Short training field for WiFi

1. A method for generating an orthogonal frequency division multiplex (OFDM) data unit for transmission via a communication channel, the OFDM data unit conforming to a first communication protocol, the method comprising:

generating a first training field to be included in a preamble of the OFDM data unit, wherein the first training field: i) conforms to a second communication protocol, and ii) has an integer number of OFDM symbols L N having a periodicity L P ;

generating a second training field to be included in the preamble after the first training field, wherein the second training field: i) conforms to the first communication protocol, ii) has an integer number of OFDM symbols H N having a periodicity H P , and iii) is based on a frequency sequence having non-zero values at an interval K, where K is an integer;

modulating the first training field using a first tone spacing L TS between adjacent OFDM tones;

modulating the second training field using a second tone spacing H TS , where the first tone spacing L TS is a multiple M of the second tone spacing H TS ;

generating the preamble to include at least the first training field and the second training field; and

generating the OFDM data unit to include at least the preamble; and

wherein generating the second training field comprises generating the second training field such that the periodicity H P : is i) proportional to the periodicity L P and the interval K, and ii) inversely proportional to the multiple M.

2. The method of claim 1 , wherein the interval K is equal to eight and the integer number L N is at least three.

3. The method of claim 2 , wherein:

the first training field is based on the frequency sequence having non-zero values at an interval K/2; and

generating the second training field comprises:

duplicating the frequency sequence to obtain a lower portion of the second training field, the lower portion have a negative frequency offset from a direct current tone; and

duplicating the frequency sequence to obtain an upper portion of the second training field, the upper portion having a positive frequency offset from the direct current tone.

4. The method of claim 1 , wherein the interval K is equal to four and the integer number L N is at least two.

5. The method of claim 1 , wherein the interval K is equal to 16 and the integer number L N is at least five.

6. The method of claim 1 , wherein:

the periodicity H P corresponds to a selected transmission mode of a plurality of transmission modes, each of the plurality of transmission modes corresponding to a different periodicity; and

generating the preamble comprises generating the preamble to indicate the selected transmission mode.

7. The method of claim 6 , further comprising selecting the selected transmission mode from the plurality of transmission modes based on a deployment usage of the communication channel.

8. The method of claim 1 , wherein the OFDM data unit is a downlink multi-user multiple input, multiple output orthogonal frequency division multiple access (MU-MIMO-OFDMA) data unit, and wherein the method further comprises causing the downlink MU-MIMO-OFDMA data unit to be transmitted to a communication device with a selective transmission power boost applied to OFDM tones of the second training field that are assigned to the communication device.

9. The method of claim 1 , wherein the OFDM data unit is an uplink orthogonal frequency division multiple access (OFDMA) data unit, and wherein the method further comprises causing the second training field and a data portion of the uplink OFDMA data unit to be transmitted to a communication device with a same normalized transmission power applied to OFDM tones that are assigned to the communication device.

10. The method of claim 1 , wherein the OFDM data unit is an uplink multi-user multiple input, multiple output (MU-MIMO) data unit, and wherein the method further comprises causing the second training field and a data portion of the uplink MU-MIMO data unit to be transmitted to a communication device with a same normalized transmission power applied to OFDM tones that are assigned to the communication device.

11. The method of claim 1 , wherein modulating the second training field comprises modulating each OFDM tone for the second training field using a steering matrix that corresponds to a same OFDM tone of a data portion of the data unit.

12. The method of claim 1 , wherein modulating the second training field comprises modulating each OFDM tone for the second training field using a steering matrix that is interpolated from steering matrices of neighboring OFDM tones of a data portion of the data unit.

13. The method of claim 1 , wherein modulating the second training field comprises modulating a plurality of consecutive OFDM tones for the second training field using a same transmit beamforming steering matrix.

14. The method of claim 1 , wherein:

generating the first training field comprises generating a legacy short training field; and

generating the second training field comprises generating a non-legacy short training field.

15. The method of claim 1 , wherein generating the OFDM data unit comprises omitting a data portion of the OFDM data unit.

16. A communication device that generates an orthogonal frequency division multiplex (OFDM) data unit for transmission via a communication channel, the OFDM data unit conforming to a first communication protocol, the communication device comprising:

a network interface device having one or more integrated circuits configured to:

generate a first training field to be included in a preamble of the OFDM data unit, wherein the first training field: i) conforms to a second communication protocol, and ii) has an integer number of OFDM symbols L N having a periodicity L P ,

generate a second training field to be included in the preamble after the first training field, wherein the second training field: i) conforms to the first communication protocol, ii) has an integer number of OFDM symbols H N having a periodicity Hp, and iii) is based on a frequency sequence having non-zero values at an interval K, where K is an integer,

modulate the first training field using a first tone spacing L TS between adjacent OFDM tones,

modulate the second training field using a second tone spacing H TS , where the first tone spacing L TS is a multiple M of the second tone spacing H TS ,

generate the preamble to include at least the first training field and the second training field, and

generate the OFDM data unit to include at least the preamble; and

wherein the periodicity H P is generated to be: i) proportional to the periodicity L P and the interval K, and ii) inversely proportional to the multiple M.

17. The communication device of claim 16 , wherein the interval K is equal to eight and the integer number L N is at least three.

18. The communication device of claim 16 , wherein the interval K is equal to four and the integer number L N is at least two.

19. The communication device of claim 16 , wherein the interval K is equal to 16 and the integer number L N is at least five.

20. The communication device of claim 16 , wherein:

the periodicity H P corresponds to a selected transmission mode of a plurality of transmission modes;

each of the plurality of transmission modes corresponding to a different periodicity; and

the one or more integrated circuits are configured to generate the preamble to indicate the selected transmission mode.

21. The communication device of claim 16 , wherein the OFDM data unit is an uplink orthogonal frequency division multiple access (OFDMA) data unit;

the one or more integrated circuits are configured to cause the second training field and a data portion of the uplink OFDMA data unit to be transmitted to a communication device with a same normalized transmission power applied to OFDM tones that are assigned to the communication device.

22. The communication device of claim 16 , wherein:

the OFDM data unit is an uplink multi-user multiple input, multiple output (MU-MIMO) data unit; and

the one or more integrated circuits are configured to cause the second training field and a data portion of the uplink MU-MIMO data unit to be transmitted to a communication device with a same normalized transmission power applied to OFDM tones that are assigned to the communication device.

23. The communication device of claim 16 , wherein the one or more integrated circuits are configured to modulate each OFDM tone for the second training field using a steering matrix that corresponds to a same OFDM tone of a data portion of the OFDM data unit.

24. The communication device of claim 16 , wherein the one or more integrated circuits are configured to modulate each OFDM tone for the second training field using a steering matrix that is interpolated from steering matrices of neighboring OFDM tones of a data portion of the OFDM data unit.

25. The communication device of claim 16 , wherein the one or more integrated circuits are configured to omit a data portion of the OFDM data unit.