## 1 Introduction  
With the development of array signal processing, the direction of arrival (DOA) estimation algorithms are used widely in radar, sonar, atmosphere, communication and so on. Since the 1980s, many high resolution 2-dim (2-D) DOA algorithms were proposed in succession. Among them, the multiple signal classification (MUSIC) algorithm [1; 2] marks a symbolic method of the spatial spectrum estimation algorithm. In ideal conditions, MUSIC algorithm has better estimation accuracy and resolution performance and nowadays there are still a lot of scholars devoting themselves to MUSIC algorithm [3; 4; 5; 6; 7]. However, in practice, MUSIC algorithm has strict requirements with the placement of array elements, which has a great influence on DOA estimation accuracy, resolution and stability [8]. As a symbolic plane array, the uniform circular array (UCA) can provide both azimuth and elevation information ranging from 0\({}^{\circ}\) to 360\({}^{\circ}\) and also has other excellent performance such as circular symmetry properties [9; 10; 11; 12; 13; 14]. Utilizing array manifold directly, the algorithm proposed in [15] could estimate the 2-D DOAs of a single extended signal combining least-squares (LS) method with weighted total LS (WTLS), which leads to a better performance. A much amplifier algorithm proposed in [16; 17] can achieve a better stability, but only apply to one single source. A unitary transformation method was proposed in [18] based on array space; this method can reduce the computational complexity and improve the practicability of the MUSIC algorithm. By base-line rotation and phase integration, the algorithm proposed in [19; 20] utilized rotating interferometer to solve the ambiguity problem. However, the algorithms mentioned above have not solved the problem as below: (1) DOA estimation performance has strict restrictions with the radius \(r\) of antenna dish in UCA; the larger the radius is, the better the resolution is; (2) More antenna elements can provide better resolution and estimation accuracy, but too many elements will cause high energy loss and that will make it difficult to calibrate channel mismatches. On the other hand, the number of elements cannot be too high because of the space restrictions of the antenna dish. To solve these issues, a virtual antenna array technique can be used by moving the receiver antenna to various locations and then measuring the static signal at those locations. In addition, another method for DOA estimation using single antenna devices was presented in [21; 22; 23].  
In this paper, a novel rotation MUSIC (R-MUSIC) algorithm for static targets based on the array rotating technique is proposed. This algorithm can obtain any even number of elements by rotating only two elements, so that it can receive the signal data at multiple positions. Besides, the R-MUSIC algorithm can break the constraints that the number of antenna elements must be more than the number of incident signals. Most importantly, the array system will be much simpler than the traditional array. Computer simulations verified the effectiveness and superior performance of the proposed method.  
The remainder of this paper is organized as follows. Section 2 introduces the MUSIC algorithm based on 2-D UCA. Section 3 elaborates the structure model of the rotation array and contains the core work of this paper, in which the R-MUSIC method is proposed for 2D-DOA estimation. Section 4 derives the Cramer-Rao bound (CRB). Section 5 presents the computer simulation result. Section 6 gives the final conclusions.