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Fast channel estimation in millimeter-wave (mmWave) systems is a fundamental enabler of high-gain beamforming, which boosts coverage and capacity. The channel estimation stage typically involves an initial beam training process where a subset of the possible beam directions at the transmitter and receiver is scanned along a predefined codebook. Unfortunately, the high number of transmit and receive antennas deployed in mmWave systems increase the complexity of the beam selection and channel estimation tasks. In this work, we tackle the channel estimation problem in analog systems from a different perspective than used by previous works. In particular, we propose to move the channel estimation problem from the angular domain into the transformed spatial domain, in which estimating the angles of arrivals and departures corresponds to estimating the angular frequencies of paths constituting the mmWave channel. The proposed approach, referred to as transformed spatial domain channel estimation (TSDCE) algorithm, exhibits robustness to additive white Gaussian noise by combining low-rank approximations and sample autocorrelation functions for each path in the transformed spatial domain. Numerical results evaluate the mean square error of the channel estimation and the direction of arrival estimation capability. TSDCE significantly reduces the first, while exhibiting a remarkably low computational complexity compared with well-known benchmarking schemes., Comment: Accepted for publication in IEEE Transactions on Wireless Communications

© . This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Molecular communications via diffusion (MCvD) systems are easily simulated for micro-scale topologies and applications. On the other hand, due to the high path loss, there is a need for the emission of a very large number of molecules to have a detectable signal for the macro-scale topologies. Therefore, the simulation of macro-scale MCvD systems or applications has its own challenges. In this work, a voxel-based simulator for MCvD is proposed and analyzed. The proposed simulator is able to consider a very large amount of molecules since it does not track every molecule, instead it simulates the aggregate behavior. We assess the correctness of such a simulation approach through comparative studies with a particle-based (i.e., per-molecule) simulation. We present the effect of voxel side-length on the modeling accuracy and devise a framework for selecting the optimal voxel side-length for high-accuracy simulations.

We propose and analyze a wireless energy and information transmission scheme in free-space optical (FSO) links. The results are presented for both quasi-static and fast-fading conditions. We derive closed-form expressions for throughput, outage probability, and optimal power allocation optimizing the system throughput/outage probability. Finally, we complement the FSO link with an additional radio frequency (RF) link to create a hybrid RF-FSO system and reduce the system outage probability. The results show that joint implementation of the RF and FSO links leads to considerable performance improvement, compared to the cases with only FSO-based communication.

To communicate at the nanoscale, researchers have proposed molecular communication as an energy-efficient solution. The drawback to this solution is that the histogram of the molecules' hitting times, which constitute the molecular signal at the receiver, has a heavy tail. Reducing the effects of this heavy tail, inter-symbol interference (ISI), has been the focus of most prior research. In this paper, a novel way of decreasing the ISI by defining a counting region on the spherical receiver's surface facing towards the transmitter node is proposed. The beneficial effect comes from the fact that the molecules received from the back lobe of the receiver are more likely to be coming through longer paths that contribute to ISI. In order to justify this idea, the joint distribution of the arrival molecules with respect to angle and time is derived. Using this distribution, the channel model function is approximated for the proposed system, i.e., the partially counting absorbing spherical receiver. After validating the channel model function, the characteristics of the molecular signal are investigated and improved performance is presented. Moreover, the optimal counting region in terms of bit error rate is found analytically., submitted to Transactions on Communications

In free space optical (FSO) communications, several factors can strongly affect the link quality. Among them, one of the most important impairments that can degrade the FSO link quality and its reliability even under the clear sky conditions consists of optical turbulence. In this work, the authors investigate the generation of both weak and moderate turbulence regimes in an indoor environment to assess the FSO link quality. In particular, they show that, due to the presence of the turbulence, the link experiences both erasure errors and packet losses during transmission, and also compare the experimental statistical distribution of samples with the predicted Gamma Gamma model. Furthermore, the authors demonstrate that the application of the RaptorQ codes noticeably improves the link quality decreasing the packet error rate (PER) by about an order of magnitude, also offering in certain cases an error-free transmission with a PER of ∼10−2 at Rytov variance value of 0.5. The results show that the recovery rate increases with the redundancy, the packet length and the number of source packets, and it decreases with increasing data rates., This work was supported by the European Space Agency under grant no. 5401001020. We are very grateful to Dr. E. Armandillo for enlightening discussions. This research project also falls within the frame of COST ICT Action IC1101 - Optical Wireless Communications - An Emerging Technology (OPTICWISE). J. Perez's work is supported by Spanish MINECO Juan de la Cierva JCI-2012-14805.

Compared to synthetic aperture radars (SARs), the angular resolution of microwave radiometers is quite poor. Traditionally, it has been limited by the physical size of the antenna. However, the angular resolution can be improved by means of aperture synthesis interferometric techniques. A narrow beam is synthesized during the image formation processing of the cross-correlations measured at zero-lag between pairs of signals collected by an array of antennas. The angular resolution is then determined by the maximum antenna spacing normalized to the wavelength (baseline). The next step in improving the angular resolution is the Doppler-Radiometer, somehow related to the super-synthesis radiometers and the Radiometer-SAR. This paper presents the concept of a three-antenna Doppler-Radiometer for 2D imaging. The performance of this instrument is evaluated in terms of angular/spatial resolution and radiometric sensitivity, and an L-band illustrative example is presented.

This paper presents a set of description schemes (DS) dealing with video programs, users and devices. Following MPEG-7 terminology, a description of an AV document includes descriptors (termed Ds), which specify the syntax and semantics of a representation entity for a feature of the AV data, and description schemes (termed DSs) which specify the structure and semantics of a set of Ds and DSs. The Program DS is used to describe the physical structure as well as the semantic content of a video program. It focuses on the visual information only. The physical structure is described by the temporal organization of the sequence (segments), the spatial organization of images (regions) as well as the spatio-temporal structure of the video (regions with motion). The semantic description is built around objects and events. Finally, the physical and semantic descriptions are related by a set of links defining where or when instances of specific semantic notions can be found. The User DS is used to describe the personal preferences and usage patterns of a user. It facilitates a smart personalizable device that records and presents to the user audio and video information based upon the user's preferences, prior viewing and listening habits, as well as personal characteristics. Finally, the Device DS keeps a record of the users of the device, available programs, and a description of device capabilities. It allows a device to prepare itself based on the existing users, profiles and available programs. These three types of DSs and the common set of descriptors that they share are designed to support personalization, efficient management of AV information and the expected variability in the capabilities of AV information access devices.

This paper proposes a trigonometric functional extension, hereafter named the Fourier model, as an alternative framework to the Volterra approach for non-linear systems modelling. This work is focused on the general advantages that trigonometric functionals show in adaptive implementations and also on the possibility they provide to reuse well-known linear processing tools in a non-linear context. The performance of the Fourier model is compared in a set of simulations that cover companders for audio and radio frequency amplifiers, probability density function (PDF) whitening and PDF estimation.

We investigate the evolution of vortex wave front dislocations in multiple-wave second-harmonic generation processes in quadratic nonlinear media. Vortices nested in finite-size host beams are shown to nucleate and to annihilate in pairs, and to move across the transverse wave front during the beam evolution. A closed-form model that holds under conditions of negligible-depletion of the pump beam is developed to describe the vortex dynamics in order to predict the number of vortices present in the wave fronts of the beams at any instance of the propagation. Results are compared with numerical simulations of the full governing equations and with experimental observations. Limitations of the model are outlined.

We address the existence and properties of three-dimensional solitons localized in space and time, or light bullets, in bulk dispersive quadratic nonlinear media under conditions for Type I second-harmonic generation, with emphasis on the general case of different group-velocity dispersions at the fundamental and second-harmonic frequencies. We find families of three-dimensional light bullets and study their features in detail, including their stability on propagation. Most of the three-dimensional spatio-temporal solitons are shown to be stable.

Amplification of spatial solitons in quadratic nonlinear media under conditions for second-harmonic generation is addressed. Under the action of gain at a given frequency band the energy has to be redistributed among the multiple-frequency waves that form the solitons, which ought to reshape accordingly. It is shown that solitons in quadratic nonlinear media appear to be well suited for such purpose. Practical implementation conditions are discussed.

This paper describes a region-based subband coding scheme intended for efficient representation of the visual information contained in image regions of arbitrary shape. QMF filters are separately applied inside each region for the analysis and synthesis stages, using a signal-adaptive symmetric extension technique at region borders. The frequency coefficients corresponding to each region are identified over the various subbands of the decomposition, so that the coding steps — namely, bit-allocation, quantization and entropy coding — can be performed independently for each region. Region-based subband coding exploits the possible homogeneity of the region contents by distributing the available bitrate not only in the frequency domain but also in the spatial domain, i.e. among the considered regions. The number of bits assigned to the subbands is optimized region by region for the whole image, by means of a rate-distortion optimization algorithm. Improved compression efficiency is obtained thanks to the local adaptativity of the bit allocation to the spectral contents of the different regions. This compensates for the overhead data spent in the coding of contour information. As the subband coefficients obtained for each region are coded as separate data units, the content-based functionalities required for the future MPEG4 video coding standard can be readily handled. For instance, content-based scalability is possible by simply imposing user-defined constraints to the bit-assignment in some regions.

In this paper, we consider a sensing-based spectrum sharing scenario in cognitive radio networks where the overall objective is to maximize the sum-rate of each cognitive radio user by optimizing jointly both the detection operation based on sensing and the power allocation, taking into account the influence of the sensing accuracy and the interference limitation to the primary users. The resulting optimization problem for each cognitive user is non-convex, thus leading to a non-convex game, which presents a new challenge when analyzing the equilibria of this game where each cognitive user represents a player. In order to deal with the non-convexity of the game, we use a new relaxed equilibria concept, namely, quasi-Nash equilibrium (QNE). A QNE is a solution of a variational inequality obtained under the first-order optimality conditions of the player's problems, while retaining the convex constraints in the variational inequality problem. In this work, we state the sufficient conditions for the existence of the QNE for the proposed game. Specifically, under the so-called linear independent constraint qualification, we prove that the achieved QNE coincides with the NE. Moreover, a distributed primal-dual interior point optimization algorithm that converges to a QNE of the proposed game is provided in the paper, which is shown from the simulations to yield a considerable performance improvement with respect to an alternating direction optimization algorithm and a deterministic game.

A family of solitary waves that describe mutually trapped fundamental and second-harmonic waves propagating in second-order nonlinear media with different total wave powers is numerically found over a range of phase-mismatches between both waves. The shape of the localized waves and the way that power is shared between the fundamental and second harmonic waves is studied as a function of the phase-mismatch and the total wave power.

Network virtualization has emerged as a solution for the Internet inability to address the required challenges caused by the lack of coordination among Internet service providers for the deployment of new services. The allocation of resources is one of the main problems in network virtualization, mainly in the mapping of virtual nodes and links to specific substrate nodes and paths, also known as the virtual network embedding problem. This paper proposes an algorithm based on optimization theory, to map the virtual links and nodes requiring a specific demand, looking for the maximization of the spare bandwidth and spare CPU in the substrate network, taking into account the CPU demanded by the hidden hops when a virtual link is mapped. The components of the virtual networks (nodes and links) that do not ask for an specific demand are then allocated following a fairness criteria

In this letter the performance of a free space optical (FSO) communication link in the presence of fog is experimentally investigated in an indoor environment. A dedicated indoor atmospheric chamber, replicating the outdoor environment, is being used to evaluate the FSO link performance under the fog condition. Theoretical analysis supported by the experimental evaluation has been carried out for the intensity modulation/direct detection on-off keying non-return-to-zero (OOK-NRZ), OOK return-to-zero (OOK-RZ) and four pulse position modulation (4-PPM) modulation formats at the Ethernet baseline data-rate. The results shown indicate that the 4-PPM signalling scheme is the most robust to the fog attenuation due to its high peak-to-average power ratio. Moreover, results also show the potential of the indoor atmospheric chamber to replicate outdoor atmospheric effects (such as fog) and be able to quickly characterise and assess the FSO system under a controlled environment.

We explore decentralized coordination of sectored cellular networks to adapt the usage of downlink resources to the instantaneous network conditions. The transmission frame consists of an orthogonal bandwidth usage phase, where sectors perform FDMA and power control over an agreed frequency chunk, and a shared bandwidth usage phase where each sector performs FDMA over the full available bandwidth without power control (interference is not controlled in this phase by any means). Decentralized network utility maximization with global optimality guarantee is enabled by fixing this structure of the transmission frame, which does not cause significant network-wide losses. Thus, the ability to better balance the resources gained from coordination generates some slack that can be used to either i) provide higher-quality access, ii) increase the number of active users, or iii) reduce deployment and maintenance costs by operating larger cells.

In this paper, we study the performance of all optical AND gates based on the four-wave-mixing (FWM) process, for application in the receivers of the spectral phase encoded optical code division multiple access (SPE-OCDMA) systems. For the performance analysis, the spectral domain auto-correlation function of the multiple access interference signal at the output of the optical AND gate is derived and then mean and variance of the photo-detector current are calculated. For solving the Fourier transform of FWM non-linear Schrodinger equations, spectral domain Volterra series are used and the solution of these coupled equations is approximated with the first three terms of the Volterra expansion. Finally, the performance of AND gating receivers in SPE-OCDMA systems is analysed and the bit error rate in terms of the pump power and the number of active users is depicted. In addition, a compression between the performance of the time-gating receivers, base on FWM in synchronous mode and other non-linear receivers such as second harmonic generation and self-phase modulation is made.

In this paper, we develop a power allocation scheme for the downlink of a two-cell two-user joint transmission system. The objective is to maximize the sum rate under per-cell power constraints. We study a worst case scenario where the carrier phases between the two base stations are un-synchronized, so that joint transmission must be performed without precoding. The derived power allocation scheme is remarkably simple, i.e., each cell transmits with full power to only one user. Note that joint transmission is still possible, when two cells select the same user for data transmission. Moreover, we prove that, in this scenario, the joint transmission case happens with higher probability when the maximum transmit power is high, or the two users are in the overlapped cell-edge area.

Wireless sensor networks (WSNs) are made up of large groups of nodes that perform distributed monitoring services. Since sensor measurements are often sensitive data acquired in hostile environments, securing WSN becomes mandatory. However, WSNs consists of low-end devices and frequently preclude the presence of a centralized security manager. Therefore, achieving security is even more challenging. State-of-the-art proposals rely on: (1) attended and centralized security systems; or (2) establishing initial keys without taking into account how to efficiently manage rekeying. In this paper we present a scalable group key management proposal for unattended WSNs that is designed to reduce the rekeying cost when the group membership changes.

A comprehensive analysis of the symmetric nonlinear directional coupler as a function of the excitation conditions is reported. The analysis is based on a coupled-mode technique in normalized (dimensionless) parameters, using the linear supermodes of the guiding structure, most of the results being derived from the Hamiltonian of the resulting nonlinear equations. The exchange of power between the branches in the coupler is graphically described by means of the trajectories of the motion in the phase space. The effect of saturation of the nonlinear index has been included.

Base station coordination is considered as a promising technique to mitigate inter-cell interference and improve the cell-edge performance in cellular orthogonal frequency division multiple-access (OFDMA) networks. The problem to design an efficient radio resource allocation scheme for coordinated cellular OFDMA networks incorporating base station coordination has been only partially investigated. In this contribution, a novel radio resource allocation algorithm with universal frequency reuse is proposed to support base station coordinated transmission. Firstly, with the assumption of global coordination between all base station sectors in the network, a coordinated subchannel assignment algorithm is proposed. Then, by dividing the entire network into a number of disjoint coordinated clusters of base station sectors, a reduced-feedback algorithm for subchannel assignment is proposed for practical use. The utility function based on the user average throughput is used to balance the efficiency and fairness of wireless resource allocation. System level simulation results demonstrate that the reduced-feedback subchannel assignment algorithm significantly improves the cell-edge average throughput and the fairness index of users in the network, with acceptable degradation of cell-average performance.

The ability to transparently detect performance degradations in all-optical networks is highly desirable. We present an optical signal monitor, based on a frequency supervision system, which can participate in some tasks for fault and performance management of transparent optical networks.

We describe large spectral broadening due to the interaction of cross-phase modulation/self-phase modulation and fiber dispersion, and explain its contribution to the penalties in standard fiber systems pre- and postcompensated by dispersion compensating fiber. To our knowledge, this is the first measurement of this effect in a dense WDM transmission system at 10 Gb/s with this dispersion management scheme and good agreement is reported with the numerical modeling results.

Demonstration of an all-fiber polarization scrambler based on the Faraday effect is carried out. The device has been constructed using dispersion-shifted fiber that has a major tolerance to bends than standard single-mode fiber. Results about the fiber Verdet constant when 1550-nm light is launched are presented. The performance of the constructed device is also shown. Main features are insertion losses as low as 0.4 dB and scrambling frequency up to 20 kHz. Although here we emphasize its application to low-frequency heterodyne detection, the scrambler is applicable to other systems that are polarization dependent. In particular, it would be useful to overcome problems originated by polarization dependent gain in erbium-doped fiber amplified systems.

The problem of designing joint power control and optimal beamforming (JPCOB) algorithms for the downlink of a coordinated multi-cell WCDMA system is considered throughout this paper. In this case, the JPCOB design is formulated as the problem of minimizing the total transmitted power in the coordinated multi-cell system, subject to a certain quality of service requirement for each user. In this paper, the performance of two JPCOB algorithms based on different beamforming approaches is compared over the coordinated multi-cell system. The first one, obtains local beamformers by means of the well-known virtual uplink-downlink duality. In contrast, the second algorithm implements multi-base beamformers, taking into account match filter equalizers at the receivers. Moreover, realistic system parameters, such as per-base station power constraints or the asynchronous nature of the signals arriving at the receivers, are taken into account. Simulation results show that the algorithm based on multi-base beamforming presents attractive properties, such as an inherent multi-base scheduling technique or a decreasing total transmitted power as the degree of coordination between base stations is increased.

[EN] The use of an ultra-wideband (UWB) radio technique is proposed as a viable solution for the distribution of high-definition audio/video content in fiber-to-the-home (FTTH) networks. The approach suitability is demonstrated by the transmission of standards-based UWB signals at 1.25 Gb/s along different FTTH fiber links with 25 km up to 60 kin of standard single-mode fiber length in a laboratory experiment. Experimental results suggest that orthogonal frequency-division-multiplexed UWB signals exhibit better transmission performance in FFTH networks than impulse radio UWB signals., This work was supported in part by the FP6-IST-33615 UROOF Project. The work of T. Alves was supported by FCT from Portugal under Contract SFRH/BD/29871/2006. The work of M. Beltran was supported by Spain MEC Grant BES-2006-12066.

In this work, it is shown experimentally that optical beams that contain a phase dislocation can be generated with a laser oscillating in the fundamental mode and a cylindrical-lens mode converter.

An all-fiber device for the control of the state of polarization (SOP) is presented. The controller is high speed (its time response is less than 50 ms in the worst case), low loss (insertion losses are 2.5 dB independently of the input SOP), and low cost (is constructed with standard single-mode fiber). The polarization controller has been implemented using two Faraday rotators and works at 1550 nm. We have also designed an algorithm that confers an endless performance to the device. The controller has been included in a 2.5 Gb/s CPFSK optical coherent system and results are presented here.

Precise estimation of propagation parameters in precipitation media is of interest to improve the performance of communications systems and in remote sensing applications. In this paper, we present maximum-likelihood estimators of specific attenuation and specific differential phase in rain. The model used for obtaining the cited estimators assumes coherent propagation, reflection symmetry of the medium, and Gaussian statistics of the scattering matrix measurements. No assumptions about the microphysical properties of the medium are needed. The performance of the estimators is evaluated through simulated data. Results show negligible estimators bias and variances close to Cramer–Rao bounds.

This paper illustrates that remote fault localisation in all-optical links with concatenated fibre amplifiers can be performed by simple monitoring of the signal and the accumulated ASE noise powers at the receiver end. The method is first presented in ideal operating conditions, and second, the effect of the spontaneous emission factor variation and the EDFA non-linear behaviour are taken into account. From the analysis, the requirements of the link design for proper supervision accuracy are derived. The presented supervisory method is a simple solution for links where a fibre break or an amplifier fault is to be detected and located with the granularity of an amplifier section without local supervisory elements within the transmission sections.

Code-division multiple-access (CDMA) schemes appear to be very promising access techniques for coping with the requirements of third-generation mobile systems, mainly because of their flexibility. This paper proposes an adaptive S-ALOHA DS-CDMA access scheme as a method for integrating nonreal-time (i.e., Internet applications) and real-time (i.e., voice) services in a multicell scenario by exploiting the potentials of CDMA under time-varying channel load conditions. The adaptive component makes data terminals autonomously change their transmission rate according to the total (voice+data) channel occupancy, so that the minimum possible data delay, which can be analytically obtained by defining a birth-death process, is almost always achieved. Moreover, by means of a simplified cellular model, the proposed algorithm revealed the same behavior, i.e., it tries to select the most suitable transmission rate at any time slot, when it is affected by intercell interference and even by power control imperfections. Finally, in order to gain more insight into the potentials of such an access strategy, the adaptive S-ALOHA CDMA scheme is then compared to a reservation time-division multiple-access (TDMA)-based protocol (PRMA++), showing the benefits of the CDMA-based solution in terms of capacity, flexibility, and data delay performance.

The correlation between the visibility samples' noise of an aperture synthesis radiometer are required for the computation of the recovered temperature noise of a given pixel and of the improvement introduced by baseline redundance. A general expression for this correlation and noise examples for a linear array are presented.

The spatial resolution of current space-borne Earth observation radiometers is limited by the physical antenna aperture. This is especially critical at L-band, which exhibits high sensitivity to soil moisture and sea surface salinity. Interferometric radiometers (InR's) are currently being studied by several space agencies as a feasible alternative to overcome this problem. However, their calibration is a crucial issue since most techniques inherited from radio astronomy cannot be directly applied. Due to the large number of receiving channels, calibration techniques based on centralized noise injection from a single noise source will require a large and stable distribution network, which is technically very complex and unacceptable from the point-of-view of mass and volume. Procedures based on distributed noise injection from a set of noise sources through smaller distribution networks have been recently proposed by the authors as an alternative to alleviate these technological problems. In this paper, the analysis of these networks, the impact of the noise generated by the network losses on the calibration, and the impact of front-end reradiated noise are analyzed. Finally, the optimum circuit topologies and tolerances to which these networks have to be characterized in order to achieve the required calibration are derived. These configurations are formed by cascading basic 1:2 and 1:3 isolated power splitters. Isolators at receiver inputs have to be included in order to minimize offsets originating from the correlation of reradiation of receiver noise. It has been found that, in order to satisfy the calibration requirements of InR's, the S-parameters of the ensemble noise-injection network plus isolators have to be known to within 0.025-0.050 dB in amplitude and 0.5/spl deg/ in phase, and their physical temperature known to within 0.5/spl deg/C.

We address the robustness of quadratic solitons with periodic non-conservative perturbations. We find the evolution equations for guiding-center solitons under conditions for second-harmonic generation in the presence of periodic multi-band loss and gain. Under proper conditions, a robust guiding-center soliton formation is revealed., 5 pages, 5 figures, submitted to Optics Communication

Formation of temporal guiding-center walking solitons in tandem structures made of quadratic nonlinear crystals is investigated. The specific new effects and possibilities arising by engineering the multifrequency periodical variation of the group-velocity, group-velocity dispersion and nonlinearity existing in the tandem structures are introduced.

Propagation of soliton signals in quadratic nonlinear media under conditions for second-harmonic generation in the presence of multiple-frequency-band absorption and amplification is addressed. The reshaping and energy redistribution of the multiple-frequency solitons under the action of the single-band loss and gain is investigated and shown in detail. New chirped localized solitary-wave solutions that exactly compensate loss and gain are presented. Practical implementation conditions are discussed.

The surface current distribution of a Sierpinski fractal antenna shows a self-similar behaviour determined by the self-similar properties of its geometry. The application of infra-red thermography to electromagnetic near field detection allows the experimental verification of the active region scaling of a fractal antenna operating at different bands.

A new wide-band mobile channel emulator for the CODIT project is designed and implemented. The UMTS code-division testbed (CODIT R2020) is a research project within the European RACE-II program set up by the Commission of the European Community. Our goal is to be able to simulate in the laboratory, in real time, the multipath propagation found in the mobile radio channel. As code-division multiple access (CDMA) is the access technique within the CODIT project, it was realized that the channel emulator must have simultaneously good delay resolution between propagation paths and long duration of the impulse response. These considerations led to a very flexible channel emulator specifically designed to host the new wide-band channel models developed within the CODIT project. Our emulator features three independent inputs and two outputs, up to 20 complex propagation paths, 10-MHz radio frequency (RF) bandwidth, a delay resolution of 50 ns, and a maximum duration of the channel impulse response of 80 /spl mu/s. Starting with an explanation of the global structure of the new channel emulator, we derive the optimum design of the interpolation procedures and present the main implementation issues arising from our initial architecture. Finally, we report the results of the laboratory tests of the first prototype of the channel emulator.

A method for optimising LPC filters in linear prediction based speech coders is described. The optimisation process compensates for errors incurred through coding the excitation signal, providing an improvement in the quality of the decoded speech, with no increase in bit rate.

We study the dynamics of the soliton–radiation interaction in the process of soliton excitation in quadratic nonlinear media. The focus is on the dynamics of initial signals that are not weakly perturbed exact solitons. We use a combination of numerical experiments and analytical tools based on the integral conserved quantities of the wave evolution. We show the rate of convergence of representative, experimentally relevant inputs, to the asymptotic soliton states. A measure of soliton content of arbitrary input signals is introduced and evaluated in several representative examples. The dynamic evolution of oscillating solitons is studied using generalized Stokes parameters.

Wind direction over the sea can be determined from radiometric measurements of the third Stokes parameter (T/sub U/). The Kirchhoff method is extended under the stationary phase approximation to compute the dependence of T/sub U/ on frequency, wind speed and incidence angle. Numerical predictions, in agreement with experimental measurements, are presented.

This paper presents a new technique for downlink transmission beamformer design in cellular mobile communication systems using antenna arrays at the base station. The method is based on estimation of an underlying spatial distribution associated with each source’s spatial downlink channel. The algorithm is ‘blind’ in the sense that it depends only on uplink spatial channel statistics, requiring no mobile-to-base station feedback in the design procedure. The assumed underlying spatial distribution models are general enough to be used in a wide variety of mobile communications scenarios (e.g., rural, urban, sub-urban, indoor). The technique is of reasonable complexity with a significant portion of the computation carried out off-line. Simulation results verify the effectiveness of the approach.

Antenna pattern mismatches are one of the most important error sources in planned Earth-observation interferometric radiometers. From a low Earth orbit, the wide field of view, about ±30°, leads to the use of antennas with a large beam. In addition, antennas must be closely spaced to avoid, or at least minimize, aliasing effects in the formation of the synthetic brightness temperature images. The accuracy demanded of these systems requires the precise knowledge of all the antenna radiation voltage patterns (amplitude and phase), which may differ from their theoretical values due to mechanical and electrical tolerances in the manufacturing process and which can change due to the proximity of other structures, i.e., other antennas of the array or the mechanical support. Two approaches are found in the literature to interpret the impact of antenna mutual coupling on the performance of an interferometric radiometer: (1) a modification of the antenna voltage pattern and (2) a mixing of the cross correlations measured between the signals collected by the antennas. The main contribution of the present work is a detailed theoretical analysis of the impact of mutual coupling effects showing the equivalence between both approaches. Theoretical results are corroborated with a set of experimental measurements with two kinds of antennas. Theoretical and experimental results can be used in the design of the antennas of interferometric radiometers in order to predict the impact of mutual coupling on the system's performance and point out the importance of an accurate antenna pattern characterization.

The derivation of soil moisture and ocean salinity contents can be performed by means of passive observations at L-band. The global coverage, the low revisit time (1-3 days), the radiometric resolution (1 K) and the spatial resolution required (10-30 Km) can be achieved by means of low orbit interferometric radiometers. Interferometric radiometers synthesize a thin beam by correlating the outputs of a sparse array of small antennas. The imaging algorithms used in radioastronomy can not be directly applied to Earth observation because of the large number of baselines, the proximity between the antennas, which increases coupling effects, and the antennas' wide beams required to cope with the large field-of-view (FOV), about 120°. Since most receiver errors and drifts can be hardware or software calibrated, inversion algorithms are focused mainly in the correction of the aberrations introduced by antenna voltage pattern mismatches and antenna positioning errors. This paper presents an extension of the CLEAN technique used in radioastronomy to map quasi-point thermal sources, i.e., stars, to the generation of brightness temperature maps of extended thermal sources, i.e., the Earth. The iterative approach used allows to account for antenna voltage pattern mismatches within the large FOV. The proposed algorithm has been optimized to process hexagonally sampled visibilities given by Y-shaped-arrays, as it is the case of the MIRAS instrument (Microwave Imaging Radiometer by Aperture Synthesis), an interferometric radiometer currently under study at the European Space Agency.

A small but long wire fractal antenna based on the Koch curve is presented. Experimental and numerical results show that the antenna improves the features of a common linear monopole. The radiation resistance is increased and the Q is reduced at each fractal iteration, approaching the fundamental limit on small antennas.

A theoretical analysis of the angular resolution of two-dimensional interferometric radiometers for Earth observation from low-orbit satellites and its degradation due to spatial decorrelation effects is presented. The analysis extends basic effects known in the context of radio astronomy (application with narrow field of view, very few baselines) and in one dimension (ESTAR L band, few baselines interferometric radiometer) to the wide-field-of-view, many-baseline, high-resolution two-dimensional system required by Earth observation applications and computes beam width, encircled energy (or main beam efficiency), and side lobe level as a function of windowing (apodization) to allow for an optimum angular versus radiometric resolution trade-off. It is found that the extension of the Barlett window (which has a poor performance in one dimensional signal processing) to two dimensions produces high-quality results, comparable or better than those of Gaussian and Blackmann windows. Theory is extended to hexagonally sampled systems based on a ¿ or Y-shaped instrument, with hexagonal- and star-shaped support regions in the visibility space, respectively. The superior performance of the latter over the former for the same number of antennas and correlators is quantified and details of the angular resolution of one instrument of this kind, MIRAS, under development by the European Space Agency, are presented. For this radiometer Gaussian or Barlett windows should be used for good radiometric sensitivity or spatial resolution, respectively. In both cases the effects of decorrelation within the small alias-free field of view are negligible. It is also found that the impact of hardware imperfections which exist within the strict requirements of the specifications have a negligible effect on the angular resolution. Finally, experimental angular resolution results with a laboratory breadboard in a focused near-field setup are presented and compared to the theoretical predictions.

We address the existence and instability of bright vortex solitary waves in quadratic non-linear media under conditions for Type II second-harmonic generation and study in detail the effects introduced by the presence of power imbalancing at the fundamental frequency. We find the families of solitary-wave solutions with different combinations of topological charges of the nested vortices and show their spontaneous azimuthal instability. The growth rates of the azimuthal perturbations are calculated as a function of the imbalancing. Experimental consequences are stressed and discussed. © 1998 Elsevier Science B.V. All rights reserved., This work has been supported by the Spanish Government under contract PB95-0768. The work of JMSC was supported by the Comunidad de Madrid under Contract 06Tr039r96 and by the CICyT under Contract TIC95-0563-03. GMT acknowledges support by the CIRIT of the Generalitat de Catalunya. The numerical work has been carried out at Centre de Computacio i Comunicacions de Catalunya.

We present a direct method for the steady-state and stability analysis of autonomous circuits with transmission lines and generic non- linear elements. With the discretization of the equations that describe the circuit in the time domain, we obtain a nonlinear algebraic formulation where the unknowns to determine are the samples of the variables directly in the steady state, along with the oscillation period, the main unknown in autonomous circuits.An efficient scheme to buildtheJacobian matrix with exact partial derivatives with respect to the oscillation period and with re- spect to the samples of the unknowns is described. Without any modifica- tion in the analysis method, the stability of the solution can be computed a posteriori constructing an implicit map, where the last sample is viewed as a function of the previous samples. The application of this technique to the time-delayed Chua's circuit (TDCC) allows us to investigate the stability of the periodic solutions and to locate the period-doubling bifurcations.

We address the effect of quintic nonlinearities in the propagation of optical solitons in cubic nonlinear media. Our focus is on the decay of higher-order solitons in the presence of self-defocusing quintic perturbations. We show that, in spite of the fact that the governing evolution equations are symmetric, the quintic nonlinearity produces the asymmetrical self-splitting of the solitons. We study in detail the self-splitting process and compare the results with the soliton evolution with self-focusing quintic perturbations.