Overcoming Interference In MIMO Cellular Systems

Multi-antenna transmission and reception (known as MIMO), which could achieve ten times higher spectral efficiency than traditional systems without additional transmit power and frequency bandwidth, is a key technique in the future mobile communication systems which is desired to high data rate. However, one of the key challenges facing the deployment of MIMO technology in cellular networks is the sensitivity of MIMO receivers to interference. Since cellular systems are inherently interference-limited, this introduces a fundamental conflict. Spatial multiplexing MIMO systems, whose principal merit is a supposed dramatic increase in spectral efficiency, lose much of their effectiveness in high levels of interference. This article focuses on the research of interference management method in cellular MIMO systems. The following issues were handledThis article investigates the cellular MIMO systems and overviews several approaches to handling interference in cellular MIMO systems. The traditional interference management techniques have limited usefulness when viewed in concert with MIMO and a single technique is hard to handle the problem of interference in MIMO systems. System-level interference reducing strategies based on cooperation, which is easy to coordinate with other interference suppressing techniques, will be important for overcoming interference in future spatial multiplexing cellular systems and valuable for system design.In this paper, an interference coordination method based on opportunistic beamforming is proposed. In the single user opportunistic beamforming systems, the scheduled beam former which is chosen by cooperation to reduce the interference is non-user specific compared with traditional beamforming systems. The interference is also more trackable and the outage caused by rate adaptation will be greatly reduced. Then the system performance is improved.Distributed multi-user precoding method for interference suppression is proposed. The interference leakage to the out-of-cell users is suppressed proactively by the base station (BS). Meanwhile, the signals of intra-cell users are orthogonal to each other. Based on the idea, two algorithms are proposed for interference suppression. One is leakage zero forcing block diagonalization (LZF-BD), in which subspaces spanned by interference channels and other intra-cell users channels are zero-forced; the other is channel projection based block diagonalization (CP-BD), in which an initial precoding, which maximizes a cell-based signal-to-leakage-plus-noise ratio (SLNR), is applied, then a traditional BD procedure is proceeded to mitigate the interference among intra-cell users. CP-BD has less limitation of degree of freedom (DoF) than LZF-BD. The analysis of theory and simulation both indicate that the proposed methods could suppress the interference efficiently, while keeping the complexity low and the topology of current systems need not be changed either.Besides, we also proposed a limited feedback method for distributed multi-user precoding in cellular MIMO systems to reduce the amount of feedback in uplink. A codebook which is known by transmitters and receivers is used to quantize the desired channel and the interference channels, and the indexes of quantization will be sent back to the BS, where LZF-BD or CP-BD will be carried out for interference mitigation. The simulation results show that the more feedback bits, the better the system performance will be.