Research On Multi-User MIMO Precoding And Inter-Cell Interference Coordination

With the rapid development of mobile devices and their applications, people are requiring much higher transmit rates, better communication quality, more users access, and more variety of quality of services(Qo S). However, the wireless spectrum resource is limited, which need to improve the utilization of spectrum resource. The base station may communicate with several users in a cell using the same channel at the same time, while the adjacent cells can use the same frequency at the same time,which cause serious inter-user interference and inter-cell interference. So multi-user multi-user multi-input multi-output(MIMO) precoding technology and inter-cell interference mitigation technology need to be developed to cancel or suppress inter-user interference within a cell and inter-cell interference.In the multi-user MIMO system, because of the separation between users, the transmitter need to precode the transmitted signal to cancel or suppress co-channel interference using the channel state information(CSI) obtained by feedback or through the channel reciprocity. The research on multi-user MIMO precoding research is mainly divided into two categories, block diagonalization(BD) precoding scheme, and signal to leakage plus noise(SLNR) based precoding scheme. BD scheme can completely eliminate inter-user co-channel interference and can transfer the multi-user MIMO channel into several single-user MIMO channel. Compared with BD precoding scheme, SLNR based precoding scheme does not exist the noise enhancement effect,and the number of antennas is not strictly restricted. SLNR based precoding scheme is one key research point of this thesis.First, using the idea of dirty paper coding(DPC), a successive singal-to-eakageplus-noise ratio(SSLNR) based precoding scheme is proposed for multi-user MIMO system. According to DPC, if the interference is known at the transmit side, it can be cancelled at the transmit side by a certain method. Thus, if the leakage is known at the transmit side, it can be cancelled at the transmitter side. In the proposed SSLNR based precoding scheme, SLNR is calculated only using the unknown leakage, ignoring the known leakage by the other users. After the precoding matrixes are obtained by maximizing SLNR, Tomlinson-Harashima precoding(THP) is applied to cancel the known leakage which are ignored at the SLNR calculation. This method can improve the BER performance compared with the traditional SLNR based on precoding scheme.Then,two modified SSLNR precoding schemes are proposed to improve the basic SSLNR precoding scheme performance. The proposed SSLNR method also inherits the advantage of no restriction on the number of transmission antennas. However, there exists gain gap between data streams as the original SLNR encoding scheme. The system bit error rate(BER) performance is mainly determined by the worst sub-channel.If adaptive modulation and coding scheme can not be used, there cause big BER performance loss. To solve this problem, per antenna based SSLNR(PA-SSLNR) and geometric mean decomposition based SSLNR(SSLNR-GMD) are developed to decrease the gain gap between data stream. By sorting antennas or users, the fairness among antennas or users is improved, so as to enhance the BER performance.And then, the proposed precoding schemes are investigated under imperfect channel state information(CSI). Some robust algorithms are proposed to improve BER performance with imperfect CSI.Finally, inter-cell interference mitigation is studied. Inter-cell interference is suppressed by the management of the inter-cell frequency, time, power, space, and code. In3 GPP LTE, inter-cell interference coordination, interference cancelation and interference randomize are used to mitigate inter-cell interference. In 3GPP LTE-Advanced,Coordinated Multi-point Transmission and Reception(Co MP) is applied to mitigate inter-cell interference. We focus on the inter-cell interference coordination technology. A major scheme for inter-cell interference coordination is soft frequency reuse(SFR). Although SFR can improve the performance of the cell edge users, it cause system throughput loss due to the multi-user diversity and frequency selective scheduling gain loss. This thesis presents a softer soft frequency reuse(Ser FR) scheme, the cell edge and the cell center users can both use the entire band, thereby enhancing the multi-user diversity and frequency selective scheduling gain.