Apparatus and method for compensating for timing offset in spread spectrum system
An apparatus and method for compensating for a timing offset in a spread spectrum system which perform an oversampling of respective symbols to obtain a plurality of samples, estimate a chip clock timing offset and a symbol clock timing offset from the plurality of samples, and compensate for the chip clock timing offset and the symbol clock timing offset.
1. An apparatus for compensating for a timing offset in a spread spectrum system, the apparatus comprising:
an oversampling unit configured to oversample, by a multiple P, data whose each of N symbols is spread over and received by C chips;
an oversampled chip collection unit configured to collect C, which is a number of chips, samples at positions apart from each of all the oversampled samples by the multiple P, which is the oversampling multiple;
a despreading unit configured to despread each of collected oversampled chips using a unique code of a corresponding symbol;
a peak value detection/storage unit configured to detect an oversampled chip having a largest correlation value among despread oversampled chips in every predetermined section, and then store sample indices of the detected oversampled chips as peak value sample indices;
a timing offset calculation unit configured to calculate a chip clock timing offset O ct and a system clock timing offset O st using the respective peak value sample indices; and
a downsampling unit configured to downsample the oversampled samples while compensating for the chip clock timing offset and the system clock timing offset.
2. The apparatus of claim 1 , wherein the predetermined section is C*P samples.
3. The apparatus of claim 1 , wherein the chip clock timing offset O ct and the system clock timing offset O st are calculated based on a number of samples between a peak value sample index of each symbol and a peak value sample index of a first symbol.
4. The apparatus of claim 3 , wherein the chip clock timing offset O ct is calculated using an equation below:
O ct ( n )= C cto *n ( n is a symbol index),
where the chip clock timing offset coefficient C cto is calculated using an equation below:
C
cto
=
N
*
δ
-
β
*
α
N
*
γ
-
β
*
β
,
and
the system clock timing offset coefficient O st is calculated using an equation below:
O
st
=
α
*
δ
-
β
*
δ
N
*
γ
-
β
*
β
,
where d[n]=q[n]−q[0] (n=0, 1, . . . , N−1), e n =d[n]−n*P*C (n=0, 1, . . . , N−1),
α
=
∑
n
=
0
N
-
1
e
n
,
β
=
∑
n
=
0
N
-
1
n
=
N
*
(
N
-
1
)
2
,
γ
=
∑
n
=
0
N
-
1
n
2
=
N
*
(
N
-
1
)
*
(
2
N
-
1
)
6
,
δ
=
∑
n
=
0
N
-
1
(
e
n
*
n
)
,
q[0] denotes the peak value sample index of the first symbol, and q[n] denotes a peak value sample index of an n th symbol.
5. The apparatus of claim 4 , wherein a downsampling time t ds is calculated using an equation below:
t ds =n*P*C+P*c+C cto *n+O st ( n is a symbol index, and c is a chip index).
6. The apparatus of claim 1 , wherein a downsampled sample value V ds is calculated using an equation below:
V ds =V I *(1− F )+( V I+1 )* F ( I is an integer part of t ds , and F is a decimal part of t ds )
where V ds denotes a downsampled sample value compensated at the downsampling time t ds , and V I and V I+1 denote sample values downsampled at a time corresponding to the integer part I of the downsampling time t ds and a time corresponding to a sum of the integer part I and one (I+1), respectively.
7. A method of compensating for a timing offset in a spread spectrum system, the method comprising:
oversampling, by a multiple P, data whose each of N symbols is spread over and received by C chips;
collecting C, which is a number of chips, samples at positions apart from each of all the oversampled samples by the multiple P, which is the oversampling multiple;
despreading each of collected oversampled chips using a unique code of a corresponding symbol;
detecting an oversampled chip having a largest correlation value among despread oversampled chips in every predetermined section, and then storing sample indices of the detected oversampled chips as peak value sample indices;
calculating a chip clock timing offset O ct and a system clock timing offset O st using the respective peak value sample indices; and
downsampling the oversampled samples while compensating for the chip clock timing offset and the system clock timing offset.
8. The method of claim 7 , wherein the predetermined section is C*P samples.
9. The method of claim 7 , wherein the chip clock timing offset O ct and the system clock timing offset O st are calculated based on a number of samples between a peak value sample index of each symbol and a peak value sample index of a first symbol.
10. The method of claim 9 , wherein the chip clock timing offset O ct is calculated using an equation below:
O ct ( n )= C cto *n ( n is a symbol index),
where the chip clock timing offset coefficient C cto is calculated using an equation below:
C
cto
=
N
*
δ
-
β
*
α
N
*
γ
-
β
*
β
,
and
the system clock timing offset coefficient O st is calculated using an equation below:
O
st
=
α
*
δ
-
β
*
δ
N
*
γ
-
β
*
β
,
where d[n]=q[n]−q[0] (n=0, 1, . . . , N−1), e n =d[n]−n*P*C (n=0, 1, . . . , N−1),
α
=
∑
n
=
0
N
-
1
e
n
,
β
=
∑
n
=
0
N
-
1
n
=
N
*
(
N
-
1
)
2
,
γ
=
∑
n
=
0
N
-
1
n
2
=
N
*
(
N
-
1
)
*
(
2
N
-
1
)
6
,
δ
=
∑
n
=
0
N
-
1
(
e
n
*
n
)
,
q[0] denotes the peak value sample index of the first symbol, and q[n] denotes a peak value sample index of an n th symbol.
11. The method of claim 10 , wherein a downsampling time t ds is calculated using an equation below:
t ds =n*P*C+P*c+C cto *n+O st ( n is a symbol index, and c is a chip index).
12. The method of claim 7 , wherein a downsampled sample value V ds is calculated using an equation below:
V ds =V I *(1− F )+( V I+1 )* F ( I is an integer part of t ds , and F is a decimal part of t ds )
where V ds denotes a downsampled sample value compensated at the downsampling time t ds , and V I and V I+1 denote sample values downsampled at a time corresponding to the integer part I of the downsampling time t ds and a time corresponding to a sum of the integer part I and one (I+1), respectively.