  This paper considers a scenario for multi-input multi-output (MIMO)
communication systems when perfect channel state information at the transmitter
(CSIT) is given while the equivalent channel state information at the receiver
(CSIR) is not available. Such an assumption is valid for the downlink
multi-user MIMO systems with linear precoders that depend on channels to all
receivers. We propose a concept called dual systems with zero-forcing designs
based on the duality principle, originally proposed to relate Gaussian
multi-access channels (MACs) and Gaussian broadcast channels (BCs). For the
two-user N*2 MIMO BC with N antennas at the transmitter and two antennas at
each of the receivers, we design a downlink interference cancellation (IC)
transmission scheme using the dual of uplink MAC systems employing IC methods.
The transmitter simultaneously sends two precoded Alamouti codes, one for each
user. Each receiver can zero-force the unintended user's Alamouti codes and
decouple its own data streams using two simple linear operations independent of
CSIR. Analysis shows that the proposed scheme achieves a diversity gain of
2(N-1) for equal energy constellations with short-term power and rate
constraints. Power allocation between two users can also be performed, and it
improves the array gain but not the diversity gain. Numerical results
demonstrate that the bit error rate of the downlink IC scheme has a substantial
gain compared to the block diagonalization method, which requires global
channel information at each node.
