Temporally Coherent Communications, 10-R8194
Travis R. Thompson
Michael S. Moore
Inclusive Dates: 11/01/10 – 11/01/11
Background — Current communications systems do not have time coherence between sender and receiver (a common sense of time) and must perform significant amounts of signal processing to identify whether a signal is present and to synchronize with it (bit synch). This processing requirement drives the size, weight, and power (SWaP) of the implementations higher. The Defense Advanced Research Products Agency (DARPA) Chip Scale Atomic Clock (CSAC) program has developed small, low-power atomic clock technology. Highly coherent atomic clocks are now available that are sufficiently small and power efficient that they can be used in mobile wireless communications. This research has developed methods for leveraging the CSAC to reduce the amount of synchronization-related processing that must be done in radios. It has also developed a new media access control (MAC) scheme that leverages time coherence, which promises to have low probability of intercept/low probability of detection (LPI/LDP) properties.
Approach — The primary objective of this project was to investigate the concept of temporally coherent communications (TCC) and to characterize the performance parameters of a novel temporally coherent waveform that leveraged the CSAC devices. Consequently, researchers were also able to obtain hands-on experience with CSAC devices that positioned the team as knowledgeable early adopters with actual device performance results to substantiate waveform development. The focus of this effort was to characterize and evaluate the CSAC technology and apply it to developing a set of waveforms for low-power, covert and interference-tolerant communications.
The technical approach was two pronged. The project team experimented with actual CSAC development boards and developed a waveform based on the model of the CSAC derived from real-world experimentation. The CSAC was leveraged to develop the TCC architecture and waveform. The approach was to develop a waveform that varied the encoding, modulation, transmit frequency, power, and time patterns in a novel way. The transmit and receive radios would each have an integrated CSAC, meaning they were temporally coherent within the bounds derived from the CSAC experimentation and modeling.
The project team designed and built a custom dual mixer time difference measuring system that mixes two CSAC time sources with the same reference signal. The CSAC timing model and clock drift values were empirically derived from multiple experiments using the dual mixer time difference measurement method. The research team used the captured test data to develop a model of the CSAC's drift with respect to temperature and time since discipline. The propagation delay can be estimated by sending custom "sounding" messages between the pairs of radios. The proposed solution operated with very low power by executing a time-synchronized, media access scheme. To functionally validate the team's research claims that the coherent waveform was feasible, base functionality was implemented in the Gnu's Not Unix (GNU) Radio framework. In both the transmitter and receiver, a CSAC model was used to simulate the clock that drives the scheduler.
Accomplishments — As a result of this research, several practical use cases were developed that led to consideration of various potential application areas and customers. Researchers conducted numerous experiments with CSAC development boards and developed a model of the clock drift over time and across temperature. These experiments were used to simulate CSAC-enabled communication and develop the temporally coherent algorithm and waveform. Several promotional opportunities have been pursued throughout the past year. The team has many promising leads and already has successfully won a Small Business Innovative Research (SBIR) directly related to the work done on this project. The team aims to transition the technology into use in Department of Defense (DoD) applications.