If some knowledge of the downlink channels of the different transmit antennas, (more specifically some knowledge of the relative channel phases) is available at the transmitter side, multiple transmit antennas can be used to shape the overall antenna beam in the direction of a target receiver. In general, such beamforming can increase the signal-strength at the receiver in proportion to the number of transmit antennas.
The overall transmission beam can be adjusted in different directions by applying different phase shifts to the signals to be transmitted on the different antennas, as shown in Figure . The adjustments are generally based on estimates of the direction to the target mobile terminal derived from feedback measurements.
Applying different phase shifts to the signals to be transmitted on the different antennas can be expressed, in vector notation, as applying a size NT ×1 precoding vector V to the signal to be transmitted.
If there is no radio channel time dispersion or if the effects of a time dispersive channel are compensated for (by using OFDM for instance), the precoding consists in rotating the transmitted signals to compensate for the instantaneous channel phase and ensure that the signals received on the different receive antennas are phase aligned.
