# US 7,613,104 B2 · Method, apparatus and computer program product providing synchronization for OFDMA downlink signal

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- Number: US 7,613,104 B2
- Document ID: US7613104B2
- Status: Granted
- Kind: B2
- Application number: 11/444,735
- Filed: 2006-05-31
- Published: 2007-12-06
- Granted: 2009-11-03
- Earliest claimed: 2006-05-31
- Assignees: Nokia Corporation
- Inventors: Tejas Bhatt; Vishwas Sundaramurthy; Jianzhong Zhang; Dennis McCain
- Web page: https://patentgrep.com/patent/US7613104B2

## CPC

- H04L 27/2656: {Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation}
- H04L 27/2659: {Coarse or integer frequency offset determination and synchronisation}
- H04L 27/266: {Fine or fractional frequency offset determination and synchronisation}
- H04L 27/2662: {Symbol synchronisation}
- H04L 27/2675: {Pilot or known symbols}
- H04L 27/2678: {using cyclostationarities, e.g. cyclic prefix or postfix}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

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).

## Claims

### Claim 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.

### Claim 2 (depends on 1)

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.

### Claim 3 (depends on 1)

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.

### Claim 4 (depends on 1)

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.

### Claim 5 (depends on 4)

The method of claim 4, further comprising computing a global maximum among all peaks that exceed a second threshold.

### Claim 6 (depends on 4)

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.

### Claim 7 (depends on 5)

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 NFFT/P from an index value returned by the conjugate symmetry search.

### Claim 8 (depends on 7)

The method of claim 7, where the cyclic prefix correlation is measured for n×NFFT/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 XCP is a cyclic prefix value.

### Claim 9 (depends on 1)

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.

### Claim 10 (depends on 9)

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, NFFT is fast fourier transform buffer size, NCP is a cyclic prefix size, and m is a subcarrier index.

### Claim 11 (depends on 1)

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.

### Claim 12 (depends on 1)

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 [−NFO, NFO], a total of (2NFO+1)×NSeq hypotheses need to be tested in order to determine both the preamble and integer frequency offset, and by letting,
RP(k)≡fft(rp(n)), k,nε[0,NFFT−1] and,
SP(k)≡fft(sp(n)), k,nε[0,NFFT−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 NSeq denotes total number of different preamble sequence present in the given preamble sequence set, and where Rp is a received preamble signal, Sp is a preamble structure, NFO is an integer frequency offset, NFFT is a fast fourier transform buffer size, NSeq 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.

### Claim 13 (depends on 12)

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, Rp is a received preamble signal, and Sp is a preamble structure.

### Claim 14 (depends on 1)

The method of claim 1, executed by a mobile station that receives a downlink signal from a base station.

### Claim 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.

### Claim 16 (depends on 15)

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.

### Claim 17 (depends on 15)

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.

### Claim 18 (depends on 15)

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.

### Claim 19 (depends on 18)

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.

### Claim 20 (depends on 18)

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.

### Claim 21 (depends on 19)

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 NFFT/P from an index value returned by the conjugate symmetry search.

### Claim 22 (depends on 21)

The computer readable medium storing the computer program of claim 21, where cyclic prefix correlation is measured for n×NFFT/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, NFFT is a fast fourier transform buffer size, XCP is a cyclic prefix value, and {circumflex over (τ)}CP is a frame start cyclic prefix.

### Claim 23 (depends on 15)

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.

### Claim 24 (depends on 23)

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, XCP is a cyclic prefix value, NFFT is a fast fourier transform buffer size, NCP is a cyclic prefix size, and m is a subcarrier index.

### Claim 25 (depends on 15)

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.

### Claim 26 (depends on 15)

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 [−NFO, NFO], a total of (2NFO+1)×NSeq hypotheses need to be tested in order to determine both the preamble and integer frequency offset, and by letting,
RP(k)≡fft(rp(n)), k,nε[0,NFFT−1] and,
SP(k)≡fft(sp(n)), k,nε[0,NFFT−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 NSeq denotes total number of different preamble sequence present in the given preamble sequence set, Rp is a received preamble signal, and Sp is a preamble structure, k is a subcarrier index, NSeq denotes a total number of different preamble sequence present in the given preamble sequence set, and m is a subcarrier index.

### Claim 27 (depends on 26)

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 NFFT is a fast fourier transform buffer size.

### Claim 28 (depends on 15)

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.

### Claim 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.

### Claim 30 (depends on 29)

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.

### Claim 31 (depends on 29)

The apparatus of claim 29, embodied in a mobile station that receives a downlink signal from a base station.

### Claim 32 (depends on 29)

The apparatus of claim 29, where at least some of said plurality of synchronization units are embodied within an integrated circuit.

### Claim 33 (depends on 29)

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.

### Claim 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.

### Claim 35 (depends on 34)

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.

### Claim 36 (depends on 34)

The mobile station of claim 34, where at least some of said plurality of synchronization units are embodied within an integrated circuit.

### Claim 37 (depends on 34)

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.