A paper discusses the optimization of the parameters of a high-rate, deep-space optical communication link that utilizes pulse-position modulation (PPM) and an error-correcting code (ECC).
The parameters in question include the PPM order (number of pulse time slots in one symbol period), the ECC rate, and the uncoded symbol error rate. In simple terms, the optimization problem is to choose the combination of these parameters that maximizes the throughput data rate at a given bit-error-rate (BER), subject to several constraints, including limits on the average and peak power and possibly a limit on the uncoded symbol error rate. This is a complex, multidimensional optimization problem, the solution of which involves computation of channel capacities for various combinations of the parameters. The paper presents extensive theoretical analyses and numerical predictions that elucidate the many facets of the optimization problem. It shows how a nearly optimum solution can be obtained by choosing the optimum PPM order for the desired number of bits per slot and concatenating the PPM mapping with an error-correction code so that the decoded bits satisfy some BER threshold.
This work was done by Bruce Moison and Jon Hamkins of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. NPO-40591
This Brief includes a Technical Support Package (TSP).
Optimizing Parameters for Deep-Space Optical Communication
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Overview
The document titled "Optimizing Parameters for Deep-Space Optical Communication" is a Technical Support Package from NASA's Jet Propulsion Laboratory, focusing on the development and optimization of optical communication systems for deep-space missions, particularly those involving satellites orbiting Mars.
The report emphasizes the importance of link budgets in deep-space optical channels, which are influenced by the choice of modulation formats and error control coding schemes. It discusses the properties of channel capacity that guide the selection of modulation formats, specifically Pulse Position Modulation (PPM), and the coding rates of error control codes. The document highlights the performance limits imposed by various constraints, such as average and peak power limits, as well as uncoded symbol error rates.
A key finding presented in the report is that concatenated convolutional codes, when iteratively decoded, can operate approximately 0.5 dB from capacity across a wide range of signal levels, outperforming Reed-Solomon codes by about 2.5 dB. This suggests that for optimal data throughput, the modulation and coding strategies should be dynamically adjusted throughout a mission based on varying conditions such as range, atmospheric interference, and angles between the sun, Earth, and the probe.
The document also outlines the anticipated data rates for deep-space communication, which can range from 10 to 100 Mbit/second, depending on the aforementioned factors. It stresses the need for careful parameterization of the transmitted signal, which is divided into time slots during which pulses may be sent.
In addition to technical specifications, the report serves as a resource for broader technological and commercial applications, aiming to disseminate aerospace-related developments. It provides references to various studies and reports that further explore the topics discussed, including the characterization of the Mars downlink budget and the performance of different coding schemes.
Overall, this Technical Support Package is a comprehensive guide for optimizing deep-space optical communication, offering insights into the interplay between modulation, coding, and environmental factors that affect communication efficacy in space missions. It serves as a valuable resource for researchers and engineers involved in the development of advanced communication systems for space exploration.
