TECHNICAL FIELD

The present disclosure relates to wireless communications, and in particular, to a hybrid transmission combining codebook-based and reciprocity-based beamforming.

BACKGROUND

In codebook-based beamforming, a base station, e.g. enhanced e-node B (eNB), base station in NR (gNB), sends reference signals in the downlink, e.g., CRS, CSI-RS. A user equipment (UE) has a pre-defined set of codebooks which is reviewed to determine the set that maximizes the UE throughput. The index of the selected codebook will be reported back to the base station in the CSI report along with RI and CQI. The base station then has the choice to use that PMI to perform precoding of the PDSCH.

In reciprocity-based beamforming, a UE sends reference signals in uplink e.g., SRS, DMRS. A base station will use these signals to estimate the channel covariance in the uplink. The base station will then use this estimation to perform the downlink (DL) beamforming. There are multiple ways to perform reciprocity-based DL beamforming including Reciprocity Assisted Transmission (RAT) and Grid of Beams (GoB). In GoB, the base station goes through a set of predefined DFT beams (codebooks) with potential oversampling rate and finds the beam that has the maximum power which reveals the information about direction of arrival (DoA). That beam will then be used to beamform the PDSCH.

There currently exist certain challenge(s).

In order to improve the performance and extract the full potential of Full Dimension Multiple Input Multiple Output (FD-MIMO), a base station needs to perform elevation beamforming. However, in certain codebook-based transmission modes, the defined Precoder Matrix Indicators (PMIs) only exploit the horizontal beamforming and do not provide elevation beamforming information to base station. Since the elevation beamforming is not fully utilized, the performance in DL is sub-optimal and could be improved.

SUMMARY

Some embodiments herein advantageously combine transmission of codebook-based and reciprocity-based one direction grid of beams (1D GoB) beamforming for codebook-based transmission modes. In general, in these embodiments, a base station estimates the channel from an uplink from a wireless device, or user equipment (UE) and determines the best beam in the vertical direction and combines it with the Precoder Matrix Indicator (PMI) that the UE reports for the horizontal beamforming. This is referred to as Hybrid Transmission. In general, these embodiments allow the base station to perform both horizontal (through PMI) and vertical (through 1D GoB) beamforming in codebook-based transmission modes which extracts the full potential of FD-MIMO.

In particular, embodiments herein include a method performed by a network node (360) to realize FD_MIMO in codebook-based transmissions. The method includes combing one or more horizontal codebooks and one or more vertical codebooks and generating a 2D beamformer for downlink transmissions based on the combination of the one or more horizontal codebooks and one or more vertical codebooks.

In some embodiments, the one or more horizontal codebooks may be based on a wireless device reported codebook, while the one or more vertical codebooks may be based on elevation beamforming at a base station side. In some of these embodiments, the elevation beamforming may be based on an 1D grid of beams (GoB) algorithm.

In some embodiments, the method may also include performing a column swap of the PMI every other sub-band to adjust a throughput between two layers in rank-2 transmissions, wherein the adjustment to the throughput of the two layers may be within a predefined range, wherein the wireless device reported codebook may include at least one of a SINR estimate and a rank estimate.

In some embodiments, the method may also include adjusting at least one of the SINR estimate and rank estimate based on a calculated BF gain.

Embodiments also include corresponding apparatus and computer programs. For example, embodiments herein include a network node. The network node is configured (e.g., via one or more processing circuits) to receive, from a wireless device, one or more horizontal codebooks. These horizontal codebooks may include a Precoder Matrix Indicator (PMI). The network node is also configured to best beam in the vertical direction. This determination may be performed through a one direction grid of beams calculation. The network node is also configured to combing one or more horizontal codebooks and one or more vertical codebooks and generating a 2D beamformer for downlink transmissions based on the combination of the one or more horizontal codebooks and one or more vertical codebooks.

In some embodiments, the method may use PMI for the vertical beam forming and reciprocity based beam forming for horizontal beamforming.

