Coarse symbol boundary detection in OFDM receivers

1. A method for determining a symbol boundary in a data packet belonging to a received Orthogonal Frequency-Division Multiplexing, OFDM, signal, said method comprising:

detecting a beginning of a data packet while a fixed reception gain is applied to the received OFDM signal, wherein the data packet comprises a first training field and a second training field beginning by a guard interval, and wherein the beginning of the data packet is determined using at least autocorrelation peaks and at least a first threshold is used for detecting autocorrelation peaks during detection of the beginning of the data packet;

starting an automatic gain control process when triggered by the detection of the beginning of the data packet; and

after said automatic gain control process is locked, determining autocorrelation peaks in the OFDM signal during reception of the first training field, and estimating said symbol boundary from times of said autocorrelation peaks, wherein a second threshold, which is lower than the first threshold, is used for determining autocorrelation peaks used for estimating the symbol boundary.

2. The method according to claim 1 , wherein said symbol boundary is estimated from a mean value of several among the autocorrelation peaks.

3. The method according to claim 1 , wherein said detecting of the beginning of the data packet involves detecting both:

a frame using a normalized autocorrelation function computed on said received OFDM signal, and

an energy saturation.

4. The method according to claim 1 , wherein the determining of the autocorrelation peaks comprises using a normalized autocorrelation function computed on said received OFDM signal.

5. The method according to claim 4 , wherein one of the autocorrelation peaks is determined above a predetermined threshold during a set of autocorrelation samples.

6. The method according to claim 1 , wherein said symbol boundary is determined between a short training field and a long training field of said data packet.

7. The method of claim 1 , wherein the fixed reception gain is a maximum reception gain.

8. The method of claim 1 , wherein a third threshold is also used for detecting autocorrelation peaks during detection of the beginning of the data packet and the third threshold is higher than the first and second thresholds.

9. The method of claim 1 , wherein the determined autocorrelation peaks are used to estimate a plurality of symbol boundaries, each corresponding to a different symbol in the first training field.

10. The method of claim 1 , wherein detection of the beginning of the data packet comprises jointly applying the following two criteria:

(1) detecting an autocorrelation peak above the first threshold during a first set of autocorrelation samples; and

(2) detecting an autocorrelation peak above a third threshold during a second set of autocorrelation samples,

wherein there are more samples in the first set of autocorrelation samples than in the second set of autocorrelation samples, and the third threshold is higher than the first threshold.

11. A non-transitory computer readable medium storing a computer program comprising program instructions, the computer program being loadable into a data processor to cause the data processor to:

detect a beginning of a data packet while a fixed reception gain is applied to the received OFDM signal, wherein the data packet comprises a first training field and a second training field beginning by a guard interval, and wherein the beginning of the data packet is determined using at least autocorrelation peaks and at least a first threshold is used for detecting autocorrelation peaks during detection of the beginning of the data packet;

start an automatic gain control process when triggered by the detection of the beginning of the data packet; and

after said automatic gain control process is locked, determine autocorrelation peaks in the OFDM signal during reception of the first training field, and estimating said symbol boundary from times of said autocorrelation peaks, wherein a second threshold, which is lower than the first threshold, is used for determining autocorrelation peaks used for estimating the symbol boundary.

12. A receiver configured to determine a symbol boundary in a data packet belonging to a received Orthogonal Frequency-Division Multiplexing, OFDM, signal, said receiver comprising:

a memory; and

a data processor coupled to the memory, wherein the data processor is configured to detect a beginning of a data packet while a fixed reception gain is applied to the received OFDM signal, wherein the data packet comprises a first training field and a second training field beginning by a guard interval, and wherein the beginning of the data packet is determined using at least autocorrelation peaks and at least a first threshold is used for detecting autocorrelation peaks during detection of the beginning of the data packet;

start an automatic gain control functional process when triggered by the detection of the beginning of the data packet; and

after said automatic gain control process is locked, determining autocorrelation peaks in the OFDM signal during reception of the first training field, and estimating said symbol boundary from the times of said autocorrelation peaks, wherein a second threshold, which is lower than the first threshold, is used for determining autocorrelation peaks used for estimating the symbol boundary.

13. The receiver according to claim 12 , wherein said symbol boundary is estimated from a mean value of several among the autocorrelation peaks.

14. The receiver according to claim 12 , wherein said detecting of the beginning of the data packet comprises detecting both:

a frame using a normalized autocorrelation function computed on said received OFDM signal, and

an energy saturation.

15. The receiver according to claim 14 , wherein said detecting of the beginning of the data packet involves detecting said beginning when either a frame or an energy saturation is detected.

16. The receiver according to claim 12 , wherein the determining of the autocorrelation peaks involves using a normalized autocorrelation function computed on said received OFDM signal.

17. The receiver according to claim 16 , wherein one of said autocorrelation peaks is determined above a predetermined threshold during a set of autocorrelation samples.

18. The receiver according to claim 12 , wherein said symbol boundary is determined between a short training field and a long training field of said data packet.

19. The receiver of claim 12 , wherein detection of the beginning of the data packet comprises jointly applying the following two criteria:

(1) detecting an autocorrelation peak above the first threshold during a first set of autocorrelation samples; and

(2) detecting an autocorrelation peak above a third-threshold during a second set of autocorrelation samples,

wherein there are more samples in the first set of autocorrelation samples than in the second set of autocorrelation samples, and the third threshold is higher than the first threshold.

20. A communication device comprising a receiver according to claim 12 .