In this report, we study the relation between the number of array antenna elements and the accuracy of DOA (Direction Of Arrival) in ESPRIT and Unitary-ESPRIT methods for efficiently constructing array system. In ESPRIT method, it is well-known that the number of incident waves plus one elements are theoretically required for the accurate DOA estimation. However, sometimes in practical situations, the accuracy becomes worse even though the number of elements satisfies the above condition. This report studies the actual number of array elements that can accurately estimate DOA in case of linear array antenna. As a result, in the case that the angle between incident waves is around 10deg, we found that the number of incident waves plus three was required for the accurate DOA estimation. Furthermore, especially in the case that the angle between incident wave is close, less than 5deg), the number of required antenna elements becomes twice to five-times of the number of incident waves.
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