Method, apparatus and computer program product providing synchronization for OFDMA downlink signal

• US 20070280098A1
• Filed: 05/31/2006
• Published: 12/06/2007
• Est. Priority Date: 05/31/2006
• Status: Active Grant

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Corporation
• Inventors: Bhatt, Tejas, Sundaramurthy, Vishwas, Zhang, Jianzhong, McCain, Dennis
• Granted Patent: US 7,613,104 B2
• Time in Patent Office: Days
• US Class Current: 370/208
• CPC Class Codes: H04L 27/2656 Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
• CPC Class Codes: H04L 27/2659 Coarse or integer frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/266 Fine or fractional frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols
• CPC Class Codes: H04L 27/2678 using cyclostationarities, e.g. cyclic prefix or postfix

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

1. A method, comprising:

obtaining time, carrier frequency and cell-specific preamble synchronization to a transmitted 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 step 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.

2. The method of claim 1, where the transmitted signal is a downlink signal transmitted into the cell 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 the use of a plurality of multiply-and-accumulate (MAC) 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 the 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 n ∈

[−

P/2, P/2] and averaged over M OFDM symbols and where the frame boundary is estimated by,

$\widehat{\mathrm{\tau <br><br>}}={\widehat{\mathrm{\tau <br><br>}}}_{\mathrm{CP}}=\underset{n}{\text{arg}<br><br>\max }<br><br>\left(\frac{{<br><br>X_{\mathrm{CP}}<br><br>\left(n\right)<br><br>}^2}{P<br><br>\left(n\right)}\right),P<br><br>\left(n\right)=\mathrm{Received}<br><br><br><br>\mathrm{Power}.$

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,

${\widehat{\mathrm{\phi <br><br>}}}_{\mathrm{\Delta <br><br>}<br><br><br><br>f}={\tan }^{-1}<br><br>\left(\underset{m=0}{\overset{M-1}{\sum <br><br>}}<br><br>X_{\mathrm{CP}}<br><br>\left(\widehat{\mathrm{\tau <br><br>}}+m<br><br>\left(N_{\mathrm{FFT}}+N_{\mathrm{CP}}\right)\right)\right),$

where the value of M is within a downlink frame boundary.

11. The method of claim 1, where performing the Fast Fourier Transform (FFT) of an identified preamble symbol performs the FFT 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,n ∈

[0, N_FFT−

1] and,

S^P(k)≡

fft(s^p(n)), k,n ∈

[0, N_FFT−

1],

where the frequency domain search yields,

$〈<br><br>\hat{i},\hat{m}〉<br><br>=\underset{i,m}{\text{arg}<br><br><br><br>\max }<br><br>\left(\frac{{<br><br>\underset{k=0}{\overset{N_{\mathrm{FFT}}-1}{\sum <br><br>}}<br><br>R^P<br><br>\left(k\right)\times <br><br>{\left(S_m^P<br><br>\left(k\right)\right)}_i<br><br>}^2}{\underset{k=0}{\overset{N_{\mathrm{FFT}}-1}{\sum <br><br>}}<br><br>{<br><br>R^P<br><br>\left(k\right)<br><br>}^2}\right),\begin{array}{c}m=0,1,\mathrm{\dots <br><br>}<br><br><br><br>N_{\mathrm{Seq}}\\ i\in <br><br>\left[-N_{\mathrm{FO}},N_{\mathrm{FO}}\right]\end{array},$

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.

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.

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

15. A computer program product embodied on a computer readable medium, execution of the computer program product by a data processor resulting in operations that comprise obtaining time, carrier frequency and cell-specific preamble synchronization to a transmitted 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 program product of claim 15, where the transmitted signal is a downlink signal transmitted into the cell from a base station that is compatible with IEEE 802.16e.

17. The computer program product 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 the use of a plurality of multiply-and-accumulate (MAC) operations per received signal sample.

18. The computer program product 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 the preamble repetition, P.

19. The computer program product of claim 18, further comprising computing a global maximum among all peaks that exceed a second threshold.

20. The computer program product 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 program product 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 program product of claim 21, where cyclic prefix correlation is measured for n×

N_FFT/P samples with n ∈

[−

P/2, P/2] and averaged over M OFDM symbols and where the frame boundary is estimated by,

$\widehat{\mathrm{\tau <br><br>}}={\widehat{\mathrm{\tau <br><br>}}}_{\mathrm{CP}}=\underset{n}{\text{arg}<br><br>\max }<br><br>\left(\frac{{<br><br>X_{\mathrm{CP}}<br><br>\left(n\right)<br><br>}^2}{P<br><br>\left(n\right)}\right),P<br><br>\left(n\right)=\mathrm{Received}<br><br><br><br>\mathrm{Power}.$

23. The computer program product 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 program product of claim 23, 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,

${\widehat{\mathrm{\phi <br><br>}}}_{\mathrm{\Delta <br><br>}<br><br><br><br>f}={\tan }^{-1}<br><br>\left(\underset{m=0}{\overset{M-1}{\sum <br><br>}}<br><br>X_{\mathrm{CP}}<br><br>\left(\widehat{\mathrm{\tau <br><br>}}+m<br><br>\left(N_{\mathrm{FFT}}+N_{\mathrm{CP}}\right)\right)\right),$

where the value of M is within a downlink frame boundary.

25. The computer program product of claim 15, where performing the Fast Fourier Transform (FFT) of an identified preamble symbol performs the FFT on a fractional frequency offset corrected received preamble symbol, based on the estimated frame start.

26. The computer program product 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,n ∈

[0,N_FFT−

1] and,

S^P(k)≡

fft(s^p(n)), k,n ∈

[0,N_FFT−

1],

where the frequency domain search yields,

$〈<br><br>\hat{i},\hat{m}〉<br><br>=\underset{i,m}{\text{arg}<br><br><br><br>\max }<br><br>\left(\frac{{<br><br>\underset{k=0}{\overset{N_{\mathrm{FFT}}-1}{\sum <br><br>}}<br><br>R^P<br><br>\left(k\right)\times <br><br>{\left(S_m^P<br><br>\left(k\right)\right)}_i<br><br>}^2}{\underset{k=0}{\overset{N_{\mathrm{FFT}}-1}{\sum <br><br>}}<br><br>{<br><br>R^P<br><br>\left(k\right)<br><br>}^2}\right),<br><br>\begin{array}{c}m=0,1,\mathrm{\dots <br><br>}<br><br><br><br>N_{\mathrm{Seq}}\\ i\in <br><br>\left[-N_{\mathrm{FO}},N_{\mathrm{FO}}\right]\end{array},$

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.

27. The computer program product 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 ^ 〉

= 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.

28. The computer program product of claim 15, where the data processor comprises part of a mobile station that receives a downlink signal from a base station.

29. A device, comprising:

a receiver for receiving a downlink signal transmitted into a cell, said receiver operable to obtain time, carrier frequency and cell-specific preamble synchronization to the received signal and including 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 device of claim 29, where the transmitted signal is a downlink signal transmitted into the cell from a base station that is compatible with IEEE 802.16e.

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

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

33. The device 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 transmitted into a cell, 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 transmitted signal is a downlink signal transmitted into the cell 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.