The Soft Decision Analyzer (SDA) is an instrument that combines hardware, firmware, and software to perform realtime closed-loop end-to-end statistical analysis of single- or dual- channel serial digital RF communications systems operating in very low signal-to-noise conditions. As an innovation, the unique SDA capabilities allow it to perform analysis of situations where the receiving communication system slips bits due to low signal-to-noise conditions or experiences constellation rotations resulting in channel polarity in versions or channel assignment swaps. SDA’s closed-loop detection allows it to instrument a live system and correlate observations with frame, codeword, and packet losses, as well as Quality of Service (QoS) and Quality of Experience (QoE) events. The SDA’s abilities are not confined to performing analysis in low signal-tonoise conditions. Its analysis provides indepth insight of a communication system’s receiver performance in a variety of operating conditions.

The SDA incorporates two techniques for identifying slips. The first is an examination of content of the received data stream’ s relation to the transmitted data content and the second is a direct examination of the receiver’s recovered clock signals relative to a reference. Both techniques provide benefits in different ways and allow the communication engineer evaluating test results increased confidence and understanding of receiver performance. Direct examination of data contents is performed by two different data techniques, power correlation or a modified Massey correlation, and can be applied to soft decision data widths 1 to 12 bits wide over a correlation depth ranging from 16 to 512 samples. The SDA detects receiver bit slips within a ±4 bits window and can handle systems with up to four quadrants (QPSK, SQPSK, and BPSK systems). The SDA continuously monitors correlation results to characterize slips and quadrant change and is capable of performing analysis even when the receiver under test is subjected to conditions where its performance degrades to high error rates (30 percent or beyond). The design incorporates a number of features, such as watchdog triggers that permit the SDA system to recover from large receiver upsets automatically and continue accumulating performance analysis unaided by operator intervention. This accommodates tests that can last in the order of days in order to gain statistical confidence in results and is also useful for capturing snapshots of rare events.

Slip and quadrant performance are displayed in real time in addition to being logged for later analysis. The SDA provides methods of evaluating the significance of BER (bit error rate) test results as well as techniques to characterize imbalances due to inter-symbol interference or inter-channel interference. Techniques employed in the SDA provide a means to statistically characterize receiver performance efficiently using a minimum of accumulated test results with a definable level of error. These advanced techniques have broad application to other fields that rely on evaluation of binomial experiments (pass/fail, true/false, go/no-go), and allow evaluators to extract more information from fewer trials.

This work was done by Glen Steele, Chatwin Lansdowne, Joan Zucha, and Adam Schlesinger of Johnson Space Center. For further information, contact the JSC Innovation Partnerships Office at (281) 483-3809.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
Johnson Space Center
(281) 483-1003.


NASA Tech Briefs Magazine

This article first appeared in the August, 2013 issue of NASA Tech Briefs Magazine.

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