Long Term Evolution (Advanced) - LTE(-A)
Beamforming

If only single antenna transmission is applied, the transmission for a user introduces interference for all other users in the same cell. If multiple antennas are used at the transceiver side, the transmission can be managed in a way to reduce interference by so-called beamforming. In case of the beamforming, direction of the transmission is controlled and adapted to focus the transmission power only to the desired user and to minimize interference to other users. It means, the transmitting power is aimed only to the direction of the receiver and the power in other directions is minimized. The beamforming is done by multiplication of the transmitted signal by complex weights to adjust magnitude and phase of individual antenna signals. To ensure transmission in a proper direction, the transmitter must be aware where the target terminal is located. Based on the knowledge of the direction, a set of complex weights is calculated or selected out of predefined codebook libraries defined by the standard. The direction can be derived from the channel quality reports collected by eNodeB from users. Nevertheless, the determination of the direction is not defined by LTE.

Principle of beamforming