Zero-forcing Beamforming (ZF-BF) is a spatial signal processing in multiple antenna wireless devices. For downlink, the ZF-BF algorithm allows a transmitter to send data to desired users together with nulling out the directions to undesired users and for uplink, ZF-BF receives from the desired users together with nulling out the directions from the interference users.
The concept of interference users in the receive mode is information theoretically dual to undesired users in the transmit mode.
This category summarizes techniques of zero-forcing and regularized zero-forcing precoding. If the transmitter knows the downlink channel status information perfectly, ZF-based precoding can achieve close to the optimal capacity especially when the number of users is sufficient. With limited channel status information at the transmitter, ZF-BF requires the amount of feedback overhead proportional to the average signal-to-noise-ratio (SNR) to achieve the full multiplexing gain. Hence, inaccurate channel state information at the transmitter may suffer the significant performance loss of the system throughput because of the interference among transmit streams is remained.
System Model for Multi-user BeamformingEdit
where is the vector of transmitted symbols, is the scalar value of the noise symbol, is the vector of downlink channel coefficients and is the linear precoding vector.
In the uplink multi-user beamforming system with receiver antennas at AP and a transmit antenna for each user where , the received signal can be written as
where is the transmitted symbol for user , is the vector of noise symbols, and is the vector of uplink channel coefficients.
ZFBF lets each beamforming vector for arbitrary user be orthogonal to other users' accurate channel state information vectors, i.e., .
The beamforming vector obtained using the perfect CSIT is denoted as .
If the perfect channel state information at the transmitter (CSIT) is assumed, the beamforming vectors are chosen to be the normalized rows of the inverse of the channel matrix . Notice that if the channel state information is not perfect, the beamforming vectors can not be orthogonal to the real channel vectors.
In order to consider more general cases, we discuss the beamforming vector obtained using the limited feedback channel information as .
The SNR of transmit stream in the zero-forcing beam forming systems is given by
Assuming is a Wishart matrix with , the distribution of is given by
- ↑ B. C. B. Peel, B. M. Hochwald, and A. L. Swindlehurst (Jan. 2005). "A vector-perturbation technique for near-capacity multiantenna multiuser communication - Part I: channel inversion and regularization". IEEE Trans. Commun. 53: 195–202. doi:10.1109/TCOMM.2004.840638.
- ↑ N. Jindal (Nov. 2006). "MIMO Broadcast Channels with Finite Rate Feedback". IEEE Trans. Information Theory. 52: 5045–5059. doi:10.1109/TIT.2006.883550.