NASA’s Deep Space Network (DSN) is looking to modernize aging downlink receivers for telemetry, tracking, and radio science. It is looking to replace multiple types of custom-built, special-purpose receivers with a unified receiver architecture that can support the various downlink data types. As part of this modernization, it is desired to only digitize the data once and then distribute the data using commercial switching network technology to multiple back-end receiver processing hardware and software. The main problem to be solved is how to distribute efficiently and flexibly high-bandwidth intermediate-frequency (100 to 600 MHz) digitized signals across a signal processing center for use in the DSN.

The solution is to implement a system that breaks an intermediate frequency (IF) signal up into multiple fixed subbands; these sub-bands are sent over a data link. The data link could be a commercial off-the-shelf (COTS) networking standard such as UDP (User Datagram Protocol) Ethernet packets. A main requirement of the sub-bands is that they can be reconstituted back into the original IF signal, or a subset of that original bandwidth, with little or no degradation in the signal quality.

This is accomplished using a structure known as a modified discrete Fourier transform (MDFT) filterbank. For this architecture, IF data from multiple antennas is digitized using analog-to-digital converters at a rate of 1280 Msamples/second or greater. The digitized data is broken up into sub-bands using a MDFT filter bank. The MDFT is a critically sampled filterbank that allows for near-perfect reconstruction. The critically sampled property provides for optimal use of limited data bandwidth resources.

The data is then packetized onto UDP Ethernet packets and transferred to back-end processing units (CPU or FPGA based) using a technology known as multicasting. Multicasting allows the back-end processing units to subscribe only to the channels that they desire. This feature is implemented in COTS network switches configured in the proper manner. Using multicasting, network bandwidth is conserved both in the switch and in the back-end processing units that receive the data. For this implementation, each transmitting front-end digitizing unit and each back-end receiver unit can handle 10 gigabits/second of network bandwidth. Allowing each receiver to subscribe only to a subset of sub-bands allows it to save bandwidth and subscribe to inputs from multiple antennas to facilitate functions such as arraying.

Using the MDFT filterbank to break the signal up into sub-bands is also a major part of the innovation. The data could be sent as just a single band, but this sacrifices flexibility. Some backend processing units may not want, need, or be able to handle the fullbandwidth data.

This work was done by Robert Navarro, Wanqing Huang, James A. O’Dea, Arby Argueta, Duo Wang, Joseph T. Trinh, Stephen P. Rogstad, Meera Srinivasan, and Andre P. Jongeling of Caltech; and Jeffrey R. Buchanan and Dean A. Miller of Santa Barbara Applied Research for NASA’s Jet Propulsion Laboratory. This software is available for license through the Jet Propulsion Lab oratory, and you may request a license at: https://download.jpl.nasa.gov/ops/request/request_introduction.cfm . NPO-49686

NASA Tech Briefs Magazine

This article first appeared in the May, 2016 issue of NASA Tech Briefs Magazine.

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