LTE system. How to compute precoder for resourse block
Think of the RB as a subset of the OFDM channel set. That reduces the complexity of finding $N_\text{FFT}$ matrix decompositions of $N_\text{TX antennas}\cdot N_\text{RX antennas}$ matrices for each user to 
$N_\text{carriers in the individual RB}$ decompositions for each user, which might well be smaller, because at user-level, often a significant amount of TX antennas can be ignored to begin with.