Research On Pilot Contamination Mitigation For Massive MIMO Systems

Massive MIMO has been a hot research topic in recent years,as one of the key enabling technologies for 5G communications.When massive MIMO technology is applied in cellular networks,the base station is equipped with hundreds even thousands of antennas which can steer the narrow beams to users.Hence,not only the received signal strength is increased,but the time and frequency reusing among multiple users is also allowed.As a result,both energy efficiency and spectrum efficiency can be greatly improved.To enable the accurate beam steering,perfect channel state information of each communication link should be known by the transmitters,i.e.BSs.Sending pilots in uplink training phase in time division duplexing mode is the most widely adopted way for acquiring the channel state information for base stations.However,the length of pilots is restricted to the duration of coherence time and consequently the number of the mutually orthogonal pilots is limited.For the case with insufficient pilot sequences,users have no alternative but to reuse the pilot resources,resulting in interfered channel estimation,which is called pilot contamination.Pilot contamination is one of the bottlenecks of massive MIMO systems limiting the increase of capacity.Considering the issue of pilot contamination in multi-cell multi-user massive MIMO systems,in this thesis,we mainly focus on the joint design of pilot contamination precoding and pilot allocation,and the dynamic pilot allocation for massive MIMO systems.The main contributions are summarized as follows:Firstly,the transmission model of massive MIMO systems is given.By introducing the uplink and downlink transmissions,the closed-form expression of system capacity is derived.Then we analyze how pilot contamination influences the system performance in detail.And these two technologies:pilot contamination precoding and pilot allocation,are also introduced,to state how they work on the system performance.Secondly,the joint design of pilot contamination precoding and pilot allocation is proposed.Pilot contamination precoding is to preprocess the interference during downlink transmission in order to decrease the effect of pilot contamination.In conventional pilot contamination precoding schemes,pilot allocation is not considered,i.e.the pilot allocation is random.It is discovered that if the pilot allocation is changed,the system performance of pilot contamination precoding schemes is also changed.To furtherly enhance the system performance,two pilot allocation schemes are proposed based on pilot contamination precoding:the method based on greedy algorithm and the method based on matrix operations.The optimization is to find the proper pilot allocation to maximize the sum rate of the pilot contamination precoding assisted system.What's more,the optimization problem aimed to ensure fairness among users is proposed and solved.The proposed schemes can achieve better performance while enjoying low complexities.Finally,the dynamic pilot allocation scheme is proposed for massive MIMO systems.In conventional research related to pilot allocation,it is assumed that the number of users in each cell is equal and fixed.So in practical communications,if one new user equipment joins in,the pilot allocation need to be operated once again,which is not convenient and requires extra high complexities.Therefore,a dynamic pilot allocation method is proposed in this thesis.When a new user equipment joins in the communication,only part of the pilot allocation information is changed,to maintain the communication quality of the existing users and ensure the new user's connection with low complexities.