A satellite broadcast network consists of a transmitting hub station and a number of receive-only
earth stations, and uses the resource of one or several channels (transponders) of a communications satellite. It relies on a star topology and point-to-multipoint connectivity. Links are unidirectional, from the hub towards the earth stations.
The hub is generally a rather large earth station while the receive-only earth stations can be very small (typical antenna size of the order of 0.5 m). The cheap cost of such small earth stations (less than 100 Euros) makes them affordable to end users who wish to receive at home television or audio programmes directly from a satellite; these end users are usually owners of their earth station.
This kind of network offers what is called a direct to home (DTH) service. The hub is located either at the broadcaster’s facilities (and is then operated by the broadcaster himself) or at some other location (when it is most often operated by the satellite operator). As per the Radio Regulations terminology, the uplink is called a feeder link, and the hub is often called the feeder earth station. The outbound link (feeder link from the hub) is received by all end-user earth stations. In such networks, there is no inbound or return link.
An evolution of this network architecture and its associated services consists in introducing
interactivity thanks to a low data rate return link transmitted from the earth stations towards the
hub. This allows the offering of interactive TV (iTV) or video-on-demand services.
DVB standards are used to deliver satellite television services throughout the world. The first DVB transmission specification was DVB-S for the satellite delivery of signals. The channel coding tools it described for the first time later became important for all other delivery media as well. DVB-S went on to become the world’s most popular system for the delivery of digital satellite television.
DVB-S supported compressed digital TV requiring a carrier whose bandwidth can be less than a transponder’s typical bandwidth (a DVB-S carrier can be as narrow as a few MHz while a transponder bandwidth can be as large as 72 MHz). Therefore DVB-S could employ two options either several carriers (programmes) per channel, or multiplexing of several (digital) TV programmes on the same carrier.
DVB-S supports MCPC (Multiple Channel Per Carrier) and SCPC (Single Channel Per Carrier), which are used to deliver broadband and private data circuits.
However, as usual with FDMA, the satellite transponder must be operated with some back-off, i.e. at reduced EIRP with respect to the maximum EIRP at saturation, which may penalise the quality of the service, unless the end customer uses a larger (and more costly) earth station, or modern satellites with very large EIRP are considered. To benefit from the maximum satellite EIRP, it is preferable to avoid FDMA access by the feeder links, and use the full channel bandwidth with a single carrier (as with Option 1) conveying a time division multiplex (TDM) of programmes from several broadcasters. With a regenerative satellite and on-board processing, it is possible tomultiplex the programmes on board the satellite.
The system also supports direct-broadcast satellite television from providers such as Sky, Astra, Dish Network, Globecast and Bell TV. With advances in technology, and the need for higher speeds and throughputs, the DVB-S standard needed to be advanced.
Second Generation Satellite (DVB-S2), developed around 2003, took advantage of advanced techniques for channel coding, modulation, and error correction to create a system that made a range of new services commercially viable for the first time. For example, when combined with the latest video compression technology, DVB-S2 enabled the widespread commercial launch of HDTV services.
Some of the improvements include:
– >Low Density Parity Check (LDPC) Forward Error Correction. This is a method for sending a message over a noisy transmission channel.
– >Variable Code Modulation (VCM) provides variable levels of error protection to different
components within the service. This is performed by permitting various combinations of modulation and FEC (Forward Error Correction) to be applied to different parts of a data stream.
– >Adaptive Coding and Modulation (ACM) provides an extension to VCM. It provides a feedback path so that various levels of error protection can be applied in near real time as signal propagation changes. In practical terms, this takes available link margin that is normally used to overcome deteriorating weather conditions and converts it to performance gains for data. ACM provided a real benefit to broadband satellite services.
Rain fade, antenna pointing errors, local noise, and interference, can all interfere and cause satellite performance to degrade. To overcome slowdowns or outages from these conditions, operators apply or reserve “link margin” or some amount of capacity that is essentially “set aside” and used as needed when conditions degrade. This works but is highly inefficient. Rather than sitting idle, ACM allows this link margin to be used to transmit data, when conditions are optimal, thus addressing both efficiency and availability.
– >DVB-S2 standard added 8-PSK, 16-APSK and 32-APSK. What these modulation options do is provide better spectral efficiency – or bits per symbol. This allows more data to be sent in the same bandwidth.
– >Support added for MPEG-2 TS based systems and MPEG-4 audio/video streams. MPEG is a standard for compressing moving pictures. MPEG 2 TS is designed for streaming live events. MPEG 2 supports data rates from 1.2 Mbps to 15 Mbps. MPEG-4 provides improved coding efficiency. It can encode mixed media such as video, audio and speech. Robust transmission is provided with advanced error resilience features.
DVB-S2 provided a 30% improvement in spectral efficiency over the original DVB-S. The standard was published as an ETSI (European Telecommunications Standards Institute) in 2005.
Vendors such as iDirect who had developed a proprietary TDM solution that outperformed DVB-S, switched to DVB-S2 when it came out, because it offered even greater efficiency.
In 2014, DVB released DVB-S2X, an extension of the DVB-S2 specification that provides additional technologies and features. S2X offers improved performance and features for the core applications of DVB-S2, including Direct to Home (DTH), contribution, VSAT and DSNG (Digital Satellite News Gathering). The specification also provides an extended operational range to cover emerging markets such as mobile applications. DVB-S2X is focused on emerging markets such as 5G.
New features include:
– Greater granularity of modulation and coding modes, supporting more “fine-tuning” of networks
– Smaller filter roll-off options of 5%, 10%, 20%, and 35%. The ROF or roll-off factor determines how much capacity can be squeezed into a particular band of spectrum. The satellite’s signal is filtered or masked and the “edges” of the signal drop off and are not used for transmission; i.e. wasted. The lower the ROF, the more of the signal that can be used to transmit data.
– Channel Bonding up to 3 channels. This provides a mechanism to deliver very large circuits with high aggregate data rates.
– More scrambling options for critical co-channel interference situations
– Very low SNR operation supporting C/N of down to -10dB
– Super Frame option is a development intended to help address beam hopping, as in a maritime vessel or airplane passing from satellite beam to satellite beam. It also supports switching in multi-spot-beam satellites, such as most of the HTS (High Throughput Satellites) being launched today.
The DVB-S2X standard was published in 2015 and has been swiftly implemented by satellite vendors. The new features enable efficiencies at higher capacities, and more granularity or control over how they are implemented. The new features pave the way to intelligent terminals that are defined by SDN (Software-Defined Architecture), allowing new capabilities to be added as they arise – for example support for LEO and MEO (Low and Medium Earth Orbit) satellites, as well as supporting increasing data rates.
DVB-S2X enables satellite networks to scale more cost effectively, managing many Mbps of capacity across multiple spot beams, creating virtual bandwidth pools. The tools are in place to support integration with other terrestrial and 5G mobile services, tying existing OSS and BSS
(Operational Support Systems/Business Support Systems) into the satellite platform. Finally, as mentioned above, the new features enable air, maritime and even “connected car” services, to maintain internet connections, as users pass from beam to beam.
Broadband satellite network
A broadband satellite network consists of one or several gateways (or hubs) and a number of satellite terminals with receive and transmit capability, and uses the resource of one or several channels (transponders) of a communication satellite. It can rely on a variety of network topologies (star, multi-star, mesh or hybrid star/mesh) and provide a variety of types of connectivity. Links are bidirectional. The characteristics of the satellite terminals and the gateways or hubs can vary a lot according to the market that is addressed. The consumer market calls for cheap and highly integrated satellite terminals, with the gateways being rather large earth stations. The professional market may address higher-end satellite terminals that have the capability to aggregate traffic generated by a LAN.
Broadband satellite networks are designed to offer most of the services that are provided by
terrestrial Internet networks. Internet service provision by satellite is mainly addressed through the widely accepted Digital Video Broadcasting (DVB) standards family, and in particular its satellite versions (DVB-S and -S2) where the data formatting, initially designed for transporting video and audio streams, is extended to carry IP datagrams. The satellite specific DVB-Return Channel Satellite (DVB-RCS) standard provides the specification for the return traffic flows from DVB-RCS terminals to gateways.
A DVB-S/S2/DVB-RCS network displays the following characteristics:
—The uplink of the RCS terminal (RCST) uses MF-TDMA according to the DVB-RCS standard,
—The downlink from satellite to RCST is fully compatible with the DVB-S/S2 standards.
—The satellite system supports symmetric predictive traffic, as well as bursty traffic generated by a large number of users, owing to dynamic allocation.
—The satellite system supports interworking with terrestrial networks such as PSTN and ISDN as
well as private IP networks belonging to service providers (SPs).
—The satellite system supports integrated IP-based data services and native MPEG video
—Satellite star connectivity network supports single-hop connectivity between satellite network
users and terrestrial network users through the GW. A satellite mesh connectivity network
supports single-hop connectivity between satellite network users; it requires an on-board
processor (OBP) that allows routing of MPEG packets from uplink to downlink beams in a
flexible way, possibly with data replication on board to support multicast services.
Management station (MS)
The management station (MS) consists of the network management centre (NMC) and the network control centre (NCC). The NMC manages all network elements and the network and service provisioning. TheNCC manages the control of the interactive network, e.g. it serves satellite access requests from the users of the system.
The GW provides interworking functions between the satellite and terrestrial networks, such as the telephony networks (including PSTN or ISDN) and Internet/intranet-oriented ground networks. The GW could be part of the hub’s functionality in the star topology of a transparent satellite access network. The GW can provide service guarantees to subscribers based on different QoS criteria and different subscription levels.
The GW is composed of different and configurable elements depending on the needs of connectivity. For example, a GW may consist of interactive receive decoders (IRD), an IP router,
an multi-conference unit (MCU), and a voice and video gateway or gatekeeper.
Return channel satellite terminal (RCST)
The user earth station is called a return channel satellite terminal (RCST) to be compliant with the terminology of theDVB-RCS standard. It consists of two main units, the indoor unit (IDU) and the outdoor unit (ODU) Uplink transmission functions consist of baseband to radio frequency (RF) up-conversion (the baseband signal modulates the carrier at intermediate frequency (IF) and then the carrier is up-converted to RF band) and RF amplification so that the signal is amplified before transmission via the transmit antenna.
The achieved EIRP must cope with the link budget requirements. On downlink reception, the RCST functions consist of RF low-noise amplification; downconversion to IF; IF DVB-S or DVB-S2 demodulation; and decoding at baseband.
The ODU consists of the RF transmitter, one or more RF receivers and the antenna. The IDU
contains the DVB-S/S2/DVB-RCS modemand the interface to the local network. The RCST may be connected to an Internet subnet in a local area network (LAN).
The RCST allows users to communicate with each other either in a single satellite hop (mesh
connectivity) or with a double satellite hop through theGW(star connectivity); it also allows users
to communicate with terrestrial network users through theGWin a single satellite hop, e.g. PSTN,
ISDN or IP-based Internet services.
On-board processor (OBP)
If a payload with on-board processing is considered, the OBP is the core of a satellite mesh system. It combines both DVB-RCS and DVB-S/S2 satellite transmission standards to allow
full cross-connectivity between the uplink and downlink beams.
The uplinkDVB-RCS carriers are down-converted from RF to a low ‘intermediate’ frequency; the
Baseband Processor (BBP) de-multiplexes, demodulates and decodes carriers at intermediate
frequency in order to generate a single multiplex of MPEG-2 packets compliant to the DVB-S/S2
standard using routing information coming from uplink packets; after channel encoding (FEC)
with selectable code rate, the modulation with QPSK (DVB-S) or higher modulations (DVB-S2) is
performed at IF to generate a carrier which is then frequency up-converted into the downlink
The components of the satellite network communicate with each other through the following
—T Interface—a user interface between RCST IDU and user terminals (hosts) or LANs;
—N Interface—the interface between NCC and RCST for control and signalling to support the
user plane (U-plane) services (synchronisation, DVB tables and connection control signalling);
—M Interface—the interface betweenNMCand RCST for management purposes (SNMP and MIB
—U Interface—the interface between the satellite payload and the RCST physical interface (the air
—P Interface—the logical interface between two RCSTs for transaction of peer layer signalling
traffic and user data traffic;
—O Interface—the interface between NCC and OBP interface for OBP control and management
Multibeam Satellite systems
TS 101 154 – DVB specification for the use of Video and Audio Coding in Broadcast and Broadband applications – is a core element of all DVB solutions. TS 101 154 is a “living” document, regularly updated to take account of new market requirements and technology developments. It provides implementation guidelines and conformance points for the use of audio and video coding utilizing MPEG-2 systems in satellite, cable and terrestrial broadcasting systems and in IP-based networks, and for the use of video coding for adaptive bitrate delivery over IP-based networks.
The 2019 revision of DVB-S2X added support for Beam Hopping. In multi-beam satellite systems, this technique enables efficient and flexible use of satellite resources for applications such as VoIP, cellular backhaul, Internet of Things, maritime and in-flight connectivity and government.
Several standards from the European Telecommunication Standardisation Institute (ETSI) and the Internet Expert Task Force (IETF) deal with the details of implementation of IP networking protocols and network architecture. These standards and technical specifications have speeded up satellite systems for both broadcasting and networking services.
References and Resources also include: