Hybrid user selection and beamforming method for multi-user MIMO

1. A method comprising, by one or more computing devices associated with a base station of a wireless network:

receiving, from a radio unit (RU) associated with the base station, sounding reference signal (SRS) received from a plurality of user equipments (UEs) associated with the base station;

computing, for each of the plurality of UEs, 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;

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

selecting a subset of the plurality of UEs to which downlink data is to be transmitted for a resource block group (RBG) in a transmission time interval (TTI) based on the estimated strengths or SNRs of the pre-determined beams for the plurality of UEs;

computing a precoding matrix for the RBG based on the selected subset;

preparing multi-layered UE data for the RBG based on the selected subset and the computed precoding matrix; and

sending, to the RU, the multi-layered UE data and the precoding matrix for the RBG, wherein the RU transmits pre-coded multi-layered UE data to the UEs in the subset using MIMO technologies.

2. The method of claim 1 , wherein the SRS-based downlink channel estimation is an SRS-based least squares channel estimation.

3. The method of claim 1 , further comprising: receiving, from the RU, channel quality indicators (CQIs) received from the plurality of UEs.

4. The method of claim 3 , wherein selecting the subset of the plurality of UEs is further based on the CQIs.

5. The method of claim 1 , wherein selecting the subset of the plurality of UEs is further based on downlink traffic information associated with each of the plurality of UEs.

6. The method of claim 5 , wherein the downlink traffic information associated with a UE comprises traffic class type, hybrid automatic repeat request (HARD) retransmission information, and any pre-scheduled persistent or semi-persistent transmission allocations.

7. The method of claim 5 , wherein selecting the subset of the plurality of UEs for the RBG comprises:

calculating a user selection metric for each of the plurality of UEs; and

determining a subset of the plurality of UEs that maximizes an estimated sum of weighted data rate for the RBG.

8. The method of claim 7 , wherein calculating a user selection metric for each of the plurality of UEs is based on a beam for the UE with a highest power and its corresponding SNR, and the downlink traffic information associated with the UE.

9. The method of claim 7 , wherein determining a subset of the plurality of UEs comprises:

sorting the plurality of UEs based on their corresponding user selection metric;

preparing an empty candidate set;

preparing an empty list of reserved beams;

adding the first UE among the sorted UEs into the candidate set; and

repeating until a finishing condition is met:

removing a first UE from the sorted UEs as a candidate UE; and

adding the candidate UE into the candidate set when (1) a beam for the candidate UE with a highest power is not in the list of reserved beams and (2) adding the candidate UE to the candidate set is determined to increase an estimated sum of weighted data rate for UEs in the candidate set.

10. The method of claim 9 , wherein adding a UE in the candidate set comprises adding a beam for the UE with a highest power to the list of reserved beams.

11. The method of claim 10 , wherein adding a UE in the candidate set further comprises adding k adjacent beams to the beam for the UE with the highest power to the list of reserved beams, wherein k is an oversampling factor.

12. The method of claim 10 , wherein adding a UE in the candidate set further comprises adding n beams for the UE with next n highest power beyond the beam for the UE with the highest power to the list of reserved beams.

13. The method of claim 9 , wherein determining whether adding the candidate UE to the candidate set increases an estimated sum of weighted data rate for UEs in the candidate set comprises:

defining a temporary set as UEs in the candidate set and the candidate UE;

adjusting a transmission signal power based on a number of UEs in the temporary set;

calculating an estimated sum of weighted data rate for UEs in the temporary set based on an assumption that a beam with a highest power for each UE is used for a transmission; and

determining whether adding the candidate UE to the candidate set increases an estimated sum of weighted data rate for the candidate set by comparing the estimated sum of weighted data rate for UEs in the temporary set and a previously-calculated estimated sum of weighted data rate for UEs in the candidate set.

14. The method of claim 9 , wherein the finishing condition comprises:

a size of the candidate set reaches a pre-determined size; or

no more UE exists in the sorted UEs.

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

establishing a massive MIMO (maMIMO) channel matrix between UEs in the selected subset and transmission antenna ports associated with the base station;

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.

16. The method of claim 1 , further comprising:

determining modulation and coding scheme (MCS) for each of the plurality of UEs for the TTI, wherein preparing the multi-layered UE data for the RBG is further based on the determined MCS values corresponding to the plurality of UEs.

17. The method of claim 1 , wherein preparing the multi-layered UE data for the RBG is further based on a number of layers for each of the plurality of UEs.

18. 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:

receive, from a radio unit (RU) associated with the base station, sounding reference signal (SRS) received from a plurality of user equipments (UEs) associated with the base station;

compute, for each of the plurality of UEs, 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;

estimate, for each of the plurality of UEs, for each of pre-determined beams, a strength and an SNR for the UE by performing a two-dimensional discrete Fourier transform (2D-DFT) on the computed channel matrix;

select a subset of the plurality of UEs to which downlink data is to be transmitted for a resource block group (RBG) in a transmission time interval (TTI) based on the estimated strengths or SNRs of the pre-determined beams for the plurality of UEs;

compute a precoding matrix for the RBG based on the selected subset;

prepare multi-layered UE data for the RBG based on the selected subset and the computed precoding matrix; and

send, to the RU, the multi-layered UE data and the precoding matrix for the RBG, wherein the RU transmits pre-coded multi-layered UE data to the UEs in the subset using MIMO technologies.

19. 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:

receive, from a radio unit (RU) associated with the base station, sounding reference signal (SRS) received from a plurality of user equipments (UEs) associated with the base station;

compute, for each of the plurality of UEs, 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;

estimate, for each of the plurality of UEs, for each of pre-determined beams, a strength and an SNR for the UE by performing a two-dimensional discrete Fourier transform (2D-DFT) on the computed channel matrix;

select a subset of the plurality of UEs to which downlink data is to be transmitted for a resource block group (RBG) in a transmission time interval (TTI) based on the estimated strengths or SNRs of the pre-determined beams for the plurality of UEs;

compute a precoding matrix for the RBG based on the selected subset;

prepare multi-layered UE data for the RBG based on the selected subset and the computed precoding matrix; and

send, to the RU, the multi-layered UE data and the precoding matrix for the RBG, wherein the RU transmits pre-coded multi-layered UE data to the UEs in the subset using MIMO technologies.