Minimizing Implementation Loss in Soft-Decision GMSK Demodulators

With more missions at high data rates demanding use of limited spectral resources, NASA's SCaN office recently coordinated a study to identify a space communications architecture to support future missions. The study recommends precoded GMSK and AR4JA LDPC codes as preferred options in most Space Network and Ground Network forward and return links and Deep Space Network return links. This modulation and coding pair provides excellent bandwidth-efficiency and greatly reduced transmitter SWaP. Unfortunately, there are no high-data-rate AR4JA LDPC devices currently available and existing GMSK receivers operate far from the performance predicted by theory, especially in the presence of severe channel and equipment impairments. Phase I provided a design of a soft-decision generating GMSK demodulator integrated with an AR4JA LDPC decoder and with estimation and compensation of a comprehensive set of severe impairments. Fixed-point simulations show performance within a small fraction of a dB of the performance with far less bandwidth-efficient modulations such as BPSK. The results of this effort show the technical and commercial viability of an integrated GMSK/AR4JA LDPC design. The proposed Phase 2 effort involves the development and delivery of a prototype transmitter and receiver to demonstrate the superior capabilities offered by this innovation and enable subsequent commercialization. A simple and highly flexible GUI system for prototype configuration and control and modular API design will allow Phase II refinement of the design and facilitate integration in future commercial products.

GMSK is employed in fielded commercial cellular systems such as GSM and military satellite communications systems such as A-EHF SATCOM. Closely related SOQPSK and Multi-h CPM is used in UHF SATCOM and Advanced Range Telemetry (ARTM) systems. LDPC codes are used in or proposed for several commercial and DoD systems. The robust and high performance, high data rate, methods developed in this effort would offer greater capabilities and size, weight, and power reduction for each of these communications systems.

Future space missions at near Earth, lunar, and deep space distances that require spectral efficient communications will benefit greatly from the robustness and size, weight, and power reduction offered by the proposed GMSK and LDPC technology. Future small satellites and other miniature platforms would be able to use much more efficient power amplifiers with the given techniques, and high data rate applications such as high definition video could be more reliably supported in limited assigned frequency bands.

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