What is DVB-RCS?
- an overview, information or tutorial about the basics of what is DVB-RCS, Digital Video Broadcast - Return Channel via Satellite.
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With DVB standards being used in many areas of television broadcasting a further standard referred to as DVB-RCS or Digital Video Broadcast - Return Satellite via Satellite is attracting a large amount of interest. Although DVB-RCS was originally intended as a broadcast technology, a number of other users from a broad spectrum of communications users are looking at the technology with a view to its use.
DVB-RCS is one of the DVB family of standards that is in widespread use for a number of television broadcast applications that can be delivered by a variety of methods. The DVB standards now dominate the television area and they have been very successfully implemented in many countries.
What is DVB-RCS?
While most television standards simply involve a one way transmission using a "one to many" concept, DVB-RCS uses a return channel to enable two way transmissions to be made.
Like other DVB standards, DVB-RCS has been designed by the DVB Project. It defines a complete air interface specification for a two way satellite broadband scheme. It uses a VSAT (Very Small Aperture Terminal - an earth station, used for the reliable transmission of data, video, or voice via geo-stationary satellite, with a relatively small dish-antenna often around 1 to 2 metres in diameter). In effect DVB-RCS provides the user with a satellite based ADSL-style link without the need for the land based cables. This makes DVB-RCS ideal for use in many areas where there is no terrestrial infrastructure installed.
Depending on link budgets between the earth station and the satellite as well as other system parameters, DVB-RCS is able to provide up to 20 Mbit/s to each terminal on the outbound link, and up to 5 Mbit/s or more from each terminal on the inbound link.
The DVB-RCS technical specification is now approved as DVB-RCS+M. It provides support for a variety of types of terminal including mobile and nomadic terminals. In addition to this, it provides enhanced support for direct terminal-to-terminal, or mesh connectivity. DVB-RCS+M includes features such as live handovers between satellite spot-beams, spread-spectrum features to meet regulatory constraints for mobile terminals, and continuous-carrier transmission for terminals with high traffic aggregation. It also includes link-layer forward error correction based on Raptor or Reed-Solomon codes, used as a countermeasure against shadowing and blocking of the satellite link.
The standards for DVB-RCS are maintained by ETSI with the actual standard number: EN 301 790.
In terms of the development of the standard, DVB-RCS was first published in 2000 and since then it has been quite stable. The first major change was the introduction of DVB-RCS+M (for mobile applications), otherwise changes have mainly been for maintenance. These changes included support for the DVB-S2 forward link standard.
DVB-RCS market drivers
The DVB-RCS standard was developed in response to a request from a number of satellite and network operators. These organizations wanted to be able to deploy VSAT systems to enable two way radio communications, i.e. not only from the satellite to the users, but also allowing the user to be able to send data back up to the satellite and hence into a data network.
At the outset of the development of the DVB-RCS standard the partners in the project wanted a standard for a satellite system that would enable two way communications. In addition to this the satellite standard would need to be open to mitigate the risks of being tied to a single vendor.
During the development of the DVB-RCS satellite communications standard a variety of trade-offs needed to be made to ensure that a satisfactory balance was made between cost and performance. Also by including the industry partners and making the standard open, consensus for DVB-RCS was gained across the industry and in this way its future was assured.
DVB-RCS user terminals
One of the major requirements for DVB-RCS is that the user terminals should be relatively small, easy to use, and be manufactured to a cost while remaining reliable. To achieve this the basic form of DVB-RCS provides what may be termed "hub-spoke" connectivity; i.e., all user terminals are connected to a central hub. This hub controls the system and it also acts as a traffic gateway between the users and the wider Internet.
The DVB-RCS user terminal sometimes known as a Satellite Interactive Terminal (SIT) or Return Channel Satellite Terminal (RCST)) comprises a number of items:
- small indoor unit
- outdoor unit which includes a diplexer, low noise amplifier and a transmit RF amplifier
- antenna - size around 1 - 2 metres in diameter
DVB-RCS technical basics
The DVB-RCS satellite system provides the user with an interactive satellite service. Accordingly there are two elements to the system:
- Receive capability
- Transmit capability
The forward link is shared among a population of terminals using either DVB-S (EN 300 421) or the highly efficient DVBS2 standard (EN 302 307). Adaptive transmission to overcome variations in channel characteristics (e.g., rain fade) can be implemented in both the forward and return links.
The DVB-RCS return link or uplink to the satellite utilises a multi-frequency Time Division Multiple Access (MF-TDMA) transmission scheme. This form of scheme enables the system to provide high bandwidth efficiency for multiple users. A key to the high efficiency of the system is the demand-assignment scheme which uses several capacity mechanisms to allow optimisation for different classes of applications, so that voice, video streaming, file transfers and web browsing can all be handled efficiently.
DVB-RCS supports several access schemes making the system much more responsive, and thus more efficient, than traditional demand-assigned satellite systems. These access schemes are combined with a flexible transmission scheme that includes turbo coding, several burst size options and efficient IP encapsulation options. These tools allow systems to be fine-tuned for the best use of the power and bandwidth satellite resources.
The user terminal offers an Ethernet interface that can be used for wired or wireless interactive IP connectivity for a local home or office network ranging from one to several users. In addition to providing interactive DVB services and IPTV, DVB-RCS systems can provide full IP connectivity anywhere there is suitable satellite coverage. As most of the satellites that can provide DVB-RCS are likely to be geostationary, this effectively means anywhere closer to the equator than either 80 degrees north in the Northern hemisphere or 80 degrees south in the Southern Hemisphere.
Beyond the basic hub-and-spoke architecture, the DVB-RCS air interface has also been deployed in systems that provide direct terminal-to-terminal "mesh" connectivity, either through satellite on-board processors that mirror the functions of a ground-based hub, or through transparent satellites, using terminals equipped with an additional demodulator.
By Ian Poole
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