Multiple antennas in LTE may also be used to transmit the same signal appropriately weighted for each antenna element such that the effect is to focus the transmitted beam in the direction of the receiver and away from interference, thereby improving the received SINR.
The beam-forming weight vector should increase the antenna gain in the direction of the desired user while simultaneously minimizing the gain in the directions of interferers.
Beamforming can provide significant improvement in the coverage range, capacity, and reliability. To perform transmit beamforming, the transmitter needs to have accurate knowledge of the channel, which in the case of TDD is easily available owing to channel reciprocity but for FDD requires a feedback channel to learn the channel characteristics so it is not implemented in LTE Release 8 or 9 yet.
As of today, beam forming is specific only to LTE TDD and can operate either under 4×4 or 8×2 configurations.
One popular beam-forming algorithm is based on Direction of Arrival where the incoming signals to a receiver may consist of desired energy and interference energy—for example, from other users or from multipath reflections.
The various signals can be characterized in terms of the DOA or the angle of arrival (AOA) of each received signal. Each DOA can be estimated by using EUTRAN signal-processing techniques as requested in 3GPP-TS 36-214. From these acquired DOAs, a beam-former extracts a weighting vector for the antenna elements and uses it to transmit or receive the desired signal of a specific user while suppressing the undesired interference signals.
Ideally, the beam-former has unity gain for the desired user and two nulls at the directions of two interferers and can place nulls in the directions of interferers. The DOA-based beam-former in this case is often called the null-steering beam-former. The null-steering beam-former can be designed to completely cancel out interfering signals only if the number of such signals is strictly less than the number of antenna elements.
Typically, there exists a trade-off between interference null and desired gain lost. Thus far, we have assumed that the array response vectors of different users with corresponding AOAs are known.
In practice, each resolvable multipath is likely to comprise several unresolved components coming from significantly different angles. In this case, it is not possible to associate a discrete AOA with a signal impinging the antenna array. Therefore, the DOA based beam-former is viable only in LOS environments or in environments with limited local scattering around the transmitter.