Method and device for transmitting data unit

1. A method of communicating over an operating bandwidth of a wireless local area network, the method comprising:

receiving a first part of a physical layer protocol data unit (PPDU)

wherein the first part comprises a legacy-short training field (L-STF), a legacy-long training field (L-LTF), a first signal field, the first signal field having a first phase rotation,

wherein the first signal field is received over the operating bandwidth that is divided into a plurality of first sub-channels for communication of the first signal field,

wherein the first phase rotation was realized by multiplying a first multiplication value to at least one sub-channel of the plurality of the first sub-channels, the first multiplication value selected from +1, −1 and +j based on a sub-carrier index value range of a corresponding first sub-channel,

receiving a second part of the PPDU,

wherein the second part comprises an STF, an LTF, a second signal field, and a data portion, the second signal field having a second phase rotation,

wherein the second signal field is received over the operating bandwidth that is divided into a plurality of second sub-channels for communication of the second signal field,

wherein the second phase rotation was realized by multiplying a second multiplication value to at least one sub-channel of the plurality of the second sub-channels, the second multiplication value selected from +1 and −1, and

wherein the data portion is received over the operating bandwidth that is divided into a plurality of third sub-channels for communication of the data portion.

2. The method recited in claim 1 further comprising:

determining whether the second part of the PPDU has an orthogonal frequency division multiple access (OFDMA) allocation by examining an indication of the first signal field before receiving the second part of the PPDU.

3. The method recited in claim 1 wherein each of the plurality of first sub-channels for transmission of the first part has a bandwidth of 20 MHz and each of the plurality of second sub-channels for transmission of the second signal field of the second part has a bandwidth less than 20 MHz.

4. The method recited in claim 1

wherein the L-LTF of the first part has a first fast Fourier transform (FFT) size and the LTF of the second part has a second FFT size; and

wherein the second FFT size is four times greater than the first FFT size.

5. The method recited in claim 4

wherein the first FFT size is 64 and the second FFT size is 256 when the operating bandwidth is 20 MHz;

wherein the first FFT size is 128 and the second FFT size is 512 when the operating bandwidth is 40 MHz;

wherein the first FFT size is 256 and the second FFT size is 1024 when the operating bandwidth is 80 MHz; and

wherein the first FFT size is 512 and the second FFT size is 2048 when the operating bandwidth is 160 MHz.

6. The method recited in claim 1 , wherein symbol duration for the L-LTF is fixed.

7. The method recited in claim 1 , wherein guard interval for the L-LTF is fixed.

8. A device configured for communicating over an operating bandwidth in a wireless local area network, the device comprising:

a radio frequency module configured to transmit and receive radio signals; and

a processor operatively coupled with the radio frequency module;

memory disposed to said processor, said memory including instructions, when executed by said processor causing the device to:

receive a first part of a physical layer protocol data unit (PPDU)

wherein the first part comprises a legacy-short training field (L-STF), a legacy-long training field (L-LTF), a first signal field, the first signal field having a first phase rotation,

wherein the first signal field is received over the operating bandwidth that is divided into a plurality of first sub-channels for communication of the first signal field, and

wherein the first phase rotation was realized by multiplying a first multiplication value to at least one sub-channel of the plurality of the first sub-channels, the first multiplication value selected from +1, −1 and +j based on a sub-carrier index value range of a corresponding first sub-channel; and

receive a second part of the PPDU

wherein the second part comprises an STF, an LTF, a second signal field, and a data portion, the second signal field having a second phase rotation,

wherein the second signal field is received over the operating bandwidth that is divided into a plurality of second sub-channels for communication of the second signal field, and

wherein the second phase rotation was realized by multiplying a second multiplication value to at least one sub-channel of the plurality of the second sub-channels, the second multiplication value selected from +1 and −1, and

wherein the data portion is received over the operating bandwidth that is divided into a plurality of third sub-channels for communication of the data portion.

9. The device recited in claim 8 wherein said memory further comprising instruction for the processor to determine whether the second part of the PPDU has an orthogonal frequency division multiple access (OFDMA) allocation by examining an indication of the first signal field before receiving the second part of the PPDU.

10. The device recited in claim 8 wherein each of the plurality of first sub-channels for transmission of the first part has a bandwidth of 20 MHz and each of the plurality of second sub-channels for transmission of the second signal field of the second part has a bandwidth less than 20 MHz.

11. The device recited in claim 8

wherein the L-LTF of the first part has a first fast Fourier transform (FFT) size and the LTF of the second part has a second FFT size; and

wherein the second FFT size is four times greater than the first FFT size.

12. The device recited in claim 11

wherein the first FFT size is 64 and the second FFT size is 256 when the operating bandwidth is 20 MHz;

wherein the first FFT size is 128 and the second FFT size is 512 when the operating bandwidth is 40 MHz;

wherein the first FFT size is 256 and the second FFT size is 1024 when the operating bandwidth is 80 MHz; and

wherein the first FFT size is 512 and the second FFT size is 2048 when the operating bandwidth is 160 MHz.

13. The device recited in claim 8 , wherein symbol duration for the L-LTF is fixed.

14. The device recited in claim 8 , wherein guard interval for the L-LTF is fixed.