Research On Uplink Pilot Allocation In Cell-free Massive MIMO Systems

In order to meet the requirements of high frequency and high energy efficiency in the new generation mobile communication system,the Cell-Free massive MIMO system has become one of the current research hotspots due to its strong macro diversity gain and antimulti-user interference characteristics.Especially in scenarios where users are highly dense and moving at a low speed,the system relies on the advantages of low AP cost,small size,and flexible deployment,and distributes APs that far exceed the number of users around users to achieve uniform coverage,which is conducive to meeting the requirements of "Usercentric" communication service requirements,while avoiding problems such as interference and frequent handovers between cellular edge users in the cellular network.However,due to the limited orthogonal pilot resources and the high density of users,serious pilot pollution of the system is caused.In addition,limited by the total power of a single AP,the AP serving too many users will not only fail to meet the power requirements of each user,but also increase the computational complexity of the AP,thereby affecting the frequency and energy efficiency of the system.Aiming at the above problems,this paper studies two aspects of pilot allocation and AP selection,and proposes a pilot allocation scheme based on a genetic map coloring algorithm and a pilot allocation scheme based on AP selection.The main research contents and innovation points are summarized as follows:(1)Aiming at the problems of uplink pilot pollution and high time complexity of pilot allocation in Cell-Free massive MIMO systems,a pilot allocation scheme based on a genetic graph coloring algorithm is proposed.Firstly,a user conflict matrix is generated according to the AP selection result.Then,the pilot allocation problem is transformed into a graph mcolor optimization problem,and the initial pilot allocation scheme is obtained through the genetic graph coloring algorithm.Finally,the pilot reuse is optimized based on the user interference coefficient based on the channel information,and the impact of the pilot pollution between users using non-orthogonal pilots on the system performance is further evaluated,so as to determine whether the secondary pilot allocation is necessary.Simulation results show that the scheme reduces pilot frequency reuse and improves the average spectral efficiency of users.(2)Aiming at the problems of high AP reuse degree and serious pilot pollution in the Cell-Free massive MIMO system,a pilot allocation scheme based on AP selection is proposed.Firstly,according to the large-scale fading coefficient,an AP selection algorithm combining exclusive AP and auxiliary AP is designed.The exclusive AP does not allow other users to reuse,and the auxiliary AP allows other users to reuse.Then,in order to reduce the time complexity of pilot allocation and the pilot pollution caused by the multiplexing of auxiliary APs by different users,the pilot allocation problem is transformed into the m-color decision problem of the graph.Finally,the problem is solved by the genetic graph coloring algorithm.Simulation results show that this scheme reduces AP multiplexing and improves system performance.