How Beamforming exactly works in 5G NR 

I don’t think trasmitting signal in Beam in high frequency deployment would be the matter of choice. It is a kind of ‘MUST’ implementation. In case of low / mid frequency region without using massive antenna array, a single transmission would cover a lot of UEs simultaneously. However, when the radiation become beam-shaped it is very difficult to cover multiple UEs in single transmission unless those multiple UEs are located in very close proximity. To handle this problem, we need a very sophisticated idea of managing/controlling the beam to cover the multiple devices scattered in all directions and the management/control mechanism should be different depending on the situtations. We will see more details in beamforming management operation part.

The beamforming technique focuses the RF energy into a narrow direction to allow the RF beam to propagate farther in that direction. Using this technique, non-line of sight (NLOS) RF communication in the mmWave spectrum may rely on reflection and/or diffraction of the beams to reach the UE. 
If the direction becomes blocked, either because of UE movement or changes in the environment , the beam may not be able to reach the UE. Thus, in order to ensure that the UE has continuous, seamless coverage, multiple beams in as many different directions as possible may be available.

Beamforming Types

In 5G NR, these beams are formed by analog beam-forming technique, but for the data transmissions 5G system dynamically uses analog or digital or the combination of analog and digital beam-forming called Hybrid beam-forming technique.

Beamforming Management Operation 

There’re 4 operations:

1. Beam sweeping:

Beam Sweeping is a technique to transmit the beams in all predefined directions in a burst in a regular interval

The gNB transmits different beams in different spatial directions. The UE listens/scans for the beam transmissions from the gNB in  different receive spatial directions. Based on the performed beam sweeps, the UE determines a channel quality associated with the performed beam sweeps. Then the UE sends the channel quality information to the gNB. 
Also the UE and the gNB exchange other information (e.g., analog or digital beamforming capabilities, beamforming type, timing information, configuration information, etc.).
Based on the received information, the gNB determines various configuration information, such as mmW network access configuration information, information for adjusting beam sweeping periodicity, information regarding overlapping coverage for predicting a handoff to another gNBs.

2. Beam measurement:

It refers to evaluation of the quality of the received signal at the gNB or at the UE. Different metrics could be used such as RSRP, RSRQ and SINR for this purpose.

3. Beam determination: 

the selection of the optimal beam (or set of beams) to set up a directional (and fully beamformed) communication.

then, The UE selects Beam Index  as it’s the “Best” beam from the UE point of view

4. Beam reporting: 

Ue after selecting the best beam from gNB, UE inform the selection by using a specific PRACH resource mapped to each DL Beam