With most internet connections being short-lived (i.e. 10 segments), it is very tempting to enlarge the TCP Initial Window (IW). This would save two of the three RTTs needed to transfer most of the web pages through a legacy slow start. However it has been demonstrated that the bursts created by a larger IW greatly impair global performance. An intuitive solution is the TCP Pacing. By spreading the transmission over the whole RTT, Pacing smoothes the bursts and delays the congestion. While postponing congestion provides good performance for short-lived connections, it could significantly deteriorate global network performance insofar as the reaction to congestion is also delayed. This paper analyzes the weaknesses of large IW and TCP Pacing and proposes a fast Start-Up mechanism to speed up short-lived connections while preserving long-term connections. Extensive simulations and analysis demonstrate that our solution is as efficient as a larger IW would be in an uncongested network and better than current mechanisms in congested environments.

Passive inter-modulation products between transmitted signals may prevent the correct operation of satellite receivers using the same antenna. In many cases, these passive products do not obey the classical rule of 3 dB/dB slope as a function of carrier input power and they cannot be modeled using the classical theory based on polynomials. This has prevented the exact computation of carrier to inter-modulation ratio in multicarrier conditions from 2-carrier measurements. This has led to the use of higher than necessary margins. We present non-analytic models that generate non-integer inter-modulation slopes identical to that obtained in measurements and permit to predict multicarrier results.

The concept of delay/Doppler radar altimeter has been under study since the mid 90’s, aiming at reducing the measurement noise and increasing the along-track resolution in comparison with the conventional pulse limited altimeters. This paper introduces a generalized semi-analytical model for the delay/Doppler echo that accounts for antenna mispointing, as well as an associated least squares estimation algorithms. The mean power of a delay/Doppler echo can be expressed by a convolution of three terms that are the probability density function (PDF) of the heights of the specular scatterers, the time/frequency point target response (PTR) of the radar and the flat surface impulse response (FSIR). The first contribution of this paper is the derivation of a generalized analytical model for the FSIR that accounts for antenna mispointing. The proposed analytical expression for the FSIR also considers Earth curvature, a circular antenna pattern and a Gaussian approximation for the antenna gain. The two dimensional delay/Doppler map (DDM) is then obtained by a numerical computation of the convolution between the proposed analytical FSIR expression, the PDF of the sea wave height and the time/frequency PTR. The resulting DDM depends on five altimetric parameters that are the epoch, the significant wave height, the amplitude, the along-track and the across-track mispointing angles. Appropriate processing, including range migration and multi-looking, is applied to the resulting DDM yielding the Doppler echo (also known as the multi-look echo). The second contribution of this paper is the derivation of estimators for the five parameters associated with the multi-look echo. A least squares approach is investigated by means of the Levenberg-Marquardt algorithm. Moreover, the study of the effect of antenna mispointing shows high correlation between the along-track mispointing and the echo's amplitude. Thus, a four parameter estimation strategy has been proposed rather than the mere estimation of the five parameters of interest. In order to evaluate these strategies, we compare their estimation performance to that obtained using the three parameter model derived in a previous paper [1]. Validation of the proposed model and the corresponding algorithms is achieved on simulated and real Cryosat-2 data. The obtained results are very promising and confirm the accuracy of the proposed model.

This paper analyses the performance of TCP over random and dedicated access methods in the context of DVB-RCS2. Random access methods introduce a lower connection delay compared to dedicated methods. We investigate the potential to improve the performance of short flows in regards to transmission delay, over random access methods for DVB-RCS2 that is currently under development. Our simulation experiments show that the transmission of the first ten IP datagrams of each TCP flow can be 500 ms faster with random access than with dedicated access making the former of interest to carry Internet traffic. Such methods, however, are less efficient in regards to bandwidth usage than dedicated access mecanisms and less reliable in overloaded network conditions. Two aspects of channel usage optimization can be distinguished: reducing the duration of ressource utilization with random access methods, or increasing the spectrum efficiency with dedicated access methods. This article argues that service providers may let low-cost users exploit the DVB-RCS2 to browse the web by introducing different services, which choice is based on the channel access method.

for Intelligent Transportation Systems (ITS). They may or may not use the infrastructure. They will consider from the traffic safety applications up to the driver’s comfort or network games. The map updates are, from our point of view, a representative application but in the other hand it can help to reduce congestion in improving efficiency in decision making. It has well-defined characteristics : high volume of data, low delay constraint, possibility of implementation of infrastructure-to-vehicle communications, between vehicles and hybrids. The objective is that the contents are fully downloaded by all vehicles in minimum time, using fewer resources and lower costs. The solutions that have emerged as the most suitable concerned the use of the technology 802.11p with or without infrastructure. In the case of solutions with infrastructure, a number of access points broadcast information with coverage areas most often disjointed. Given the size of area used and/or flow devoted to this type of applications, the transition to a single access point is not enough to download these maps. It is then to define strategies of information dissemination. A first study was to compare a unicast strategy face to broadcast/multicast strategy. The latter appears largely improved. A combination of these principles does not improve system performance, because the flow devoted to unicast transmission does not compensate for the flow not used by the broadcast. The problem is duplicate chunks received by vehicles passing from several consecutive access points. To mitigate the phenomenon of duplication, we used the linear network coding pseudorandom. The idea is that the access point broadcasts linear combinations of chunks of files. The large number of these linear combinations significantly reduces this phenomenon. In a complementary manner, we investigated the use of ad hoc communications to fill the missing chunks of file, particularly in the absence of infrastructure. We verified that we could achieve good results in this context based on the diversity of chunks of files which are owned by the encountered vehicles.

The evolution of wireless communications systems have enabled to consider many applications for Intelligent Transportation Systems (ITS). They may or may not use the infrastructure. They will consider from the traffic safety applications up to the driver’s comfort or network games. The map updates are, from our point of view, a representative application but in the other hand it can help to reduce congestion in improving efficiency in decision making. It has well-defined characteristics : high volume of data, low delay constraint, possibility of implementation of infrastructure-to-vehicle communications, between vehicles and hybrids. The objective is that the contents are fully downloaded by all vehicles in minimum time, using fewer resources and lower costs. The solutions that have emerged as the most suitable concerned the use of the technology 802.11p with or without infrastructure. In the case of solutions with infrastructure, a number of access points broadcast information with coverage areas most often disjointed. Given the size of area used and/or flow devoted to this type of applications, the transition to a single access point is not enough to download these maps. It is then to define strategies of information dissemination. A first study was to compare a unicast strategy face to broadcast/multicast strategy. The latter appears largely improved. A combination of these principles does not improve system performance, because the flow devoted to unicast transmission does not compensate for the flow not used by the broadcast. The problem is duplicate chunks received by vehicles passing from several consecutive access points. To mitigate the phenomenon of duplication, we used the linear network coding pseudorandom. The idea is that the access point broadcasts linear combinations of chunks of files. The large number of these linear combinations significantly reduces this phenomenon. In a complementary manner, we investigated the use of ad hoc communications to fill the missing chunks of file, particularly in the absence of infrastructure. We verified that we could achieve good results in this context based on the diversity of chunks of files which are owned by the encountered vehicles.

The evolution of wireless communications systems have enabled to consider many applications for Intelligent Transportation Systems (ITS). They may or may not use the infrastructure. They will consider from the traffic safety applications up to the driver’s comfort or network games. The map updates are, from our point of view, a representative application but in the other hand it can help to reduce congestion in improving efficiency in decision making. It has well-defined characteristics : high volume of data, low delay constraint, possibility of implementation of infrastructure-to-vehicle communications, between vehicles and hybrids. The objective is that the contents are fully downloaded by all vehicles in minimum time, using fewer resources and lower costs. The solutions that have emerged as the most suitable concerned the use of the technology 802.11p with or without infrastructure. In the case of solutions with infrastructure, a number of access points broadcast information with coverage areas most often disjointed. Given the size of area used and/or flow devoted to this type of applications, the transition to a single access point is not enough to download these maps. It is then to define strategies of information dissemination. A first study was to compare a unicast strategy face to broadcast/multicast strategy. The latter appears largely improved. A combination of these principles does not improve system performance, because the flow devoted to unicast transmission does not compensate for the flow not used by the broadcast. The problem is duplicate chunks received by vehicles passing from several consecutive access points. To mitigate the phenomenon of duplication, we used the linear network coding pseudorandom. The idea is that the access point broadcasts linear combinations of chunks of files. The large number of these linear combinations significantly reduces this phenomenon. In a complementary manner, we investigated the use of ad hoc communications to fill the missing chunks of file, particularly in the absence of infrastructure. We verified that we could achieve good results in this context based on the diversity of chunks of files which are owned by the encountered vehicles.

This paper addresses the problem of ship localization by using the messages received by satellites and transmitted by the automatic identification system (AIS). In particular, one considers the localization of ships that do not transmit their actual position in AIS signals. The proposed localization method is based on the least squares algorithm and uses the differences of times of arrival and the carrier frequencies of the messages received by satellite. A modification of this algorithm is proposed to take into account the displacement model of the ships as additional measurements. This modification shows a significant localization improvement.