Multi-user data unit arrival time adjustment

1. A method for adjusting an arrival time of a first orthogonal frequency division multiplex (OFDM) data unit of an uplink multi-user (MU) data unit, the method comprising:

determining, by a first communication device, respective round trip times (RTTs) for multiple communication devices that communicate with the first communication device;

determining, by the first communication device, a first time offset for a second communication device of the multiple communication devices based on the respective RTTs, wherein the first OFDM data unit is to be transmitted by the second communication device using the first time offset, the first time offset being determined so that the first OFDM data unit and a second OFDM data unit of the uplink MU data unit have a substantially same arrival time at the first communication device, the second OFDM data unit being transmitted by a third communication device of the multiple communication devices;

generating, by the first communication device, a first timing control message that indicates the first time offset; and

causing, by the first communication device, the first timing control message to be transmitted to the second communication device.

2. The method of claim 1 , wherein the method further comprises:

causing, by the first communication device, a trigger frame to be transmitted to the multiple communication devices; and

receiving, by the first communication device, the uplink MU data unit;

wherein:

the uplink MU data unit includes i) the first OFDM data unit transmitted by the second communication device using the first time offset, and ii) the second OFDM data unit transmitted by the third communication device; and

the first time offset is determined so that the arrival time of the first OFDM data unit and an arrival time of the second OFDM data unit are within a guard interval duration.

3. The method of claim 1 , wherein the method further comprises:

determining, by the first communication device, a second time offset for the second communication device based on the first time offset, wherein the second time offset is a differential offset to be combined with the first time offset;

generating, by the first communication device, a second timing control message that indicates the second time offset; and

causing, by the first communication device, the second timing control message to be transmitted to the second communication device.

4. The method of claim 1 , wherein determining the respective RTTs for the multiple communication devices comprises receiving respective control frames that include the respective RTTs from the multiple communication devices.

5. The method of claim 4 , wherein determining the respective RTTs for the multiple communication devices further comprises sequentially requesting a next control frame of the respective control frames from a corresponding communication device of the multiple communication devices in response to receipt of a previous control frame of the respective control frames.

6. The method of claim 4 , wherein:

the uplink MU data unit includes OFDM data units from a plurality of communication devices; and

the multiple communication devices are a subset of the plurality of communication devices.

7. The method of claim 6 , wherein determining the respective RTTs for the multiple communication devices further comprises:

generating a downlink MU data unit that indicates a request for the respective control frames that include the respective RTTs from the multiple communication devices; and

causing the downlink MU data unit to be transmitted to the multiple communication devices.

8. The method of claim 6 , wherein the downlink MU data unit is a multicast frame that indicates the request.

9. A first communication device for adjusting an arrival time of a first orthogonal frequency division multiplex (OFDM) data unit of an uplink multi-user (MU) data unit, the first communication device comprising:

a network interface device having one or more integrated circuits configured to

determine respective round trip times (RTTs) for multiple communication devices that communicate with the first communication device,

determine a first time offset for a second communication device of the multiple communication devices based on the respective RTTs, wherein the first OFDM data unit is to be transmitted by the second communication device using the first time offset, the first time offset being determined so that the first OFDM data unit and a second OFDM data unit of the uplink MU data unit have a substantially same arrival time at the first communication device, the second OFDM data unit being transmitted by a third communication device of the multiple communication devices,

generate a first timing control message that indicates the first time offset, and

cause the first timing control message to be transmitted to the second communication device.

10. The first communication device of claim 9 , wherein:

the one or more integrated circuits are configured to:

cause a trigger frame to be transmitted to the multiple communication devices, and

receive the uplink MU data unit;

the uplink MU data unit includes i) the first OFDM data unit transmitted by the second communication device using the first time offset in response to the trigger frame, and ii) the second OFDM data unit transmitted by the third communication device in response to the trigger frame; and

the first time offset is determined so that the arrival time of the first OFDM data unit and an arrival time of the second OFDM data unit are within a guard interval duration.

11. The first communication device of claim 9 , wherein the one or more integrated circuits are configured to:

determine a second time offset for the second communication device based on the first time offset, wherein the second time offset is a differential offset to be combined with the first time offset;

generate a second timing control message that indicates the second time offset; and

cause the second timing control message to be transmitted to the second communication device.

12. The first communication device of claim 9 , wherein the one or more integrated circuits are configured to receive respective control frames that include the respective RTTs from the multiple communication devices.

13. A method for adjusting an arrival time of a first orthogonal frequency division multiplex (OFDM) data unit of an uplink multi-user (MU) data unit, the method comprising:

determining, by a first communication device, a round trip time (RTT) for communications with a second communication device;

causing, by the first communication device, the RTT to be transmitted to the second communication device;

receiving, by the first communication device and from the second communication device, a first time offset that is based on the RTT;

generating, by the first communication device, the first OFDM data unit; and

causing, by the first communication device, the first OFDM data unit to be transmitted to the second communication device after a first transmission delay that is based on i) an interframe space associated with the first OFDM data unit, and ii) the first time offset, so that the first OFDM data unit and a second OFDM data unit of the uplink MU data unit have a substantially same arrival time at the second communication device, the second OFDM data unit being transmitted by a third communication device.

14. The method of claim 13 , wherein the method further comprises:

receiving, by the first communication device and from the second communication device, a first trigger frame that indicates a request to transmit the first OFDM data unit; and

determining the transmission delay to start at an end of the trigger frame and to include a sum of the interframe space and the first time offset.

15. The method of claim 13 , wherein the method further comprises:

receiving, by the first communication device and from the second communication device, a first trigger frame that indicates a request to transmit a second OFDM data unit;

receiving a second time offset that is a differential offset to be combined with the first time offset; and

determining a second transmission delay to start at an end of the second trigger frame and to include a sum of the first transmission delay and the second time offset.

16. The method of claim 13 , wherein the method further comprises receiving, by the first communication device and from the second communication device, a multicast frame that includes a request to determine the RTT for communications with the second communication device.

17. The method of claim 13 , wherein causing the first OFDM data unit to be transmitted to the second communication device includes causing the first OFDM data unit to be transmitted as a portion of a multiple user, orthogonal frequency division multiple access (MU-OFDMA) data unit.

18. The method of claim 13 , wherein causing the first OFDM data unit to be transmitted to the second communication device includes causing the first OFDM data unit to be transmitted as a portion of a multiple user, multiple input multiple output (MU-MIMO) data unit.

19. A first communication device for adjusting an arrival time of a first orthogonal frequency division multiplex (OFDM) data unit of an uplink multi-user (MU) data unit, the first communication device comprising:

a network interface device having one or more integrated circuits configured to

determine a round trip time (RTT) for communications with a second communication device,

cause the RTT to be transmitted to the second communication device,

receive, from the second communication device, a first time offset that is based on the RTT,

generate the first OFDM data unit, and

cause the first OFDM data unit to be transmitted to the second communication device after a first transmission delay that is based on i) an interframe space associated with the first OFDM data unit, and ii) the first time offset, so that the first OFDM data unit and a second OFDM data unit of the uplink MU data unit have a substantially same arrival time at the second communication device, the second OFDM data unit being transmitted by a third communication device.

20. The first communication device of claim 19 , wherein the one or more integrated circuits are configured to:

receive, from the second communication device, a first trigger frame that indicates a request to transmit the first OFDM data unit; and

determine the transmission delay to start at an end of the trigger frame and to include a sum of the interframe space and the first time offset.

21. The first communication device of claim 19 , wherein the one or more integrated circuits are configured to:

receive, from the second communication device, a first trigger frame that indicates a request to transmit a second OFDM data unit;

receive, from the second communication device, a second time offset that is a differential offset to be combined with the first time offset; and

determine a second transmission delay to start at an end of the second trigger frame and to include a sum of the first transmission delay and the second time offset.

22. The first communication device of claim 19 , wherein the one or more integrated circuits are configured to receive, from the second communication device, a multicast frame that includes a request to determine the RTT for communications with the second communication device.