International Journal of Research and Reviews in Wireless Communication Communication Vol. 1, No. 1, March 2011 Copyright © Science Academy Publisher, United Kingdom
Science Academy Publisher
Direct Broadcast Satellite (DBS) Television Systems
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Lukman Sharif , Munir Ahmed , and Nauman Sharif 1
School of Computing, London College of Research, Reading, United Kingdom
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College of Computer Science and Engineering, Taibah University, Al-Madinah Al-Munawwarah, Saudi Arabia
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Department of Computer Science, Mohammad Ali Jinnah University, Islamabad, Pakistan
Correspondence should be addressed to Lukman Sharif
[email protected] Sharif
[email protected]
Abstract - Consumers around the world enjoy digital television from a variety of sources including terrestrial, cable, satellite and broadband Internet broadcast systems. However, it is satellite broadcast systems that have provided consumers real widespread opportunity to enjoy digital television. This paper presents an overview of direct broadcast satellite (DBS) systems used in the delivery of digital television. The key DBS system building blocks are identified including the broadcaster as well as the consumer side of the communication link. This paper also discusses the key technology evolutions that facilitated the introduction of DBS services, the most common services provided through DBS TV technology and the future directions that this technology is likely to take. Keywords: Digital TV, direct broadcast satellite (DBS), GEO satellite, DVB-S2, subscription-based TV, interactive TV
1.
Introduction
Digital Television is one of the most prevalent broadcast platforms around the world. The use of satellites to transmit digital TV directly to homes is perhaps one of the most successful commercial applications of this communications technology. The history of this particular application of satellite technology can be traced back to Arthur C. Clarke’s truly visionary idea first proposed in an article called “Extraterrestrial Relays” in “Wireless World Magazine” in 1945 [10]. Clarke proposed that special space platforms (now commonly referred to as satellites) could be launched into orbits at some 22,200 miles above the earth’s equator and used as relay stations to achieve worldwide communication. The Russians were the first to explore Clarke’s idea and their efforts culminated in the launch of the first man-made satellite, called Sputnik, in 1957 [10]. The first commercial application of direct broadcasts using a satellite came in 1976, when Home Box Office (HBO) started programme delivery to cable using satellite technology [13]. Satellite TV is essentially a wireless system used to deliver TV programs to viewers all over the world. The signals used by broadcast TV satellites are typically digitally compressed which allows multiple programs to be transmitted from a single transponder onboard the satellite. The type of program content delivered by satellite TV could include existing cable services, movies, sports, Pay per View (PPV) and a variety of other services. The term Direct to Home (DTH) services and Direct Broadcast Satellite (DBS) services is often used to describe this technology.
This paper provides an overview of DBS systems used in the delivery of TV content to consumers. DBS is a relatively new TV distribution technology. The term is often used to describe the actual communications satellites themselves as well as the TV service received by the consumers on earth. This paper provides a technical overview of the technology used in DBS TV delivery, on the broadcaster/operator side as well as the consumer side of the communication link. This paper also discusses the most common services provided through DBS TV technology and the future directions that this technology is likely to take.
2.
Overview of DBS TV systems
The signal containing the actual TV content originates on earth from broadcast stations. Due to the fact that this signal can only travel in straight lines, it is not possible for viewers’ receiving equipment to be in the required Line of Sight (LoS) to be able to capture and decode the signal. Satellite technology provides the solution to this particular problem. As first envisioned by Clarke, the DBS TV service involves the use of satellites launched into orbit at 22,200 miles above the earth’s equator. These satellites are said to be in geostationary or geosynchronous orbits (called GEO geostationary earth orbit) around the earth because the speed of their orbit is exactly the same as that of the earth itself. As a result, these satellites appear to be stationary above the earth. A GEO satellite orbits directly above the earth's equator at 0° latitude. As such the location of GEO satellites is described in terms of the longitude only, e.g. 28.2° East for
International Journal of Research and Reviews in Wireless Communications the SES Astra satellite used to broadcast Sky digital satellite television and radio services [6]. This particular feature of GEO satellites allows TV broadcast stations to direct the TV program signals at the satellites from an uplinking facility. The GEO satellites essentially relay the signal back to earth which can then be picked up by the consumer receiving equipment such as the parabolic shaped mini-dishes and converted back to the original TV programs using Integrated Receiver Decoder (IRD) devices. When compared with traditional TV delivery platforms such as terrestrial and cable, DBS systems offer numerous advantages. These include consistently high picture quality and a very wide range of channels. The use of satellites as the delivery mechanism ensures fast and comprehensive coverage [9], making it possible to reach low density population areas as well as parts of the world which may not yet have the basic cable or terrestrial TV infrastructure [2]. The receiving and decoding equipment required by consumers is readily available which allows the DBS service to be deployed in a very short period of time. The origins of DBS service could be traced back to 1975 when HBO first started to use satellite technology to deliver program content to cable TV companies for broadcast. This led to individuals, often those living in rural areas, installing their own dishes in order to capture the signals intended for distribution by cable companies [16]. These dishes became known as Television Receive Only (TVROs) in order to differentiate them from the commercial satellite TV systems used at the time. The TVRO systems operated in the C-band frequencies which typically range from 3.7–4.2 GHz for downlinks and 5.925–6.425 GHz for uplinks [1]. Due to these lower frequencies, much larger dish sizes were required on earth in order to adequately receive the signal. Typical TVRO dishes were often in excess of 4m in diameter [16]. The GEO satellites are generally classified as either Fixed Satellite Service (FSS) or Broadcasting Satellite Service (BSS) [1]. FSS tends to use C band frequencies for open satellite communications. This typically involves the use of the service to relay raw feeds and program content from the originating studios to cable head-ends and DBS uplink facilities to be rebroadcast as DBS services to viewers [16]. In contrast BSS services tend to use Ku-band frequencies in a closed system to deliver subscription based program content directly to small dishes installed at the consumer premises. The 11.7 to 12.5 GHz range is reserved for BSS [1]. Most modern DBS systems use the Ku-band frequencies. The higher power used in Ku-band means that the signal from satellite to receiver can be better directed allowing much smaller dish sizes to be used [16]. Most modern domestic dishes used for satellite TV services are between 43 cm to 80 cm (18in to 31in) in diameter. Before the advent of DBS, viewers would typically have motorised C-band dishes in order to capture the signal from different satellites [16]. Modern Ku-band dishes however, can be fixed in one position and still receive the full signal provided there is a clear LoS fix on the transmitting GEO satellite. Although modern consumer DBS receiving dishes tend to be fixed in one position, it is possible for them to be blown out of alignment due to bad weather, e.g. thunderstorms, resulting in
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degradation or loss of signal [16]. The presence of moisture (in the form of rain) along the LoS is one of the main issues for the smaller sized dishes. This generally results in signal fades. In DBS systems, there is also the issue of interference from other satellites at neighbouring orbits which could be picked up by the consumer dishes. In general, close coordination with the ITU and other operators can ensure that such interference is minimised [1].
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Technical overview of DBS TV systems
From a technical perspective, components of a DBS service: • • •
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Broadcasting / uplink facility GEO broadcasting satellites Consumer premise equipment
Broadcasting / uplink facility Like various other forms of satellite communication, the DBS service signal originates on earth. The DBS subscription based channels are typically sent over to the uplink facility through fibre connections or other satellite backhaul links [1]. The latter are generally Fixed Satellite Service (FSS) systems making use of the Ku-band and C-band uplink frequencies. The backhaul signals may also be used to deliver program content to other distributors (typically cable and satellite companies) [5]. It is increasingly common for DBS service providers to offer local off-the-air channels as part of their service [6]. For example, in the UK the main DBS provider, Sky, offers the digital public service channels such as BBC, ITV, and Channel 4 as part of its service. This requires the use of digital facilities to backhaul the public service channels to Sky’s uplink facilities. The uplink satellite dishes are generally quite large, typically around 9 to 12 meters in diameter [16]. This plays an important role in aiming and delivering higher signal strength to the relevant satellite. The signals are uplinked at a particular frequency range and can only be picked up by the satellite onboard transponder that is correctly tuned to that frequency range [1]. Figure 1 provides an overview of the process involved in the backhaul of signals to the DBS uplink facility.
Figure 1 [1]: Backhaul of program signals to a DBS uplink facility
In general, the content received by the uplink facility is not altered. However, the uplink facility does provide a number
International Journal of Research and Reviews in Wireless Communications of important functions. This includes adjustment and resynchronization of the incoming signal [1]. In the case of pre-recorded content, this also involves quality control and playback functions. The program content is also copied from master tapes and stored on video servers to be broadcast on the relevant satellite channel in accordance with the schedules / Electronic Program Guide (EPG). Conditional access forms a very important part of the DBS service business model. As such, the DBS service provider needs to supply the necessary equipment in order for the consumer to receive and pay for the service (discussed later). The broadcasting facility also provides important signal processing functions such as compression of video and audio content. The program content is typically compressed (from around 270 Mbps) to between 1-10 Mbps before transmission [1]. This also has the effect of dramatically increasing the number of channels for a given bandwidth. MPEG-2 is the most common encoding standard used [7] whilst Quadrature Phase Shift Keying (QPSK) is the most common modulation scheme used by DBS systems [8]. GEO broadcasting satellites The signal broadcast from the DBS uplink facility is picked up by the appropriate RF transponder (part of the frequencytranslating repeater) onboard the satellite. Most communications satellites are simply microwave radio relay stations and have dozens of onboard transponders. Each transponder could have a bandwidth in the region of tens of megahertz. The typical operation of a transponder is often referred to as bent-pipe because of the fact that the uplink signals are simply amplified and shifted to different frequency (called translation) in order to avoid interference with the uplink signal before being sent back trough the downlink [1]. In general, for a BSS Ku-band, the 11.7 to 12.5 GHz range is used.
Figure 2 [1]: A typical GEO satellite deployed for DBS services
The GEO satellites used for DBS services tend to be very similar to those used for traditional communications (see Figure 2). Since the mid 1990s, the satellites deployed for DBS services have greatly increased in size and weight.
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However this increase in size and weight offers many benefits for DBS services. The large onboard solar panels allow more DC power to be generated [1] and larger antennas facilitate better shaping of the downlink microwave beams.
Figure 3 [1]: Repeater configuration for one RF carrier onboard a GEO satellite
As shown in Figure 3, each DBS service satellite has a frequency-translating repeater. There is a front-end broadband receiver which converts to downlink frequency. The latter also drives multiple RF chains (transponders) with each transponder having high-power Travelling Wave Tube (TWT) transmitter [14]. Each TWT amplifier typically has a maximum power rating of 240W [15]. Consumer premise equipment The typical equipment used by the consumer to receive and decode the DBS signal is illustrated in Figure 4. The equipment consists of a parabolic shaped dish used to reflect the satellite signal to the feed-horn. The latter is the dish’s focal point and front-end of a waveguide that is used to pass the signal to a Low Noise Block (LNB) down-converter where the signal is down-converted to L-band IF of 950– 1450MHz [2].
Figure 4 [1]: A typical dish and Integrated Receiver Decoder (IRD) box at consumer premises
The dish also receives low-voltage DC power through the coaxial cable which is used to deliver the converted signal to
International Journal of Research and Reviews in Wireless Communications the IRD box inside the consumer premises [6]. The IRD box has a number of very important components required for a DBS service. This includes a QPSK demodulator, TV signal regenerator, an IF tuner, FEC decoder, stream de-multiplexer, decryptor (for conditional access), and a MPEG decoder (video/audio) [1]. In the UK, DBS receivers typically make use of smart cards with embedded chips that can be used for generating cryptographic keys in order to decrypt the program content for consumer viewing. For the purpose of viewing the program content, the IRD box decrypts and outputs the content to a SD or HDTV. The use of a smart card and encryption technology plays an important role in implementing the conditional access system for DBS providers. The conditional access system ensures that the consumer’s account is up to date with payments and that the consumer is in the correct geographical location in order to receive the content [6].
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High-definition television (HDTV) HDTV is rapidly becoming the norm for television viewing. HDTV offers a significantly enhanced viewing experience with notably higher picture quality than standard definition television (SDTV). DBS TV has been one of the earliest adopters of HDTV technology [4]. Satellite receiving and decoding equipment for HD content has been available since 1999 [2]. In the UK, the majority of HDTV services tend to be HD simulcasts of subscription-based, PPV, and free-to-air digital channels which are already available as Standard Definition TV (SDTV). The components of a typical DBS HDTV system are illustrated in Figure 5.
DBS consumer services and applications
Unlike terrestrial broadcasting which is traditionally supported through advertising revenues [3], the DBS industry has successfully created and sustained a consumer subscription based business model. Typically, in this type of model, the DBS service provider designs and implements subscription based services to be offered to consumers. The main characteristics of this type of business model are that the service offering has to be compelling and cost effective for the consumer in order to achieve widespread take-up. Some of the most commonly available DBS services and application are discussed below. Subscription-based TV DBS TV providers typically offer tier based subscription services. The vast majority of customers tend to subscribe to the most basic package and perhaps one or two premium packages. In the UK, Sky offers a basic starter package and premium content such as sports and movies is marketed as additional add-ons that consumers can opt for at additional cost. Pay per-view (PPV) This type of service allows DBS TV consumers to choose and pay for specific programs as one-off transactions. The chosen program is typically broadcast at the same time to everyone and could include concerts, sports events such as boxing and football matches. Public service channel rebroadcasts Most DBS TV providers offer packages that include the local free-to-air channels. In the UK, Sky is the dominant DBS service provider. Currently Sky offers around 240 free-to-air digital channels included as part of any Sky TV package chosen by consumers. This includes the major public service channels such as BBC, ITV and Channel 4.
Figure 5 [1]: Components of a typical DBS HDTV system
In addition to the main services discussed above, DBS providers also offer a number of very useful ancillary applications. Digital video recorders (DVR) A DVR is essentially a device that allows the recording of video content in a digital format. The content is typically stored on a disk drive but USB memory drives, SD memory cards and various other storage media could also be used. There are two very important aspects of the DBS service that make DVR devices an attractive ancillary application. The first is the Electronic Program Guide (EPG) that provides the consumers with comprehensive program schedules. The second is the availability of program content in a digital format. This allows consumers to select a program from within the EPG and record it directly to the DVR onboard hard disk in a digital format [1]. Support for IP-based home networking Modern DBS receiving and decoding equipment such as settop boxes provide integrated IP networking capabilities. This typically involves support for an internet connection through a broadband network such as cable or ADSL. The internet connection provides consumers remote DVR scheduling capabilities.
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expected to play an important role in future DBS services.
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Advances in DBS technology and future directions
Early efforts to create DBS services in the 1980s were not particularly successful. Most of the early applicants of the DBS service were forced to either postpone or completely abandon their plans due to the high start up costs involved [16]. However, by the early 1990s, there had been significant developments in the area of digital satellite communications. In particular, progress in four important areas of the technology paved the way for a more practical and economically viable launch of the DBS service [1]: Figure 6 [1]: Program content sharing between multiple set-top boxes over an IP network
In a traditional DBS service setup, there is typically one antenna dish installed at the consumer premises. This dish may be connected through a multi-switch to numerous set-top boxes in the consumer’s home. This would require multiple connections (typically using coaxial cable) originating from the dish (multi-switch) to various locations in the house [1]. However, the IP-based home networking feature allows multiple set-top boxes to be connected through an IP network and enables digital TV programs to be recorded on one main set-top box and distributed to multiple boxes in the house over a wired or wireless network. Interactive services Interactive television (also called iTV) is an increasingly common feature allowing viewers to interact with the television content. The interactivity level of the service varies depending on the delivery platform being used, e.g. Digital Terrestrial TV (DTT), cable, or satellite (DBS). At the low end of the interactivity scale, this could involve simple controls, e.g. on/off, volume, browsing channels, etc to more moderate levels of interaction involving features such as movies on demand, etc. However, in order to be truly interactive, the viewer must be able to have some influence over the viewing experience [1]. Some examples of this include, being able to choose the angle to view a football match from or participating in a live program through voting in order to influence the outcome. This involves the use of some kind of return path to send information back to the broadcaster. With DBS services, this typically involves the use of broadband connections such as cable or ADSL. Sky offers one of the most advanced iTV services through DBS in the UK [11]. There is an increasing trend with DBS operators to offer more complex interactive features which are integrated within the actual program content. To date, most of the technologies used by DBS operators to offer interactive services have been proprietary implementations. There have been numerous efforts to standardise the delivery of interactive services on various TV platforms. The Advanced Common Application Platform (ACAP) standard by the Advanced Television Systems Committee (ATSC) is a well known example of such efforts [12]. This standard aims to provide the technical basis for the development of interoperable interactive services and products and is
Advances in video and audio encoding techniques – The development of the MPEG-2 standard for source coding offered significant gains in compression. Advances in modulation and error correction techniques – DBS technology provided the basis for the first mass-market application of concatenated coding (FEC). Advances in consumer electronics – Rapid improvements in digital circuit design and decreasing cost of consumer electronics equipment. Advances in satellite platforms – Increases in satellite size and weight, higher power generation using more efficient solar technology, support for up to 240W transponders and subsequent increased signal strength.
A second phase of the technology evolution is making it possible for DBS providers to offer bigger and better services to consumers. There haven’t been any major breakthroughs and much of the basic DBS technology has remained the same. However, much of the change has been in the way that existing technology can be customised and deployed for specific applications. With regards to video and audio encoding techniques, the advent of HDTV has given rise to the MPEG-4 AVC standard which is expected to deliver even greater performance. Similarly, DVB-S2 is being promoted as the successor to the DVB-S standard and will also provide improved modulation and coding. The existing trend of cost vs performance is expected to continue and consumers are likely to benefit from declining cost of electronics equipment. Consumers are also expected to benefit from smaller parabolic dish sizes and improved signal reception. In particular, the addition of multiple feed-horns and LNBs to give consumers access to signals from multiple satellites is also likely to continue and improve. Technical improvements in satellite platforms are also expected to continue and there is a trend of increasing number of onboard transponders and greater bandwidth per satellite. The use of larger solar panel area has also led to greater DC power levels.
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Conclusions
DBS TV is an increasingly popular platform for television viewing around the world. In comparison with other TV delivery platforms such as terrestrial and cable, DBS systems offer a number of advantages. The bandwidth available to DBS providers is generally much higher than that available
International Journal of Research and Reviews in Wireless Communications for cable TV. This allows a consistently higher picture quality and a wide range of channels to be delivered. DBS offers fast and comprehensive coverage for consumers all over the world. Due to the wireless nature of the technology, there is no need for physical infrastructure such as cable deployment to the consumer premises. This makes DBS technology an ideal candidate for low population d ensity / rural areas as well as developing countries where the traditional terrestrial and cable infrastructure may not be available. The DBS industry has managed to create and sustain a successful subscription based business model. Although early efforts to create economically viable DBS services failed due to high start-up costs, later advances in technology paved the way for practical and economically feasible DBS services to be deployed. Advances in four particular areas of satellite technology have helped to transform DBS TV into one of the fastest growing TV platforms: video and audio encoding techniques, modulation and error correction techniques, technical improvements in satellite platforms and cheaper and more efficient consumer electronics. Most of the major DBS operators are expected to increase their fleet of satellite platforms and delivery capacity in the near future. There is strong evidence of growing consumer demand for HD content and with more and more local and free-to-air channels being rebroadcast over satellite, DBS networks are expected to play an increasingly important role.
References [1] [2] [3]
[4] [5] [6] [7] [8]
[9] [10] [11] [12]
[13] [14]
[15]
[16]
Dulac, S. P., & Godwin, J. P. (2006). Satellite direct-to-home. Proceedings of the IEEE, Vol. 94, No.1 Pritchard, W. L., & Ogata, M. (1990). Satellite direct broadcast. Proceedings of IEEE, Vol. 78, No. 7, pp. 1116–1140 Clement- Jones, T. (1983). Cable and satellite TV in the UK and Europe - the emerging legal issues, Online Conference, Cable 83, 1012 May, London Peltor, H. (1998). Digital television broadcasting - the UK start-up, IEEE Review Ameil, C. (2005). Developments in satellite broadcasting, IEE seminar on broadcasting spectrum, 1st June, London Peterson, R. R. (1997). Direct broadcast satellite – a new generation of television in America. From: www.nmia.com/~roberts/DBSV32.DOC Reimers, U. H. (2006). DVB - the family of international standards for digital video broadcasting, Proceedings of IEEE, Vol. 94, No. 1 Kratochvíl, T. (2008). From analog to digital television - the c ommon way to digitize European broadcasting, IEEE HISTORY OF ELECTROTECHNOLOGY Conference, 11-12 September 2008, Paris Johnson, L. L., & Castleman, D. R. (1991). Direct broadcast satellites – A competitive alternative to cable television?, RAND Publishing Marples, G. (2008). The history of satellite TV – a vision for the future. From: http://www.thehistoryof.net/history-of-satellite-tv.html Direct broadcast satellite (DBS). The interactive television dictionary. From: http://www.itvdictionary.com/dbs.html ATSC to hold ACAP Interactive TV Summit. (2006). Broadcast Engineering. From: http://broadcastengineering.com/hdtv/atsc-acapsummit-1219/ Satellite TV: how did it start? (2008). From: http://kfarr.com/2008/01/26/satellite-tv-how-did-it-start/ Godwin, J.P. (1999). Direct satellite television broadcasting. In Wiley Encyclopedia of Electrical and Electronics Engineering, J. G. Webster, Wiley, pp. 590–602 Satellites to deliver TV direct to home viewers. Boeing Company. From: http://www.boeing.com/defense space/space/bss/factsheets/601/dbs/dbs.html Garay, R. Direct broadcast satellite - satellite delivery technology. From: http://www.museum.tv/eotvsection.php?entrycode=directbroadc
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Lukman Sharif is a professional member of the Institution of Engineering and Technology (IET). He gained his Bachelor’s degree from London Metropolitan University in Computer Networking. He has worked in the IP communications industry for over a decade as a Network Architect and Consultant. He is currently a Senior Lecturer in Computer Networking and Information Security at London College of Research, UK. His research interests include IP routing and security in Mobile Ad-Hoc Networks. Professor Dr Munir Ahmed is a professional member of the Institution of Engineering and Technology (MIET). He is currently pursuing his DProf (Doctorate in Professional Studies) in Computer Communications Engineering Specialisation in Information Security with Middlesex University, London, UK. He earned his PhD in Digital Communications Systems Engineering from London Institute of Technology, UK (in collaboration with London South Bank University, UK) in 1997 and his MSc in Information Systems Engineering from London South Bank University, UK in 1994. Before joining Taibah University as Professor of Computer Networks and Communications Engineering, he was a Professor of Computing and Telecommunications at The American University, London, UK. He has extensive experience in the commercial sector and has held a variety of high-level positions in the industry, including Chief Executive Officer (CEO), Chief Operations Officer (COO), Training Director and Chief Network Architect in the UK. His current research interests include Wireless Sensor Networks, Mobile Ad-Hoc Networks, Routing Protocols, Network/Information Security and Digital Modulations. Prof. Ahmed is author or co-author of 6 books and more than 25 papers. Nauman Sharif is a professional member of the British Computer Society (BCS). He holds M.Sc. in Internet Engineering from University of East London, PGCE from University of Greenwich and B.Sc. in Computing from University of Westminster, UK. He is currently an Assistant Professor with the Department of Computer Science at Mohammad Ali Jinnah University, Islamabad, Pakistan. Before joining academia, he worked in the IT and Computer Networking industry for many years in a variety of roles including Systems Administrator, Consultant and Technical Author. His research interests include wireless networks, ubiquitous computing and the semantic web.
