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What Is Beamforming:
Beamforming is a downlink multi-antenna feature. With Beamforming, an eNodeB weights downlink data before transmission to form a narrow beam towards target UEs, thereby increasing the signal strength on the UE side. The direction of incoming wave and the path loss information are obtained by measuring the uplink received signal. Beamforming is particularly important for the time division duplex (TDD) mode in LTE and 5G NR.
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| Weighting Process |
Benefits:
An eNodeB direct beams towards target UEs in real time, increasing SINR especially for UEs at the cell edge.
Beamforming Technologies:
Single-Stream Beamforming:
- Single-stream beamforming means transmission of a single data stream in the same OFDM resource block.
- It is suitable for situations of poor channel quality.
- Single-stream beamforming achieves diversity gain by 1 dB by increasing the SNR.
Dual-Stream Beamforming:
- Dual-stream beamforming means transmission of two data streams in the same OFDM resource block, leading to spatial multiplexing.
- It is suitable for situations of good channel quality.
MU beamforming:
- When MU beamforming is enabled, the eNodeB selects the UEs with high antenna channel correlations and low UE channel correlations for pairing due to the following reasons:
- If antenna channels are highly correlated and dual-stream beamforming is used, the data streams interfere with each other. In MU beamforming mode, the UEs with low channel correlations are paired. In this situation, spatial multiplexing gains are offered.
- If antenna channels are lowly correlated, each UE can transmit multiple data streams and MU beamforming offers lower gains than dual-stream beamforming.
Beamformig TMs:
In LTE, usually they use multiple Antenna for downlink (at least from Category 3 UE and higher), meaning that eNode use multiple Tx Antenna and UE use multiple Rx antenna.
Now you almost automatically think about 'MIMO', but in reality 'multiple antenna' does not automatically mean 'MIMO'. For example, you have two downlink antenna. You can use these two antenna in various ways. Of course, one ways is to use it as 2 x 2 MIMO, but this is not the only way. You can use the two antenna in diversity configuration rather than MIMO configuration. Or you can just use only one of the antenna and sometimes you would like to use various different multiplexing, precoding methods etc.
The following table describes the available downlink transmission modes (TM) in LTE that are used with Beamforming as specified in 3GPP :
| Transmission Mode | 3GPP-Defined MIMO Technique | Description |
|---|---|---|
| TM7 | Single antenna port (port 5) | In this mode, RSs are transmitted over antenna port 5 for single-stream beamforming. |
| TM8 | Dual-layer transmission (ports 7 and 8) | In this mode, RSs are transmitted over ports 7 and 8 for dual-stream beamforming |
| TM9 | 8 layer transmission | Up to 8 layers, antenna ports 7 - 14 |
