Improved Linear Precoding over Block Diagonalization in Multi-cell Cooperative Networks

In downlink multiuser multiple-input multiple-output (MIMO) systems, block diagonalization (BD) is a practical linear precoding scheme which achieves the same degrees of freedom (DoF) as the optimal linear/nonlinear precoding schemes. However, its sum-rate performance is rather poor in the practical SNR regime due to the transmit power boost problem. In this paper, we propose an improved linear precoding scheme over BD with a so-called “effective-SNR-enhancement” technique. The transmit covariance matrices are obtained by firstly solving a power minimization problem subject to the minimum rate constraint achieved by BD, and then properly scaling the solution to satisfy the power constraints. It is proved that such approach equivalently enhances the system SNR, and hence compensates the transmit power boost problem associated with BD. The power minimization problem is in general nonconvex. We therefore propose an efficient algorithm that solves the problem heuristically. Simulation results show significant sum rate gains over the optimal BD and the existing minimum mean square error (MMSE) based precoding schemes. Index Terms Linear precoding, block diagonalization, network MIMO, multi-cell cooperation, per-base-station power constraint, convex optimization The authors are with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639801 (email: {ze0003ng, egunawan, eylguan}@e.ntu.edu.sg) This work was supported in part by the Advanced Communications Research Program DSOCL06271, a research grant from the Directorate of Research and Technology (DRTech), Ministry of Defence, Singapore. February 23, 2012 DRAFT ar X iv :1 20 2. 48 56 v1 [ cs .I T ] 2 2 Fe b 20 12