IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS, VOL. XX, NO. YY, AUGUST 2014 3
Fig. 2. The permissible regions for (a) the elevation angle θ
k
(n) and the
azimuth angle ϕ
k
(n) and (b) the elevation angle θ
k
(n) and the projected
azimuth angle
¯
ϕ
k
.
In this paper, the following basic assumptions are made on
the data model.
A1) The mathematical model of array response matrices
(i.e., A(θ) and A(ϕ)) is known, and the sensor spacing
d satisfies 0 < d ≤ λ/2 to avoid angle ambiguity.
A2) For facilitating the theoretical performance analysis,
the incident signals {s
k
(n)} are temporally complex
white Gaussian random processes with zero-mean and
the variance are given by E{s
k
(n)s
∗
k
(t)} = r
s
k
δ
n,t
and E{s
k
(n)s
k
(t)} = 0 ∀n, t for 1 ≤ k ≤ p.
A3) The additive noises {w
z
i
(n)} and {w
x
i
(n)} are tem-
porally and spatially complex white Gaussian ran-
dom processes with zero-mean and the covariance
matrices are given by E{w
z
(n)w
H
z
(t)} = E{w
x
(n)
·w
H
x
(t)} = σ
2
I
M
δ
n,t
and E{w
z
(n)w
T
z
(t)} =
E{w
x
(n)w
T
x
(t)} = O
M×M
∀n, t, and they
are statistically independent with each other, i.e.,
E{w
z
(n)w
H
x
(t)} = O
M×M
.
A4) The additive noises {w
z
i
(n)} and {w
x
i
(n)} at two
ULAs are statistically independent with the incident
signals {s
k
(n)}.
A5) The number of incident signals p is known or estimated
by number detection techniques in advance (cf. [42],
[43], and it satisfies the inequality that p < M.
From the relationship cos(ϕ
k
(n)) = cos(
¯
ϕ
k
(n)) sin(θ
k
(n))
[10], [37], we easily find the permissible region for θ
k
(n) and
ϕ
k
(n) and that for θ
k
(n) and
¯
ϕ
k
(n) as shown in Fig. 2(a)
and 2(b), respectively, while the geometry restrictions require
that the parameters θ
k
(n) and ϕ
k
(n) lie in the square region
defined by
−θ
k
(n) + 90
◦
≤ ϕ
k
(n) ≤ θ
k
(n) + 90
◦
,
for 0
◦
≤ θ
k
(n) ≤ 90
◦
θ
k
(n) − 90
◦
≤ ϕ
k
(n) ≤ −θ
k
(n) + 270
◦
,
for 90
◦
≤ θ
k
(n) ≤ 180
◦
.
(3)
The classical 1-D subspace-based direction estimation meth-
ods with eigendecomposition (e.g., MUSIC [26], estimation
of signal parameters via rational invariance techniques (ES-
PRIT) [44]) and the computationally simple 1-D subspace-
based direction estimators without eigendecomposition (e.g.,
propagator method (PM) [45], subspace-based method without
eigendecomposition (SUMWE) [39]) can be applied to each
ULA to obtain the reliable estimates of azimuth and elevation
angles separately. However, in general, there are p! possible
combinations between the estimates {
ˆ
ϕ
k
(n)} and {
ˆ
θ
i
(n)}.
Consequently the crux of 2-D direction estimation is the
pair-matching of the azimuth and elevation angles estimated
independently, which can require a tremendous computational
burden when the number of incident signals is large. Except
for the CODE [40], most of the existing techniques for pair-
matching or automatic pairing involve the computationally
extensive eigendecomposition process [5]–[8], [10], [29], [30],
[32]–[34], [36], [37]. Even though the pair-matching is accom-
plished correctly, the separate estimation of the azimuth and
elevation angles may cause the estimated angles
ˆ
θ
k
(n) and
ˆ
ϕ
k
(n) lie outside the permissible region shown in Fig. 2(a),
or equivalently we may have |cos(
ˆ
ϕ
k
(n))/sin(
ˆ
θ
k
(n))| > 1 for
sin
ˆ
θ
k
= 0, and obviously the estimate of the conventional
azimuth angle
¯
ϕ
k
(n) is unavailable with the relation
¯
ϕ
k
(n) =
arccos( cos(ϕ
k
(n))/sin(θ
k
(n)) ). Thus the estimation failure
occurs in this situation [10]. Unfortunately, the estimation
failure has not been resolved yet for the L-shaped array.
Therefore in order to overcome the aforementioned pairing
and estimation failures, we focus our attention on the joint
azimuth-elevation DOA estimation without eigendecomposi-
tion process and pair-matching procedure.
Remark 1: For decoupling the 2-D DOA estimation prob-
lem into 1-D estimation problems and fully exploiting the
well-known property of the ULA in a more straightforward
way, we parameterize the 2-D direction of the incident signals
by (θ
k
(n), ϕ
k
(n)) instead of (θ
k
(n),
¯
ϕ
k
(n)). In fact, by re-
defining the parameter θ
k
(n) as the angle of the signal s
k
(n)
with respective to the y axis, the proposed CODEC method is
also applicable to the L-shaped array placed in the x–y plane.
However in order to compare the performance of the CODEC
method with the existing methods, we still concentrate on the
problem of 2-D DOA estimation with L-shaped array placed
in the x–z plane in this paper.
III. BATCH METHOD FOR 2-D DOA E STIMATION
A. Estimation of Elevation Angles
Firstly by assuming the elevation and azimuth angles be
time-invariant, i.e., θ
k
(n) = θ
k
and ϕ
k
(n) = ϕ
k
, we can pro-
pose a new computationally efficient batch 2-D DOA estima-
tion method without eigendecomposition and pair-matching,
which is suitable for DOA tracking. Under Assumption A5, we
can divide the ULA along the z axis into two nonoverlapping
forward subarrays with p or M − p sensors, and the received
signal vector z(n) in (1) can be rewritten as
z(n) = [
¯
z
T
1
(n),
¯
z
T
2
(n)]
T
= [A
T
1
(θ), A
T
2
(θ)]
T
s(n) + [w
T
¯z
1
(n), w
T
¯z
2
(n)]
T
(4)
where
¯
z
1
(n)
△
= [z
0
(n), z
1
(n), · · · , z
p−1
(n)]
T
,
¯
z
2
(n)
△
=
[z
p
(n), z
p+1
(n), · · · , z
M−1
(n)], w
¯z
1
(n)
△
= [w
z
0
(n), w
z
1
(n),
· · · , w
z
p−1
(n)]
T
, and w
¯z
2
(n)
△
= [w
z
p
(n), w
z
p+1
(n), · · · ,
w
z
M−1
(n)]
T
, while A(θ) in (1) is divided into two sub-
matrices A
1
(θ) and A
2
(θ) with the columns given by
a
1
(θ
k
)
△
= [1, e
jα
k
, · · · , e
j(p−1)α
k
]
T
and a
2
(θ
k
)
△
= [ e
jpα
k
,
e
j(p+1)α
k
, · · · , e
j(M−1)α
k
]
T
. Then under the assumptions of
data model, from (1), (2) and (4), we easily obtain the cross-
correlation matrix R
zx
between the received signals of two