Sounding method

1. A sounding method by a first receiving device, the method comprising:

receiving a null data packet announcement (NDPA) frame from a transmitting device;

receiving a null data packet (NDP) frame from the transmitting device after receiving the NPDA frame; and

transmitting to the transmitting device a feedback frame including subchannel information measured on a first subchannel after receiving the NDP frame, the first subchannel being a subchannel that is allocated to the first receiving device among a plurality of subchannels into which a predetermined band is divided,

wherein transmitting the feedback frame includes:

transmitting the feedback frame to the transmitting device while a second feedback frame including subchannel information measured on the second subchannel is transmitted to the transmitting device by a second receiving device, the second subchannel being a subchannel that is allocated to the second receiving device among the plurality of subchannels.

2. The method of claim 1 , wherein the subchannel information includes an average signal-to-noise ratio (SNR) of the first subchannel.

3. The method of claim 1 , wherein the subchannel information includes average signal-to-noise ratios (SNRs) of the first subchannel for a plurality of space-time streams.

4. The method of claim 1 , wherein the feedback frame further includes subchannel information measured on other subchannels excluding the first subchannel.

5. The method of claim 1 , wherein transmitting the feedback frame includes

adding pad bits to a data field of the feedback frame when a length of data to be transmitted by the feedback frame is shorter than a predetermined length, or partitioning the data into a plurality of fragments and inserting any one of the fragments to the data field of the feedback frame when the length of the data is longer than the predetermined length.

6. The method of claim 5 , wherein the NDPA frame indicates information corresponding to the predetermined length.

7. The method of claim 1 , wherein the NDPA frame includes allocation information of the first subchannel.

8. The method of claim 1 , wherein the subchannel information is measured on each of a predetermined number of subchannels including the first subchannel, and

wherein the first subchannel is allocated based on the subchannel information.

9. The method of claim 8 , wherein the NDPA frame includes information on the predetermined number.

10. The method of claim 8 , further comprising receiving a frame including selected subchannel information from the transmitting device, the selected subchannel information indicating a subchannel that is selected by other receiving device among the plurality of subchannels,

wherein the predetermined number of subchannels do not include the subchannel that that is selected by the other receiving device.

11. The method of claim 10 , wherein the selected subchannel information is represented by a bitmap having a plurality of bits that correspond to the plurality of subchannels respectively, and

wherein a predetermined value in each of the plurality of bits indicates that a corresponding subchannel is selected by the other receiving device.

12. The method of claim 1 , wherein transmitting the feedback frame includes:

transmitting a subchannel information frame to the transmitting device after receiving the NDP frame, the subchannel information frame including measurement information that is measured on each of the plurality of subchannels by the first receiving device;

receiving from the transmitting device a channel feedback trigger frame including allocation information of the plurality of subchannels; and

transmitting the feedback frame to the transmitting device after receiving the channel feedback trigger frame.

13. The method of claim 12 , wherein transmitting the subchannel information frame includes

transmitting the subchannel information frame to the transmitting device before a second subchannel information frame is transmitted to the transmitting device by the second receiving device or after the second subchannel information frame is transmitted to the transmitting device by the second receiving device, the second subchannel information frame including measurement information that is measured on each of the plurality of subchannels by the second receiving device.

14. The method of claim 12 , wherein transmitting the subchannel information frame includes

transmitting the subchannel information frame to the transmitting device while a second subchannel information frame is transmitted to the transmitting device by the second receiving device, the second subchannel information frame including measurement information that is measured on each of the plurality of subchannels by the second receiving device.

15. The method of claim 1 , further comprising:

receiving a trigger frame from a transmitting device; and

transmitting a frame including a plurality of long training fields to the transmitting device after receiving the trigger frame,

wherein the plurality of long training fields are multiplied by a predetermined identification code allocated to the first receiving device among a plurality of receiving devices.

16. The method of claim 15 , wherein a number of the plurality of long training fields is equal to a number of the plurality of receiving devices.

17. The method of claim 15 , wherein the trigger frame includes information on a predetermined identification code allocated to each of the plurality of receiving devices.

18. A sounding method by a transmitting device, the method comprising:

transmitting a null data packet announcement (NDPA) frame to a plurality of receiving devices;

transmitting a null data packet (NDP) frame to the plurality of receiving devices after transmitting the NPDA frame; and

receiving from each receiving device a feedback frame including subchannel information measured on a subchannel that is allocated to each receiving device among a plurality of subchannels into which a band is divided, after transmitting the NDP frame,

wherein the plurality of feedback frames from the plurality of receiving devices are received at a same time, and the NDPA frame includes allocation information of the plurality of subchannels.