To demodulate a communication signal, a receiver must recover and synchronize to the symbol timing of a received waveform. In a system that utilizes digital sampling, the fidelity of synchronization is limited by the time between the symbol boundary and closest sample time location. To reduce this error, one typically uses a sample clock in excess of the symbol rate in order to provide multiple samples per symbol, thereby lowering the error limit to a fraction of a symbol time. For systems with a large modulation bandwidth, the required sample clock rate is prohibitive due to current technological barriers and processing complexity. With precise control of the phase of the sample clock, one can sample the received signal at times arbitrarily close to the symbol boundary, thus obviating the need, from a synchronization perspective, for multiple samples per symbol.
Sample-clock phase-control feedback was developed for use in the demodulation of an optical communication signal, where multi-GHz modulation bandwidths would require prohibitively large sample clock frequencies for rates in excess of the symbol rate. A custom mixed-signal (RF/digital) offset phase-locked loop circuit was developed to control the phase of the 6.4-GHz clock that samples the photoncounting detector output. The offset phase-locked loop is driven by a feedback mechanism that continuously corrects for variation in the symbol time due to motion between the transmitter and receiver as well as oscillator instability. This innovation will allow significant improvements in receiver throughput; for example, the throughput of a pulse-position modulation (PPM) with 16 slots can increase from 188 Mb/s to 1.5 Gb/s.
The novelty of this innovation is precise control of the sample-clock phase supports synchronization to the symbol timing of the received waveform without the use of a sample clock in excess of the symbol rate. This can reduce the required sample clock frequency for demodulation of a communication signal, and thereby reduce the processing complexity as well as permit demodulation of large bandwidth signals for which there was a technological barrier to a sample frequency in excess of the symbol rate.
Sample-clock phase-control feedback has direct applications in optical and radio frequency communication systems for satellite and deep space applications, as well as other applications in high-precision timing.
This work was done by Kevin J. Quirk, Jonathan W. Gin, Danh H. Nguyen, and Huy Nguyen of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47663
This Brief includes a Technical Support Package (TSP).
Sample-Clock Phase-Control Feedback
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Overview
The document titled "Technical Support Package for Sample-Clock Phase-Control Feedback" outlines advancements in the demodulation of optical communication signals, particularly focusing on high modulation bandwidth systems. Developed by researchers at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology, the technology aims to enhance the synchronization of communication signals, which is crucial for effective data transmission.
The core of the technology involves a mixed-signal (RF/digital) offset phase-locked loop circuit that controls the phase of a 6.4 GHz clock. This clock samples the output from a photon-counting detector, which is essential for processing optical communication signals. The offset phase-locked loop employs a feedback mechanism to correct variations in symbol timing caused by the relative motion between the transmitter and receiver, as well as any instability in the oscillator. This ensures that the sampling occurs at the optimal time, close to the symbol boundary, thereby improving the accuracy of the demodulation process.
The document emphasizes the importance of using a sample clock that exceeds the symbol rate, allowing for a higher resolution in symbol timing. This is achieved through a feedback mechanism that continuously adjusts the phase of the sample clock. The approach minimizes digital processing complexity and speed by utilizing a single sample per symbol, which is particularly beneficial for systems with pulse shaping, such as RF Nyquist pulse shaping.
Significantly, the technology allows for the implementation of high modulation bandwidth systems, which can lead to substantial improvements in receiver throughput. For instance, the throughput of a pulse-position modulation (PPM) system with 16 slots can increase dramatically from 188 Mb/s to 1.5 Gb/s, showcasing the potential for enhanced data rates in optical and high-rate RF communications.
Overall, this document serves as a technical brief on the innovative methods developed for sample-clock phase control, highlighting their applications in improving communication systems' efficiency and performance. The research is supported by NASA's Commercial Technology Program, aiming to make aerospace-related developments accessible for broader technological and commercial applications.
