In the next few years, Low Earth Orbit (LEO) constellations will become key enablers for the deployment of global Internet of Things (IoT) services. Due to their proximity to Earth, LEO satellites can directly communicate with ground nodes and, thus, serve as mobile gateways for IoT devices deployed in remote areas lacking terrestrial infrastructure. Within this Direct-to-Satellite IoT (DtS-IoT) context, LoRa (Long Range) technology, capable of providing long range connectivity to power-constrained devices, has received great attention. However, serious scalability issues have been observed in LoRa-based DtS-IoT networks when a high number of LoRa devices perform uplink transmissions driven by the straightforward Aloha protocol during the short visibility periods of the passing-by satellites. In this paper, we evaluate some Aloha-based protocols suitable for this kind of networks and present a new adaptive variant that allows LoRa devices to dynamically adjust their uplink transmission rates in order to make the network work near its optimal operating point. Simulation results show that our proposal is able to significantly improve the network throughput in overloaded scenarios without the need for coordination among LoRa devices, listening to the channel nor gateway support.

Accurately estimating node density in Vehicular Ad hoc Networks (VANETs) is a challenging and crucial task. Various approaches exist yet none are able to deliver an accurate estimation in all situations especially in presence of collision in highly saturated networks. This paper describes an adaptive framework that allows individual nodes to estimate the real-time node density of their surrounding network. Our solution performs well in all situations even in chaotic channel conditions by retrieving information from collisions. This provides every vehicle on the road with two equations indicating the relation between 1) received signal strength versus simultaneously transmitting nodes, and 2) the relation between simultaneously transmitting nodes versus node density. These equations and frequently sampled received power levels enable individual nodes to estimate their real-time surrounding node density. Our proposal adapts to different environments and can address fast changes in configuration of the network and property of the channel in VANETs. Our experimental results indicate that the method yields reliable estimation in an acceptable time frame.

We present the statistical analysis of the output from three timeseries generators of the dual polarised multiple input, multiple output (MIMO) land mobile satellite (LMS). The Liolis- CTTC model [1] is the baseline for statistical modelling for the dual polarised LMS channel, leading to the development of the Enhanced Statistical Model [2] which addresses a number of issues including eliminating the unwanted high frequency components associated with low-pass filtering to impose temporal correlation on the slow variations, the introduction of Doppler effects including Doppler shaping of the fast variations, the implementation of a smooth state transition process and also the introduction of an interpolation process to sample the channel at the required sub-symbol rate for transmission. The QuaDRiGa model [3] is a geometric, ray-tracing model and the product of the ESA's extensive measurement campaign of the dual polarised LMS channel. In this paper, the timeseries of each of the three models are compared and particular focus is given to the the statistical properties of each model's output.

Biological research in agriculture needs a lot of specialized electronic sensors in order to fulfill different goals, like as: climate monitoring, soil and fruit assessment, control of insects and diseases, chemical pollutants, identification and control of weeds, crop tracking, and so on. That research must be supported by consistent biological models able to simulate diverse environmental conditions, in order to predict the right human actions before risky biological damage could be irreversible. In this paper an experimental distributed network based on climatic and biological wireless sensors is described, for providing real measurements in order to validate different biological models used for viticulture applications.

This paper presents some results obtained from a first approach to a point-scatter simulator for multiple-input multiple-output (MIMO) radio channel modeling. Specifically, we present the comparison of capacity obtained using the point-scatter simulator with that obtained with the Kronecker model. Capacity results obtained from measured propagation data are also presented. Ideas to enhance the simulator, which include the development of a statistical-based generative channel model for MIMO systems, are discussed.

This study aims to characterize and model the rain fields in the North-West of Spain from weather radar images. The study includes the modeling of these images according to two well known rain cell models: EXCELL and HYCELL. For this analysis, a procedure to detect rain fields and an algorithm for identifying rain cells within the images is presented. This work is part of an on-going activity trying to develop dynamic space-time models for rain fields for simulating rain induced effects on millimeter wave communication systems.

Biological research in agriculture needs a lot of specialised electronic sensors in order to fulfil different goals, like as: climate monitoring, soil and fruit assessment, control of insects and diseases, chemical pollutants, identification and control of weeds, crop tracking, and so on. That research must be supported by consistent biological models able to simulate diverse environmental conditions, in order to predict the right human actions before risky biological damage could be irreversible. In this paper an experimental distributed network based on climatic and biological wireless sensors is described, for providing real measurements in order to validate different biological models used for viticulture applications. First, the experimental network for field automatic data acquisition is introduced, as a system based in a distributed process. Following, the design of the wireless network is explained in detail, with a previous discussion about the state-of-the-art, and some measurements for viticulture research are pointed out. Finally future developments are stated.

The classical Mie scattering theory together with the bistatic radar equation has been applied in this paper to quantify interference levels in broadband fixed wireless access (BFWA) for two different network topologies: mesh and cellular. The complete scattering analysis takes into account the realistic variations of rainfall along the propagation paths and inside the common volume. The interfering element is a rain cell model that follows closely the experimental characteristic dimension, decay and movement obtained in recent research by the authors. The signal-to-interference power ratios (S/I) for two different frequencies, 28 and 42 GHz, and several scattering angles have been calculated. They show that, when two beams cross each other, rain-induced interference must be taken into account for an efficient planning of radio communication systems operating at these millimeter-wave frequencies as well as for an adequate design and implementation of fade mitigation techniques.

Classical models for mobile satellite channels are based on Markov chains where each state is linked to special propagation conditions. In this paper, we focus on the methodology of extraction of hidden Markov model (HMM) from experimental data to describe the time fluctuations of received power in a mobile satellite service (MSS) context. The method developed in this paper is based on a MCMC (Monte Carlo Markov Chain) training phase associated with a k-means classification. Its complexity is reduced when compared to traditional MCMC method. Contrary to existing detection methods, nearly no prior are necessary and it enables an accurate estimation of the HMM parameters and the possibility to detect the evolution of the channel sequentially. Furthermore, a sequential method, based on estimated HMM parameters, is presented and enables to follow the on-line variation of the state sequence.

This work presents a rain rate time series model based on a two-level Markov model structure. 'Rain' or 'no-rain' events are generated in two steps: in a first time, the 'rain' or 'inter-rain' duration of the considered event is determined according to the experimental data series. This corresponds to the first level of the model that is a semi-Markov model. In a second time, the rain rate intensities are generated resorting to an N states hidden Markov model based on the conditional probabilities of experimental rain samples (modeling the dependence of the current rain intensity with the previous one). This two-level model produces simulated rain samples whose statistics fit very accurately those of the experimental data without using any stored rain events in the generation step when the model is known.

The wireless access networks based on millimetre wavelength technologies are mainly impaired by rain. To evaluate the rain effects over a communication system, it is essential to know the temporal and spatial evolution of rainfall rate. For this reason it is necessary to develop an experimental network which provides the adequate data to study, prevent and compensate the rain fade. In this paper, an experimental rain gauge network is presented. This network comprises weather stations capable of measuring rainfall rate, temperature and humidity. The paper describes first the experimental network for automatic data acquisition as a system based in a distributed process. The design of the experimental network is explained in detail and finally the interest for millimetre wavelength applications is pointed out.

In this paper three statistical models frequently used to describe the land mobile satellite (LMS) propagation channel under narrowband conditions are compared. The models are those of Loo (1985), Corazza and Vatalaro (1994), and Suzuki (1979). Statistical models are very well suited for the production of time-series (generative models) for simulation studies. Methodologies to produce synthetic time-series corresponding to the three models are discussed and results are compared.

A simple three-segment model quantifying the cross-correlation coefficient of shadowing effects in multiple satellite systems for "street canyons" is presented. The assumption of uncorrelated blockage in two satellite-to-mobile links is not always accurate: positive or, even negative correlation may exist. Shadowing cross-correlation information is needed in the assessment of multi-satellite system availability.

In this paper a simple methodology to assess the availability of multi-satellite systems for terminals in urban areas is presented. Urban environments are defined in terms of a mixture of basic or canonic scenarios, namely, street canyons, street crossings T-junctions, single walls. For each of these basic scenarios "masking functions" are defined which are well suited for a quick, approximate evaluation of the availability.

We focus on a non-GEO scenario and investigate the effect of actual power control performance on the return capacity dynamics of a DS-CDMA system. We consider mobile users at different speeds and transmission rates yielding several classes of services. We compute analytically the ideal maximum return capacity of the system which is shown not to be fixed but limited by a hyperplane in the space of the considered quality of service (QoS), so that different resource assignment criteria can be applied. At maximum ideal capacity, for the different percentages of users that can be served, it is shown that the more restraining the QoS, the more resources that must be assigned. We then explore the appropriate efficiency of the system, i.e. the actual used resources as a function of the distribution of QoS. We finally look into the inter-beam effect by applying different degrees of diversity gains and we show the trade-off between improving the capacity efficiency which is poor due to power control error and the net capacity achieved by the overall system.

We present a wideband land mobile satellite channel model based on a three-state Markov chain model valid for any kind of satellite constellation and for L, S, and Ka bands. Data from measurement campaigns was used to extract the parameters for the statistical distributions involved in the model. We also present a methodology describing how to use the proposed channel model to interface with system-level analysis. We use the time series generated by the model to parameterize power control (PC) imperfection as a function of the environment and user speed by acting upon the time series through a seven-level PC algorithm. We then apply the obtained parameterization to the analysis of a direct sequence-code division multiple-access scenario with users requiring different quality-of-service (QoS). It is shown that request of different QoS produces an unbalance in the users power distribution and less users can be served with the most demanding QoS. PC error worsens this effect and considerably reduces capacity efficiency which is highly dependent on the environment and mobile speed. Bit-error rate statistics are also investigated through the log-normal characterization obtained by using the channel model obtaining results on the effect of required E/sub b//(N/sub o/+I/sub o/) of a particular user.

Abstract This paper reviews the state of the art and current trends towards 3G and 3G+ mobile satellite systems that are being planned in parallel or within the International Mobile Telecommunications (IMT‐2000) framework. The subject is firstly approached by introducing the key and enabling technologies both for satellites and high altitude platform stations. After having introduced an overall picture, another review is presented on physical layer research activities including propagation and channel modeling. Finally, the use of the reviewed channel models for Code Division Multiple Access system level analysis is introduced and some current efforts towards system design are presented. Copyright © 2002 John Wiley & Sons, Ltd.