Timing control in orthogonal frequency division multiplex systems based on effective signal-to-noise ratio

• US 20060245347A1
• Filed: 04/29/2005
• Published: 11/02/2006
• Est. Priority Date: 04/29/2005
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Black, Peter J., Xu, Hao, Namgoong, June, Jayaraman, Srikant, Mangan, Michael John
• Granted Patent: US 7,564,775 B2
• Time in Patent Office: Days
• US Class Current: 370/208
• CPC Class Codes: H04L 1/20 using signal quality detector
• CPC Class Codes: H04L 2001/0093 Point-to-multipoint
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 27/2695 with channel estimation, e.g. determination of delay spread, derivative or peak tracking channel estimation H04L25/0202
• CPC Class Codes: H04L 5/0048 Allocation of pilot signals, i.e. of signals known to the receiver

Abstract

A spread-spectrum communication system provides an effective signal-to-noise ratio (SNR) of received orthogonal frequency division multiplex (OFDM) slots in the presence of timing errors. Effective SNR can serve as a diagnostic tool for determining whether there was a timing error when a measured packet error rate (PER) remains high, and a predicted PER from the effective SNR remains low. A loop can use the effective SNR to control a time reference used by an OFDM decoder.

Claims

1. A method comprising:

using a current time reference to decode a plurality of orthogonal frequency division multiplex (OFDM) symbols in a plurality of time slots;

measuring a packet error rate of the decoded OFDM symbols;

determining an effective signal-to-noise ratio (SNR) metric from pilot tones of the demodulated OFDM symbols;

based on the determined effective SNR metric, predicting a supportable packet error rate;

comparing the measured packet error rate and the predicted packet error rate; and

based on said comparing, adjusting the current time reference before decoding a next OFDM time slot.

2. The method of claim 1, wherein comparing the measured packet error rate and the predicted packet error rate comprises:

determining if the measured packet error rate is greater than a first threshold value; and

determining if the predicted packet error rate is less than a second threshold value.

3. The method of claim 1, wherein based on said comparing, adjusting the current time reference comprises if the measured packet error rate is greater than the first threshold value, and the predicted packet error rate is less than the second threshold value, then adjusting the current time reference before decoding a next OFDM time slot.

4. The method of claim 1, wherein determining an effective signal-to-noise ratio (SNR) metric comprises using:

$\mathrm{SNR}=C^{-1}<br><br>\left(\frac{1}{P}<br><br>\underset{k=1}{\overset{P}{\sum <br><br>}}<br><br>C<br><br>\left(\frac{{<br><br>H_k<br><br>}^2}{{\mathrm{\sigma <br><br>}}^2}\right)\right)$

where C(x) is a capacity of a Gaussian Channel with SNR x and input constrained to a selected type of modulation, where σ

² denotes an estimated noise variance, where P is the number of pilot tones, and where Hk represents an amplitude response of a channel.

5. The method of claim 4, further comprising using a lookup table to implement the function C(x), and estimating the noise variance.

6. The method of claim 1, further comprising:

determining whether the current time reference does not change for a number of consecutive OFDM time slots; and

based on said determining, advancing the current time reference by a configured number of chips for the next OFDM time slot to be decoded.

7. The method of claim 1, further comprising:

determining whether the current time reference does not change for a configured number of chips from a previous time reference; and

based on said determining, advancing the current time reference by a configured number of chips for the next OFDM time slot to be decoded.

8. The method of claim 1, further comprising:

wirelessly receiving multipath transmissions containing the OFDM symbols in the time slots; and

searching the multipath transmissions to determine the current time reference.

9. The method of claim 1, wherein adjusting the current time reference comprises advancing the current time reference by a configured number of chips.

10. The method of claim 1, further comprising repeating said adjusting the current time reference until the measured packet error rate is close to the predicted packet error rate.

11. The method of claim 1, further comprising repeating said adjusting the current time reference until adjusting the current time reference exceeds a threshold number of chips determined a priori based on a maximum possible delay in a channel.

12. The method of claim 1, further comprising repeating said adjusting the current time reference until a relative timing of a first arriving path changes more than a threshold number of chips.

13. The method of claim 1, wherein the OFDM symbols comprise broadcast content intended for a plurality of mobile stations.

14. The method of claim 1, wherein the times slots containing OFDM symbols are time division multiplexed with time slots comprising user-specific data.

15. An apparatus configured to receive signals from a base station, the apparatus comprising:

means for using a current time reference to decode a plurality of orthogonal frequency division multiplex (OFDM) symbols in a plurality of time slots;

means for measuring a packet error rate of the decoded OFDM symbols;

means for determining an effective signal-to-noise ratio (SNR) metric from pilot tones of the decoded OFDM symbols;

based on the determined effective SNR metric, means for predicting a supportable packet error rate;

means for comparing the measured packet error rate and the predicted packet error rate; and

based on said means for comparing, means for adjusting the current time reference before decoding a next OFDM time slot.

16. The apparatus of claim 15, wherein the means for comparing comprises:

means for determining if the measured packet error rate is greater than a first threshold value;

means for determining if the predicted packet error rate is less than a second threshold value; and

if the measured packet error rate is greater than the first threshold value, and the predicted packet error rate is less than the second threshold value, means for adjusting the current time reference before decoding a next OFDM time slot.

17. An apparatus configured to receive orthogonal frequency division multiplex (OFDM) symbols from a base station, the apparatus comprising:

a timing control module configured to;

determine if a measured packet error rate of the received OFDM symbols is greater than a first threshold value;

determine if a predicted packet error rate is less than a second threshold value; and

if the measured packet error rate is greater than the first threshold value, and the predicted packet error rate is less than the second threshold value, then adjust a current time reference before decoding a next OFDM time slot.

18. The apparatus of claim 17, further comprising:

a receiver configured to wirelessly receive multipath transmissions containing OFDM symbols in a plurality of time slots; and

a searcher to search the multipath transmissions to determine the current time reference.

19. The apparatus of claim 18, wherein the times slots containing OFDM symbols are time division multiplexed with time slots comprising user-specific data.

20. The apparatus of claim 17, further comprising:

a decoder operable to;

use a current time reference to decode a plurality of OFDM symbols in a plurality of time slots;

measure a packet error rate of the decoded OFDM symbols;

determine an effective signal-to-noise ratio (SNR) metric from pilot tones of the decoded OFDM symbols; and

based on the determined effective SNR metric, predict a supportable packet error rate.

21. The apparatus of claim 17, wherein the timing control module is further configured to:

determine whether the current time reference does not change for a number of consecutive OFDM time slots; and

based on said determining, advance the current time reference by a configured number of chips for the next OFDM time slot to be decoded.

22. The apparatus of claim 17, wherein the timing control module is further configured to:

determine whether the current time reference does not change for a configured number of chips from a previous time reference; and

based on said determining, advance the current time reference by a configured number of chips for the next OFDM time slot to be decoded.

23. The apparatus of claim 17, wherein adjusting the current time reference comprises advancing the current time reference by a configured number of chips.

24. The apparatus of claim 17, wherein the timing control module is configured to repeat said adjusting the current time reference until the measured packet error rate is close to the predicted packet error rate.

25. The apparatus of claim 17, wherein the timing control module is configured to repeat said adjusting the current time reference until adjusting the current time reference exceeds a threshold number of chips determined a priori based on a maximum possible delay in a channel.

26. The apparatus of claim 17, wherein the timing control module is configured to repeat said adjusting the current time reference until a relative timing of a first arriving path changes more than a threshold number of chips.

27. The apparatus of claim 17, wherein the OFDM symbols comprise broadcast content intended for a plurality of mobile stations.