Method, apparatus and computer program product providing synchronization for OFDMA downlink signal
View Patent ↗Disclosed is a method, a computer program product and a device that includes a receiver for receiving a downlink signal transmitted into a cell. The receiver is operable to obtain time, carrier frequency and cell-specific preamble synchronization to the received signal and includes a plurality of synchronization units that include a first detector to detect a frame boundary using preamble delay correlation; a second detector to detect the frame boundary with greater precision using a conjugate symmetry property over a region identified by the first detector; a cyclic prefix correlator to resolve symbol boundary repetition; an estimator, using the cyclic prefix, to estimate and correct a fractional carrier frequency offset; an operator to perform a Fast Fourier Transform of an identified preamble symbol and a frequency domain cross-correlator to identify cell-specific preamble sequences and an integer frequency offset in sub-carrier spacing. The transmitted signal may be a downlink signal transmitted into the cell from a base station that is compatible with IEEE 802.16e (WiMAX).
1. A method, comprising:
obtaining time, carrier frequency and cell-specific preamble synchronization to a signal received by a radio frequency receiver by
detecting by the radio frequency receiver a frame boundary using preamble delay correlation;
detecting by the radio frequency receiver the frame boundary with greater precision using a conjugate symmetry property over a region identified during the first step of detecting the frame boundary;
the radio frequency receiver using cyclic prefix correlation to resolve symbol boundary repetition;
the radio frequency receiver using the cyclic prefix to estimate and correct a fractional carrier frequency offset;
the radio frequency receiver performing a fast fourier transform of an identified preamble symbol; and
the radio frequency receiver performing a frequency domain cross-correlation to identify cell-specific preamble sequences and an integer frequency offset in sub-carrier spacing.
2. The method of claim 1 , where the received signal is a downlink signal received from a base station that is compatible with IEEE 802.16e.
3. The method of claim 1 ,
where detecting the frame boundary using preamble delay correlation provides an estimate of an initial start of the frame, and comprises a use of a plurality of multiply-and-accumulate operations per received signal sample.
4. The method of claim 1 ,
where detecting the frame boundary with greater precision comprises performing a conjugate symmetry search over indices that exceed a preamble delay correlation threshold, where the preamble exhibits conjugate symmetric structure, and where the conjugate symmetry search returns peaks at interval defined by a preamble repetition, P.
5. The method of claim 4 , further comprising computing a global maximum among all peaks that exceed a second threshold.
6. The method of claim 4 , comprising reducing a complexity of the conjugate symmetry search by at least one of: optimizing the threshold, performing a partial search, and performing the conjugate symmetry search to identify conjugate symmetry within a 1/P-th portion of the preamble symbol that is repeated P-times to make a single preamble symbol.
7. The method of claim 5 ,
where using cyclic prefix correlation to resolve symbol boundary repetition comprises resolving a condition where the global maximum does not correspond to an exact preamble start but instead corresponds to a peak that is at a distance of integer multiple of 1/P-th of the preamble symbol from the correct preamble start, and further comprises executing the cyclic prefix search only for those received signal samples that are at a distance multiple of N FFT /P from an index value returned by the conjugate symmetry search.
8. The method of claim 7 , where the cyclic prefix correlation is measured for n×N FFT /P samples with nε[−P/2, P/2] and averaged over M OFDM symbols and where the frame boundary is estimated by,
τ
^
=
τ
^
CP
=
arg
max
n
(
X
CP
(
n
)
2
P
(
n
)
)
,
P
(
n
)
=
Received
Power
,
where {circumflex over (τ)} is an estimated frame start, {circumflex over (τ)} CP is a frame start cyclic prefix, and X CP is a cyclic prefix value.
9. The method of claim 1 , where using the cyclic prefix to estimate and correct the fractional carrier frequency offset comprises estimating the fractional frequency offset in the time domain by estimating a phase of the cyclic prefix correlation.
10. The method of claim 9 , where the cyclic prefix correlation is averaged for M OFDM symbols, where {circumflex over (τ)} is the estimated frame start, and where the normalized phase due to frequency offset is estimated as,
ϕ
^
Δ
f
=
tan
-
1
(
∑
m
=
0
M
-
1
X
CP
(
τ
^
+
m
(
N
FFT
+
N
CP
)
)
)
,
where the value of M is within a downlink frame boundary, {circumflex over (τ)} is an estimated frame start, {circumflex over (φ)} Δf is a normalized phase due to frequency offset, N FFT is fast fourier transform buffer size, N CP is a cyclic prefix size, and m is a subcarrier index.
11. The method of claim 1 , where performing the fast fourier transform of an identified preamble symbol performs the fast fourier transform on a fractional frequency offset corrected received preamble symbol, based on the estimated frame start.
12. The method of claim 1 , where performing the frequency domain cross-correlation to identify cell-specific preamble sequences and the integer frequency offset in sub-carrier spacing is accomplished using an assumption that an integer frequency offset range of [−N FO , N FO ], a total of (2N FO +1)×N Seq hypotheses need to be tested in order to determine both the preamble and integer frequency offset, and by letting,
R P ( k )≡ fft ( r p ( n )), k,nε[ 0 ,N FFT −1] and,
S P ( k )≡ fft ( s p ( n )), k,nε[ 0 ,N FFT −1],
where the frequency domain search yields,
〈
i
^
,
m
^
〉
=
arg
max
i
,
m
(
∑
k
=
0
N
FFT
-
1
R
P
(
k
)
×
(
S
m
P
(
k
)
)
i
2
∑
k
=
0
N
FFT
-
1
R
P
(
k
)
2
)
,
m
=
0
,
1
,
…
N
Seq
i
∈
[
-
N
FO
,
N
FO
]
,
where (v(n)) i denotes the vector v(n), circularly shifted by i values and N Seq denotes total number of different preamble sequence present in the given preamble sequence set, and where R p is a received preamble signal, S p is a preamble structure, N FO is an integer frequency offset, N FFT is a fast fourier transform buffer size, N Seq denotes a total number of different preamble sequence present in the given preamble sequence set, k is a subcarrier index, and m is a subcarrier index.
13. The method of claim 12 , where to counter an effect of multipath coherent combining is implemented only over adjacent sub-carriers, and where correlation values of different groups of sub-carriers are combined in a non-coherent manner, where if G consecutive sub-carriers are grouped together, then,
〈
i
^
,
m
^
〉
=
arg
max
i
,
m
(
∑
q
=
0
N
FFT
G
-
1
∑
g
=
0
G
-
1
R
P
(
qG
+
g
)
×
(
S
m
P
(
qG
+
g
)
)
i
2
∑
k
=
0
N
FFT
-
1
R
P
(
k
)
2
)
,
m
=
0
,
1
,
…
N
Seq
i
∈
[
-
N
FO
,
N
FO
]
,
and where the value of G is determined for a channel having a longest delay spread, G is a guard interval, R p is a received preamble signal, and S p is a preamble structure.
14. The method of claim 1 , executed by a mobile station that receives a downlink signal from a base station.
15. A computer readable medium storing a computer program, execution of the computer program by a data processor resulting in operations that comprise obtaining time, carrier frequency and cell-specific preamble synchronization to a received signal by:
detecting a frame boundary using preamble delay correlation;
detecting the frame boundary with greater precision using a conjugate symmetry property over a region identified during the first operation of detecting the frame boundary;
using cyclic prefix correlation to resolve symbol boundary repetition;
using the cyclic prefix to estimate and correct a fractional carrier frequency offset;
performing a fast fourier transform of an identified preamble symbol; and
performing a frequency domain cross-correlation to identify cell-specific preamble sequences and an integer frequency offset in sub-carrier spacing.
16. The computer readable medium storing the computer program of claim 15 , where the received signal is a downlink signal received from a base station that is compatible with IEEE 802.16e.
17. The computer readable medium storing the computer program of claim 15 , where the operation of detecting the frame boundary using preamble delay correlation provides an estimate of an initial start of the frame, and comprises a use of a plurality of multiply-and-accumulate operations per received signal sample.
18. The computer readable medium storing the computer program of claim 15 , where the operation of detecting the frame boundary with greater precision comprises performing a conjugate symmetry search over indices that exceed a preamble delay correlation threshold, where the preamble exhibits conjugate symmetric structure and where the conjugate symmetry search returns a peak at interval defined by a preamble repetition, P.
19. The computer readable medium storing the computer program of claim 18 , further comprising computing a global maximum among all peaks that exceed a second threshold.
20. The computer readable medium storing the computer program of claim 18 , comprising reducing a complexity of the conjugate symmetry search by at least one of: optimizing the threshold, performing a partial search, and performing the conjugate symmetry search to identify conjugate symmetry within a 1/P-th portion of the preamble symbol that is repeated P-times to make a single preamble symbol.
21. The computer readable medium storing the computer program of claim 19 , where the operation of using cyclic prefix correlation to resolve symbol boundary repetition comprises resolving a condition where the global maximum does not correspond to an exact preamble start but instead corresponds to a peak that is at a distance of integer multiple of 1/P-th of the preamble symbol from the correct preamble start, and further comprises executing the cyclic prefix search only for those received signal samples that are at a distance multiple of N FFT /P from an index value returned by the conjugate symmetry search.
22. The computer readable medium storing the computer program of claim 21 , where cyclic prefix correlation is measured for n×N FFT /P samples with nε[−P/2, P/2] and averaged over M orthogonal frequency division multiplexing symbols and where the frame boundary is estimated by,
τ
^
=
τ
^
CP
=
arg
max
n
(
X
CP
(
n
)
2
P
(
n
)
)
,
P
(
n
)
=
Recieved
Power
where {circumflex over (τ)} is an estimated frame start, {circumflex over (φ)} Δf is a normalized phase due to frequency offset, N FFT is a fast fourier transform buffer size, X CP is a cyclic prefix value, and {circumflex over (τ)} CP is a frame start cyclic prefix.
23. The computer readable medium storing the computer program of claim 15 , where the operation of using the cyclic prefix to estimate and correct the fractional carrier frequency offset comprises estimating the fractional frequency offset in the time domain by estimating a phase of the cyclic prefix correlation.
24. The computer readable medium storing the computer program of claim 23 , where the cyclic prefix correlation is averaged for M orthogonal frequency division multiplexing symbols, where {circumflex over (τ)} is the estimated frame start, and where the normalized phase due to frequency offset is estimated as,
ϕ
^
Δ
f
=
tan
-
1
(
∑
m
=
0
M
-
1
X
C
P
(
τ
^
+
m
(
N
F
F
T
+
N
C
P
)
)
)
,
where the value of M is within a downlink frame boundary, {circumflex over (φ)} Δf is a normalized phase due to frequency offset, X CP is a cyclic prefix value, N FFT is a fast fourier transform buffer size, N CP is a cyclic prefix size, and m is a subcarrier index.
25. The computer readable medium storing the computer program of claim 15 , where performing the fast fourier transform of an identified preamble symbol performs the fast fourier transform on a fractional frequency offset corrected received preamble symbol, based on the estimated frame start.
26. The computer readable medium storing the computer program of claim 15 , where performing the frequency domain cross-correlation to identify cell-specific preamble sequences and the integer frequency offset in sub-carrier spacing is accomplished using an assumption that an integer frequency offset range of [−N FO , N FO ], a total of (2N FO +1)×N Seq hypotheses need to be tested in order to determine both the preamble and integer frequency offset, and by letting,
R P ( k )≡ fft ( r p ( n )), k,nε[ 0 ,N FFT −1] and,
S P ( k )≡ fft ( s p ( n )), k,nε[ 0 ,N FFT −1],
where the frequency domain search yields,
〈
i
^
,
m
^
〉
=
a
r
g
max
i
,
m
(
∑
k
=
0
N
F
F
T
-
1
R
P
(
k
)
⨯
(
S
m
P
(
k
)
)
i
2
∑
k
=
0
N
F
F
T
-
1
R
P
(
k
)
2
)
,
m
=
0
,
1
,
⋯
N
S
e
q
i
∈
[
-
N
F
O
,
N
F
O
]
,
where (v(n)), denotes the vector v(n), circularly shifted by i values and N Seq denotes total number of different preamble sequence present in the given preamble sequence set, R p is a received preamble signal, and S p is a preamble structure, k is a subcarrier index, N Seq denotes a total number of different preamble sequence present in the given preamble sequence set, and m is a subcarrier index.
27. The computer readable medium storing the computer program of claim 26 , where to counter an effect of multipath coherent combining is implemented only over adjacent sub-carriers, and where correlation values of different groups of sub-carriers are combined in a non-coherent manner, where if G consecutive sub-carriers are grouped together, then,
〈
i
^
,
m
^
〉
=
a
r
g
max
i
,
m
(
∑
q
=
0
N
F
F
T
G
-
1
∑
g
=
0
G
-
1
R
P
(
q
G
+
g
)
×
(
S
m
P
(
q
G
+
g
)
)
i
2
∑
k
=
0
N
F
F
T
-
1
R
P
(
k
)
2
)
,
m
=
0
,
1
,
⋯
N
S
e
q
i
∈
[
-
N
F
O
,
N
F
O
]
,
and where the value of G is determined for a channel having a longest delay spread, and N FFT is a fast fourier transform buffer size.
28. The computer readable medium storing the computer program of claim 15 , where the data processor comprises part of a mobile station that receives a downlink signal from a base station.
29. An apparatus, comprising:
at least one processor and at least one memory including computer program code;
the at least one memory and the computer program code stored on the memory is configured, with the at least one processor, to cause the apparatus, in response to a received downlink signal, to obtain time, carrier frequency and cell-specific preamble synchronization to the received signal said apparatus comprising a plurality of synchronization units comprising a first detector to detect a frame boundary using preamble delay correlation;
a second detector to detect the frame boundary with greater precision using a conjugate symmetry property over a region identified by the first detector;
a cyclic prefix correlator to resolve symbol boundary repetition;
an estimator, using the cyclic prefix, to estimate and correct a fractional carrier frequency offset;
an operator to perform a fast fourier transform of an identified preamble symbol; and
a frequency domain cross-correlator to identify cell-specific preamble sequences and an integer frequency offset in sub-carrier spacing.
30. The apparatus of claim 29 , where the received signal is a downlink signal received from a base station that is compatible with IEEE 802.16e.
31. The apparatus of claim 29 , embodied in a mobile station that receives a downlink signal from a base station.
32. The apparatus of claim 29 , where at least some of said plurality of synchronization units are embodied within an integrated circuit.
33. The apparatus of claim 29 , where at least some of said plurality of synchronization units are embodied as computer program code stored in a memory device.
34. A mobile station, comprising:
means for receiving a downlink signal, said receiving means operable to obtain time, carrier frequency and cell-specific preamble synchronization to the received signal and comprising a plurality of synchronization units comprising first means for detecting a frame boundary using preamble delay correlation;
second means for detecting the frame boundary with greater precision using a conjugate symmetry property over a region identified by the first means for detecting;
means for cyclic prefix correlation for resolving symbol boundary repetition;
means for estimating and correcting, using the cyclic prefix, a fractional carrier frequency offset;
means for performing a fast fourier transform of an identified preamble symbol; and
means for frequency domain cross-correlation for identifying cell-specific preamble sequences and an integer frequency offset in sub-carrier spacing.
35. The mobile station of claim 34 , where the received signal is a downlink signal received from a base station that is compatible with IEEE 802.16e.
36. The mobile station of claim 34 , where at least some of said plurality of synchronization units are embodied within an integrated circuit.
37. The mobile station of claim 34 , where at least some of said plurality of synchronization units are embodied as computer program code stored in a memory device.