What's the capacity of the mufti-user MIMO downlink and how to get the Capacity are the hot research topics in the latest yeais.The BD(Block Diagonalization) algorithm

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based on the constraint which the interferences between different users are zero

have two drawbacks.One is the capacity loss because the sisal spaces of different uses are overlapped

the other is that it is required that the number of the access point's antennas must larger than the sum of the num of users' antennas.This paper presents two linear precoding algorithms.One is the optimal algorithm based on the maximization of SJNR

the other is a modified suboptimal algorithm which can improve the downlink capacity.By capacity analysis and simulation

the performances of the two algorithms both are better than BD algorithm.