Short training field for WiFi

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 LN having a periodicity LP,
• 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 HN 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 LTS between adjacent OFDM tones,
• modulate the second training field using a second tone spacing HTS, where the first tone spacing LTS is a multiple M of the second tone spacing HTS,
• 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 HP is generated to be:
• i) proportional to the periodicity LP and the interval K, and ii) inversely proportional to the multiple M.