Frame formats for distributed MIMO

1. A method for a multiple-input multiple-output (MIMO) communication for a first access point and a second access point, the method comprising:

transmitting, during a first time period before a second time period, a trigger frame to the second access point, the second access point to estimate, according to the trigger frame, synchronization information for a first null data packet announcement;

transmitting, during a third time period between the second time period and the first time period, the first null data packet announcement to a device; and

transmitting, during the second time period after the first time period, by the first access point, a portion of a null data packet to the device, the second access point transmitting the portion of the null data packet to the device, according to the synchronization information for a joint transmission by the first access point and the second access point to the device, the device to determine steering information for the MIMO communication, according to the null data packet.

2. The method of claim 1 , wherein the second access point estimates the synchronization information by estimating a carrier frequency offset or a sampling frequency offset with respect to the first access point.

3. The method of claim 1 , further comprising:

transmitting, by the second access point during the third time period between the first time period and the second time period, a second null data packet announcement to the device according to the synchronization information, the device to prepare for the null data packet, according to the first and second null data packet announcement transmitted by the first access point and the second access point.

4. The method of claim 1 , wherein the second access point transmits the portion of the null data packet during the second time period.

5. A first access point comprising:

a transceiver; and

circuitry configured to:

cause the transceiver to transmit to a second access point during a first time period, information for a joint transmission by the first access point and the second access point to a device, the second access point to estimate synchronization information for the joint transmission according to the information for the joint transmission, and

cause the transceiver to transmit to the device during a second time period after the first time period, a portion of a null data packet, while the second access point transmits the portion of the null data packet to the device according to the synchronization information, the device to determine, according to the null data packet, steering information for a multiple-input multiple-output (MIMO) communication to determine steering vectors for the joint transmission by the first access point and the second access point to the device, wherein the circuitry is further configured to cause the transceiver to:

transmit, during a third time period before the first time period, a trigger frame to the second access point, the second access point to estimate, according to the trigger frame, additional synchronization information for a null data packet announcement, and

transmit, during a fourth time period between the third time period and the first time period, the null data packet announcement to the device, while the second access point transmits the null data packet announcement to the device during the fourth time period according to the additional synchronization information, the device to prepare for the null data packet, according to the null data packet announcement transmitted by the first access point and the second access point.

6. The first access point of claim 5 , wherein the circuitry is further configured to cause the transceiver to transmit the information for the joint transmission in a preamble of the null data packet.

7. The first access point of claim 5 , wherein the circuitry is further configured to cause the transceiver to transmit the information for the joint transmission in the null data packet announcement, the device to prepare for the null data packet, according to the null data packet announcement.

8. The first access point of claim 7 , wherein the circuitry is further configured to cause the transceiver to transmit, during a fifth time period between the first time period and the second time period, a training field to the device, while the second access point transmits the training field to the device during the fourth time period, the device to adjust a gain setting to receive the portion of the null data packet, according to the training field transmitted by the first access point and the second access point.

9. The first access point of claim 5 , wherein the circuitry is further configured to cause the transceiver to:

transmit, during the fourth time period between the first time period and the second time period, the null data packet announcement to the device, while the second access point transmits the null data packet announcement to the device during the fourth time period according to the synchronization information, the device to prepare for the null data packet, according to the null data packet announcement transmitted by the first access point and the second access point.

10. The first access point of claim 5 , wherein the information for the joint transmission includes two or more long training field (LTF) symbol groups that are separated by at least a symbol.

11. The first access point of claim 5 , wherein the second time period is after the first time period.

12. A first access point comprising:

a transceiver; and

circuitry configured to:

cause the transceiver to receive, during a first time period, from a second access point, information for a joint transmission by the first access point and the second access point to a device,

estimate synchronization information for the joint transmission, according to the information for the joint transmission, and

cause the transceiver to transmit, during a second time period after the first time period, a portion of a null data packet to the device according to the synchronization information, while the second access point transmits the portion of the null data packet to the device during the second time period, the device to determine, according to the portion of the null data packet, steering information for a multiple-input multiple-output (MIMO) communication to determine steering vectors for the joint transmission by the first access point and the second access point to the device; and

cause the transceiver to receive, during a third time period before the first time period, a trigger frame to the second access point, the second access point to estimate, according to the trigger frame, additional synchronization information for a null data packet announcement, and

cause the transceiver to transmit, during a fourth time period between the third time period and the first time period, the null data packet announcement to the device.

13. The first access point of claim 12 , wherein the circuitry is configured to estimate the synchronization information for the joint transmission by estimating a carrier frequency offset, a sampling frequency offset, a first reference value for a common phase offset, or a second reference value for a timing offset with respect to the second access point.

14. The first access point of claim 12 , wherein the circuitry is configured to cause the transceiver to receive the information for the joint transmission in a preamble of the null data packet.

15. The first access point of claim 12 , wherein the circuitry is configured to cause the transceiver to receive the information for the joint transmission in the null data packet announcement, the device to prepare for the null data packet, according to the null data packet announcement.

16. The first access point of claim 15 , wherein the circuitry is configured to cause the transceiver to transmit, during a fifth time period between the first time period and the second time period, a training field to the device, while the second access point transmits the training field to the device during the third time period, the device to adjust a gain setting to receive the portion of the null data packet, according to the training field transmitted by the first access point and the second access point.

17. The first access point of claim 12 , wherein the circuitry is configured to cause the transceiver to:

transmit, during the third time period between the first time period and the second time period, the null data packet announcement to the device, while the second access point transmits the null data packet announcement to the device during the third time period according to the synchronization information, the device to prepare for the null data packet, according to the null data packet announcement transmitted by the first access point and the second access point.

18. The first access point of claim 12 , wherein the second time period is after the first time period.

19. A method for a multiple-input multiple-output (MIMO) communication, the method comprising:

transmitting, during a first time period before a second time period, a first null data packet announcement to a device from a first access point;

transmitting by the first access point to a second access point during the second time period, information for a joint transmission by the first access point and the second access point to the device, the second access point to estimate synchronization information for the joint transmission according to the information for the joint transmission; and

transmitting, during a third time period after the second time period and the first time period, by the first access point, a portion of a null data packet to the device, the second access point to transmit the portion of the null data packet to the device, according to the synchronization information for the joint transmission by the first access point and the second access point to the device, the device to determine steering information for the MIMO communication, according to the null data packet.

20. The method of claim 19 , wherein the first null data packet announcement and the information for the joint transmission are provided in same transmission.

21. The method of claim 19 , wherein the first null data packet announcement and the information for the joint transmission are provided in different transmissions.

22. The method of claim 20 , further comprising:

transmitting, by the second access point during the first time period a second null data packet announcement to the device according to the synchronization information, the device to prepare for the null data packet, according to the first and second null data packet announcement transmitted by the first access point and the second access point.

23. A first access point comprising:

a transceiver; and

circuitry configured to:

cause the transceiver to receive, during a first time period, from a second access point, information for a joint transmission by the first access point and the second access point to a device,

estimate synchronization information for the joint transmission, according to the information for the joint transmission, and

cause the transceiver to transmit, during a second time period after the first time period, a portion of a null data packet to the device according to the synchronization information, while the second access point transmits the portion of the null data packet to the device during the second time period, the device to determine, according to the portion of the null data packet, steering information for a multiple-input multiple-output (MIMO) communication to determine steering vectors for the joint transmission by the first access point and the second access point to the device, wherein the circuitry is further configured to:

cause the transceiver to receive, during a third time period before the first time period, from the second access point, a trigger frame, and

estimate additional synchronization information for a null data packet announcement, according to the trigger frame, and

cause the transceiver to transmit, during a fourth time period between the third time period and the first time period, the null data packet announcement to the device according to the additional synchronization information, while the second access point transmits the null data packet announcement to the device during the fourth time period, the device to prepare for the null data packet, according to the null data packet announcement transmitted by the first access point and the second access point.