A report discusses the performance of a soft digital-data-transition tracking loop (DTTL) in a radio receiver that recovers digital data conveyed by binary phase-shift keying. The DTTL is used as a symbol synchronizer; it provides symbol timing to essential parts of the receiver. The DTTL includes a quadrature channel and an in-phase channel, which contains a transition detector with a hyperbolic-tangent response. The DTTL is said to be "hard" or "soft" in the special case of high or low signal-to-noise ratio (high or low SNR, respectively), for which the hyperbolic tangent can be approximated as a hard-limiting or a linear function, respectively.
This work was done by Samson Million and Sami Hinedi of Caltech for NASA's Jet Propulsion Laboratory. NPO-20154
This Brief includes a Technical Support Package (TSP).
Performance of a soft digital-data-transition tracking loop
(reference NPO20154) is currently available for download from the TSP library.
Don't have an account?
Overview
The document presents a technical report on the performance of a Soft Digital-Data-Transition Tracking Loop (DTTL), which is utilized in radio receivers to recover digital data transmitted via binary phase-shift keying. The DTTL functions as a symbol synchronizer, providing essential timing information to various components of the receiver. The report, authored by Sami Hinedi and Samson Million from Caltech for NASA's Jet Propulsion Laboratory, focuses on the soft DTTL's performance, particularly in low signal-to-noise ratio (SNR) environments.
The DTTL comprises two channels: an in-phase channel and a quadrature channel. The in-phase channel includes a transition detector that employs a hyperbolic-tangent response. The performance of the DTTL is categorized as "hard" or "soft" based on the SNR; at high SNRs, the hyperbolic tangent can be approximated as a hard limiter, while at low SNRs, it behaves more like a linear function.
The report introduces a mathematical model for the soft DTTL, which is used to derive two key performance metrics: the loop S curve, representing the normalized expectation value of symbol error as a function of timing error, and the two-sided spectral density of the equivalent additive noise. These metrics allow for a comparative analysis of the soft DTTL's performance against that of the hard DTTL.
Findings indicate that at low symbol SNRs, the soft DTTL exhibits less timing jitter compared to the hard DTTL. Specifically, at a symbol SNR of -10 dB, the soft DTTL demonstrates a twofold improvement in timing jitter. The report concludes that both the soft and hard DTTL have comparable tracking performance at certain SNR levels (2.5 dB and -1 dB for specific window sizes), but the soft DTTL outperforms the hard DTTL in lower SNR conditions.
The document emphasizes the importance of the soft DTTL in future space missions, where higher-rate codes and lower SNRs are expected. The acquisition performance of the soft DTTL is noted as a topic for future research. Overall, this report contributes valuable insights into the design and performance of digital data tracking systems, particularly in challenging communication environments.
