Abstract: In this paper, an optimal precoding is designed for multi-cell multi-user MIMO networks. It is proposed to expand a block diagonalization method of single cell to the multi-cell situation. Considering interference leakage to out-of-cell users, a channel projection based block diagonalization (BD) method is used in designing the precoding. Using the extra-space freedom degree offered on the transmitting side, the transmit signal is preprocessed with a cell-based signal-to-leakage-plus-noise ratio (SLNR) maximization criterion, which can compromise the target user signal and leakage interference to suppressed interference. Moreover, a traditional subspace zero forcing procedure is proceeded to mitigate the interference among intra-cell users. Thus,
negative factors of the inter-cell interference for the multi-user MIMO system can be suppressed. Compared to the classical interference leakage zero forcing algorithm, it can flexibly adjust the freedom for inter-cell interference suppression, balance relationship between interference suppression and system capacity enhancement. Moreover, under the same interference, the capacity can be doubled. Both theoretical analysis and simulation
indicate that the proposed methods can efficiently suppress interference while keeping low complexity without changing topology of the current systems.

Key words: precoding design, multiple input multiple output (MIMO), cooperation communication, inter-cell co-channel interference