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 randomized 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, the project team was also able to obtain hands-on experience with CSAC devices that positioned them 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: experiments were conducted on actual CSAC development boards and a waveform was developed 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, and transmit frequency, power, and time patterns over time, based on a priori arranged pseudo-random sequences. 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. A custom dual-mixer, time-difference measuring system was designed and developed 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 special "sounding" messages between the pairs of radios. The proposed solution operated with very low power by executing a time-synchronized, pre-planned media access scheme. To functionally validate the research claims that the coherent waveform was feasible, the 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 for various potential application areas and customers. Numerous experiments with CSAC development boards were conducted, and a model of the clock drift over time and across temperature was developed. 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. There are many promising leads, and the team has been awarded a Small Business Innovative Research (SBIR) directly related to the work done on this project.