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Space Modem GMSK Modulator

NASA’s Jet Propulsion Laboratory, Pasadena, California

A high-rate GMSK (Gaussian Minimum Shift Keying) modulator was developed for space operation. Currently, multi-user modems are under development, and the GMSK modulator provides a way of packing more users within a space environment, especially for Mars exploration.

The modulator architecture is based on a ROM (read-only memory) lookup technique and consequently occupies very little space on flight-qualified FPGAs (field-programmable gate arrays). The GMSK modulator is implemented on a Virtex-2 3000 FPGA used for MAVEN (Mars Atmosphere and Volatile EvolutioN) and TGO (Trace Gas Orbiter). The modulator utilizes approximately 4% of the Virtex-2 resources (slices).

The GMSK modulator does not require any arithmetic devices (multipliers/adders), only sin/cos lookup tables and counters. The sin/cos tables are generated based on a depth 5 memory with four phase states (a 0.5 mod index is programmed, but can be reduced by increasing the tables to depth 6 memory), and up to 4, 8, or 16 samples per BPSK (Binary Phase Shift Keying) symbol. The result is an 8-kbyte ROM with 16 bits of precision. As configured, the modulator can generate up to 20 Mbps GMSK. This work enables more bandwidth-efficient modulation waveforms that will benefit current and future NASA missions.

This work was done by Mazen M. Shihabi, Wanqing Huang, Biren N. Shah, and Edgar H. Satorius of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it.. NPO-49320