Partial FFT processing and demodulation for a system with multiple subcarriers

• US 7,720,162 B2
• Filed: 03/08/2006
• Issued: 05/18/2010
• Est. Priority Date: 03/10/2005
• Status: Expired due to Fees

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Krishnamoorthi, Raghuraman
• Primary Examiner(s): Pathak; Sudhanshu C
• Application Number:
  US11/372,397
• Publication Number:
  US 20060215777A1
• Time in Patent Office: 1,532 Days
• Field of Search: 375/316, 375/295, 708/404, 708/405, 370/210
• US Class Current: 375/260
• CPC Class Codes: G06F 17/142 Fast Fourier transforms, e.g. using a Cooley-Tukey type algorithm
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 27/26522 using partial FFTs
• CPC Class Codes: H04L 27/26526 with inverse FFT [IFFT] or inverse DFT [IDFT] demodulators, e.g. standard single-carrier frequency-division multiple access [SC-FDMA] receiver or DFT spread orthogonal frequency division multiplexing [DFT-SOFDM]
• CPC Class Codes: H04L 5/023 Multiplexing of multicarrier modulation signals multicarrier modulation H04L27/2601

Abstract

Techniques for efficiently performing partial FFT for subcarriers of interest are described. The N total subcarriers may be arranged into M sets. Each set may contain K subcarriers uniformly distributed across the N total subcarriers, where M·K=N. For the partial FFT, pre-processing is initially performed on time-domain samples to obtain intermediate samples. The pre-processing may include performing M-point FFTs on the time-domain samples and multiplying the FFT outputs with unit complex values. For each set of subcarriers of interest, a K-point FFT is performed on a set of intermediate samples to obtain a set of frequency-domain symbols for that set of subcarriers. Since K is typically much smaller than N, substantial savings in computation and power may be realized when only one or few sets of subcarriers are of interest.

Claims

1. An apparatus comprising:

at least one processor configured to select at least one set of subcarriers from among all sets of subcarriers where the at least one set is less than the all sets, to perform pre-processing on time-domain samples to obtain intermediate samples wherein the intermediate samples are defined by

$y_m<br><br>\left(n\right)=\underset{i=0}{\overset{M-1}{\sum <br><br>}}<br><br><br><br>x<br><br>\left(n+K·<br><br>i\right)·<br><br>W_N^{m<br><br>\left(n+K·<br><br>i\right)},\mathrm{for}<br><br><br><br>n=0,\mathrm{\dots <br><br>}<br><br>,K-1,$

with x representing the time-domain samples, m representing the at least one set of subcarriers, M representing the all sets, K representing a quantity of the intermediate samples and W is a phasor that rotates around a unit circle, and to perform at least one fast Fourier transform (FFT) on the intermediate samples to obtain frequency-domain symbols for the at least one set of subcarriers; and

a memory coupled to the at least one processor;

wherein the memory is configured to store the time-domain samples in a first direction, and to provide the time-domain samples for pre-processing in a second direction and further wherein the first direction is column-wise and the second direction is row-wise.

2. The apparatus of claim 1, wherein the at least one processor is configured to perform M-point FFTs on the time-domain samples, where M is the number of sets of subcarriers, and to multiply outputs of the M-point FFTs with unit complex values to obtain the intermediate samples.

3. The apparatus of claim 1, wherein the at least one processor is configured to rotate the time-domain samples to obtain rotated samples, and to accumulate the rotated samples to obtain the intermediate samples.

4. The apparatus of claim 1, wherein the memory is configured to store the intermediate samples in the second direction, and to provide the intermediate samples for FFT processing in the first direction.

5. The apparatus of claim 1, wherein the at least one processor is configured to perform pre-processing in accordance with a pre-processing scheme selected based on the number of sets of subcarriers selected.

6. The apparatus of claim 1, wherein for each of the at least one set of subcarriers the at least one processor is configured to perform a K-point FFT on K intermediate samples to obtain K frequency-domain symbols for the set of subcarriers, where M×

K=N, M is the number of sets of subcarriers, K is the number of subcarriers in each set, and N is the total number of subcarriers.

7. The apparatus of claim 1, wherein the at least one processor is configured to perform a first FFT on a first set of intermediate samples to obtain a first set of frequency-domain symbols for a first set of subcarriers used for pilot transmission, and to perform a second FFT on a second set of intermediate samples to obtain a second set of frequency-domain symbols for a second set of subcarriers used for data transmission.

8. The apparatus of claim 7, wherein the at least one processor is configured to perform a third FFT on a third set of intermediate samples to obtain a third set of frequency-domain symbols for a third set of subcarriers used for data transmission.

9. The apparatus of claim 7, wherein the at least one processor is configured to derive channel gain estimates for the second set of subcarriers based on the first set of frequency-domain symbols, and to perform detection on the second set of frequency-domain symbols with the channel gain estimates.

10. The apparatus of claim 1, wherein the multiple sets include equal number of subcarriers, and wherein the subcarriers in each set are uniformly distributed across system bandwidth.

11. The apparatus of claim 1, wherein the multiple sets of subcarriers are obtained with orthogonal frequency division multiplexing (OFDM).

12. The apparatus of claim 1, wherein the multiple sets of subcarriers are obtained with single-carrier frequency division multiple access (SC-FDMA).

13. A processor configured

to select at least one set of subcarriers from among all sets of subcarriers where the at least one set is less than the all sets, to store time-domain samples in a first direction, and to provide the time-domain samples for pre-processing in a second direction and further wherein the first direction is column-wise and the second direction is row-wise, to perform pre-processing on time-domain samples to obtain intermediate samples wherein the intermediate samples are defined by

$y_m$

( n ) = ∑

i = 0 M - 1 ⁢

⁢

x ⁡

( n + K ·

i ) ·

W N m ⁡

( n + K ·

i ) , for ⁢

⁢

n = 0 , …

⁢

, K - 1 ,

with x representing the time-domain samples, m representing the at least one set of subcarriers, M representing the all sets, K representing a quantity of the intermediate samples and W is a phasor that rotates around a unit circle, and

to perform at least one fast Fourier transform (FFT) on the intermediate samples to obtain frequency-domain symbols for the at least one set of subcarriers.

14. The processor of claim 13, and configured to perform M-point FFTs on the time-domain samples, where M is the number of sets of subcarriers, and to multiply outputs of the M-point FFTs with unit complex values to obtain the intermediate samples.

15. The processor of claim 13, and configured to perform a K-point FFT on K intermediate samples for each of the at least one set of subcarriers to obtain K frequency-domain symbols for the set of subcarriers, where M×

K=N, M is the number of sets of subcarriers, K is the number of subcarriers in each set, and N is the total number of subcarriers.

16. The processor of claim 13, and configured to perform a first FFT on a first set of intermediate samples to obtain a first set of frequency-domain symbols for a first set of subcarriers used for pilot transmission, and to perform a second FFT on a second set of intermediate samples to obtain a second set of frequency-domain symbols for a second set of subcarriers used for data transmission.

17. The processor of claim 16, and configured to derive channel gain estimates for the second set of subcarriers based on the first set of frequency-domain symbols, and to perform detection on the second set of frequency-domain symbols with the channel gain estimates.

18. A method comprising:

selecting at least one set of subcarriers from among all sets of subcarriers where the at least one set is less than the all sets;

storing time-domain samples in a buffer in a first direction;

retrieving the time-domain samples from the buffer in a second direction for pre-processing, wherein the first direction is column-wise and the second direction is row-wise;

performing pre-processing on time-domain samples to obtain intermediate samples wherein the intermediate samples are defined by

$y_m<br><br>\left(n\right)=\underset{i=0}{\overset{M-1}{\sum <br><br>}}<br><br><br><br>x<br><br>\left(n+K·<br><br>i\right)·<br><br>W_N^{m<br><br>\left(n+K·<br><br>i\right)},\mathrm{for}<br><br><br><br>n=0,\mathrm{\dots <br><br>}<br><br>,K-1,$

with x representing the time-domain samples, m representing the at least one set of subcarriers, M representing the all sets, K representing a quantity of the intermediate samples and W is a phasor that rotates around a unit circle; and

performing at least one fast Fourier transform (FFT) on the intermediate samples to obtain frequency-domain symbols for the at least one set of subcarriers.

19. The method of claim 18, wherein the performing pre-processing comprises performing M-point FFTs on the time-domain samples, where M is the number of sets of subcarriers, and multiplying outputs of the M-point FFTs with unit complex values to obtain the intermediate samples.

20. The method of claim 18, further comprising:

storing the intermediate samples in the buffer in the second direction; and

retrieving the intermediate samples from the buffer in the first direction for FFT processing.

21. The method of claim 18, wherein the performing at least one FFT comprises performing a first FFT on a first set of intermediate samples to obtain a first set of frequency-domain symbols for a first set of subcarriers used for pilot transmission, and performing a second FFT on a second set of intermediate samples to obtain a second set of frequency-domain symbols for a second set of subcarriers used for data transmission.

22. The method of claim 21, further comprising:

deriving channel gain estimates for the second set of subcarriers based on the first set of frequency-domain symbols; and

performing detection on the second set of frequency-domain symbols with the channel gain estimates.

23. An apparatus comprising:

means for selecting at least one set of subcarriers from among all sets of subcarriers where the at least one set is less than the all sets;

means for storing time-domain samples in a first direction;

means for providing the time-domain samples in a second direction for pre-processing, wherein the first direction is column-wise and the second direction is row-wise;

means for performing pre-processing on time-domain samples to obtain intermediate samples wherein the intermediate samples are defined by

$y_m<br><br>\left(n\right)=\underset{i=0}{\overset{M-1}{\sum <br><br>}}<br><br><br><br>x<br><br>\left(n+K·<br><br>i\right)·<br><br>W_N^{m<br><br>\left(n+K·<br><br>i\right)},\mathrm{for}<br><br><br><br>n=0,\mathrm{\dots <br><br>}<br><br>,K-1,$

with x representing the time-domain samples, m representing the at least one set of subcarriers, M representing the all sets, K representing a quantity of the intermediate samples and W is a phasor that rotates around a unit circle; and

means for performing at least one fast Fourier transform (FFT) on the intermediate samples to obtain frequency-domain symbols for the at least one set of subcarriers.

24. The apparatus of claim 23, wherein the means for performing pre-processing comprises means for performing M-point FFTs on the time-domain samples, where M is the number of sets of subcarriers, and means for multiplying outputs of the M-point FFTs with unit complex values to obtain the intermediate samples.

25. The apparatus of claim 23, further comprising:

means for storing the intermediate samples in the second direction; and

means for retrieving the intermediate samples in the first direction for FFT processing.

26. The apparatus of claim 23, wherein the means for performing at least one FFT comprises means for performing a first FFT on a first set of intermediate samples to obtain a first set of frequency-domain symbols for a first set of subcarriers used for pilot transmission, and means for performing a second FFT on a second set of intermediate samples to obtain a second set of frequency-domain symbols for a second set of subcarriers used for data transmission.

27. The apparatus of claim 26, further comprising:

means for deriving channel gain estimates for the second set of subcarriers based on the first set of frequency-domain symbols; and

means for performing detection on the second set of frequency-domain symbols with the channel gain estimates.

28. Computer-readable medium encoded with a computer program to:

select at least one set of subcarriers from among all sets of subcarriers where the at least one set is less than the all sets;

store time-domain samples in a first direction;

provide the time-domain samples for pre-processing in a second direction and further wherein the first direction is column-wise and the second direction is row-wise;

perform pre-processing on time-domain samples to obtain intermediate samples wherein the intermediate samples are defined by

$y_m<br><br>\left(n\right)=\underset{i=0}{\overset{M-1}{\sum <br><br>}}<br><br><br><br>x<br><br>\left(n+K·<br><br>i\right)·<br><br>W_N^{m<br><br>\left(n+K·<br><br>i\right)},\mathrm{for}<br><br><br><br>n=0,\mathrm{\dots <br><br>}<br><br>,K-1,$

with x representing the time-domain samples, m representing the at least one set of subcarriers, M representing the all sets, K representing a quantity of the intermediate samples and W is a phasor that rotates around a unit circle; and

perform at least one fast Fourier transform (FFT) on the intermediate samples to obtain frequency-domain symbols for the at least one set of subcarriers.

29. The computer-readable medium of claim 28, and further encoded with the computer program to:

perform M-point FFTs on the time-domain samples, where M is the number of sets of subcarriers, and multiply outputs of the M-point FFTs with unit complex values to obtain the intermediate samples.