Uplink training for MIMO implicit beamforming

1. A method for beamforming in a multiple input multiple output (MIMO) communication system, the method comprising:

receiving a data unit from a communication device via a MIMO communication channel;

determining whether the data unit satisfies one or more selection criteria, wherein at least one of the one or more selection criteria i) is fixed, and ii) relates to one or more of a) a maximum number of spatial streams supported by the communication device and b) channel bandwidth being used for transmission to the communication device; and

when it is determined that the data unit satisfies the one or more selection criteria, selecting the data unit to be used in developing a steering matrix for transmitting data units to the communication device,

wherein developing the steering matrix comprises:

developing, based on an uplink data unit, an estimate of a reverse channel via which the data unit was received;

developing an estimate of a forward channel based on the estimate of the reverse channel; and

developing the steering matrix to perform beamforming in the forward channel based on the estimate of the forward channel.

2. The method of claim 1 , further comprising dynamically determining at least one selection criterion of the one or more selection criteria.

3. The method of claim 2 , wherein dynamically determining the selection criterion comprises determining the selection criterion based on a signal strength associated with one or more data units received from the communication device.

4. The method of claim 2 , wherein dynamically determining the selection criterion comprises:

determining a number of data units (i) that were received from the communication device during a time period of a certain duration and (ii) that were transmitted using at least a certain number of spatial streams; and

setting the selection criterion based on the number of data units.

5. The method of claim 4 , wherein dynamically determining the selection criterion comprises:

determining whether the number of data units is greater than or equal to a predetermined number; and

setting the selection criterion based on whether the number of data units is greater than or equal to the predetermined number.

6. The method of claim 5 , wherein at least one of i) the duration of the time period and ii) the predetermined number is adjustable.

7. The method of claim 1 , wherein the data unit is a non-sounding data unit.

8. An apparatus for use in a communication system, the apparatus comprising:

a network interface device configured to

receive a data unit from a communication device via a multiple input multiple output (MIMO) communication channel,

determine whether the data unit satisfies one or more selection criteria, wherein at least one selection criterion of the one or more selection criteria i) is fixed, and ii) is based on one or more of a) a maximum number of spatial streams supported by the communication device and b) channel bandwidth being used for transmission to the communication device,

when it is determined that the data unit satisfies the one or more selection criteria, select an uplink data unit to be used in developing a steering matrix for transmitting data units to the communication device,

develop, based on the data unit, an estimate of a reverse channel via which the data unit was transmitted,

develop an estimate of a forward channel based on the estimate of the reverse channel, and

develop the steering matrix to perform beamforming in the forward channel based on the estimate of the forward channel.

9. The apparatus of claim 8 , wherein the network interface device is further configured to dynamically determine at least one selection criterion of the one or more selection criteria.

10. The apparatus of claim 9 , wherein the network interface device is configured to dynamically determine the selection criterion at least based on a signal strength associated with one or more uplink data units received from the communication device.

11. The apparatus of claim 9 , wherein the network interface device is configured to dynamically determine the selection criterion at least by

determining a number of data units (i) that were received from the communication device during a time period of a certain duration and (ii) that were transmitted using at least a certain number of spatial streams; and

setting the selection criterion based on the number of data units.

12. The apparatus of claim 11 , wherein the network interface device is configured to dynamically determine the selection criterion at least by

determining whether the number of data units is greater than or equal to a predetermined number; and

setting the selection criterion based on whether the number data units is greater than or equal to the predetermined number.

13. The apparatus of claim 12 , wherein at least one of i) the duration of the time period and ii) the predetermined number is adjustable.

14. The apparatus of claim 8 , wherein the data unit is a non-sounding data unit.

15. A wireless communication system comprising:

a first communication device having

a plurality of antennas, and

a first network interface device; and

a second communication device having a second network interface device,

wherein the first network interface device of the first communication device is configured to determine steering matrices for use in transmitting data from the first communication device to the second communication device via a wireless communication channel, wherein the steering matrices are determined based on data units received from the second communication device, and

wherein the second network interface device of the second communication device is configured to, when communicating with the first communication device using a number of spatial streams that is less than a maximum number of spatial streams supported by the second communication device, transmit, during a time period of a certain duration, a minimum number of data units using the maximum number of spatial streams supported by the second communication device.

16. The communication system of claim 15 , wherein at least one of i) the duration of the time period and ii) the minimum number of data units is adjustable.

17. The communication system of claim 14 , wherein the second network interface device of the second communication device is configured to, during the time period of duration T,

transmit, in accordance with a first modulation and coding scheme (MCS), data units using the maximum number of spatial streams supported by the second communication device,

wherein, the first MCS corresponds to a first data rate that is lower than a second data rate corresponding to a second MCS used by the second communication device to transmit data units using the number of spatial streams that is less than the maximum number of spatial streams supported by the second communication device.

18. A method for beamforming in a multiple input multiple output (MIMO) communication system, the method comprising:

receiving a data unit from a communication device via a MIMO communication channel;

determining whether the data unit satisfies one or more selection criteria;

dynamically determining at least one selection criterion of the one or more selection criteria based on a signal strength associated with one or more data units received from the communication device;

when it is determined that the data unit satisfies the one or more selection criteria, selecting the data unit to be used in developing a steering matrix for transmitting data units to the communication device,

wherein developing the steering matrix comprises:

developing, based on an uplink data unit, an estimate of a reverse channel via which the data unit was received;

developing an estimate of a forward channel based on the estimate of the reverse channel; and

developing the steering matrix to perform beamforming in the forward channel based on the estimate of the forward channel.

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

receiving a data unit from a communication device via a MIMO communication channel;

determining whether the data unit satisfies one or more selection criteria; and

dynamically determining at least one selection criterion of the one or more selection criteria, including

determining a number of data units (i) that were received from the communication device during a time period of a certain duration and (ii) that were transmitted using at least a certain number of spatial streams, and

setting the selection criterion based on the number of data units;

when it is determined that the data unit satisfies the one or more selection criteria, selecting the data unit to be used in developing a steering matrix for transmitting data units to the communication device,

wherein developing the steering matrix comprises:

developing, based on an uplink data unit, an estimate of a reverse channel via which the data unit was received;

developing an estimate of a forward channel based on the estimate of the reverse channel; and

developing the steering matrix to perform beamforming in the forward channel based on the estimate of the forward channel.

20. The method of claim 19 , wherein dynamically determining the selection criterion comprises:

determining whether the number of data units is greater than or equal to a predetermined number; and

setting the selection criterion based on whether the number of data units is greater than or equal to the predetermined number.

21. The method of claim 20 , wherein at least one of i) the duration of the time period and ii) the predetermined number is adjustable.

22. An apparatus for use in a communication system, the apparatus comprising:

a network interface device configured to

receive a data unit from a communication device via a multiple input multiple output (MIMO) communication channel,

determine whether the data unit satisfies one or more selection criteria,

dynamically determine at least one selection criterion of the one or more selection criteria based on a signal strength associated with one or more data units received from the communication device,

when it is determined that the data unit satisfies the one or more selection criteria, select an uplink data unit to be used in developing a steering matrix for transmitting data units to the communication device,

develop, based on the data unit, an estimate of a reverse channel via which the data unit was transmitted,

develop an estimate of a forward channel based on the estimate of the reverse channel, and

develop the steering matrix to perform beamforming in the forward channel based on the estimate of the forward channel.

23. An apparatus for use in a communication system, the apparatus comprising:

a network interface device configured to

receive a data unit from a communication device via a multiple input multiple output (MIMO) communication channel,

determine whether the data unit satisfies one or more selection criteria,

dynamically determine at least one selection criterion of the one or more selection criteria, including

determining a number of data units (i) that were received from the communication device during a time period of a certain duration and (ii) that were transmitted using at least a certain number of spatial streams, and

setting the selection criterion based on the number of data units,

when it is determined that the data unit satisfies the one or more selection criteria, select an uplink data unit to be used in developing a steering matrix for transmitting data units to the communication device,

develop, based on the data unit, an estimate of a reverse channel via which the data unit was transmitted,

develop an estimate of a forward channel based on the estimate of the reverse channel, and

develop the steering matrix to perform beamforming in the forward channel based on the estimate of the forward channel.

24. The apparatus of claim 23 , wherein the network interface device is configured to dynamically determine the selection criterion at least by

determining a number of data units (i) that were received from the communication device during a time period of a certain duration and (ii) that were transmitted using at least a certain number of spatial streams; and

setting the selection criterion based on the number of data units.

25. The apparatus of claim 24 , wherein the network interface device is configured to dynamically determine the selection criterion at least by

determining whether the number of data units is greater than or equal to a predetermined number; and

setting the selection criterion based on whether the number data units is greater than or equal to the predetermined number.

26. The apparatus of claim 25 , wherein at least one of i) the duration of the time period and ii) the predetermined number is adjustable.