JIANG et al.: JOINT DOD AND DOA ESTIMATION FOR BISTATIC MIMO RADAR 5115
two receiver subarrays, the received signal at the th snapshot
= 1, 2 ...Lfrom all the P targets is given by
X
()
1
X
()
2
=
A
r1
(φ
1
)
A
r2
(φ
2
)
B
()
A
T
t1
(θ
1
)A
T
t2
(θ
2
)
S
1
S
2
+
Z
()
1
Z
()
2
(2)
where X
()
i
is the N
i
× K received data matrix in
the ith receiver subarray at the th snapshot, B
()
=
diag(β
1
()
...β
P
()
) is the target matrix with β
p
()
being the
zero-mean Gaussian reflection coefficient of the pth target at
the th snapshot, and A
ti
(θ
i
) is the transmitter steering matrix
for the ith transmitter subarray formed by all the P directional
vectors, i.e., A
ti
(θ
i
)=[a
ti
(θ
i1
) a
ti
(θ
i2
) ···a
ti
(θ
iP
)]. Likewise,
A
ri
(φ
i
)=[a
ri
(φ
i1
) a
ri
(φ
i2
) ···a
ri
(φ
iP
)] is the receiver
steering matrix, and Z
i
is the zero-mean complex N
i
× K
(space–time) noise matrix in the ith subarray at the th
snapshot, the covariance of which is such that
E
Z
()
i
Z
()H
j
=
R
z
i
z
i
i = j
0 i = j
,i,j=1, 2(3)
i.e., the two receiver subarrays are sufficiently separated such
that the noise in the two subarrays are uncorrelated. In addition,
R
z
i
z
i
, i = 1, 2 are assumed arbitrary and unknown covariance
matrices.
The received signal in (2) can be written in a more compact
form such that
X
()
= A
r
(φ)B
()
A
T
t
(θ)S + Z
()
(4)
where X
()
=[X
()T
1
X
()T
2
]
T
, S =[S
T
1
S
T
2
]
T
, Z
()
=
[Z
()T
1
Z
()T
2
]
T
, A
t
(θ)=[A
t1
(θ
1
)
T
A
t2
(θ
2
)
T
]
T
, and
A
r
(φ)=[A
r1
(φ
1
)
T
A
r2
(φ
2
)
T
]
T
. We note that, since A
r
(φ)
is (N
1
+ N
2
) × P , A
t
(θ) is (M
1
+ M
2
) × P , and S is
(M
1
+ M
2
) × K, then X
()
is of dimension (N
1
+ N
2
) × K,
and so is Z
()
. From (3) above, we can see that the (N
1
+
N
2
) × (N
1
+ N
2
) covariance matrix of Z
()
is given by
R
zz
=E
Z
()
1
Z
()
1
H
Z
()
1
Z
()
2
H
Z
()
2
Z
()
1
H
Z
()
2
Z
()
2
H
=
R
z
1
z
1
0
0R
z
2
z
2
. (5)
Since the M
i
complex waveforms transmitted by the ith trans-
mitter subarray are mutually orthogonal, then we can also see
S
i
S
H
j
=
KI
M
i
i = j
0 i = j
,i,j= 1, 2. (6)
This yields SS
H
= KI
M
.
B. Pulse Compression, Vectorization, and
Noise Characteristics
The purpose of pulse compression is to allow the radar to
employ a longer pulse to have larger radiated energy while
achieving the range resolution of a shorter pulse [14]. This
is accomplished by having the received signal processed by
a matched filter so that the duration of the long pulse is
compressed. Thus, pulse compression can be carried out by
applying the matched filter matrix (1/
√
K)S
H
on the received
signal in (4) so that the data matrix at the th snapshot, after
pulse compression, is given by
Y
()
=
√
KA
r
(φ)B
()
A
T
t
(θ)+
1
√
K
Z
()
S
H
=
Y
()
11
Y
()
12
Y
()
21
Y
()
22
(7)
where Y
()
ij
=
√
KA
ri
(φ
i
)B
()
A
T
tj
(θ
j
)+(1/
√
K)Z
()
i
S
H
j
,
i, j = 1, 2 are the four submatrices of Y
()
.
Denoting the vectorization operator of a matrix [15] by
vec(·), a composite data vector can be constructed by vector-
izing the four submatrices Y
()
ij
,giving
η
()
=
⎡
⎢
⎢
⎢
⎣
vec(Y
()
11
)
vec(Y
()
12
)
vec(Y
()
21
)
vec(Y
()
22
)
⎤
⎥
⎥
⎥
⎦
=
⎡
⎢
⎢
⎣
A
11
(φ
1
, θ
1
)
A
12
(φ
1
, θ
2
)
A
21
(φ
2
, θ
1
)
A
22
(φ
2
, θ
2
)
⎤
⎥
⎥
⎦
b
()
+
⎡
⎢
⎢
⎢
⎣
ζ
()
11
ζ
()
12
ζ
()
21
ζ
()
22
⎤
⎥
⎥
⎥
⎦
(8)
where A
ij
is (N
i
M
j
×P ) such that A
ij
(φ
i
, θ
j
)=[a
tj
(θ
j1
) ⊗
a
ri
(φ
i1
) ···a
tj
(θ
jP
) ⊗ a
ri
(φ
iP
)],b
()
=
√
K[β
()
1
β
()
2
···β
()
P
]
T
,
and ζ
()
=[ζ
()T
11
ζ
()T
12
ζ
()T
21
ζ
()T
22
]
T
, with ζ
()
ij
=(1/
√
K)
vec(Z
()
i
S
H
j
).
Let us examine the statistical characteristics of the noise
vector at the ith receiver array mixed with the jth matched
filter signal. We note that this vectorized subarray noise can be
written as [15]
ζ
()
ij
=
1
√
K
vec
Z
()
i
S
H
j
=
1
√
K
S
∗
j
⊗ I
N
j
vec
Z
()
i
.
(9)
Therefore, using some basic properties of Kronecker products
[15], their autocovariance and cross-covariance matrices are
given by
R
ζ
ij
ζ
mn
=E
ζ
()
ij
ζ
()
H
mn
=
1
K
S
∗
j
⊗ I
N
j
× E
vec
Z
()
i
vec
H
Z
()
m
S
T
n
⊗ I
N
n
=
1
K
S
∗
j
⊗ I
N
j
(I
K
⊗ R
z
i
z
m
)
S
T
n
⊗ I
N
n
=
1
K
S
∗
j
⊗ R
z
i
z
m
S
T
n
⊗ I
N
n
=
1
K
S
∗
j
S
T
n
⊗ R
z
i
z
m
=
I
M
j
⊗ R
z
i
z
i
, if i = m, j = n
0, if i = m or j = n
(10)
where, in the last step, the results of (3) and (6) have been used.