High-throughput Hardware Decoder Implementation for Optical Deep-space Communications

Future space missions will have to meet the demand for very high data rates. Transmission of scientific data and other high-bandwidth communications from deep space back to Earth are among the key challenges coming along with next-generation missions to the Moon and beyond. Optical communication has gained significant interest recently as a promising candidate to offer these high data rates. Compared to today’s deep-space communication systems that operate in the radio frequency (RF) spectrum, the small diffraction losses of an optical link yield significantly higher concentration of received signal energy. This improved signal level enables corresponding space equipment to operate at lower transmit power and, at the same time, to achieve higher data rates. Furthermore, optical transmit modules are much smaller in size than state-of-the-art RF transmit antennas, which entails a potential reduction in both dimension and weight of space equipment. In order to proof the feasibility of such an optical link, the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) have been collaborating on the Lunar Laser Communications Demonstration (LLCD) project [1] with the goal to demonstrate an optical communication link from NASA’s LADEE lunar-orbit spacecraft to Earth. This link has been specified to offer up to 16 multiplexed subchannels, where each individual subchannel provides a data rate of 38.55Mbps, resulting in a maximum aggregate data rate of roughly 620Mbps. In order to achieve high link reliability, the proposed link is based on NASA’s capacity-approaching modulation and coding scheme, which comprises a serial concatenation of an inner accumulate pulse-position modulation (PPM) and an outer convolutional code (SCPPM) [2]. Among several existing approaches to decode the specified SCPPM code, employing the turbo principle has proved to yield best results in terms of error-rate performance [2]. Unfortunately, turbo decoding entails significant computational complexity due to the fact that costly soft-input soft-output (SISO) maximum a-posteriori decoders for the inner and outer code are required, exchanging soft information about the transmitted code bits in an iterative fashion. In contrast to the prominent class of binary parallel concatenated turbo codes well-established in cellular communication systems, the use of higher-order PPM further exacerbates the implementation. In fact, the high implementation costs of turbo decoding in conjunction with the high throughput requirements render the SCPPM turbo-decoder a key challenge in the receiver design for the LLCD project and future space missions.