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Demonstration of Multi-Gigabit Per Second Data Rates Through Ka-Band Frequencies

satellite

Future NASA missions will require an integrated space communication network capable of delivering orders-of-magnitude higher data rates. The driving requirements for the network are quite broad, covering space-based communication and navigation for manned and robotic missions from near-Earth to deep space, all with emphasis on higher data rates in the Gigabit per second (Gbps) range. The justification for Gbps data rates is closely linked to the capabilities of remote spacecraft sensors, which are quickly outstripping the ability of current data links to rapidly move the sensor information back to Earth.

NASA, along with several other U.S. government agencies, recently conducted a decadal survey to generate consensus recommendations from earth science communities regarding a system approach to space-based observations. The survey focused on climate change, water resources, ecosystem health, human health, solid-Earth natural hazards, and weather. At the conclusion of the survey, the committee recommended that the U.S. continue investing in Earth-observing systems and also provide leadership through the implementation of 14 new Earth science missions over the 2010-2020 decade. The data volume generated during these missions is on the order of several hundred Gigabits per orbit (Gb/orbit); and, with several orbits per day, the total volume is on the order of several Terabits per day (Tb/day). The current state-of-practice data throughputs for NASA’s deep space and Earth-observing spacecrafts typically range from a few Mbps to about 150 Mbps (or 300 Mbps dual polarization) at X-band frequencies. Hence, a downlink with Gbps speed and operating at Ka-band frequencies would be advantageous.

The high data rate Ka-band demonstration was targeted toward enhancing the radio frequency (RF) communications capability of NASA’s future spacecraft-to-Earth data links for the applications described above. Shifting the downlink frequency from the current X-band to the future Ka-band has several advantages, including greater available bandwidth, which enables increased data throughput. This is extremely important; a faster data rate reduces ground resources, mission operation support, and cost. The two critical components are the Gbps software-defined modem (SDM) with bandwidth efficient modulation/pre-compensation for distortion mitigation and a high power RF amplifier. In this demonstration, L-3 Communications Systems-West developed the SDM with NASA Glenn Research Center developing the space traveling-wave tube amplifier (TWTA). This was a collaborative effort among NASA Glenn, NASA Goddard Space Flight Center (GSFC), and L-3 Communications. The software-defined hardware modem demonstrated an unprecedented 4.5-Gbps data transmission through the space Ka-band high power amplifier in the laboratory. In addition, the hardware modem was used to demonstrate in the field 1.5 Gbps (8-PSK) and 2.0 Gbps (16-APSK) through NASA’s TDRSS F10 spacecraft with low BER of 10-9. This demonstration involved transmitting data from NASA GSFC to the TDRSS GEO spacecraft and receiving the data at NASA’s White Sands Complex (WSC). The advanced breadboard modem demonstrated 5.7 Gbps (QPSK) and 20 Gbps (128-QAM) data transmission through the Ka-band high power amplifier in the laboratory with low BER of 10-9. The results of the experiments, when included in an RF link budget analysis, show that the throughput of the low Earth orbit (LEO), lunar relay satellites (LRS), spacecrafts stationed at the second Lagrangian point (L2), and spacecrafts in deep space (DS) can be enhanced by an order of magnitude (10X) or higher over the current state-of-practice.

The key improvements demonstrated over the current state-of-practice are: first, the capability to compensate for the system nonlinearity, including the residual nonlinearity of the high power amplifier using software techniques in the modem. This involves acquiring knowledge of the overall system nonlinearity and then creating a predistorted version of the desired waveforms, which after transmission through the link emerges from the receiver in an ideal state with negligibly small bit error rate (10-9). This feature enables the high power amplifier to operate with the peak signals at or near saturation for all modulations and data rates for maximum power efficiency, and also eliminates need for an external linearizer. By operating the amplifier at peak efficiency, the waste heat that is generated is small, which helps to improve thermal reliability and also enables easier spacecraft integration.

Second is the capability to generate and implement waveforms, which are bandwidth efficient using higher order modulation techniques. Bandwidth efficiency is measured by the number of bits per second that can be packed in one hertz bandwidth (bits/sec/Hz).

The advanced breadboard modem is capable of generating a suite of waveforms, including QPSK, 8-PSK, 16-APSK, 32-APSK, 64-APSK, and 128-QAM. Measured bandwidth efficiency for these waveforms ranges from 1.74 bits/sec/Hz for QPSK to 6.1 bits/sec/Hz for 128-QAM, respectively, and the corresponding maximum data rates range from 5.7 Gbps QPSK to 20 Gbps 128-QAM. The increased bandwidth efficiencies are needed for ubiquitous less expensive satellite broadband Internet, HDTV, and voice services everywhere, even to remote or rural locations. This will be a critical technology to meet goals of the National Broadband Plan.

Third, the modem operates with the widely applicable digital video broadcast satellite second generation standard (DVB-S2). The performance of DVB-S2 is close to the theoretical ultimate limit, defined as the Shannon limit, the theoretical maximum information transfer rate in a channel for a given noise level. This standard is widely adopted by the commercial satcom industry. The advanced breadboard modem is frequency-agnostic and can be readily adapted for high data rate communications at the emerging higher frequencies at Q-band and V-band. At these frequency bands, up to 4 GHz of spectrum is available, and therefore they are under consideration by the commercial satellite industry for the needed expansion of satellite broadband services. Hence, the multi-Gbps modem/transmitter technology has applications in commercial fixed, mobile, broadcast, broadband and imaging satellite services.

By comparing and contrasting the above improved RF high data rate transmission technology with alternative communications technologies, one can understand the importance and need of RF technology. An example of an alternative is optical technology based on laser communications. However, for space communication, using lasers to become a reality several challenges have to be addressed, e.g., overcoming the attenuation due to the Earth’s atmosphere, beam-pointing accuracy, and efficiency and reliability of the laser system.

Enhancing the rate of data transfer from Earth-observing satellites benefits humanity by the quick dissemination of meteorological data and disaster warning information. Transferring the high data rate transmission technology to the commercial satellite communications industry will enable greater access to satellite broadband services, with accompanying benefits in the areas of education, health, employment, entertainment, and use of government services.

This product offers a cost effective path for migrating from the current X-band to the future Ka-band. In addition, the design of the high power amplifiers available at Ka-band can be scaled to the Q-band and the V-band frequencies. Furthermore, the product allows for pre-compensation of the end-to-end system nonlinearity, which allows correction for the high power amplifier imperfections using software techniques. This feature enhances the yield and lowers the manufacturing cost of the high power amplifiers.

For more information, contact Rainee Simons at 216.433.3462 or Rainee.n.simons@nasa.gov

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