Method for processing an OFDM signal
View Patent ↗When decoding an OFDM signal, frame and frequency synchronisation is necessary. The invention shows a method enabling a high accuracy for frame and frequency synchronization, which leads to a very low error rate in comparison to prior art. The inventive method for processing an OFDM (Orthogonal Frequency Division Multiplex)-signal is based on calculating (S 5 ) a maximum of metric values (Λ) for arrangements between a reference pilot pattern (REF-PP) and a received pilot pattern (REC-PP), on determining the respective maximizing arrangement, and on determining (S 7 ) and maximizing integer frequency ({circumflex over (f)} I ) and time offsets ({circumflex over (T)} sy ) for frame synchronization and/or frequency synchronization (S 8 ) of said OFDM-signal, wherein the calculation of said metric values (Λ) is based on the evaluation of pairs (REF-PC-PAIR (P,(k, l); P(k, l+N l ))) of reference pilot cells (REF-PC), the members of said pairs (REF-PC-PAIR (P,(k, l); P(k, l+N l ))) corresponding to the same discrete frequency (k) and to different discrete times (l), and based on the signals (R(k, l), R(k, l+N l )) of corresponding received cells with respect to said discrete frequencies (k) and sad discrete times (l).
1. Method for processing an OFDM (Orthogonal Frequency Division Multiplex)-signal comprising the following steps:
receiving (S 1 ) an OFDM signal comprising received cells (REC-PC, REC-DC), each received cell (REC-PC, REC-DC) corresponding to a discrete frequency (k) and to a discrete time (l), wherein each of said received cells (REC-PC, REC-DC) is a received pilot cell (REC-PC) or a received data cell (REC-DC), said received pilot cells (REC-PC) corresponding to a received pilot pattern (REC-PP) of an OFDM-frame,
providing and/or generating (S 3 ) a reference pilot pattern (REF-PP) of reference pilot cells (REF-PC) each reference pilot cell (REF-PC) corresponding to a discrete frequency (k) and to a discrete time (l),
generating (S 4 ) at least one arrangement (XY) between said received pilot pattern (REC-PP) and said reference pilot pattern (REF-PP), each arrangement (XY) corresponding to an integer frequency offset (f I ) and an integer time offset (T sy ) between said received pilot pattern (REC-PP) and said reference pilot pattern (REF-PP),
calculating (S 5 ) a metric value (Λ) for each arrangement of said at least one arrangement (XY),
determining the maximum value of said metric values (Λ) and the respective maximizing arrangement,
determining (S 7 ) a maximizing integer frequency offset ({circumflex over (f)} I ) and a maximizing integer time offset ({circumflex over (T)} sy ) corresponding to the integer frequency offset and the integer time offset of the maximizing arrangement,
using said maximizing integer frequency offset ({circumflex over (f)} I ) and/or said maximizing integer time offset ({circumflex over (T)} sy ) for frequency offset compensation and/or time offset compensation (S 8 ) of said OFDM-signal,
wherein the calculation of said metric values (Λ) is based on the evaluation of groups (REF-PC-PAIR (P,(k, l); P(k, l+N l ))) of reference pilot cells (REF-PC), wherein at least two members of said groups (REF-PC-PAIR (P,(k, l); P(k, l+N l ))) correspond to the same discrete frequency (k) and to different discrete times (l), and based on the signals (R(k, l), R(k, l+N l )) of corresponding received cells (REC-PC, REC-DC) with respect to said discrete frequencies (k) and said discrete times (l).
2. Method according to claim 1 , characterized by
calculating a remaining fractional frequency error ({circumflex over (f)} f ),
wherein the calculation (T 9 ) of said remaining fractional frequency error ({circumflex over (f)} f ) is based on said metric value (Λ).
3. Method according to claim 2 , characterized by using said remaining fractional frequency error ({circumflex over (f)} f ) for an adaptation of a frequency offset compensation in time domain (T 2 , T 10 ).
4. Method according to claim 2 , characterized in that said calculation of said remaining fractional frequency error ({circumflex over (f)} f ) is based on the formula
f
^
f
=
-
1
T
s
·
N
l
·
2
π
·
∠
{
Λ
·
ⅇ
+
j
·
2
π
·
f
^
I
·
N
l
·
T
g
/
T
u
}
wherein
Λ denotes said metric value,
T s denotes the length of time of an OFDM symbol,
N l denotes a predetermined pilot cell time difference, which is the time distance between two neighboring reference pilot cells of said reference pilot pattern in time direction,
{circumflex over (f)} I denotes said integer frequency offset,
T g denotes the length of time of a guard interval of said OFDM symbol,
T u denotes length of time of a useful part of said OFDM symbol, and
∠{z} denotes the argument, i. e. the angle, of the complex number z.
5. Method according to claim 1 , characterized in that, said groups are pairs (REF-PC-PAIR (P,(k, l); P(k, l+N l ))) of reference pilot cells (REF-PC), the members of said pairs (REF-PC-PAR (P,(k, l); P(k, l+N l ))) corresponding to the same discrete frequency (k) and to different discrete times (l).
6. Method according to claim 1 , characterized in that, said first and second members of said pairs (REC-PC-PAIR (P,(k, l); P(k, l+N l ))) are in each case temporally separated by a predetermined pilot cell time distance (N l ).
7. Method for processing an OFDM (Orthogonal Frequency Division Multiplex) signal comprising the following steps:
receiving (T 1 ) an OFDM signal comprising received cells (REC-PC, REC-DC), each received cell (REC-PC, REC-DC) corresponding to a discrete frequency (k) and to a discrete time (l), wherein each of said received cells (REC-PC, REC-DC) is a received pilot cell (REC-PC) or a received data cell (REC-DC), said received pilot cells (REC-PC) corresponding to a received pilot pattern (REC-PP) of an OFDM-frame,
providing and/or generating (T 3 ) a reference pilot pattern (REF-PP) of reference pilot cells (REF-PC), each reference pilot cell (REF-PC) corresponding to a discrete frequency (k) and to a discrete time (l),
receiving, providing, and/or computing an integer frequency offset ({circumflex over (f)} I ) and an integer time offset ({circumflex over (T)} sy ),
receiving, providing, and/or computing a metric value (Λ) based on said received pilot pattern (REC-PP) and said reference pilot pattern (REF-PP), which metric value (Λ) depends on said integer frequency offset ({circumflex over (f)} I ), and said integer time offset ({circumflex over (T)} sy ) between said received pilot pattern (REC-PP) and said reference pilot pattern (REF-PP),
calculating a remaining fractional frequency error ({circumflex over (f)} f ),
wherein the calculation of said remaining fractional frequency error ({circumflex over (f)} f ) is based on said metric value (Λ).
8. Method according to claim 7 , characterized by using said remaining fractional frequency error ({circumflex over (f)} f ) for an adaptation of a frequency offset compensation in time domain (T 2 ,T 10 ).
9. Method according to claim 7 , characterized in that said calculation of said remaining fractional frequency error ({circumflex over (f)} f ) is based on the formula
f
^
f
=
-
1
T
s
·
N
l
·
2
π
·
∠
{
Λ
·
ⅇ
+
j
·
2
π
·
f
^
I
·
N
l
·
T
g
/
T
u
}
wherein
Λ denotes said metric value,
T s denotes the length of time of an OFDM symbol,
N l denotes a predetermined pilot cell time difference, which is the time distance between two neighboring reference pilot cells of said reference pilot pattern in time direction,
{circumflex over (f)} I denotes said integer frequency offset,
T g denotes the length of time of a guard interval of said OFDM symbol,
T u denotes length of time of a useful part of said OFDM symbol, and
∠{z} denotes the argument, i. e. the angle, of the complex number z.
10. Method according to claim 1 , characterized in that, said calculation (S 5 , T 5 ) of said metric values (Λ) is based on correlation techniques.
11. Method according to claim 1 , characterized in that, said calculation (S 5 , T 5 ) of said metric values (Λ) is based on products of said corresponding signals (R(k, l)) of received cells (REC-PC, REC-DC) and/or its conjugate complex values (R*(k, l+N l )).
12. Method according claim 1 , characterized in that, said calculation (S 5 , T 5 ) of said metric values (Λ) is based on products of exponential values of phase differences of reference pilot cells (REF-PC) of said groups (REF-PC-PAIR (P,(k, l); P(k, l+N l ))).
13. Method according to claim 1 , characterized in that, said calculation (S 5 , T 5 ) of said metric values (Λ) is based on a process of summation over at least two groups (REF-PC-PAIR (P,(k, l); P(k, l+N l ))) of reference pilot cells (REF-PC) within said reference pilot pattern (REF-PP).
14. Method according to claim 1 , characterized in that, said calculation (S 5 , T 5 ) of said metric values (Λ) is based on a process of summation over selected groups of reference pilot cells (REF-PC) within said reference pilot pattern (REF-PP), which groups are selected according to a selection criterion.
15. Method according to claim 14 , characterized in that, said selection criterion assures that only groups are chosen containing reference pilot cells (REF-PC) of said reference pilot pattern (REF-PP), which reference pilot cells (REF-PC) correspond to received cells (REC-PC; REC-DC) that have been received before a certain discrete time (l).
16. Method according to claim 14 , characterized in that, said selection criterion assures that only groups are chosen containing reference pilot cells (REF-PC) of said reference pilot pattern (REF-PP), which reference pilot cells (REF-PC) correspond to received cells (REC-PC; REC-DC) that correspond to discrete frequencies, at which a transmission channel has good transmission properties.
17. Method according to claim 1 , characterized in that, said calculation (S 5 , T 5 ) of said metric values (Λ) is based on a process of summation over all groups, in particular over all pairs (REF-PC-PAIR (P,(k, l);
P(k, l+N l ))), of reference pilot cells (REF-PC) within said reference pilot pattern (REF-PP).
18. Method according to claim 1 , characterized in that, said metric value is calculated (S 5 , T 5 ) by using the formula
Λ
=
∑
∀
(
k
,
l
)
∈
Γ
R
(
k
,
l
)
·
R
*
(
k
,
l
+
N
l
)
·
exp
(
j
·
2
π
·
(
∠
{
P
(
k
,
l
+
N
l
)
}
-
∠
{
P
(
k
,
l
)
}
)
)
wherein
R(k, l) denotes a signal of a received cell at discrete frequency k and discrete time l,
R*(k, l+N l ) denotes a conjugate complex signal corresponding to a signal of a received cell at discrete frequency k and discrete time (l+N l )
P(k, l) denotes a signal of reference pilot cell at discrete frequency k and discrete time l,
P(k, l+N l ) denotes a signal of reference pilot cell at discrete frequency k and discrete time (l+N l ),
Γ is the set of couples (k, l) of all discrete frequencies and all discrete times of said reference pilot pattern,
∠{z} denotes the angle of the complex number z, and
N l denotes the predetermined pilot cell time difference.
19. Method according to claim 1 , characterized in that, said metric value is calculated (S 5 , T 5 ) by using the formula
Λ
(
x
,
y
)
=
∑
∀
(
k
-
x
,
(
l
-
y
)
mod
N
sy
)
∈
Γ
[
R
(
k
,
l
)
·
R
*
(
k
,
l
+
N
l
)
·
exp
(
j
·
2
π
·
(
∠
{
P
(
k
-
x
,
(
l
+
N
l
-
y
)
mod
N
sy
)
}
-
∠
{
P
(
k
-
x
,
(
l
-
y
)
mod
N
sy
)
}
)
)
]
wherein
R(k, l) denotes a signal of a received cell at discrete frequency k and discrete time l,
R*(k, l+N l ) denotes a conjugate complex signal corresponding to a signal of a received cell at discrete frequency k and discrete time (l+N l )
P(k, l) denotes a signal of reference pilot cell at discrete frequency k and discrete time l,
P(k, l+N l ) denotes a signal of reference pilot cell at discrete frequency k and discrete time (l+N l ),
Γ is the set of couples (k, l) of all discrete frequencies and all discrete times of said reference pilot pattern,
∠{z} denotes the angle of the complex number z,
N l denotes the predetermined pilot cell time difference,
N sy denotes the number of OFDM symbols in one OFDM frame,
x denotes said integer frequency offset between said received pilot pattern (REC-PP) and said reference pilot pattern (REF-PP),
y denotes said integer time offset between said received pilot pattern (REC-PP) and said reference pilot pattern (REF-PP), and
mod denotes the modulo operator.
20. Method according to claim 1 , characterized in that, said received OFDM signal is subjected to a pre-processing method, wherein a fractional frequency offset and a fractional time offset is determined.
21. Method according to claim 20 , characterized in that, said pre-processing method (S 2 ) is based on an auto-correlation of said received OFDM signal in the time domain.
22. Method according to claim 1 , characterized in that, said received pilot cells (REC-PC) correspond to boosted pilot cells.
23. A receiver for performing or realizing a method for processing an OFDM signal according to claim 1 and/or the steps thereof, in particular according to the Digital Radio Mondiale (DRM) standard, the Digital Video Broadcasting-Terrestrial (DVB-T) standard, the Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) standard.
24. Signal processing system, for realizing a method for processing an OFDM signal according to claim 1 and/or the steps thereof.
25. Signal processing system according to claim 24 , comprising or based on an Application-Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA).
26. Computer program product, comprising computer program means adapted to perform and/or to realize the method for processing an OFDM signal according to claim 1 and/or the steps thereof when it is executed on a computer, a digital signal processing means.
27. Computer readable storage medium, comprising a computer program product according to claim 26 .