Short training field for WiFi

1. A method, comprising:

receiving, at a communication device, a first physical layer (PHY) data unit via a communication channel, wherein the first PHY data unit corresponds to a trigger frame that is configured to prompt the communication device to transmit a second PHY data unit in response to receiving the first PHY data unit, wherein the first PHY data unit includes:

a first PHY preamble having a legacy portion and a non-legacy portion,

a first training field in the legacy portion of the first PHY preamble, wherein the first training field is for packet detection and for automatic gain control (AGC) adjustment, and wherein the first training field includes a first training signal having a periodicity L P , and

a second training field in the non-legacy portion of the first PHY preamble, wherein the second training field includes a second training signal having the periodicity L P ;

generating, at the communication device, the second PHY data unit, wherein the second PHY data unit includes:

a second PHY preamble having a legacy portion and a non-legacy portion,

a third training field in the legacy portion of the second PHY preamble, wherein the third training field is for packet detection and for AGC adjustment, and wherein the third training field includes a third training signal, and

a fourth training field in the non-legacy portion of the second PHY preamble, wherein the fourth training field includes a fourth training signal having a periodicity 2*L P ;

wherein generating the second PHY data unit comprises:

modulating the third training field using a first tone spacing L TS between adjacent OFDM tones, and

modulating the fourth training field using a second tone spacing equal to L TS /4 between adjacent OFDM tones; and

transmitting, by the communication device, the second PHY data unit in response to the first PHY data unit.

2. The method of claim 1 , wherein:

the periodicity L P of the second training signal is 0.8 microseconds; and

the periodicity of the fourth training signal is 1.6 microseconds.

3. The method of claim 1 , wherein:

the first training signal has the periodicity L P ; and

the third training signal has the periodicity L P .

4. The method of claim 1 , wherein:

the first tone spacing L TS is 312.5 kilohertz; and

the second tone spacing is 78.125 kilohertz.

5. The method of claim 1 , wherein generating the second PHY data unit comprises:

generating a first OFDM symbol corresponding to the third training field, wherein the first OFDM symbol includes i) non-zero value tones at intervals of K/2 tones, and ii) zero value tones between the non-zero value tones, wherein K is a positive even integer; and

generating a second OFDM symbol corresponding to the fourth training field, wherein the second OFDM symbol includes i) non-zero value tones at intervals of K tones, and ii) zero value tones between the non-zero value tones.

6. The method of claim 5 , wherein K is one of 4, 8, or 16.

7. The method of claim 1 , wherein generating the second PHY data unit comprises:

generating, at the communication device, a data portion of the second PHY data unit.

8. The method of claim 7 , wherein generating the data portion of the second PHY data unit comprises:

modulating information in the data portion using the second tone spacing.

9. The method of claim 1 , wherein:

the first PHY data unit is received from an access point (AP) of a wireless local area network (WLAN); and

the second PHY data unit is transmitted in an uplink (UL) transmission to the AP.

10. The method of claim 9 , wherein transmitting the second PHY data unit comprises transmitting the second PHY data unit as part of an UL multi-user transmission to the AP.

11. An apparatus, comprising:

a network interface device having one or more integrated circuit (IC) devices configured to:

receive a first physical layer (PHY) data unit via a communication channel, wherein the first PHY data unit corresponds to a trigger frame that is configured to prompt the network interface device to transmit a second PHY data unit in response to receiving the first PHY data unit, wherein the first PHY data unit includes:

a first PHY preamble having a legacy portion and a non-legacy portion,

a first training field in the legacy portion of the first PHY preamble, wherein the first training field is for packet detection and for automatic gain control (AGC) adjustment, and wherein the first training field includes a first training signal having a periodicity L P , and

a second training field in the non-legacy portion of the first PHY preamble, wherein the second training field includes a second training signal having the periodicity L P ;

wherein the one or more IC devices are further configured to:

generate the second PHY data unit, wherein the second PHY data unit includes:

a second PHY preamble having a legacy portion and a non-legacy portion,

a third training field in the legacy portion of the second PHY preamble, wherein the third training field is for packet detection and for AGC adjustment, and wherein the third training field includes a third training signal, and

a fourth training field in the non-legacy portion of the second PHY preamble, wherein the fourth training field includes a fourth training signal having a periodicity 2*L P ;

wherein generating the second PHY data unit comprises:

modulating the third training field using a first tone spacing L TS between adjacent OFDM tones, and

modulating the fourth training field using a second tone spacing equal to L TS /4 between adjacent OFDM tones; and

wherein the one or more IC devices are further configured to transmit the second PHY data unit in response to the first PHY data unit.

12. The apparatus of claim 11 , wherein:

the periodicity L P of the second training signal is 0.8 microseconds; and

the periodicity of the fourth training signal is 1.6 microseconds.

13. The apparatus of claim 11 , wherein:

the first training signal has the periodicity L P ; and

the third training signal has the periodicity L P .

14. The apparatus of claim 11 , wherein:

the first tone spacing L TS is 312.5 kilohertz; and

the second tone spacing is 78.125 kilohertz.

15. The apparatus of claim 11 , wherein the one or more IC devices are further configured to:

generate a first OFDM symbol corresponding to the third training field, wherein the first OFDM symbol includes i) non-zero value tones at intervals of K/2 tones, and ii) zero value tones between the non-zero value tones, wherein K is a positive even integer; and

generate a second OFDM symbol corresponding to the fourth training field, wherein the second OFDM symbol includes i) non-zero value tones at intervals of K tones, and ii) zero value tones between the non-zero value tones.

16. The apparatus of claim 15 , wherein K is one of 4, 8, or 16.

17. The apparatus of claim 11 , wherein the one or more IC devices are further configured to:

generate a data portion of the second PHY data unit.

18. The apparatus of claim 17 , wherein the one or more IC devices are further configured to:

modulate information in the data portion using the second tone spacing.

19. The apparatus of claim 11 , wherein:

the first PHY data unit is received from an access point (AP) of a wireless local area network (WLAN); and

the one or more IC devices are further configured to transmit the second PHY data unit in an uplink (UL) transmission to the AP.

20. The apparatus of claim 19 , wherein the one or more IC devices are further configured to transmit the second PHY data unit as part of an UL multi-user transmission to the AP.

21. The apparatus of claim 11 , wherein:

the network interface device comprises a physical layer (PHY) processing unit implemented on the one or more IC devices; and

the PHY processing unit is configured to:

receiver the first PHY data unit,

generate the second PHY data unit, and

transmit the second PHY data unit.

22. The apparatus of claim 21 , wherein the network interface device further comprises a media access control (MAC) processing unit implemented on the one or more IC devices, and wherein the MAC processing unit is coupled to the PHY processing unit.

23. The apparatus of claim 22 , wherein the PHY processing unit comprises one or more transceivers.

24. The apparatus of claim 23 , further comprising:

one or more antennas coupled to the one or more transceivers.