This study aims at summarising the work done in the SatNEx project for the development and the performance evaluation of a new access scheme for multimedia data transmissions in digital video broadcasting-return channel via satellite (DVB-RCS)/satellite second generation (S2), applied to a mobile environment. The work has been stimulating because, as far as the authors know, this research topic has not yet been fully addressed in the literature. The authors started thinking about a realistic scenario of mobility, where novel services could be offered; having defined the scenario, the authors developed a satellite access scheme suitable for transmission control protocol (TCP) sessions and video streams in mobility. In this study, the authors present the reference architecture, the access scheme, the choice of the most suitable transmission parameters and the simulation results that are obtained when transmitting the two different types of data.

Terrestrial and satellite networks have dominated the telecommunications arena for years. In parallel with these two well-established technologies, a new alternative based on platforms located in the stratosphere, termed high altitude platforms (HAP's), has gained considerable interest in the past few years. In this paper, we consider an integrated HAP-satellite architecture and propose a reliable multicast protocol that capitalizes upon the HAP-layer to perform local retransmissions. Our protocol relies on a progressive packet-level forward error correction (FEC) scheme. The performance of this multicast protocol has been supported thorough the comparison with a theoretical bound in the literature.

The goodput of long-lived TCP connections can be maximized in error-prone channels by trading bandwidth for Bit Error Rate (BER). This fact can be exploited in the bandwidth and transmission parameters' assignment, by means of Adaptive Coding and Modulation (ACM), in satellite systems; in such a case, multiple source- destination (SD) pairs actually "see" channels with diverse characteristics, owing to unequal weather conditions. However, when dealing with multiple SD assignments, both aspects of goodput-maximization and fairness must be dealt with. Improving the effectiveness of TCP may be relatively easy, but maintaining the effectiveness while maintaining the fairness is a challenge. The paper numerically analyzes the fairness behavior of two bandwidth allocation methods to TCP connections, experiencing diverse fading conditions.

The TCP efficiency degradation in high delay- bandwidth product and error-prone channels is a well-known problem. To reduce this degradation, we utilized a hybrid wireless network architecture, where a geostationary bent-pipe satellite channel is used for forward high bit-rate transmissions, while the return link is realized through a terrestrial 3G segment. The performance of four of the most popular TCP versions are tested and compared via simulations in terms of goodput. Our aim is choosing which version performs better by using an Adaptive Coding and Modulation (ACM) technique, both alone and together with an Automatic Repeat reQuest (ARQ) scheme of the Selective Repeat (SR) type. ARQ is used with and without transmit and receive timeouts. The obtained results show that such a hybrid architecture is advantageous for TCP transmissions in terms of average goodput, and that ACM is effective only if used together with ARQ schemes.

Optimizing the end-to-end throughput of a TCP connection (goodput) over geostationary satellite links is a challenging research topic. This is because the high delay-bandwidth product, together with a non-negligible random loss of packets, is a condition that considerably differs from the environments TCP was originally designed for. As a result, TCP performance is significantly impaired by the channel bit error rate. The literature is full of suggestions for improving TCP goodput, most based on modifications of the protocol itself. We only investigated the application of different FEC (forward error correction) types for TCP goodput optimization, leaving the end-to-end protocol unaltered. Using a method midway between analysis and simulation to evaluate the goodput of TCP long-lived connections, we first studied the influence of packet loss rate, introduced by errors on the channel, on the TCP goodput. We showed that, in some cases, the packet loss rate does not need to be negligible with respect to that caused by congestion, as it is widely-held opinion. We then applied physical-level FEC techniques, such as convolutional encoding/Viterbi decoding, Reed Solomon, link-level erasure codes and their combinations, over a wide field of signal to noise conditions of the satellite channel. For each FEC type, we found the FEC rate that maximizes the TCP goodput, in each channel condition. We finally compared the results of all FECs used between them, and presented the case of multiple TCP connections sharing the same link as well

Video-coding techniques that are widely used in videoconferencing and streaming applications over the Internet may face degradation in performance when traversing satellite channels. The aim of the present paper is to provide objective measurements of the quality of video-encoded sequences for two of the most popular video coders in this setting [namely, H.261 and moving picture experts group (MPEG)-2], after transmission over faded satellite channels. In particular, the emphasis is on the Ka band, which is becoming an attractive alternative for commercial applications but is more prone to quality degradation than the currently widely used Ku band. In order to test the behavior of the coders in repeatable experimental conditions and over different data link platforms, two transmission chains have been implemented in the laboratory by careful emulation of the required environment. The data link layer has been based on HDLC-like and direct video broadcasting protocols, respectively. The experiments have been performed in the presence of additive white Gaussian noise (AWGN) and by using fading patterns derived by real-world data. The results obtained highlight some often neglected aspects in the behavior of the coders under examination, both in relation to their comparative performance and to their adaptability to different underlying data link protocols.

This paper shows how the adoption of a dynamic rate shaping (DRS) to variable bit rate (VBR) MPEG-2 (Motion Pictures Expert Group-2) coded video data, in addition to different levels of data protection, adds more flexibility in the way a video data stream can be sent over a faded satellite channel. DRS allows the satellite bandwidth required to be continuously trimmed, within a certain range. Also, some bandwidth can be saved when the video quality is maintained as close as possible to a required level during changing fade conditions. The performance of DRS applied to a VBR MPEG-2 coded video stream is shown. An application of this technique is discussed, and some criteria are given which help the user to choose the most suitable way to send video coded data, according to the service quality requirements.

We propose a model that describes the signal fading process due to scintillation in the presence of rain. We analyzed a data set of uplink (30 GHz) and downlink (20 GHz) attenuation values averaged over 1 s intervals. The data are samples relative to ten significant events, for a total of 180 000 s recorded at the Spino d'Adda (North of Italy) station using the Olympus satellite. Our analysis is based on the fact that the plot of attenuation versus time recalls the behavior of a self-similar process. We then make various considerations, and propose, a fractional Brownian motion model for the scintillation process. We describe the model in detail, with pictures showing the apparent self-similarity of the measured data. We then show that the Hurst parameter of the process is a simple function of the rain fade. We describe a method for producing random data that interpolate the measured samples, while preserving some of their interesting statistical properties. This method can be used for simulating fade countermeasure systems. As a possible application of the model, we show how to optimize fade measurement times for fade countermeasure systems.

We describe a method for estimating satellite link quality for use with rain fade countermeasures. The method applies to convolutionally encoded bit streams. It is based on the measurement of the uncoded data bit-error rate (BER) which is estimated by monitoring the errors recovered by a Viterbi decoder. Using a previously developed attenuation model, the measurement time intervals are optimized for a simple application, and the performance of the estimator is shown.

A description is given of FODA-TDMA (FIFO ordered demand assignment-TDMA), a satellite access scheme designed for mixed traffic (patent N. 9373A/89). The study of the system is presented and the choice of some parameters is justified. A simplified analytic solution is found, describing the steady-state behavior of the system. Some results of the simulation tests for an already existing hardware environment are also presented for channel speeds of 2 and 8 Mb/s, considering both the stationary and the transient cases. The results of experimentation at 2 Mb/s on the EUTELSAT-F2 satellite are also presented and compared with the results of the simulation.>

STELLA (Satellite Transmission Experiment Linking Laboratories) is a European wide-band data transmission experiment. STELLA makes use of the European Orbital Test Satellite (OTS) which provides a 2 Mb/s broadcast data transmission channel. The first phase of the STELLA project (STELLA/I) is summarized. The more important design characteristics of an improved version (STELLA/II) of STELLA/I are emphasized. Collaboration between STELLA/II and UNIVERSE projects in the framework of COST-11Bis is then outlined.