Pilot Random Access Schemes In Massive MIMO Systems

With the popularization of the internet of things,massive Machine Type Communications(MTC)need to be connected through the network.For the channel estimation in massive multiple input multiple output(MIMO)systems,it is infeasible to allocate each terminal a dedicated pilot via the conventional pilot allocation mechanism.Thus,the base station(BS)can not estimate the data correctly,which is called pilot contamination.Pilot contamination is regarded as the main challenge in massive MIMO systems.It is important to study the efficient schemes to resolve the pilot contamination.Pilot random access(PRA)scheme is one of the effective solution to allocate the pilot proposed by researchers in recent years,which can alleviate the pilot contamination efficiently.In this paper,we mainly focus on the PRA schems in the massive MIMO systems under the crowded envionments.For deeply understanding the PRA process,we first introduce the random access procedures in Long Term Evolution(LTE)systems,and mainly focus on the basic procedures of the contention-based random access.Then,we briefly describe six solutions for the overload problems in the Pysical Random Access Channel(PRACH).At last,we introduce two kinds of PRA schems,Strongest User Contention Resolve(SUCR)and Coded Pilot Access(CPA)scheme in detail.Then,the advantages and disadvantages of these schemes are analyzed.To alleviate the pilot contamination problem in massive MIMO systems,we propose a novel pilot random access scheme,called as the strongest user collision resolution combined timing advance(SUCR-TA)scheme.This sheme can reduce the number of contenders who will perform the SUCR algorithm by selecting an appropriate Timing Advance(TA)information,and then improve the probability of collision resolution.Simulation and theoretical results demonstrate that,compared with the SUCR algorithm,SUCR-TA scheme can significantly improve the number of successful terminals.Based on the above SUCR-TA scheme,we propose a SUCR-TA-IPA(SUCR-TA-Idle Pilot Access)scheme,whose key idea is to make full use of the idle pilots.Simulation results show that,compared with the SUCR-TA algorithm,SUCR-TA-IPA scheme can improve the pilot utilization efficiency and the number of successful devices.Besides,we also introduce SUCR,SUCR-TA and SUCR-TA-IPA schemes into multi-time slot access system.After simulating the expected number of devices that can be allocated pilots successfully of these schemes,we can know the SUCR-TA-IPA scheme can obtain the best performance.