Re: precoding....
Hi,
Precoding is a technique that exploits channel information at Transmitier. CSI (Channel State Information) is mandatory because precoding matrix is a function of the mimo channel.
To obtain CSI, in TDD systems it is possible to exploit the concept of reciprocity. In FDD systems, a feedback control channel is used to provide channel information to the base station.
About matrix structure...
Y= HFS+V eq (1)
where F is an Mt x M precoding matrix, S is an Mt x T space-time block codeword, and V is an Mr x T noise matrix. The precoder parameter M is chosen so that M<=Mt.
The precoder is chosen using a function "f" that maps an Mr x Mt channel realization to an Mt x M precoding matrix F= f(H).
The equation 1 covers a large range of closed-loop MIMO techniques, including beamforming (that convert a MIMO channel into an equivalent SISO channel), precoded spatial multiplexing, precoded orthogonal space-time block codes and antenna selection techniques -where M out of Mt antennas are selected for transmission (in that case F consists of M different columns of the MxMt identity matrix).
so.. the structure depends on the technique that is used.
Added after 12 minutes:
.......
"my question is that actually what precoding doing at the transmitter side... "
Precoding helps to customize the transmitted waveforms to provide higher link capacity and throughput. Using CSI at transmitter improve system capacity by sharing spatial channel with multiple users simultaneously , simplify multi-users receivers through interference avoidance, and provide a general means to exploit spatial diversity.