A simulation of multipath effects on the cross-polarization discrimination in microwave line-of-sight links

A ray-tracing computer model, combining the effects of the curvature of the Earth, of the radiation patterns of the antennas, as well as those of the reflection at the Earth’s surface, has been developed in order to simulate multipath effects on the relationship between cross-polarization discrimination (xpd)and co-polar attenuation (cpa)values occurring in frequency-reuse line-of-sight microwave links. Measured refractive-index vertical profiles have been used as input data to this model, in order to study the dependence of the parameters of the law relating xpd and cpa on the characteristics of the link. Based on the observed dependence, experimental values for the parameters of this law, available in the literature, have then been analyzed.     

Ice depolarization at 11.7 and 19 GHZ on slant paths in Virginia,USA

Measurements of attenuation and cross-polarization on slant paths from the Cts and Comstar D2 satellites to Blacksburg, VA, have been in progress for three years. The data for 1977 and 1978 have been analyzed to determine the extent of severe ice depolarization on these paths, and a number of events have been identified. The behaviour of the observed cross-polarization at 11.7 and 19 GHz in two events has been compared with current theory for ice depolarization.     

Rain depolarization measurement using INTELSAT-IV satellite in 4 GHz band at a low elevation angle

The degradation of cross polarization discrimination (XPD)due to rain has been measured at 4 GHz using the Indian Ocean INTELSAT-IV satellite at Yamaguchi in Japan, where the satellite elevation angle is as low as 9 degrees. Based on the data obtained from August 1975 to October 1976, this paper describes the analyses on the cumulative time distribution of XPD,the correlation of XPD degradation with surface rain rate and rain attenuation, and the duration characteristics of XPD degradation.     

LDPC-based space-time coded OFDM systems: Turbo-EM receiver design with channel state information guessing algorithms

In this paper we study some turbo receiver architectures employing low-density parity check (Ldpc) codes together with orthogonal frequency division multiplexing (Ofdm) for high data rate wireless transmissions. Different demodulation schemes based on expectation-maximization (Em) algorithm are studied along with the channel impulse response (Em) algorithms. We studied differentCir guessing algorithms including the EM-based algorithms such as a space-alternating generalized expectation-maximization algorithm (Sage). It is shown that the proposed turbo-Em receiver employing a soft maximum a posteriori (Map)Em demodulator and a belief propagationLdpc decoder can perform within 1 dB from the ergodic capacity of the studiedMimo ofdm channels. Besides, we find that a suboptimum structure based on a soft interference cancellationMmse filtering demodulator exhibits negligible loss in non-correlated fadingMimo channels but suffer extra performance loss in highly correlatedMimo channels.     

Analyse de la stabilité spectrale d’un laser DFB à trois sauts de phase (3PS-DFB)

The aim of this article is to analyze the spectral stability of the three phase shift distributed feedback (3ps-dfb) laser by using Time domain model (tdm). We have developed a software that simulates static and dynamic properties of distributed feedback (dfb) multi-sections laser at large signal. The best single-mode stability operation up to 18mW of the 3ps-dfb is achieved for three phase shifts at (π, π, π) whatever the phase position. These results showed that the 3ps-dfb laser was a much more suitable structure to realize stable single-mode high-power operation for a coherent optical communication system.     

Robust time-frequency distributions based on the robust short time fourier transform

Design of time-frequency distributions (Tfds) that are robust to the impulse noise influence is considered. The robustTfds based on the robust short-time Fourier transform (Stft) are proposed. An efficient procedure to evaluate the robustStft is given. RobustTfds based on the robustStft have better energy concentration around the signal instantaneous frequency (If) than the robustStft itself. Also, theseTfds are more resistant to higher impulse noise than the robustTfds obtained using the local autocorrelation function (Laf) based minimization problem.     

Modulation par codage spectral

Using the time-frequency duality principle, the author gives a relation between the so-called Pulse Code Modulation system (Pcm)and a system in which the spectral lines are coded in a similar way (Fcm or Frequency Code Modulation system). The latter appears to be quite comparable to a four phase digital modem, the carrier frequencies of which are multiples of a common basic frequency. It is showed that a Fcm frequency coded multiplexing device may be built, which is quite comparable to a Pcm equipment, as far as the bandwith requirement is concerned. The Fcm terminal equipment is much more expensive and sophisticated than the Pcm one, but the Fcm line amplifiers are simpler than the Pcm regenerative repeaters. The development of the Fcm system relies upon further technical and economical progress of the technology of digital discrete Fourier transformer operating at very high speed.     

A taxonomy for medium access control protocols in wireless sensor networks

Wireless sensor networks (Wsns) tend to be highly optimized due to severely restricted constraints. Various medium access control (Mac) protocols forWsns have been proposed, being specially tailored to a target application. This paper proposes a taxonomy for the different mechanisms employed in those protocols. The taxonomy characterizes the protocols according to the methods implemented to handle energy consumption, quality of service and adaptability requirements. We also present an overview of the transceptors found inWsns, identifying how events on communication affect the energy consumption. Based on the taxonomy, we classify existingMac protocols. Finally, we discuss challenging trends inMac protocols forWsns, such as security issues and software radios.     

Optimisation de la quantification vectorielle codée par treillis: application au codage des paramètres LSF

Speech coders operating at low bit rates necessitate efficient encoding of the linear predictive coding (Lpc) coefficients. Line spectral Frequencies (Lsf) parameters are currently one of the most efficient choices of transmission parameters for theLpc coefficients. In this paper, an optimized trellis coded vector quantization (Tcvq) scheme for encoding theLsf parameters is presented. When the selection of a proper distortion measure is the most important issue in the design and operation of the encoder, an appropriate weighted distance measure has been used during theTcvq construction process. We further applied the optimizedTcvq system for encoding theLsf parameters of the us Federal Standard (Fs1016) 4.8 kbps speech coder. At lower bit rates, objective and subjective evaluation results show that the incorporatedLsf tcvq encoder performs better than the 34 bits/frameLsf scalar quantizer used originally in the fs1016 coder. The subjective tests reveal also that the 27 bit/frame scheme produces equivalent perceptual quality to that when theLsf parameters are unquantized.     

A MC-CDMA system analysis in a software radio context

This paper presents a Multi-Carrier Code Division Multiple Access (Mc-Cdma) system analysis in a software radio context. Based on a combination of multi-carrier modulation and code division multiple access,Mc-Cdma benefits from the main advantages from both schemes: high spectral efficiency, high flexibility, multiple access capabilities, etc. It is firstly shown why, nowadays,Mc-Cdma is undoubtedly a high potential candidate for the air interface of the 4G cellular networks. TheMc-Cdma concept and the block-diagrams of the transmitter and the receiver are presented first. Afterwards, the technical issues concerning the processing devices for the implementation ofMc-Cdma systems in a software radio context are analysed. The advantages and disadvantages of Digital Signal Processors (Dsps) and Field Programmable Gate Arrays (Fgpas) components are discussed. The implementation ofMc-Cdma systems and the integration of signal processing algorithms as Fast Hadamard Transform (Fht) and Inverse Fast Fourier Transform (Ifft) are considered and analysed for the first time. Finally, implementation results with a mixed prototyping board are presented. Then, it is shown that a new combination of the flow graphs ofFht andIfft leads to interesting computation savings and that hardware structures asFgpas are more adapted thanDsps to those intensive computation functions. Finally, for the completeMc-Cdma modem implementation, the necessity of a Co-Design methodology is highlighted in order to obtain the best matching between algorithms and architecture.     

Cross-polarization measurements at Martlesham Heath (UK) using OTS

Slant-path propagation measurements using transmissions to and from the Ots satellite are being carried out at Martlesham Heath (Uk)at an elevation angle of 29.9°. The measurements allow the determination of the attenuation and cross-polarization for circularly-polarized signals at 14.5 and 11.8 GHz and attenuation and cross-polarization for a linearly-polarized signal at 11.6 GHz. This paper reports the results of cross-polar measurements made during the summer of 1978 together with examples of particularly severe cross-polar effects from 1979. The measurements are compared with both a Ccir curve relating cross-polar discrimination to fade depth for rain and theoretical curves supplied by Essex University. The results clearly demonstrate the superior cross-polar performance of linear polarization compared to circular polarization.     

Codage conjoint source-canal des paramètres lsf du codeur FS1016: application de la quantification vectorielle par treillis optimisée pour un canal bruité

In this paper, we present an optimized trellis coded vector quantization (tcvq) coding system designed for the effective and robust coding of lsf spectral parameters at low bit rate. The aim of this system, called at the beginning « lsf-otcvq Encoder », is to achieve a low bit rate transparent quantization of lsf parameters of the us Federal Standard (fs1016) speech coder. Once the effectiveness of our lsf-otcvq encoders (with weighted distance) was proven in the case of ideal transmissions over noiseless channel, we were interested after in the improvement of their robustnesses for real transmissions over noisy channel. To implicitly protect the transmission indices of our lsf-otcvq coders incorporated in the Fsl016, we used a joint source-channel coding carried out by the channel optimized vector quantization (covq) method. In the case of transmissions over noisy channel, we will show that our new encoding system, called “covq-lsf-otcvq Encoder”, would be able to contribute significantly in the improvement of the fs1016 performances by ensuring a good coding robustness of its lsf spectral parameters.     

Mastering quality of service in GPRS/UMTS an overview

Quality of Service (QoS) has become a very important issue in networking, covering many performance aspects and numerous measures. The deployment of next generation wireless system includes 2.5G General Packet Radio Service (Gprs), which is the packet-switched extension of the Global System for Mobile communications (Gsm), and Third-Generation (3G) Universal Mobile Telecommunications System (Umts) to meet the needs of larger capacity and higher bit rates. AnUmts packet core network is an IP-based network. The Internet Engineering Task Force (Ietf) Forum developed several IP QoS related mechanisms available for IP transport networks. Service Quality Management (Sqm), one component of Telecommunication Management Network (Tun), will enable providers to manage QoS against objectives set out in customer Service Level Agreements (Slas) and will enable customers to compare the service offerings of different service providers.     

Modélisation de la technique temporelle asynchrone

The evolution towards the integrated broadband communication network is undertaken by several countries. In France, system designers ofCnet in Lannion have assembled an experimental integrated broadband communication network (Ibcn)called Prelude based upon a 4.5 Gbit/s switching matrix and a new and promising transfer mode called the asynchronous time-division technique (Atd).During the project, performance evaluation studies have been carried through, aiming to validate the Atd concepts and to dimension the switching matrix. This paper reviews the Ibcn lab experiments in the world, with operational characteristics;Atd concepts are detailed and the most important features of Ibcn modelling are given. Finally, the network dimensioning is carried through giving packet size, buffers length, loss probabilities and waiting times.     

Bandwidth sharing under the assured forwarding Per-Hop behavior

The DiffServ’s Assured Forwarding (af) Per-Hop Behavior (phb) Group defines a differentiated forwarding of packets in four independent classes, each class having three levels of drop precedence. Specific end-to-end services based on thisphb are still being defined. A particular type of service that could assure a given rate to a traffic aggregate has been outlined elsewhere. In such a service, a fair distribution of bandwidth is one of the main concerns. This paper presents experimental work carried out to evaluate howaf distributes bandwidth among flows under different load conditions and traffic patterns. We focused on the effect that marking mechanisms have on bandwidth sharing among flows within a singleaf class. The traffic types we used includeudp flows, individual and aggregatedtcp flows, mix oftcp andudp, tcp sessions with heterogeneous round-trip times, as well as color-blind and color-aware re-marking at the aggregation point fortcp flows. Tests were performed on real and simulated networks. We have found certain conditions under whichaf distributes bandwidth fairly among nonadaptiveudp flows andtcp aggregates. Finally, we evaluate a basic rule for setting the parameters of the two-rate Three-Color Marker conditioning algorithm (trtcm) in order to achieve a better bandwidth distribution fortcp flows.     

Improving parallel EKF-based nonlinear channel equalization using unscented transformation

The paper presents a new review of parallel Kalman filtering for nonlinear channel equalization. A Network of Extended Kalman Filters (nekf) has already been suggested for this purpose. This equalizer gives recursively a minimum mean squared error (mmse) estimation of a sequence of transmitted symbols according to a state formulation of a digital communication scheme. It is essentially based on two mechanisms: the approximation of the non Gaussiana posteriori probability density function (pdf) of the symbol sequence by a Weighted Gaussian Sum (wgs); and the local linearization of the nonlinear channel function for each branch of the network. Since the linearization, bearing on scattered symbol states, is one of the major limitations of thenekf, a new Kalman filtering approach, the Unscented Kalman Filter (ukf) suggested by Julier and Uhlman is considered in this paper for an interesting adaptation to the equalization context. Theukf algorithm is based on the equations of a Kalman filter, as the optimal linear minimum variance estimator, and on determining conditional expectations based on a kind of deterministic Monte-Carlo simulations. The new equalizer referred to as the Network ofukf (nukf), thus combines density approximation by awgs and the Unscented Transformation (ut) principle to circumvent the linearization brought within eachekf and is shown to perform better than thenekf based equalizer for severe nonlinear channels. Also, an adaptive version of thenukf is developed using the k-means clustering algorithm for noise-free channel output identification, since thenukf-based algorithm does not require the knowledge of the channel nonlinearity model.     

Iterative low-complexity receiver for the UMTS downlink

In this paper, an iterative low-complexity receiver is proposed for Code Division Multiple Access (cdma) systems with small spreading factors. Theumts (Universal Mobile Télecommunication System) radio interface based oncdma has been designed to offer a wide range of data rates using variable spreading factors. High data rate services are obtained by using small spreading factors. For such services, the spreading sequences have bad autocor-relation properties causing the degradation of the Rake receiver performance because of the InterSymbol Interférence (isi). In order to improve the receiver performance, we propose to add a Decision Feedback Sequence Estimation (dfse) equalizer at the Rake receiver output. Thedfse is a low complexity equalizer which is able to take into accounta priori probability ratios and to deliver a posteriori probability ratios on bits in order to exchange soft information with the channel decoder, so that the proposed receiver benefits from the turbo-processing gains. Channel estimation is also treated in an iterative fashion. The complete receiver is well suited to theumts downlink system as it drastically reduces theisi while keeping a reasonable computational complexity.     

On estimation of the power density spectrum with particular reference to a non stationary phenomenon

Results of measurement of the Power Density Spectrum (Pds)of the cable impedance profile (reflection density function) are reported. Spectral analysis techniques using the modified periodogram method (Mpm)and the maximum entropy method (Mem)have been used to obtain estimates of the relevant Pds from the measured data. The results of the two methods are compared. Resolution, bias and stability are examined. The results indicate that the resolution and stability obtainable using Mem are superior but with short records there is evidence of loss of detail.     

Propagation oblique dans les bandes de fréquences des 11 et 14 GHz

After describing the characteristics of the experimental satellite telecommunications earth-station at the Cnet,the article sets out the results of propagation measurements carried out on the one hand with the Italian experimental communications satellite Sirio I between November 1977 and November 1978, and on the other hand with the European Ots satellite between January and November 1979. 8 353 hours of useful data recording have been obtained using Sirio Iand 7 879 hours with Ots.     

Characterization and evaluation of TCP and UDP-based transport on real networks

StandardTcp (RenoTcp) does not perform well on fast long distance networks, due to its AMD congestion control algorithm. In this paper we consider the effectiveness of various alternatives, in particular with respect to their applicability to a production environment. We then characterize and evaluate the achievable throughput, stability and intra-protocol fairness of differentTcp stacks (Scalable,Hstcp,Htcp, FastTcp, Reno,Bictcp, hstcp-lp andLtcp) and aUdp based application level transport protocol (Udtv2) on both production and testbed networks. The characterization is made with respect to both the transient traffic (entry and exit of different streams) and the steady state traffic on production Academic and Research networks, using paths withRtts differing by a factor of 10. We also report on measurements made with 10 Gbit/secNics with and withoutTcp Offload Engines, on 10 Gbit/s dedicated paths set up forSc2004.