Why does 5G require beam management?
The 5G band is higher, especially in the millimeter wave band, and the coverage is smaller. In order to enhance the 5G coverage, beam shaping emerges as the times require.
Beamforming technology, which adjusts the amplitude and phase of multiple antennas, gives the antenna radiation pattern a specific shape and direction, and concentrates the wireless signal energy on a narrower beam to enhance coverage and reduce interference.
The beamforming direction is controllable, and the direction of the beam can be adjusted following the moving terminal. In non-line of sight (NLOS) scenarios, beamforming can also use beam reflection or refraction to get the signal to the terminal.
However, since the terminal is often in a moving state, high-frequency signals (especially millimeter waves) are susceptible to the wireless environment, such as being blocked by buildings, rainwater, etc., and it is easy for the beam signal to reach the terminal.
Therefore, in order to ensure continuous seamless coverage, it is necessary for the base station side to transmit multiple beams in different directions as much as possible. To manage multiple beams, beam management techniques are needed.
Beam management mainly includes four steps:
Within the beam coverage, a set of beams is transmitted and received according to a predefined time interval and direction.
The quality of the received signal is evaluated. The evaluation indicators include RSRP (Reference Signal Received Power), RSRQ (Reference Signal Received Quality), and SINR (Signal to Interference plus Noise Ratio).
The optimal beam (or beam set) is selected based on the beam measurements.
The UE reports the beam quality and beam decision information to the base station to establish beam-oriented communication between the base station and the terminal.
Beam sweeping is the first step in beam management. gNB (5G base station) transmits meters in different spatial directions, and UE (terminal) monitors/scans beam transmission from GNB in a different receiving space direction (thus total There are m * n secondary beam scans).
Based on the beam sweeping, the UE determines the channel quality of the beam and reports the channel quality information to the gNB. The surrounding buildings, weather conditions, UE moving speed and direction, and even the way of holding the UE will affect the beam Channel quality.
After receiving the beam quality information, the gNB adjusts various configuration parameters based on the reported beam quality status, such as adjusting the beam sweeping period and switching threshold decisions.
The entire beam sweeping process uses an exhaustive search method. The so-called exhaustive search method, also called violent search, as the name implies, lists all the possibilities and traverses all the possibilities. The exhaustive search method is used for beam sweeping, which means that the direction codebook is pre-defined for the entire coverage angle space for the UE and the gNB, and then the transmission/reception synchronization and reference signals are traversed in order.
As shown in the figure below, a beam group can consist of eight beams in different spatial directions. The UE determines the beam index based on the received Beam Reference Signal (Beam 1-8).
The UE selects a beam by measuring the received power or quality of each beam. The beam receiving power parameter is BRSRP, which is the received power of the beam reference signal.
The UE selects the best beam, such as the beam with the highest measured BRSRP value. As shown in the figure below, in this example, the UE selects beam 6.
After the UE selects the best beam, the beam quality and the beam decision information are reported to the base station by performing a random access procedure to implement beam alignment between the UE and the gNB to establish directional communication.
During the beam reporting process, the UE must wait for the gNB to schedule the RACH (Random Access Channel) opportunity to perform random access on its selected optimal beam direction. Therefore, if in the SA (Independent Networking) mode, the gNB may need to perform a complete beam sweep again. In the NSA (non-independent networking) mode, the gNB can be directly notified through the LTE connection.
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