Enhanced Signal Copy in High Noise Environments, 16-9436

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Principal Investigator
Don R. Van Rheeden

Inclusive Dates:  10/01/03 - 10/01/04

Background - Enhanced signal copy approaches utilize the diversity of an N-element antenna array connected to a set of N receivers to concurrently enhance a signal-of-interest (SOI) while suppressing interfering signals. If an enhanced copy system is located in a noisy environment, suppressing unwanted interference can present an enormous challenge. For example, on a building rooftop there may be radio frequency (RF) emitters present such as emergency services transmitters, cellular transmitters, PCS transmitters, and paging transmitters. If the total number of signals impinging the antenna array is greater than the number of array elements, then it becomes difficult to separate the SOI from the interfering signals. Recently, researchers have discovered that array processing based on higher order statistics yields a concept known as the virtual array. A virtual array is one that has virtual sensors in addition to the physical array elements. This concept has two significant implications. First, the virtual elements extend the aperture of an array, producing lower sidelobe levels and better signal copy performance. Second, the virtual array allows enhanced signal copy to take place when there are more signals than physical array elements present. Both of these factors have the potential to reduce the cost of signal copy (and direction finding) systems by requiring fewer antenna elements, fewer receivers, and less real estate.

Approach - The approach to this problem consists of four main efforts. First, we are investigating blind enhanced signal copy approaches that exploit second and higher order statistical processing. Second, the impact of the virtual array concept on signal copy performance is being determined. Third, a data collection exercise to capture on-air signals that are affected by multipath, co-channel interference, and noise will be performed. Fourth, the copy algorithm will be optimized so a real-time software implementation can be developed.

Accomplishments - To date, we have studied and derived the theoretical background that supports the virtual array concept. We have done preliminary testing using HF co-channel signals collected by an 8-element HF spiral array connected to an 8-channel HF receiver. In the original experiment, there were three co-channel signals present. Two of the signals were pairs of PSK, FSK, and SSB voice signals transmitted from two known locations. The third signal was a broadband interferer from a local manufacturing facility. Using a blind signal copy approach, we have been able to separate the pairs of co-channel PSK/FSK/SSB voice signals and the broadband interferer using only three of the spiral array's eight antenna elements. The next step in the development will be to determine how well the approach works when the number of co-channel signals exceeds the number of array elements. Both simulated signals and on-air signals will be used to test and refine the approach.

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