Research On Allocation Strategy Of Mitigating Pilot Contamination In Massive MIMO Systems

By deploying a large number of antennas at the base station,massive multiple-input multiple-output(MIMO)technology,which has become an important guarantee for 5G systems,can significantly improve the channel transmission environment and multiply the spectrum efficiency of the system.With the continuous popularization of wireless communication technologies,especially the development of massive machine type communication(mMTC),the number of user equipments has rapidly increased,making the limited orthogonal pilot resources become scarcer.Different users have to reuse the same pilot sequence,which creates the problem of pilot contamination.Although pilot contamination is not a unique problem for massive MIMO systems,its impact on massive MIMO is much more serious.With the increasing number of base station antennas,channels of different users will tend to be orthogonal,noise and other non-coherent interference will gradually disappear,and pilot contamination will become a major factor limiting the performance of massive MIMO systems.This paper will study the data transmission and random-access process of massive MIMO systems separately and propose some novel schemes to reduce pilot contamination from the perspective of pilot allocation strategy.The main contributions of this article are as follows:(1)This article introduces the basic principle and system model of massive MIMO technology in detail,and theoretically derives the causes of pilot contamination and its impact on system performance.Theoretical results show that,in the data transmission phase,when the number of base station antennas tends to infinity,pilot contamination will become the main factor limiting the achievable rate of uplink and downlink transmission.Meanwhile,in the random-access phase,pilot contamination may directly cause the user's random-access request to fail.(2)For the pilot contamination in data transmission,a pilot allocation strategy based on user location information is proposed.This scheme sorts the users and allocates the pilots sequentially according to the polar angle of the user's location in the polar coordinate system with the base station of the residential cell as the pole.The proposed scheme combines the characteristics of directional antennas to control the pilot contamination between users at a low level.Theoretical and simulation results show that,under the proposed pilot allocation strategy,pilot contamination between cells is effectively mitigated,the achievable rate of user transmission is significantly improved,and the bit error rate(BER)of data transmission is also dramatically decreased.(3)For the pilot contamination in the data transmission process,a pilot packet allocation strategy based on the change of user quantity is proposed.The scheme can dynamically adjust the multiplexing of pilots as the number of users changes and achieve soft handover between different pilot re-use factors,thereby combining the advantages of different pilot re-use factors.The proposed scheme can reduce pilot contamination in the case of a small number of users and provide services for more users in the case of a large number of users.(4)For the pilot contamination in the random-access process,a random-access protocol based on the idle pilot reallocation strategy is proposed.In this protocol,for the cases with known and unknown number of pilot users,corresponding idle pilot reallocation strategies are proposed to improve the effective utilization of idle pilots.