Jean-Didier Gayrard

Applications of Optical Techniques in Future Communication Payloads

Optics and photonics bring unusual but attractive and performing solutions for implementing functions in communication payload. For the time being, these techniques have been mostly put aside for various reasons, but the technical world changing very fast, it is advisable to check if they could be competitive alternatives. Moreover, new functions will appear in the next generation of broadband satellites in response to the increasing needs for bandwidth, connectivity and flexibility. Optical techniques should be both viable solutions to enhance existing functions and the only solutions to create the new functions of broadband satellites to come. Alcatel Space research department has identified functions and equipment where optical techniques could be either a significant and necessary breakthrough or simply a technical and cost improvement. For mid term, Alcatel Space has selected and studied three functions where optics is particularly attractive for performances, mass saving and cost. First application is backplane interconnections. This technology allows gigabit connections required inside onboard large digital processors. A second application is the transport and distribution of microwaves onto optical fibers. Fibers are an interesting substitution for coaxial harness in payloads. Associated with the emerging Optical Micro-Electro-Mechanical Systems (OMEMS), this technique will extend the routing capability of payloads. A third application is intrasatellite data link. Optic fibers or wireless infrared links are promising choices to build onboard data handling system. In the long term, optical techniques could change the very aspect of repeaters and antennas of communication payloads. However, the introduction of optics for space applications is neither easy nor rapid. This paper summarizes Alcatel Space research effort and the main results in this domain. I. Optics in broadband payloads In the next five to ten years, satellite operators are anticipating the creation of a new market for satellite: broadband communications. Typically, broadband systems should offer high data rate connections to very large numbers of low price terminals. The present generation of satellite is not really suited for broadband missions involving millions of end users. Ku and C bands are too congested to offer sufficient room for growth. Receive performances of the satellite are not sized for the use of small and power limited terminals. Multiple access schemes are not adapted, as there are, for managing tens of thousand simultaneous links. A new family of satellite shall be designed to cope with these unusual requirements. Lastly, to be economically viable, communication price shall be as low as possible. Only satellites with very large capacity in term of bandwidth or number of circuits would achieve low communication price. Several techniques are investigated to increase significantly the satellite capacity [1]. Broadband payloads will have complex multi-beam antennae, hundreds of channels to receive, to route and to transmit, large onboard processors for regenerating and switching digital signals... Optics and photonics bring unusual but attractive and performing solutions for implementing these functions. Optical techniques should be viable solutions to enhance existing functions in mid term. In the long term, optical techniques will be the only solution to create the new functions of the broadband satellites to come. II. Mid term utilization of optical techniques In mid term, optical techniques could complete or replace existing functions in communication payloads: digital intra-satellite links (Onboard Data Handling System), microwaves on optical fibers, gigabit backplane interconnections in high throughput On-Board Processor (OBP), high speed data packet switching, inter-satellite and inter-orbit links, pyrotechnic harness, signal interfaces of cryogenic electronic units... 21st International Communications Satellite Systems Conference and Exhibit AIAA 2003-2297 Copyright

Future trends in digital On-Board Processors for telecommunication satellites

Since about 10 years, digitalization of signal processing in communication satellites has turned into reality. Digital On-Board Processors (OBP) are handling filtering, channel routing, beam forming, demodulation and decoding… They bring sensible benefits: enhanced performances, flexibility and reconfiguration capability. The digitalization is nevertheless limited to few channels in a payload due to the additional mass and power consumption of the Digital OBP. Digital OBP mainly call for two semiconductor technologies: the technology of analog to digital (ADC) and digital to analog (DAC) conversion and the technology of silicon-based digital integrated circuits (ASIC). The evolution of these two technologies are strongly linked with the Moore’s law. During the last four decades, Moore’s law has became a self-fulfilling prophecy for the semiconductor industry and forecasts the exponential evolution of transistor density (gate length), operation frequency, production cost and even computing power and memory capacity… It seems possible to assess a similar predictive law in a form of an observation and forecast for the performances of Digital OBP in space telecommunications. From the performances of past space digital processors and by considering what semiconductor technology they used, the predictive law enables to estimate the performances of a processor using a more advanced semiconductor technology. When applying the Moore’s law to the digital parts, it becomes possible to forecast the overall performances of digital OBP through time. Considering the full digitalization of communication payloads becomes a realistic goal for the end of the next decade (after 2015).