APPARATUS AND METHOD FOR TRACKING SYMBOL TIMING OF OFDM MODULATION IN A MULTI-PATH CHANNEL

• US 20070092013A1
• Filed: 09/06/2006
• Published: 04/26/2007
• Est. Priority Date: 09/07/2005
• Status: Active Grant

• Current Assignee: CSR Technology Incorporated (Qualcomm, Inc.)
• Original Assignee: SIRF Technology Incorporated (Qualcomm, Inc.)
• Inventors: Ananth, Sharath, Cahn, Charles
• Granted Patent: US 7,813,454 B2
• Time in Patent Office: Days
• US Class Current: 375/260
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2676 Blind, i.e. without using known symbols
• CPC Class Codes: H04L 27/2678 using cyclostationarities, e.g. cyclic prefix or postfix
• CPC Class Codes: H04L 27/2688 Resistance to perturbation, e.g. noise, interference or fading

Abstract

Methods and Systems for tracking symbol timing of an OFDM signal are disclosed. In one embodiment, a wireless receiver includes signal tracking and timing logic to facilitate the timing of a demodulation operation such that the demodulation operation occurs on the proper symbol boundary, particularly when the OFDM signal has multipath components and the second component has greater power than the first.

Claims

1. A wireless signal receiver to demodulate an OFDM signal having multipath components, said receiver comprising symbol timing logic to control the timing of a Fourier Transformation operation by Fast Fourier Transform (FFT) logic, wherein the symbol timing logic is to drive the Fast Fourier Transform logic to perform the Fourier Transformation operation on signal data synchronized to the first multipath component received earlier in time than a second multipath component.

2. The wireless signal receiver of claim 1, wherein the symbol timing logic adjusts a result of an autocorrelation operation by a fixed design constant to generate a signal used to drive the timing of the FFT logic, wherein the autocorrelation operation determines an amount of correlation between a cyclic prefix and a later portion of the OFDM signal that repeats the cyclic prefix.

3. The wireless signal receiver of claim 2, wherein the result of the autocorrelation operation is smoothed over a specified number of samples of the OFDM signal.

4. The wireless signal receiver of claim 2, wherein the autocorrelation operation is normalized by dividing by the power of the signal averaged over the number of samples in the total symbol duration including the cyclic prefix.

5. The wireless signal receiver of claim 2, wherein the autocorrelation operation is normalized by dividing by the instantaneous power of the signal.

6. The wireless signal receiver of claim 2, wherein the first multipath component of the signal is received positioned as early as possible within a guard interval, thereby maximizing the tolerable time delay spread between the two multipath components.

7. The wireless signal receiver of claim 2 wherein the first multipath component has a lower signal strength than the second multipath component.

8. In a wireless signal receiver, a method of tracking symbol timing in a received signal having multipath components, comprising:

setting FFT timing in the receiver near a leading edge of a portion of a nonzero autocorrelation operation due to a first multipath component by shifting a result of the autocorrelation operation based on a fixed design constant.

9. The method of claim 8, wherein the autocorrelation operation determines an amount of correlation between a cyclic prefix and a later portion of the received signal that repeats the cyclic prefix.

10. The method of claim 8, wherein the autocorrelation operation is smoothed over a specified number of samples of the received signal.

11. The method of claim 8, wherein the autocorrelation operation is normalized by dividing by the power of the received signal averaged over the number of samples in the total symbol duration including the cyclic prefix.

12. The method of claim 8, wherein the autocorrelation operation is normalized by dividing by the instantaneous power of the received signal.

13. The method of claim 8, wherein the first multipath component of the signal is received positioned as early as possible within a guard interval, thereby maximizing the tolerable time delay spread between two or more multipath components.

14. The method of claim 8 wherein the first multipath component has a lower signal strength than a second multipath component.

15. A wireless signal receiver to demodulate an OFDM signal with our without multipath components, said receiver comprising:

power monitoring logic to determine the average power in a predetermined number of samples from the OFDM signal; and

edge detection logic to detect rising edges of average power measured, wherein each rising edge determines the location of a multipath component, and the location of the first rising edge indicates the first received multipath component which is used to control the timing of a Fourier Transformation operation by Fast Fourier Transform (FFT) logic.

16. The wireless signal receiver of claim 15 further comprising firmware to determine the predetermined number of samples from the OFDM signal to evaluate.

17. The wireless signal receiver of claim 15, wherein each change in average energy level in a null carrier represents effects of inter-symbol interference (ISI) due to different multipath components.

18. The wireless signal receiver of claim 15, wherein the power monitoring logic includes a sliding DFT logic to perform a sliding DFT operation on the predetermined number of samples.

19. In a wireless signal receiver, a method of tracking symbol timing in a received signal having multipath components, comprising:

monitoring the power within a null carrier portion of the received signal to detect a first increase in power caused by a time delayed null carrier of a multipath component; and

driving a Fast Fourier Transorm (FFT) operation on the signal based on the detection of the first increase in power.

20. The method of claim 19 wherein the first increase in power caused by the time delayed null carrier of a multipath component creates ISI in the null carrier portion of the received signal.

21. The method of claim 19 wherein the first increase in power is identified by locating the first positive edge occurring after a negative edge in a waveform representing the power within a null carrier portion of the received signal.

22. The method of claim 19 wherein a Sliding DFT is used to efficiently monitor the power in every individual null carrier of interest over every input sample.