Short training field for wifi

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 LP, 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 LP;
• 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*LP;
• wherein generating the second PHY data unit comprises:
• modulating the third training field using a first tone spacing LTS between adjacent OFDM tones, and
• modulating the fourth training field using a second tone spacing equal to LTS/4 between adjacent OFDM tones;
• and
• transmitting, by the communication device, the second PHY data unit in response to the first PHY data unit.