In communications systems, there must be a method to identify the beginning of a message at the receiving end, to know when the data stream of a specified structure begins. This is conventionally done by inserting attached sync markers (ASMs) between codewords, and looking for these ASMs in the transmitted data. Instead, a means has been developed to detect the positions of codewords directly, using the decoder to partially process a decoder attempt at each candidate synchronization position. A short decoding operation can successfully synchronize codewords in about two seconds in software.
This method is a variation of the brute force approach in which decoding is halted prematurely. At the correct offset, the messages passed in the decoding algorithm begin to converge in a fundamentally different way than they do when the offset is incorrect. This difference can be exploited by forming an appropriate metric that discriminates between the correct offset and the incorrect offsets.
A brute force way to synchronize frames is to buffer two frame-lengths of symbols — a length sufficient to guarantee capture of at least one full frame — and attempt decoding at each possible offset until an offset is found for which decoding is successful. This decode-at-all-offsets approach was used in the Mars Laser Communications Demonstration (MLCD), for example, and works well if the decoder is many times faster than the data rate of the link. For CCSDS LDPC (Consultative Committee for Space Data Systems Low-Density Parity-Check) codes, the decoder would need to operate four to five orders of magnitude faster than the data rate in order for it to be able to acquire the correct frame offset without dropping or buffering additional codewords during the synchronization process.
This method is computationally more complex and doesn’t perform as well as frame synchronizers that utilize an ASM; nevertheless, the new synchronizer acquires frame synchronization in about two seconds when using a 600-kbps software decoder, and would take about 15 ms on prototype hardware. It also eliminates the need for the ASMs, which is an attractive feature for short uplink codes whose coding gain would be diminished by the overheard of ASM bits.
This work was done by Jon Hamkins of Caltech for NASA’s Jet Propulsion Laboratory.
The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at
This Brief includes a Technical Support Package (TSP).
Frame Synchronization Without Attached Sync Markers
(reference NPO47920) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA, specifically addressing "Frame Synchronization Without Attached Sync Markers" (ASMs). It is part of the NASA Tech Briefs and aims to disseminate information about aerospace-related developments that have potential applications beyond their original context.
The primary focus of the document is on frame synchronization techniques used in communication systems, particularly in the context of space missions. Frame synchronization is crucial for ensuring that data is accurately received and interpreted, especially in environments where signal integrity can be compromised. The document discusses the conventional method of using Attached Sync Markers, which are inserted between codewords in a data stream to help receivers identify the start of each frame of data.
The document outlines the process of how ASMs work: they are sent alongside codewords without gaps, allowing for effective synchronization at the receiving end. This method, while effective, may not always be feasible or optimal in certain scenarios, particularly in high-speed or high-noise environments where the overhead of additional markers can be detrimental to performance.
In contrast, the document explores alternative synchronization methods that do not rely on ASMs. These methods aim to enhance the efficiency and reliability of data transmission by reducing the complexity and potential points of failure associated with traditional synchronization techniques. The document emphasizes the importance of innovative technology in improving communication systems, particularly for space applications where reliability is paramount.
Additionally, the document provides contact information for further inquiries and assistance related to research and technology in this area, specifically through the Innovative Technology Assets Management at the Jet Propulsion Laboratory (JPL).
Overall, the Technical Support Package serves as a resource for engineers, researchers, and technologists interested in advancing frame synchronization techniques and understanding the implications of moving away from conventional methods. It highlights NASA's commitment to technology transfer and the broader impact of aerospace innovations on various fields.
