Preamble design for frequency offset estimation and channel equalization in burst OFDM transmission system

• US 7,554,902 B2
• Filed: 04/23/2003
• Issued: 06/30/2009
• Est. Priority Date: 08/31/2002
• Status: Active Grant

• Current Assignee: Hanwha Systems Co., Ltd.
• Original Assignee: Samsung Thales Company Limited (Hanwha Corporation)
• Inventors: Kim, Ki-yun, Choi, Hyung-jin, Kim, Ho
• Primary Examiner(s): Yao; Kwang B
• Assistant Examiner(s): Lai; Andrew
• Application Number:
  US10/420,693
• Publication Number:
  US 20040042385A1
• Time in Patent Office: 2,260 Days
• Field of Search: 370/208, 370/210, 370/343, 370/203
• US Class Current: 370/208
• CPC Class Codes: H04L 27/2607 Cyclic extensions
• CPC Class Codes: H04L 27/2657 Carrier synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols

Abstract

Provided is a transmission data frame in an orthogonal frequency division multiplexing (OFDM) burst mode transmission, and more particularly, a frequency offset estimating method and a channel equalizing method using a transmission data frame. In the transmission data frame used in the orthogonal frequency division multiplexing burst mode transmission, a length of a guard interval in a preamble area for frequency offset estimation and channel estimation varies depending on a channel environment not being limited in consideration of a size of a data symbol, and only data of ending part of the guard interval in the preamble is used to estimate frequency offset in consideration of intersymbol interference. In the frequency offset estimating method, desired frequency estimation efficiency can be satisfied with the small number of samples. Also, when channel equalization is performed, two symbols are averaged in a time area to reduce the accuracy of channel estimation due to Gaussian random noise. In a frequency area, by referring to weight placed on an adjacent subcarrier, a channel equalization coefficient is obtained to efficiently reduce Gaussian random noise.

Claims

1. A method of estimating frequency offset in an orthogonal frequency division multiplexing system, the method comprising:

calculating a normalized frequency offset for each of a known experimental sample signal when a correlative distance is N subcarriers, and the known experimental sample signal when a correlative distance is 2N subcarriers, where N is a number greater than zero; and

subtracting or adding a predetermined value from or to the normalized frequency offset calculated when the correlative distance is 2N, in dependence upon a range of the normalized frequency offset value calculated when the correlative distance is N so as to estimate the frequency offset.

2. The method of claim 1, wherein in the step of calculating normalized frequency offset, the normalized frequency offset is calculated by using the amount of phase change between data of guard interval in a preamble area for frequency offset estimation and channel estimation and data of ending parts of the known experimental sample signals.

3. The method of claim 1, wherein the step of calculating the normalized frequency offset further comprises calculating a normalized frequency offset for the known experimental sample signal when a correlative distance is 3N subcarriers.

4. The method of claim 3, wherein the step of subtracting or adding the predetermined value from or to normalized frequency offset calculated comprises subtracting or adding a predetermined value from or to the normalized frequency offset calculated when the correlative distance is 3N, in dependence upon a range of the normalized frequency offset value calculated when the correlative distance is N so as to estimate the normalized frequency offset.

5. The method of claim 1, wherein in the step of subtracting or adding the predetermined value from or to the normalized frequency offset calculated, when the frequency offset value calculated from the correlative distance N is within a range of −

0.5 to −

0.25, 0.5 is subtracted from the frequency offset value calculated when the correlative distance is 2N so as to estimate the frequency offset.

6. The method of claim 1, wherein in the step of subtracting or adding the predetermined value from or to the normalized frequency offset calculated, when the frequency offset value calculated from the correlative distance N is within a range of 0.25 to 0.5, 0.5 is added to the frequency offset value calculated when the correlative distance is 2N so as to estimate the frequency offset.

7. The method of claim 1, wherein in the step of subtracting or adding the predetermined value from or to normalized frequency offset calculated, when the frequency offset value calculated from the correlative distance N is not within the range of −

0.5 to −

0.25 and the range of 0.25 to 0.5, the frequency offset value calculated when the correlative distance is 2N is used as it is so as to estimate the frequency offset.

8. A method of equalizing a channel in an orthogonal frequency division multiplexing system, the method comprising:

averaging two signals, which are transmitted in the same known pattern and received via a wireless channel, in a time area to obtain an average signal, with the average signal being established by;

$y_{\mathrm{Ave}}<br><br>\left(n\right)=\frac{y_0<br><br>\left(n\right)+y_1<br><br>\left(n\right)}{2}$

where n=0, 1, . . . , N−

1, N is the total number of samples in each signal, y₀(n) and y₁(n) are the two signals, where N is a number greater than zero; and

calculating a channel coefficient by performing fast Fourier transform for the obtained average signal.

9. The method of claim 8, after obtaining the channel coefficient by performing the fast Fourier transform for the obtained average signal, further comprising recalculating a channel coefficient by using a result obtained when weight is given to a channel coefficient of an adjacent subcarrier and a result obtained when weight is given to a channel coefficient of a current subcarrier.

10. The method of claim 9, wherein when recalculating the channel coefficient, the channel coefficient is recalculated by using equation below after calculating the channel coefficient by performing the fast Fourier transform for the obtained average value:

{tilde over (C)}_m(k)=α

Ĉ

_m(k)+β

{Ĉ

_m(k−

1)+Ĉ

_m(k+1)}+γ

{Ĉ

_m(k−

2)+Ĉ

_m(k+2)}

wherein, Ĉ

_m(k) denotes the channel coefficient obtained by performing the fast Fourier transform, and weight factors α

, β

, and γ

are in relations of α

≧

β

≧

γ and

α

+2(β

+γ

)=1.

11. The method of claim 10, wherein the weight factors α

, β

, and γ

are all ⅕

.

12. The method of claim 10, wherein the weight factor α

is ½

, the weight factor β

is 3/16, and the weight factory γ

is 1/16.

13. A computer-readable medium, comprised of a transmission data frame structure encoded on the computer-readable medium and enabling orthogonal frequency division multiplexing burst mode communications in an orthogonal frequency division multiplexing system, wherein a length of a guard interval in a preamble area of the transmission data frame structure for frequency offset estimation and channel estimation varies depending on a channel environment and not being limited in consideration of a size of a data symbol, and only data of ending part of the guard interval in the preamble is used to estimate frequency offset in consideration of intersymbol interference.

14. The computer-readable medium encoded with the transmission data frame structure of claim 13, wherein the length of the guard interval is four times a length of a guard interval of a transmission data frame used in an existing orthogonal frequency division multiplexing burst mode.

15. The computer-readable medium encoded with the transmission data frame structure of claim 13, wherein the length of the guard interval is at least about four times a length of a guard interval of a transmission data frame used in an existing orthogonal frequency division multiplexing burst mode.

16. A computer-readable recording medium encoded with a computer program for executing a frequency offset estimating method performed in an orthogonal frequency division multiplexing system in a computer is recorded, the frequency offset estimating method comprising:

calculating a normalized frequency offset for each of a known experimental sample signal in which a correlative distance is N subcarriers, and the known experimental sample signal in which the correlative distance is 2N subcarriers, where N is a number greater than zero; and

subtracting or adding a predetermined value from or to the normalized frequency offset calculated when the correlative distance is 2N in dependence upon a range of the normalized frequency offset calculated when the correlative distance is N so as to estimate normalized frequency offset.

17. A computer-readable recording medium encoded with a computer program for executing a channel equalizing method performed in an orthogonal frequency division multiplexing system in a computer is recorded, the channel equalizing method comprising:

averaging two signals, which are transmitted in the same known pattern and received via a wireless channel, in a time area to obtain an average signal; and

calculating a channel coefficient by performing fast Fourier transform for the obtained average signal.

18. The method of claim 17, comprised of subtracting or adding a predetermined value from or to the normalized frequency offset calculated when a correlative distance is 2N, in dependence upon a range of a normalized frequency offset value calculated when the correlative distance, where N is a number greater than zero is N so as to estimate the frequency offset, wherein in the step of subtracting or adding the predetermined value from or to the normalized frequency offset calculated, when the frequency offset value calculated from the correlative distance N is within a range of −

0.5 to −

0.25, 0.5 is subtracted from the frequency offset value calculated when the correlative distance is 2N so as to estimate the frequency offset.

19. A computer-readable medium having stored thereon a data structure comprising:

a first field containing data representing a calculating of normalized frequency offset for each of a known experimental sample signal when a correlative distance is N subcarriers, and the known experimental sample signal when the correlative distance is 2N subcarriers, where N is a number greater than zero; and

a second field containing data representing a changing of a predetermined value from the normalized frequency offset calculated when the correlative distance is 2N in dependence upon a range of the normalized frequency offset calculated when the correlative distance is N accommodating to estimate normalized frequency offset.

20. The computer-readable medium having stored thereon a data structure of claim 19, further comprising:

a third field containing data representing averaging two signals, which are transmitted in the same known pattern and received via a wireless channel, in a time area to obtain an average signal; and

a fourth field containing data representing calculating a channel coefficient by performing fast Fourier transform for the obtained average signal.

21. A method, comprising:

calculating normalized frequency offset for each of a known experimental sample signal when a correlative distance is N subcarriers, and the known experimental sample signal when a correlative distance is 2N subcarriers, where N is a number greater than zeso;

changing by a predetermined value of the normalized frequency offset calculated when the correlative distance is 2N, in dependence upon a range of the normalized frequency offset value calculated when the correlative distance is N so as to estimate the frequency offset;

averaging two signals, which are transmitted in the same known pattern and received via a wireless channel, in a time area to obtain an average signal; and

calculating a channel coefficient by performing fast Fourier transform for the obtained average signal.

22. The method of claim 21, wherein in the step of calculating normalized frequency offset, the normalized frequency offset is calculated by using the amount of phase change between data of guard interval in a preamble area for frequency offset estimation and channel estimation and data of ending parts of the known experimental sample signals.

23. The method of claim 22, wherein the step of calculating normalized frequency offset further comprises a calculating normalized frequency offset for the known experimental sample signal when the correlative distance is 3N subcarriers.

24. The method of claim 23, wherein the step of changing by a predetermined value of a normalized frequency offset comprises:

changing by a predetermined value, the normalized frequency offset calculated when the correlative distance is 3N, in dependence upon the range of the normalized frequency offset value calculated when the correlative distance is N so as to estimate the normalized frequency offset.

25. The method of claim 24, wherein in the step of changing by a predetermined value of a normalized frequency offset, when the frequency offset value calculated from the correlative distance N is within a range of approximately −

0.5 to −

0.25, approximately 0.5 is subtracted from the frequency offset value calculated when the correlative distance is 2N to accommodate estimating of the frequency offset.

26. The method of claim 25, wherein in the step of changing by a predetermined value of a normalized frequency offset, when the frequency offset value calculated from the correlative distance N is within a range of approximately 0.25 to approximately 0.5, approximately 0.5 is added to the frequency offset value calculated when the correlative distance is 2N so as to estimate the frequency offset.

27. The method of claim 21, wherein in the step of changing by a predetermined value of a normalized frequency offset, when the frequency offset value calculated from the correlative distance N is not within the range of about −

0.5 to about −

0.25 and the range of about 0.25 to about 0.5, the frequency offset value calculated when the correlative distance is 2N is used as it is to accommodate an estimate of the frequency offset.

28. The method of claim 27, after obtaining the channel coefficient by performing the fast Fourier transform for the obtained average signal, further comprising recalculating a channel coefficient by using a result obtained when weight is given to a channel coefficient of an adjacent subcarrier and a result obtained when weight is given to a channel coefficient of a current subcarrier.

29. The method of claim 26, wherein when recalculating the channel coefficient, the channel coefficient is recalculated by using equation below after calculating the channel coefficient by performing the fast Fourier transform on the obtained average signal:

{tilde over (C)}_m(k)=α

Ĉ

_m(k)+β

{Ĉ

_m(k−

1)+Ĉ

_m(k+1)}+γ

{Ĉ

_m(k−

2)+Ĉ

_m(k+2)}

wherein, Ĉ

_m(k) denotes the channel coefficient obtained by performing the fast Fourier transform, and weight factors α

, β

, and γ

are in relations of α

≧

β

≧

γ and

α

+2(β

+γ

)=1.

30. The method of claim 29, wherein the weight factors α

, β

, and γ

are all about ⅕

.

31. The method of claim 29, wherein the weight factor α

is about ½

, the weight factor β

is about 3/16, and the weight factory γ

is about 1/16.

32. The method of claim 31, further comprising of a transmission data frame used in a orthogonal frequency division multiplexing burst mode system including a length of a guard interval in a preamble area for frequency offset estimation and channel estimation varies depending on a channel environment not being limited in consideration of a size of a data symbol, and only data of ending part of the guard interval in the preamble is used to estimate frequency offset in consideration of intersymbol interference.

33. The method of claim 32, comprised of subtracting or adding a predetermined value from or to normalized frequency offset calculated when the correlative distance is 2N, in dependence upon a range of a normalized frequency offset value calculated when the correlative distance is N so as to estimate the frequency offset, wherein in the step of subtracting or adding the predetermined value from or to the normalized frequency offset calculated, when the frequency offset value calculated from the correlative distance N is within a range of 0.25 to 0.5, 0.5 is added to the frequency offset value calculated when the correlative distance is 2N so as to estimate the frequency offset.

34. The method of claim 32, wherein in the step of subtracting or adding the predetermined value from or to normalized frequency offset calculated, when the frequency offset value calculated from the correlative distance N is within a range of 0.25 to 0.5, 0.5 is added to the frequency offset value calculated when the correlative distance is 2N so as to estimate the frequency offset.