Adaptive channel estimator and adaptive channel estimation method
An adaptive channel estimator for estimating channel frequency responses when receiving OFDM symbols. The adaptive channel estimator comprises a channel estimator and a channel tracking module. The channel estimator generates one first channel frequency response for each OFDM symbol when operated in the first mode, and generates the channel frequency responses of a second channel frequency response for every N-th OFDM symbols when operated in the second mode, wherein N is a constant integer exceeding one and j is a non-negative integer. The channel tracking module indicates a channel status, and controls the channel estimator to operate in a first mode or a second mode according to the channel status.
1. An adaptive channel estimator for estimating channel frequency responses when receiving OFDM symbols, comprising:
a channel estimator generating a first channel frequency response for each one of a plurality of first OFDM symbols when operated in a first mode wherein the first OFDM symbols are continuous in time axis, and generating a second channel frequency response for each one of a plurality of second OFDM symbols when operated in a second mode, wherein each one of the second OFDM symbols is separated from the other one of the second OFDM symbols by M−1 non-second symbols in time axis and M is a constant integer exceeding one; and
a channel tracking module indicating a channel status, and controlling the channel estimator to selectively operate in the first mode or the second mode according to the channel status, wherein the channel status is obtained by the channel tracking module, which monitors a function of time variation of the channel.
2. The adaptive channel estimator as claimed in claim 1 , wherein each of the first OFDM symbols, the second OFDM symbols and the non-second OFDM symbols comprises a plurality of pilot sub-carriers, a plurality of data sub-carriers and pilot sub-carriers, or a plurality of data sub-carriers.
3. The adaptive channel estimator as claimed in claim 1 , wherein the channel status can be identified according to a channel status indicator C I (m) which can be expressed as:
C
I
(
m
)
=
(
1
-
λ
)
·
C
I
(
m
-
1
)
+
λ
·
W
(
m
)
D
(
m
)
,
where
W
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
Re
(
h
k
(
m
,
0
)
)
2
+
Im
(
h
k
(
m
,
0
)
)
2
}
,
D
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
[
Re
(
h
k
(
m
,
0
)
)
-
Re
(
h
k
(
m
,
L
)
)
]
2
+
[
Im
(
h
k
(
m
,
0
)
)
-
Im
(
h
k
(
m
,
L
)
)
]
2
}
,
P represents the summation number of pilot sub-carriers, h k (m,n) denotes the channel estimation of the n-th symbol in the m-th frame, k is sub-carrier index, and λ is a smoothing factor.
4. The adaptive channel estimator as claimed in claim 3 , wherein when the C I (m) is less than a predetermined threshold, the channel estimator is set to operate in the first mode; when the C I (m) exceeds the predetermined threshold, the channel estimator is set to operate in the second mode.
5. The adaptive channel estimator as claimed in claim 1 , wherein the channel status can be identified according to a moving velocity of the channel estimator.
6. The adaptive channel estimator as claimed in claim 1 , wherein the channel status is updated when receiving a new OFDM frame.
7. The adaptive channel estimator as claimed in claim 1 , wherein the channel estimator further comprises a filter linearly filtering the second channel frequency responses of the second OFDM symbols to generate the second channel frequency responses of the non-second OFDM symbols.
8. The adaptive channel estimator as claimed in claim 7 , wherein the second channel frequency responses of the non-second OFDM symbols are generated by performing the following equation:
h
k
(
j
*
N
-
i
)
=
N
-
i
N
h
k
(
(
j
-
1
)
*
N
)
+
i
N
h
k
(
j
*
N
)
,
wherein h k (0) and h k (M) of the equation are the second channel frequency responses of k-th sub-carrier in the 0-th and M-th OFDM symbol, respectively, and M is the span of the second OFDM symbols performing the equation.
9. The adaptive channel estimator as claimed in claim 7 , wherein the second channel frequency responses of the non-second OFDM symbols are generated by performing the following equation:
h
k
(
j
*
N
-
i
)
=
N
+
i
N
h
k
(
(
j
-
1
)
*
N
)
-
i
N
h
k
(
(
j
-
2
)
*
N
)
,
wherein h k (0) and h k (M) of the equation are the second channel frequency responses of k-th sub-carrier in the 0-th and M-th OFDM symbol, respectively, and M is the span of the second OFDM symbols performing the equation.
10. The adaptive channel estimator as claimed in claim 7 , wherein the adaptive channel estimator can be further operated in a third mode; when operated in the third mode, the adaptive channel estimator generates the second channel frequency responses for the second OFDM symbols and generates the second channel frequency responses of the non-second OFDM symbols by utilizing the second channel frequency responses of one of the second OFDM symbols.
11. The adaptive channel estimator as claimed in claim 10 , wherein the channel status can be identified according to a channel status indicator C I (m) which can be expressed as:
C
I
(
m
)
=
(
1
-
λ
)
·
C
I
(
m
-
1
)
+
λ
·
W
(
m
)
D
(
m
)
,
wherein
W
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
Re
(
h
k
(
m
,
0
)
)
2
+
Im
(
h
k
(
m
,
0
)
)
2
}
,
D
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
[
Re
(
h
k
(
m
,
0
)
)
-
Re
(
h
k
(
m
,
L
)
)
]
2
+
[
Im
(
h
k
(
m
,
0
)
)
-
Im
(
h
k
(
m
,
L
)
)
]
2
}
,
P represents the summation number of pilot sub-carriers, h k (m,n) denotes the channel estimation of the n-th symbol in the m-th frame, k is sub-carrier index, λ is a smoothing factor, and L is a constant exceeding one, when the C I (m) is less than a first predetermined threshold, the channel estimator is set to operate in the first mode; when the C I (m) is not lower than the first predetermined threshold for consecutive J frames and the C I (m) of the previous L consecutive frames are also higher than a second predetermined threshold, the channel estimator is set to operate in the third mode; otherwise, the channel estimator is set to operate in the second mode, wherein J and L are positive integers.
12. The adaptive channel estimator as claimed in claim 1 , wherein M is 4.
13. An adaptive channel estimating method, comprising:
detecting a channel statistic metric;
determining one operation mode from a plurality of operation modes according to the channel statistic metric, wherein the channel statistic metric is a function of time variation of a channel;
generating a first channel frequency response for each one of a plurality of first OFDM symbols when operating in a first mode, wherein the first OFDM symbols are continuous in time axis; and
generating a second channel frequency response for each one of a plurality of second OFDM symbols when operating in a second mode, wherein each one of the second OFDM symbols is separated from the other one of the second OFDM symbols by M−1 non-second symbols in time axis and M is a constant exceeding one.
14. The adaptive channel estimating method as claimed in claim 13 , wherein each of the first OFDM symbols, the second OFDM symbols and the non-second OFDM symbols comprises a plurality of pilot sub-carriers, a plurality of data sub-carriers and pilot sub-carriers, or a plurality of data sub-carriers.
15. The adaptive channel estimating method as claimed in claim 13 , further comprising determining to operate in the first mode when the channel statistic metric is less than a predetermined threshold value; otherwise, determining to operate in the second mode.
16. The adaptive channel estimating method as claimed in claim 13 , wherein the channel statistic metric of the m-th OFDM frame C I (m) can be expressed as:
C
I
(
m
)
=
(
1
-
λ
)
·
C
I
(
m
-
1
)
+
λ
·
W
(
m
)
D
(
m
)
,
wherein
W
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
Re
(
h
k
(
m
,
0
)
)
2
+
Im
(
h
k
(
m
,
0
)
)
2
}
,
D
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
[
Re
(
h
k
(
m
,
0
)
)
-
Re
(
h
k
(
m
,
L
)
)
]
2
+
[
Im
(
h
k
(
m
,
0
)
)
-
Im
(
h
k
(
m
,
L
)
)
]
2
}
,
P represents the summation number of pilot sub-carriers, h k (m,n) denotes the channel estimation of the n-th symbol in the m-th frame, k is sub-carrier index, and λ is a smoothing factor.
17. The adaptive channel estimating method as claimed in claim 16 , wherein when the C I (m) is less than a predetermined threshold, the operation mode is set to operate in the first mode; otherwise, the operation mode is set to operate in the second mode.
18. The adaptive channel estimating method as claimed in claim 13 , wherein the operation mode is updated when receiving a new OFDM frame.
19. The adaptive channel estimating method as claimed in claim 13 , further comprising linearly filtering the second channel frequency responses of the second OFDM symbols to generate the second channel frequency responses of the non-second OFDM symbols.
20. The adaptive channel estimating method as claimed in claim 19 , wherein the second channel frequency responses of the non-second OFDM symbols are generated by performing the following equation:
h
k
(
n
)
=
M
-
n
M
h
k
(
0
)
+
n
M
h
k
(
M
)
,
n
=
1
,
2
,
…
,
M
-
1
,
wherein h k (0) and h k (M) of the equation are the second channel frequency responses of k-th sub-carrier in the 0-th and M-th OFDM symbol, respectively, and M is the span of the second OFDM symbols performing the equation.
21. The adaptive channel estimating method as claimed in claim 19 , wherein the second channel frequency responses of the non-second OFDM symbols are generated by performing the following equation:
h
k
(
n
)
=
M
+
n
M
h
k
(
0
)
-
n
M
h
k
(
M
)
,
n
=
1
,
2
,
…
,
M
-
1
,
wherein h k (0) and h k (M) of the equation are the second channel frequency response responses of k-th sub-carrier in the 0-th and M-th OFDM symbol, respectively, and M is the span of the second OFDM symbols performing the equation.
22. The adaptive channel estimating method as claimed in claim 19 , further comprising generating the second channel frequency responses for the second OFDM symbols and generating the second channel frequency responses of the non-second OFDM symbols in a third mode by utilizing the second channel frequency response of one of the second OFDM symbols.
23. The adaptive channel estimating method as claimed in claim 22 , wherein the channel statistic metric can be identified according to a channel status indicator C I (m) which can be expressed as:
C
I
(
m
)
=
(
1
-
λ
)
·
C
I
(
m
-
1
)
+
λ
·
W
(
m
)
D
(
m
)
,
wherein
W
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
Re
(
h
k
(
m
,
0
)
)
2
+
Im
(
h
k
(
m
,
0
)
)
2
}
,
D
(
m
)
=
1
P
∑
k
=
0
P
-
1
{
[
Re
(
h
k
(
m
,
0
)
)
-
Re
(
h
k
(
m
,
L
)
)
]
2
+
[
Im
(
h
k
(
m
,
0
)
)
-
Im
(
h
k
(
m
,
L
)
)
]
2
}
,
P represents the summation number of pilot sub-carriers, h k (m,n) denotes the channel estimation of the n-th symbol in the m-th frame, k is sub-carrier index, λ is a smoothing factor, and L is a constant exceeding one, when the C I (m) is less than a first predetermined threshold, the operation mode is set to operate in the first mode; when the C I (m) is not lower than the first predetermined threshold for J consecutive frames and the C I (m) of the previous L consecutive frames are also higher than a second predetermined threshold, the operation mode is set to operate in the third mode; otherwise, the operation mode is set to operate in the second mode, wherein J and L are positive integers.
24. The adaptive channel estimating method as claimed in claim 13 , wherein M is 4.