Padding for orthogonal frequency division multiplexing (OFDM) symbols in a wireless communication system

1. A method for generating a multi-user physical layer (PHY) data unit for transmission via a communication channel, the method comprising:

determining, at a first communication device, a number of orthogonal frequency division multiplexing (OFDM) symbols for the multi-user PHY data unit based on respective sets of information bits corresponding to respective second communication devices;

determining, at the first communication device, a common boundary within a last OFDM symbol of the PHY data unit based on the respective sets of information bits corresponding to respective second communication devices;

adding, at the first communication device, respective numbers of pre-encoder padding bits to the respective sets of information bits corresponding to respective second communication devices to generate respective sets of padded information bits corresponding to the respective second communication devices, wherein the respective numbers of pre-encoder padding bits are determined such that the padded information bits, after being encoded, fill one or more OFDM symbols up to the common boundary within the last OFDM symbol;

encoding, at the first communication device, the respective sets of padded information bits to generate respective sets of coded bits;

generating, at the first communication device, the one or more OFDM symbols of the PHY data unit to include (i) the respective sets of coded information bits in an initial portion of the one or more OFDM symbols up to the common boundary within the last OFDM symbol, and (ii) respective sets of post-encoder padding bits, added to the respective sets of coded bits, in a remaining portion of the last OFDM symbol following the common boundary; and

including, at the first communication device, an indication of the common boundary in a PHY preamble of the PHY data unit to inform the second communication devices of the common boundary so that the second communication devices can stop decoding the one or more OFDM symbols after decoding the initial portion of the one or more OFDM symbols.

2. The method of claim 1 , further comprising:

for each of the second communication devices, determining, at the first communication device, a respective initial boundary within the last OFDM symbol; and

determining, at the first communication device, the common boundary based at least in part on the determined respective initial boundaries within the last OFDM symbol.

3. The method of claim 2 , wherein:

the method further comprises, for each of the second communication devices, determining, at the first communication device, a respective excess number of information bits that do not fit in a respective maximum integer number of OFDM symbols that can be entirely filled by the corresponding set of information bits; and

determining the initial boundary for each of the second communication devices comprises determining the respective initial boundary based at least in part on the determined respective excess number of information bits.

4. The method of claim 3 , wherein determining the common boundary comprises:

selecting, as the common boundary, one of the respective initial boundaries that corresponds to a maximum of the respective excess numbers of information bits.

5. The method of claim 1 , further comprising:

selectively adding, at the first communication device, an extension field after a data portion of the PHY data unit based on a position of the common boundary within the last OFDM symbol.

6. The method of claim 5 , further comprising:

determining, at the first communication device, a duration of the extension field based on the position of the common boundary within the last OFDM symbol.

7. The method of claim 1 , wherein including the indication of the common boundary in the PHY preamble comprises:

including, at the first communication device, the indication of the common boundary in a signal field of the PHY.

8. The method of claim 1 , further comprising:

generating, at the first communication device, each respective set of post-encoder padding bits to include a repetition of a subset of coded bits in the respective set of coded bits.

9. An apparatus, comprising:

a network interface device associated with a first communication device, wherein the network interface device includes one or more integrated circuit (IC) devices configured to:

determine a number of orthogonal frequency division multiplexing (OFDM) symbols for a multi-user physical layer (PHY) data unit based on respective sets of information bits corresponding to respective second communication devices, wherein the multi-user PHY data unit is for transmission via a communication channel,

determine a common boundary within a last OFDM symbol of the PHY data unit based on the respective sets of information bits corresponding to respective second communication devices,

add respective numbers of pre-encoder padding bits to the respective sets of information bits corresponding to respective second communication devices to generate respective sets of padded information bits corresponding to the respective second communication devices, wherein the respective numbers of pre-encoder padding bits are determined such that the padded information bits, after being encoded, fill one or more OFDM symbols up to the common boundary within the last OFDM symbol,

encode the respective sets of padded information bits to generate respective sets of coded bits,

generate the one or more OFDM symbols of the PHY data unit to include (i) the respective sets of coded information bits in an initial portion of the one or more OFDM symbols up to the common boundary within the last OFDM symbol, and (ii) respective sets of post-encoder padding bits, added to the respective sets of coded bits, in a remaining portion of the last OFDM symbol following the common boundary, and

include an indication of the common boundary in a PHY preamble of the PHY data unit to inform the second communication devices of the common boundary so that the second communication devices can stop decoding the one or more OFDM symbols after decoding the initial portion of the one or more OFDM symbols.

10. The apparatus of claim 9 , wherein the one or more IC devices are further configured to:

for each of the second communication devices, determine a respective initial boundary within the last OFDM symbol; and

determine the common boundary based at least in part on the determined respective initial boundaries within the last OFDM symbol.

11. The apparatus of claim 10 , wherein:

the one or more IC devices are further configured to: for each of the second communication devices, determine a respective excess number of information bits that do not fit in a respective maximum integer number of OFDM symbols that can be entirely filled by the corresponding set of information bits; and

determine the respective initial boundary based at least in part on the determined respective excess number of information bits.

12. The apparatus of claim 11 , wherein the one or more IC devices are further configured to:

select one of the respective initial boundaries that corresponds to a maximum of the respective excess numbers of information bits.

13. The apparatus of claim 9 , where the one or more IC devices are further configured to:

selectively add an extension field after a data portion of the PHY data unit based on a position of the common boundary within the last OFDM symbol.

14. The apparatus of claim 13 , wherein the one or more IC devices are further configured to:

determine a duration of the extension field based on the position of the common boundary within the last OFDM symbol.

15. The apparatus of claim 9 , wherein the one or more IC devices are further configured to:

include the indication of the common boundary in a signal field of the PHY.

16. The apparatus of claim 9 , wherein the one or more IC devices are further configured to:

generate each respective set of post-encoder padding bits to include a repetition of a subset of coded bits in the respective set of coded bits.

17. The apparatus of claim 9 , wherein the network interface device comprises:

a medium access control (MAC) processing unit implemented on the one or more IC devices; and

a physical layer (PHY) processing unit implemented on the one or more IC devices;

wherein the MAC processing unit is coupled to the PHY processing unit.

18. The apparatus of claim 17 , wherein:

the MAC processor is configured to:

determine the number of OFDM symbols for the multi-user PHY data unit,

determine the common boundary within the last OFDM symbol of the PHY data unit, and

add pre-encoder padding bits to the respective sets of information bits corresponding to respective second communication devices.

19. The apparatus of claim 18 , wherein the PHY processor is configured to generate the one or more OFDM symbols of the PHY data unit.

20. The apparatus of claim 19 , wherein the PHY processor comprises one or more transceivers implemented on the one or more IC devices.

21. The apparatus of claim 20 , further comprising:

one or more antennas coupled to the one or more IC devices.