Research On The Spectral Efficiencies Of Cell-free Massive MIMO With Underlaid D2D Communications

In order to meet the ever-increasing demand for higher service quality and more device connections in wireless networks,many key technologies and solutions have been introduced,such as Cell free massive MIMO and device-to-device(D2D)technologies,etc.Cell-Free Massive MIMO combines the advantages of Massive MIMO and distributed antennas,and eliminates the cell and cell edge.More specifically,it can avoid handover,effectively combat fading,suppress interference,solve the problem of poor signal quality for users at the edge of the cell,and provide users with a more balanced and unified high-quality service.In addition,D2 D technology can offload part of the traditional communication traffic to the D2 D link through resource reuse and direct user connection,thereby improving the spectrum efficiency of the system and reducing the transmission delay and energy consumption of communication.However,the reuse of resources will also lead to additional interference while the large number of APs in the Cell-Free Massive MIMO system can effectively eliminate some of the interference and provide more stable performance for the communication network.In order to combine the D2 D communication technology and cell-free massive MIMO,the pilot design and power control are important for reducing the co-channel interference between users.This thesis studies the spectral efficiency of the hybrid wireless networks with cell-free massive MIMO and D2 D technologies.First,this thesis studies the analysis and optimization of the spectrum efficiency of the hybrid network,considering an uplink scenario.The closed-form expressions of the uplink spectrum efficiency of the two types of users under the condition of imperfect channel state information are derived.Aiming at the pilot contamination problem caused by orthogonal pilots reuse,a greedy pilot assignment algorithm is proposed by iteratively updating the UE's pilot sequence which has the worst channel condition.For the co-channel interference problem caused by users sharing the same time-frequency resource,the power control optimization problem is formulated,which maximizes the minimum user spectrum efficiency to ensure the fairness of user service quality.Finally,the correctness of the analysis is verified by the simulation.It is shown that the introduction of D2 D communication into the cell-free massive MIMO system can significantly improve the overall spectrum efficiency performance of the system.Moreover,the proposed pilot allocation scheme and power control scheme in the thesis can effectively improve the system performance.Then,this thesis studies the pilot power control optimization of the hybrid wireless networks with hardware impairments.Considering more practical communication scenarios,the hardware impairments of transmitters will constraint the spectrum efficiency.The impact of hardware impairments on signal transmission is analyzed and the closed-form expressions of the uplink spectrum efficiency of the two types of users are derived.In order to reduce the channel estimation error caused by pilot reuse and hardware impairments,we propose a pilot power control design,which chooses the pilot power control coefficients to minimize the mean-squared error of the channel estimation.This is achieved via the sequential convex approximation method.Finally,the effectiveness of the scheme is verified by simulation.The results show that the pilot power control scheme proposed in this thesis can effectively reduce users' channel estimation errors and improve the minimum user's spectrum efficiency.