Research On Directional Interference Suppression For MIMO System

With the development of wireless communication techniques, the demands of all kinds of new services are booming. The problem how to use the limited spectrum resources to achieve higher spectrum efficiency and communication reliability of wireless communications is urgent to be resolved. Multiple input multiple output (MIMO) technique, one of the most significant breakthroughs in wireless communications field, offers an effective solution to this problem. MIMO technique is able to multiply the spectrum efficiency and communication reliability by taking advantage of multi-path fading without additional frequency band and power expends.Unfortunately, MIMO system may face many kinds of interferences in a practical scenario, such as multi-user interferences and directional interferences. These interferences may cause MIMO system to suffer significant performance loss and even make MIMO system fail to transmit useful information. Therefore, effective interference suppression is necessary to avoid system performance degradation. However, even though directional interference exists widely, especially in military communication scenario, discussion focusing on suppression of directional interference in MIMO system is limited. There are still many problems need be solved. Therefore, it is very important and necessary to build a MIMO system with high anti-interference ability.This dissertation focuses on the suppression of the directional interference in MIMO system, and is concerned with systematic and thorough researches on many aspects, such as the capacity of the MIMO system with directional interferences, various interference suppression schemes and the effective realization of channel estimation in interference scenario.Main contents of this dissertation are as follows:Firstly, the capacity of the MIMO system with directional interferences is researched. The capacity for deterministic and random fading MIMO channels is derived respectively. Moreover, the impacts of the interference-to-noise ratio (INR), correlation and directions of the interferences on capacity are discussed respectively. It is indicated that the relationship between the numbers of receive antennas and interferences is critical to the capacity in high INR. The number of receive antennas should be greater than that of the interferences in order to ensure the system work normally. It is also seen that higher correlation and closer directions of the interferences both yields higher capacity. Monte Carlo simulation results are given and well coincide with the theoretical analysis.Secondly, the interference suppression schemes based on beam forming for MIMO system with space-time block coding (STBC) in directional interference scenario are researched. The principle of two kinds of interference suppression schemes based on beam forming are presented, one is the kind of interference suppression schemes based on minimum mean square error (MMSE) criterion and beam forming, the other is the kind of interference suppression schemes based on least squares (LS) criterion and beam forming. And four realizations of these two kinds of interference suppression schemes are discussed in detail, which are the interference suppression scheme using direct matrix inversion (DMI) algorithm, the interference suppression scheme using normalized least mean square (NLMS) algorithm, the interference suppression scheme using LS batch algorithm and the interference suppression scheme using recursive LS (RLS) algorithm. These four schemes are valid to suppress interferences and realize the detection of the transmit blocks symbol by symbol through beam forming without performing channel estimation.Thirdly, the interference suppression schemes based on maximum likelihood decoding (MLD) are researched. A simplified interference suppression scheme based on MLD is derived after the conventional interference suppression scheme based on MLD is given. Compared with the conventional scheme, the simplified scheme is able to realize MLD of the transmit blocks symbol by symbol with much less searching calculations through linear processing. And in the pactical applications of these two schemes, how to utilize the equivalent system model to perform effective channel estimation in directional interference scenario is concerned. The theoretical SER performances of these two schemes are analyzed and compared with those of the schemes based on MMSE criterion and beam forming in detail.Fourthly, the interference suppression schemes based on noise subspace projection are researched. Utilizing the characteristic that the INR of the directional interference is usually high, the interference suppression scheme based on smplified MLD and noise subspace projection and the interference suppression scheme based on iterative MLD and noise subspace projection are presented. These two schemes both use the estimation of the noise subspace projected channel matrix instead of the estimation of the equivalent channel matrix to perform MLD, and simplify the processing of effective channel estimation in strong directional interference scenario. Moreover, the scheme based on iterative MLD and noise subspace projection reduces the bad effect of channel estimation error on decoding performance, and realizing MLD of the transmit blocks block by block instead of frame by frame by accumulating the increment of cost function iteratively. The SER performance of these two schemes are also analyzed and compared.Lastly, plenty of simulation results verify the interference suppression abilities of all the schemes mentioned above, and well accord with the conclusions drawn by theoretical analysis. The impacts of the length of the pilot matrix, the INR and directions of the interferences on the performance of these schemes are also discussed through simulations. The simulation platform of the MIMO system with a high anti-interference ability is developed by the author and others, and several directional interference suppression schemes mentioned above are realized on the platform. Static image transmission experiments in interference scenario are carried out on the platform, and the results show that the platform has high anti-interference ability.In this dissertation, the explorative researches on directional interference suppression for MIMO system provide more choices and references for the development of the MIMO system with high anti-interference ability.