Beam-forming in LTE

In LTE, various antenna technologies are used to provide better SNR. Transmit and receive diversity can be used to increase SNR by factor of number of transmit/receive antennas. Beam-forming can be used to provide better performance in low SNR and fading conditions.

Beam-forming involves shaping of the signal in the direction of the device. By applying appropriate precoding matrix, the strength of the signal can be increased in a particular direction. This is particularly useful for cell edge users.

The knowledge of the downlink channel is very essential for the eNodeB to apply appropriate precoding. In case of TDD, due the channel reciprocity, uplink channel quality estimation done by eNodeB can be used as estimation of downlink channel quality. In case of FDD however, that is not true. Beam-forming, is therefore, used particularly for TDD.

There are different ways to achieve beam-forming in different scenarios.. If the antenna signals are highly correlated, beam-forming can be achieved by applying different phase shifts to highly correlated signals. The signals are, therefore same, the only difference being phase shift. As the signals are highly correlated, this kind of beam-forming does not provide much protection against fading, it only provides better SNR.

If the antenna signal paths have low correlation, precoding matrix can be applied in such a way that precoded signal has both amplitude gain and phase difference. This kind of beam-forming provide much better results in faded channel conditions but it needs more detailed knowledge of the channel conditions.