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.