User grouping for multi-user MIMO

1. A method comprising, by a computing device associated with a base station of a wireless network:

sending, to a distributed unit (DU) associated with the base station, sounding reference signal (SRS) received from a plurality of user equipments (UEs) associated with the base station;

receiving, from the DU, information regarding a subset of the plurality of UEs selected for downlink data transmissions for a resource block group (RBG), multi-user data to be transmitted to UEs in the subset, and identities of selected beams among a plurality of pre-determined beams to be associated with the UEs in the subset, wherein each of the plurality of pre-determined beams corresponds to a discrete Fourier transform (DFT) vector;

computing a precoding matrix for the RBG based on inverse-DFT (IDFT) vectors corresponding to the selected beams;

preparing pre-coded multi-user data by applying the precoding matrix to the multi-user data; and

transmitting the pre-coded multi-user data to the UEs in the subset for the RBG using Multiple-Input Multiple-Output (MIMO) technologies.

2. The method of claim 1 , wherein computing a precoding matrix for the RBG comprises:

establishing a maMIMO channel matrix by calculating the IDFT of the DFT vectors corresponding to the selected beams;

calculating a regularized pseudo-inverse of the maMIMO channel matrix; and

normalizing power of each column of the regularized pseudo-inverse of the maMIMO channel matrix such that a transmit power level for each UE in the selected subset equals to each other.

3. The method of claim 2 , wherein calculating an IDFT vector is calculating a complex conjugate of the DFT vector.

4. The method of claim 1 , wherein the subset of the plurality of UEs is selected based on estimated strengths or signal-to-noise ratios (SNRs) for the pre-determined beams for each of the plurality of UEs.

5. The method of claim 4 , wherein estimating strengths or SNRs for the pre-determined beams for a UE comprises:

computing a channel matrix for corresponding to the UE by performing SRS-based downlink channel estimations based on the received SRS, wherein the channel matrix is between an antenna array for the UE and an antenna array for the base station; and

estimating, for each of the pre-determined beams, a strength or an SNR for the UE by performing a two-dimensional discrete Fourier transform (2D-DFT) on the computed channel matrix.

6. The method of claim 5 , wherein the SRS-based downlink channel estimation is an SRS-based least squares channel estimation.

7. The method of claim 4 , further comprising sending channel quality indicators (CQIs) received from the plurality of UEs to the DU.

8. The method of claim 7 , wherein the subset of the plurality of UEs is selected further based on the CQIs.

9. The method of claim 4 , wherein the subset of the plurality of UEs is selected further based on downlink traffic information associated with each of the plurality of UEs.

10. The method of claim 9 , wherein the downlink traffic information associated with a UE comprises traffic class type, hybrid automatic repeat request (HARQ) retransmission information, and any pre-scheduled persistent or semi-persistent transmission allocations.

11. One or more computer-readable non-transitory storage media embodying software that is operable when executed, by one or more computer devices associated with a base station of a wireless network, to:

send, to a distributed unit (DU) associated with the base station, sounding reference signal (SRS) received from a plurality of user equipments (UEs) associated with the base station;

receive, from the DU, information regarding a subset of the plurality of UEs selected for downlink data transmissions for a resource block group (RBG), multi-user data to be transmitted to UEs in the subset, and identities of selected beams among a plurality of pre-determined beams to be associated with the UEs in the subset, wherein each of the plurality of pre-determined beams corresponds to a discrete Fourier transform (DFT) vector;

compute a precoding matrix for the RBG based on inverse-DFT (IDFT) vectors corresponding to the selected beams;

prepare pre-coded multi-user data by applying the precoding matrix to the multi-user data; and

transmit the pre-coded multi-user data to the UEs in the subset for the RBG using Multiple-Input Multiple-Output (MIMO) technologies.

12. The media of claim 11 , wherein computing a precoding matrix for the RBG comprises:

establishing a maMIMO channel matrix by calculating the IDFT vectors corresponding to the selected beams;

calculating a regularized pseudo-inverse of the maMIMO channel matrix; and

normalizing power of each column of the regularized pseudo-inverse of the maMIMO channel matrix such that a transmit power level for each UE in the selected subset equals to each other.

13. The media of claim 12 , wherein calculating an IDFT vector is calculating a complex conjugate of the DFT vector.

14. The media of claim 11 , wherein the subset of the plurality of UEs is selected based on estimated strengths or signal-to-noise ratios (SNRs) for the pre-determined beams for each of the plurality of UEs.

15. The media of claim 14 , wherein estimating strengths or SNRs for the pre-determined beams for a UE comprises:

computing a channel matrix for corresponding to the UE by performing SRS-based downlink channel estimations based on the received SRS, wherein the channel matrix is between an antenna array for the UE and an antenna array for the base station; and

estimating, for each of the pre-determined beams, a strength or an SNR for the UE by performing a two-dimensional discrete Fourier transform (2D-DFT) on the computed channel matrix.

16. The media of claim 15 , wherein the SRS-based downlink channel estimation is an SRS-based least squares channel estimation.

17. The media of claim 14 , further comprising sending channel quality indicators (CQIs) received from the plurality of UEs to the DU.

18. The media of claim 17 , wherein the subset of the plurality of UEs is selected further based on the CQIs.

19. The media of claim 14 , wherein the subset of the plurality of UEs is selected further based on downlink traffic information associated with each of the plurality of UEs.

20. A system associated with a base station of a wireless network comprising:

one or more processors; and

one or more computer-readable non-transitory storage media coupled to one or more of the processors and comprising instructions operable when executed by one or more of the processors to cause the system to:

send, to a distributed unit (DU) associated with the base station, sounding reference signal (SRS) received from a plurality of user equipments (UEs) associated with the base station;

receive, from the DU, information regarding a subset of the plurality of UEs selected for downlink data transmissions for a resource block group (RBG), multi-user data to be transmitted to UEs in the subset, and identities of selected beams among a plurality of pre-determined beams to be associated with the UEs in the subset, wherein each of the plurality of pre-determined beams corresponds to a discrete Fourier transform (DFT) vector;

compute a precoding matrix for the RBG based on inverse-DFT (IDFT) vectors corresponding to the selected beams;

prepare pre-coded multi-user data by applying the precoding matrix to the multi-user data; and

transmit the pre-coded multi-user data to the UEs in the subset for the RBG using Multiple-Input Multiple-Output (MIMO) technologies.