SBIR-STTR Award

Gaussian Minimum Shift Keying (GMSK) is a CCSDS standard scheme for space missions requiring very bandwidth efficient communications using transceivers with strict size, weight, and power restrictions. GMSK provides a constant envelope signal that passes unaltered through system nonlinearities, allowing efficient power amplifier operation without extensive filtering and predistortion. Unfortunately, current off-the-shelf GMSK and related continuous phase modulation (CPM) receivers (e.g., multi-h CPM) operate far from predicted theoretical performance and suffer from frequent link dropouts. CPM performance issues are mostly attributable to a failure to adequately model and mitigate against transmitter, channel, and receiver impairments, including fixed-point processing effects, and to generate reliable soft-decision information for decoding. To maximize performance given CCSDS turbo or LDPC coding and eliminate communications dropouts, a soft-decision generating GMSK demodulator with carefully designed impairment estimation and compensation is proposed.

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.