Satellite Implementation

Satellite Implementation
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Published Date:25-10-2017
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CHAPTER 3 Issues in Space Segment and Satellite Implementation With the global adoption of the technology, satellite communications has brought with it a number of issues that must be addressed before an application can be implemented. Satellite capacity is only available if the right satellites are placed in service and cover the region of interest. Considering the complexity of a satellite and its supporting network, applications can be expensive to install and manage. If the issues are addressed correctly, however, the economic and functional needs of the application will be satisfied. A viable satellite communications business is built on a solid technical founda- tion along the lines discussed in the previous two chapters. In addition to frequency band and bandwidths, such factors as orbit selection, satellite communications pay- load design, and the network topology have a direct bearing on the attractiveness of service offerings. The satellite operator must make the decision whether to launch a satellite with one frequency band or to combine payloads for multiple-frequency operation (called a hybrid satellite); whether or not to design the payload and net- work around an onboard digital processor is another question. As the payload becomes more unique, the demands on the market and supporting technologies increase. In addition, the business and operation should consider and properly address all of the issues that this chapter raises. We also review the current state of the art in bus design as it has a bearing on payload power and flexibility. The remainder of this chapter goes into contingency planning from the perspec- tive of the operator and the user. The reliability and flexibility of satellite applica- tions cannot be assured without thorough analysis and proper implementation. For example, a satellite operator should implement a system with multiple satellites so that no single event can terminate vital service to users. Users, on the other hand, must approach satellite communications with an open mind and open eyes. They might arrange for backup transponder capacity for use in the event of some type of failure. Both parties may also need to obtain insurance to reduce financial loss. The information that follows provides background on some of the more critical areas that often hamper the introduction and smooth operation of effective systems. Readers should also consider how other potential problems not identified here could adversely impact their services and plan accordingly. 6768 Issues in Space Segment and Satellite Implementation 3.1 Satellite Selection and System Implementation Many of the issues that must be considered by the operators of terrestrial telephone, television, and cellular networks must also be faced by providers of satellite applica - tions. What is different is the need to split the application between space and ground segments. The most basic type of space segment, shown in Figure 3.1, employs one or more GEO satellites and a tracking, telemetry, and command (TT&C) ground station. The associated ground segment can contain a large quantity of Earth sta - tions, the specific number and size depending on the application and business. For example, there would be as few as 10 Earth stations in a backbone high-speed data network, but in the millions of TV receive-only terminals in a major DBS system. The ground segment is very diverse because the Earth stations are installed and oper - ated by a variety of organizations (including, more recently, individuals). Impor - tantly, we have moved out of the era when the space and ground segments are owned and operated by one company. Due to the size of the investment and the complexity of the work, the satellite operator is usually a tightly organized company with the requisite financial and technical resources. It engages in the business of providing satellite capacity to the user community within the area of coverage. There are more than 50 commercial satellite operators in 25 different countries; however, the industry is dominated by six companies who provide most of the global transponder supply. Capacity can be offered on a wholesale basis, which means that complete transponders or major por- tions thereof (even the entire operating satellite, in some cases) are marketed and Space segment (Satellite operator) TT&C Earth station Satellite control center Hub or gateway Earth station Ground segment (network operator or user) VSATs or other user terminals Figure 3.1 Elements of a satellite system, including the space segment and the ground segment.3.1 Satellite Selection and System Implementation 69 sold at a negotiated price. Each deal is different, considering the factors of price (lease or buy), backup provisions, and the term. The retail case comes into play where the satellite serves the public directly, such as in MSS and BSS networks. We consider such business issues in detail in Chapter 13. To create the space segment, the satellite operator contracts with one of the approximately 12 spacecraft manufacturers in the world for many of the elements needed for implementation. Historically, most operators took responsibility for putting the satellite into operation, including the purchase and insurance of the launch itself. More recently, some contracts have required in-orbit delivery of the satellite, which reduces the technical demand and some of risk on the satellite pur - chaser. However, satellite buyers still need a competent staff to monitor the con - struction of the satellites and ground facilities, and to resolve interface and specification issues. This can be accomplished with consultants, the quality of which depends more on the experience of individuals than on the cost or size of the con - sulting organization. The experienced spacecraft consultants include Telesat Can - ada, The Aerospace Corporation, and SESG Global. Individuals, such as retirees from spacecraft manufacturers, can provide excellent assistance at much lower cost. However, they can be difficult to find. The capacity demands of cable TV and DTH systems are pushing us toward operating multiple satellites in and around the same orbit position. Successful satel- lite TV operators like SES and PanAmSat have been doing this for some time, devel- oping and improving the required orbit determination and control strategies. This considers accurately determining the range of the satellite, since we are talking about separating satellites by tenths of degrees instead of multiple degrees. A few of the smaller operators of domestic satellites like Telenor, Thaicom, and NHK double the capacity of an orbit slot by operating two smaller satellites rather than launching a single satellite with the larger combined payload capacity. On the other hand, employing a larger satellite with double or quadruple the number of transponders will generally significantly reduce incremental costs at some increase in risk. Implementation of the Earth station network can follow a wide variety of paths. One approach is to purchase the network as a turn-key package from a manufac- turer such as ViaSat (Carlsbad, California), Hughes Network Systems (German- town, Maryland), Alcatel (Paris, France), or NEC (Yokohama, Japan). This gives good assurance that the network will work as a whole since a common technical architecture will probably be followed. There are systems integration specialists in the field, including L3 Communications STS, Globecomm Systems, Inc. (both of Hauppauge, New York), IDB Systems (Dallas, Texas), and ND SatCom (Friedri- chshafen, Germany), which manufacture and purchase the elements from a variety of manufacturers and perform all of the installation and integration work, again on a package basis. The application developer may take on a significant portion of implementation responsibility, depending on its technical strengths and resources. Another strategy for the buyer is to form a strategic partnership with one or more suppliers, who collectively take on technical responsibility as well as some of the financial risk in exchange for a share of revenue or a guarantee of future sales. Some of the smaller and very capable satellite communications specialists, such as Shiron Satellite Communications ( and EMS Technologies, Inc. (Norcross, Georgia), can provide a targeted solution.70 Issues in Space Segment and Satellite Implementation The operations and maintenance phase of the application falls heavily on the service provider and in many cases the user as well. The service may be delivered and managed through a large hub or gateway Earth station. This facility should be sup - ported by competent technical staff on a 24-hour per day, 7-day per week basis (called 24–7)—either on site or remotely from an NOC. Such a facility might be operated by the integrator or supplier and shared by several users or groups of users. This is a common practice in VSAT networks and cable TV uplinking. Inexpensive user terminals, whether receive-only or transmit and receive, are designed for unat - tended operation and would be controlled from the hub. The systems integrator can operate portions or the entire network, including maintenance and repair of equip - ment. A properly written contract or Service Level Agreement (SLA) with a compe - tent supplier often gives functional advantages for the buyer, such as backup services and protection from technical obsolescence. Other risks to be addressed are reviewed at the end of this chapter. The satellite communications industry keeps evolving as satellite operators dis - cover how to enter the businesses of their users and as users experiment with becom - ing satellite operators. In the case of the former, Hughes Communications created the DIRECTV service to produce much more revenue than would be possible through the wholesale lease of the required Ku-band satellite capacity. On the other hand, PanAmSat was started up by the former management of the Spanish Interna- tional Network, which was the leading Spanish language network in the United States. As these companies have discovered, their counterpart’s business is quite dif- ferent in the nature of the respective investment. A basic issue on the space segment side is the degree to which the satellite design should be tailored to the application. Historically, C- and Ku-band satellites in the FSS are designed for maximum flexibility so that a variety of customer’s needs can be met. A typical FSS transponder may support any one of the following: an ana- log TV channel, 4 to 10 digital TV channels, a single 60-Mbps data signal such as would come from a wideband TDMA network, or an interactive data network of 2,000 VSATs. The satellite operator may have little direct involvement in these applications. Alternatively, they may invest in these facilities to provide value-added services. The alternative is to design the payload to meet the requirements for a specific type of signal, tailored for one application. The latest generation of BSS satellites provides high levels of RF power to deliver the signal to very small antennas, and may have less uplink than downlink coverage. These satellites are very effective for broadcasting but would be less suitable for two-way communications from VSATs. At the other end of the range are the MSS constellations, where the antennas and transponder electronics are specifically designed to receive the very low power sig- nals coming from portable terminals and handheld phones similar to cellular phones. The downlink transmit power, on the other hand, is much higher per voice channel than is typical of an FSS design. With all of this, the satellite must provide a high degree of frequency reuse since the bandwidth available at L-band is only on the order of 30 MHz. As a consequence of specialization, systems like DIRECTV and Globalstar are inflexible when it comes to adapting to substantially different applications. On the other hand, these tailored satellites deliver the service that is intended and do it with relative efficiency.3.2 Communications Payload Configurations 71 One trend is toward more highly tailored satellites, which could accelerate as the transponder itself becomes digital in nature. Digital processing offers many benefits, such as much greater flexibility in channel routing and antenna beam con - trol. The trade-off is that once internals of the repeater are made of silicon, the sig - nal format may need to remain nearly constant throughout the life of the satellite. Looking ahead a decade or more, satellite technology will evolve so that the flexibil - ity of today’s fixed bandwidth analog transponder might be available in the digital repeater mode. The ground segment is also undergoing an evolutionary process, where the first designs were versatile, multipurpose (voice, TV, and data), and expensive. Over time, the Earth station has become more specific to the purpose and lower in price. Digital implementation of analog functions like the modem now permit very sophis - ticated small Earth stations that can sell for about the price of a PC. The choice of level of integration of the ground and space segment activities is a strategic decision of the satellite operator and application developer. There are some operators who have launched spacecraft with no specific thought as to how their space segment services will function with the ground segment of prospective users. In contrast, other operators have simulated their hypothetical communication net- work performance at every step from when the concept is defined to factory tests to in-orbit. These results have been fed back into the design of critical elements of the satellite and ground user equipment. If the integration between these is very tight, such efforts are mandatory. The choice somewhat depends on how unique the spacecraft payload and frequency band are in relation to alternative systems and services. We begin the discussion of satellite system issues with a review of some of the most important trade-offs in satellite design. The two halves of the satellite—the communications payload and the spacecraft bus—are presented in Table 3.1 and reviewed next. 3.2 Communications Payload Configurations Communications payloads are increasing in capability and power, getting more complex as time passes. Examples include the newer DTH missions, which are designed to maintain maximum EIRP and digital throughput for up to 32 high- power transponders in the downlink. Likewise, state-of-the-art MSS satellites have large, deployable antennas to allow portable terminals and handheld phones to operate directly over the satellite path. Onboard digital processors are likewise included as a means to improve the capacity and flexibility of the network, a capa- bility for multibeam Ka-band satellites. Some traditional issues are still with us. One of the most basic is the selection of the frequency band, which was addressed in Chapter 2. Suppose that the operator wants to be able to address the widest range of applications and has consequently decided to implement both C- and Ku-band. With today’s range of spacecraft designs, one can launch independent C- and Ku-band satellites. This was done in the first generation of the Ku-band SBS and C-band Galaxy systems in the United States. Each satellite design can be optimized for its particular service. The alterna- tive is to build a larger spacecraft that can carry both payloads at the same time. The72 Issues in Space Segment and Satellite Implementation Table 3.1 Subdivision of a Typical Communications Satellite into the Subsystems of the Communications Payload and Spacecraft Bus Major Subsystem Element Function Communications Repeater and Provide all communications relay and processing payload antenna system functions Microwave repeater Basic weak signal reception of the uplink and high- power transmission to the downlink Onboard processor May be analog or digital; intended to divide band- widths and communications traffic for proper routing between the uplink and downlink Antenna system Captures electromagnetic waves on the uplink side and radiates electromagnetic waves on the downlink side Spacecraft Power, control, The vehicle that powers and protects the payload bus and structure of the through all phases of the mission and operation spacecraft vehicle Power subsystem Provides prime power from solar arrays and energy storage using batteries to provide power during eclipse Telemetry and Allow ground operations to monitor and control command the entire satellite (payload and bus) Attitude control Manage and control the spacecraft vehicle in orbit and propulsion and keep it properly aligned with the Earth throughout its lifetime Thermal control Maintain a suitable temperature environment for all subsystems Structure Contain all of the subsystems and protect them during launch and deployment on orbit first such hybrids were purchased in the early 1980s by INTELSAT (Intelsat V), Telesat Canada (Anik B), and Southern Pacific Railway (Spacenet). The hybrid satel- lites launched by INTELSAT also permit cross-band operation, with the uplink Earth station at one band (e.g., Ku-band) and the downlink at the other (C-band). A third overall design issue deals with the coverage footprint, which has a direct bearing on the design of the spacecraft antenna system. The two basic alternatives are to either create a single footprint that covers the selected service area or to divide the coverage up into regions that each gets its own spot beam. These two approaches have significant differences in terms of capacity, operational flexibility, and techni- cal complexity. 3.2.1 Single-Frequency-Band Payload The single-frequency payload represents the most focused approach to satellite design. The concept was first introduced by Early Bird (Intelsat I) in 1965 and advanced in 1975 by the 24-transponder Satcom spacecraft built by RCA Astro- space (now absorbed into Lockheed Martin). Beyond 2000, single-band payloads have become targeted toward specific applications in TV and mobile communica- tions. The TV marketplace is dominated by cable TV and DTH, where the quantity of available TV channels at the same orbit position becomes important. The3.2 Communications Payload Configurations 73 majority of the cable TV satellites for the United States, including some of the Gal - axy and AMC series, are single-frequency designs, optimized to the requirements of the cable TV networks. This considers all of the technical, operational, and financial factors in providing service. From a technical prospective, the transponder gain and power is made to match the Earth stations used to uplink and receive the signals. What is more important to this class of customer is that the capacity must be there when needed. Because nearly all U.S. families receive satellite-delivered program - ming, the cable TV networks put a very high value on the reliability of getting the capacity to orbit and operating once it gets there. The satellite operators that address cable markets therefore need excellent plans for launch and on-orbit backup (discussed further at the end of this chapter). In brief, experience has shown that the best way to do this is to construct a series of identical satellites and launch them according to a well-orchestrated plan. This must consider how the capacity is sold to the cable programmers as well as the strategy for replacing existing satellites that reach end of life. The single-band satellite can fit well into such a plan. Matters are more complicated when an operator wishes to replace individual C- and Ku-band satellites with a dual-frequency hybrid satellite. The benefit of doing this is reduced investment cost per transponder and simpler operation, but the common timing and orbit slot required might be incompatible. A second area of the TV market where single-band satellites are preferred is in DTH. Spacecraft for SES-Astra, DIRECTV, and EchoStar/DISH are single-band designs tailored to the specific requirements of their respective DTH networks. This considers the quantity of transponders, the size of the receiving antenna (and there- fore the satellite EIRP), the signal format (which determines the transponder band- width, channel capacity, and quality), and the coverage area. Taken together, these factors have enormous leverage on the economics and attractiveness of the service, second only to the programming. The delivery of the signal to millions of small dishes demands the highest EIRP that is feasible with a given state of the art. DBS satellites tend to push the limits on power, as opposed to mass (often referred to as weight, but technically mass since the satellite is in a zero-g environment). This leaves little left over for C- or L-band repeater elements, which, if included, could force a compromise of some type. (A possible exception, for smaller markets, is con- sidered in the next section.) The cost of a high-power DTH satellite is often more than that of, say, a C-band satellite that serves cable TV. This should not be a concern because of the much larger quantity of receiving antennas. In fact, in 1 we demonstrate that to achieve an optimum G/T for a network of 10 million receivers, the satellite EIRP must be approximately 50 dBW. By optimum, we mean that the total cost of the satellite and all of the receiving Earth stations is at a minimum. Lower EIRP demands that dishes must be larger, raising the cost of the ground segment faster than the savings in the satellite. Going in the direction of increasing satellite EIRP is likewise unattractive since the increase in satellite cost outweighs the savings in receive dishes. In other words, the high investment cost of the satellite is ultimately very economical on a cost-per-user basis. Increasing satellite EIRP allows smaller receive dishes; however, a limit at around 55 dBW and 45 cm, respectively, is imposed by the adjacent satel- lite interference. This happens to be the design point for most major DBS networks, reflecting the preference of consumers for compact antennas.74 Issues in Space Segment and Satellite Implementation 3.2.2 Multiple-Frequency-Band Hybrid Payloads Hybrid satellites were first introduced by INTELSAT at C- and Ku-bands with the launch of Intelsat V. A third L-band payload was added to Intelsat V-A for use by Inmarsat. The first domestic hybrid, Anik B, was operated by Telesat Canada in the late 1970s; and two American companies—Sprint Communications and American Satellite Corporation (both since merged into GTE and the satellites subsequently sold to Americom)—were also early adopters. The idea behind the use of the hybrid was to address both the C- and Ku-band marketplaces at a reduced cost per trans - ponder. During the 1990s, satellite operators pursued much larger spacecraft plat - forms like the 8-kW Lockheed Martin A2100, the Boeing 601-HP, and the Astrium Eurostar. Even higher powers are provided by the Boeing 702 and Loral 1300S series, which reach 15 to 20 kW of prime power. Illustrations of these spacecraft are shown in Figure 3.2. This class of vehicle can support almost 100 transponders, allowing a full DBS repeater to be combined with the high end of C-band services. A criticism leveled at the 15 kW and greater design is that the operator may be putting too many eggs in one basket. However, the other side of the coin is that these designs simplify operation (only one spacecraft need be operated at the orbit position) and markedly reduce the cost per transponder. 3.2.3 Shaped Versus Spot Beam Antennas The coverage pattern of the satellite determines the addressable market and the flexibility of extending services. The traditional and most successful approach to SS/Loral 1300S Astrium-Space 9kW 19 kW 6200 kg at launch 3200 kg at launch LM A 2100 AX 3600 kg at launch Figure 3.2 Large-capacity GEO spacecraft.3.2 Communications Payload Configurations 75 date is the shaped area-coverage beam that serves a country or region of a hemi - sphere. This type of antenna pattern permits one signal to be delivered across the entire footprint from a bent-pipe transponder. While versatile, this approach limits the overall satellite throughput bandwidth as well as the effective spacecraft antenna gain (and hence EIRP) at the boundary. The opposite principle of frequency reuse through multiple spot beams is gaining favor for high EIRP MSS satellites like Thuraya and Inmarsat 4; in addition, systems that employ Ka-band to provide broadband Internet access likewise use the multiple spot beam approach. This sec - tion reviews the characteristics and trade-offs between these two means of serving users on the ground. For a constant transponder output power, the EIRP varies inversely with the beam area. Stated another way, for a given spacecraft antenna configuration, the product of gain (as a ratio) and area is a constant. We can estimate the gain of any particular area of coverage using the following relationship: 2 G ≅ 27,000 φ (3.1) where G is the gain as a ratio, and φ is the average diameter of a circular coverage area, measured from GEO in degrees. Measuring coverage in degrees comes about because the full Earth extends across approximately 17° as viewed from GEO, 2 resulting in a minimum gain at beam edge of 27,000 / 17 = 93.4 or 19.7 dBi. This value would be further reduced by the aperture efficiency, which depends on the design of the antenna (horn, reflector, or array). A beam of one-tenth this angular diameter would have one-hundredth the area, but the gain would increase by 100 (or 20 dB) to a total of 39.7 dBi. Figure 3.3 provides an illustration of how the gain and area are related for two differing coverage areas: the country of Colombia and the continent of South Amer- ica. The Colombian market would be served with a national beam that is directed exclusively toward this country, delivering high gain and no direct frequency reuse Figure 3.3 Coverage options: single country (Columbia) versus entire continent (South America).76 Issues in Space Segment and Satellite Implementation (other than through cross-polarization). From an orbit position of 65 WL, the pic - ture of the land area shows heavy grid lines that are 1° apart. By counting grid 2 squares, we can estimate the landmass of Colombia to cover approximately 2.8 deg . A national coverage antenna would of necessity reach beyond the border and is 2 slightly larger at 4.6 deg . In comparison, the landmass and example antenna cover - 2 age of South America are approximately 40 and 52 deg , respectively. Because the area in square degrees of the South American beam is 10 times that of Colombia’s, the gain over the entire continent is a full 10 dB less. One could, of course, maintain the same level of EIRP by increasing downlink transmitter power by 10 dB as well. An alternative that is shown in Figure 3.4 subdivides the coverage area many times over using small spot beams. Assuming that each beam is 0.4° in diameter, it will take approximately 38 such spots to provide the full national coverage. As the 2 size of the spot is only 0.126 deg , the gain increases substantially by approximately 15 dB compared to the area beam. The 28 spot beams are arranged in a 7-beam reuse pattern with one-seventh of the allocated spectrum assigned into each spot. Spots that reuse the same piece of spectrum are separated by two adjacent spots that are noninterfering. This need to isolate spots applies to FDMA and TDMA; CDMA offers the possibility of not subdividing the spectrum but rather allowing interfer - ence to overlap in adjacent beams. To use these beams more effectively, the satellite can have an onboard beam-routing scheme. The general relationship among the coverage area, beam size, and number of beams is indicated in Figures 3.5 and 3.6. The first graph allows us to estimate the directivity of a beam of a given area, measured in square degrees. For the case of 2 Colombia, which extends approximately 2.8 deg as viewed from GEO, the satellite can deliver about 40-dBi gain. A doubling of the area to include, say, Venezuela will reduce gain by 3 dB to about 37 dBi. Extending further to cover all of South America 2 in one 50-deg beam pushes the gain down all the way to 27 dBi. A word of caution about the directivity numbers: these are approximate values that do not include the relevant losses in a real antenna system. Also, we have not evaluated the actual beam shaping that would be provided during the design of the antenna system. These numbers are intended to provide a general feel for the relationships. Figure 3.4 Comparison of multiple spot beam coverage versus single country shaped beam.3.2 Communications Payload Configurations 77 60.0 55.0 50.0 45.0 40.0 35.0 30.0 25.0 20.0 0.01 0.1 1 10 100 Area of each beam, square degrees Figure 3.5 An estimate of directivity in dBi versus beam area in square degrees. The second figure plots the number of beams required to cover either Colombia or all of South America. This is the concept behind Figure 3.4, which indicates that it would take about 38 spots of 0.4° each to fully cover the country. Beam area is plotted along the x-axis to be consistent with the previous figure. The information tells us that it can take a very large number of beams to cover a large landmass. On the other hand, the gain of such beams is substantially higher and the potential for frequency reuse much greater when considering a high density of small spot beams. There are two application areas where the multiple-beam approach appears to be 1000 100 South America coverage 10 Colombia coverage 1 0.1 0.01 0.01 0.1 1 10 100 Area of each beam, square degrees Figure 3.6 Approximate number of beams versus area covered and beam size in square degrees. Beam edge directivity, dbI Number of beams required to cover area78 Issues in Space Segment and Satellite Implementation the most appropriate: L-band MSS networks to serve handheld phones, and Ka- band FSS networks for advanced broadband communications to inexpensive per - sonal VSATs. As discussed in Chapter 2, L-band spectrum is very limited and we must incorporate as much frequency-reuse as possible. This, coupled with the diffi - cult requirement of serving low-power handheld phones, demands a large reflector antenna with many small spot beams. Service to mobile users would be restricted by the resulting link budget to something in the range of 50 to 150 Kbps. The band - width is more ample at Ka-band, so the major concern is with delivering high digital bandwidths (up to 20 Mbps) to an antenna of less than 1m. The choice among the coverage alternatives depends on the interaction of the technical and business factors that confront the satellite operator (who may also be the application provider). From a pure marketing perspective, the single area cover - age approach is the most flexible since you can deliver both individual services and broadcast services as well. The wide beamwidth produces a relatively low antenna gain, and the only frequency reuse is from cross-polarization. Moving toward multi - ple spot beams can greatly improve the attractiveness of the service to large quanti - ties of simple, inexpensive Earth terminals. The ultimate example is the handheld satellite phone, which demands the greatest quantity of beams and their correspond- ing high gain. When we move in this direction, we restrict the range of services that can be delivered. Broadcasting of video and other content is impractical because bandwidth must be provided within each and every beam to be served. A way around this might be to use dynamic beam forming to create an area footprint for the transmission in question. 3.2.4 Analog (Bent-Pipe) Repeater Design The repeater is that portion of the communications payload that transfers communi- cation carriers from the uplink antenna to the downlink antenna of the spacecraft. In established C- and Ku-band satellite systems, the repeater is divided into transpond- ers, each of which can transmit a predefined amount of bandwidth and downlink power. It is common practice to call a repeater a transponder and vice versa, although repeater is the more general term. Transponder, on the other hand, more typically refers to one RF channel of transmission, which can be assigned to one cus- tomer or group of customers for a common purpose (transmitting a multiplex of TV channels or providing a VSAT network). In the following, we review the traditional type of transponder, called the bent pipe, along with newer concepts employing digital onboard processing (OBP). An OBP repeater may provide a more sophisticated system for routing analog channels (and hence can offer greater flexibility for bent-pipe services) or may demodulate the bit streams onboard for efficient routing, multiplexing, or additional processing. As one moves toward increasing levels of complexity, the satellite becomes more and more a part of an overall network of ground stations and is inseparable from it. This tends to increase performance and effectiveness for a specific network implementa- tion but renders the satellite less flexible in terms of its ability to support different traffic types not considered prior to launch. The development time for an OBP repeater will generally take extra months or years as compared to the bent pipe, introducing the risk that the market for the planned application could be missed.3.2 Communications Payload Configurations 79 Each transponder of a bent-pipe repeater receives and retransmits a fixed- bandwidth segment to a common service area. There is a simple mathematical rela - tionship between the number of transponders and the total available bandwidth that is provided by the particular spectrum band. Simply stated, the number of transponders equals the total bandwidth divided by the bandwidth per transponder. There will be 10% to 15% guard band due to filtering at the edges of each trans - ponder. The example of a six-transponder design in Figure 3.7 has a single wide - band receiver that takes the entire uplink frequency band, typically 500-MHz wide, amplifies and transfers the same 500 MHz to the corresponding downlink band. The bank of input filters, labeled F1 through F6, subdivides the total bandwidth into 72-MHz segments (11.3 MHz less than straight division would indicate), each amplified to a high level by a dedicated power amplifier. The individual outputs of six amplifiers (each on a different frequency) are summed with minimum loss in an output multiplexer composed of six reactively coupled waveguide filters. The result - ing spectrum of 500 MHz (less the guardbands) is applied to the transmitting antenna system of the satellite, which typically broadcasts these signals across a common footprint. The engineering design of the transponder channel is a high art because a multi- tude of specifications and manufacturing issues must be considered. Parameters in the link budget like receiveG/T, transmit EIRP, transponder bandwidth, and inter- modulation distortion have a direct impact on users. These should be specified for every application. A multitude of others, like gain flatness, delay distortion, spuri- ous and phase noise, and AM-to-PM conversion, are often of less concern to some applications but potentially vital to others. Wideband digital transmission at 155 Mbps in a 54-MHz transponder is an exception because these distortions can sig- nificantly reduce throughput or increase the EIRP requirement for the same throughput. The driver/limiter/amplifier (DLA) in Figure 3.7 provides a degree of control over data transfer by adjusting the input power and possibly correcting some of the nonlinear distortion. A F1 DLA F1 A F2 DLA F2 A F3 DLA F3 A F4 DLA F4 A F5 DLA F5 F6 A DLA F6 Wideband To From receiver LPF transmit receive (500-MHz antenna antenna bandwidth) Legend: F1 Bandpass filter for channel 1 DLA Driver/limiter/amplifier A High-power amplifier LPF Lowpass filter Figure 3.7 A simple bent-pipe satellite repeater with six transponders.80 Issues in Space Segment and Satellite Implementation Typical transponder characteristics for the bent-pipe design are listed in Table 3.2. Actual values will vary from design to design, in response to the type of ampli - fier, the frequency of operation, and design choices for the intended service. Some examples of how repeater parameters can be related to particular signal types are shown in Table 3.3. To do this properly, the designer must fully understand the sig - nals being transferred and the distortions to those signals caused by the various ele - ments of the transponder. With the advent of 1-GHz PCs and signal analysis software, this optimization can be performed in minutes. However, the issue remains about whether it is wise to design the transponder for a particular signal and corresponding application. A useful alternative is to utilize a compromise design to accommodate a variety of expected signal types. This is actually how the first 36- MHz transponders were designed for INTELSAT IV, which was based on transfer - ring one analog FM TV carrier or a multiplexed telephone baseband containing up to 1,600 voice channels. Today, the 36-MHz transponder is the standard for bent- pipe satellites that serve analog and digital applications. If the signal format does not change during the lifetime of the satellite, the trans - ponder is eligible for optimization. Consider first if you will operate the transponder with only a single carrier over a wide bandwidth or if you will carry multiple carriers in the same transponder. The DTH transponder has characteristics that are deter- mined first by the frequency assignments filed for (see Chapter 7) and second by the type of signal modulation (analog or digital). Advanced repeaters that use digital sig- nal processing offer a great deal of flexibility in routing traffic and permitting inex- pensive user terminals to gain access to a range of mobile and fixed services; however, they may not have the greater than 30-dB range level flexibility (dynamic range) of the standard bent-pipe transponder. Alternatively, the processor function can be implemented with analog components that nevertheless have a degree of flexibility. The MSAT satellites operated by AMSC and Telesat Mobile contain Table 3.2 Typical Transponder Characteristics in a Bent-Pipe Repeater Characteristic Typical Value 2 Gain (saturation flux density) −96.0 dBW/m Linearity for multiple carriers −10 dB with respect to saturation (C/3IM) at saturation by two equal carriers Linearity for multiple carriers –20 dB at 8-dB IBO, (with (C/3IM) with backoff linearizer on) Noise power ratio (NPR) 16 dB at 4-dB output backoff Nonlinear phase shift 40° from saturation to –20-dB (AM-to-PM conversion) input backoff Amplitude frequency response ±−0.25 dB over useful bandwidth (gain flatness) Out-of-band attenuation −30 dB in adjacent channel (input channel separation) Cross-polarization isolation 30 dB (linear) (XPOL) −6 Frequency tolerance and stability 10 in the translation frequency Gain (attenuation) control 0 to 18 dB in 2-dB steps Gain stability ±1.5 dB over lifetime3.2 Communications Payload Configurations 81 Table 3.3 Specific Signal Types and the Transponder Characteristics That Can Be Optimized for Improved Performance FM video TWT AM/AM and AM/PM; filter group delay characteristics Wideband digital data Gain slope, group delay; TWT AM/AM and AM/PM SCPC Gain flatness and amplitude nonlinearity Wideband TDMA Linearity, gain flatness, and transient response of the power supply VSAT operations Gain flatness/linearity and frequency stability Mobile SCPC Noise power ratio surface acoustic wave (SAW) filters that, when combined with commandable down/upconverters and switching, allow the ground network operator to alter the bandwidths and routing of SCPC bandwidth segments. This controls the balance of traffic and permits the operator to isolate sources of interference. Processors of this design have been offered to the market by ComDev of Canada, and one is carried on Anik F2. The selection of the bent-pipe transponder has implications for users of satellite capacity. Bent pipes are nearly transparent to the user and can be subdivided in power and bandwidth, as discussed previously in this section. Moving toward the more sophisticated designs, the satellite becomes integrated with the network and transparency is lost. One of the first specialized commercial repeaters was carried aboard the Spacenet IIIR satellite. This was the Geostar payload, which introduced an L-band vehicular position determination service in the United States. Geostar contracted with the spacecraft manufacturer to have additional antennas and receivers installed on the satellite and paid Spacenet for the operation and use of their payload. Later, Geostar failed as a business. The special transponder could not be reassigned to some other revenue producing application although it was kept operating. The spectrum for this application was subsequently reallocated by the ITU. 3.2.5 Digital Onboard Processing Repeater The digital OBP repeater is a significant advancement from the analog versions that merely interconnect frequency channels using microwave filters and mechanical switches. At the core of OBP is digital signal processing (DSP), a computational process reduced to solid-state electronics that converts an information signal from one form into another unique form. Historically, the DSP was programmed on a multipurpose digital computer as a way to save the time and energy of doing the transform mathematically with integral calculus. The most well-known DSP process is the fast Fourier transform (FFT), which is related to both the Fourier transform and Fourier series taught to all electrical engineering students. It takes a signal in the time domain (i.e., a waveform) and converts it into a collection of fre- quencies (i.e., a frequency spectrum). The inverse FFT does just the opposite, trans- forming a frequency spectrum into a time waveform. When in either digital format, we can multiply, filter, and modulate the signals to produce a variety of alternate signal types. In this manner, a digital processor can perform the same functions in software that would have to be done with physical hardware elements like mixers, filters, and modulators. Modern DSP chips and82 Issues in Space Segment and Satellite Implementation systems can operate over many megahertz of bandwidth, which is what we need to build an effective digital repeater. To do this, the calculation speed must be in the gigahertz range. More recently, OBP has taken on many other roles where the actual bits on the RF carrier are recovered and reconstructed with minimum error, switched and routed, and remodulated onto other RF carriers in the downlink. This permits the OBP to act as a conventional packet switch and multiplexer, common to what is employed in land-based data communications networks. The specific con - figuration of the OBP repeater is created for the expected network environment, including the specific telecommunications applications to be provided to end users. Generic Processing Repeater Architecture A block diagram of a hypothetical digital processing repeater is shown in Figure 3.8. The antenna and wideband receivers perform their traditional analog functions, while filtering and switching occur in the digitized sections of the repeater. This is indicated within the box at the center of the figure. The majority of OBP repeaters are used to transfer traffic between multiple beams on the uplink and downlink, as would be the case in an MSS L-band or broadband Ka-band satellite. Each uplink beam is first low noise amplified and then down-converted to an intermediate fre- quency (IF) that is suitable for input to the digital onboard processor. The first function at the input of the processor is to convert the incoming fre- quency spectrum into a digital data stream. This is accomplished by the analog-to- digital (A/D) converter using pulse code modulation (PCM). For an IF bandwidth of 50 MHz, the A/D converter must sample at a speed greater than 100 MHz and con- vert each sample thus taken into a specified number of bits. The number of bits, in turn, is determined by such factors as the acceptable signal-to-noise ratio (inversely, Low-power transmission line High-power transmission line Upconv Rcvr HPA or driver Upconv Digital Rcvr HPA or driver processor: A/D Upconv Rcvr Demod HPA Rcv or driver Tx Routing and feed feed Switching network network Upconv HPA Rcvr Multiplexing or driver Modulation Beam forming Upconv Rcvr HPA D/A or driver Upconv Rcvr HPA or driver Active redundancy not shown Figure 3.8 Block diagram of a generic digital processing repeater with multiple spot beams, demodulation/remodulation, packet switching, and low-level beam forming. Receive feeds and apeture Transmit feeds and aperture3.2 Communications Payload Configurations 83 quantization error) and the dynamic range of input signals. The selection of the number of quantization levels and hence the number of bits per sample determines the amount of degradation to signal quality attributable to the processor. For exam - ple, if we assume 150 million samples per second and 10 bits per sample, then the A/D converter must output data at the speed of 1.5 Gbps. Since this is the data per beam, the total data processing capability of the A/D function is 1.5 Gbps times the number of beams. This does not include the processing associated with either the inverse digital-to-analog (D/A) conversion process, nor any of the processing done within the OBP itself. We can see that the processing power of a broadband repeater can be high indeed. The digitized channels can be routed either as narrow frequency bands or pack - ets. To route frequency bands, the OBP needs to select specific channels, cross con - nect them to the associated downlink, and reassign the frequencies so as to create a contiguous band. The process for packets requires the additional step of demodula - tion and potentially forward error correction to deliver the appropriate bit streams to packet switching elements. Other functions that are possible include automatic gain control, phase adjustment (as part of a phased array antenna system), channel multiplexing, linearization, and interference cancellation. After all of the processing is complete, the data is reconverted back into its analog form (i.e., D/A conversion). While the functions are shown as discrete components, they are actually per- formed mathematically by the processor and memory chips that implement the desired functionality. This means that the processor is designed for a specific pur- pose. To convert the processor analog output to the transmit band, each channel is fed to a hardware upconverter that translates from the intermediate frequency range to the RF downlink band. From this point, the signals are amplified in a conven- tional power amplifier and applied to the appropriate transmit antenna of the satellite. The onboard digital processor is controlled from the ground to set up the rout- ing instructions and other aspects appropriate to the network. As development con- tinues, it will be possible to transfer the entire uplink/downlink bandwidth through each port on the processor. This only requires greater processor speed, something that one expects to see as the technology is improved over time. The digital processor repeater was applied commercially in Iridium, which went into service in 1999–2000 incorporating routing, demod-remod, and packet switch- ing functions. Subsequently, ACeS was launched into GEO MSS service with the ability to route narrowband frequency channels between remote handheld termi- nals and the gateway; likewise, the Thuraya GEO MSS satellite added functionality for dynamic beam forming and direct handheld-to-handheld connectivity. OBP- based repeaters have also been developed for Ka-band satellites that could reach orbit in the mid-2000s. High capacity and sophistication for the processing function translate into the size, mass, and power consumption of the processor itself. In com- parison to the typical microprocessor found in a personal computer, the digital repeater processor needs substantially more capacity and tailored functionality. To accomplish this, the OBP employs an architecture that is typically composed of general purpose computer processors, programmable gate arrays, DSP chips, and specialized very large scale integrated (VLSI) circuits and application-specific inte- grated circuits (AISCs). These tend to run at a much higher speed and therefore84 Issues in Space Segment and Satellite Implementation consume more power. Table 3.4 suggests the technical issues and trade-offs required in the design, manufacture and test of the modern OBP repeater 2. Another consideration is the degree of redundancy that needs to be included. Certain common functions, like clocks, memory, and power supplies, can be made redundant. But the actual channel processing elements would normally be single string. Redundancy must then be provided by including extra strings such that excess capacity may be reallocated in case of a partial failure. The OBP must also be adaptable to ground control and management, a function associated with all land- based digital networks. This must be extended to the payload from the conventional NOC used by telecommunications operators, which in this case, are charged with providing an end-to-end service that meets commercial quality of service (QoS) objectives. This will involve expanding the TT&C link for network management and optimization functions and extending to the network provider. Classification of Processing Repeater Designs Digital onboard processing repeaters are individually designed for a specific mission and therefore are not interchangeable unless produced from the exact same design. We can try to put them into classifications regarding the manner in which the uplink signals are transferred to the downlink, the types of signal processing on board, and whether the carriers are demodulated to recover the bit streams. A classification matrix of OBP repeaters is presented in Table 3.5, based on mis- sions that have been defined and in some cases launched as of this writing. Within each, there are a wide variety of alternatives, and, in fact, some from one category intersect with others. In the limit, one can imagine an OBP repeater with the Table 3.4 Considerations in the Design and Production of a Functional Onboard Processing Repeater  Design and performance considerations  Analog to digital quantizing (bits per sample)  Number of A/D and D/A operations  Fast Fourier transform size  Number of points  Amplitude granularity  Sampling rate  Frame overlapping factor  Time window  Ripple  In-band interference  Physical and electrical considerations  Proper interfacing of analog IF circuitry to the A/D function of the processor  Design and specification of ASICs and multichip modules (MCMs)  Control signals: parallel and serial buses, precise timing  Power distribution: voltage and regulation  Reliability: 20-year lifetime, radiation shielding, thermal, redundancy, monitoring  Gain and phase matching of IF upconversion and downconversion  Testing: complex processing and connection, different techniques at different stages  Packaging: consumption within allowable budget, integration of thermal control heat pipes, location of mounting hardware3.2 Communications Payload Configurations 85 Table 3.5 General Classification of Onboard Digital Processing Repeaters Packet Routing RF Switching IF Routing Beam Forming Demod/remod (with demod/remod) FDMA ACeS, ICO Thuraya Skyplex mulitplexer Iridium Inmarsat 4 (HOT BIRD 5) TDMA Intelsat 6 Iridium CDMA Voice-span Military intelligence and speed to be able to detect any type of uplink signal, recover the bits, and dynamically transfer the resulting data to the most appropriate downlink chan - nel. This is, after all, the role of routers on the Internet and it is possible that a simi - lar type of device could ultimately find its way on board a satellite. Satellites that provide frequency reuse through multiple spot beams are candi - dates for OBP because any analog approach is inherently inefficient and inflexible (a possible exception is the Beamlink FDMA routing repeater by Com-Dev). The proc - essor on board Intelsat 6 performed a basic time-division switching function on the full 250-MHz bandwidth of the uplink. TDMA is used on the uplink side to sepa- rate carrier bursts according to the desired downlink beam. Switching is done at RF using PIN diode switches, which chop the time frame according to a prestored defi- nition of traffic flow. See our previous work for a more complete description of this somewhat basic approach, which was installed on the ACTS satellite as well 1. This approach, while efficient in terms of channel capacity (because the downlink amplifier is operated at saturation), had the disadvantage that Earth stations trans- mit at 100 Mbps or greater. Thus, it was intended for large Earth stations, which were rather expensive, and the entire strategy of RF switching has largely been retired by commercial industry in favor of long-haul fiber. The U.S. military introduced CDMA onboard a satellite during the 1990s to provide secure and antijam communications. The OBP was one of the first to include A/D conversion as part of the repeater. Subsequently, AT&T Bell Laborato- ries proposed to combine CDMA with demod/remod as part of the now-defunct Voice-span Ka-band project. The complexity and cost of this approach was beyond what commercial industry could produce at the time in 1997. At the same time, Hughes Space and Communications adapted their military processor experience to the MSS market with the IF routing repeater design. Lacking the complexity and features of CDMA and demod/remod, this approach permits low-power (and there- fore low-cost) user terminals to transmit narrowband information to the OBP wherein channels are selected and routed at IF to the appropriate downlink. This is reviewed further in Section A very useful step that was introduced by Boeing Satellite Systems was digital beam forming on Thuraya. This takes a fixed feed array and through appropriate phase and amplitude adjustment, permits the satellite operator to shape beams to meet traffic requirements after placement into service. Inmarsat selected EADS to produce a similar OBP for their fourth generation satellite. An early demod-remod repeater was put on the HOT BIRD 5 satellite of Eutelsat, thus offering format con- version in space. The concept is that individual uplinks can transmit one video chan- nel per carrier (e.g., SCPC) and the OBP demodulates and multiplexes as many as86 Issues in Space Segment and Satellite Implementation six into one TDM stream. Motorola empolyed demod/remod in Iridium by putting a packet switch inside the repeater. Properties of Demodulation/Remodulation OBP As shown in Figure 3.9, a demod/remod repeater looks nearly identical to the bent pipe, but with a demodulator and modulator added to each channel. The minimum function of this combination is to prevent the direct addition of uplink noise to the downlink noise. Instead, uplink RF noise is transferred to the baseband of the signal where it causes a specific amount of impairment such as increased error rate. The uplink will threshold at a point determined by the demodulator on board the satel - lite, while the downlink will threshold at a point determined by the demodulator in the receiving Earth station. The only impairment is the additional errors caused by uplink noise, which in many cases is substantially fewer than result in the downlink. Another benefit is that the downlink EIRP will be stable because the carrier that is applied to it is generated in the satellite modulator and not the uplinking Earth sta - tion. This same effect is produced by a limiter on the input side of the TWT; how- ever, a limiter is highly nonlinear and cannot be used with multiple carriers. Some missions might suffice with demod/remod capability alone. For example, we could build a very effective satellite that broadcasts data to millions of receivers where the uplinks come from a variety of locations and sources. The OBP provides the integration of data and proper formatting for distribution. The only variation in downlink received power will be that caused by fading along the path between the satellite and the receiving Earth station. Uplink RF noise will introduce errors in the satellite demodulator, which will be transferred directly to the downlink. For exam- –7 ple, if the uplink produces an error rate of 10 and the downlink produces an error –6 –6 rate of 10 , then the combined error rate is 1.1 × 10 . This condition might corre- spond to the uplinkC/N being only 1 dB greater than the downlink. Figure 3.10 pro- vides an example of how this compares to the bent-pipe repeater, offering up to a 3-dB improvement. The increase in error rate of 10% is almost immeasurable in the recovered data. In comparison, without demod/remod, a 1-dB difference would reduce the totalC/N by 2 dB. The error rate for the received data would now be two –4 orders of magnitude less than the downlink by itself, or 10 . Once we have recovered the original data in the satellite, it is likely that we would want to do some additional digital processing and switching. Figure 3.8 shows a baseband switch inside the repeater, similar in function to a digital Repeater Remod Demod Uplink Uplink Downlink BER up C/N up BER = BER + BER up down C/N down Figure 3.9 Features of demod/remod repeater; bit errors are transferred from uplink to downlink, not noise.

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