture, the eavesdropper is strongly affected by the inter-cell interference. This paper presents an overview of HetNets intrinsic security benefits, mainly focusing on users association and resource allocation policies. In particular, allocation of radio resources is a poorly investigated topic when related to information security. However, in systems with a large radio resource reuse like HetNets, co-channel interference can be suitably exploited to resist to the eavesdropper. This paper presents a new framework for radio resources allocation using reinforcement learning (Q-learning) to increase the security level in HetNets. A coordinated scheduling among different cells using the same radio resources is proposed based on the exploitation of the spatial information. The goal is to optimize the security at physical layer. The reinforcement learning approach represents a feasible and efficient solution to the proposed problem.

Although combining Full Duplex (FD) and Cognitive Radio (CR) technologies can provide several benefits, the presence of the residual self-interference (RSI) at the FD transceiver may influence the spectrum sensing capabilities. In particular, focusing on the blind eigenvalue-based detectors (EVDs), this paper shows that operating at mmWave widebands, the RSI is a colored noise strongly affecting the detection performance even when it is characterized by low power. Consequently, we propose here a whitened detector whose novelty consists in the real-time computation of the whitening matrix, differently from previous works which employ an offline computation. The proposed whitening matrix computation uses a sample covariance matrix (SCM) regularization tailored for the FD scenario, instead of the conventional SCM estimator. Particularly, we prove that the proposed whitened detector is robust against RSI power uncertainty, attaining a constant false alarm rate under typical FD operating conditions. Finally, we show that our method provides computational load reduction and faster convergence to the theoretical false alarm probability characteristic of the white noise case. Numerical simulations validate the theoretical analysis confirming the better performance of the proposed whitening approach, particularly when applied to the sphericity test, in comparison to the offline-based approach.

Recently, with the advances in self-interference cancellation, full-duplex (FD) techniques have become a feasible approach to improve the spectrum usage in new generation wireless systems. In particular, in cognitive radio networks, FD allows a secondary user to simultaneously sense the spectrum and transmit, improving the network efficiency. However, the residual self interference (RSI) arising from an imperfect cancellation, represents a bottleneck for the spectrum sensing performance. This paper analytically demonstrates that RSI represents a colored noise which leads to the SNR Wall phenomenon making the typical OFDM signal detectors no longer robust. To overcome this drawback, we propose a generalized likelihood ratio test detector exploiting the multipath correlation due to the primary user’s (PU) channel. It is proven that the proposed method is not affected by the SNR Wall, hence it is robust w.r.t an imperfect self-interference cancellation. Specifically, best detection performance is achieved when an approximated knowledge of the maximum delay spread of the PU channel is available, otherwise it is shown that a limited performance degradation occurs. Moreover, provided results highlight that our approach outperforms the classical alternatives in different scenarios, including when PU employs a mixed numerology OFDM signal, which characterizes the 5G New Radio waveform.

Spectrum sensing plays a key role in cognitive radio technology to acquire information on the occupancy status of the channel. Cyclostationary spectrum sensing is considered one of the most promising approaches. However, its performance is severely degraded by a mismatch between the actual and the nominal cyclic frequency [i.e. cyclic frequency offset (CFO)]. This is particularly true for vehicular networks, due to the high mobility and particularly to the presence of a non‐zero radial acceleration between the transmitter and the receiver that makes the signal filtered out by the Doppler channel chirped, thus introducing the CFO. Consequently, the signal to be detected has to be properly modelled as a generalised almost‐cyclostationary process. The proposed approach performs a maximum‐likelihood estimation of the CFO jointly to the detection exploiting multiple cycles, through a maximisation of the generalised likelihood ratio test. The closed‐form of false alarm probability is derived highlighting that this approach represents a constant false alarm rate detector. Numerical evaluations are provided to show the correctness of the theoretical analysis and that the performance is approximatively constant for a large range of CFO values. Moreover, comparisons with existing algorithms are provided.

The explosive growth of mobile traffic needs that the fifth generation of wireless systems supports a thousand-fold increase of network capacity, thanks to new paradigms such as network densification. However, in dense networks interference can be a limiting factor, and must be suitably managed. In this paper multiple cells coordinate to mitigate the mutual interference: cross-tier interference is limited by means of the traditional enhanced Inter Cell Interference Coordination approach, while a new co-tier interference management strategy based on Matching Theory is proposed. The objective is to provide a stable matching between the set of small cells and the set of available subframes left almost blank by the macrocell and used by the small cells to communicate with their most vulnerable users. The proposed solution aims at improving the throughput of most critical users with an affordable computational complexity. The effectiveness of the proposed scheme has been verified in comparison with benchmark methods.

One of the main challenges of supporting massive machine-type-communications (MTCs) by future mobile networks, is the radio access network (RAN) congestion. A viable solution is a hierarchical architecture where cluster-grouped machines are indirectly connected to the mobile network through a cluster gateway. This paper considers such an architecture and focuses on radio resource allocation to machine-clusters where resources can be spatially reused for concurrent transmissions. In particular, resources can be accessed simultaneously by machines of different clusters and by different communication phases, thus resulting in an interference-limited system. Three different resource partitioning schemes are proposed and evaluated, mainly differing on how interference is taken into account. The aim is to minimize the packet loss probability, and thus maximizing the spectral efficiency. Numerical results show the effectiveness of the proposed schemes in comparison to benchmark methods. In particular, by joining resource reuse and interference-mitigation, the efficiency improves significantly.

The use of satellite communications to support the Internet of Things paradigm is of paramount importance for application scenarios where machines are spread in remote and wide areas. In such context, this paper proposes an efficient hierarchical radio access scheme. Ground machines randomly access the radio channel to communicate with their cluster heads, which in turn relay the cluster's aggregated traffic toward the satellite. The aim is optimizing the resource allocation to different communication stages, allowing resource reuse among clusters, in order to minimize the packet drop probability, and hence, to maximize the system throughput. First, the optimal solution is derived for the single cluster, then it is extended to the multi-cluster scenario. Numerical results show the effectiveness of the proposed method, also in comparison with benchmark approaches.

Ultra Dense Networks (UDNs) are considered a key element to address the expected growing of communication capacity demand in an affordable and sustainable way, improving both spectral and energy efficiency. This paper focuses on the energy efficiency of a coordinated multicell multicast transmission in UDNs. In particular, a new iterative algorithm is proposed, whose goal is to maximize the energy efficiency of a single-group multicast service. Toward this goal, the cells are activated opportunistically, only when needed, using suitable transmission power. Moreover, small cells use adaptive beamforming weights in order to increase the efficiency of the radiated power. The effectiveness of the proposed method is proven in terms of energy efficiency and in comparison with different benchmark solutions. Benefits and limits of network densification are evaluated.

Energy efficiency (EE) is one of the main requirements for future fifth generation wireless networks to allow a sustainable network development. Toward this goal, this study proposes a novel iterative algorithm to maximise the EE of multicast services in ultra dense networks. Macrocells and small cells cooperate using a multicast beamforming strategy to efficiently radiate the power in the considered area. Moreover, small cells are activated opportunistically, and their power levels are adjusted suitably. Due to the high complexity of the optimisation problem, the proposed algorithm works calculating iteratively beamforming weights and power adjustments, thus reducing the set of possible solutions. Then, an exhaustive search is performed among the elements of this restricted set. This solution has reduced computational complexity, and the achieved EE is higher than benchmark alternatives. Performance in terms of EE and complexity is provided together with a solution feasibility evaluation.

Packet collision is a well-known severe impairment in ALOHA-based networks and, more generally, in any random access network. As a consequence, several methodologies have been considered with the aim of lowering the resulting performance degradation. A novel multiuser detection scheme exploiting channel diversity to separate collided signals is proposed here to enable multiple packet reception capability in random access schemes. The performance of this approach is analytically derived and optimized in terms of network throughput, and compared with that of different alternatives. The overall performance in terms of throughput shows the effectiveness of the proposed scheme in different operating conditions.

Power line communication technology receives today a renewed attention for a wide class of advanced applications and services, thanks to the advantage of using the existing electrical infrastructure, which requires only limited deployment costs. However, wires have been originally designed for power and not for data, and signal propagation is affected by several impairments such as impulsive noise and frequency selectivity. On the other end, new emerging application perspectives-mainly related to the smart grid or Internet of Things paradigms-need reliable unicast and/or multicast information delivery methodologies. This paper deals with an efficient multicast power line communication system based on the use of a decision direct impulsive noise mitigation scheme and of multiple transmissions of symbols in the frequency domain, allowing an efficient exploitation of the communication channel frequency diversity. The performance of the reliable multicast power line communication system is derived by resorting to the application of the absorbing Markov chain theory in terms of mean overall delivery delay and throughput, in relation to different application scenarios. Moreover, a suitable optimization procedure is also proposed here in order to achieve the best performance. The advantages of the proposed optimized reliable multicast power line communication system are finally validated by comparisons with a classical system.

The most recent trend in the Information and Communication Technology world is toward an ever growing demand of mobile heterogeneous services that imply the management of different quality of service requirements and priorities among different type of users. The long-term evolution (LTE)/LTE-advanced standards have been introduced aiming to cope with this challenge. In particular, the resource allocation problem in downlink needs to be carefully considered. Herein, a solution is proposed by resorting to a modified multidimensional multiple-choice knapsack problem modeling, leading to an efficient solution. The proposed algorithm is able to manage different traffic flows taking into account users priority, queues delay, and channel conditions achieving quasi-optimal performance results with a lower complexity. The numerical results show the effectiveness of the proposed solution with respect to other alternatives.

Heterogeneous network deployment represents one of the main enhancements of future LTE-A networks to increase spectrum efficiency. However, the achievable improvements are limited by the interference that arises from a co-channel allocation of the cells. This paper proposes two cognitive resource allocation methods that work using the angle of arrival of the signals instead of the channel state information. Both methods reduce the interference towards the macro-cell User Equipments (MUEs) using Zero Forcing Beamforming to avoid transmission in the MUEs direction. At the same time they maximize the capacity of the femto-cell with a suitable allocation of the time-frequency resources to the femto-cell UEs (FUEs). The performance of the proposed methods is compared with that of a conventional maximum gain beamforming and conventional zero forcing beamforming. The results show that the proposed methods achieve a good trade off between maximization of the femto-cell capacity and reduction of the interference level at the MUEs side.

We consider a heterogeneous Long Term Evolution Advanced (LTE-A) network where a cognitive femtocell operates in the coverage area of a traditional macrocell sharing the same frequency band. A new method is proposed here to avoid the interference generated by the femtocell on the macrocell downlink communications. In particular, the estimation of the direction of arrival of the primary user signals is used to place nulls in the femtocell transmission paths by means of a suitable beamforming algorithm. The novelty of the proposed approach is that it works on a physical resource basis, taking into account the specific LTE-A access scheme. In this way it is able to manage the interference towards several macrocell users characterized by a high angle dispersion of the multipath components. The obtained numerical results show that the system is able to significantly reduce the interference in different operational conditions, especially when the physical resource blocks belonging to the same macrocell user can be clustered, thus increasing the dimension of the snapshot used for the direction of arrival estimation.

Heterogeneous network deployment is one of the key issues of future Long Term Evolution Advanced systems. However, the coexistence of high power macro-cells and low power femto-cells leads to co-channel interference problems that must be suitably counteracted. Enhanced Inter-Cell Interference Coordination allows a joint management of the resources among the cells so as to reduce the interference. The behaviour of this approach depends on the choice of some parameters, and in particular on the Almost Blank Subframes muting ratio. This paper proposes a simple approach to determine adaptively the most suitable value of the muting ratio as a function of the network traffic load, based on suitable threshold values. The performance is given in terms of network throughput percentage gain.

Resource reuse among heterogeneous cells in LTE-A networks permits to increase the spectral efficiency but requires suitable interference mitigation techniques. Among these Coordinated scheduling and beamforming (CS/CB) is a promising solution. This paper proposes a new CS/CB scheme aiming to maximizing the mean system throughput taking into account the fairness among the cells. First the most suitable CB technique selection is analysed and a sub-optimum criterion based on the reciprocal angular distance between two concurrent users is proposed. Then a smart CS algorithm that allows to pair users with sufficient angular distance is introduced. The results show that the proposed scheme achieves performance close to the optimum CB scheme selection and the smart allocation permits to increase significantly the system throughput.

This paper deals with a cognitive approach used to reduce the co-channel interference arising from the coexistence of an LTE macrocell and an LTE femtocell sharing the same frequency band. Multiple-antennas technology is considered to exploit the angle dimension as a new spectrum opportunity: femtocell acts as a secondary system and inserts nulls in the directions of the UE connected to the macrocell thus protecting the downlink transmission of the primary base station. MuSiC algorithm is used to detect the DOA of the UE signal replicas in an indoor environment assuming a variable number of multipath components characterized by a very large angle spread. The proposed scheme permits to increase the signal to interference ratio at the UE side thus reducing the achieved error rate on the downlink connection even when the number of resolvable paths is higher than the number of antennas. Finally the UE mobility is considered to take into account the effect of variable DOAs.

The radio resource shortage is one of the most important issues to be taken into account when deploying modern wireless communication systems. A novel communication paradigm named cognitive radio, has been introduced in the last years for a more efficient exploitation of the limited available spectrum and cope with the inefficiency in the spectrum usage. Its main aim is to allow the co-existence of different wireless systems on the same spectral resources by limiting the mutual interference. The aim of this paper is to design a cognitive networking environment where the primary network is based on the OFDMA principle. The proposed Medium Access Control (MAC) technique for the secondary network, named Data Aided Cognitive Technique (DACT), aims to exploit the framing information broadcast by the primary network in order to setup transparently an independent network with a particular focus on Machine to Machine (M2M) communications.

This paper deals with the performance evaluation of Decode and Forward Physical Layer Network Coding schemes under multipath fading channel propagation conditions. Physical Layer Network Coding in Two Way Relay Networks ideally allows to drastically increase the network throughput. However, under the assumption of actual channel propagation conditions, the performance quickly degrades due to high decoding error probability. With the aim of counteracting this drawback, the paper proposes a suitable approach for Orthogonal Frequency Division Multiplexing systems based on a signal pre-equalization technique. Furthermore, in order to lower the decoding error probability, three subcarriers suppression methods are introduced. These methods differ from each other for the performance achieved and the required amount of signaling that nodes have to exchange. An important result obtained here is that the combined use of channel inversion and subcarriers suppression allows to enhance the system reliability with a slight decrease of the throughput.

3GPP's Long Term Evolution (LTE) represents the one of the most valuable alternatives to offer a wireless broadband access in fully mobile network context. In particular LTE is able to manage several communication flows characterized by different QoS constrains. This paper deals with a network topology where the mobile users are clustered in Multicast Groups and the base station broadcasts a different traffic flow to each cluster. In order to improve the network throughput on a per-user basis, all communications rely on a Random Linear Network Coding (RLNC) scheme. A key aspect in the QoS management is represented by the power adaptation strategy in use. This paper proposes a novel convex formulation to the power adaptation problem for the downlink phase taking into account the specific RLNC scheme adopted by each communication flow. By the proposed convex formalization, an optimal solution of the problem can be early found in real time. Moreover, the proposed power adaptation strategy shows good performance for what concern throughput and fairness among the users when compared with other alternatives.

This paper analyzes the performance of the Physical Layer Network Coding (PLNC) approach based on Decode-and-Forward (DF) and operating in multipath fading channel. In actual channel affected by multipath fading the DF method has poor performance since the relay node cannot properly perform data decoding. In addition the decoding at the relay requires the knowledge of the channels involved in the communication with a consequent signaling overhead. In order to overcome these problems this paper proposes an Orthogonal Frequency Division Multiplex (OFDM) system where the pre-emphasis approach is considered jointly with an efficient method of sub-carrier suppression named Blind Subcarrier Suppression (BSS). Simulations results show that BSS achieves high throughput with a Bit Error Probability (BER) near to that of a classical OFDM system in multipath channel.

Long Term Evolution (LTE) is considered one of the main candidate to provide wireless broadband access to mobile users. Among main LTE characteristics, flexibility and efficiency can be guaranteed by resorting to suitable resource allocation schemes, in particular by adopting adaptive OFDM schemes. This paper proposes a novel solution to the sub-carrier allocation problem for the LTE downlink that takes into account the queues length, the QoS constraints and the channel conditions. Each user has different queues, one for each QoS class, and can transmit with a different data rate depending on the propagation conditions. The proposed algorithm defines a value of each possible sub-carrier assignment as a linear combination of all the inputs following a cross-layer approach. The problem is formulated as a Multidimensional Multiple-choice Knapsack Problem (MMKP) whose optimal solution is not feasible for our purposes due to the too long computing time required to find it. Hence, a novel efficient heuristic has been proposed to solve the problem. Results shows good performance of the proposed resource allocation scheme both in terms of throughput and delay while guarantees fairness among the users. Performance has been compared also with fixed allocation scheme and round robin.

Abstract Wireless communications have achieved a great attention during the last decades due to the easy implementation, possibility of delivering multimedia services to rural communities, suitability for public safety and emergency communications. In particular, a wireless network designed for an emergency scenario seems to be the best solution to manage all the phases of an emergency situation, from the prevention to the post‐emergency management. This paper discusses the main features of a such wireless network aiming to interconnect several heterogeneous systems and providing multimedia access to groups of people involved in emergency operations as foreseen by the In.Sy.Eme. (Integrated System for Emergency) project. In particular, the focus here will be on a communication infrastructure able to efficiently support a pervasive high performance distributed elaboration structure based on the Next Generation Grid principle. Main scope of the proposed system is that of allowing functional integration of new technologies with actual or off‐the‐shelf technologies to provide fast responses to any emergency situations and efficient use of all available resources. Copyright © 2009 John Wiley & Sons, Ltd.

Wireless communications have achieved a great attention during the last decades due to the easy implementation, possibility of delivering multimedia services to rural communities, suitability for public safety and emergency communications. In particular, a wireless network designed for an emergency scenario has to be capable of managing all the phases of an emergency situation, from the prevention to the post-emergency management. This paper discusses the main features of a wireless network aiming to interconnect several heterogeneous systems and providing multimedia access to groups of people by introducing the main outcome of the In.Sy.Eme. (Integrated System for Emergency) project. Particular attention will be devoted to the approach of the project, aiming to consider a communication infrastructure strictly connected with an information processing side.

Cooperative relaying systems represent a promising solution for future wireless communications. In this paper simple cooperative relaying strategies are proposed in order to improve the performance of a WiMAX network, overcoming the drawbacks of a non-line-of-sight (NLOS) propagation. The cooperative schemes proposed combine the amplify and forward (AAF) and the decode and forward (DAF) algorithm with delay diversity (DD) and cyclic delay diversity (CDD) techniques, in order to exploit the features of the wireless communication channel. We investigated the pros and cons of each scheme, comparing the assumptions which are required and the simulation results with respect to two cooperative implementations of the Alamouti scheme.

WiMax is one of the most important technologies for providing a broadband wireless access (BWA) in a metropolitan area. The use of OFDM transmissions has been proposed to reduce the effect of multipath fading in wireless communications. Moreover, multiple access is achieved by resorting to the OFDMA scheme. Adaptive subcarrier allocation techniques have been selected to exploit the multiuser diversity, leading to an improvement of performance by assigning subchannels to the users accordingly with their channel conditions. A method to allocate subcarriers is to assign almost an equal bandwidth to all users (fair allocation). However, it is well known that this method limits the bandwidth efficiency of the system. In order to lower this drawback, in this paper, two different adaptive subcarrier allocation algorithms are proposed and analyzed. Their aim is to share the network bandwidth among users on the basis of specific channel conditions without loosing bandwidth efficiency and fairness. Performance comparisons with the static and the fair allocation approaches are presented in terms of bit error rate and throughput to highlight the better behavior of the proposed schemes in particular when users have different distances from the BS.

The demand for high-speed services and multimedia applications anywhere and anytime has led to the rise of wireless communications. In particular, WiMAX technology is nowadays considered one of the most prominent solutions capable to provide a Broadband Wireless Access (BWA) in metropolitan areas with a simpler installation and lower cost than traditional wired alternatives. This paper deals with the proposal of efficient adaptive modulation and coding techniques to be used in WiMAX based wireless networks, that allow to improve network performance in the case of Non Line-of-Sight communications, which are typical in urban environments. Through these techniques it is possible to switch the modulation order and coding rate in order to better match the channel conditions, and, hence, obtaining better performance both in terms of error probability and data throughput.

This paper deals with the problem of finding an optimal subcarrier allocation strategy for uplink and downlink communications in an OFDMA metropolitan wireless system. In particular a two steps approach is considered; at the first step the scheduler establishes the amount of resources to assign to users on the basis of their quality of service (QoS) constraints, while, at the second step, channel conditions are used to allocate subcarriers to users. A simple strategy for choosing the appropriate modulation and coding scheme according to the channel conditions of the assigned subcarriers is also proposed.

The demand for high-speed services and multimedia applications in wider wireless environments leads to the growth of wireless communications, due to the simplicity of installation and the reduction of costs with respect to traditional cabled links. In particular, WiMAX technology is considered one of the most prominent solutions capable to provide a Broadband Wireless Access in metropolitan areas. In this paper two schemes of adaptive modulation and coding are proposed with the aim of improving performances in Non Line-of-Sight communications, typical of urban environments. Through these techniques it is possible to switch the order of the modulation and the coding rate to better match the channel conditions, obtaining comforting results in terms of probability of error and throughput. The system has been modeled with a finite state structure in which every state consists in a possible scheme of transmission (i.e. a specific modulation and coding rate), and the switch among different states happens when multiple thresholds on channel attenuation are reached. The adaptation is realized at the physical level of the transmission for a WiMAX OFDMA structure. The two proposed techniques are suitable for different kind of traffic and, therefore, can be used for respecting different QoS requirements.

WiMAX is one of the most important technologies for providing a Broadband Wireless Access (BWA) in a metropolitan area. The use of OFDM transmission has been proposed to reduce the effect of multipath fading in wireless communications; moreover, multiple access is achieved by resorting to the use of OFDMA. Adaptive subcarrier allocation techniques have been selected to exploit the multiuser diversity, leading to an improvement of performances by assigning subchannels to the users accordingly with their channel responses. In this paper, three adaptive subcarrier allocation algorithms have been proposed; they are based on channel capacity and aim to assign an equal amount of capacity to all the users or to distribute the capacity proportionally to the users' channel conditions. The algorithms performances have been valued and compared in terms of bit error rate and throughput. The proposed strategies show a substantial improvement with respect to a static allocation, turning the multiuser diversity into a significant increase of data rate.

WiMAX is considered one of the most prominent technologies for providing a broadband wireless access (BWA) in a metropolitan area. Among several strategies, the adaptive modulation techniques has been selected for providing an efficient non line of sight (NLOS) coverage. Adaptive modulation allows the system to adapt the modulation scheme according to the channel conditions in order to enhance the system performance. This paper deals with the proposal of a state model to be used for the performance comparison of two different adaptation algorithms based on the maximization of different functional costs suitable for use in WiMAX system with an OFDMA physical structure. The algorithms' performance has been derived and compared in terms of error rate and throughput. The algorithms show a significant improvement of the system performance compared with the static case, i.e., no adaptive modulation; in particular each algorithm is suited for satisfying different QoS requirements.

In this paper, a selection of different multiantenna (MIMO) systems will be presented. The adopted MIMO configurations with four and two transmit antennas and two receive antennas are selected according to the IEEE 802.16e standard: diversity scheme, hybrid diversity scheme and spatial multiplexing. Computer results present an adaptive selection strategy among the considered MIMO schemes along with their performance.

WiMAX is considered one of the most prominent technologies for providing a Broadband Wireless Access (BWA) in a metropolitan area. Among several strategies, the adaptive modulation techniques has been selected for providing an efficient non line of sight (NLOS) coverage. Adaptive modulation allows the system to adapt the modulation scheme according to the channel conditions in order to enhance the system performance. This paper deals with the proposal of a state model to be used for the performance comparison of two different adaptation algorithms based on the maximization of different functional costs suitable for use in WiMAX system with an OFDMA physical structure. The performance of the algorithms has been derived and compared in terms of error rate and throughput. The algorithms show a significant improvement in the system performance compared with the static case, i.e., no adaptive modulation; in particular each algorithm is suitable for satisfying different QoS requirements.

Future ultra-wideband systems will be applied for Internet access, multimedia services, wireless interfaces and location-aware services. This paper deals with the impulse radio (IR) UWB systems with high spectral efficiency. Though the IR systems are based on the use of baseband signals, the introduction of a proper orthogonal basis permits to derive higher order constellations as a linear combination of the elements of the basis with no additional bandwidth and a higher system throughput

Ultra-wideband (UWB) systems have been envisaged as optimal candidates to provide high data rate communications within short range. In this paper, a novel cross-layer approach suitable for TH-UWB systems is proposed, named punctured hopping (PH). PH scheme permits to limit the throughput reduction of the basic full rate turbo coding scheme, while maintaining the same code free distance. The proposed coding scheme performs parity bits puncturing. However, different from the rate compatible punctured turbo codes (RCPTC) scheme, it works on the idea that the less significant bits in a coded frame are not completely discarded as only half of their impulses are punctured. Thus preserving the error correcting capabilities jointly with a better spectral efficiency. The performance of the proposed PH coding scheme has been examined using theoretical analysis in additive white Gaussian noise channel and simulations in a realistic multipath fading channel. It has been compared with the uncoded, the basic full rate, and the RCPTC systems in order to underline its effectiveness. The results show a significant performance gain in comparison with the RCPTC and, hence, a significant reduction of the performance loss in comparison with the basic full rate turbo coding scheme, while maintaining the same throughput of the RCPTC.

In orthogonal frequency division multiple access systems clusters of subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapped, but orthogonal to each other such that there is no intercarrier interference (ICI). However, synchronization errors among users cause the loss of the orthogonality and introduce ICI resulting in multiple-access interference. Synchronization between users is particularly difficult in the uplink channel where the user signals are potentially asynchronous and affected by different frequency offsets due to misalignment in carrier frequencies and Doppler shifts. This paper proposes a method to lower the effects of different frequency offsets among user signals in an OFDMA uplink system. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. An approach based on the selective cancellation method, is proposed and its performance is analyzed by means of theoretical analysis and computer simulations. The effectiveness of the proposed system has been evaluated in the case of ideal and no-ideal frequency offset estimation and has been compared with that of the classical successive and parallel cancellation schemes. Simulation results show that the proposed approach allows performance close to the ideal case, i.e., with ideal frequency synchronization among users, with a low increase of the implementation complexity. Moreover, it is also highlighted here, that the successive cancellation method slightly outperforms the selective scheme, at the expense of a higher computational complexity and processing delay

This paper deals with a novel cross-layer approach, involving both physical and data link layers, suitable for TH-UWB systems, named punctured hopping (PH). PH scheme permits to limit the throughput reduction of the basic full rate turbo coding scheme, while maintaining the same code free distance. The proposed coding scheme performs a puncturing of the parity bits, but, differently from the rate compatible punctured turbo codes (RCPTC) scheme, the less significant bits within a coded frame are not completely discarded as only half of their impulses are punctured, thus preserving the error correcting capabilities jointly with achieving a better spectral efficiency. The performance of the proposed PH coding scheme has been derived in an indoor multipath fading channel and compared with the uncoded, the basic full rate and the RCPTC systems. The results show a significant performance gain in comparison with the RCPTC and, hence, a significant reduction of the performance loss in comparison with the basic full rate turbo coding scheme, while maintaining the same throughput of the RCPTC

This paper deals with a variable rate discrete multi-tone modulation (DMT) system for broadband power-line communications (PLCs), based on the bit-loading algorithm proposed by Leke and Cioffi. In the proposed system a suitable LMS channel estimator is considered, which is based on the insertion of a training sequence (TS). The proposed approach will be compared with the ideal channel estimates, showing its effectiveness. Moreover, different TS lengths will be compared. The performance of the system, expressed in terms of bit-rate and bit error rate (BER), with and without an estimation error is derived by simulation under the assumption of frequency-selective multipath fading channel and additive colored Gaussian noise, according to the in-building networks model.

In multiple access OFDM systems (OFDMA) the separation of different users is achieved by exploiting the orthogonality of the subcarriers. However, a well-known problem in OFDM-based systems is the vulnerability to synchronization errors which leads to a loss of orthogonality. In this paper, a method is proposed to counteract the effect of different frequency offsets among users in an OFDMA uplink system with a frequency selective Rayleigh fading channel. The multiple access interference due to the user frequency misalignments is reduced by reconstructing and removing the interfering signals in the frequency domain. This is performed by means of a successive or selective cancellation method. In particular, the performance of these cancellation schemes is analyzed by assuming either an ideal or estimated frequency offset. Simulation results show that it is possible to achieve, with the interference cancellation, a performance close to the ideal case, i.e., without frequency offset among users.

This paper deals with an efficient receiver for applications in direct sequence-code-division multiple access wireless communication systems. This receiver combines a modified scheme of a parallel interference cancellation detector with antenna arrays with optimum beamforming and is used at the base station for the detection of asynchronous user signals. Each user's transmission channel is assumed to be a multipath frequency-selective independent Rayleigh fading channel. The receiver operates by dividing the signals into reliable and unreliable sets following space-time combining. The reliable signals are detected and canceled from the whole signal received at each sensor. Unreliable signals are then detected to improve the decision reliability. The receiver performance in terms of bit-error probability is analytically derived and optimized. According to the analytical and simulation results, this receiver outperforms previously proposed schemes and, thanks to its low implementation complexity, real-time operation is achieved.

In this paper, the performance of a multiuser detection (MUD) receiver is investigated for a multi-carrier-code division multiple access (MC-CDMA) system in an indoor environment. Furthermore, low complexity pilot-aided methods are introduced for estimating channel parameters in an effort to study the resistance of the proposed solution to imperfect channel estimates. Simulation results show that benefits of MC-CDMA schemes, in terms of capacity and probability of error (P/sub e/), are enhanced by means of a selective parallel interference cancellation (S-PIC) receiver (at the expense of moderate increases in overall system complexity). The proposed channel estimation methods, based on a classical mean and a newly adaptive mean square error (AMSE) algorithm, introduce low performance loss, in terms of P/sub e/ and throughput, with respect to the ideal channel estimate situation. Moreover, the AMSE shows better accuracy and faster convergence than the mean algorithm.

This paper deals with the proposal and performance evaluation of a fixed microwave communication (FMC) system that shares the same bandwidth as that of an existing UMTS network. The main application of the FMC system is for wireless connections between remote base stations and a core network access point for an UMTS network implementation in a dense urban environment, where a wired connection could be very expensive for service providers. Besides, sharing of the same frequency band could be attractive for the actual cost and lack of frequency spectrum. The mutual interference effects between the FMC and the existing UMTS systems are investigated by focusing on typical application scenarios. The performance for the two systems under consideration has been evaluated in terms of bit error rate by means of computer simulations for proposed receiving scheme where mutual interference are, firstly, detected and then canceled from the other system. The obtained results between the proposed FMC system and an existing UMTS network and the improvement of performance in terms of bit error rate with proposed receiving system.

Link-level simulations are essential in the design of UMTS communication systems. The large number of inter-dependent variables makes it impossible to derive easy design steps without an efficient modeling of the environments and the implemented reception schemes. In this paper, a novel tool for UMTS design is presented. The tool includes a fast C++ simulation engine and a complete 3GPP library to model the uplink transmission chain. As an example, a series of Monte Carlo performance simulations are described in the paper for a UMTS Rake and S-PIC receivers with real channel estimation in a multi-path fading channel.

This paper deals with a multiuser cancellation detection scheme combined with antenna arrays, for wideband code division multiple access (WCDMA) communication systems where different traffic load and different data rate are considered. The proposed receiver, defined as space time-selective parallel interference cancellation detector, is designed to be used at the base station of a WCDMA communication system. Asynchronous users and multipath fading phenomena are considered in our analysis. The proposed receiver basic assumption is to divide signals into two different groups according to the received power level. The reliable ones are directly detected and cancelled from the whole received signal, by each sensor, before making the decision on not reliable signals, without any further processing delay. Performance evaluation has been carried out in terms of P/sub e/ (bit error probability) by means of computer simulations. In particular, it is shown here that the proposed receiver exhibits a good behaviour and lowers the computational complexity in comparison with previously proposed alternatives.