5G NR BFR - Beam Failure Recovery

Beam steering and Beam Switching

Beam steering is achieved by changing the phase of the input signal on all radiating elements. Phase-shifting allows the signal to be targeted at a specific receiver. An antenna can employ radiating elements with a common frequency to steer a single beam in a specific direction. Different frequency beams can be steered in different directions to serve different users. The direction a signal is sent in is calculated dynamically by the base station as the endpoint moves, effectively tracking the user. 

Beam Management Techniques

Beam steering and Beamforming are additional techniques that use multiple antennas to create directional transmissions that accurately point at the receiving device. Beam steering is a set of techniques used to focus the direction and width of a radiation pattern. In wireless communications, beam steering changes the direction of the signal while beaming refinement narrows the width of the transmitted signal. These actions are typically performed by manipulating the phase shift of the signal through an array of multiple antenna elements. 

Beamforming applies different phase shifts to each antenna element to shape and provide discrete control of the direction of a transmitted beam. Beamforming requires communication channel feedback to implement real-time control of the beam. 

Both beamforming and beam steering incorporates channel feedback to manipulate the beam shape and direction in real-time purposes.  

For beam failure detection, the gNB configures the UE with beam failure detection reference signals (SSB or CSI-RS), and the UE declares beam failure when the number of beam failure instance indicators from the physical layer reaches a configured threshold before a configured timer expires. SSB-based beam failure detection is based on the SSB associated with the initial DL BWPs and for DL BWPs containing the SSB associated with the initial DL BWP. For other  DL BWPs, Beam Failure Detection can only be performed based on CSI-RS. 

After beam failure is detected on Pcell, the UE:

• Triggers beam failure recovery by initiating a Random Access procedure on the Pcell
• Selects a suitable beam to perform beam failure recovery 
• Includes an indication of a beam failure on Pcell in a BFR MAC CE if the Random Access procedure involves contention-based random access. 

Upon completion of the Random Access Procedure, beam failure recovery for Pcell is considered complete.

After beam failure is detected on a Scell, the UE: 

• Triggers beam failure recovery by initiating a transmission of a BFR MAC CE for this Scell.\
• Selects a suitable beam for this Scell and indicate it along with the information about the beam failure in the BFR MAC CE. 

Upon reception of a PDCCH  indicating an uplink grant for a new transmission for the HARQ. Multi-antenna schemes at the transmitter and/or receiver can enhance the cellular system’s performance in a different fashion. By using different antenna elements to transmit multiple streams of data -” layers” - over the same time-frequency resources, higher data rates can be achieved which is referred to as spatial multiplexing. 

Once UE gets into a connection state with a network, at least one beam is properly in the connection between UE and the network. There can be so many different ways in which UE and Network beam is connected. 

Cases

• UE and Network are connected through a single TRP (Tx/Rx point) and a single beam. 
•  UE and Network are connected through multiple TRP (Tx/Rx point) and a single beam for each TRP.
• UE and Network are connected through a single TRP (Tx/Rx point) and multiple beams. 
• UE and Network are connected through multiple TRP (Tx/Rx point) and multiple beams for each TRP. 

The general idea of the beam management during the connected states would be: 

• Network transmit a specific reference signal for beam management. 
• UE detects the signal and perform some measurement and send feedback to the network. 

Process

P1/P2/P3 is a set of processes that are designed for beam management while in a connected state. These all are related to downlink beam management. It is a mechanism for UE to better receive the downlink beam (data). There is another mechanism for uplink beam management as U1, U2, and U3. 

P1: It is for Beam selection- gNB sweeps beam and UE selects the best one and reports it to gNB.

P2: It is Beam refinement for transmitter (gNB Tx)- gNB refine beam and UE detects the best one and reports it to the gNB.

P3: It is Beam refinement for the receiver (UE Rx)-  gNB fixes a beam and UE refines its receiver beam. It sets a spatial filter on the receiver antenna array. This is used only when UE supports beamforming.