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AIAA_1111 1 02S00729.154 American Institute of Aeronautics and Astronautics
NEXT GENERATION BROADBAND SATELLITE COMMUNICATION SYSTEMS Satchandi Verma, SMIEEE, Senior Staff, Eric Wiswell, MIEEE, Technical Fellow
TRW Inc. One Space Park
Redondo Beach, CA 90278 310-812-1742
ABSTRACT 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.
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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-to-back in tension across the rims of a deployable graphite-epoxy 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 Ka-Band 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 side-lobes, 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 geo- political 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.
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Figure 2 – Satellite Coverage Area Frequency Reuse
The system capacity is increased by reusing the available frequency spectrum through optimum allocation of frequencies and polarizations in the communication links. Multi Beam coverage and Multi color frequency schemes further expands the systems capacity. The frequency reuse in the system for the Asia/Pacific regions is shown in Figure 3.
Figure 3 (a) – Singe Wide Area Coverage
The region coverage is compared for both a single wide area beam and the set of multi spot beams. For example for a system using dual polarization links and 50 spot beams the Effective System Bandwith is substantially increased. This frequency reuse produces 310 equivalent transponders (36
MHz) which is ten times of the transponders available in the satellite using a single wide area beam:
ESB = (Number of Cells) x (Allocated Bandwidth)/ (Frequency Reuse Factor)
= (50x2)x (500 MHz)/4 =12.5 GHz The Equivalent Number of 36 MHz Transponders
(ENT): ENT = 12.5 GHz/40 MHz = 310
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Figure 3 (b) – Multi Spot Beam Coverage Multi Color Frequency Plans
The multi color schemes assign same frequency to different sections of the coverage area having locations with spatial diversity. The application of these techniques further enhances the system capacity. For example the six regional beams in Europe (Figure 2) could use three-color schemes in association with twice the frequency reuse to obtain the larger system capacity (Table 1). Table 1 – European Regional beam Color and Frequency reuse scheme
Region Beams Spatial Frequency UK Green X Italy Green X Spain Blue X Germany Blue X France Red X Scandinavia Red X
By incorporating this scheme to a typical satellite (with no frequency reuse and color scheme) the system capacity could be increased over 58% (56 to 96) using the same assigned frequency slots without changing any ground equipment. The actual system capacity improvement depends on the capability of the spacecraft bus to implement the required hardware and operating power changes. The system provides higher capacity in six regions speaking six different languages.
AIAA_1111 4 02S00729.154 American Institute of Aeronautics and Astronautics
SATELLITE COMMUNICATION PAYLOADS Most of the satellite communication payloads operate in
C and Ku frequency bands. Recently few Ka-Band transponders are added in some satellites to support the development of future Ka-Band systems. The payloads use 36 to 72 MHz bandwidth with both linear and circular polarizations for providing “Bent Pipe” transponders.
Traveling Wave Tube Amplifiers (TWTA) are extensively utilized in transponders for obtaining the downlink EIRPs in the range of 30-40 dBW (C-Band) and 40-50 dBW (Ku-Band). The next generation satellites employ communications payloads with advanced antenna, on board switching and digital processing. The Onboard Processors enables the use of Solid State Power Amplifiers (SSPA’s) to enhance the system performance and capacity. On board switching provides the needed network flexibility both in analog and digital transponders. Payload System Configuration
Generally the communication payloads system consists of antenna, onboard processor and RF systems as described in Figure 4. The antenna subsystem includes the antenna (transmit, receive) and the associated feeds, control electronics and the distribution networks (OMT, BFN). Depending on the reflector size and operating frequencies these TRW advanced shaped antenna could provide up to 15dB larger EIRP per beam than the current payloads of equivalent power. RF Subsystem
The RF subsystem includes the required RF components for receiving and transmitting the signals in the payloads. The receive RF subsystem uses low noise amplifiers and down converts for changing the power levels and frequencies of the signals received from antenna. The signals are formatted to the required input interface of the processor. The transmit RF subsystem up converts and amplifiers the power of signals to produce the desired EIRP at the payload antenna output. Using advanced technologies
in Gallium Arsenide (GaAs) high electron mobility transistor (HEMT) devices TRW has produce compact low noise amplifiers (LNA), downconverters and upconverters for next generation satellites. The use of advances extremely small MMIC (millimeter/microwave integrated circuit) packaging provides small size LNA/Downconverter modules, which are easily mounted directly on the antenna, feed horns. This ability to collocate the first stage of processing with the antenna feed eliminates the waveguides, (prevents the loss of received signal power in the waveguides) and reduces the payload mass and complexity. For transmitting the downlink signals the upconverters are mounted directly on the TWTAs reducing the signal losses and payload complexity. On Board Processor Subsystem
The next generation satellites extensively need to use On Board Processors (OBP) to design cost effective system solution for the customer needs. The OBP in satellites eliminates the inherent disadvantages of the “Bent Pipe” transponders. The payloads either employ partial or full on board processing depending upon the system design requirements and cost. Full on board processing enables an ATM like switching process in payloads. The partial processing on board is used in “Digital Transponders”. Generally the processor in satellite provides the following on board features and capabilities for system enhancements [5]: • Full network connectivity (Mesh, Star) • On board switching of signals, beams & coverage areas • Simplified payloads with efficient use of TWTA /SSPA • Efficient bandwidth and power level control
(Automatic, Selectable) • On orbit management of network traffic, capacity and
QOS • Flexible system integration, operation and tests Full On Board Processor
In full OBP satellite system the payloads transforms the incoming signals into fixed length packets using range of
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Figure 4 – Communication Payload System Configuration
AIAA_1111 5 02S00729.154 American Institute of Aeronautics and Astronautics
protocols. The payload interprets and processes the data to provide efficient downlink bandwidth, connectivity and QOS. The block diagram of the on board processor using ATM (asynchronous transfer mode) cells. is shown in Figure 5. The router consists of an ATM switch and an on-board computer for performing the on board control and processing functions. The packets (Cells) are dispatched through the switch to the desired destinations. The signal Figure 5 – Gen*Star On Board Processor Block Diagram [4]
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processing requires efficient channel routing, power control and dynamic bandwidth reallocation. The ATM cell switching provides flexibility and compatibility with existing protocols and interfaces. The scaleable feature of switch meets the needs of both the users requiring very little bandwidth as well as those with high bandwidth demands (large business users over a single network infrastructure).
For Gen*star payload TRW has built the broadband
packet switch processor using a distributed system architecture [6]. The processor design provides both the point-to-point and multicast services up to 10 Gbps network data rates with 128 ports.
The data information is routed through a self-routing, non-blocking cross bar switch to provide the throughput capacity up to 6.5 Gbps. Digital Transponder
Digital transponders in communication satellite payloads use circuit switching and data processing for designing cost effective network services between different satellite coverage beams (Area, Spot). The various system advantages provided by digital transponder are [7]: • Transponder bandwidth flexibility and control.
Transponder provides circuit switching between different spot beams in fractions of transponder bandwidth (0.5 MHz increments). Capacity allocation at the transponder level provides the flexibility of selling fractional bandwidth to customers on demand.
• Reconfigurable Network Connectivity with Backward Compatibility.
The transponders can easily be reconfigured at the sub-transponder level to respond the system connectivity requirements for changing markets and traffic patterns. The backward compatibility to analog transponders in legacy systems is obtained by the programmable bandwidth feature of (27, 36, 54, 72 MHz) in the design.
• On Board Channel Aggregation. This feature provides the on board capability of combining the Signals from multiple uplinks located in different coverage regions into a single downlink channel. The system efficiency is improved by simpler frequency translation and time slot (TDM) switching.
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Figure 6 – Digital Transponder Payload Configuration
AIAA_1111 6 02S00729.154 American Institute of Aeronautics and Astronautics
• System Capacity and performance Enhancements. The system capacity is increased by using more efficient modulation (8-PSK, 16 -QAM) and reducing the channel guard bands (filtering, channel to channel amplitude power leveling). The system performance is improved by using the digital pre-distortion to HPA for increasing the TWTA amplitude and phase linearity.
The block diagram of the Digital Transponder is described in Figure 6. The antenna subsystem includes the offset feed spot beam antenna (transmit, receive) including the associated control electronics and distribution networks (OMT, BFN). The RF subsystem includes the required RF components for receiving and transmitting the signals in the payloads for each beam. Using low noise amplifiers and down converts the received signal power levels and frequencies are changed. The transmit RF subsystem up converts and amplifiers the power of signals to produce the desired EIRP. Digital Transponder Processor
The processor in Digital transponders performs five major functions as shown in the concept diagram [7] of Figure 6. The incoming analog signal is converted into 125 MHz digital signals at 640 MHz intermediate frequencies. The digital channelizer divides the complex baseband input signal into 250 overlapping 0.5 MHz sub channels. The user signal power level is normalized to a constant level by the Automatic Level control function in the digitizer. Using subchannel mapping the digital switch routes subchannel to the appropriate ports for broadcasting or multicasting. The subchannels are reconstructed and recombined in the recombiner to form one signal with a 125 MHz bandwidth. Finally the digital samples are converted into analog signal centered on 512 MHz
Digitizer* Analog to digital conversion* 125 MHz signal @ 512 Msps
Digital Channelizer* Divides inputsignal intosub-channels* ALC function levels the power
Recombiner* Subchannels reconstructed* Recombines user channelsinto a one signal
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Figure 7 Digital Processor Concept Diagram
The Uplinked signals are downconverted at the antenna and input to the digitizer in 64 channels using a transmission bandwidth of 125 MHz. The digitizer samples each channel at the rate of 512 Msps (mega samples per second) and sends signals to the Channelizer. The Channelizer divides the baseband input signal, containing the user channels, into 250 sub-channels. An automatic level control function
(ALC) normalizes the average power of the signal received from each user to a constant level. Input power levels being time-varying: rain attenuation, atmospheric scintillation, etc., the control is needed to increase TWTA efficiency on the downlink. The recombiner performs the inverse function of the channelizer. It combines the 250 sub-channels into 125 MHz bandwidth channels. Reconstruction filter design constraints ensure minimal signal distortion during this process. The switch, commanded by a control processor, routes sub-channel data from the channelizer to the appropriate ports in the recombiner. Figure 8 – 36 MHz & 72 MHz Channel Sharing The switch is reconfigurable approximately every microsecond and supports both point-to-point, multicast and broadcast operations. Switch reconfiguration is performed by the ground command. High-speed, indium phosphide, integrated circuit devices convert the digital output of the recombiner to 125 MHz analog beams
The channel bandwidth of 125 MHz can further be used for carrying multiple transponder channels (36 MHz, 72 MHz). Figure 8 shows the frequency response of locating 36 and 72 MHz channels in 125 MHz using digital filters. The use of digital filters provides improved performance of insertion loss and frequency response over analog SAW filters. The performance a 36 MHz bandpass SAW filter (Best effort analog filter) is compared with an achievable digital filter (using 4096) taps in Figure 9. The digital filter provides substantial improvement in performance as compared to analog SAW filters.
CONCLUSIONS
For next generation satellite systems to provide cost effective and profitable network service it is necessary to apply new technologies in satellite antenna and communication payload designs. The satellite network
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AIAA_1111 7 02S00729.154 American Institute of Aeronautics and Astronautics
implementation requires upgrading the legacy systems and adding new payloads with advanced features for system enhancements. This paper discusses the different technologies and designs employed in providing the improved network flexibility, capacity and performance. The key drivers for the designs and implementation of next generation satellite systems employ: (a) Shaped and spot beams large deployable mesh reflectors antenna (b) High gain Solid Reflector Multiple Beam satellite antenna (c) Satellite Coverage Flexibility for Local, Regional and Global services (d) Satellite capacity enhancements through frequency reuse (e) Onboard Analog and Digital Processing Payloads.
Figure 9 – Analog & Digital Filter Comparison
This paper reviews architecture and technologies developed by TRW for the Broadband satellite systems. The critical features, concepts, performance and benefits of advanced satellite antenna, payload processor, and digital transponder are summarized. It is shown that the design and fabrication of multibeam systems designs using large, deployable mesh antenna reflectors could provide higher EIRP with increased system capacity and operation flexibility.
The digital payload processors and switches on board satellites also effectively increase the system network capacity, billable bits and Quality of Service (QoS). The network flexibility is further achieved through on board programmable configurable processor for obtaining the various point-to-point, star and full mesh connectivity in the network. The multiplexing of multiple signals destined for a downlink beam into a single high-rate data stream permits use of high-efficiency (lower DC power consumption) saturated TWTA operation.
Finally it is concluded that similar benefits of the large spot beam antenna aperture coverage and onboard processing can also be achieved in upgrading the legacy satellite systems operating at C and Ku-Band. The same
onboard processors developed for Ka-Band satellites in along with multibeam antenna designs can easily provide increased capacity and cost savings at these lower frequency systems. The use of spot beams provides much higher antenna gain with larger effective isotropic radiated power (EIRP) and G/T than the conventional C and Ku-Band systems supporting high performance communication links.
REFERENCES
[1] Roger J. Rush “Success factors for Broadband Satellite Systems”, Seventh Ka-Band Utilization Conference, Taromina, Italy, October, 2001.
[2] H. J. Morgan, Eric R. Wiswell, Joseph Freitag, “The Gen*Star Program,” 18th AIAA International Communication Satellite Systems Conference & Exhibit,
April 10-14, 2000.
[3] E. Wiswell, Z. Stroll, A. Baluch, J. Freitag, H.J. Morgan, ”Gen*Star Results Applicable to Ka-Band”, Proc. of the Fifth Ka-Band Utilization Conference, Taromina, Italy, October 18-20, 1999.
[4] Massih Hamidi, Eric Wiswell, Alan Cherrette, Oliver Saunders, Hau Ho “TRW’s Broadband Communication Payloads at C and Ku Frequency Bands”, 2002 IEEE Aerospace Conference, Bigsky, Montana, USA March 9-16, 2002.
[5] Mark Bever, Scott Willoghby, Eric Wiswell, Kenton Ho, and Stuart Linsky, “Broadband Payloads for The Emerging Ka_Band Market,” Seventh Ka-Band Utilization Conference, Santa Margherita, Italy, September 26-28, 2001
[6] S. Mishima, L. Moy-Yee, G. Yee-Madera, E. Yousefi , “Broadband Packet Switch Processor,” Seventh Ka-Band Utilization Conference, Santa Margherita, Italy, September 26-28, 2001.
[7] Mark Bever, Eric Wiswell, Kenton Ho, and Stuart Linsky, “TRW Broadband Payloads for Emerging Markets” PTC Conference Hawaii, USA, February, 2002.
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