The problem is then generally solved for a analytically by [19]
a ¼ðF
T
WFÞ
1
F
T
Wd: ð8Þ
In the WLS design method described above, the length and width of the frequency grid are
recommended as M
1
Z 4N
1
and M
2
Z 4N
2
for a tradeoff between the approximation accuracy
and computational complexity. When M
1
and M
2
are large, a large memory space and long CPU
time would be needed to compute the high-dimensional coefficient vector a from (8).
2.2. DCT-based GCG algorithm
The impulse response coefficients, hðn
1
; n
2
Þ's, of a 2-D FIR filter and their associated
coefficients, a
ij
's, are naturally in matrix forms. In the following sections, the WLS design
problem of 2-D FIR filters will be reformulated in the way that the filter coefficients a
ij
's are in a
matrix form, rather than the vector form as in the previous subsection. To that end, we formulate
the magnitude response (2) of a quadrantally symmetric filter as
Mðω
1
; ω
2
Þ¼ψ
T
ðω
1
; N
1
ÞAψðω
2
; N
2
Þ; ð9Þ
where ψ ðω; NÞ is a vector given by
ψω; NðÞ¼
1
ffiffi
2
p
; cos ωðÞ; cos 2ωðÞ; …; cos
N 1
2
ω
hi
T
; for odd N;
cos
ω
2
; cos
3ω
2
; …; cos
N 1
2
ω
T
; for even N;
8
>
>
>
<
>
>
>
:
and A is the L
1
L
2
real coefficient matrix of the filter with its ij-th entry being a
ij
.
Aravena and Gu [13] considered the WLS approximation of 2-D linear-phase FIR filters on a
uniform frequency grid:
ω
1i
; ω
2j
¼
2i1
2M
1
π;
2j1
2M
2
π
; i ¼1; 2; …; M
1
; j ¼ 1; 2; …; M
2
: ð10Þ
For quadrantally symmetric filters, the approximation problem formulated in [13] can be
reformulated as follows:
min
A
1
2
〈A; p AðÞ〉〈A; D
w
〉 þ c ð11Þ
where c is a constant, D
w
is an L
1
L
2
matrix given by
D
w
¼
4
M
1
M
2
X
M
1
i ¼ 1
X
M
2
j ¼ 1
D ω
1i
; ω
2j
W ω
1i
; ω
2j
ψω
1i
; N
1
ðÞψ
T
ω
2j
; N
2
;
pðÞ is a linear operat or acting on the L
1
L
2
real matrix space R
L
1
L
2
defined by
p XðÞ¼
4
M
1
M
2
X
M
1
i ¼ 1
X
M
2
j ¼ 1
ψ
T
ω
1i
; N
1
ðÞXψω
2j
; N
2
Wðω
1i
; ω
2j
Þψðω
1i
; N
1
Þψ
T
ðω
2j
; N
2
Þ; XA R
L
1
L
2
; ð12Þ
R. Zhao et al. / Journal of the Franklin Institute 353 (2016) 1759–1780 1763