Next Generation Broadband Satellite Communication Systems

For next generation satellite systems to provide cost effective network service it is essential to use efficient and advanced technologies for adding new satellites or upgrading legacy systems. This paper discusses the various technologies that are being developed and utilized for increasing the network capacity, improving service performance and reducing the cost of satellite systems. An overview of enabling technologies is presented describing the key architecture, capability and performance of the broadband satellite payload processor, digital transponder and satellite antenna developed by TRW. The various benefits of using these advanced features in satellites over the conventional “Bent Pipe” satellite systems are summarized. INTRODUCTION Over the last two decades the communication satellites have extensively used “Bent Pipe” transponders at C and Ku-Band frequencies to provide the audio, data, video and VSAT services using narrow and wideband transmission channels. These satellite systems are characterized by broad regional coverage, rigid network configurations, relatively low satellite antenna gain, EIRP and G/T with modest channel data capacity rates. The network throughput capacity is mostly limited by the availability of small numbers of transponders in the satellite. In recent years cost effective solutions for Multimedia Broadband Global Communication systems are being developed using next generation of communication satellite designs. These systems require high quality of service, affordable prices and good matching of customer demand with the satellite system capacity for successful and profitable business operation. The growth of satellite data services in the next decade is estimated to be substantial during this decade [1]. To meet the needs of these cost effective next generation systems requirements the satellite may employ following advanced techniques in antenna designs, onboard payload processing and frequency reuse: • Deployable large mesh reflector (Shaped and spot beams) satellite antenna • High gain Solid Reflector Multiple Beam Antenna (MBA) for Satellites • Satellite Coverage Flexibility (Local, Regional, Global) • Larger satellite capacity (higher link frequencies with frequency reuse) • Onboard Processing Payloads (Analog and Digital) To meet the requirements of future systems TRW has applied these design enhancements in developing the Gen*Star [2, 3] payloads for next generation satellites. The satellite payload was designed for operation at Ka band and included the companion network and terminal infrastructure. The first payload using this antenna design was completed in December 2001 for the Astrolink satellite. Presently this payload design is expanded to provide the efficient cost effective system solution for the replacement and or enhancement of Ku and C-Band satellite networks. NEXT GENERATION SATELLITE ANTENNA Most of the present satellites use relatively small size solid antenna reflectors to provide the desired coverage. The antenna size (2-3m) is limited by the launch vehicle fairings and packaging constraints. Regional coverage is provided by using the antenna shaped beam (wide) while the spot area coverage is obtained by the high gain multiple narrow spot beams. The radiated power coverage efficiency of these shaped beam antenna is considerable reduced from the loss of energy (power emission) in the undesired coverage regions (desert, ocean) with no source of revenue. The small gain reduction (slow antenna side-lobe roll offs) at the coverage area edges also further degrades the system performance. Deployable Mesh Satellite Antenna The next generation satellite antenna design will use the lightweight large deployable reflector antenna to provide high performance shaped and multi spot beam coverage’s. For improving the system performance TRW Astro Aerospace has developed the technology for manufacturing lightweight, shaped beam, deployable mesh reflectors antenna systems. These antenna (Figures 1) designs use deployable mesh reflectors ranging from 6m to 30m in diameters to provide 60-100% improvement in the shaped directivity over the solid reflector antennas [4]. The antenna is designed to provide regional or global service coverage at both C and Ku Band frequencies. The antenna performance is improved by flattening the coverage area radiation pattern and creating the rapid gain reduction at the edge of coverage. The sharper antenna side-lobe roll offs and higher cross polarization which further enhances the system capacity and reduction in satellite DC power consumption. 20th AIAA International Communication Satellite Systems Conference and Exhibit 12-15 May 2002, Montreal, Quebec, Canada AIAA 2002-1999 Copyright © 2002 by the American Institute of Aeronautics and Astronautics, I c. All rights reserved. AIAA_1111 2 02S00729.154 American Institute of Aeronautics and Astronautics Recently launched Thuraya (L-Band) satellite uses a 12.25m mesh parabolic reflector while a 9m (L-Band) reflector is in production for the INMARSAT4 satellite. Figure 1 – Mesh Reflector Deployable Antenna The mesh shaped antenna reflector consists of a pair of doubly curved geodesic trusses, which are placed back-toback in tension across the rims of a deployable graphiteepoxy ring truss. This light and inherently stiff drum-like structure provides high efficiency, thermal dimensional stability, and stiffness-to-weight ratios. These L-Band antenna designs and manufacturing processes are being further optimized for the production of C and Ku Band satellite systems. High Gain Multiple Beam Antenna TRW has developed Multi Beam Antenna (MBA) to provide the high capacity flexible coverage beams for KaBand satellite systems. These MBA antenna designs provide high-gain, multiple-hopping spot beams for national, regional and global service coverage’s. The desired coverage area is tiled with narrow beams using frequency reuse and multi color operation schemes for enhancing the system capacity. These antennas also provide low sidelobes, higher cross-polarization isolation and high degree of network coverage flexibility to meet the dynamic market demands from the customers [3]. Satellite systems using Multi Beam Antenna have additional advantages of larger channel capacity and on orbit coverage adaptability for the changing usage patterns. SATELLITE SYSTEM COVERAGE The, types of network services, customer population and required system performance, governs the design of satellite system coverage. The satellite payloads are developed to provide the services in Local, Regional and Global areas for transmission of specific contents. Local Coverage The local coverage contains transmissions in the selected areas (City or country) to provide the local content delivery services to meet the demand of customers located in the city. The antenna spot beams in the satellite provide high gain directivity in specific coverage areas. The beams are optimized for the maximum system capacity and performance for local service contents. Regional Coverage The regional coverage contains transmissions in specific regions based on the language (eg: Spanish, German) or country (eg: Spain, Germany). This coverage is provided by a single shaped antenna beam or multiple narrow spot beams covering the required regions. The regional coverage is tailored to radiate the power for customer in conformance with language needs and geopolitical concerns (Frequency coordination, content restrictions). The regional beams provide higher directivity, which could be used for increasing the satellite power efficiency or reducing the user system costs on the ground. Global Coverage The satellite global coverage provides the transmissions in the area covered by multiple regions (language, Geo political) or countries. For example the Pan European coverage beam provides the same service contents to all customers located in different Europe countries The wide shaped antenna beam covering the European countries provides the Pan European coverage. A typical example of Local, Regional and Global (Pan European) coverage beams for satellites are shown in Figure 2. Using multi spot beams for global coverages could also develop cost effective systems LARGER SATELLITE SYSTEM CAPACITY The satellite system capacity is enhanced by using larger useable bandwidth at higher frequency bands (Ka,V) and by applying the frequency reuse including optimal multicolor channel transmissions schemes. The number and complexity of transponder implementation in satellite limits the improvement in system channel capacity. The maximum number of transponder in a satellite is controlled by the capabilities (mass, power, ferrying size) of the spacecraft bus used.