Abstract
Most popular techniques for super-resolution direction of arrival (DOA) estimation rely on an eigen-decomposition (EVD) or a singular value decomposition (SVD) computation to determine the signal/noise subspace, which is computationally expensive for real-time applications. A two-step root multiple signal classification (TS-root-MUSIC) algorithm is proposed to avoid the complex EVD/SVD computation using a uniform linear array (ULA) based on a mild assumption that the number of signals is less than half that of sensors. The ULA is divided into two subarrays, and three noise-free cross-correlation matrices are constructed using data collected by the two subarrays. A low-complexity linear operation is derived to obtain a rough noise subspace for a first-step DOA estimate. The performance is further enhanced in the second step by using the first-step result to renew the previous estimated noise subspace with a slightly increased complexity. The new technique can provide close root mean square error (RMSE) performance to root-MUSIC with reduced computational burden, which are verified by numerical simulations.
1. Introduction
Subspace-based algorithms for estimating the direction of arrivals (DOAs) of multiple signals impinging on array of sensors have drawn considerable interests due to their super-resolution abilities and high estimation accuracies [1–6]. Over several decades, this topic has been extensively studied and varieties of techniques including multiple signal classification (MUSIC) [7], minimum norm (MN) [8], estimation of the signal parameters via rotational invariance techniques (ESPRIT) [9], subspace fitting (SF) [10], and maximum likelihood (ML) [11] have been developed. In subspace methods, the data matrix or a statistics matrix of the data is normally decomposed into the signal and noise subspaces. Traditionally, this decomposition is usually carried out using an eigen-decomposition (EVD) or a singular value decomposition (SVD) computation, which is computationally intensive and time-consuming. In applications like high-resolution passive sonar systems where the number of sensors is much larger than that of signals [12], the use of such methods is unattractive owing to their intensive computational implementation, and therefore, there is a need for techniques demanding less computations.
Several techniques that seek to determine signal subspace-based estimates without EVD/SVD have been developed. These include the propagator method (PM) [13], the bearing estimation without eigen-decomposition (BEWE) [14], subspace methods without eigen-decomposition (SWEDE) [15], and subspace-based method without eigen-decomposition (SUMWE) [16], in which the signal or the noise subspace is extracted by a linear operator from the array output data and DOA is finally estimated by a cost function similar to MUSIC [17] or root-MUSIC [18]. These methods are found to have significant computational saving over those that explicitly compute EVD or SVD. However, the root mean square error (RMSE) and resolution performances of linear operation-based techniques degrade severely when the signal-to-noise ratio (SNR) is low [19].
Another cross-correlation-based 2D DOA estimation (CODE) algorithm was recently proposed [20] based on an L-shaped array composed of two uniform linear arrays (ULAs). In the CODE method, the noise space is obtained through a linear operation of matrices formed from the forward cross-correlation matrix. It has been shown that CODE can provide good performance with low SNRs and small numbers of snapshots [16]. However, the accuracy of CODE is still inferior to potentially more computationally methods such as MUSIC and root-MUSIC.
The purpose of this paper is to propose an alternate low-complexity subspace-based algorithm without EVD/SVD computation to reduce the computational burden while providing acceptable accuracy for DOA estimation. We present a novel two-step root-MUSIC (TS-root-MUSIC) algorithm to achieve performances closer to the standard root-MUSIC while keeping computational complexity comparable with CODE and being lower than root-MUSIC. In particular, we consider a single ULA which can be possibly regarded as part of a more complex array such as an L-shaped one. We propose to use both the forward and backward cross-correlation matrices [21, 22] and performance enhancement by a further second-step process to improve the estimation accuracy. Numerical simulations are finally provided to illustrate the degree of performance enhancement.
This paper is organized as follows. Section 2 describes the data model of DOA estimation problem using a ULA as well as the background of a standard root-MUSIC algorithm. The proposed TS-root-MUSIC is presented in Section 3, in which the computation of forward and backward cross-correlation matrices, subspace extraction without EVD/SVD, and performance enhancement using the two-step process are discussed in detail. Section 4 summarizes the implementation of the new method and analyzes its computation complexity with comparisons to other methods including root-MUSIC and CODE. Finally, Section 5 conducts numerical simulation results to show that our algorithm costs a less computation burden than root-MUSIC while providing a comparable performance which is much better than CODE.
2. Signal Model and Standard Root-MUSIC
Assume that there are narrowband signals with unknown DOAs simultaneously impinging from far-field on a ULA composed of sensors. Without loss of generality, assume that is an even number, that is, ; the array output vector at time can be written as where is the source waveform vector, is the additive white Gaussian noise (AWGN) vector, and is the array steering matrix. Define , where , is the center wavelength and is the array interval; the steering vector can be written as
The array covariance matrix can be written as where is the source covariance matrix, is the power of AWGN, and is the identity matrix. For full-rank AWGN, the EVD/SVD of matrix can be written in a standard way as where and are the two diagonal matrices of dimensions and , respectively, and and are the so-called signal and noise matrices, which are of dimensions and and contain the eigenvectors relating to the significant and smallest eigenvalues of , respectively. In real-world applications, the theoretical is unavailable, and it is usually estimated by snapshots of array observed data as
Therefore, the EVD of the array covariance matrix is in fact given by
Based on the special geometry of a ULA as well as the orthogonality between span () and span (), the standard root-MUSIC algorithm transforms the tremendous spectral search step involved in MUSIC into a simplified polynomial rooting as [18] where is the projection matrix of the noise subspace. For (7), there are generally pairs of roots that symmetrically distribute around the unit circle. By choosing the roots which are situated the most closely to the unit circle, one can estimate signal DOAs by
Note that the computational complexity of this polynomial rooting step is substantially lower than that of the spectral search [18]. However, as EVD computation requires about flops [20, 23], the complexity of root-MUSIC is still high, especially when large numbers of sensors are used [18].
3. The Proposed Algorithm
3.1. Noise Subspace Extraction without EVD/SVD Computation
As shown in Figure 1, we divide the ULA into two subarrays along its center position without overlapping such that there are elements in each subarray (Note that for the case , the center sensor is shared by the two subarrays and there will be sensors in each subarray.) Thus, both the output vectors of the two subarrays are of dimensions , which can be written as where are the steering matrices of the two subarrays, respectively. Due to the Vandermonde structure, and satisfy where is a exchange matrix with ones on its antidiagonal and zeros elsewhere and is a diagonal matrix, given by
The forward cross-covariance matrix of the two subarrays can be computed by
Define two new virtual signal vectors as follows:
Using (11) and (12), the forward cross-covariance matrices between and and the backward cross-covariance matrix between and can be written as
Note that all the three theoretical cross-covariance matrices , , and are noise-free in nature, which makes it possible to extract a signal or noise subspace from those three matrices with only linear operations.
Under the assumption , we can divide into two submatrices along its row direction as where and are the and submatrices consisting of the first and last rows of , respectively. Since is a Vandermonde matrix with full rank, we have . Therefore, the rows of can be expressed as linear combinations of those of , and we further have where is an unknown matrix of dimensions and is of dimensions . Combining , , and , we can form two new noise-free cross-correlation matrices as follows:
Inserting (18) into (19) and (20) leads to where and are of dimensions while and are of dimensions , respectively, given by
Define a new matrix and a new matrix . Because and , we have . Since , we have
Now, we have computed matrix . According to (18), the projection matrix of the noise subspace can be extracted without EVD/SVD computation by with which we can construct a root-MUSIC-like polynomial where . The signal DOAs can be found by finding the phases of the roots of that lie closest to and inside the unit circle.
3.2. Performance Enhancement Using the Two-Step Process
As mentioned above, the ideal cross-covariance matrix is noise-free in theory. However, with snapshots in real-world applications, it is usually estimated by
It can be observed by comparing (30) with (14) that the last three terms of (30) are undesirable by-products that can be taken as estimates for the correlation between the signal and noise vectors [24]. This is mainly caused by a finite number of snapshots, and only for a large enough , the last three terms of (30) tend to zero. According to [24], the accuracy of can be enhanced by a two-step process as follows: where is a real scaling factor between zero and one, which can be predetermined by minimizing the stochastic maximum likelihood (SML) objective function [24]. , , and are least square (LS) estimates of the last three terms in (30), given by where are the projection matrices of the signal and noise subspaces extracted from subarray , respectively, and is a rough DOA estimation result reduced by using root-MUSIC with the extracted noise projection matrix given by (28).
Similarly, with snapshots, the other two cross-covariance matrices and are estimated by
By comparing the last three items in and with that in , it is easy to obtain the two-step enhancements for and as follows:
With the three accuracy enhanced cross-covariance matrices, we can use , , and instead of , , and to compute a new noise projection matrix by using (19), (20), (21), (22), (23), (24), (25), (26), (27), and (28), and obtain another polynomial as follows:
A more accurate DOA can be estimated by finding the phases of the roots of that lie closest to and inside the unit circle.
4. Summary and Complexity Analysis
With snapshots of the observed data, detailed steps for implementing the proposed two-step root-MUSIC algorithm for DOA estimation without EVD/SVD computation are summarized in Table 1.
Table 2 compares the primary computational complexity of diffident algorithms including root-MUSIC, CODE [20], and the proposed method, where is the sensor used for root-MUSIC for a fair comparison.
For root-MUSIC, estimation of the covariance matrix requires flops while its EVD costs flops [23]. As the polynomial in (7) is of order , the rooting step costs flops [16, 20]. For both CODE and the proposed method, as , , and are all of reduced dimensions , estimations of the three cross-covariance matrices cost flops in total. According to (27) and (28), the computation of needs about flops since it occurs often in application of DOA estimation that [16, 20, 25]. Finally, as the polynomial order is reduced to half in the proposed method, the rooting step for the new method costs flops.
Based on the above analysis and notice that , it can be concluded from Table 2 that the proposed method has a comparable computational complexity to CODE and is much lower than root-MUSIC.
5. Simulation Results
Numerical simulations are conducted to assess the performance of the proposed method with comparisons among CODE and root-MUSIC. Throughout the simulations, 500 independent Monte-Carlo trials have been used on a ULA with sensors. For the RMSE comparison, the unconditional Cramer-Rao lower bound (CRLB) given in [26] is applied for a common reference, where the RMSE for the estimates of source incident angle is defined as where is the true DOA and represents the estimated value of the DOA of the trial. For sources, the SNR is defined as where denotes the average power of all sources, and is the power of the source. According to the discussions in [24], the parameter in step 2 is fixed as 0.5 throughout the simulations.
In the first simulation, we examine the maximum number of signals that can be detected by the proposed method. To this end, we plot the MUSIC spectrums computed by using the noise subspace given by CODE with that by our method in Figure 2, where sources at , , and are detected by a half-wavelength ULA with sensors. It is seen from Figure 2 that both the two methods can resolve the three sources well. It is also verified by Figure 2 that the maximum number of signals that can be detected by the proposed method is less than half that of the number of sensors, which is the same as CODE.
In the second simulation, we evaluate the RMSE performance of DOA estimation by the proposed method and compare it with those by CODE [20] and standard root-MUSIC. For fair comparisons, we exploited the two-step process for both CODE and the proposed method. In the simulation, sources are estimated by using a nonuniform linear array (UNLA) with 10 sensors spaced by a half-wavelength.
First, we consider fixed sources at and . In Figure 3, we plot the RMSEs as functions of the SNR, where the number of snapshot is fixed as . In Figure 4, we plot the RMSEs as functions of the number of snapshot, where we set .
Next, to see more clearly the performance of the new method, we consider varying sources at and , where varies from to . We plot the RMSEs as functions of angle separation in Figure 5, where we set and .
It can be seen clearly from the three figures that the RMSE curves of the three methods including CODE, root-MUSIC, and the proposed TS-root-MUSIC decrease dramatically as the SNRs, the number of snapshots, and the values of angle separation increase. However, the RMSE curves of our method locate below that of CODE all along in all the three figures, which indicates that our method is superior to CODE. In addition, the proposed method in step 2 also outperforms CODE with the two-step process. This is because TS-root-MUSIC uses both the forward and backward cross-covariance matrices to exploit more information for DOA estimate. It is also seen from the three figures that both CODE and the proposed TS-root-MUSIC algorithm in step 2 shows a much better accuracy than those in step 1, which verifies the performance enhancement by the two-step process.
In the last simulation, we verify the efficiency of the proposed approach by comparing the simulation times of DOA estimates by CODE, root-MUSIC, and TS-root-MUSIC as functions of the number of sensors in Table 3. The simulated results are given by a PC with Intel® Core™ Duo T5870 2.0 GHz CPU and 1 GB RAM by running the Matlab codes in the same environment. It can be seen from Table 3 that the proposed method in both step 1 and step 2 costs lower simulation time than the standard root-MUSIC, especially for large numbers of sensors. Since TS-root-MUSIC in step 1 has a similar complexity as CODE, the new method costs a similar CPU time to CODE in step 1 while it costs approximately twice that in step 2. Considering that TS-root-MUSIC has a much better accuracy than CODE, our method trades off MSEs by complexity efficiently as compared to CODE.
6. Conclusions
We have proposed a new computationally efficient algorithm for DOA estimation using a ULA, in which the ULA is divided into two subarrays and both the forward and backward cross-covariance matrices are exploited to obtain a signal subspace without EVD/SVD computation. A two-step process is also applied for further performance enhancement with a slightly increased complexity. Numerical simulations demonstrate that the new technique has a much lower computational complexity than root-MUSIC and CODE have while the RMSE performance of the new method is comparable to root-MUSIC and is much better than CODE.
Data Availability
The data sets used or analyzed during the current study are available from the corresponding author on reasonable request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (61501142) and the Science and Technology Program of WeiHai and project supported by the Discipline Construction Guiding Foundation in Harbin Institute of Technology (Weihai) (WH20160107).