Research On Capacity Improving Method Of Multi-user Massive MIMO System

In recent years, multi-antenna communication system has not only applied to theoretical stage, but also already applied to modern wireless cellular systems massively. Today’s wireless communication systems, whether in practice or in theory, the inter-cell interference have become a bottleneck, this bottleneck severely limits the performance of the communication system. In 2010,some researchers put forward a system which need to set up large-scale antenna to replace existing multiple antennas on the base station side, and by using TDD mode, the base station can estimate downlink channel state information through the uplink pilot sequences,owing to the uplink channel state information and downlink channel state information is reciprocity, then we called this system large-scale MIMO communication system. Large-scale MIMO technology is different from the traditional technology in interference coordination, it use a lot of antennas to reduce the influence of irrelevant interference and noise on the useful signal, it realize the interference suppression by using space wireless resources, so as to achieve higher spectrum efficiency and power efficiency. In addition, the large-scale MIMO system can also launch the downlink signal in the direction of the target user through the downlink pre-coding technology, and then improve the system performance.Previous studies have shown that large-scale MIMO system can serve more users on the same time-frequency resources, restrain interference between cells effectively, and has a higher data rate and lower power consumption. However, due to pilot frequency reuse condition existence between cells in the system, pilot pollution problems in large-scale MIMO system can lead to the limitation of the system performance. In the other hand, because of the finite channel coherence time, the system is not able to serve much more users at the same time, which make the system throughput is limited. This paper mainly studied the spectrum efficiency of large-scale MIMO TDD system, and aimed at the above performance limiting factors, do a research on the spectrum efficiency enhancement method based on pilot scheduling, and studies show that the level of the system performance affected by pilot pollution associated with the distribution of pilot, let users whose interference with each other is serious use orthogonal pilot, and users whose interference with each other is small use same pilot, in this way, the pilot pollution between users can be effectively reduced, and we can achieve the purpose of improving system performance. Besides, this paper put forward a spectrum efficiency enhancement method based on user classification. Due to the limited channel coherence time, the time can be used to send pilot is limited, the maximum number of user terminals one cell can serve on the same time-frequency resources is limited, so that the channel capacity of one cell is limited. This method classify users according to the different length of channel coherence time, make users belong to different categories use different length of time slot and different frequency spectrum resource to transmit information, so as to increase the maximum number of users one cell can serve, and then achieve the goal of channel capacity enhancement.