Reconfigurable Communication Equipment on SmartSAT-1
Smart Satellite Technology Group
Wireless Communications Department
National Institute of Information and Communications Technology
This paper will discuss development of Reconfigurable Communication Equipment (RCE) on Smart SAT-1. The RCE is an implementation of the onboard Software Defined Radio (SDR), and this is consist of five components, that is, an onboard SDR, a high power amplifier, an IF converter, a transmit antenna, and a receive antenna. The smartSAT-1 to plan to launch FY2008 is small experiment satellite. The mass of the satellite is less than 100 kg. Some mission equipment including this RCE will be installed on the satellite.
The demand for geostationary satellite communications continues to rise, and more efficient satellite communication methods have been proposed as a means to meet the increased demand. For example, a regenerative relay and baseband switching system is now being developed -. The regenerative relay will demodulate the received signal into baseband data, modulate the data, and then transmit it. The ground segmentfs antenna can be downsized and the link made more efficient through this method. To establish broadband satellite links, Ka (20-30 GHz) and higher bands must be used. However, a large rain margin (to allow for signal attenuation caused by rain) is required to use these bands. Because the uplink and downlink can be designed independently, the regenerative relay has advantages relative to a bent-pipe relay in this case. Such systems have already been implemented in experimental satellites and their effectiveness demonstrated . Looking forward to the 2030s, we expect to see a movement towards the development of terabit-class satellites . Such satellites will require high-speed, large-scale onboard processors.
There has been a trend towards designing commercial satellites with a 10- to 20-year lifetime in recent years to reduce launch risks and costs. The development of a large-capacity satellite bus is also moving ahead . This technology is necessary to build a high-data-rate communications system. Emphasis has also been placed on minimizing the size, weight, and power consumption of onboard electrical power amplifiers because of the launch weight limitations.
Implementing the latest techniques onboard a satellite is difficult, though, because of the enormous budgets and long lead times required for satellite development. The technology of onboard devices for satellites is thus several years behind that of general terrestrial devices. Even if the latest communications technology can be implemented at the time of a launch, the technology cannot follow subsequent paradigm shifts in terrestrial communication technology because payloads generally cannot be repaired and replaced in orbit. Although some efficient communication technologies are being applied in experimental satellites, most current commercial satellites are equipped with only bent-pipe repeaters capable of simple frequency changes, not with regenerative repeaters or baseband switches. There are valid reasons why regenerative relay systems are not generally used. One is that the onboard modulation and demodulation technology is not as mature as the bent-pipe relay technology. Also, the modem for the regenerative relay is heavy and consumes a lot of power. Thus, loading more output amplifiers, which are based on a well developed technology, is considered a better way to accommodate more users compared to loading the modem. In addition, since the onboard equipment cannot be replaced, the regenerative relay system lacks flexibility.
A flexible communication system can be implemented by using the SDR system as the onboard modem to overcome the issues. SDR systems that can respond to various communication systems have been studied extensively . The required techniques must be applied not only for the terminals , but also for the base station. Implementing an onboard SDR system will enable implementation of variable rates, variable modulation types, and variable error-correcting systems. This will make it possible to establish efficient communication links that are robust with respect to rain attenuation.
The RCE has two objectives. One is to demonstrate flexibility of onboard SDR system, and the other is to implement gracefully degradable equipment. It is well known that semiconductor devices are weak against radiation effect, and all the function of the device may be lost on an orbit. A device failure may lead to whole function loss as for non-reconfigurable equipment. Reconfigurable equipment, however, may avoid the crisis with reconfiguration. The required complexities (i.e. logic cells) for onboard modulator and demodulator are not identical. In general, demodulation function needs more computational power than modulation function, and decoding error correcting code also needs more than encoding. With self resource evaluation, the RCE can recognize the function, i.e. modulation, demodulation, decoding, and encoding, which can be realize simultaneously by own resource. The RCE can also respond to two service class. The onboard SDR consists of one non-volatile FPGA controller and two S-RAM type FPGAs banks and each bank consists of three identical S-RAM type FPGAs. Triple module redundancy system can be configured with three FPGAs in a bank for high reliable service including the configuration uploading. For high throughput service, the three FPGAs are connected serially.
A Bread Board Model (BBM) of the onboard SDR has already developed and Engineering Flight Model (EFM) design will be started from 3Q 2004. Virtex II PRO FPGA (XC2VP series) was used for BBM and will be also used for EFM. Since this FPGA is not sold as a radiation hardened, two heavy ion testing were conducted. Same testing of Virtex II FPGA (XC2V series) had been conducted in , and some comparative results will be shown.
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