This article presents a mathematical model for the traffic congestion caused by the interference due to a sudden increase in the number of users that access a social network at a given time. The rapid increase in the number of users causes a non-stationary traffic in the network, which can be modeled using a stochastic differential approach. An upper bound on the channel capacity is presented.

Cell-free systems are those in which the users are served by a large number of low cost and low power distributed antennas (Access Points - AP). These systems are no longer divided into cells and each user is served by all APs simultaneously. This paper analyses cell-free systems assuming the small-scale fading characterized by the generalized η-μ and κ-μ distributions, which proved to fit more precisely simulated data. The analyses considers the achievable rate of the users, assuming a homogeneous small-scale fading scenario. The results show that the presence of the signal specular component, expressed by the κ parameter, strongly affects the system performance.

This paper examines the capacity of millimeter waves systems which appear as an attractive alternative band for the next generation of mobile communications. The wide bandwidths available in this spectrum range along with the directional beamforming differentiate millimeter waves systems from previous mobile systems promising to dramatically increase the capacity of 5G systems. This study aims to quantify the capacity of millimeter waves channels described by a deterministic channel model based on ray tracing technique. It suggests a different way of displaying capacity estimations, which are presented as a heat map to allow easy and simple assessment. The impacts of beamwidth on directional transmissions and antenna height on the base station are also investigated.

The pursuit of improving the performance of communications systems has, in recent years, led to the popularization of the concept of cell-free system. In this system, division of space in cells is abolished and base stations are replaced by low-cost access points serving all mobile terminals in the system. In the literature, the behavior of such systems has been studied considering the non line-of-sight (NLoS) condition and, more recently, LoS condition. In this work, we propose, for cell-free systems, the use of a mixed channel model, where the occurrence of both conditions is possible, depending on the distances of the links. Analyzes were performed based on the number of devices, the density of users and access points and the quality of the channel estimation. The results indicate that the densification of the system, by reducing the area, affects the three scenarios in different ways. The mixed scenario surpassed, in terms of achievable rates, the homogeneous scenario with line-of-sight.

Cell-free wireless communications systems are characterized by the absence of base stations responsible for covering a specifically region and, then, all access points ideally can communicate with all user equipments. In this work, the performance analysis of cell-free systems subject to gain mismatches of the transceiver radio frequency circuits is presented at the access points, when they operate in the downlink mode. We also adapt a LMMSE channel estimation for this scenario. The analyses consider scenarios with two levels of mismatch, as well as the presence of pilot interference. The results have shown that the average system transmission rates are reduced when there is RF mismatch at the access points operating in transmission mode. It also demonstrated that increasing of mismatch level reduces system's achievable rates. The comparison between curves which consider or not RF mismatch during precoding have shown that the precoder which considers the mismatch gives achievable rates slightly higher.

In this paper, new exact expressions for evaluating the bit error probability (BEP) of the M ‐ary quadrature amplitude modulation scheme ( M ‐QAM) for a channel model characterized by double gated additive white Gaussian noise (G 2 AWGN) and Nakagami fading are presented. The derivation of the BEP is performed considering a method in which the multiplicative fading is transformed in an additive noise R obtained by dividing the received signal by the estimated fading envelope. In addition, the probability density function (PDF) and cumulative density function (CDF) of the random variable R that represents the aforementioned noise are obtained for the G 2 AWGN. The BEP curves as a function of the signal to permanent noise ratio for several values of the signal to impulsive noise ratio, modulation order, and fading parameters are also presented.

This paper presents a new algorithm based on the Jarque-Bera (JB) test for spectrum sensing in cognitive networks subject to Rayleigh fading. In the proposed method, based on JB statistical test, fading that affects the spectral detection is considered. The performance of the method is evaluated, using Monte-Carlo simulations, and compared with the Energy Detector (ED) performance. It is shown that the detection probability performance of the proposed method outperforms the original JB test and ED detection when the channel is subject to Rayleigh fading.

The technology of Wireless Sensor Networks (WSNs) has been applied to automatic monitoring systems, and used in a wide variety of environments. In this paper, the authors evaluate the performance of a cooperative technique, applied to heterogeneous WSNs. The main contribution is the analysis of the impacts between the energy factor, the channel Signal-to-Noise Ratio (SNR) and the packet loss rate. The analysis also considers the energy consumption aspect. The cooperative modulation diversity performance is evaluated for different propagation scenarios and for two values of interleaving depth (K). A large value for K, leads the network to present lower values of packet lost rate. Some limiting conditions are obtained and the retransmission algorithm is discussed.

A way to deal with the variations in the link quality of Wireless Sensor Networks (WSN) is the use of strategies for Dynamic Channel Allocation (DCA). The first step to perform DCA is estimating the channel quality, so that the network nodes can decide if a channel change is needed, and the best channel to be used. This paper proposes a distributed approach with nodes dedicated to monitor channel quality, by using the Received Signal Strength Indication (RSSI) and the Link Quality Indicator (LQI) to identify low quality channels. This approach is acceptable in industrial WSN, since the network deployment can be performed with adequate planning. Furthermore, the sensor nodes do not need to stop their operation for monitoring the channel quality. As a first step, experiments were performed in a real industrial environment to identify the relation between RSSI and LQI traces, and the Packet Error Rate for different channels, by using IEEE 802.15.4 radios operating in the 2.4 GHz band.

The future Internet scenario consists of a higher number of users and applications, which demand more resources from the communication infrastructure. Techniques for providing performance and scalability, such as Traffic Engineering (TE), will always be necessary even if the transmission rate is very high, because of such demands. Quality of Service is one of the solutions that can be used to improve the traffic engineering in the Internet, with the most referenced architecture: DiffServ. In general, TE needs traffic classification to accurately identify the input traffic and manage it properly. However, the current DiffServ port traffic classifier is considered outdated. This paper presents a performance evaluation of machine learning traffic classification solutions applied to DiffServ, and investigates their benefits on network performance. For a backbone network with 40 nodes, the performance of the network can increase up to 15% for both data and voice traffic.

Biologically inspired networking is becoming a very active area of research and development because methods from biology have attractive features such as scalability and resilience to changes in the environment. In this paper, the authors propose a cross-layer protocol for cooperative wireless sensor networks, inspired by the collaborative behaviour of ant groups. The fuzzy heuristic information modelling is proposed to assist the clustering and routing processes at network layer. A resultant relaying threshold is combined with an adaptive invoking of cooperative modulation diversity, at link and physical layers. Simulation results show the performance enhancement in network lifetime and packet loss rate, compared with another cross-layer cooperative system. The evaluation of energy consumption distribution and a comparison between the performance of fuzzy and crisp modelling are also provided.

In this paper, the authors propose the LF-Ant protocol for multi-hop wireless sensor networks, a cross-layer design inspired in the organized and collaborative behaviour of natural ants. At the network layer, the heuristic information is modelled by a fuzzy inference system to assist a cluster-head election and the routing process. A resultant relaying threshold is combined with an adaptive invoking of cooperative modulation diversity, at link and physical layers. Simulation results show the performance enhancement in network lifetime and packet loss rate, compared with another cross-layer cooperative system.

Wireless Image Sensor Network (WISN) is a type of sensor network, which has the ability of capturing, processing and transmitting images from a monitored environment. The need to improve the performance of such systems can be attained by using the Selective Cooperative diversity with ARQ (SCA) technique, which is basically a cross-layer design that combines Truncated ARQ at the link layer and cooperative diversity at the physical layer. In addition, the LEACH protocol has the advantage of utilizing randomized rotation of local cluster-heads to evenly distribute the energy load among the sensors in the network. This paper proposes an integrated system, SCA with LEACH, and evaluates its performance with LEACH with Truncated ARQ system. A comparison is obtained based on network lifetime, under varying propagation scenarios, and on the quality of transmitted images in a WISN, using both techniques. Simulation results show an improved performances for the proposed system.

The effect of the mutual coupling (MC) between the randomly located array elements on the performance of an adaptive antenna array (AAA) is investigated. The AAA is designed with N elements at locations xNon a line of length L. The xNare chosen from a set of independent random variables described by a probability distribution, assumed to be uniform over L. The network formulation of antenna arrays is found to be a convenient and useful approximation. A theory of MC for random arrays is formulated to analyze the spacing between the antenna elements. Two main effects of MC are considered. The first is the increase in the sidelobe level. The second effect is the mainlobe beamwidth broadening.

In this paper a closed-form expression for the bit error probability (BEP) of M -ary pulse amplitude modulation (M-PAM) subject to Rayleigh fading is derived. This expression is used to derive another general result: a closed-form expression for the BEP of rectangular quadrature amplitude modulation(R-QAM) subject to Rayleigh fading. In this work the fading channel is seen as an additive noise channel whose additive noise is modeled as the ratio between Gaussian and Rayleigh random variables.

The mobile cellular network is normally represented by means of hexagonal topology. This structure is useful for planning frequency reuse but not appropriate for the analysis of handoff and coverage. This paper presents the cellular network as a multiplicatively weighted Voronoi diagram. Radio parameters such as antenna height, transmission power and specific-environment propagation characteristics are used as the basis to define the proximity rule in order to generate the Voronoi diagram. The cell boundaries are the edges of the Voronoi diagram. They are defined by comparison of the radii of adjacent cells. The proximity between a mobile and a base station is determined by means of an Euclidean distance weighted by propagation parameters.

The performance of wireless communications systems can be significantly improved using the modulation diversity technique, which is basically based on the combination of a suitable choice of the reference angle of an MPSK constellation with independent interleaving of the symbol components. This technique presents good performance assuming the absence of estimation errors for channels characterized by flat fading. In this paper, the performance of this technique is analyzed taking into account the effects of channel estimation errors. It is shown, by simulation, that the efficiency of this technique is maintained even under this assumption. Additionally, the impact of the Doppler effect on the system performance is treated and a tradeoff between the interleaving depth and the error probability is achieved.

The outage contour is the boundary surrounding a cell, where the signal to interference plus noise ratio reaches a minimum acceptable value. In a mobile communication network, this contour is used in planning frequency reuse, dynamic or fixed channel allocation and cell breathing. This work presents these outage contours as a multiplicatively weighted Voronoi diagram. Each base station is a generating point of this diagram, with a corresponding weight. The weights are obtained by use of radio parameters as antenna heights, transmission power, receiver sensitivity and specific-environment propagation characteristics. The cells outage contours are represented by the edges of the Voronoi diagram, obtained by comparison between the ranges of the base stations radio signal from adjacent cells. The results show that the signal to interference plus noise ratio can be mapped to a distance ratio. The distance ratio, instead of power ratio, makes it possible to represent the outage, a signal service dependent parameter, by means of the outage contour, a geometric approach, using the geometric structure called Voronoi diagram.

The performance of the least mean square (LMS) algorithm on the estimation of time-varying channels is analytically evaluated, using the estimation error correlation matrix, the mean-square weight error (MSWE) and the mean-square estimation error (MSE) as parameters. Expressions for those parameters are obtained from a set of hypotheses usually adopted in the communication systems context. The channel is modeled as a wide sense stationary (WSS) discrete time stochastic field with known autocorrelation. The expressions for the steady-state MSWE and MSE are particularized for the class of WSS channel models, and an original analysis of the optimum LMS step-size parameter for usual channel models is addressed. For the sake of comparison with other works, the analytical step-size optimization for random-walk models is also considered. Several estimates of MSWE curves obtained by computer simulation are compared with analytical results for validation purposes. A very good agreement between simulated and analytical results for both the MSWE expressions and the optimum value of the LMS step-size parameter is shown.

This paper proposes a new strategy for establishing the LMS step-size parameter in the context of adaptive MLSE-PSP reception over time-varying frequency-selective fading channels. This strategy is based on the optimization of the step-size in the sense of minimizing the LMS steady-state mean square error (MSE) in the estimation of the channel impulse (IR) response. Analytical expressions of the optimum step-size are derived assuming Doppler spectrum models usually adopted for mobile communication channels. These expressions are used to choose the step-size of LMS based MLSE-PSP receivers under different conditions of Doppler spread. Several computer simulation results are provided which show the performance benefits so obtained.

This article proposes the use of denoising techniques at the receiver front-end in order to combat the effects of additive noise in digital transmission over frequency-selective channels. The proposed technique does not require knowledge of the channel impulse response, and is, therefore, potentially suitable for application in several receiver structures, including those with blind equalizers and adaptive receivers for mobile communications.

The performance of the least-mean-square (LMS) algorithm on the estimation of time-varying channels is analytically evaluated, using the estimation error correlation matrix and the mean square error (MSE) as parameters. Expressions for those parameters are obtained from a set of hypotheses usually adopted in the communication systems context. The channel is modeled as a wide sense stationary discrete time stochastic field with known autocorrelation. The expression for the steady state MSE is particularized assuming wide sense stationary-uncorrelated scattering (WSS-US) channel model. Several estimates of MSE curves obtained by computer simulation are compared with analytical results for validation purposes. A very good agreement between simulated and analytical results is shown.

This paper presents a geometrical approach to generate 3D radiation patterns for cellular antennas. The procedure is based on rotation of the vertical radiation pattern around the z-axis taking into account the horizontal radiation pattern. This method is useful in order to estimate the electromagnetic field intensity in points outside the vertical or horizontal planes.

This paper addresses the issue of blind adaptive maximum likelihood sequence estimation (BMLSE) using per survivor processing (PSP) over a fast frequency selective fading (FFSF) channel. Special care is dedicated to the choice of the adaptive filtering (AF) algorithm used and its influence on the receiver symbol error rate (SER) performance. An in-depth investigation of least mean square (LMS) and Kalman filtering (KF) performance is attempted. Regarding the KF algorithm two channel models were considered: first-order autoregressive (AR1) and second-order autoregressive (AR2) models. Three reception BMLSE schemes are discussed: BPSP-LMS, which uses LMS, and BPSP-KF1 and BPSP-KF2, which employ KF, according to the channel model used. For all the simulations accomplished in this work, the M-algorithm was used to obtain the survivors path. Results of several simulations, under varying signal-to-noise ratio (SNR) conditions and maximum Doppler shift (f/sub D/), are reported.

Intersymbol interference and additive noise are two of the main factors that limit the system communication performance for frequency selective channels. In particular, for applications over fast fading channels and blind reception, the noise level imposes severe performance degradation. This paper proposes the use of denoising techniques at the receiving end to reduce the noise level. The proposed technique does not require knowledge of the channel impulse response, being therefore suitable for blind equalization and mobile communications. The simulation accomplished in this work indicates that the proposed technique presents better performance than the one obtained with matched transmission and reception filters.

Image compression using the combination of the discrete wavelet transform (DWT) with vector quantization (VQ) has been considered in many works. Most of the studies have been dedicated to evaluating the choice of the wavelet filters employed in the multiresolution wavelet image decomposition or to developing bit-allocation schemes. It is worth mentioning, however, that the VQ codebook design plays a crucial role in the quality of the reconstructed image. A competitive neural network algorithm (synaptic space competitive algorithm, SSC) has already been successfully applied for voice waveform VQ codebook design (Vilar Franca and Aguiar Neto, 1994). In the present work, the SSC algorithm is applied in a wavelet/VQ image coding framework. The SSC codebooks are used to code the image subbands that result from the multiresolution decomposition. The coding results show that the SSC multiresolution codebooks lead to better reconstructed image quality than that obtained by using JPEG and conventional (spatial domain) VQ.

This paper presents a comparison of performance for FH-CDMA systems using either algebraic geometric (AG) or Reed-Solomon (RS) codes, for error correction. The performance measures of interest are the achievable region and the channel throughput. The achievable rate region is defined as the set of all pairs of code rate and number of users for which communication is possible, with error probability below a fixed value. The throughput measures the expected number of successful codeword transmissions per unit bandwidth.

Reed-Solomon (RS) codes present some desirable properties that make them useful in the generation of hopping sequences, for frequency hopping code division multiple access (FH CDMA). The algebraic geometric codes include the RS codes as a special case, therefore it is natural to propose the former as a candidate to FH CDMA. In this article, a description of such codes, along with the relationship between the RS codes and the proposed algebraic geometric codes is presented. Algebraic geometric codes constructed over Hermitean curves are used to generate the frequency hopping sequences. A comparison with sequences generated using RS codes reveals that longer sequences can be obtained for the same number of hits and number of sequences. Some of the properties of algebraic geometric codes, which make them suitable to use in CDMA systems, will be discussed.

One of the proposed techniques for the third generation of mobile communications is code division multiple access (CDMA). Some attributes of CDMA which are important to indoor systems include desirable anti-multipath properties, the lack of a need for frequency management or assignment, coexistence with other systems and suitability for micro-cell and in-building systems. The anti-multipath feature turns out to be a key issue for indoor systems and derives from the property that a signal with path delay greater than one chip interval is treated much like an interfering user. The number of users that share the same spectrum, and still maintain an acceptable performance, is determined by the interference generated by the set of remaining users and leads to soft degradation as the number of users increases. This work describes and analyzes a model for the discrete-input continuous-output Gaussian multiple-access channel, that uses spread spectrum techniques. The notion of sum capacity is used to characterize the system performance. Bounds on the sum capacity of this essentially binary input, continuous output channel, where the variance of the output noise is dependent on the number of users present, are obtained. The users are noncooperative in that no common codebook or synchronization is assumed. A novel feature of the work is the use of the modeling of the activity of the user community as a birth-death process. An information theoretic approach is used and the sum capacity is then considered in light of the various regimes to determine the effect of this modeling.>

The problem of finding the capacity region of a multiple access channel occupies one of the cornerstones of information theory. In this type of communications system several users transmit simultaneously to a common receiver. The main multiplexing strategies that fall in that category include frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). The indoor wireless channel can be analysed through the use of an information theoretic approach, whose main objective is to find the set of information rates at which the simultaneous reliable communication of the messages of each user is possible. This article presents an analysis of the capacity region for the multiaccess Gaussian channel, when subject to random fading.>

In order to provide the decoder with essential information on the channel state, and improve the effectiveness of the communication, a probabilistic analysis of the communication channel is needed. Such modeling must include fading and multipath, as well as interference. An insight into the behavior of the channel can be gained by attempting an information theoretical approach for the multipath model of the wireless indoor channel. The main objective of the information theoretic approach of the multipath Gaussian channel is to find its capacity region, i.e. the set of information rates at which the simultaneous reliable communication of the messages of each user is possible.>