Satellite Data Communications using VSAT Systems – Extending IT Networks to the Global Context

 

Bruce Elbert

Application Technology Strategy, Inc.

 

Abstract

This paper provides a review and assessment of satellite technologies for providing broadband data communications using very small aperture terminal (VSAT) network systems. In this context, “broadband” means that the application requires a data transfer rate greater than 100 kbps and should allow broadcast, multi- and uni-cast, and interactive bi-directional services to fixed locations worldwide. The applications considered are: Internet access over satellite, digital content distribution, wide area network (WAN) connectivity, video teleconferencing, distance learning, and telephony. The systems examined include digital broadcasting (e.g., DVB) with IP encapsulation, and bi-directional VSAT star networks. Detailed comparisons of various transmission parameters are provided to help evaluate currently available satellite and ground equipment capabilities. It is observed that improved forward correction is desirable, namely the turbo product codes now being introduced widely in satellite ground equipment.

1                    Broadband service – a definitionA broadband data communications service is one that requires a transfer rate greater than that afforded by a dial-up telephone line using a V.92 modem. This places the minimum data transfer rate at about 100 kbps, which is typical of current high-speed access (HSA) services from Digital Subscriber Line (DSL) in its many forms, cable modems, and comparable fixed wireless and satellite HSA services. There is also the question of whether the two directions of transmission are of equal speed (symmetrical) or asymmetrical such that the inbound speed from server to user is greater than the outbound speed from user to server. From an IT perspective, broadband service supports standard office applications including email and file transfer, and major software systems like Enterprise Resource Planning and distance education. Organizations are structuring many of their IT applications for use within a standard Web browser, allowing employees and partners to access services within the Intranet and from the external Internet as well. This makes applications seem relatively similar to the network, but the detailed structure cannot be ascertained in general. HSA can provide video distribution, telephony and video conferencing, although these may not be deliverable through a browser since they require specialized user terminal devices or other appliances.

2                    Role of Operating GEO satellitesThe single most critical element and technology in broadband satellite communications is the satellite itself, since every link within a common footprint must pass through it. Spacecraft designed and constructed in recent years are larger in physical size and mass, and provide

substantially more power than their predecessors. This results from improvements in launchers, on-board power systems, high performance components, and radio-frequency high power amplifiers. Other components used within the microwave repeater have improved as well, with benefits showing up in reduced component mass, lower signal loss, and enhancement of transmission quality. A typical satellite weighs almost 5000 kg on top of the launch vehicle, has a lifetime of 15 years, and provides between 50 and 90 channels of wideband transmission (commonly referred to as transponders) with individual power levels of up to 200 RF watts, each. A modern GEO satellite may serve relatively small antennas throughout a large area such as the entire Euro-Asia continent or the full breath of the Pacific region (illustrated in Figure 1 for the PanAmSat 2 satellite).

 

Figure 1. A typical satellite transmit (downlink) footprint of the PanAmSat 2 satellite, located at 169º East Longitude; satellite radiated EIRP values are approximate (courtesy PanAmSat Corp).

 

 

Digital communications and GEO satellites have long partnered and in fact innovations such as TDMA, CDMA, digital speech interpolation and video compression were applied to space-ground links well ahead of terrestrial networks. The Digital Video Broadcast (DVB) standard fits tightly to the satellite’s natural ability to transmit the same high-quality signal across a wide region, rendering the cost per location to an infinitesimally small number (not including the cost of the dish and set-top box installation). Moving forward, this platform provides broadband data delivery and facilitates return channel service if remote sites are suitably configured. Competition from new entrants like PanAmSat, SES and Loral has impacted satellite operation such that quasi-governmental operators like Intelsat and Eutelsat have

become commercial enterprises. Suitable GEO satellite capacity is now available throughout Asia-Pacific for networks that serve most any location.

3                    Application Interface StandardsA summary of applications and interface standards is provided in Table 1. For digitized content, quality is set at the source encoder and transmission only introduces time delay. For a satellite hop, this delay is small compared to that of compression/decompression. The terrestrial interface concerned with DVB and digital TV in general is called ASI, a high speed serial connection used primarily on the uplink side. Telephone service is usually delivered on an analog basis (2-wire or 4-wire). A properly engineered satellite voice circuit meets the currently accepted standard of 400 ms total delay, including the added delay for speech processing (e.g., compression and decompression, if applied), routing and switching. Importantly, such a satellite circuit will sound better to subscribers than casual Voice over IP connections through the Internet.

 

Table 1. User applications and their interface standards applied in satellite communication networks.

User application Network TechnologyInternet access (one user; small group; remote site)

High speed access to Internet backbone; TCP/IP

One way over satellite; terrestrial return

    Two way over satellite; broadcast outbound with multiple access inbound

Remote access to corporate Intranet (LAN extension)

High speed access to private network infrastructure; web-based applications; TCP/IP

One way over satellite; terrestrial return

    Two way over satellite; broadcast outbound with multiple access inbound

Remote access to corporate business applications

Medium to high speed access to private network infrastructure; applications employ client/server or mainframe style; may employ proprietary protocol

Two-way over satellite; broadcast outbound with multiple access inbound

    Two way over satellite; point-to-point circuit, either pre-assigned or demand assigned

Content distribution Multi-cast uplink for wide area distribution to PCs and content caching servers; UDP/IP and Multicast Transport Protocol (MYP

One way over satellite; verification of 100% reception via terrestrial or satellite return

Video teleconferencing High speed access to private network infrastructure or public  ISDN; H.320 or H.323 standards

Two-way over satellite; broadcast outbound with multiple access inbound

    Two way over satellite; point-to-point circuit, either pre-assigned or demand assigned

Telephone Low to medium speed access to private network infrastructure or PSTN; POTS or VoIP standards

Two-way over satellite; broadcast outbound with multiple access inbound; echo cancellation

    Two way over satellite; point-to-point circuit, either pre-assigned or demand assigned; echo cancellation

Leased line Medium to high speed connection; T1/E1

Two way over satellite; point-to-point circuit, pre-assigned

 

4                    Internet ProtocolThe Internet itself is the last and probably most important interface in the context of data communications. Organizations in the private and public sector have either converted their data communications over to the Internet Protocol, or are in the process of doing so. The interface that is growing to dominate the data world is the simple RJ-45 modular jack associated with the Ethernet standards, 10baseT and 100baseT. Higher rates than 100 Mbps demand Gigabit Ethernet or the optical speeds of the Synchronous Digital Hierarchy (e.g., OC-3, OC-48 and the like). Such speeds are presently beyond a practical HSA service from currently operating C and Ku band GEO satellites. This could be the domain of the coming generation of broadband satellites employing Ka band spot beams and on-board processing.

 

4.1             Broadcast, Multicast and Unicast

 

Terrestrial networks, including the Internet, are effective for point-to-point transfer of digital media and content. Multicast service over the Internet must employ several point-to-point links to emulate a broadcast system, and therefore has difficulty assuring timely delivery of content to all receivers. A broadcasting station from a local radio tower or GEO satellite affords timely delivery of content with a consistent bandwidth. Guaranteeing delivery is usually less of a problem because receivers are designed to directly play the content (a local recording device can allow later playback, if desired).

 

Included in the DVB standard is a data transfer capability called Internet Protocol Encapsulation (IPE). This allows a single broadcast carrier to transfer both television programming and Internet content on the same transport stream. At the subscriber end, the carrier is detected by an integrated receiver decoder (IRD) that extracts the data and delivers it a local PC or LAN. This vehicle allows satellite broadcasters to introduce broadband data into their multiplexed transmissions. The data that rides the MPEG stream may be encrypted along with the digital video and audio, or can be processed with its own unique encryption system. To this may be added a terrestrial return channel for bi-directional service to the desktop or other computational device. Many applications can be supported in this asymmetrical manner since the greater demand is for megabit per second transfer over the satellite in the outbound direction. One must not neglect the potential of this mode for reaching locations that cannot transmit directly over the satellite. Provision of a satellite return channel, in the inbound direction, is discussed next.

 

4.2             Interactive bi-directional data

 

Interactive data communications are the foundation of must corporate and government uses of telecommunications. These needs can be addressed by properly engineered bi-directional satellite links that involve multiple transmitting earth stations. The Very Small Aperture Terminals (VSATs) used by fueling stations and discount department store chains in the Americas, Europe and parts of Asia demonstrate that such networks are practical (e.g., easy to install and centrally manage), reliable (e.g., 99.9% availability) and cost/effective (e.g., saving users as much as 20% over what an equivalent terrestrial network would cost). The architecture of a typical VSAT network employing a star topology (e.g., all communications through a central hub) is illustrated in Figure 2.

 

Figure 2. Architecture for a typical VSAT network employing a common hub and star topology.

 

 

In 2002, VSATs are becoming attractive to smaller enterprises and for big organizations that wish to push the use of satellite communication down further in their operation. The cost of equipment per site has dropped from over US$10,000 in 1998 to around US$2000 in 2002. Consumer versions that provide HSA to the Internet are offered in the US for under $500.

5                    Review of equipment and network suppliers in the marketplace

The market for satellite communications ground equipment as introduced above is served by specialist manufacturers and systems integrators. A partial listing is provided in Table 2. Several have been in business for more than a decade; however, some of the more interesting technology is offered by relatively new companies that didn’t exist prior to 1998. This can make it more challenging to convert architecture into a real network, but the methodology reviewed here should make the task less formidable.

 

Table 2. Suppliers of Technology for Broadband Satellite Communications.

Service Provider Approach Product Base Experience

One –way BroadLogic Receivers and IRDs   DVB-SPC cards and IRDs

  ViaCastEncapsulators, receivers, IRDs

  DVB-SSupplier to many CDNs

  SkyStream Encapsulators and IRDsSatellite Media Router

DVB-SOriginator of opportunistic encapsulation

 Logic Innovations

Encapsulators   DVB-SSupplier to integrators

 International Datacasting

Encapsulators and IRDs  DVB-S and others

Experienced supplier and integrator

 Global Telemann

Receivers and IRDs   DVB-S

Distributor of products for data distribution and access

  Helius Receiver and router   DVB-SStand-alone remote site receiver/router

Two-wayHughes Network Systems

Full range of VSAT and wireless products

PES, TESProprietary, TDMA

Leading supplier to the global market

  Gilat SatelliteFull range of VSAT and wireless products

SkyStar Advantage, SkyBlaster

Proprietary and DVB, TDMA

Leading supplier to the global market

  ViaSat VSAT productsLinkStar, ArcLight

Proprietary and DVB, TDMA and CDMA

Growing supplier to commercial and gov. markets

  STM WirelessFull range of VSAT and wireless products

 Proprietary and DVB, TDMA

Supplier to global market

  ShironInteractive VSAT product line

InterSKYProprietary and DVB, FDMA

Field proven DVB-S product line

  iDirectInteractive VSAT product line

Netmodem IIProprietary, IP-based

High-speed IP networking over satellites

Integrator Bit CentralDistributor and integrator for private networks

  DVBExperienced integrator

  AlcatelMajor telecom manufacturer/integrator, satellite experience

 

Proprietary and DVB, uses products from STM and others

Leading telecom equipment supplier, also active in satellite communications

 

5.1             Basic multiple access and modulation schemes

 

Effective and efficient satellite communications depends on the type of modulation and multiple access used by transmitting user terminals and earth stations. The staunch support by suppliers of their particular approach often produces interesting and confusing debate within the technical community. Mirroring the dialog of the digital mobile (cellular) standards, satellite multiple access techniques run the gamut of time division, frequency division and code division approaches. Figure 3 shows how these techniques occupy the two key dimensions of satellite capacity: frequency spectrum and time. The suppliers of two-way products in Table 2 each have chosen a scheme for reasons of experience and capability. Evaluation of these systems is ongoing, and each can demonstrate satisfactory operation in a live network. Any of the three can be made to work; however, it is likely that one or two may be superior for a specific defined application. Beyond the theory, it is the product design and protocol operation that matter as to how well the multiple access system delivers information in an effective and manageable way.

 

Figure 3. Illustration of the time and frequency utilization of basic multiple access techniques, indicating how four earth stations would share the overall channel bandwidth.

 

The primary modulation method in use over satellites is phase shift keying (PSK). Adopted by satellite engineers in the 1960s, PSK has found its way into all wireless systems as it is nearly optimum with regard to the use of bandwidth and power. Variants like minimum shift keying (MSK) and Gaussian MSK (GMSK) that are applied in different situations, and some have gained in popularity due to increased importance of using low-power transmitters on the ground.

 

The key FEC techniques employed by current VSAT equipment include:

Convolutional encoding with Viterbi decoding, long a favorite in satellite and terrestrial wireless communications, is available in coding rates (R) between 7/8 (minimum gain) and 1/2 (maximum gain);

Reed-Solomon (R-S) code, a block coding scheme with excellent properties and popularized by DIRECTV;

Concatenated (combined in a serial manner) convolutional and  R-S, provided by the DIRECTV Satellite System (DSS) and the Digital Video Broadcast (DVB) system; very effective for all services, particularly digital TV using MPEG 2 and Internet data transfer;

Turbo Product Code (TPC), similar in concept to the concatenated code but based on combining two like encoding mechanisms  that employ feedback and iterations to boost FEC performance..

 

A comparison of typical implementations of these FEC techniques is provided in Figure 4. Plotted on the X axis is the ratio (in dB) of the ratio of energy per bit to the noise density; the Y axis indicates the estimated probability of bit error, which approximates the bit error rate (BER) delivered at the receiver. It is clear from these data that TPC is to be favored on performance alone; however, its computational complexity limits data rate to about 10 Mbps, appropriate for inbound service. Modems to support concatenated coding, particularly within the DVB standard, are available up to about 100 Mbps, making this system desirable for high speed outbound transmission from the hub and for the broadcast of both digital video and Internet content.

 

Figure 4. Performance characteristics of current forward error correction techniques, provided for comparison purposes only.

5.2             Comparing Technical Performance

 

A global comparison of technology and its implementation by developers and manufacturers is probably impractical. However, if the basic requirements for the network are known, it is possible to narrow the possibilities and make choices of equipment and operating parameters. Most generalized comparisons resort to the basic linear equations for the wireless line-of-sight path, e.g.,

where C is the received carrier power, Pt is the transmitter output power, Gt is the transmit gain, Gr is the receive gain, and R0 is the range from the transmitting antenna to the receiving antenna. From the basic geometry of the geostationary orbit, the line-of-site path length (R0) can be estimated from:

where is the latitude and is the longitude of the earth station relative to that of the satellite.

The measure of link performance, C/N, is computed as the ratio of C to the noise (N) in the signal RF bandwidth (B). For pure thermal noise as produced within the receiving earth station,

where k is Boltzmann’s constant and T is the equivalent noise temperature of the receiving system (composed of contributions from the antenna, coupling loss and low noise amplifier).

Conversion to Eb/N0 amounts to multiplying the C/N by the ratio of the bandwidth, B, to the information bit rate.

 

This simple calculation is not sufficient to account for a variety of other noise sources and impairments that significantly affect the satellite channel. These include:

Uplink noise, which is computed in the same manner as above; Propagation effects due to the various layers of the atmosphere, particularly absorption

by clear air and rain (which introduces substantial power loss at frequencies above about 10 GHz), and scintillation fading due to the troposphere and ionosphere;

Transponder intermodulation distortion, which may add noise products into the spectrum of the carrier;

Interference from cross polarized signals on the same satellite (XPOL) and from adjacent satellites (ASI)

Direct distortion to the signal as it passes through the uplink earth station, satellite transponder, and receiving earth station; the principal impact is called intersymbol interference (ISI), an impairmentwhich causes the required Eb/No to increase for the same probability of bit error.

 

Satellite communications engineers most often use the link budget to identify and combine the various gains, losses and margins in the uplink and downlink path. The practice of link budget formulation involves both science and art. Individuals who routinely compile them have their own unique formats, typically embodied in personalized Microsoft Excel spreadsheets. There are a myriad of calculations and assumptions for individual entries, and engineers typically include margins anywhere in the range of 0.5 to 3 dB to cover factors not known with sufficient accuracy.

 

The following is an example of the type of analysis that would be performed in studying the properties of a particular network, considering the candidate satellite and techniques for multiple access, modulation and forward error correction. This is for illustrative purposes and not to derive conclusions applicable to a different network design.

 

Fixed in this analysis is the satellite, assumed to be at 169º EL and operating at C-band (e.g., PAS 2) with the uplink emanating from San Francisco, CA; the receiving earth stations are located at Suva, Fiji, Manila, Philippines, and Palembang, Indonesia. The cities where chosen to measure the impact of different local climate situations and geometry to the satellite, where Suva is the most favorable in terms of elevation angle and rainfall; Palembang is the least

favorable, having the lowest angle and being situated in a region of intense tropical rainfall, and Manila being about in the middle in terms of these issues. While PAS 2 provides a fixed coverage to these locations, we have chosen to make the downlink effective isotropic radiated power (EIRP) an independent variable in the range of 25 to 40 dBW. The bit error probability at the receiver, including all impacts on the uplink and downlink, is held constant at 10-8 at 99.9% availability. The results of the analysis in terms of receive antenna diameter are shown in Figures 5 a, b and c, for Suva, Manila and Palembang, respectively. The separation between each set of curves reflects the required Eb/N0 for the particular FEC technique, i.e., concatenated R-S with R=3/4 or R=1/2 convolutional, and TPC. Also factored into the analysis are realistic values for XPOL, ACI and ICI.

 

Figure 5. Receive antenna diameter for the hypothetical video distribution network consisting of an uplink from San Francisco and downlinks to (a) Suva, Fiji, (b) Manila, Philippines, and (c) Palembang, Indonesia, versus satellite EIRP and forward error correction technology - Rate 3/4 and 1/2 convolutional concatenated with R-S (204,188), and turbo product code.

 

(a) Suva, Fiji

(b) Manila, Philippines

(c) Palembang, Indonesia

The curves reveal some interesting aspects of this evaluation:

A 2.8 meter antenna at Suva would require 35 dBW using concatenated R-S and R=3/4 convolutional coding, while about 3 dB less EIRP would suffice using TPC;

For 35 dBW at Manila, antenna diameters range from 3 meters for R=3/4 concatenated coding to 2.2 meters for TPC

EIRP values of up to 2 dB greater are required at Palembang to use antenna sizes similar to Manila and Suva; a larger diameter would  therefore be recommended for use in the same network.

 

The analysis was extended to include uplinking from these sites to create a star VSAT network back to a hub located in San Francisco, CA. The assumed antenna sizes of 2.8 meters for Suva and Manila, and 4.8 meters for Palembang were determined both by the outbound downlink requirements and inbound transmit needs. Table 3 presents the results of this evaluation, assuming the actual satellite uplink characteristics for these three locations in Asia Pacific. For an information transfer rate of 256 kbps for the inbound link, we see significant improvement from the application of TPC as compared to convolutional FEC (at the time of this writing, concatenated convolutional with R-S was not available on the market for the inbound channel). In addition to the antenna, the other critical VSAT design parameter is the solid state power amplifier (SSPA) power output. We see that this power is held to a reasonable value (under 4 watts) for both Palembang and Manila, resulting from the satellite providing good uplink performance. On the other hand, the Suva location happens to be situated in an unfavorable part of the PAS 2 uplink pattern for the Vertical uplink beam, which causes the transmitter requirement to balloon to tens of watts.

 

Table 3. Excel spreadsheet containing parameters for VSAT inbound link evaluation

Paramter Units      Data rate kbps 256    Eb/No dB      

R=1/2 (without R-S)   6.4    Turbo   4.5    

Transponder capacity channels      R=1/2 (without R-S)   161    Turbo   225    

         

   Suva, Fiji

Manila, Phil

Palembang, Indon

SFD dBW/m2 -80.0 -90.0 -88.9G/T dB/K -12.0 -1.6 -2.9Elevation angel degrees 66.1 33.1 17.5Rain rate at 99% mm/hr 83.1 110.9 121.4Uplink fade (rain + tropo) dB 0.6 1.3 3.8Dish size meters 2.8 2.8 4.8SSPA power Watts      

R=1/2   30.7 3.6 3.0Turbo   22.0 2.6 1.9

 

A study of this type can be quite involved as there are potentially many links, different modulation and FEC methods, and a variety of satellite coverage options to consider. The difficulty one faces in this type of effort is that there are almost always more equations than knowns; therefore, a unique solution cannot be obtained. Many important factors, such as ASI and ISI, can only be estimated and then budgeted in the link budget in terms of additional margin. It is always a good practice to test the proposed solution using a comparable satellite link and equipment of the proposed design. Also, satellite performance should be based on high-quality measurements taken from calibrated earth stations at critical points of the coverage. The best source of this type of data is the appropriate manufacturer or operator; however, the network developer may need to perform some of this work themselves.

6                    ConclusionWe have identified and reviewed many topics that are central to the development of cutting-edge satellite broadband networks. New configurations and applications using interactive satellite networks are being devised to address a hungry market for communications and information applications; some will succeed and, unfortunately, some will fail. One can only increase the probably of success by considering a sufficiently wide range of technologies and their providers. However, gaining a firm base in the technical performance of the different options is a key to developing a satellite network that satisfies users.

 

Endnotes

Much of the analysis was performed using software developed by Derek Stephenson, who heads Arrowe Technical Services of the UK.

 

References

Kadish, Jules E. and Thomas W.R. East, Satellite Communications Fundamentals, Artech House, Boston, MA, 2001.

Elbert, Bruce R., The Satellite Communication Applications Handbook, Artech House, Boston, MA, 1997.

Elbert, Bruce R., Introduction to Satellite Communication – second edition, Artech House, Boston, MA, 1999

Elbert, Bruce R., The Satellite Communication Ground Segment and Earth Station Handbook, Artech House, Boston, MA, 2001

Elbert, Bruce R., Networking Strategies for Information Technology, Artech House, Boston, MA, 1992

 

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