TUTORIALS

Digital Satellite Broadcasting with DVB-SH

For ITS systems demanding for large territorial coverage even in remote areas with sparse population, satellite-based broadcasting systems offer an attractive advantage over pure terrestrial systems by alleviating from the need to build-up cost intensive terrestrial infrastructure.

For SafeTRIP, digital satellite broadcasting using DVB-SH has been adopted. DVB-SH is a European standard for the provision of mobile satellite services [DVB-SH-SYS], [DVB-SH-WF]. A DVB-SH system consists of a satellite component (SC) providing large territorial coverage that can be accompanied by a complementary ground component (CGC) to enhance reception in areas where satellite reception may be compromised (typically in urban areas with a lot of high buildings). Such a hybrid (combined satellite and terrestrial) broadcasting system offers additional flexibility in finding the optimal trade-off between system capacity and deployment costs, by varying density and transmit-power of the CGC network.

In contrast to typical broadcasting applications, where latency (from transmission in the Hub Station) to reception in the On-Board Unit is irrelevant, in an ITS domain, there is a need for data distribution with low latency, e.g., in case of an emergency or for severe road alerts. The optional low-latency extension of the latest DVB-SH standard [DVB-SH-WF] is therefore adopted in the SafeTRIP platform. It allows simultaneous transmission of regular broadcasting content (without specific latency requirements) together with time critical low-latency data. Note that a higher latency for non-critical content is usually desirable as it allows enhanced protection against impairments of the physical transmission channel.

In the SafeTRIP platform, the broadcasting forward-link serves three main purposes:

-     Live streaming of multi-media content mainly for the provision of live TV/Audio services

-     Data casting for the transfer of any type of “file” from the service centre to the vehicle

-     Low-latency transmission of content like e.g. for time-critical control information.

Messaging from the Car to the Satellite with the S-MIM Standard

The broadcasting forward-link is supplemented with a message-based return link directly from the vehicle via the satellite to the Hub Station. As already stated for the forward-link, using satellite technology is attractive in the ITS domain as it offers large territorial coverage without the need for deployment of a terrestrial network.

For SafeTRIP, the Enhanced Spread Spectrum Aloha (E-SSA) waveform according to the S-MIM standard [S-MIM] is used. It consists of a single transport channel, the so-called Random Access Channel (RACH). Three different packet sizes are supported by the S-MIM standard; the size of a typical message can be compared with an SMS. Typical data rates per vehicle are in the range of a few kBit/s during transmission of messages.

The advantage of E-SSA is that it is an asynchronous burst transmission protocol allowing un-solicited and un-synchronized transmission of messages. In addition, the waveform is defined in such a way that simultaneous transmissions from a large number (up to several thousands) of terminals can be handled. The transmission is suitable for vehicular applications as it is:

-      designed to tolerate typical mobile channel impairments;

-      optimized for low transmission power (only slightly above GSM);

-      suited for use of low-cost omni-directional antennas.

In the SafeTRIP platform, the subtleties of messaging via satellite are hidden from the application developer by the Service Enabling Layer middleware on the OBU side and the Service Enabling Platform on the server side. Applications need to deal with messaging only on the level of an IP-based API (see The SafeTRIP Middleware for details).