Army

Sensor network with signal fusion center

Use of an echo signal to synchronize multiple signal collections from sensors in order to pull out signals of interest

Communications Sensors

Photo: David Becher

When a signal of interest (SOI) is transmitted with low-power or in a remote location (the SOI is transmitted from a ship or in a space vehicle and the sensors are along the shore or in an isolated geographical area or when the sensors are in unfavorable or noisy receiving locations), the SOI copies collected at distributed sensors may be too weak to analyze. In a dumb sensor and smart fusion center scenario, the sensors are asynchronous and the fusion center combines the signal copies after a post-synchronization process. When the asynchronous signal copies are very weak, this post-synchronization becomes very difficult and unreliable.

To address synchronization of weak signals and enable enhanced sensing, Army scientists and engineers have developed a system using asynchronous low-cost sensors deployed in distributed locations. In this schema, the SOI may not be identifiable through the use of a single sensor alone (due to the weak nature of the signal) but can be detected and classified by fusing multiple weak signal copies together, where the signal copies are collected by the sensor network. These asynchronous signal copies have relative offsets in both frequency and phase due to the diversities and drifts in the distributed local oscillators. The Army’s novel synchronization method estimates the relative offsets by transmitting an echo signal (ES) to the asynchronous sensors. The returning echo signals which have been contaminated by local oscillators can be used for estimating the relative offsets. The weak signal is compensated by using the estimated offsets of the echo signal from multiple sensors and coherently combined to achieve a higher processing gain for reliable signal exploitation.

In this invention, the ES is transmitted by the fusion center to the sensors together with a requesting signal (RS) for collecting SOI. The fusion center comprises a filter module, SOI Snapshot module, ES Snapshot module, a frequency and phase compensation module, a frequency and phase estimators module, a time, frequency, and phase fine-tuning module, a signal fusion module, and a spectrum sensing and signals classification module. The ES can be transmitted in the same frequency as the SOI, or RS, or in different frequencies. The RS itself can serve as the ES in that one signal is transmitted by the fusion center.

This approach can be used for detecting remote weak signals from outer space, receiving low power transmission signals, demodulating a cell phone signal transmitted at a location far away from towers, military or police surveillance of hostile transmission, spectrum survey of weak signals, and other applications.

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