Secure Wireless Communication Using Coherent Distributed Transmission and Spatial Signal Decomposition

We present a new approach to secure wireless communications using coherent distributed transmission of signals that are spatially decomposed between a two-element distributed antenna array. High-accuracy distributed coordination of microwave wireless systems supports the ability to transmit different parts of a signal from separate transmitters such that they combine coherently at a designated destination. In this paper we explore this concept using a two-element coherent distributed phased array where each of the two transmitters sends a separate component of a communication signal where each symbol is decomposed into a sum of two pseudo-random signal vectors, the coherent summation of which yields the intended symbol. By directing the transmission to an intended receiver using distributed beamforming, the summation of the two vector components is largely confined to a spatial region at the destination receiver. We implement the technique in a 50 wavelength array operating at 3 GHz. We evaluate the symbol error ratio. (SER) in two-dimensional space through simulation and measurement, showing the approach yields a spatially confined secure region where the information is recoverable(i.e., the received signal has low SER), and outside of which the information is unrecoverable (high SER). The proposed system is also compared against a traditional beamforming system where each node sends the same data. We validate experimentally that our approach achieves a low SER of 0.0082 at broadside and a SER above 0.25 at all other locations compared to a traditional beamforming approach that achieves a SER of 0 at all locations measured.

Real-Time High-Accuracy Digital Wireless Time, Frequency, and Phase Calibration For Coherent Distributed Antenna Arrays

This work presents a fully-digital high-accuracy real-time calibration procedure for frequency and time alignment of open-loop wirelessly coordinated coherent distributed antenna array (CDA) modems, enabling RF phase coherence of spatially separated commercial off-the-shelf (COTS) software-defined radios (SDRs) without any cables or external references such as global navigation satellite system (GNSS). Building on previous work using high-accuracy spectrally-sparse time of arrival (ToA) waveforms and a multi-step ToA refinement process, a high-accuracy two-way time transfer (TWTT)-based time-frequency coordination approach is demonstrated. By using a high-accuracy time estimation approach, frequency estimates can be derived over long observation intervals leading to a high-accuracy frequency estimate, without the requirement for long pulse durations as is required for direct spectral frequency estimation techniques, minimizing coordination overhead. Furthermore, due to the two-way nature of the high-accuracy TWTT approach, the time and frequency estimates are Doppler and multi-path tolerant, so long as the channel is reciprocal over the synchronization epoch. This technique is experimentally verified by demonstrating wireless distributed array coordination using COTS SDRs in a lab environment in static and dynamic scenarios and with significant multipath scatterers. Time, frequency, and phase stability were measured over coaxial cables to an oscilloscope and achieved time and phase coordination precision of ~60-70 ps, with median coherent gains above 99% using optimized parameters, and a beamforming frequency RMSE of 3.73 ppb in a dynamic scenario. Finally, experiments are conducted to compare the performance of this technique with previous work works using an analog continuous-wave two-tone (CWTT) frequency reference technique in both static and dynamic settings as a benchmark.