IEEE TRANSACTIONS ON SIGNAL PROCESSING, VOL. XX, NO. YY, SEPTEMBER 2010 2
response vector of two overlapping subarrays of each ULA,
a modified propagator method (MPM) with eigendecompo-
sition was presented to estimate the elevation and azimuth
angles independently from the covariance matrix of combined
received data of two overlapping subarrays of each ULA [3].
However unlike the ordinary propagator method (PM) [9], the
least-square (LS) estimation of the linear propagator is seri-
ously affected by the non-zero correlations of additive noises
between two overlapping subarrays, and correspondingly the
angle estimates are biased regardless of the signal-to-noise
ratio (SNR). Furthermore a pair-matching procedure is still
required to associate the elevation and azimuth estimates due
to the independent orderings of the array response vectors of
two ULAs in terms of the elevation or azimuth angles and that
of eigenvalues of two above-mentioned covariance matrices
for 1-D DOA estimation with respect to each ULA [4], [8].
By dividing one ULA into two overlapping subarrays to gain
the shift invariance of these subarrays, a joint singular value
decomposition (SVD) based method (JSVD) was suggested
to 2-D DOA estimation with automatic pairing by using
two cross-correlation matrices (CCMs) between two subarrays
and another ULA [5], where the effect of additive noise is
mitigated. The key point of [5] and its variants [35], [36] is the
ordering exchange of a product of the source signal covariance
matrix and a diagonal matrix of the phase delays with respect
to the elevation or azimuth angle, which is only valid when
the the source signal covariance matrix is diagonal matrix.
Unfortunately the JSVD [5] performs worse in the estimation
of azimuth angle even in the case of uncorrelated signals
when the number of snapshots is small, because the nonzero
cross-correlations in the estimated source signal covariance
matrix couple up the array response vectors in terms of the
azimuth angle, and it results in inaccurate parameter estimate
even at high SNR. An alternative cross-correlation matrix and
SVD based method was proposed for 2-D DOA estimation
with automatic pairing [7], but it involves the minimization
of a constrained nonlinear function and necessitates a prior
knowledge of the powers of incident signals. By a introducing
an auxiliary electric angle as a function of the elevation and
azimuth angles and adopting the 1-D generalized ESPRIT
method [34], a joint estimation method was proposed with-
out pair-matching procedure [31], however it requires two
computationally intensive eigenvalue decomposition (EVD)
processes and a restriction on the elevation and azimuth angles.
In addition, an elaborative CCM based pair-matching method
was developed [4], but it is seriously affected by the estimation
of the “virtual angles” of incident signals even though the
elevation and azimuth angles are estimated well [5].
Therefore the purpose of this paper is to investigate the
2-D DOA estimation of noncoherent narrowband signals
with the L-shaped array placed in the 𝑥–𝑧 plane in a new
computationally efficient way. Since the eigendecomposition
(EVD or SVD) process is computationally intensive and
time-consuming [37], some computationally simple subspace-
based 1-D DOA estimation methods were proposed without
eigendecomposition (see [10] and references therein). In order
to eliminate the effect of additive noise and to reduce the com-
putational burden involved by the eigendecomposition, we pro-
Fig. 1. The geometrical configuration of the L-shaped array [4].
posed a subspace-based method without eigendecomposition
(SUMWE) for 1-D DOA estimation with the ULA [10], where
the full ULA is divided into overlapping forward and backward
subarrays, and the null space is obtained through a linear
operation of the combined Hankel correlation matrix,which
is formed from the cross-correlations of some senor data by
exploiting the shift invariance of ULA. Inspired the SUMWE,
we propose a new computationally efficient cross-correlation
based 2-D DOA estimation (CODE) method without eigen-
decomposition by exploiting the array geometry of L-shaped
array. Firstly the elevation and azimuth angles are estimated
independently by a 1-D subspace-based estimation technique,
where the null spaces are obtained through a linear operation
of the matrices formed from the cross-correlation matrices
between the array data received by two ULAs, and then the
pair-matching can be conducted by searching the minimums of
a cost function of the azimuth and elevation angles, where the
eigencomposition is avoided in the both procedures. Moreover
the statistical analysis of the proposed DOA estimation method
is studied, and the explicit expressions of the asymptotic mean-
square-errors (MSEs) of the elevation and azimuth estimates
are derived. The effectiveness of proposed method and the the-
oretical analysis are substantiated through numerical examples.
The simulation results show that the proposed CODE method
has good estimation with a small number of snapshots and at
low SNR and there is good agreement between the theoretical
analyzes and empirical results.
II. DATA MODEL AND ASSUMPTIONS
As shown in Fig. 1, we consider the L-shaped array consist-
ing of two ULAs in the 𝑥-𝑧 plane, where each ULA has 𝑀
omnidirectional sensors with spacing 𝑑 and the sensor at the
origin of coordinates 𝑧
0
is the reference one for each ULA,
and suppose that 𝑝 noncoherent narrowband signals {𝑠
𝑘
(𝑛)}
with the wavelength 𝜆 are in the far-field and imping on the
array from distinct directions with the elevation and azimuth
angles {𝜃
𝑘
, 𝜙
𝑘
}. Then the received signals at the ULAs along
the 𝑧 and 𝑥 axes are given by
𝒛(𝑛) = 𝑨(𝜃)𝒔(𝑛) + 𝒘
𝑧
(𝑛) (1)
𝒙(𝑛) = 𝑨(𝜙)𝒔(𝑛) + 𝒘
𝑥
(𝑛) (2)