Extended guard interval for outdoor WLAN

1. A method for generating a data unit for transmission via a communication channel, the method comprising:

generating a data portion of the data unit, including generating orthogonal frequency division multiplexing (OFDM) symbols of the data portion using a guard interval selected from a set of guard intervals that includes (i) a normal guard interval, (ii) a short guard interval, and (iii) a long guard interval;

generating a preamble of the data unit, including generating the preamble to indicate whether at least OFDM symbols of the data portion are generated using the normal guard interval, the short guard interval, or the long guard interval, wherein, when the at least OFDM symbols of the data portion are generated using the long guard interval, generating the preamble includes (i) generating two or more repetitions of a legacy signal field, and (ii) including the two or more repetitions of the legacy signal field in the preamble, wherein presence of the two or more repetitions of the legacy signal field in the preamble serves as an indication that the at least OFDM symbols of the data portion are generated using the long guard interval; and

generating the data unit to include the preamble and the data portion.

2. The method of claim 1 , wherein generating the preamble of the data unit includes generating a non-legacy signal field of the preamble, wherein the non-legacy signal field includes a guard interval indication set to indicate whether at least the OFDM symbols of the data portion are generated using the normal guard interval, the short guard interval, or the long guard interval.

3. The method of claim 2 , wherein the guard interval indication comprises two bits.

4. The method of claim 2 , wherein generating the preamble includes:

generating a first portion of the preamble, wherein the first portion of the preamble is (i) generated using the normal guard interval and (ii) includes the non-legacy signal field, and

generating a second portion of the preamble using the guard interval indicated by the guard interval indication in the non-legacy signal field.

5. The method of claim 1 , wherein generating the preamble of the data unit comprises generating one of (i) a regular guard interval mode preamble when the at least OFDM symbols of the data portion are generated using (a) the normal guard interval or (b) the short guard interval or (ii) an extension guard interval mode preamble when the at least OFDM symbols of the data portion are generated using the long guard interval, wherein generating the preamble includes formatting the preamble such that a receiving device can automatically detect whether the preamble corresponds to the regular guard interval preamble or to the extension guard interval preamble.

6. The method of claim 5 , wherein generating the extension guard interval mode preamble includes:

generating a non-legacy signal field to be included in the preamble, and

modulating the non-legacy signal field differently from a corresponding non-legacy signal field in the regular guard interval mode preamble, and

wherein the receiving device can automatically detect that the preamble corresponds to the extension guard interval mode preamble by detecting the modulation of the non-legacy signal field.

7. The method of claim 6 , wherein generating the non-legacy signal field includes generating the non-legacy signal field using the long guard interval.

8. The method of claim 5 , wherein generating the extension guard interval mode preamble includes generating, using the long guard interval, a non-legacy signal field to be included in the preamble, and wherein the receiving device can automatically detect that the preamble corresponds to the extension mode guard interval preamble by comparing results of autocorrelation of the non-legacy signal field performed using the long guard interval and autocorrelation of the non-legacy signal field performed using the normal guard interval.

9. The method of claim 1 , wherein the data unit conforms to a first communication protocol, and wherein generating the preamble further comprises generating the preamble such that (i) a legacy receiver configured to operate according to a legacy communication protocol but not according to the first communication protocol can determine a duration of the data unit and (ii) a receiver configured to operate according to the first communication protocol can detect that the data unit conforms to the first communication protocol.

10. An apparatus comprising:

a network interface configured to:

generate a data portion of the data unit, including generating orthogonal frequency division multiplexing (OFDM) symbols of the data portion using selected from a set of guard intervals that includes (i) a normal guard interval, (ii) a short guard interval and (iii) a long guard interval;

generate a preamble of the data unit, including generating the preamble to indicate whether at least OFDM symbols of the data portion are generated using the normal guard interval, the short guard interval, or the long guard interval, wherein, when the at least OFDM symbols of the data portion are generated using the long guard interval, generating the preamble includes (i) generating two or more repetitions of a legacy signal field, and (ii) including the two or more repetitions of the legacy signal field in the preamble, wherein presence of the two or more repetitions of the legacy signal field in the preamble serves as an indication that the at least OFDM symbols of the data portion are generated using the long guard interval; and

generate the data unit to include the preamble and the data portion.

11. The apparatus of claim 1 , wherein the network interface is further configured to generate a non-legacy signal field of the preamble, wherein the non-legacy signal field includes a guard interval indication set to indicate whether at least the OFDM symbols of the data portion are generated using the normal guard interval, the short guard interval, or the long guard interval.

12. The apparatus of claim 11 , wherein the guard interval indication comprises two bits.

13. The apparatus of claim 11 , wherein the network interface is further configured to:

generate a first portion of the preamble, wherein the first portion of the preamble is (i) generated using the normal guard interval and (ii) includes the non-legacy signal field, and

generate a second portion of the preamble using the guard interval indicated by the guard interval indication in the non-legacy signal field.

14. The apparatus of claim 10 , wherein generating the preamble of the data unit comprises generating one of (i) a regular guard interval mode preamble when the at least OFDM symbols of the data portion are generated using (a) the normal guard interval or (b) the short guard interval or (ii) an extension guard interval mode preamble when the at least OFDM symbols of the data portion are generated using the long guard interval, wherein generating the preamble includes formatting the preamble such that a receiving device can automatically detect whether the preamble corresponds to the regular guard interval preamble or to the extension guard interval preamble.

15. The apparatus of claim 14 , wherein generating the extension guard interval mode preamble includes:

generating a non-legacy signal field to be included in the preamble, and

modulating the non-legacy signal field differently from a corresponding non-legacy signal field in the regular guard interval mode preamble, and

wherein the receiving device can automatically detect that the preamble corresponds to the extension guard interval mode preamble by detecting the modulation of the non-legacy signal field.

16. The apparatus of claim 15 , wherein generating the non-legacy signal field includes generating the non-legacy signal field using the long guard interval.

17. The apparatus of claim 14 , wherein generating the extension guard interval mode preamble includes generating, using the long guard interval, a non-legacy signal field to be included in the preamble, and wherein the receiving device can automatically detect that the preamble corresponds to the extension mode guard interval preamble by comparing results of autocorrelation of the non-legacy signal field performed using the long guard interval and autocorrelation of the non-legacy signal field performed using the normal guard interval.

18. The apparatus of claim 10 , wherein the data unit conforms to a first communication protocol, and wherein generating the preamble further comprises generating the preamble such that (i) a legacy receiver configured to operate according to a legacy communication protocol but not according to the first communication protocol can determine a duration of the data unit and (ii) a receiver configured to operate according to the first communication protocol can detect that the data unit conforms to the first communication protocol.