Satellite Communications Theory - Book 2

File Name: Practical VSAT Training Revision: 1 Author: Stadler Brits Date Updated: 2006/03/05

ComSatPro LTD

SATELLITE COMMUNICATIONS

AN OVERVIEW

BOOK 2

Stadler Brits

ComSatPro LTD PO Box 10604

Centurion Gauteng

South Africa 0046

Tel/Fax: +27 (0) 12 998 4577 E-mail: [email protected]

SATELLITE COMMUNICATION

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TABLE OF CONTENTS 1 VSAT Systems 1 1.1 The market for VSAT Applications 1 1.2 The users 1 1.3 One-way VSAT services 1 1.4 Two-way VSAT Services 2 1.5 The drivers 2 1.6 Advantages 3 1.7 Disadvantages 3 1.8 Benefits of One-Stop Shop Solutions to customers 4 1.9 The value derived by end-users 4 1.10 A Definition of VSAT's 4 1.11 VSAT Network Topologies or Architectures 5 1.11.1 System types 5 1.11.2 Connectivity 5 1.11.3 Architecture issues 7 1.12 Gateway, Hub and Hubless networks 8 1.12.1 Hubs 8 1.13 VSAT Technology Developments 10 1.14 IPVSAT Systems 10 1.15 VSAT Network implementation 10 1.16 VSAT Network Design 10 1.17 User Requirement Specifications 11 1.18 Functionality Specifications 12 1.19 Capacity planning and sizing 12 1.20 The network design phase 12 1.21 VSAT Network Economics 12 1.21.1 Capital Cost items 12 1.21.2 Operational costs 13 1.22 Pilot Network Installations 13 1.23 A Typical VSAT Terminal 14 2 Open standards 15 3 SATELLITE orbits 16 3.1 Geosynchronous orbit 16 3.2 Geostationary Orbit (GSO) 17 3.2.1 What is the GSO? 17 3.2.2 Orbital Positions 18 3.2.3 Hot Orbital Positions 19 3.2.4 Hotbird 19 3.3 LEO's MEO's & GEO's & HEO's 20 3.3.1 Low Earth Orbit (LEO) 20 3.4 Space Debris 22 3.5 Inclined Orbits, Orbital positions and GSO Slot Positions 23


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3.6 Summary 23 4 Some More on satellites 24 5 some more on frequencies 24 5.1 The Major Frequency Bands 24 5.1.1 Polarisation 27 6 Terminology 27 6.1 Bandwidth (BW) 27 6.2 Footprints and coverage 28 6.3 "Uplinks" and "Downlinks" 28 6.4 Transponders 28 6.5 Link budgets 28 6.6 Redundancy 29 6.7 "Bent Pipe" 29 6.8 Regenerative 29 6.9 On-Board processing 29 6.10 On-Board switching 29 6.11 Modulation Schemes 29 6.11.1 Amplitude Modulation 30 6.11.2 Frequency Modulation 30 6.11.3 Phase modulation 30 6.12 Multiple Access Schemes 30 6.13 Multibeam Satellite Systems 32 6.13.1 Beams 33


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File Name: Practical VSAT Training

1 VSAT SYSTEMS

1.1 The market for VSAT Applications VSAT technologies have evolved to the point where VSATs are able to address very effectively

most of the connectivity needs of ICT users, from the residential market to through to the corporate market.

VSAT systems have traditionally only seen application within large corporates because of the costs involved. This has been changed substantially since the advent of DTH television broadcasting, where it has been shown that VSAT receive only technologies can be applied in the consumer end-user markets. The success of these deployments has been largely due to:

• The development of open standards that have lowered the costs of TVRO terminals

• The launching of GSO satellites utilising Ku band frequencies that in turn allow the deployment of smaller remote terminals

• Modern GSO satellites deliver high power spot beams that are able to focus directly on specialised market areas

• The development of multibeam satellites with improved connectivity arrangements

1.2 The users Any requirement for connectivity in remote areas or areas where other technologies cannot provide

services are prime targets for the use of VSAT technologies.

The user list is long and detailed but can be grouped as follows according to the types of ICT services required.

1.3 One-way VSAT services The delivery of television broadcasting direct to end-user through the use of Direct-to-home (DTH)

services is the obvious one that everyone knows about. But there are others and the following list is a sample of where one-way VSAT service are used.

• Stock market and other closed user group information broadcasting

• Distance education and continued education services for large corporate companies

• The distribution of financial analysis services in real time

• The distribution of market information to remote offices located in geographically dispersed locations

• The updating of market related information, the distribution of new product catalogues and pricing information to outlets in the retail industries.

• The distribution of audio broadcasting services in shops and public areas

• The introduction of DAB services even direct to end-users (GlobalStar)

• The relay of advertising material to shops and direct to electronic signs

• The relay of information to motor vehicle drivers along highways

Revision: 1 Author: Stadler Brits Date Updated: 2006/03/05

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1.4 Two-way VSAT Services By far the most significant applications are those that allow for two-way communication via satellite,

especially in areas considered to be rural or so remote that normal ICT services are not possible. The most extreme examples available are the delivery of modern ICT services to places such as the Antarctic and to shops at sea using two-way VSAT systems. These so-called Earth Stations on Vessels (ESV's) are now able to maintain all the normal services available on land. The demand for ESV services on cruise ships extends to the delivery of:

• Normal voice, fax and data connectivity

• Access to internet services and the WWW

• The provision of connectivity to the cellular industry so that mobile GSM services remain active even on the oceans.

The list below is some of the more traditional applications for two-way VSAT systems.

• Interactive computer transaction services

• Low rate video conferencing

• High rate real time video monitoring services (e.g. the control of undersea diamond mining operations from land based security centres).

• Database enquiry services

• Bank transactions and ATM services in remote locations

• Tourism reservation systems

• Distributed process control systems especially in the oil industries

• Distributed telemetry systems used for monitoring power networks, water management systems, traffic control on highways and disaster monitoring systems

• Voice, data, fax communication services in rural areas

• Emergency services

• Electronic funds transfer services direct at point of sale

• E-mail systems

• Medical data transfer and distance or tele-medicine

• Sales monitoring and stock control

• Satellite news gathering services

• The provision of Wide Area Networks (WAN's) and the extension of LAN's to remote locations

• The delivery of interactive multimedia services

• Large ICT networks for embassies

Now why are VSAT systems able to provide these types of services? The following section show that any ICT service that builds on the inherent strengths of satellite communication systems will benefit from VSAT applications.

1.5 The drivers The main drivers of the VSAT industry are all market related and are there because the market

requires needs to be addressed. Some of these drivers are:

• The desire of governments to ensure that all their citizens are able to obtain equivalent ICT services regardless of their geographical location. This is also driven by a desire to stop the urbanisation process that is so destructive, especially in the developing world.


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• The desire by large corporations for redundant and alternative networks, independent of terrestrial infrastructure.

• The need for connectivity that is always available, regardless of geographical and political constraints.

• The increasing need for high-speed services and multimedia connectivity direct to end-users

• The needs for more and more connectivity

1.6 Advantages Satellite communication systems have some advantages that can be exploited for the provision of

connectivity. These are:

• Costs Insensitive to Distance

• Single Platform service delivery (one-stop-shop)

• Flexibility

• Upgradeable

• Network life cycle costs are lower than some alternatives

• Fast Deployment:

• Install & Commission: 1/2 - 2 Days

• Reliable Service: 99.9% for Data; 99.5%+ for Voice

• MTBF > 25,000 Hours (3 Years)

• Proven in 100+ Countries

• Unrestricted ubiquitous coverage of large geographical areas by single systems

• Low incremental costs per unit

1.7 Disadvantages However like all systems there are disadvantages also. Some of these are

• High start-up costs (hubs and basic elements must be in place before the services can be provided)

• Lack of adequate coverage in some parts of the world (Africa is the last continent that does not have dedicated GSO satellite bandwidth available that will allow single satellite connectivity anywhere on the continent).

• Higher than normal risk profiles

• Severe regulatory restrictions imposed by countries that prevent VSAT networks and solutions from reaching critical mass and therefore profitability

• Some service quality limitations such the high signal delays (latency)

• Natural availability limits that cannot be mitigated against

• Lack of skills required in the developing world to design, install and maintain satellite communication systems adequately

Some of these limitations and disadvantages are nothing else than opportunities that can be overcome.




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1.8 Benefits of One-Stop Shop Solutions to customers VSAT systems and especially the new IPVSAT systems have a trump card at their disposal. VSAT

systems have for many years been able to deliver all customers ICT needs from one platform. Service convergence for voice, fax, data and video has been accomplished. Even convergence between mobile and so-called fixed line services has been achieved within the VSAT industries.

Customers are able to develop a universal set of applications that deliver all their needs on one platform. The benefits are obvious in that overall connectivity costs are lower for similar or comparable ICT networks.

1.9 The value derived by end-users End-users derive value from VSAT networks in a number of ways. Some of these are:

• Lowered overall per unit connectivity costs

• Improved total ICT network performance

• Loss mitigation under network failure conditions

• Improved response times to major business failures because of improved connectivity.

• Increased market share and increased revenues for companies large and small

• Improved provision of essential services by governments especially in remote areas

• Improved quality of life for all the people of a country as a result of increased economic activity

• Improved response to emergency situations as satellite systems van be restored quickly and effectively

1.10 A Definition of VSAT's The term VSAT is an acronym for Very Small Aperture Terminals. The term was initially a

trademark for small earth stations marketed by Telcom General of the USA in the '80's. Today the term is used to describe a family of satellite systems and applications, where the terminal equipment use an antenna with a diameter of less than about 4,5 meters, varying down to about 1,2 meters.

This course covers VSAT systems and applications that make use of geostationary satellites (GSO). These are satellites placed in the geostationary orbit over the equator of the earth at an altitude of 35 786 km. Satellites that are placed in this orbit appear to "stand still" relative to any position on the earth and thus can be used to implement communication systems using non-tracking antennas.

VSATs are at the lower end of a product line of communication systems used for providing communication services that were traditionally only used for international services between earth stations serving countries. These earth stations typically make use of parabolic antennas with diameters between 7 and 32 meters.

VSAT systems are aimed at delivering services direct to end users as is shown in the diagram below.


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Satellite Systems: Source: VSAT Networks: Maral, 1995

1.11 VSAT Network Topologies or Architectures

1.11.1 System types VSAT systems fall into 2 broad categories:

• One-way systems, using powerful (large antennas with high power transmitters, 100's of Watts) at central hub stations, transmitting information to a large number of low sensitivity VSAT's (small antennas with low sensitivity) receiving stations. TVRO (television receive only) or DTH (direct to home) systems fall into this category.

• Two-way systems, where the remote VSAT's are able to transmit and receive, but the transmit power at the remote terminals is limited to a few Watts. All VSAT systems used by corporates or any network providing two way services such as voice, data, fax, fall into this category.

1.11.2 Connectivity Satellite systems are all about the provision of connectivity over vast distances, unaffected by

geographical constraints such as terrain, rivers, oceans etc, and man-made constraints, such as borders. There are three categories of connectivity.




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• Point-Point (PTP) connectivity, where communication services is provided between two points. In a network when many independent PTP links are provided, we talk of Mesh Network Architecture.

• Point-MultiPoint (PTMP) connectivity, where communication services are provided from/to a central point (hub), to many remote sites. In this case we talk of a Star Network Architecture.

• MultiPoint-MultiPoint (MPT-MPT) connectivity, where communication services is provided from many to many sites. In this case we talk of Hybrid Network Architecture. (Also known as Multipoint Interactive Architecture.

The drawings below illustrate these various

Mesh Connectivity: Source: Maral 1995




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Star Connectivity Source: Maral 1995

1.11.3 Architecture issues There are a few issues and generic terms that need now to be put into perspective.

Firstly, meshed networks imply that the services to be offered are two-way services such as voice, data and fax. Meshed networks are mostly symmetrical, where the bandwidth requirements and quality requirements are the same in both directions. Voice and fax services are always symmetrical services. Meshed networks can also be asymmetrical, where the bandwidth and quality requirements in the two directions are not equal, such as for some data services, Internet access for end-users and in high-speed data delivery systems.

Secondly, star networks are mostly asymmetrical by definition where large amounts of information are to be transmitted from a central point to many remotes, with very little (if any) return information flows. Again star networks can also be symmetrical and this is the case for some voice and fax networks where the services are required for connection of remotes sites to a central location with no allowance for remote-to-remote connectivity. Star networks can be used for remote-to-remote connectivity BUT services provided in this way will involve "double hop" communications with adverse effects on performance. For example the Round Trip Time (RTT) immediately becomes more than 1 second or more.




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Thirdly, star VSAT networks use terms such as "outbound" and "inbound" routes to try to differentiate between the bandwidths and link qualities offered by these two services. The drawing below illustrates these concepts. These terms only have real relevance when used within the context of Star topologies.

INBOUND & OUTBOUND Source: Maral 1995

1.12 Gateway, Hub and Hubless networks VSAT networks where originally designed on the premise that all would be Star networks, used for

communication between a central station or Hub and many remote sites. Users of these networks were not impressed as it soon became apparent that many closed user groups required remote-to-remote connectivity, and that 'double hop" was just not good enough.

As a result VSAT networks are now available in the many architectures as described above. New terminology was the needed to describe the functionality of some of the elements.

1.12.1 Hubs The term "Hub" is used to describe Star networks and also to describe networks that require one of

the earth stations in the network to manage and control the services provided. It is important to understand the context within which the word "hub" is used.

Star networks

Users of star network topologies have two choices. They can use networks designed around a dedicated company owned hub or "lease their services from a network provided by a service operator on a "shared hub" facility.

In a closed user group star network providing services to only that group, the "Hub" could mean simply the station that monitors and controls (manages) the services being provided. That same station would (in most cases) also be the central station in a star network. In a "shared" VSAT service, the term "hub" is used almost exclusively to describe the station that manages the services provided to many users. If the shared VSAT service is based on a Star topology, then that station is also the central station of the network. However if the service being provided is based on meshed or hybrid topologies, then the term again is used to describe the management station.

A "controlling hub" is another way to describe the management hub. A VSAT network designed to provide billed voice services will always have a station called the hub, because it is at that point where the service management systems will be installed, in particular the "billing" system.

The following guidelines can be used to try to decide between a dedicated and a shared hub service.




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This assumes that a customer has the regulatory freedom to choose in the first place, and that if not that the licensed VSAT service providers are prepared to allow customers to operated dedicated hubs under their licences!

Dedicated Hubs are desirable if

1. The services to be provided are provided from a centrally located point where all connectivity and data requirements reside.

2. That the customer wishes to eliminate (or is allowed to) the need for a long backhaul link from the central offices to the location of a shared hub facility

3. The total number of remote VSATs in the network is large, certainly more than about 200.

4. The applications are considered to be strategic or total redundancy is required from terrestrial infrastructure.

5. The highest possible level of availability and performance is required.

6. The customer is prepared to take on total network responsibility and train personnel to operate a VSAT hub.

7. High bandwidth services are required (such as full broadcast quality video)

8. The customer is prepared to spend the money or can justify the life cycle costs in some way.

Shared Hubs are desirable if

1. The company wishes to become a VSAT service provider and can obtain the much-needed licences to operate VSAT services. Star networks generally are expensive and the fixed costs can then be shared between many customers.

2. Customers are happy and can get suitable (reliable) backhaul circuits to connect their centrally based networks to the hub facility.

3. The company is prepared to satisfy the individual needs of customers (availability, performance, data throughput, etc.).

4. Satellite bandwidth costs are high and only available in large blocks (complete transponder)

5. Individual customers requiring less than 200 VSATs are looking for services.

6. Technical expertise is scarce.

7. Technologies are changing rapidly.

8. Capital resources are not available to individual customers.

Gateways

The term "gateway" is relatively new. It should be used to describe the station in a VSAT network that provides connectivity to other networks such as PSTN's PLMN's, Internet networks (WWW) and data networks. VSAT networks of all types are connected to other networks in a country or anywhere in the world through "gateways".

VSAT networks do not have to only have one "gateway" either. Thus it is possible to define a station in a VSAT network as a "local" or "national" gateway and another (or the same one) as the "international" gateway.

The only controlling factor is the services that are provided by the VSAT network.

"Hubless" networks




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VSAT network technologies are traditionally complex, requiring a large amount of resources to configure, monitor and control the services provided. Technological developments have reduced the complexity of these systems to the point where it is now possible for any station on a network to undertake the network management functions and thus the term "hubless" networks. In truth there is no such thing as a "hubless" network. Please note that "hubless" networks are almost always meshed or hybrid networks, because of the quality requirements needed to be able to achieve all the management functionality needed. "Hubless" VSAT networks also imply that the networks are "self healing" in that a failure at the "designated hub" station will be detected and a standby hub station will be able to take over network management automatically, resulting in less total network downtime.

1.13 VSAT Technology Developments Technological developments in VSATs involve a process of continually providing for more flexible

service independent systems and a process of continually developing new and better techniques to improve on bandwidth efficiency. The most exciting developments have been in the area of combining digital broadcasting technologies with traditional mature VSAT technologies. Further the combining of satellite coding schemes and modulation systems with TCP/IP protocols has resulted in a new family of VSAT systems called IPVSAT systems.

1.14 IPVSAT Systems IPVSAT systems are generally but not always based on hybrid network topologies. IPVSAT

system have been designed to deliver all services on a truly OSS platform based on direct connectivity to LAN's, with the voice services delivered via VoIP.

The mature VSAT technologies are known to work, are tried and tested and are well understood. The newer technologies such as DVB and DVB-RCS plus TCP/IP bring to the table the synergies of fully integrated services. IPVSAT systems are all able to deliver high-speed services. The following drawing illustrates how this has been achieved.

1.15 VSAT Network implementation Modern VSAT network designs now follow the so-called ISO stack model. It is therefore relatively

easy to decide if a VSAT solution will suit a particular application or service requirements that has been specified in terms of the ISO stack. VSAT networks provide for layers 1 (physical), 2 (data link connectivity), 3 network, and possibly, 4 (transport) some suppliers have developed systems that over the remaining layers, 5 (session), 6 presentation an 7 (application) as an integral part of the total package.

1.16 VSAT Network Design VSAT network design involves a few very crucial steps that if properly completed almost guarantee

success. The most crucial steps are:




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• A proper evaluation of end-user requirements and the development of a URS.

• The development of a functionality specification that is needed to correctly select technologies, frequencies, network topologies and service mix.

• The development of evaluation criteria tables against which proposals can be measured.

• The preparation of Requests For Proposals

• The preparation of marketing plans, business plans, funding plans and on going operational support plans.

This course serves only to highlight the importance of all these steps and does not try to address all of them.

1.17 User Requirement Specifications This is the most crucial step and applies equally to the development of shared VSAT services as it

does to private networks or to delivery of specific customer requirements. The process requires an in depth understanding of all the customers requirements for services. Some of the crucial issues are:

• Data flows must be estimated or measured,

• Network size and location of terminals must be determined. This will affect the selection of a satellite operator, the coverage requirements of a footprint and the power available to be utilised in the VSAT network design.

• Possible future growth must be determined.

• Customer's assessment of the strategic value of the network must be evaluated. If the network has strategic or mission critical elements these must be high-lighted as this will affect the choice of network topology, location of hub and gateways and network ownership issues.

• The nature and type of services required. For example, a voice requirement will need to determine not only where the calls will be made to but also the number of simultaneous calls that need to be catered for. If voice traffic or any traffic is required between remote sites a mesh network will be required.

• Interface requirements are the next most crucial item to be evaluated. This includes an assessment of the legacy systems already in place. One of the most difficult aspects to correct at a later stage, are mistakes that have been made regarding interfaces.

• An assessment of the customers needs for flexibility of the base technologies.

• Assessments of the customer's requirements for data transfer services, access to the Internet and the delivery of Intranet services. This includes an evaluation of existing ICT systems and LAN's.

• An assessment of overall network availability, performance and reliability requirements is also required.

• An assessment of the value in monetary terms of the network. This is crucial to determine the "opportunity cost" of the network so that a financial return on investment study can be completed. It is not sufficient just to look at the direct capital and operational costs involved. It is essential to estimate what the true value of improved connectivity means to the customer. It is in this area where most of the financial justification for the network will lie.




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1.18 Functionality Specifications Functionality specifications can then be developed as first step in the process of preparing an RFP

that includes the evaluation criteria. This step is crucial for determining the overall functionality requirements and should be written in "plain English" avoiding technicalities as afar as possible. This document is then used as a first cut report to the customer to test the requirements that need to be catered for in the VSAT design.

1.19 Capacity planning and sizing The greatest challenge that VSAT network designers face is in this aspect of the functionality

specification. The process is not an exact science. The information that is gathered at the assessment stages is at best incomplete and vague. However a methodical approach will provide the basis and will show up areas in the design that are sensitive to incorrect needs evaluation. Plans can then be developed in advance to address the issues that may arise at a later stage.

Some of the issues that need to be addressed are:

• The number of users and locations. This is crucial for defining the geographical coverage of the VSAT system and will influence the choice of satellite and frequencies to be used. Also the basic network topology will flow out of this step.

• Traffic and information flows need to measured or estimated. This step impacts many technical issues but the most important is the future satellite bandwidth requirements for the VSAT network. Data flows and voice call handling capacities are the biggest drivers for bandwidth requirements.

• Throughput estimates need to be made as this will influence the choice of modulation schemes, number of carriers, number of outroutes and inroutes and the choice of frequencies to some extent.

• Latency issues: Customer equipment requirements as well as the sensitivity of software packages to latency must be known. This will adversely affect the customer perceptions if not correctly evaluated and design for.

• Network response times need to be estimated as this also affects customer perceptions. A choice of the suitability of an MA system will be determined by this measurement as well as the suitability of using a shared VSAT network will be determined by this step.

1.20 The network design phase This now involves the services of a professional VSAT network design house as many of the

choices are involved with many layers of analysis. Technical compromises and choices have to be made at every step of the way and these have to be properly evaluated and documented.

1.21 VSAT Network Economics This involves the determination of the total costs of implementing the VSAT network. All relevant

costs must be taken into account, including the life cycle costs.

There are many trade offs that have to be made. Demand for high network performance will influence capital expenditures as well as operational costs. Short cuts in capital expenditures will increase on –going operational costs. High initial capital costs will tend to decrease operational expenditures but will lead to longer payback periods.

The costs that need to be considered are:

1.21.1 Capital Cost items • Initial capital costs for all parts of the network. This includes replacement costs for legacy

systems that are not compatible with the VSAT network being proposed.


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• Remote terminal costs. This must include critical items such as installation, on site spares holdings, power supply upgrades and housekeeping items such as air-conditioning and accommodation requirements.

• Initial licence fees for VSAT terminals and hubs.

• System redundancy costs

• Gateway equipment costs

• Overall network installation and commissioning costs.

• Supply of interface equipment to other networks.

• Network management equipment and systems.

• Personnel training costs that include the cost of training VSAT service users.

1.21.2 Operational costs • By far the largest cost element is the on-going cost of bandwidth for the VSAT network

(transponder lease costs). These costs may also include an allowance to be made for bandwidth reservation for future growth.

• The next item is remote site maintenance costs. These costs are affected by the nature of the support required including allowances for fast response times under failure conditions.

• All VSAT networks have to be licensed with annual charges being made for frequency use and in some cases for VSAT service providers, a percentage of the turnover that must be paid to regulators.

• Another crucial item is an allowance for network replacement that needs to be made. VSAT technologies are continually changing and the effect is to place large demands on network upgrades as service requirements change.

• Operational support costs by suppliers includes the costs of system upgrades and repairs.

• Network management service charges

• Network usage charges that may be payable when shared hub services are used.

1.22 Pilot Network Installations Many customers insist on pilot networks to be installed for service testing before commitment to the

full network installation and delivery. The reasons why pilot networks are installed are:

• To assess the suitability of VSAT's for their ICT needs. This is a poor reason as if the functionality specifications were correctly developed it would be plainly obvious why VSATs are the right decision or not.

• To assess the compatibility of VSATs to existing systems. Again in the initial phases simulations can be done that should remove the needs for this phase altogether. Simulations are now possible to test for all the sensitivities on a VSAT system before implementation.

Pilot networks should only be a consideration in the most extreme of new applications, where simulations have still left unanswered questions. Pilot network installations for customers on shared hub facilities are relatively easy to undertake because the basic resources are already available. Private networks are entirely another matter because a large outlay is required before the system can be tested effectively. It is far more beneficial to be able to provide case study information to customers regarding similar applications instead.

Pilot networks should be covered by very clear contracts in which the following matters have been attended to:

• Clear acceptance criteria for the overall design must be specified.




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• Clear time limits for the acceptance testing process must be laid down.

• The end-user customer must accept responsibility for all costs related to the provision of the VSAT services for the duration of the test such as the bandwidth costs.

• Acceptance of the network by the customer must also mean that the services will have been deemed to be provided as of the start of the pilot test phase.

VSAT service providers offering pilot networks on shared hubs are able to recover the costs of shared resources from other customers but need to ensure that they are not left with unrecoverable costs associated with equipment installed at new customer premises.

1.23 A Typical VSAT Terminal VSAT terminals consist an antenna, an outdoor unit (ODU), and an indoor unit (IDU). The

following sketch shows a typical VSAT terminal unit block diagram.




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2 OPEN STANDARDS The success of the GSM open standards showed that it is possible to create an open standard that

many suppliers could build equipment for. The same has now happened with the development of the so-called DVB-RCS standard. For the first time, inter-operability between VSAT equipment from numerous suppliers is now possible, although only a few niche market suppliers have started to produce systems based on these standards.

By far the biggest driver has been the successes that have been achieved in the DTH VSAT market where the combination of MPEG standards with the work done on Digital Video Broadcasting standards, have shown that open standards in VSATs are possible and very desirable.

The DVB standards are designed to optimise bandwidth utilisation with any system designed to deliver vast quantities of data to remote sites. The DVB standards provide a common transport stream layer allowing for many MPEG signals to be multiplexed together onto a single carrier for delivery to large numbers of remote sites. The standards provide for common service information interfaces, common Reed Solomon Forward error correction coding schemes. DVB further supports media specific modulation schemes, thus making the standard ideal for combination with FDM TDM and CDM satellite modulation schemes. DVB also allows for common conditional access encryption schemes to be imbedded in the main baseband.

A family of standards, each one optimised (but mutually compatible at baseband level) for different media has been developed, some of which are listed below:

• DVB-S/S2: configured for use across different frequency bands, bandwidths and power levels on satellite systems

• DVB-C: for cable television systems

• DVB-T: for terrestrial systems

The DVB-S standard is ideally suited to VSAT applications where demand is for high information delivery in one direction is required. Some examples are, data broadcasting and Internet information delivery to remote sites. The standard ensures that a SCPC system fully utilising a complete transponder can deliver up to 45 Mbps over a vast geographical area. This has allowed the integration of many ICT service with normal digital broadcasting through a common DVB multiplex unit.

The DVB-RCS standard has been specifically developed for use in Star networks for the delivery of low cost Internet access services direct to the home market, as an integral part of normal DTH services or as stand alone systems. This standard should allow for the mass manufacture of low cost VSAT terminals. The standard is ideally suited for Star network topologies and services. The standard is not suitable for use in the corporate, SME and Soho market segments because these networks generally require larger return channels. However the combination of DVB with traditional ICT SCPC DAMA systems provides an alternative method to deliver return path bandwidths of up to 2 Mbps. The following drawing shows how the various systems can be provided to address the new IP based markets.




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The sketch shows how successfully the mature tried and tested technologies have been combined

with the new standards to produce truly remarkable new solutions for digital IP based services. The services are fully integrated OSS services. Most suppliers are providing a family of products that can be provided over any type of network topology. Some systems are even able to adapt to the type of traffic delivery form a Star to a mesh to a hybrid topology on demand.

3 SATELLITE ORBITS Satellite communication systems make use of satellites that orbit the earth at different altitudes

above the surface of the earth. Many orbits are used for communication systems. The section below describes the most important orbits used by communication systems.

3.1 Geosynchronous orbit A geo-synchronous orbit is any type of orbit that produces a repeating ground track. This is

achieved with an orbit period which is an integer multiple or sub-multiple of a sidereal day. Please note that there are many geosynchronous orbits around the earth. These orbits are circular orbits and are not dependent on the direction in which the satellites move around the earth either. Satellites in geosynchronous orbits do not appear to be stationary in the sky, when viewed from the earth.

Those that move around the earth in the same direction as the earth rotates will paint out a "figure eight" in the sky. The size of the "figure eight" will depend on the inclination of the orbital plane with respect to the equator. Such satellites do not set either.

Geosynchronous satellites that orbit the earth in the opposite direction will rise and set, as the sun does.




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3.2 Geostationary Orbit (GSO)

3.2.1 What is the GSO?

The geostationary orbit, also named the Clarke orbit in honour of Arthur C Clarke, as he was the first person to postulate the idea of using this orbit for communication satellites.

The geostationary orbit is a special case of the general geosynchronous orbit. The GSO meets certain fundamental conditions. These are:

36,000 km36,000 km

• The GSO's orbital plane is the same as the earth's equatorial plane. This means that the inclination angle, with respect to the equatorial plane, is zero�.

• A satellite in GSO moves in the same direction as the earth rotates, i.e. from West to East. • The angular velocity of the satellite is the same as that of the earth. • The altitude of the satellite above the equator is 35 786 km, (note this is the average value).

A satellite in a geostationary orbit will appear fixed above the surface of the Earth. In practice, the orbit has small non-zero values for inclination and eccentricity, causing the satellite to trace out a small figure of eight in the sky.

The other properties or characteristics associated with the GSO and GSO satellites are:

• The maximum angle that the earth is viewed from the satellite is 17,4º. • The footprint, or service area of a geostationary satellite covers almost 1/3 of the Earth's

surface (from about 75º South- to about 75º North-latitude), so that near-global coverage is possible, with a minimum of three satellites in orbit.




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• The propagation delay of a radio signal transmitted earth-to-satellite-earth lies between 238 milliseconds and 278 milliseconds.

Minimum = 2R0/C

Maximum = 2{(R0 + RE)/C} cos(17,4/2)

Where:

Ro is the satellite altitude measured from a point on the equator directly below the satellite, = 35 786 km

RE is the radius of the earth, = 6 378 km

C is the speed of light, a constant, = 300 000 km/sec

The GSO has some advantages:

• Satellites in the GSO appear to be stationary when viewed from a point in their footprint area on the Earth

• 42,4% of the earth's surface can be seen from a single satellite • Only three satellites are required to "see" all of the earth • As the satellites do not move relative to the earth, no Doppler shift occurs • No tracking is required for ground station antennas and the smaller the antenna, the less need

there is for tracking to be provided. • Earth stations scattered over the entire footprint area can be interconnected in a single

network.

Like with all things there are also disadvantages.

• A geostationary satellite communication system experiences a Round Trip Time (RTT) of approximately 500 milliseconds.

• The signal loss measured from the earth to the satellite is about 200 dB. • GSO satellites cannot cover the Polar Regions of the earth. • System availability is limited by two factors that are not controllable. These are

o The twice-yearly equinoxes, when the solar power installations stop charging the on-board batteries; and,

o When the sun and the satellites line up right behind each other as seen from an earth station. At these times, the noise generated by the sun makes satellite communication systems using the GSO orbit, unworkable.

3.2.2 Orbital Positions The geostationary orbit (GSO) is divided up into what is known as Orbital Positions.

The three regions of the World all have been allocated an "orbital arc", which is a section of the GSO orbit. In theory, only position within the allocated orbital arc for that region may be used for satellites serving that region. Obviously there are exceptions as some satellites can and do serve countries in adjacent regions. This is by agreement between the administrations and most of these satellites are designated as providing international communication services. Clearly, satellites providing domestic or regional services should only be placed into orbital positions within the respective orbital arc allocated to the region in which the country or region is located.

The issues around orbital arcs arose when GSO satellites started providing broadcast services. Before that GSO satellites were used predominantly for point-to-point services (fixed communication links). Broadcasting Satellite Services (BSS) was seen as a direct attack on the sovereign rights of countries and a debate started that still continues. The resolution of the debate was to develop Nominal Band Plans where countries were assured that they would retain a share of the GSO orbit and that they could be certain that when the time came they would have a position available for a broadcast satellite.




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Later the idea of nominal band plans was extended to the FSS services as well for basically the same reasons because very few broadcast services (if any) have been provided using the BSS frequencies allocated. Most broadcast services in use today use FSS allocations simply because equipment is available for these portions of the band AND because BSS and FSS are able to occupy one satellite AND because in most cases the only way to make a satellite economically viable is to combine BSS and FSS services.

Now that service convergence is a de facto reality between broadcasting, voice data and Internet, there is even less motivation for separating out the BSS services into their own frequencies and band plan allocations.

At first it was believed that the closest that satellites could be placed would every 5º. Modern methods used in station keeping now suggest that the spacing could be reduced to 2º apart.

No an orbital position represent a "cube of space nominally centred every 2º apart on the GSO extending vertically above and below the GSO nominal orbit and East and West by + and – 0.2 degrees.

Orbital positions are always quoted relative to The Greenwich meridian as X degrees West or Y degrees EAST of GMT. A look at any of the websites of a typical satellite operator will clearly show what this means.

The orbital positions are approximately 1472 km apart, on the GSO.

3.2.3 Hot Orbital Positions It was also believed that only one satellite could be placed into each orbital position. However

today some slot positions have up to 9 satellites placed in the cube of space. Such a slot position is known as being a "hot slot" because of its popularity. Now why is this?

Well this is because of the cost of ground station equipment and because when satellites are used for broadcasting services it is better to offer the market place more TV channels in the same position. Simply put, the residential market will not tolerate a situation where individual users are expected to have more than one dish on their homes! (A change in slot position requires either that the ground antennas have to be re-pointed at the new satellite OR a second dish is required).

3.2.4 Hotbird Some satellite operators have been able to corner the market for broadcasting within a certain

geographical area. When this happens, all the broadcasters providing material for reception in that area try to get capacity on the same satellite and hence the satellite then becomes known as a "Hotbird", very much in demand.




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3.3 LEO's MEO's & GEO's & HEO's

3.3.1 Low Earth Orbit (LEO)

LEO's are either elliptical or circular orbits at a height of less than 2,000 km above the surface of the earth. The orbit period varies between 90 minutes and 2 hours. Satellites in LEO are affected by atmospheric drag, which causes the orbit to gradually deteriorate. The radius of the footprint of a communications satellite in LEO varies from 3 000 to 4 000 km. In other words, a LEO satellite using an antenna that generates a circular footprint will provide coverage in a circular area that moves over the earth in step with the movement of the satellite. The maximum time during which a satellite in LEO orbit is above the local horizon for an observer on the earth is up to 20 minutes. Any communications system using this type of orbit requires a large number of satellites in a number of different inclined orbits to provide the same coverage, as that would be achieved by a GSO satellite. When a satellite serving a particular user moves below the local horizon, it needs to be able to hand over the service to a succeeding one in the same or adjacent orbit. Satellite communication systems such as the Iridium and GlobalStar systems involved many satellites placed in complex orbit patterns to provide coverage for the target markets that these systems proposed to, or tried to serve. There are many technical challenges that these systems faced. One of these related to the chosen orbit (LEO) is known as Doppler Shifts. The effect of the movement of the satellite is to increase the frequency of the received signal as a satellite approaches a ground station and to decrease the frequency of the received signal as a satellite moves away from a ground station. The GlobalStar system originally proposed to use 48 satellites in 8 orbital planes (at about 1 400 km above the earth) whereas the Iridium system proposed to use 66 satellites in 6 orbital planes (at about 780 km above the earth). Ground feeder link stations and remote terminals would have to "track" the satellites for optimal performance. Hand held terminals using omni-directional antennas similar to those used by normal cellphones. Thus, it is possible to make dual handsets capable of connecting to LEO systems and to terrestrial cellular networks.

Many other systems were proposed none of which saw the light of day, and now that both GlobalStar and Iridium are no longer seen as viable projects in the long-term.

3.3.1.1 Intermediate Circular Orbits (ICO), or Medium Earth Orbits (MEO) ICO's are circular orbits at an altitude of around 10 000 km. Their orbital period measures about 6

hours. A global communications system using this type of orbit (such as the proposed ICO system) requires a modest number of satellites in 2 to 3 orbital planes to achieve global coverage. ICO satellite communications systems operate in much the same way as LEO systems with a few fundamental differences related to the chosen orbit. Hand-over is less frequent but propagation delay and free-space-loss are greater.




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Once again the problems of implementing these systems were under estimated even if shown to be technically feasible and the only project that came close to implementation was The ICO project, consisting of 10 satellites in inclined orbits at about 10 000 km above the earth.

3.3.1.2 Highly Elliptical Orbits (HEO) HEO's typically have a perigee at about 500 km above the surface of the earth and an apogee as

high as 50,000 km. Systems based on HEO orbits are aimed at providing communication services in the far north and south latitudes, where coverage by satellites in GSO orbits are either non-existent or very marginal. HEO orbits are inclined at 63,4º. The particular inclination value is selected in order to avoid rotation of the apses, i.e. the intersection of a line from earth centre to apogee and the earth surface will always occur at a latitude of 63,4º North/South. The orbit period varies from eight to 24 hours.

Why choose a HEO? Well the reason is that a HEO satellite will spend about 67% of its orbit period close to or at it apogee (apogee dwell) and thus would in effect appear to be stationary to a ground station. Ground station antennas therefore would require minimal tracking capability. After this period, a switchover needs to occur to another satellite in the same orbit in order to avoid loss of communications. Free-space-loss and propagation delay for this type of orbit is comparable to that of geostationary satellites. However, due to the relatively large movement of a satellite in HEO with respect to an observer on the earth, satellite systems using this type of orbit need to be able to cope with very large Doppler shifts. Some of the systems that were proposed at the time are listed below but none of these have been placed into commercial use.

• The Russian Molniya system, using 3 satellites in three 12 hour orbits separated by 120º around the earth, with apogee distance at 39 354 km and perigee at 1 000 km;

• The Russian Tundra system, using 2 satellites in two 24 hour orbits separated by 180º around the earth, with apogee distance at 53,622 km and perigee at 17,951 km;

• The Loopus system, using 3 satellites in three 8 hour orbits separated by 120º around the earth, with apogee distance at 39,117 km and perigee at 1,238 km, and;

• The European Space Agency's (ESA's) Archimedes system using a so-called "M-HEO" 8-hour orbit. This would produce 3 apogees spaced at 120º. Each apogee corresponding to a different service area covering a major population centre, for example the full European continent, the Far East and North America.

None of these systems exist today in practice. Most were abandoned before implementation due to technical difficulties and moist of all financing issues.

The idea is however far from dead and forgotten because a new variant of the idea has emerged recently. This system proposes to create equivalent two Quasi-stationary orbits on each side of the equator over the tropics.




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3.3.1.3 Polar Orbit

A polar orbit is inclined at about 90º to the equatorial plane, covering both poles. The orbit is fixed in space, and the Earth rotates underneath. A single satellite in a Polar Orbit provides in principle coverage to the entire globe although there are long periods during which the satellite is out of view of a particular ground station. Communications systems of the store-and-forward type can make use of satellites in Polar Orbits, but without many satellites being place in these orbits continuous coverage would not be possible.

Most Small-LEO systems employ polar, or near-polar orbits. An example of such a system is the COSPAS-SARSAT Maritime Search and Rescue system. This system uses 8 satellites in 8 near-polar orbits: Four SARSAT satellites move in 860 km orbits, inclined at 99º, which makes them sun-synchronous. Four COSPAS satellites move in 1000 km orbits, inclined at 82º.

3.3.1.4 Sun-Synchronous Orbit A Sun-synchronous or Helio-synchronous orbit requires that the angle between the orbital plane

and Sun remains constant, which results in consistent light conditions for the satellite.

3.4 Space Debris NASA and the ITU became concerned a few decades back regarding the vast quantities of "space

junk that was accumulating in orbit around the earth. Each time a rocket is launched, various bits and pieces end up being discarded and continue to orbit the earth. There have been attempts to address the issue by the countries involved in space and the ITU. The USA has taken on the task of monitoring the situation. NORAD tracks each and very piece of space junk satellites that orbit the earth. Space junk consists of discarded rocket stages, satellites shields and used satellites, especially LEO satellites.

The items and satellites that are in LEO orbits pose the most serious risk, especially for LEO satellite constellations. The probability of a collision is still considered to be low, but if one accident happens involving a working satellite, others are bound to follow.

The Issues is that it takes a large amount of energy to get these items into space in the first place. For example, an object in a LEO, travelling at 10 km/s, with a mass of 1 kg, is equivalent to a 35 000 kg truck travelling at 190 Km/hr! Imagine the consequences of a collision with another piece of junk or a working satellite! The resultant impact will create many smaller pieces of junk that then become an even bigger problem.

The problem is being addressed by mainly the USA (The FCC) and the ITU. Rules have been drafted requiring LEO constellation system operators to set aside sufficient fuel on each satellite, to be used to place the satellite into either a "Safe orbit" or to ensure that the satellite returns to earth in such a way that "guarantees" it will "burn up" in the atmosphere.




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The problem is no less serious for the GSO orbit. The GSO orbit and its slot positions are a scarce resource; so used "end of life" satellites cannot be allowed to remain in place. This is besides the fact that station keeping requires fuel on board the satellite to fire rockets to keep the satellite in place. The ITU and the US government have a set of rules that require all GSO satellite operators to place "end of life satellites into a "graveyard orbit" about 300 km above the GSO orbit. However the Inter-Agency Space Debris Co-ordinating Committee (IADC) is in the process of changing the rules to allow satellites to placed into an orbit only 235 km above the GSO. It is believed that these satellites will slowly be "captured" by the sun and drawn ever so slowly towards it until finally they will burn up in the sun.

Not all operators have followed the rules however. Some have allowed their satellites to be used until all fuel resources have been used up and none was available for transfer into their "end-of-life orbits". In 1997-1998 38 GSO satellites were taken out of service. 13 were placed into an orbit way beyond the IADC requirement, 9 beyond the 300 km ITU/US requirement and 12 were abandoned in GSO. Of these, only one belonged to a USA operation. The others were all either Russian or Chinese.

The issue is that the fuel that has to be kept in reserve for the placing of the satellite into the graveyard orbit comes at a cost. That cost is measured in terms of the number of weeks of useful life that has to be sacrificed, and the higher the orbit the more it costs. Each 25 km of altitude required by the rules means one week less of life.

3.5 Inclined Orbits, Orbital positions and GSO Slot Positions The GSO orbit is used extensively for communication satellites today. The ITU co-ordinates and

manages the GSO orbit. All satellites that are launched for placement in the GSO orbit must be co-ordinated by the ITU.

GSO satellites are placed into the geostationary orbit at very specific locations, precisely over the equator at 2º intervals. The orbital positions or slot positions are measured relative to the Greenwich meridian, in degrees East or West. Every country in the world has been allocated at least one "nominal orbit" position, and sometimes more than one.

GSO satellites have to be accurately maintained in their orbital positions. This process will be described later. As satellites age, satellite operators extend the life of satellites by placing satellites in "inclined" orbits. The inclination is measured in degrees away from the equatorial plane. These satellites are said to "drift" North or South, and no longer appear stationary. Satellite transponder capacity on these satellites is normally cheaper than normal, but users must make use of tracking antennas to be able to provide continuous communication links using such satellites.

3.6 Summary The types of orbit available for satellite communication systems are numerous as can be seen from

the above. The selection of an appropriate orbit for any satellite system is dependent on the system that is to be provided and the functionality required of the system. Not all orbit types are equally suited to all systems. The most favourable orbits are:




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• The HEO orbits inclined at about 64� to the equatorial plane. This orbit is stable with respect to irregularities in terrestrial gravitational forces. The orbit is useful for mobile communication systems where low elevation angles very often result in shielding effects from buildings and terrain factors. Also such system can be very effectively used to provide coverage of the Polar Regions.

• ICO circular inclined orbits are very popular in earth observation and imaging systems. The altitude of the satellite is constant and with near 90º inclination angles, such a satellite system will pass over every region of the earth according to a regular and predictable timetable. An example is the SPOT system at an altitude of 830 km an inclination angle of 98,7º and a period of 101 minutes.

• The GSO orbit, with an inclination angle of zero degrees, an altitude of 35 786 km and the most useful orbit for stable high quality communication and broadcasting systems.

The choice of an orbit depends on the purpose of the system to be provided. Some of the parameters that must be considered when making a design choice are:

• The coverage required especially in the Polar Regions.

• The altitude of the satellite has minimal effect on overall performance and quality of communication systems for a given coverage area. The signal attenuation varies according to the inverse square of the distance that would tend to make one believe that LEO satellites offer better performance because of the low attenuation that has to be taken into account in the link budgets. However when one factors in the coverage area requirements then some areas will involve longer paths with more atmospheric interference and lower performance.

• The elevation angles of earth stations. HEO and Polar Orbit systems offer very high elevation angles and thus do not suffer from blocking factors, but are not always visible. GSO satellites placed at some locations may offer very low "look angles" where local obstacles come to play. The elevation angles are affected by the location of the earth station on the earth's surface.

• By far the most over played aspect is that of propagation delay. GSO offers a constant but long 250 ms whereas LEO systems promise shorter RTT's but this has to be weighed against the increased complexity of the systems if continuous communication facilities are required.

• Interference with other systems using the radio frequency spectrum. GSO systems involve satellites in known locations and thus non-interfering bandplans have been developed. Interference probabilities are much higher and more complex to deal with for orbiting satellite systems.

• The costs involved in launching satellites. The costs are almost directly proportional to the total mass of the satellite, but GSO satellites require two launches for a given coverage area with an in-orbit spare, but a LEO system requires many launches to achieve the same result even though the mass of the satellites are much less.

4 SOME MORE ON SATELLITES

5 SOME MORE ON FREQUENCIES

5.1 The Major Frequency Bands The radio frequency spectrum is subdivided into a number of bands for easy reference. The main

Table of Frequency Allocations (TFA) has a few conventions that are related to this subdivision. The Frequency spectrum above 3 GHz is further subdivided into bands that have been given somewhat arbitrary names. How these names were derived is steeped in mystery and intrigue with one popular view being that the names originated during the Second World War when attempts were made to confuse the enemy by allocating the letters to parts of the band.

The band is subdivided into 9 frequency bands. The frequencies are expressed in Hertz (Hz) according to the following rules:




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Band Unit of measurement0 – 3 000 kHz Kilohertz (kHz)

> 3 MHz – 3 000 MHz Megahertz (MHz) > 3 GHz – 3 000 GHz Gigahertz (GHz)

The following table shows the ITU band numbers and a few other designations. There is even an attempt underway to give the bands new codes just to add to the confusion!




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ITU Band

No

ITU Symbol

Frequency Band (Lower limit excl.)(Upper limit incl.)

Alt Symbol Typical Usage

0-3 kHz ELF Normal Speech 4 VLF 3-30 kHz 9-540 kHz Radio Navigation 5 LF 30-300 kHz 6 MF 300-3 000 kHz 540-1630 kHz AM Radio broadcast 7 HF 3-30 MHz Short-wave radio broadcast 8 VHF 30-300 MHz 54-88 MHz TV Channels 2-6 88-174 MHz FM Radio broadcast 174-216 MHz TV Channels 7-13 216-600 MHz TV Superband 9 UHF 300-3 000 MHz 470-806 MHz TV UHF Channels 14-70

10 SHF 3-30 GHz 11 EHF 30-300 GHz 12 300-3 000 GHz

The following table shows the frequency bands specifically used for satellite communications and includes the popular symbols.

ITU Band

No ITU

Symbol

Frequency Band Lower limit exclusive Upper limit inclusive

GHz

Alt Symbol

Satellite Link

Satellite Service

Available Bandwidth

MHz

1 500-1 600 MHz Down 100 1 600-1 700 MHz L Up Mobile 100 9 UHF 2 500 2 600 MHz S Down Broadcast

(BSS) 100

3,4-4,8 Down Fixed satellite (FSS) 1100

5,9-7,0 C

Up 1100 7,2-7,7 Down Military 500 7,9-8,4 X Up Military 500

10,7-11,7 Down FSS 1000 11,7-12,5 Down Broadcast 800

12,5-12,75 Down FSS 250 12,75-13,25 Up FSS 250

14,0-14,8 Up FSS 800 17,3-18,3

Ku

Up FSS 1000 17,7-20,2 Down FSS 2500 20,2-21,2 Down Mobile 1000 22,5-23,0 Down Broadcast 500

10 SHF

27,0-30,0 Up FSS 3000 11 EHF 30,0-31,0

Ka

Up Mobile 1000

Chapter 2 of the radio regulations gives the complete TFA for each region of the World. This table is the most used portion of the radio regulations and is continually being modified as new allocations are made for new services and as requirements in countries change.




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The other very crucial sections of the radio regulations referring to satellite communication matters are the famous (or infamous) Appendix 30, Appendix 30A and appendix 30B. These appendices contain the details of the nominal GSO satellite band plans that govern all slot position allocations for each and every country. The nominal band plans were created to try to ensure that all countries get ("their fair share" of the available slot positions. The sad part is that very few countries have to date been in a position to launch their own satellites! Thus many of the satellites that are in orbit at the moment make use of slot positions that have been "ceded" to those countries that can afford their own satellites. The USA, the European Community, Russia, and a very small handful of domestic satellite programs dominate the GSO orbit.

5.1.1 Polarisation An electromagnetic wave radiated by an antenna consists of an electric field and a magnetic field.

These two fields are at right angles to each other and to the direction of propagation.

Now it is possible to design a transmitter to transmit what is known as circularly polarised signals where the wave "rotates" either right-hand (Clockwise) or left-hand (anticlockwise) as well as non-rotating waves (linearly polarised signals) that are either vertically polarised or horizontally polarised.

The significance of all this (besides the highly complex antenna design, the mathematics and other technical issues) is that:

1. Antennas designed for transmission and reception of circular polarised waves cannot receive or transmit linearly polarised signals.

2. Antennas designed for linearly polarised signals cannot transmit or receive circular polarised signals.

3. Antennas can however be designed to receive both left-hand and right-hand signals or both vertical and horizontal polarisations.

4. This allows for the same frequency to be used at the same site for both modes of transmission thus improving on the total utilisation of the frequency spectrum.

5. Antennas can also be designed to receive all forms of polarisation. These antennas are costly and are not generally used for commercial systems anymore.

The ITU uses this property of circular and linear polarisation in the band plans by allocating the same frequencies to many satellites in such a way that there would be no interference between adjacent services.

The commercial impact of all of this is that:

• Network designers must know, before any ground station equipment is purchased, which satellite is to be used so that the correct antennas can be specified

• Network designers must know if both polarisations are to be used on the same antenna, also before the antenna is specified

• Network designers cannot move services from one satellite to another that uses a different polarisation scheme on a whim.

For example, most Intelsat satellites in operation use circular polarisation, and PanAmSat uses linear polarisation. In South Africa, DSTV services on PAS 10 are all linearly polarised. Most point-to-point services offered by Telkom make use of circular polarisation.

6 TERMINOLOGY

6.1 Bandwidth (BW) Bandwidth is a generic term used to describe a portion of the frequency spectrum that has a lower

limit and an upper limit. It is used in many different contexts, sometimes to illustrate that a transmission is restricted to certain values and other times to indicate what bandwidth is required for a particular service.




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6.2 Footprints and coverage These two terms are used interchangeably. However the purist will use footprint to describe the

particular area that is provided with coverage by an antenna on a satellite. Coverage is used by some to indicate the total area in which a satellite provides services and is a summation of all the footprints of the individual antennas on a satellite.

6.3 "Uplinks" and "Downlinks" The uplink is that part of the space segment that transmits the signal to a satellite including the

reception equipment on the satellite.

The downlink is that part of the space segment that transmits the signal back to the ground including the transmitter on the satellite.

6.4 Transponders A satellite payload is made up of paired transmit/ receive devices known as transponders. It

includes any equipment involve in the frequency conversion process and also any regenerative equipment that may also be provided. There are different transponder types. A regenerative transponder includes a regeneration stage and a hopping transponder is frequency agile and able to be remotely reconfigured for a different bandwidth and frequency.

6.5 Link budgets Probably the most mysterious element in the vocabulary of satellite experts. It is always made out

to be very difficult to undertake but in reality is nothing else than a calculation done according to very strict rules of the transmission losses and noise associated with a satellite link.

The word "budget" is used because the whole exercise is to "balance" the losses against the gains available to ensure that the design system and network parameters are achieved.

Most of the process is mechanical and thus programmable into software and there are now many packages available for this. One is required to enter in the fixed values for elements of the link, the desired performance criteria and out will come the required transmit power levels, the receiver sensitivities needed and the antenna sizes required to meet the performance objectives of the link. Conversely a know network's parameters can be entered and an estimate of what the performance criteria are likely to be is the result.




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Satellite operators invariably insist on being provided with the results of the Link budget calculations so that they can verify that the planned link will not overload a transponder. This is not so important if the transponder is entirely dedicated to one user as if this happens only he will be affected. A shared transponder however means that all the users must jointly stay within the limits set by the operator and thus he will insist on the calculations. He may even demand some changes to ensure the total limits are not exceeded. Many users get the satellite operator to do the calculations for them as a result and then just implement the "link" according to the operators' requirements.

The whole purpose of the link budget therefore is to get optimal performance from the link at the lowest possible transponder utilisation level using the lowest possible power levels but still meeting the link performance criteria required for the service.

The only parameters available to the end user are antenna sizes, output powers of the transmitters and receiver and demodulator performance criteria.

6.6 Redundancy Redundancy is used to describe any and all network elements or components provided in a design

for back-up purposes. Satellites provide spare transponders that can be used for back-up and may other components that can be remotely switched into a circuit if a component fails. This is of course totally dependent on the equipment on the satellite that controls this process!

6.7 "Bent Pipe" A "bent pipe" satellite is one where effectively the signals transmitted to the satellite are simply

turned around and sent back to earth, without regeneration. Most of the satellites systems available today are of the type. They do not process any information received on the uplink. They merely amplify the signal convert the frequency to the correct downlink frequency and retransmit the signal. The designated receiver tuned to the correct frequency will receive the signal.

6.8 Regenerative Regenerative satellites do process the signals. In effect the satellite then becomes an earth station

in the sky because the signal is demodulated and re modulated after processing for retransmission. This process allows in digital systems for all interference and noise added to the signal in the uplink stage to be removed and transmitted afresh as if it came direct from the original earth station. Such satellites are more complex than bent pipe satellites. They also have to be designed for very specific services in mind and are not as flexible as "bent pipe" satellites. Users of such satellites have to be very sure of the market they want address and the services they want to provide, as changes are not possible after the satellite is launched.

6.9 On-Board processing Regenerative satellites can be simple or complex. The more complex ones provide for more

service flexibility in that the "on-board processor" can be instructed what to do with signals on a packet-by-packet basis. Generally we then talk of on-board processing satellites to make the distinction.

6.10 On-Board switching On-board switching is used to describe any functions that allow signals to be rerouted on

instructions from a service controlling station.

6.11 Modulation Schemes Modulation is the term used to describe the process whereby a "baseband signal" is super imposed

on to a carrier signal before the carrier signal is transmitted from one site to another. Modulation takes place in a number of domains. These are described in simple terms below




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6.11.1 Amplitude Modulation The baseband signal is used to vary the signal strength of the carrier signal. Thus the information

contained in the baseband is converted to a variation of the strength of the carrier wave. Amplitude modulation is not extensively used in satellite communication systems because of the vast distances over which the signals have to be transmitted. Amplitude modulation systems are prone to being disrupted by other radio signals and noise.

6.11.2 Frequency Modulation In frequency modulation the baseband is used to vary the frequency of the carrier wave. This

variation in the frequency of the carrier wave is proportional to the baseband. Frequency modulation schemes are less affected by noise and other signals, but are more difficult to maintain and operate. The amplitude of the carrier wave thus contains no information. Frequency modulation is the basis for most communication systems.

6.11.3 Phase modulation Phase modulation is very similar to frequency modulation but in this case the baseband is used to

vary the phase of the carrier signal, instead of the frequency. Phase modulation is the most common form of modulation used for digital systems as each digital state or code can be allocated a specific phase. Digital phase modulation schemes are the least affected by propagation conditions and noise, and are thus ideally suited for satellite systems.

6.12 Multiple Access Schemes Multiple access systems are another cornerstone development that first came out of the satellite

industries before it was introduced into terrestrial systems. Multiple access systems exploit some factors that are common to voice communication services and also data communication services to a lesser extent.

The three basic techniques are:

• Frequency Division Multiple access (FDMA)

• Time Division Multiple Access (TDMA)

• Code Division Multiple Access (CDMA)

All of these techniques are based on the fact that communication channels are not fully occupied in both directions continuously because certainly for voice, when one direction is talking the other direction is not and that there are pauses in between where both directions are not using a channel at all. Thus it is possible to temporarily allocate these channels to other users in these silence periods.

All this is encompassed in the work done statistically by Erlang in developing the famous rule of telephony that one Erlang of traffic is where one channel is fully occupied for one hour.

The other important aspect as related to satellite communications is that of routing where there are two possibilities available. A channel can be allocated on the basis of one carrier per link to every other station I a network or, that one carrier can be allocated per carrier station.

The first one means that every channel that is required in a network will require a dedicated carrier and in a three-station network this means at least two carriers are required for station and each direction!

The second one states that only three carriers are required for each direction and that that carrier will be shared somehow between each of the stations. The techniques for implementing this sharing process is known as Multiple Access Routing and uses either FDMA, TDMA or CDMA to do the sharing.

The choice as always was driven by economics as the second option will reduce the bandwidth required for such a network because less capacity is need for three carriers as is needed for 6 carriers.


THEORY BOOK 2


A further issue that is relevant to satellite systems is that Multiple access can also be implemented by providing multiple access to a particular transponder or to a particular channel on a transponder. The three techniques differ only in that the division is done by either using frequencies or by using or by using the same frequency at different times or by applying a coding scheme to the transmission and using the same frequency with different codes.

All that has to be addressed is how to prevent the wrong messages from being passed onto other channels at the other end.

In FDMA the signals occupy different sub-bands, in the time domain the times are carefully synchronised across the links and in the coding system each signal is identified by a unique signature.

The next step in the process is to add the aspect of a single channel per carrier (SCPC) or a multiple channel per carrier (MCPC) to the equation.

This in satellite system we talk of SCPC/FDMA or MCPC/FDMA and similarly for the others.

FDMA is simple to implement but lacks flexibility and capacity is lost when loads increase and the carriers have to always be transmitted in the same order

TDMA requires time synchronisation to achieve the use of the time domain. The advantages are that only a single carrier has to be transmitted occupying all of the bandwidth available. Transmission throughput remains high even under load conditions. There is no need to control transmitter power and all stations transmit and receive on the same frequencies. The disadvantages are the need for timing synchronisation and the need to correctly dimension the network to achieve the desired throughput.

CDMA is also simple to implement once the technique is understood. It does not require synchronisation and it offers protection against interference and interception.

The last aspect is that of Demand Assignment, where channels are only allocated if and when required on a per channel basis and on a per repeater basis. Again two methods can be used, that of fixed assignment and on demand assignment.

Thus we get to the point where some services are referred to as DAMA Demand Assignment Multiple Access or PAMA permanently assigned Multiple Access. The difference is simply that the one only assigns channels when required and the other.

The diagram below best describes these techniques.




THEORY BOOK 2


6.13 Multibeam Satellite Systems Single beam satellite networks have disadvantages. These are:

• Large single so-called global beams cover a complete hemisphere visible from the satellite. The RF energy transmitted by the antenna is spread over a large area affecting the antenna gain of the transmitting antenna. A GSO satellite with an aperture angle of 17,5º to the 3 dB points implies an antenna gain of only 20 dB.

• Narrow beam systems may provide coverage over smaller portions of countries but are then unable to provide services outside of the coverage area.

The solution to this matter is the use of multibeam satellite networks. The advantages are:

• Improved link performance and hence lower earth station costs

• Improved frequency reuse as each of the multibeams are able to use the same frequency spectrum.

The disadvantages is increased interference between the beams under failure conditions. This can be mitigated against by careful but possibly costly design implementations.

Interconnection is achieved by using three techniques. These are Transponder hopping systems where transponders are divided into sub bands that are interconnected in such a way that all possible combinations are catered for. The next method used is to provide for complex on-board switching matrices that allow transponders to cross-strapped according to customer requirements. The third method is to provide inter-satellite links that allow cross connection of uplinks and downlinks on beams on other satellites




THEORY BOOK 2


The most popular method is a combination of the first two where some transponders are used for cross-strapped connections either fixed or variable and others using transponder hopping techniques.

6.13.1 Beams There are five types of beams:

• Global beams that provide coverage of the complete portion of the earth that the satellite can "see". Global beams are usually circular beams produced by reflecting antennas.

• Zone beams that provide coverage of only a portion of the earth. Shaped beams may be a combination of reflecting antennas and/or phased array antennas produce these.

• Spot beams that deliberately focus energy on a particular area of the earth. Spot beams are normally circular beams produced by reflecting steerable antennas.

• Multiple beam coverage implies many beams using different frequency bands and different polarisations.

• Lattice beams using a defined frequency pattern (3 or 7 pattern).

Shaped beams are produced by altering the shape of the reflector, or by using a horn feed array driven by mechanical phase shifters, the so-called phased array antenna.

Phased array antennas are more efficient at concentrating power within the desired footprint with less interference outside of the footprint. Phased array antennas allow electronic manipulation of the antenna shape by electronically changing the phase distribution and the amplitude of the radiating elements. Such methods allow "fine tuning of footprints after satellite launch, by telecommands sent form the TT&C station.

The disadvantage of using phased array antennas and electronic manipulation of the elements is complexity, increased power drain and increased risk of component failure.

Multiple beam coverage can be done using separate beams with normally circular footprints all using the same frequencies but different polarisations in a grid format, or by contiguous beams that may overlap each other to some extent but using different frequencies and different polarisations.

Lattice beams are used in mobile satellites in particular where generally only the frequencies are changed to allow mobile earth station equipment and handsets to roam within the larger "footprint". Note that the higher the number of frequencies used in the pattern, the less interference possibilities exist but this goes hand in hand with reduced overall capacity per beam is possible.



Satellite Communications Theory - Book 2

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