Methods and apparatus for multiplexing reference signals for multiple input multiple output (MIMO) layers

1. A method of wireless communication, comprising:

assigning resources for Demodulation Reference signals (DMRS) corresponding to each of two or more data streams, wherein the resources assigned to each of the data streams are staggered in frequency and span two or more Orthogonal Frequency Division Multiplexing (OFDM) symbols, wherein each of the assigned resources is assigned to multiplexed DMRS corresponding to a combination of the two or more data streams, wherein the multiplexed DMRS corresponding to the two or more data streams are multiplexed using time-domain cyclic shifts over each of the two or more OFDM symbols, wherein a different code sequence is used for each of the two or more data streams in each of the two or more OFDM symbols,

wherein the resources assigned to the multiplexed DMRS corresponding to a same combination of two or more data streams occupy all tones of a resource block such that a different set of tones of the resource block is assigned to each of two different OFDM symbols; and

transmitting the DMRS using the assigned resources.

2. The method of claim 1 , wherein the time-domain cyclic are achieved by phase ramping in frequency domain.

3. The method of claim 1 , further comprising assigning the same code sequence to DMRSs corresponding to the two or more data streams.

4. The method of claim 1 , wherein the two or more OFDM symbols comprise non-adjacent OFDM symbols.

5. The method of claim 1 , further comprising multiplexing DMRSs corresponding to a different combination of data streams in different OFDM symbols.

6. The method of claim 1 , wherein the two or more OFDM symbols comprise adjacent OFDM symbols.

7. The method of claim 1 , further comprising, for a group of multiplexed DMRSs corresponding to multiplexed data streams, repeating DMRS pattern of a first set of OFDM symbols over a second set of OFDM symbols.

8. An apparatus for wireless communication, comprising:

means for assigning resources for Demodulation Reference signals (DMRS) corresponding to each of two or more data streams, wherein the resources assigned to each of the data streams are staggered in frequency and span two or more Orthogonal Frequency Division Multiplexing (OFDM) symbols, wherein each of the assigned resources is assigned to multiplexed DMRS corresponding to a combination of the two or more data streams, wherein the multiplexed DMRS corresponding to the two or more data streams are multiplexed using time-domain cyclic shifts over each of the two or more OFDM symbols, wherein a different code sequence is used for each of the two or more data streams in each of the two or more OFDM symbols,

wherein the resources assigned to the multiplexed DMRS corresponding to a same combination of two or more data streams occupy all tones of a resource block such that a different set of tones of the resource block is assigned to each of two different OFDM symbols; and

means for transmitting the DMRS using the assigned resources.

9. The apparatus of claim 8 , wherein the time-domain cyclic shifts are achieved by phase ramping in frequency domain.

10. The apparatus of claim 8 , further comprising means for assigning the same code sequence to DMRSs corresponding to the two or more data streams.

11. An apparatus for wireless communication, comprising:

at least one processor configured to assign resources for Demodulation Reference signals (DMRS) corresponding to each of two or more data streams, wherein the resources assigned to each of the data streams are staggered in frequency and span two or more Orthogonal Frequency Division Multiplexing (OFDM) symbols, wherein each of the assigned resources is assigned to multiplexed DMRS corresponding to a combination of the two or more data streams, wherein the multiplexed DMRS corresponding to the two or more data streams are multiplexed using time-domain cyclic shifts over each of the two or more OFDM symbols, wherein a different code sequence is used for each of the two or more data streams in each of the two or more OFDM symbols,

wherein the resources assigned to the multiplexed DMRS corresponding to a same combination of two or more data streams occupy all tones of a resource block such that a different set of tones of the resource block is assigned to each of two different OFDM symbols; and

a transceiver for transmitting the DMRS using the assigned resources; and

a memory coupled to the at least one processor.

12. The apparatus of claim 11 , wherein the at least one processor is further configured to assign the same code sequence to DMRSs corresponding to the two or more data streams.

13. A non-transitory computer-readable medium for wireless communication, storing instructions which when executed by at least one processor performs a method comprising:

assigning resources for Demodulation Reference signals (DMRS) corresponding to each of two or more data streams, wherein the resources assigned to each of the data streams are staggered in frequency and span two or more Orthogonal Frequency Division Multiplexing (OFDM) symbols, wherein each of the assigned resources is assigned to multiplexed DMRS corresponding to a combination of the two or more data streams, wherein the multiplexed DMRS corresponding to the two or more data streams are multiplexed using time-domain cyclic shifts over each of the two or more OFDM symbols, wherein a different code sequence is used for each of the two or more data streams in each of the two or more OFDM symbols,

wherein the resources assigned to the multiplexed DMRS corresponding to a same combination of two or more data streams occupy all tones of a resource block such that a different set of tones of the resource block is assigned to each of two different OFDM symbols; and

transmitting the DMRS using the assigned resources.

14. The computer-readable medium of claim 13 , further comprising instructions for assigning the same code sequence to DMRSs corresponding to two or more data streams.