Minimizing Implementation Loss in Soft-Decision GMSK Demodulators

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.

GMSK is employed in fielded commercial cellular systems such as GSM and military satellite communications systems such as A-EHF SATCOM. Multi-h CPM is used in UHF SATCOM and Tier 2 Advanced Range Telemetry (ARTM) systems. Robust and high performance CPM 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 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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