Beamforming is a technique that uses multiple antennas to steer the radio waves in a desired direction, improving the signal quality and reducing the interference.
There are two main types of beamforming techniques in 5G:
- Codebook-based
- Reciprocity-based
1) Codebook-based beamforming is a technique that uses a predefined set of beamformers, known as a codebook, to determine the optimal beamforming direction. A beamformer is a vector that defines the phase and amplitude of the signal transmitted by each antenna. The codebook contains a number of candidate beamformers that are designed to match the measured downlink channel at the UE. The downlink channel is the channel from the base station (gNB) to the user equipment (UE). To acquire the downlink channel state, the gNB transmits a Channel State Information Reference Signal (CSI-RS) in the downlink, which is then measured by the UE. The UE uses this information to determine the most suitable beamformer from the codebook and sends feedback to the gNB recommending the chosen beamformer. The feedback is typically encoded into a few bits and sent in the uplink control information. The uplink channel is the channel from the UE to the gNB. The limited feedback format ensures robust coverage and reduces overhead. However, since the feedback is limited, only simple classical beamforming techniques can be used, which work well for low-rank transmission in line-of-sight channels. Low-rank transmission means that the channel has only one or a few dominant directions. Line-of-sight channels means that there is a direct path between the gNB and the UE, without any obstacles or reflections. Codebook-based beamforming is more suitable for single-user MIMO (SU-MIMO) transmission and provides superior coverage of the channel state information acquisition. SU-MIMO means that the gNB communicates with only one UE at a time, using multiple antennas.
2) Reciprocity-based beamforming is a technique that utilizes the reciprocity property of the uplink and downlink channels in a TDD system. TDD stands for Time Division Duplex, which means that the uplink and downlink transmissions use the same frequency band, but at different time slots. The reciprocity property means that the uplink and downlink channels have similar characteristics, such as the path loss, the delay spread, and the angle of arrival. In this approach, the downlink channel state is estimated by sounding the uplink channel. Sounding means that the UE transmits a Sounding Reference Signal (SRS) in the uplink, and the gNB uses this signal to estimate the downlink channel state. Since the uplink and downlink channels are reciprocal, the gNB can use the information from the uplink channel to determine the downlink channel properties. This includes spatial and frequency domain information, as well as fast-fading properties of the channel. Fast-fading means that the channel varies rapidly due to the movement of the UE or the environment. However, it is important to note that the reciprocity-based beamforming requires calibration between the uplink and downlink chains, as the channel effects due to RX/TX processing are generally not reciprocal. RX/TX processing means the processing of the signal at the receiver (RX) and the transmitter (TX), such as filtering, amplification, and modulation. Calibration means that the gNB and the UE need to measure and compensate for the differences between the uplink and downlink chains, to ensure accurate estimation of the downlink channel state from the uplink signal. Reciprocity-based beamforming can achieve higher performance in multi-user MIMO (MU-MIMO) scenarios, where the gNB communicates with multiple UEs at the same time, using multiple antennas. This is because reciprocity-based beamforming can exploit the spatial diversity and multiplexing gains of the channel, and suppress the interference between the UEs. Spatial diversity means that the channel has multiple independent paths, which can improve the reliability of the transmission. Multiplexing gain means that the channel can support multiple data streams, which can increase the capacity of the transmission. Interference suppression means that the gNB can use the beamforming to nullify the signals from the undesired UEs, and focus on the signals from the desired UEs.
